The Alpha Magnetic Spectrometer (AMS) Collaboration is excited to announce the publication of its first physics results in the prestigious journal Physical Review Letters. The AMS experiment, renowned as the most powerful and sensitive particle physics spectrometer ever deployed in space, has been operating aboard the International Space Station (ISS) since its installation on May 19, 2011. During this period, AMS has measured over 30 billion cosmic rays at energies up to trillions of electron volts, capturing data with unprecedented precision and accuracy.

AMS-02, located on the exterior of the ISS as shown in Figure 1, uses its permanent magnet and array of precision particle detectors to identify charged cosmic rays from the far reaches of space. With its ability to record signals from 16 billion cosmic rays annually, AMS-02 is a cornerstone of space-based particle physics research. Since its installation, AMS has transmitted vast amounts of data back to Earth for analysis by the international AMS Collaboration, and this publication marks the first of many physics results to come.

During the initial 18 months of operation, from May 19, 2011, to December 10, 2012, AMS-02 analyzed 25 billion primary cosmic ray events. Of these, 6.8 million were unambiguously identified as electrons and their antimatter counterparts, positrons, across an energy range of 0.5 to 350 GeV. This analysis, focused on electrons and positrons, is the subject of the first published study. The AMS instruments provide accurate and redundant measurements that allow for the clear identification of these particles against a large background of protons, with a rejection power of more than one in a million.

The initial findings reveal that the positron fraction (the ratio of positrons to the combined flux of positrons and electrons) exhibits intriguing behavior across different energy levels. From 0.5 to 10 GeV, the fraction decreases with increasing energy, while from 10 GeV to approximately 250 GeV, the fraction steadily increases. The rate of increase, however, slows significantly from 20 to 250 GeV, and the spectrum appears to flatten above 250 GeV. The positron fraction spectrum shows no structure or time dependence, and the positron to electron ratio indicates no preferred directional source for these energetic positrons, suggesting evidence of new physical phenomena.

These observations align well with current theoretical models, including one described in the paper. The model incorporates diffuse electron and positron components along with a common source component, fitting the AMS data remarkably well. This indicates that a significant portion of high-energy electrons and positrons may originate from a common source, potentially shedding light on the origins of dark matter or the role of pulsars in cosmic ray production.

Over the past few decades, the positron fraction from primary cosmic rays has garnered significant interest from both particle physicists and astrophysicists. The AMS-02 findings contribute to this ongoing research, offering the most detailed measurements to date and paving the way for a deeper understanding of cosmic ray origins and the fundamental nature of our universe.

The AMS Collaboration comprises hundreds of scientists, engineers, technicians, and students from 16 countries, working under the leadership of Nobel Laureate Samuel Ting of MIT. The team has spent over 18 years bringing AMS to fruition, with the collaboration extending across Europe, Asia, and North America. The construction, assembly, and ongoing data analysis of AMS have involved numerous universities and research institutes worldwide, demonstrating the power of international cooperation in advancing scientific knowledge.

AMS is sponsored by the U.S. Department of Energy and NASA, and its successful deployment and operation on the ISS were made possible through the efforts of many, including the NASA AMS Project Management team from Johnson Space Center. AMS was transported to Kennedy Space Center in August 2010 and launched aboard the final mission of the Space Shuttle Endeavour (STS-134) on May 16, 2011. Since then, AMS has been continuously collecting data from space and transmitting it to the AMS Payload Operations Control Center (POCC) at CERN, Geneva, Switzerland.

The first results from AMS represent a significant milestone, reflecting the culmination of years of hard work and dedication. With AMS-02 continuing its mission on the ISS, the Collaboration looks forward to further exploring the mysteries of the cosmos, from dark matter to antimatter and beyond. The wealth of data provided by AMS will continue to advance our understanding of fundamental physics and contribute to the discovery of new phenomena in the universe.