Relativistic Heavy Ion Group

Hard Probes 2020 in Austin, Texas (Online)

The 10th edition of the Hard and Electromagnetic Probes International Conference series was hosted as an online conference from June 1st until June 5th 2020 due the to coronavirus pendamic over the world. Everyone in the MIT Heavy Ion Group connected to the meeting and many of them presented the latest results for CMS and sPHENIX experiments in the meeting. We have given 5 parallel talks online. All students attened the online student lecture on May 31. The next day, in the afternoon of June 1, our almuni Chris McGinn gave a presentation on his work at MIT entitled "Measurement of Jet Nuclear Modification Factors with Large-R with the CMS Detector". Next, Molly reoprted the CMS photon-jet correlations results "Study of in-medium momentum broadening with photon-jet momentum correlations in PbPb collisions at 5.02 TeV with the CMS experiment".

On the next day, our PhD student Michael gave a parallel talk on the overview of sPHENIX heavy flavor program with the MAPS upgrade "Heavy Flavor Physics with the sPHENIX MAPS Vertex Tracker Upgrade".

On June 3, our group presented two open heavy flavor parallel talks. Our PhD student Zhaozhong first reported the new CMS B mesons results and the studies of beauty in-medium hadronization with the CMS 2018 PbPb dataset. His talk is "Measurements of nuclear modification factors of Bs and B+ mesons in PbPb Collisions with the CMS Experiment". After that, our group postdoc Jing gave a nice presentation the studies of X(3872) internal structure using the CMS 2018 PbPb dataset: "Evidence of X(3872) production in PbPb collisions at 5.02 TeV with CMS".

Finally, on June 4, Kaya gave an excellent talk on the new and exciting Z-hadron correlation studies with the 2018 PbPb entitled: "Parton modification studies using EW-boson-tagged hadrons with pp and PbPb collisions at 5.02 TeV with the CMS experiment".

This was our first time to participate in online major international conferences. All speaks in our group did excellent jobs in giving the presentations and delivered the physics messages to the audiences. We hope the pandemic will soon be over so that we can present our nice results and meet other physicists in person in future conferences.

Yen-Jie Lee awarded tenure from MIT

Great News: Yen-Jie was Awarded Tenured at MIT

(May 2020) Our group CMS analysis leader associate professor Yen-Jie was award tenured at MIT. Yen-Jie got his PhD from MIT and joined MIT as the Class of 1958 Career Development Associate Professor of Physics in September 2013. He is a young leader in heavy ion physics and has made great contribution understand QCD under extremely hot and dense conditions. He started the heavy flavor physics research program in heavy-ion physics at MIT and has trained many excellent PhD students and postdocs. He has won many prestigious awards such as the DOE Early Career Award, Alfred P. Sloan Prize, and Presidential Early Career Award. He has also served as the committee member in many international conferences and referee in many renowned journals. Let's congratulate Yen-Jie for his truly exceptional achievement and tenure at MIT!

Quark Matter 2019 in Wuhan, China

The XXVIIIth International Conference on Ultra-relativistic Nucleus-Nucleus Collisions - was held in Wuhan, China, during November 4-9, 2019. The MIT Heavy Ion Group sent a 10-member delegation to the conference and presented 2 plenary talk, 4 parallel talks, and 2 posters. Early in the morning of October 31, all students and postdocs attened the student lecture at the Science Hall of Central China Normal University.

On Nov 4, our students Ran, Michael, and Molly presented their posters during the poster section. On Nov 6, our group gave 4 parallel talks. In the morning, Molly presented the new CMS results on Jet nuclear modification factor using the 2015 CMS PbPb datasets entitled "Mapping the redistribution of jet energy in PbPb collisions using jets with various radius parameters with CMS". Then, in the early afternoon, Ran gave a parallel talk on "Study of in-medium momentum broadening with photon-jet momentum correlations in PbPb collisions at 5.02 TeV in the CMS experiment", showing the photon-jet correlations resulstion with the CMS 2015 PbPb datasets. Next, Zhaozhong presented the Non-Prompt J/ψ, B meson, and the new Ds studies entitled "Studies of Strange and Non-Strange Beauty Productions in PbPb Collisions with the CMS Detector ". Followed by Zhaozhong's presentation, Yen-Jie reported the first exciting observation of exotic hadron X(3872) in PbPb collision with CMS experiment. His talk "Evidence of X(3872) and studies of its prompt production in PbPb collisions". has received a lot of attention and lead to discussions from the heavy-ion physics community.

Yen-Jie is presenting his parallel talk in QM 2019

In the morning of On Nov 8, our group postdocs gave two plenary talks. The first talk was given by Yi on the experimental overview of jet substructure entitled "Jet substructure and parton splitting: an experimental overview".

Yi is giving her plenary talk in QM 2019

After that, Jing presented the experimental overview of open heavy flavor physics: "Heavy Quark production and energy loss: Experiments". Both of the talk were excellent and led to extensive discussions among the audience.

Jing is showing the audience about her plenary presentation

Everyone in our group worked hard on their analysis to prepare for Quark Matter 2019. Some measurements using CMS 2018 PbPb new data have been presents. We will present more important and exciting results with the 2018 CMS PbPb dataset in Hard Probes 2020!

Our new convener, Yi Chen.

New CMS Heavy Ion Convener

(September 2019) The renowned jet expert Dr. Yi Chen will be the new CMS Heavy Ion convener starting September 1st, taking over for the exiting convener, Camelia Mironov, the Queen, who did an excellent job over the last four years.

Yi is looking forward to guiding the CMS heavy ion group towards new discoveries with heavy ion data. Congratulations Yi!

Dr. Jing Wang

Great News: Jing Wang Won The 2018 Buechner Student Teaching Prize

(August 2019) Dr. Jing Wang received the award for her outstanding performance as a Teaching Assistant in 8.03 "Vibrations and Waves" with Prof. Yen-Jie Lee.

The Buchner Student Teaching Prize awarded to a graduate student for outstanding contributions to the educational program of the Physics Department during the past academic year. The $1,000 prize was established in 1987 by the late Mrs. Christina Buechner in memory of her husband Prof. William Buechner, who served as Physics department head from 1962-67.

Yen-Jie Lee receiving the PECASE award

Great News: Yen-Jie Lee won a 2019 Presidential Early Career Award

(July 2019) Professor Yen-Jie Lee received a 2019 Presidential Early Career Award for Scientists and Engineers (PECASE), the highest honor bestowed by the U.S. government to science and engineering professionals in the early stages of their independent research careers. The PECASE Awards are intended to recognize some of the finest scientists and engineers who, while early in their research careers, show exceptional potential for leadership at the frontiers of scientific knowledge during the twenty-first century. The Awards foster innovative and far-reaching developments in science and technology, increase awareness of careers in science and engineering, give recognition to the scientific missions of participating agencies, enhance connections between fundamental research and national goals, and highlight the importance of science and technology for the nation's future.

Yen-Jie was nominated by the Department of Energy, Office of Science "for innovative scientific leadership in the field of relativistic heavy ion nuclear physics investigating the energy loss of jets originating with charm and bottom quarks to determine the properties of the Quark Gluon Plasma".

Yen-Jie was among the seven MIT faculties who won the PECASE awards this year. Congratulations to Yen-Jie!

