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Pions as a catalyst: Microscopic study of deuteron production in lead collisions


It’s an exciting field of research for physics: quark-gluon plasma, the state of matter that existed in the universe until fractions of a second after the big bang, that can be generated and studied by collisions of heavy lead ions. Experimental observations show that these collisions produce light nuclei such as deuterons, tritons and helium. Researchers, however, don’t agree on the theoretical explanation for their production. A group of physicists including Professor Hannah Elfner from the GSI Helmholtzzentrum für Schwerionenforschung and her former PhD candidate Dr. Dmytro Oliinychenko from the Lawrence Berkeley National Laboratory in California as well as other partners recently published in the “Physical Review C” journal regarding new results on the microscopic understanding of deuteron production.

Deuteron is the atomic nucleus of deuterium (“heavy hydrogen”). Deuterons play a role in nuclear fusion reactions in stars. “Like snowballs in hell” is how some researchers describe the fact that light nuclei like deuterons are recognizable in the quark-gluon plasma. In actuality, the high temperatures of the fireballs emanating from the collisions should melt the nuclei into their subatomic constituents though that’s not exactly what they seem to do. Elfner, Oliinychenko and his colleagues are now proposing a microscopic mechanism that could explain why the nuclei don’t disappear.

They start from an already existing qualitative explanation for the observation of these nuclei. This proposal postulates that the light nuclei created in the fireball are destroyed by high temperatures and are recreated over and over again by flying protons and neutrons as the fireball cools down. The microscopic mechanisms behind this scenario were unclear up until now. This is where Elfner, Oliinychenko and colleagues started and set about finding the mechanism by analyzing a series of reactions that could form deuterons. They identified a possible reaction in which protons and neutrons form deuterons in the presence of pions or quark-antiquark pairs. The pions could serve as a kind of catalyst for reactions between protons and neutrons, thereby enabling the stable production of deuterons in high-energy nuclear collisions.

The team simulated similar conditions to a CERN experiment recently conducted by the ALICE collaboration that accurately characterized collisional light nuclei. Then the comparison followed: the calculated deuteron yield and energy spectra were consistent with ALICE observations. The conclusion: if Elfner, Oliinychenko and the team’s idea is correct, it should also explain the formation of other observed nuclei such as tritons.

The authors now plan to review this possibility in upcoming calculations and to further substantiate their findings. In addition, they’re considering how to conduct further studies at lower radiation energies. Such considerations are also relevant for the HADES experiment at GSI and for the CBM experiment at the future FAIR Accelerator Center currently being developed at GSI. The topic of Elfner, Oliinychenko and the group is also presented in Bari, Italy, at this year's "Strangeness in Quark Matter" conference, one of the largest conferences in this field of research. (BP)

More information

Article in Physical Review C

ALICE detector
Important comparative data: The team of researchers including Professor Hannah Elfner and her former PhD candidate Dr. Dmytro Oliinychenko simulated similar conditions to a CERN experiment recently conducted by the ALICE collaboration that accurately characterized collisional light nuclei.
Photo: CERN