NERSC Helps Shed Light on the Nature of Antimatter

Using the National Energy Research Scientific Computing Center's (NERSC) Parallel Distributed Systems Facility (PDSF) and the Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC), physicists have detected and confirmed the first-ever antimatter hypernucleus, called "antihypertriton."

Translated, the newly detected "antihypertriton" means a nucleus of antihydrogen containing one antiproton and one antineutron—plus one heavy relative of the antineutron, an antilambda hyperon.

A typical event in the STAR detector that includes the production and decay of an antihypertriton candidate (H3 with a bar indicates an antithydrogen 3 nucleus; the lambda with a bar indicates that it contains an antilambda). The dashed black line is the trajectory of the candidate, which cannot be directly measured. The heavy red and blue lines are the trajectories of an antihelium nucleus and a pion, decay daughters which are directly measured.

Most of the objects in the cosmos today consists of matter, comprised of "normal" particles like positively charged protons and negatively charged electrons. Each of these fundamental particles has a corresponding "antiparticle." Antiparticles have primarily the same properties as their normal counterparts, with a few reversals. For example, antiprotons have the same mass as protons but a negative charge, and positrons have the same properties as electrons but with a positive charge. Just as most "normal" hydrogen is comprised of a proton and electron, antihydrogen is comprised of an antiproton and positron.

"STAR is the only experiment that could have found an antimatter hypernucleus," says Nu Xu of the Lawrence Berkeley National Laboratory's Nuclear Science Division, the spokesperson for the STAR experiment. "We've been looking for them ever since RHIC began operations. The discovery opens the door on new dimensions of antimatter, which will help astrophysicists trace back the story of matter to the very first millionths of a second after the Big Bang."

Cosmologists believe that equal quantities of matter and antimatter were created in the Big Bang, yet most of the cosmic objects observed today are made of matter. So why is there more matter than antimatter in the universe? This is one of the greatest mysteries in science, and solving it could tell us why human beings, indeed why anything at all, exists today.

Computers and Colliders Collaborate to Find Antimatter

The diagram above is known as the 3-D chart of the nuclides. The familiar Periodic Table arranges the elements according to their atomic number, Z, which determines the chemical properties of each element. Physicists are also concerned with the N axis, which gives the number of neutrons in the nucleus. The third axis represents strangeness, S, which is zero for all naturally occurring matter, but could be non-zero in the core of collapsed stars. Antinuclei lie at negative Z and N in the above chart, and the newly discovered antinucleus (magenta) now extends the 3-D chart into the new region of strange antimatter.

So far, scientists have only been able to study antimatter by colliding particles in accelerators. By colliding gold ions at high energies in RHIC, the STAR collaboration is attempting to recreate what is believed to be the conditions in the universe just microseconds after the Big Bang. The enormous energy density that existed at that time would have separated the constituents of protons and neutrons, called quarks.

This very hot cosmic stew of free floating fundamental particles, including quarks, antiquarks and gluons is known as the quark-gluon plasma. As the universe expanded and cooled, the quarks recombined in a variety of ways to make protons and neutrons (consisting solely of up and down quarks), hyperons (which contain strange quarks) and all of the associated antiparticles. Because quarks and antiquarks exist in equal numbers in the quark-gluon plasma, the cooling gas produces both matter and antimatter. Eventually, a small fraction of these particles combined to form light nuclei and their antiparticles like the antihypertritons detected by the STAR collaboration. To identify this hypernucleus, physicists used supercomputers at NERSC and other research centers to painstakingly sift through the debris of some 100 million collisions.

The team also used NERSC's PDSF system to simulate detector response. These results allowed them to see that all of the charged particles within the collision debris left their mark by ionizing the gas inside RHIC's time projection chamber, while the antihypertritons revealed themselves through a unique decay signature— the two tracks left by a charged pion and an antihelium-3 nucleus, the latter being heavy and so losing energy rapidly with distance in the gas.

"These simulations were vital to helping us optimize search conditions such as topology of the decay configuration," says Zhangbu Xu, a physicist at Brookhaven who is part of the STAR collaboration. "By embedding imaginary antimatters in a real collision and optimizing the simulations for the best selection conditions, we were able to find a majority of those embedded particles."

Physicists agree that the discovery also extends human knowledge of the nuclear terrain. Physicists represent this terrain graphically by placing each kind of nucleus on a three-dimensional graph with the three axes being Z, the number of protons in a nucleus; N, the number of neutrons; and S, the degree of strangeness. Each of these three axes has positive and negative sections, allowing for the representation of both particles and antiparticles. This latest result extends the nuclear terrain below the N–Z plane for the first time.

Jinhui Chen, a postdoctoral researcher at Kent State University and currently a staff scientist at the Shanghai Institute of Applied Physics, and Zhangbu Xu were among the lead authors of the paper that was published in the March issue of Science Express. Their work utilized more than 100,000 processor hours on NERSC's PDSF and was partially supported by the Offices of Nuclear Physics and High Energy Physics in the Department of Energy's Office of Science. Data generated by RHIC's STAR experiment, located at the Brookhaven National Laboratory in New York, travels to NERSC, managed by Lawrence Berkeley National Laboratory in Berkeley, Calif., via DOE's high-bandwidth Energy Sciences Network (ESnet).

This story was adapted from an article published in Physics World and the Berkeley Lab feature "STAR Discovers the Strangest Antimatter Yet."

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