Dr. Jing Wang giving a plenary presentation in IS2019

Initial Stages 2019 Conference

MITHIG group members participated in the Initial Stages 2019 conference at Columbia University in New York City, which is the fifth installment on the physics of the initial stages of high energy nuclear collisions.

On behalf of the CMS collaboration, Dr. Jing Wang from our group presented the physics results from the CMS collaboration in a plenary talk, highlighting several publications from our group including the dijet angular distributions in proton-lead collisions (Phys.Rev.Lett. 121 (2018) no.6, 062002), which provided strong constraints on the gluon nuclear parton distribution functions, and the nuclear modification of D0 mesons in jet (CMS-PAS-18-007), which is a new approach developed in MITHIG for the detection of the heavy quark diffusion and energy loss inside the Quark-Gluon Plasma.

Dr. Austin Baty, who recently graduated from MITHIG and is now a postdoc at Rice University, presented the two-particle correlation function results from ALEPH archived data, an effort led by Prof. Yen-Jie Lee, in a plenary talk. The highlight of this result is a stringent limit on the long-range associated yield, which is closely related with the Quark-Gluon Plasma formation in heavy ion collision. The "non-observation" of this signal (for the first time in high energy particle collider) brought new insights to the understanding of the long-range correlation observed in proton-proton, proton-lead and lead-lead collisions. Prof. Yen-Jie Lee was also invited to chair the plenary session on Thursday.

Anthony Badea at the graduation ceremony

June 5th, 2019: Great News: Anthony Badea won a 2019 Malcolm Cotton Brown Award

Anthony Badea finished his undergraduate thesis and graduated this summer. Due to his outstanding works on reanalysis of the ALEPH archive data (See the recent paper submitted to PRL), he received a 2019 Malcolm Cotton Brown Award from MIT with the following quote:

"This award is presented to a senior in high academic standing in physics and outstanding research in experimental physics. Anthony was selected as the winner for his works using electron-position annihilation data to make a connection to larger collision systems like heavy ion collisions."

Congratulations to Anthony and good luck in Harvard!

Feb 25, 2019: Ta-Wei Wang defends thesis

Congratulations to Ta-Wei on his successful thesis defense on "Probing Quark-Gluon Plasma with Beauty Quarks". Many thanks to Ta-Wei for the good works in CMS on heavy flavor trigger, analyses, and MC production! Ta-Wei has accepted a Quantitative Researcher position in DRW!

Feb 5, 2019: Great News: Gunther Roland elected sPHENIX Co-Spokesperson

Professor Gunther Roland

The 2019 sPHENIX Co-Spokespersons electron closed on February 1st. Prof. Gunther Roland and Dave Morrison (BNL) were elected to served a second three-year term as sPHENIX cospokespersons with unanimous YES votes. sPHENIX is a new detector planned for the RHIC facility at BNL. sPHENIX will provide state-of-art capabilities for studies of the strongly interacting quark-gluon plasma using jet and heavy-flavor observables. The goal of sPHENIX is to understand the microscopic structure of the plasma and reveal how its strongly interacting nature arises from the underlying interactions of quarks and gluons described by quantum chromodynamics. We look forward to the first data-taking in 2022/23 under Prof. Roland's leadership.

Many congratulations to Gunther!

Hard Probes 2018

The 9th International Conference on Hard and Electromagnetic Probes of High-Energy Heavy-Ion Collisions (Hard Probes 2018) was hosted in Aix-Les-Bains (Savoie, France) from October 1st-5th, 2018, with a student lecture day hosted at CERN on September 30th. Hard Probes focuses on experimental and theoretical developments on perturbative probes of hot and dense QCD matter as studied in high-energy nucleus-nucleus, proton-nucleus and proton-proton collisions. The 6-member delegation from the MIT Heavy Ion Group presented several high-impact results, distributed over many areas of interest. The group's contributions to the conference began with Professor Yen-Jie Lee's plenary talk showcasing the "Highlights on Hard Probes from CMS" on the morning of October 1st. This was followed by several parallel talks on October 2nd: early that morning, Austin Baty presented the results of measurements of "Charged Particle Nuclear Modification Factors in pPb, PbPb, and XeXe Collisions with CMS;" the comparison of XeXe and PbPb collisions was used to constrain the path-length dependence of various energy loss models. In the afternoon, Michael Peters presented the results of the first measurement of "D-meson production in jets in pp and PbPb collisions with the CMS detector," providing insights into the dynamics of charm-quark propagation in heavy-ion collisions. Afterward, Kaya Tatar presented measurements of "Photon-tagged jet fragmentation functions and jet shapes in pp and PbPb collisions with the CMS detector," in which the final version of the jet shape measurement was presented for the first time; by measuring jets associated with isolated photons, these measurements constrain the initial parton kinematics and energy loss of quark jets in-medium.

Group photo of MIT Heavy Ion Hard Probes delegation

The next day, October 3rd, Yi Chen presented "A step towards tagging of quenched jets", which detailed the efforts in the implementation of techniques to attempt to study the initial hard splitting in jet fragmentation. The group's contributions were closed with Professor Gunther Roland's plenary talk "Everything you always wanted to know about sPHENIX at RHIC," which summarized developments in the sPHENIX detector commissioning. Prof. Roland also participated in a panel discussion regarding the future of the field of experimental heavy ion physics.

ICHEP 2018

The 39th edition of the International Conference on High Energy Physics (ICHEP 2018) was hosted in Seoul, South Korea from July 4th - 11th. ICHEP is the largest high energy physics conference in the world and serves as a vital platform for exchanging recent achievements and ideas in particle physics, astrophysics, and cosmology. During the heavy ion session, undergraduate Anthony Badea gave a very interesting talk on the measurement of the "Two-Particle Correlation in e+ e- Collisions at 91.2 GeV with ALEPH Archived Data". This result serves as an important step towards understanding the origin of collective flow phenomenon observed in small systems.

Anthony presenting the results on two-particle correlations in e+e-

Distribution of azimuthal angle correlation in archived e+e- data compared to Pythia simulation

Quark Matter 2018

The XXVII Quark Matter conference on ultrarelativistic heavy ion collisions was held recently in Venice, Italy from May 12 to May 19. The MIT Heavy Ion Group sent a 14-member delegation to the conference, and set a record with 8 parallel talks and 2 posters being accepted. Early on May 15, Austin Baty kicked off the proceedings with his talk on "Charged particle nuclear modification factors in pPb, PbPb and XeXe collisions with the CMS experiment", showing for the first time, measurements of charged-hadron suppression in xenon-xenon collisions at the LHC. He was followed by Jing Wang, who gave her talk on "D-meson production in jets in pp and PbPb collisions with the CMS detector", an effort to better understand the production of heavy-flavor mesons. Kaya Tatar presented measurements of jet substructure in photon-tagged events, making use of the photon to trace the energy of jets back to the original hard scattering: "Jet fragmentation and shapes for inclusive, b-tagged, and photon-tagged jets in pp and PbPb collisions with the CMS detector". To round up, Christopher McGinn presented various improvements to jet reconstruction techniques in the high-background environment found in heavy ion collisions: "Measurement of Jets in PbPb Collisions at 5.02 TeV with CMS".

Anthony and his poster

Professor Yen-Jie Lee gave a very interesting presentation on "Long-range angular correlations of charged particles in high multiplicity e+e- collisions using archived data from the ALEPH detector at LEP", framed in the context of trying to understand the origin of flow in small systems. This measurement was also the subject of a poster by Anthony Badea, an undergraduate student in our group who made leading contributions to this analysis.

