MIT Physics News Spotlight

The AMS Collaboration Releases New Results

Results from the Alpha Magnetic Spectrometer (AMS) on the International Space Station (ISS) were presented April 15 – 17, 2015, in Geneva, Switzerland, during “AMS Days at CERN.”

By S. Ting and P. Zuccon for the AMS Collaboration
April 24, 2015

The AMS-02 detector on the International Space Station (NASA)
The AMS-02 detector on the International Space Station (NASA)
Click image for high resolution version

The occasion brought together leading theoretical physicists and principal investigators of some of the major experiments exploring the field of cosmic ray physics: IceCube, Pierre Auger Observatory, Fermi-LAT, H.E.S.S. and CTA, the Telescope Array, JEM-EUSO, and ISS-CREAM.

AMS is the only major particle physics experiment on the ISS. In its first four years on orbit, AMS has collected more than 60 billion cosmic ray events (electrons, positrons, protons, antiprotons, and nuclei of helium, lithium, boron, carbon, oxygen, and heavier elements), up to multi-TeV energies.

As an external payload on the ISS through at least 2024, AMS will continue to collect and analyze an increasing volume of cosmic rays at very high energies. Along with in-depth knowledge of the detector and systematic errors, these data will produce valuable insight into the origin and propagation of cosmic rays, anti-matter and possibly dark matter.

The AMS measurements of the ratio of positrons to electrons, the electron and positron spectra, and the combined electron plus positron spectrum are consistent with dark matter collisions and cannot be explained by existing models of the collision of ordinary cosmic rays. However, there are many new models showing that the results may alternatively be explained by astrophysical sources (such as pulsars) or new acceleration and propagation mechanisms. To determine whether the observed new phenomena are from dark matter, measurements are underway by AMS to determine the rate at which the positron fraction falls beyond its maximum. The measurement of the antiproton to proton ratio may also shed light on this question.

The newly presented AMS measurement of the antiproton to proton ratio (Figure 1) stays constant from 20 GV to 450 GV rigidity (ratio of momentum to charge). Secondary production of antiprotons from ordinary cosmic ray collisions should lead to a decreasing antiproton/proton ratio with increasing rigidity, which is not observed in the data.

Figure 1: Antiproton to proton ratio measured by AMS. As seen, the measured ratio
becomes flat at high rigidities, where existing models of secondary production 
predict a decrease. (AMS Collaboration)
Figure 1: Antiproton to proton ratio measured by AMS. As seen, the measured ratio
becomes flat at high rigidities, where existing models of secondary production
predict a decrease. (AMS Collaboration)
Click image for high resolution version

The AMS data seem to point to a new source of antiprotons active in the galaxy. However, the theoretical uncertainties in the prediction from secondary production are substantial, and it is not yet clear whether such a source is required. Other complementary cosmic-ray measurements from AMS-02 will help clarify this question. If an excess is confirmed, its origin will still be an open question. While pulsars can readily produce energetic positrons, they are not expected to generate high-energy antiprotons.

Understanding the AMS results for electrons, positrons and antiprotons will require a thorough understanding of the propagation of ordinary cosmic rays, including secondary production. At the AMS Days, the AMS Collaboration reported on their most recent precision studies of nuclei spectra (such as protons, helium and lithium) up to multi-TeV energies.

The latest data on the precision measurement of proton flux in cosmic rays from 1 GV to 1.8 TV rigidity (momentum/charge based on 300 million events) will appear shortly in Physical Review Letters.

Figure 2: Measured proton fluxes as a function of rigidity. (AMS Collaboration)
Figure 2: Measured proton fluxes as a function of rigidity. (AMS Collaboration)
Click image for high resolution version

AMS has found that the proton flux is characteristically different from all prior experimental results. The AMS measurement (Figure 2) shows that the measured proton flux changes its behavior at ~300 GV rigidity. The solid line is a fit to the data. The dashed line is the proton flux expected with no change in behavior; it does not agree with the data at high energy.

Most surprisingly, AMS has also found, based on 50 million events, that the helium flux exhibits nearly identical and equally unexpected behavior (Figure 3). AMS is currently studying the behavior of other nuclei in order to understand the origin of this unexpected change. These new observations will provide important insight into cosmic ray production and propagation.

Figure 3: Measured helium flux as a function of rigidity. (AMS Collaboration)
Figure 3: Measured helium flux as a function of rigidity. (AMS Collaboration)
Click image for high resolution version

According to MIT Professor of Physics Tracy Slatyer, “Measurements of antiprotons and positrons can also provide powerful constraints on the properties of dark matter at lower masses, particularly below 150 GeV. AMS-02 has the sensitivity to probe the hypothesis that excess gamma rays in the inner Galaxy originate from dark matter annihilation, and more generally to test for the existence of light thermal relic dark matter.”

“However,” Slatyer continued, “at present, such tests are limited by systematic uncertainties in both the background from secondary production, and the propagation of the dark matter annihilation products. The unprecedented precision and accuracy of the new AMS-02 cosmic ray data will be key in reducing these uncertainties, by allowing better modeling of both the background and the signal.”

The latest AMS measurements of the positron fraction, the antiproton/proton ratio, and the fluxes of electrons, positrons, protons, helium, and other nuclei provide accurate, precise and unexpected information. The new data, spanning many different types of cosmic rays, require a comprehensive model to ascertain if their origin is from dark matter, astrophysical sources, acceleration and/or propagation, or some combination of these effects.