Laboratory for Nuclear Science

The Electromagnetic Interactions (EMI) Group is led by Nobel Laureate Professor Samuel Ting. The EMI group initiated and led the development of the Alpha Magnetic Spectrometer (AMS), a multi-purpose magnetic spectrometer installed on the U.S. National Laboratory of the International Space Station in May 2011 and, for the foreseeable future, the only magnetic spectrometer in space. The detector precisely measures charged particles travelling through the cosmos before they interact with the Earth's atmosphere. Cosmic rays can have energies of more than 10²⁰ eV, so space is providing particles with much higher energy than can be produced by any accelerator on Earth. AMS is a large international collaboration of physicists from 46 institutes representing 15 countries on 4 continents. The EMI group, as the leading institute in AMS, is responsible for the operation of the detector in space, which is performed from the Payload Operations and Control Center (POCC) at CERN in Geneva, Switzerland. The detector operates on a 24 h/day 365 days/year basis and has already recorded more than 170 billion charged cosmic ray events, far more than in the entire history of cosmic ray physics.

The EMI group leads the data analysis efforts of the AMS collaboration. The physics objective of AMS is the measurement of the fluxes of individual cosmic ray species with percent level precision up to trillion electron volt energies. Our precise measurements of many different types of cosmic rays provide key information on the origin, acceleration and propagation of cosmic rays.

Conventionally, it was assumed that there are just two types of cosmic ray nuclei: primary and secondary. Primary nuclei (H, He, C, ..., Fe) are produced during the lifetime of stars and accelerated to high energies by the explosion of stars (supernovae). Secondary cosmic nuclei (Li, Be, B, …) are produced by the collision of primary cosmic rays with the interstellar medium. Precision AMS measurements reveal a much more complex picture, as, for instance, the recently announced and unexpected discovery of two distinct subclasses in primary cosmic rays {He, C, O} and {Ne, Mg, Si}. A surprising recent observation by AMS is that much heavier iron primary cosmic rays belong to the light subclass of primary cosmic rays {He, C, O} and not to the heavier subclass of {Ne, Mg, Si}.

AMS measurements of cosmic radiation are vital for space exploration. AMS provides information for low-Earth orbit activities as well as for a long-term base on the Moon.

In addition, these measurements allow us to search for signals of the annihilation or decay of dark matter particles in the Galaxy. To date close to 2 million energetic positrons and 560,000 antiprotons have been identified. Their measured energy dependences are in good agreement with models of annihilation of dark matter. Another objective of AMS is to look for primordial antimatter in cosmic rays to address the long-standing mystery of the excess of matter over antimatter in our universe. The most exciting objective of AMS, though, is to probe the unknown; to search for phenomena which exist in nature that we have not yet imagined nor had the tools to discover. Indeed, all the results show extraordinary and unexpected behavior.

AMS has published nineteen major physics results in Physical Review Letters, of which most were selected as Editors’ Suggestions. Special AMS sessions are organized at major international conferences to review AMS results. The precise AMS data are contradicting all previous measurements and current theories on cosmic ray origin and propagation. AMS is the only precision magnetic spectrometer in space. It is exploring and discovering many new physics phenomena and it is an ideal experiment for graduate students.

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Figure 1. AMS is a unique precision magnetic spectrometer on the ISS. AMS will operate on the ISS for the Station’s lifetime.

Figure 2. The rigidity dependence of the primary cosmic nuclei Ne, Mg, and Si fluxes compared to rigidity dependence of the primary He, C, O and Fe fluxes. This shows two distinct subclasses of primary cosmic rays. Surprisingly, Fe belongs to the light subclass of primary cosmic rays.

Figure 3. AMS measurements of the charge of cosmic rays using the time of flight detector (TOF) and the silicon tracker. As seen, AMS is measuring radiation of all cosmic ray nuclei up to iron and above.

Figure 4. The positron spectrum measured by AMS. Also shown are the expected positron flux from the collision of cosmic rays with the interstellar medium and a model predicting an additional contribution of positrons from dark matter annihilations. Projection of the AMS results to 2028 is also shown.