Zhaozhong's talk in the open heavy flavor physics parallel session

In the open heavy flavor sessions, Zhaozhong Shi presented results on measurements of D-mesons: "Measurements of D meson nuclear modification factors and of direct and elliptic flow of D0 mesons in pPb and PbPb collisions at 5.02 with CMS". Ta-Wei Wang subsequently presented the first measurement of Bs mesons in heavy ion collisions "Measurements of strange and non strange beauty production in PbPb collisions at 5.02 TeV with the CMS detector", helping to further the understanding of strangeness enhancement in heavy ion collisions. Last but not least, the measurements of charged-hadron multiplicities in xenon-xenon collisions was presentedby Ran Bi: "Multiplicity and transverse energy measurements from pp, pPb, PbPb and XeXe collisions with the CMS experiment"

The past year has been a fruitful one for the MIT heavy ion group, and also for the heavy ion community in general. Looking to the future, there will be a last round of heavy ion collisions before the LHC takes a break and shuts down for 3 years. We expect to collect 3-4 times the total amount of data we currently have, and we hope that the following years will be equally productive as the past few.

MIT/CMS heavy ion group photo at the banquet at Quark Matter

Zhaozhong Shi

Great News: 2018 NSF Fellowship

Our second-year graduate student Zhaozhong has been awarded the National Science Foundation Graduate Research Fellowship. The NSF fellowship is a highly competitive fellowship supporting to U.S. graduate student working in STEM and social sciences. The fellowship recipients will be awarded stipends as well as the education allowance. The length of the fellowship is three years.
Zhaozhong's proposal plans to use charm quarks to probe the quark-gluon plasma with the CMS experiment. He will carry out the research on this proposal as well as working on the detector development for the sPHENIX experiment. Let's congratulate our for being awarded the NSF fellowship and hope him to have greater achievement as an NSF fellow in the future!

Wit Busza, Francis Friedman Professor of Physics, Emeritus

Heavy Ion Collisions: The Big Picture, and the Big Questions

Professor of Emeritus Wit Busza, along with Professor Krishna Rajagopal from Center of Theoretical at MIT, and Postdoctoral researcher Dr. Wilke van der Schee from Center of Theoretical Physics at MIT, were invited to write a review paper for Annual Review of Nuclear and Particle Science.
The preprint version of this paper, Heavy Ion Collisions: The Big Picture, and the Big Questions, is available on arXiv. This paper reviews the theoretical and experimental studies of relativistic heavy-ion physics at RHIC and LHC. It is an excellent introductory material for students to learn the physics motivations, general formulations, big questions, and future directions of relativistic heavy ion physics. We are proud of Wit's great contributions in relativistic heavy-ion physics!

August 23, 2017: Z+jet paper is on the cover of PRL

PRL Coverpage and Figure 2-bottom in the paper. Distribution of the transverse momentum ratio x_{jZ} between the jet and the Z boson. Compared to pp data, the distribution in PbPb is shifted to left indicating that the jets lose energy after interacting with the QCD medium.

Our measurement of jet quenching using Z+jet correlations has been published in PRL and chosen as the cover of corresponding issue (See the synopsis from the PRL journal).

Properties of the quark-gluon plasma can be inferred from measurements of jets and Z bosons simultaneously produced in the ion collisions that create the plasma. Z bosons interact with the QGP much more weakly than the partons do, so they don't lose energy as they travel through it. Thus the bosons energy is very close to the initial energy of the partons emerging from the ion collisions. And the difference between the boson and parton energies can reveal the QGP's properties.

The excitement is that the experimental uncertainties in identifying a Z boson are smaller compared to that for an isolated photon due to the complicated procedure of photon identification in heavy ion collisions. On the other hand, Z bosons are produced much more rarely compared to isolated photons. This was why the first measurement of Z+jet correlations in PbPb collisions is so hard and it only became possible thanks to the high statistics PbPb data collected by the CMS detector at the end of 2015.

2018 sPHENIX Test Beam at Fermilab

The Fermilab Test Beam Facilities where sPHENIX test beam was carried out.

In addition to the CMS experiment at CERN, the MIT Heavy Ion Group also involves in the planned sPHENIX experiment at RHIC. Currently, Professor Gunther Roland is serving as the spokesperson for the sPHENIX experiment. Graduate student Zhaozhong Shi has been working on the sPHENIX electromagnetic calorimeter. The 2018 sPHENIX Test Beam (T-1044), which tests the prototypes of the EMCAL, HCAL, MVTX, and INTT subdetectors, has been successfully carried out at Fermilab Test Beam Facilities from Feb 21 to Mar 28. This Test Beam project served as an important part of the validation and testing process of the sPHENIX experiment

The setup of the sPHENIX test beam in the enclosure.

Zhaozhong Shi participated in the test beam at Fermilab. He took the EMCAL position scan data and analyzed its energy uniformity. His results will help sPHENIX physicists to understand the current performance of EMCAL so that one could optimize the EMCAL detector design for future photon and jet measurements with sPHENIX. We look forward to the construction and data taking of the sPHENIX expeeriment in the future!

Great News: CERN Staff Position

Dr. Gian Michele Innocenti in our group got a staff scientist position at CERN. He will start working in September 2018. Congratulations on Gian Michele and wish him to have greater achievement at CERN in the future.

Mexico Workshop 2017

The 2nd International Workshop on QCD Challenges from pp to AA was hosted in Puebla City (Mexico) from 31 Oct to 3 Nov 2017. More than fifty high energy and heavy ion theoretical and experimental physicists were in attendance discussing the progress on the understanding of the collective, and possible QGP signals in the hot dense QCD systems of the fundamental strongly interacting matter. Prof. Gunther Roland was invited to give a plenary talk discussing the development of jets study as probes of QGP. Several important measurements were presented by MITHIG members.

Gunther answering questions during the plenary talk.

Jing presented the measurements of D meson nuclear modification factor and vn harmonics in PbPb collisions with CMS. These measurements deepen our understanding of the interactions between heavy quarks and the medium, and provide insights into the degree of the thermalization of the bulk medium. Ta-Wei presented results on open beauty production and modifications in PbPb collisions with CMS, that includes the measurement of J/psi mesons from B meson decays, as well as fully reconstructed B mesons in PbPb collisions. They are one of the key measurements to address the flavour-dependence of in-medium energy loss.

Nuclear modification factor of D mesons, charged particles, B mesons and nonprompt J/psi mesons in PbPb collisions at 5.02 TeV.

Ran presented the results on energy loss in photon-jet and Z-jet events in PbPb collisions with CMS. These measurements provide strong evidence for medium-induced jet energy loss by using the energy of these electroweak bosons that do not participate in the strong interaction as a reference. Zhaozhong presented the test beam experiment of sPHENIX, a proposed experiment to succeed the current PHENIX experiment, at Fermilab Test Beam Facilities. This experiment studies the performance of the 2017 prototype electromagnetic calorimeter and hadronic calorimeter.

Quark Matter 2017

The XXVI Quark Matter conference on ultrarelativistic heavy-ion collisions was held recently in Chicago, USA from 5 Feb to 11 Feb 2017. The MITHIG sent a 11-member delegation to the conference and contributed to the proceedings with a plenary talk by Prof. Yen-Jie Lee summarizing the recent exciting results from the CMS heavy-ion group, 2 talks in the parallel sessions and 3 posters. Prof. Gunther Roland also gave a student lecture on the evolution of jet measurements as probes of the Quark Gluon Plasma, as a prelude to the conference.

