The Alpha Magnetic Spectrometer (AMS) Collaboration celebrates the fifth anniversary of the International Space Station (ISS) experiment and summarizes its major scientific achievements.

The AMS (shown in Figure 1) is the most sensitive particle detector ever deployed in space and opens an exciting new frontier in physics. As a magnetic spectrometer, AMS is unique in studying charged particles and nuclei in the universe before they are annihilated in Earth’s atmosphere. Its long time in space, large aperture, built-in redundant systems, and careful calibration on a test beam at CERN provide improvements in measurement accuracy of up to ~1%. The first five years of AMS data on the ISS are beginning to unlock the mysteries of space.

Since its installation on the ISS in May 2011, AMS has collected data from more than 90 billion cosmic rays with energies up to several TeV and published its main physical results in Physical Review Letters.

About cosmic rays

Primary cosmic rays, which are the products of supernova explosions, can travel millions of years through the galaxy before reaching the AMS. Secondary cosmic rays are produced by the interaction of primary cosmic rays with the interstellar medium. The AMS, located on the ISS, studies cosmic rays passing through its precision detectors, determining the charge, energy, and momentum of the particles to understand dark matter, the existence of complex antimatter in space, the properties of primary and secondary cosmic rays, and new unexpected phenomena.

There are many kinds of charged elementary particles, but only four of them – electrons, protons, positrons, and antiprotons – have infinite lifetimes, allowing them to travel through space indefinitely. Electrons and positrons, having less mass compared to protons and antiprotons, lose more energy in the galactic magnetic field due to synchrotron radiation.

Key findings

Recent decades have generated a great deal of interest in understanding the nature of dark matter. When dark matter particles collide, energy is released and transformed into ordinary particles such as positrons and antiprotons. Characteristic of dark matter is an increase in flux with energy and then a sharp drop on the dark matter mass, as well as an isotropic distribution of the arrival directions of excess positrons and antiprotons.

Figure 4 shows the latest AMS results for the positron flux. After rising from 8 GeV above the expected level from cosmic ray collisions, the spectrum shows a sharp drop at high energies, which is exactly in line with the predictions of dark matter models with ~1 TeV masses. This is of great interest to physicists, although there is an alternative suggestion that this spectrum behavior may be caused by new astrophysical phenomena such as pulsars.

AMS has also studied the ratio of antiprotons to protons. The excess of antiprotons observed by AMS cannot be easily explained by pulsars, but can be explained by dark matter collisions or new astrophysical models. Antiprotons are very rare in space: there are 10,000 protons per antiproton, so an accurate experiment requires a one-in-a-million background selection. Over five years, AMS collected a clean sample of 349,000 antiprotons, of which 2,200 had energies above 100 billion electron volts. Experimental data on antiprotons in cosmic rays are critical to understanding their origin and new physical phenomena.

Nuclei Survey

AMS includes seven instruments for the independent identification of various elementary particles and nuclei. Helium, lithium, carbon, oxygen, and heavier nuclei up to iron have been studied by AMS. Helium, carbon, and oxygen are thought to be produced directly from primary sources in supernova remnants, while lithium, beryllium, and boron are produced by collisions of primary cosmic rays with the interstellar medium.

The ratio of secondary to primary cosmic ray fluxes, such as boron and carbon, provides information on the distribution and average amount of interstellar matter (ISM) through which cosmic rays pass in the galaxy. Surprisingly, the B/C (boron/carbon) ratio shows no significant structures, which contradicts many cosmic ray models.

The mystery of antigelium

The origin of the Universe as a result of the Big Bang requires equal amounts of matter and antimatter at the beginning. The search for an explanation for the absence of antimatter in complex form is known as baryogenesis. Despite considerable experimental effort, no evidence for strong symmetry breaking or proton decay has been found. The observation of a single antigelium event in cosmic rays is of great significance.

In five years, the AMS has collected 3.7 billion helium events (charge Z = +2). To date, several events with Z = -2 and a mass similar to 3He have been observed. At a rate of about one antigelium candidate per year and a necessary signal/background rejection rate of one in a billion, a detailed understanding of the instrument is required. In the coming years, with the increasing amount of data, one of the main challenges will be to establish the origin of events with Z = -2.