Where did all the antimatter go?

Dubbed the Hubble telescope of cosmic rays, a state-of-the-art particle physics detector attached to the International Space Station (ISS) is looking for a needle’s worth of antimatter in a haystack of cosmic rays. The detector is also searching for elusive evidence of dark matter.

 

Recent ground-breaking observations about the nature of our universe share a common thread—the support of the Open Science Grid (OSG). The confirmation of the Higgs Boson showed us that theories about particle physics turned out to be correct. The first ever observation of an astrophysical high-energy neutrino flux was made by the IceCube detector at the South Pole. And the first direct observation of gravitational waves by the LIGO observatory proved that Einstein’s theory was correct. In all cases, the OSG was there to provide the needed computing support.

 

The OSG is also helping scientists search for dark matter and antimatter. They are trying to better understand what happened to all the antimatter after the formation of the universe, when equal amounts of matter and antimatter existed. In what could turn out to be another major physics breakthrough, the OSG is critical to supporting the research.

 

The unique environment of outer space gives researchers a better opportunity to explore what makes up the universe’s invisible mass. Dubbed the Hubble telescope of cosmic rays, the Alpha Magnetic Spectrometer (AMS) is a state-of-the-art particle physics detector attached to the International Space Station (ISS). It is designed to help scientists measure antimatter in cosmic rays and search for evidence of dark matter.

 

Figure 1: International Space Station. Photo courtesy AMS Project.

Figure 1: International Space Station. Photo courtesy AMS Project.

The AMS project is an international collaboration of more than 50 institutions from 16 countries represented by the United States Department of Energy (DoE). The principal investigator is Nobel laureate particle physicist Samuel Ting.

In 2013, AMS scientists reported that hints of dark matter may have been detected with an unexplained excess of high-energy positrons coming from cosmic rays.

In 2014, the project announced new results on energetic cosmic ray electrons and positrons based on the first 41 billion events measured with the AMS. Precise measurement of the positron fraction—the ratio of the number of positrons to the combined number of positrons and electrons—is important for understanding the origin of dark matter. Their results again point to evidence of dark matter.

In 2015, the project once again presented results that were consistent with dark matter collisions that cannot be explained by existing models of the collision of ordinary cosmic rays.

The search continues, and the volume of data requires a massive analysis effort. The team relies on the OSG to run simulations.

Figure 2: Baosong Shan, Vitali Choutko, with colleague Hung-Te Lee. Courtesy photo

Figure 2: Baosong Shan, Vitali Choutko, with colleague Hung-Te Lee. Courtesy photo

 

“We want to reach the highest precision on our data analysis,” said Baosong Shan, a lecturer at Beihang University, China. Shan has been the principle user behind the AMS computing workload on OSG. “To achieve this with the very precise detector we have, we require a very large amount of computing power for data production and simulation.”

OSG computing is giving them a boost. “The OSG is helping us reach the amount of simulation data we require without delaying our analysis work,” said Vitali Choutko, a senior research scientist at MIT who has been coordinating much of the computing effort. Just since mid-February 2016, the AMS project has used more than 11.5 million CPU hours on the OSG. “In the future,” added Choutko, “we hope to use many millions of CPU hours per month as opportunistic resources are available.”

While the environment of outer space provides a unique opportunity, it also presents some real challenges. The detector’s computers must use special radiation-tolerant chips. “Because the detector is mounted on board the ISS,” added Shan, “we have to deal with a constantly changing and hostile environment.”

Will the AMS help scientists observe dark matter and antimatter? The evidence is mounting. Orbiting the Earth on the ISS at an altitude of about 300 kilometers, the detector is collecting hundreds of millions of primary cosmic rays that traveled hundreds of millions of light years before reaching it. And it’s scheduled to operate for the lifetime of the ISS.

The huge amount of AMS data on cosmic ray fluxes and their composition could yield unexpected results in the ISS’s remaining eight (or more) years. A leading candidate for dark matter is the neutralino. If they exist, they could collide and produce excesses of charged or neutral particles which can be detected by the AMS. If that happens, the OSG will be there to support the science.

 

-Greg Moore