Ta-Wei introducing the motivation behind open beauty measurements in heavy-ion collisions.

Ta-Wei Wang presented results on open beauty production and modification in PbPb collisions with CMS, including the first measurement of fully reconstructed B mesons in PbPb collisions. These measurements are important for their ability to determine the flavour-dependence of energy loss in the medium. A strong suppression of both J/psi particles from B meson decays and fully reconstructed B mesons was observed. Ran Bi presented the latest results on energy loss in photon-jet and Z-jet events in PbPb collisions. These measurements are able to precisely characterize the jet energy loss by using the energy of the unquenched boson as a reference. The results show a clear dependence of the jet energy loss on the transverse momentum of the photon and the centrality of the collisions.

In the poster session, Jing Wang presented the latest results on the D meson nuclear modification factor (RAA) in PbPb at 5.02 TeV, Austin Baty presented the latest results on the charged particle nuclear modification factor in both pPb and PbPb at 5.02 TeV while Dr. Gian Michele Innocenti presented the latest results on the B meson nuclear modification factor in PbPb at 5.02 TeV. Gian Michele's poster was also selected as one of the best posters out of around 300 total posters, and was promoted to a plenary flash talk that was given on the last day of the conference.

Gian Michele giving his flash talk on the D meson RAA measurement to close out the conference.

Hard Probes 2016

The 8th edition of the International Conference on Hard and Electromagnetic Probes of High-energy Nuclear Collisions (Hard Probes 2016) was hosted in Wuhan (China) on September 23 - 27. Hard Probes is one of the most important conference in our field and it is particularly focused on the recent experimental and theoretical developments on hard and electromagnetic probes in heavy-ion collisions. Several new measurements were presented by MITHIG members in parallel and plenary talks.

Distribution of xJ? of photon+jet pairs of pp and PbPb collisions normalized by the number of photon+jet pairs in central PbPb collisions at 5.02 TeV.

Chris presented new results on photon-jet correlations in pp and PbPb collisions at 5.02 TeV. This measurement allowed to characterise jet quenching precisely as a function of centrality and photon pT using the unquenched reference given by electromagnetic probes. Kaya presented the measurement of Z+jet correlations in pp and PbPb collisions at the same energy that provided further insights on our understanding of jet quenching in heavy-ion collisions. Yen-Jie presented the recent results of pseudo-rapidity distribution of di-jet in proton-proton and proton-nucleus. This very precise measurement allowed to provide very important constraints on the parton distribution functions used in theoretical calculations for pp and pPb collisions.

B meson nuclear modification factor in 0-100% PbPb collisions at 5.02 TeV.

Ta-Wei presented the first measurement of B meson nuclear modification factor in PbPb collisions that confirmed a strong energy loss of b-quarks in the medium and provided further insights on the flavour dependence of energy loss in heavy-ion collisions. Jing presented the recent results on D meson nuclear modification factor in PbPb collisions that showed an impressive agreement between the RAA of charmed and light particles up to very high pT, providing strong constraints on theoretical calculations. Gian Michele gave an overview of heavy-flavour results at the LHC in the experimental wrap-up session.

Initial Stages 2016

Austin presenting an exciting new result at 5 TeV.

(May 2016) Many members of the MITHIG group attended the Initial Stages 2016 conference in Lisbon, Portugal. There were two plenary talks and two parallel talks given by the MITHIG group members. Between enjoying the Portuguese seafood and Fado music, they presented exciting new results coming from the data that was gathered in the fall of 2015.

Kaya Tatar presented a new measurement of jet quenching in PbPb which uses events where a high energy jet is emitted along with a Z boson. This is the first time that a significant modification of Z+jet correlation, which is consistent with jet quenching, was observed. Austin Baty showed a measurement of the PbPb charged particle nuclear modification factor up to 400 GeV/c, significantly increasing the high pt reach of previous measurements. The nuclear modification factors extracted from Austin's studies were found to be increasing as a function of charged particle transverse momentum, reaching a value very close to 1.

Kaya showing off a Z+Jet event display.

Dr. Gian Michele Innocenti presented a new measurement of the supression of D0 meson production in PbPb collisions in his plenary talk, elucidating the behavior of heavy flavor quarks in the medium. This is the widest D0 meson kinematics range ever measured in PbPb collisions. Surprisingly, the D0 nuclear modification factors wer found to be similar to the ones from charged particles. All three measurements were the first measurements performed with PbPb collision data at 5 TeV, significantly improved the statstical reach of previous measurements at lower energies, and will help elucidate the behavior of high momentum particles traversing the quark gluon plasma.

Nuclear modification factor for charged particles and D mesons.

A comparison of the remarkably similar nuclear modification factors for charged particles and D mesons, shown by Gian Michele.

Prof. Yen-Jie Lee was also invited to give a plentary talk discussing the current status of nuclear parton distribution function measurements, their parameterizations and their importance for looking at small systems. It was shown in the talk that measurements of electroweak bosons and high pt jets, which could be performed with high accuracy, will be able to further improve our understanding of nuclear parton distribution functions. Studies of various processes will also help to verify the factorization assumptions and the universality of the nuclear parton distribution functions.

Our new convener, Camelia Mironov.

New Heavy Ion Convener

(May 2015) The renowned Camelia Mironov will be the new CMS Heavy Ion convener starting September 1st, taking over for the exiting convener, Yen-Jie Lee. The self-styled 'royal highness' is looking forward to guiding the CMS heavy ion group towards new beautiful results in the years to come. Congratulations Camelia!

2016 Sloan Research Fellowship!

Sloan Fellow Yen-Jie Lee.

(April 2016) Yen-Jie Lee has been named a 2016 Sloan Research Fellow! Since 1955, the Sloan Research Fellowships are given to early-career scientists and scholars whose achievements and potential identify them as rising stars among the next generation of scientific leaders. Yen-Jie is one of the 126 researchers to earn this prestigious two-year fellowship, which will allow him to make even more contributions to our field, particularly in the study of jet quenching and heavy flavor dependence of parton energy loss. Congratulations Yen-Jie!

News from LHCC Poster Session

(Upper Pannel) Chris, Krisztian, Kaya
(Lower Pannel) Gian Michele, Austin

(March 2016) MITHIG made a strong showing in the 2016 LHCC poster session!

There are four posters presented in the session, including "Triggering on PbPb events with CMS in 2015" which showed the trigger performance in the 2015 heavy ion run (by Krisztian Krajczar and Kaya Tatar), "Heavy Flavor Meson Production in pp, pPb and PbPb collisions with the CMS detector" which featured the D meson production in pp and PbPb collisions at 5 TeV (by Gian Michele Innocenti, Ta-Wei Wang and Jing Wang), "Angular distributions of the quenched energy flow from dijets with different radius parameters in CMS" which summarized our published missing transverse momentum paper (by Chris McGinn) and "Fragmentation Functions of jets in pPb collisions in CMS" which presented the jet-track correlations measured in proton-lead collisions (by Austin Baty).

The posters can be found in this indico page.

Great news: Infinite Kilometer Award

Ivan Cali, MITHIG Research Scientist

(Jan 2016) Congratulations to Ivan Cali for receiving a 2016 Infinite Kilometer Award of the MIT School of Science for his outstanding work in MIT community and CMS experiment!

