“AMS Days at CERN” and Latest Results

April 15th, 2015

Results from the Alpha Magnetic Spectrometer (AMS) on the International Space Station (ISS) will be the focus of the three day “AMS Days at CERN” meeting, an occasion that brings together many of the world’s 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).

The main objective of this scientific exchange is to understand the interrelation between AMS results and those of other major cosmic rays experiments and current theories. The latest results (published and to be published) from AMS will be presented by members of the AMS international collaboration during the three day event.

 

» Download the AMS Collaboration press release

» Webcast of all the talks

» The program of the “AMS days”

 

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, …) 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 statistics at highest energies which, combined with in-depth knowledge of the detector and systematic errors, will produce valuable insight.

The AMS results on the positron fraction, the electron spectrum, the positron spectrum, 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. There are many new models showing that the results may be explained by new astrophysical sources (such as pulsars) or new acceleration and propagation mechanisms (such as supernova remnants).

To distinguish if 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, as well as the measurement of the antiproton to proton ratio. As seen in Figure 1, the antiproton to proton ratio stays constant from 20 GeV to 450 GeV kinetic energy. This behavior cannot be explained by secondary production of antiprotons from ordinary cosmic ray collisions. Nor can the excess of antiprotons be easily explained from pulsar origin. The latest results on these studies will be reported by the AMS Collaboration during “AMS Days at CERN” and in future publications.

Figure 1. Antiproton to proton ratio measured by AMS. As seen, the measured ratio cannot be explained by existing models of secondary production.

Figure 1. Antiproton to proton ratio measured by AMS. As seen, the measured ratio cannot be explained by existing models of secondary production.

In addition, a thorough understanding of the process involved in the collision of ordinary cosmic rays is a requirement in understanding the AMS results mentioned above. The AMS Collaboration will also report on the most recent results on the 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) will appear shortly in Physical Review Letters. These results are based on 300 million proton events. AMS has found that the proton flux is characteristically different from all the existing experimental results. As seen in Figure 2, the AMS result shows the measured flux changes its behavior at ~300 GV rigidity. The solid line is a fit to the data. The dashed line in Figure 2 is the proton flux expected with no change in behavior; as seen, it does not agree with the data.

Figure 2. Measured proton flux as a function of rigidity.

Figure 2. Measured proton flux as a function of rigidity.

Most surprisingly, AMS has also found, based on 50 million events, that the helium flux exhibits nearly identical and equally unexpected behavior as the proton flux (see Figure 3). AMS is currently studying the behavior of other nuclei in order to understand the origin of this unexpected change.

Figure 3. Measured helium flux as a function of rigidity.

Figure 3. Measured helium flux as a function of rigidity.

These unexpected new observations provide important information on the understanding of cosmic ray production and propagation.

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