Ivan Cali led the L1 trigger upgrade efforts in CMS and played a very important role in the 2015 heavy ion data-taking period. Together with friends in the CMS collaboration, we had a very successful heavy ion run and collected high statistics pp and PbPb samples at 5 TeV.

News from Quark Matter 2015

With preparations for the upcoming LHC heavy ion Run 2 in full swing, the heavy ion community took a brief break to review exciting new results from Run 1 data at Quark Matter XXV. This year's conference was held on Rokk? Island in Kobe, Japan, from September 27 to October 3.

Members of the MIT Heavy Ion Group were in attendance, with everyone contributing to the proceedings. Gunther Roland gave a lecture on hard probes of the Quark Gluon Plasma (QGP) to many bright-eyed graduate students and postdocs at the preparatory student day. The next day, Yen-Jie Lee successfully managed to summarize all the new CMS results in only five minutes during his 'teaser talk.' Yen-Jie also presented an impressive poster detailing CMS's ability to reconstruct D mesons in PbPb events.

Austin explains a novel interpolation technique used in the pPb fragmentation function measurement.

Austin Baty presented an important measurement of jet fragmentation functions in pPb events, exploring the relationship between jets and the charged particles they produce. The result indicates that the fragmentation function modification previously observed by the MIT group in PbPb events (see the June 4, 2014 entry on this page) is indeed a signature of the QGP and not due cold nuclear effects. Finally, Chris McGinn showed a large update of the 'missing pt' measurement previously shown at Quark Matter 2014. In particular he emphasized nontrivial dijet structures present in both PbPb and (perhaps surprisingly) pp collisions caused by the existence of three or more jets in the event. The result is now submitted to JHEP and is available at arXiv:1509.09029.

CMS also presented multiple exciting new results on collective behavior in small systems - a topic which was heavily discussed and debated at the conference. The group is now working hard to ensure Run 2 goes smoothly this November.

Missing pt for different jet clustering distance parameters.

A comparison of the 'missing pt' in PbPb and pp events, calculated after clustering jets with different radii, as shown in Chris's talk.

Hard Probes 2015

Doga giving a presentation in the plenary session on jets (chaired by Gunther on the right).


The 7th edition of the Hard Probes conference was hosted in the beautiful Montreal, Quebec, Canada (June 27-July 3). In one of the most important heavy ion physics conferences, many new experimental and theoretical developments were presented and discussed. Among the presentations were talks by Yue-Shi Lai, Yen-Jie Lee, Ta-Wei Wang and Dragos Velicanu on CMS data, and overview talks by Doga Gulhan on jets and the strongly interacting medium and Camelia Mironov on quarkonium production in pp, pA and AA collisions.

Camelia giving her unique perspective on quarkonium physics in heavy ion collisions.

First ever direct B meson reconstruction in nucleus nucleus collision, shown in Ta-Wei's parallel talk.

One of the highlights for our group was shown on the very last slide of Ta-Wei Wang's talk, presenting the first-ever reconstruction of exclusive B-meson decays in heavy-ion collisions. While the result is only a CMS "detector performance plot" at this time, it points to the exciting new possibilities for understanding heavy-quark energy loss based on the upcoming LHC Run 2 data.

New Phobos paper submitted

Before joining CMS, the Relativistic Heavy Ion Group was a member of the PHOBOS collaboration at the RHIC accelerator, a part of Brookhaven National Lab on Long Island, NY. Working with current group member George Stephans, Corey Reed (one of the group's former graduate students now an Associate Project Scientist at the university of California at Irvine) has just completed a new publication using PHOBOS data. The analysis used forward calorimeters to "tag" the subset of deuteron+gold collisions in which only the proton or neutron in the deuteron interacted with the gold nucleus. This is currently the only way to study proton+gold collisions at RHIC energies and the only way to study neutron+gold collisions at any highly relativistic energy. The arXiv version of the paper can be found here.

Bolek Wyslouch

Bolek Wyslouch named director of LNS

(May 2015) From the MIT News office: Boleslaw "Bolek" Wyslouch, professor of physics, has been named the new director of the Laboratory of Nuclear Science (LNS), effective July 1. "Bolek is a superb physicist who has a clear vision for the future of LNS and the strong support of its members," Michael Sipser, dean of the School of Science, says. "I am delighted that he has agreed to be the next LNS director, and I look forward to working with him." Congratulations, Bolek!!

Great news: Early Career Award

Yen-Jie giving a plenary talk at Quark Matter 2014.

(May 2015) Congratulations to Yen-Jie Lee for receiving a 2015 DOE Early Career Award for his proposal "Study of Heavy Flavor Mesons and Flavor?Tagged Jets with the CMS Detector"!

More on these competitive awards can be found at here. Among the 44 awards nation-wide, selected from 620 proposals, Yen-Jie's is the only one in the field of heavy-ion physics.

Great news: Infinite Kilometer Award

Gian Michele Innocenti

(Dec 2014) Congratulations to Gian-Michele Innocenti for receiving a 2014 Infinite Kilometer Award of the MIT School of Science for his outstanding contributions to MIT's mission and community!

Gian Michele has been working on the L1 trigger upgrade and heavy flavor meson analyses in heavy ion collisions in the CMS collaboration.

Great news: Henry Kendall Teaching and Deutsch Awards

Award Alex won was named after Henry Kendall.

(Sep 2014) Congratulations to Alex Barbieri for winning the Henry Kendall Teaching Award for his great work as a teaching assistant of the Vibrations and Waves course and " for going out of his way to help students ".

Congratulations to Doga Gulhan for winning Deutsch Award for Excellence in Experimental Physics " for the excellence that characterizes her detector and analysis work and everything she does, leading to the most sophisticated CMS heavy ion analysis to date ".

More information about the awards can be found here

Lisbon Jet Workshop 2014

Doga presents a study of dijet events in pPb collisions, making good use of the analog pointer.

The third Heavy Ion Jet Workshop was held recently in Lisbon, sponsored by MIT and Instituto Superior Tecnico. Over three days, more than twenty five theoretical and experimental physicists gathered and discussed recent results in jet physics and the modifications observed when jets propagate through the strongly coupled medium created in the high energy collisions of nuclear matter, or Quark Gluon Plasma (QGP). Of particular interest at this workshop were new experimental results recently presented at the Quark Matter conference in Darmstadt, new background subtraction techniques for a many particle environment, and the introduction of several Monte Carlo generators modeling heavy ion collisions.

Jets are of particular interest for study of the QGP, as in dijet and gamma-jet events they make excellent probes of medium properties. Jets correspond to high momentum partons scattered perpendicular to the beam pipeline, producing a highly collimated spray of particles which can be reconstructed with our detector and then clustered into the final object. As opposed to the vacuum case studied by our colleagues in particle physics, a loss of jet energy in the QGP has been observed. This energy loss is dependent on the size of the produced medium, as an increasing number of nuclear participants in the collision corresponds to greater energy loss. New experimental results and Monte Carlo generators seek to characterize this effect, and thus develop our understanding of the produced medium.

Chris presents recent results in PbPb collisions. Much like background, a blurred photo can obscure the signal.

Doga Gulhan, Yen-Jie Lee, and Chris McGinn presented recent results on behalf of the MIT Heavy Ions Group, showing respectively results from dijet events in pPb collisions, an exploration of nPDF sensitive observables at the CMS detector, and a missing momentum study of dijet events in PbPb collisions. Additionally, Doga Gulhan presented a generator level study of dijet system eta distributions in pPb events, showing that the effect can be reproduced under the assumption that the proton is largely driving the shift. These experimental results in heavy ion jet physics can constrain the space of current explanations for observed jet quenching, and perhaps even inspire new ideas for further measurement.

An illustration of the new powerful HF/Voronoi background subtraction. After subtraction, momentum in eta-phi space is largely consistent with zero.

To study jets and the impact of the QGP, a rigorous background subtraction method specific to the high particle multiplicity environment of heavy ion collisions is needed. The large numbers of particles produced in collisions of nuclear matter can obscure a signal from a jet, or enhance said signal if found on an upward fluctuation of the background. Thus, a jet must be corrected to more accurately reflect the true momentum and energy. To this end MIT postdoc Yue-Shi Lai presented the new HF/Voronoi subtraction technique. The panel here from his slides shows a map of momentum in eta-phi space before and after applying HF/Voronoi background subtraction.

The workshop concluded with some discussion of standardizing Monte Carlo output such that these new generators can be more readily incorporated into existing software frameworks. With standardization in place, cross comparisons between different generators and data will hopefully teach us more about the properties of jets in the QGP.

Quark Matter 2014

Here we see relative to proton-proton systems, dijet imbalance is recoved in central PbPb systems by low momentum particles distributed through large angles.

The Quark Matter conference returned to its birthplace, Darmstadt, in its 24th edition. Results from proton-Lead collisions at the LHC continued to generate interest, with contributions from MIT and CMS at the forefront. The most intriguing results from the previous Quark Matter conference concerned energy loss in the hot dense medium formed in heavy ion collisions. This conference continued in that vein, and MIT presented exciting new results exploring the origin of this energy loss in proton-Lead and Lead-Lead collisions.

In PbPb collisions, Doga Gulhan presented an extensive study of parton energy loss traversing the quark gluon plasma in dijet events as part of a parallel talk. In dijet events of high momentum imbalance, the missing momentum is recovered by summing over tracks scattered through large angles in the medium. By projecting the momentum of tracks onto an axis symmetric with respect to the dijet system, we are able to study the angular distribution of particles in both the leading and subleading jet hemispheres. The accompanying slide of particle angular distribution is taken directly from Doga?s presentation.

Doga giving a talk on the interesting dijet imbalance result in PbPb collisions.

Yet another way to investigate this energy loss is to use heavy-flavored particles. By looking at the spectra of B-mesons, we can try to understand whether or not the energy lost by a particle in the medium depends on the flavor of the quarks which constitute it. Gian Michele?s poster, discussing B-meson reconstruction and spectra in proton-Lead collisions was one of 5 posters selected (from nearly 400 entries) for a five minute flash talk on the final day of the conference.

While dijet and heavy-flavor systems can offer insight into the state of the probe after interaction with the quark gluon plasma, it is difficult to disentangle what the state of the probe was before traversing the medium. One solution is to use a much rarer event topology using photons. These photon-jet events allow an unbiased characterization of the energy both before and after interaction with the medium. Alex Barbieri presented updated results of our previous photon-jet paper confirming the earlier result (significant quenching in PbPb) with higher statistics pp reference data. In addition, no modification of the probe was observed in the new pPb data.

Alex presenting dijet and photon-jet results in pPb and PbPb collisions.

Using the new pPb data collected in 2013, a very sensitive measurement of the change of parton distribution functions (PDFs) due to nuclear effects was conducted using the distribution of the average eta of dijet systems as the ruler. Alex presented this result as part of his parallel talk, and there was significant interest from the theoretical community for this result, as it is one of the only measurements sensitive enough to differentiate between multiple competing models of the nuclear PDFs.

Another highlight of the conference was the ATLAS confirmation of a baffling result first presented by CMS at the Hard Probes 2013 conference which shows that there is an excess of high transverse momentum charged particles in proton-Lead collisions as compared to proton-proton collisions. We hope to explore in further detail these, and other interesting phenomena in heavy ion collisions in the next few months!

June 4, 2014: Fragmentation function measurement in PbPb and pp collisions

Figure 1

Figure 1 in the paper: Fragmentation functions (top) and PbPb-to-pp ratio (bottom) vs. ξ for 100-300 GeV/c jets and charged track pT > 1 GeV/c.

Our measurement of the fragmentation function in PbPb and pp collsions is submitted to arXiv:1406.0932 and for publication in Physical Review C. This paper is the second PbPb/pp fragmentation function measurement from CMS, and the first paper to observe a significant modification in PbPb collisions when compared to pp. The evolution of this difference is further explored as function of both the the collision centrality, and for the first time, jet momentum.

January 20, 2014: Dijet measurement in pPb collisions

EPS09 comparison

A comparison between dijet eta distributions in data and in NLO calculations are shown.

Our paper on first event activity dependent jet measurements in pPb collisions is submitted to arXiv:1401.4433. The dijet transverse momentum ratio is found to be independent of event activity in pPb collisions. This confirms that the dijet transverse momentum imbalance enhancement in central PbPb collisions is not originating from initial-state effects. It also shows that any final state effect on jets in pPb collisions is small, justifying the usefulness of jets in studying initial state effects. Initial state effects due to nuclear parton distribution functions(nPDFs) are quantified by the measurement of dijet pseudorapidity in minimum bias events. The measurement is agreement with EPS09 nPDF, while it disfavors the calculation with CT10 proton PDF. The observation of a large modification in dijet pseudorapidity and the shift in its mean towards the lead beam direction as a function of event activity sets a new puzzle for the field. Some clues to understand the shift are included in the paper by the measurement of dijet pseudorapidity as a function of activity in the lead direction for fixed the activity in the proton direction.

Great News!

(Dec 2013) Victoria Zhukova was one of the winners of the 2014 MIT School of Science "Infinite Kilometer" awards, recognizing her contributions to MITs mission in education and research. Congratulations, Victoria!

Bolek Wyslouch and Gunther Roland were named Fellows of the American Physical Society. Bolek was cited "For his leadership role in the PHOBOS experiment and in creating a world-class heavy ion research program within the CMS Collaboration at the LHC" and Gunther "For his pioneering work on particle correlations in high-energy nuclear interactions, which led to the discovery of triangular flow, and his role in steering the PHOBOS and CMS heavy-ion physics programs." Congratulations!

July 1-4, 2013: Paris Jet Workshop

Proton-lead collision

An event display of a heavy ion collision containing two reconstructed jets with unequal energies.

At the start of July, MIT and Universite Pierre and Marie Curie jointly organized a workshop on Jets which took place in Paris. About twenty high energy and heavy ion theoretical and experimental physicists were in attendance discussing the challenges and new opportunities of doing jet measurements and predictions in heavy ion collisions. A jet is the experimental signature of a quark or gluon that is scattered off at very high momentum perpendicular to the direction of motion of the colliding particles, the beampipe, and into our detector. However a single quark or gluon can not freely propagate through the vacuum to reach our detector but will transform into more quarks and gluons sharing the original momentum, in a process called fragmentation. It will also pull particles from the vacuum to create a collimated spray of color neutral particles that exist for timescales observable by our detectors, in a process called hadronization. This spray of particles is what we call a jet and it carries exactly the energy and momentum of the initial parton, so by reconstructing the jet in our detector we can infer these kinematic properties of the partons that we can't directly observe.

Jets are also the link between the theory and experiment of high energy quarks and gluons. One challenge is for experimentalists to make sure their measurements of jets are calculable theoretically, as well as theorists making jet predictions that are measurable. For this we have jet algorithms that both theorists and experimentalists agree upon and use, the anti-kt algorithm is most commonly used by our group to reconstruct jets. At this workshop we discussed how to expand the ways in which theorists and experimentalists can compare data and theory in more effective ways.

Physicists clustered by the anti-kt algorithm.

Physicists clustered by the anti-kt algorithm.

In the figure on the left are displayed two jets reconstructed with this algorithm in a collision recorded by the CMS detector. One of the jets here has almost three times the energy of the of the second one but is completely back-to-back , pi units apart in azimuth. This means the jets coming from a pair of particles scattering off one another conserve momentum in direction but not in magnitude, which implies the quark gluon plasma seems to slow down high energy quarks and gluons but not significantly change their direction.

However alongside the spray of particles originating from the jet, in heavy ion collisions there is also a large spray of particles everywhere in the detector from the expanding quark gluon plasma. This makes reconstructing jets to accurately reflect the properties of the original parton much more difficult in heavy ion collisions since it becomes ambigous which particles around the jet come from the quark gluon plasma and which come from the parton. At this workshop we discussed various techniques and new ideas to subtract these background effects and understand the nature of jet reconstruction in a dense environment.

May 13, 2013: Frank Ma defends thesis

Congratulations to Frank on his successful thesis defense on "Detailed Characterization of Jets in Heavy Ion Collisions Using Jet Fragmentation Functions". In the fall Frank will continue his studies in Boston at the Gordon Conwell Theological Seminary.

March 28, 2013: Yongsun Kim defends thesis

Congratulations to Yongsun for succesfully defending his thesis on "Study of jet quenching using gamma-jet events in Heavy Ion Collisions at 2.76TeV". Yongsun will soon go back to Korea where he will continue to work on CMS as a postdoc at Korea University.

February 28, 2013: Yetkin Yilmaz defends thesis

Congratulations to Yetkin for succesfully defending his thesis on "Jet quenching in heavy-ion collisions at LHC with CMS detector". Yetkin will now be continuing to work on the CMS experiment as a postdoc in the heavy ion group at LLR.

Spring 2013: Colliding different particle species: the LHC run 1 ends with proton-lead collisions

Dijet in pPb collision

A dijet event in a proton-lead collision as seen by the CMS experiment.

The new year brought a new type of collision at the LHC: the accelerator smashed protons and lead nuclei together. Although we already caught a glimpse of these asymmetric proton-lead (pPb) collisions during a pilot run last September, the data collected early this year was the first sustained pPb run at LHC energies.

The high luminosity pp data taking aimed at precision measurements addressing the properties of the recently discovered higgs boson ended in mid december 2012 with a total integrated luminosity of 23fb-1. After three weeks of downtime the recommissioning of the accelerator and the CMS detector for pPb data taking started on January 4th during the annual CERN Christmas break. Commissioning the detector involves making sure all of the subdetectors of the CMS experiment are operating within their expected parameters. On 7 January, when everyone returned to CERN, cooling and power were up and running. Since the collision rate in the pPb run was lower than that in the pp one, two forward subdetectors that couldn't withstand the radiation under the harsher conditions of proton collisions have been reinstalled in CMS: CASTOR and ZDC. These subdetectors will help measure the collision remnants that travel very close to the beampipe, crucial to the study of pPb data.

The biggest challenge in preparing the CMS experiment for pPb collisions was to configure the trigger system that selects the collisions to be recorded to mass storage. Compared to proton proton or lead-lead collisions we are looking at different physics processes happening at different beam intensities. This requires dedicated trigger menus to be developed by the CMS Heavy Ion group, which tell the data aquisition system how many events of which type (e.g events containing high pT jets or pairs of muons) to keep and which events to reject. When colliding lead nuclei, there are around 4000 interactions each second, of which around 200 are selected to be recorded to tape. The proton-lead collisions, on the other hand, happened at around 2,000,000 interactions each second, and CMS recorded around 1000 of these. This of course requires a carefull selection of events to ensure all future physics analysis will have a sufficient number of events to work with, while making sure not to exceed the available output bandwidth.

For this run, CMS also joined forces with the TOTEM experiment to cover a greater range of collision data. The two are essentially separate entities -- independent experiments that use different analysis software -- and they are fully complementary. CMS measures in the central region and TOTEM exclusively measures in the very forward region. Combining information from both allows us to perform a lot of physics studies that previously were impossible to do by correlating proton remnants seen in TOTEM with objects such as jets and Upsilon particles observed in the central part of CMS.

Proton-lead collision

A proton-lead collision as seen by CMS from a side view, here the protons come from the right and lead nuclei come from the left.

During the 2013 Ion beam period the LHC delivered 31.3 nb-1 of pPb collisions to CMS and also provided a short run of pp collisions at the center of mass energy of the PbPb collisions collected in 2011. The pPb data sample corresponds to about 60 billion collisions sampled by the CMS experiment and will now serve as a reference to the PbPb collision data.

In PbPb collisions a hot and dense system of strongly interacting matter is expected to be produced, which in turn we study by means of hard interactions produced inside the hot medium. Fast partons produced in these hard interactions traverse the hot medium and are modified by it, e.g. they lose energy due to the interaction with the medium. To perform precision studies to quantify these medium modifications it is essential to disentangle the effects on the production of these probes due to the initial state, i.e the parton distribution function of a highly Lorentz contracted lead nucleus, and the final state effect due to the presence of the medium. At this stage the pPb data comes into play, since colliding a proton with a lead ion will expose all initial state effects while the system size of the interaction in expected to be too small to form an extened volume of a strongly interacting medium. Apart from serving as a reference sample to PbPb collisions the analysis of pPb data is already starting to yield results that also make the study of these collisions a very interesting topic in its own right, like the recent analysis of two particle correlations that shows an unexpectedly large ridge like structure.

Going into the first long shutdown period of the LHC lasting until end of 2014 the CMS Heavy Ion group is now well equipped with a large PbPb collision data set and matching statistics pPb and pp reference samples. The quiet time during the LHC shutdown is eagerly awaited by all group members to dig into the wealth of data, produce exciting new physis results and further our understanding of nuclear matter at extreme conditions.

[Note about all event displays: The red and blue boxes show energy deposits from particles produced in the collisions in the electromagnetic and hadronic calorimeters, respectively. As the inner tracker was not activated during this run, charged particles could not be reconstructed as charged tracks.]

Fall 2012: Discovery in the proton lead test run

Ridge in high multiplicity pPb collisions.

The biggest physics surprise of the 2012 pPb test run.

Shortly after the August Quark Matter conference, the Large Hadron Collider geared up for a test run of proton on lead (pPb) collisions. Proton nucleus collisions are the intermediate stage between the short length scale proton proton physics and finite sized physics of lead lead collisions. These types of collisions are going to be the main focus of the January running period and this short test was meant to get us ready for what's to come. One of the test fills, a few hour continuous running period from midnight to 6am on September 13, gave us about two million good proton lead collisions.

It was very exciting as this is the first look at these collisions at such a high energy, 5TeV center of mass, producing never-before-seen particle densities in proton nucleus collisions. We were curious not only to see what reference measurements we can obtain from these collisions, but also if there is any new undiscovered physics lurking around. Indeed last time we saw unprecedented particle densities in proton proton collisions when the LHC turned on two years ago our group discovered a surprise in the form of a "ridge" in two particle correlations. This "ridge" indicates that something is driving particles produced from these collisions which have very little time to talk to each other, i.e. they are causally disconnected shortly after the collision, to have some slight preference in which plane they are produced. To see this effect in proton proton we had to look at extreme rare collisions in which high enough particle densities were produced, picking out the top 300K of over 150 billion events.

So while we expected to see some kind of ridge since it's there both in pp and PbPb collisions we didn't quite expect to see a strong signal in just the two million events we recorded. However when we looked at pPb collisions with similar particle densities as those which showed a ridge in pp collisions, we saw a huge ridge correlation! The effect is the same as described above for the pp ridge. We have something driving produced particles to align to some sort of plane, except in a manner much stronger than observed in the high multiplicity pp collisions, so strong it resembles PbPb collisions more than pp. This generated quite a bit of excitement, the unveiling of the result at the Hot Quarks conference was met with applause. CMS posted the following article on this result, which was the top headline in the physicsworld magazine a few days ago.

This exciting news may be just the tip of the iceberg of what's to come from the full proton lead run in January. The entire statistics from the test run that yielded this result, two million good collisions, is just one second of running time in 2013!

Quark Matter 2012

Gunther at Quark Matter

Gunther presenting an overview of all the results from CMS heavy ions on the opening day of Quark Matter. See more photos here.

The 2011 PbPb run was the run for rare physics and pushing the boundaries of what can be measured and high precision. With an arsenal of dedicated triggers we showed a whole range of interesting new results at this years Quark Matter conference.

By looking at the top 0.01% of collisions which deposited the highest amount of energy in our detector we selected almost fully overlapping nuclear geometries. Measuring the fourier spectra of these collisions with the tight geometric control we provide some of the best constraints to the viscosity of the hot quark gluon plasma produced at the LHC.

By selecting jets and high momentum tracks right as the collisions occur in the machine, we have aquired and analyzed a dataset that paints a comprehensive picture of jet quenching and parton energy loss from many angles. We showed the azimuthal anisotropy of charged particles of momentum above 20 GeV/c up to 60 GeV/c which probes the path length dependence of energy loss in the quark gluon plasma. We showed a complete picture of particle suppresion in PbPb compared to proton proton collisions for both charged particles and jets, as well as bottom quarks through j/psi's and tagged jets, which all show significant suppression. However bosons which don't interact strongly with the medium have been measured to have no significant suppression compared to proton proton, in the isolated photon, and W and Z channels.

We have shown the direct energy loss of colored partons traversing the medium by precisely measuring the initial energy of the outgoing photon in gamma-jet events. In addition we have studied the detailed structure the constituents of jets both in momentum and angular redistributions compared to similar jets found in proton proton collisions. These measurements further probe in what ways the quark gluon plasma responds to high energy particles.

May 5, 2012

Our measurement of medium induced energy loss in QGP using photon-jet events was finally submitted to arXiv:1205.0206 and for publication in Physics Letters B (PLB). This paper has been long awaited since the conception of the heavy ion program at CMS more than 12 years ago (See early CMS Note, p122). It is the first time in the heavy ion field where the medium induced energy loss can be directly studied with a fully unbiased probe (photon).

PhotonJet energy loss vs npart

Figure 3 in the paper. The measured distribution of fraction (x) of remaining jet energy after losing energy to the QGP, plotted in 4 bins of increasing amount of medium interaction.

Also today our dihadron correlations paper got accepted into EPJC!

April 2012

EPJC front cover

Our paper featured on the cover of European Physical Journal C.

Our paper on the measurement of high transverse momentum charged particle suppression, accepted by the European Physical Journal C (EPJC) in February 2012, was recently published and featured in the cover of the new EPJC issue (March 2012). The figure that was featured in the cover is the main result of the paper showing the suppression of the charged particles over a large transverse momentum range, where a number of theoretical predictions show large variation, in particular at high transverse momentum regime, exhibiting large theoretical uncertainty. Together with other jet quenching related quantities that we have measured, this measurement should help elucidate the mechanism of jet quenching and the properties of the medium produced in heavy-ion collisions at collider energies.

March 2012

We have three new papers about to be published that mark some of the most important Heavy Ion measurements to date, addressing how colored particles lose energy as they traverse the quark gluon plasma created in our relativistic heavy ion collisions. The first measurement we made probes how high energy quarks and gluons fragment as they pass through the hot colored medium and into the vacuum. The second clarifies how these high energy particles lose energy depending of how much medium they pass through. The final paper uses back-to-back pair of photon and quark or gluon to directly probe the absolute amount of energy loss.

February 2012

2011 / 2010 Data Taking

We recorded 7 inverse microbarns in all of 2010, which was surpassed in 1 day of 2011 data taking.

For the month of November the LHC switched to colliding 2.76 TeV lead nuclei. The heavy ion group was heavily involved in the data taking which resulted in a very rich, high statistics dataset. This year's recorded luminosity is 20 times higher than last year's data giving us access to more precise measurements of known phenomena as well as new measurements and probes available for the first time ever in heavy ion physics.

In a little over two months after the end of data taking we published an updated dijet imbalance paper taking advantage of the full 2011 high statistics dataset. In addition to analyzing new 2011 data we finalized the measurements first shown at Quark Matter and published those in three new papers. The first paper of 2012 is the measurement of isolated photon production in pp and PbPb collisions at 2.76 TeV , where the transverse energy distributions are found to be in good agreement with next-to-leading-order perturbative QCD predictions and the ratio of PbPb to pp isolated photon ET-differential yields is consistent with unity for all PbPb centralities.

Next came the study of dihardon correlations and azimuthal anisotropy harmonics where we show the evolution of short (jet) and long range range correlations with increasing centrality and transverse momentum, as well as the single particle azimuthal harmonics up to fifth order from a Fourier analysis.

The most recent completed paper is the measurement of high transverse momentum charged particle suppression. We find the charged particle yield is suppresed by a factor of 5 compared to pp collisions in the transverse momentum range of 5-10 GeV/c and rises to a factor of 2 in the 40-100 GeV/c range.

Quark Matter 2011

Yenjie at Quark Matter

Yen-Jie giving a talk in front of the full quark matter audience shortly after completing his PhD.

After barely a year of pp and a month of PbPb collisions, several exciting new results emerged from analyses led by our group. From the first 7 TeV pp collisions two summers ago novel correlations were seen in collisions producing the highest number of particles. These were never before seen in pp collisions nor were they predicted by any of the existing Monte Carlo models. This was the first manifestation of unexpected physics at the LHC which has gathered significant interest in both the media and the pp and heavy ion community.

From the start of the PbPb run we saw striking evidence in some of the first event displays of a phenomenon called jet quenching, where a colored quark or gluon passing through the QGP loses a significant fraction of its energy by interacting with the medium. While jet quenching has been observed before, the LHC was the first time you could really see it "with the naked eye". A closer investigation of these types of collisions revealed that we may need to rethink how colored partons really interact with the QGP, since it turns out a quenched jet looks no different than a lower energy jet produced in a vacuum.

These are just a small fraction of the new and exciting discoveries and measurements being performed by the MIT group at the LHC. In less than two months a new PbPb run is coming up with an order of magnitude more collisions than last year, giving access to more precise measurements, rarer events, and potentially new discoveries.