Quark-gluon plasma may have been created by LHC in collisions of lighter particles

Brazilian researcher studied LHC experiment and found a signature highly similar to that of quark-gluon plasma in lead atom proton collision. Previous studies were based on collisions of heavy nuclei.

By José Tadeu Arantes  |  Agência FAPESP – A tiny fraction of a second after the Big Bang, the material universe consisted of a plasma made up of the elementary particles known as quarks and gluons. This is what the so-called Standard Model of the origin of the universe proposes.

As it rapidly expanded and cooled, this formless and intensely dynamic medium fragmented, and each small group of quarks and gluons gave rise to a compound particle called a hadron. One result was the formation of protons, each comprising two up quarks and one down quark bound together by gluons. Up and down quarks have the lowest masses of all quarks.

This primordial situation has been reproduced in the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN), on the Franco-Swiss border near Geneva, and in the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in the United States.

Quark-gluon plasma was first detected when two atomic nuclei of heavy elements such as lead and gold were collided. Now the ALICE collaboration, one of the international groups of researchers involved with the LHC, has obtained a characteristic “signature” of quark-gluon plasma by colliding protons in lead nuclei.

This result was achieved thanks to a major boost to luminosity and energy, which reached 5.02 teraelectron volts (TeV) for each colliding pair of nucleons. One TeV is equal to 10¹² electron volts.

Brazilian physicist Henrique Zanoli, a member of the ALICE collaboration, studied these collisions for his PhD research, and the results are published in Physical Review Letters.

Zanoli is currently a postdoctoral fellow at Utrecht University in the Netherlands. He holds a PhD from the University of São Paulo, Brazil, where his thesis advisor was Professor Alexandre Suaide. He has received funding from FAPESP via a direct doctorate scholarship and a scholarship for a research internship abroad.

“The experiment displayed azimuthal anisotropy in the distribution of the particles created by the collision. This means that the particles resulting from the collision were not produced in the same quantities in all directions. The electron distribution pattern we observed can be characterized as a quark-gluon plasma signature,” Zanoli told Agência FAPESP.

Zanoli supplied the following illustration to help readers understand what he said.

The diagram on the far left shows a proton (p) and a lead atom (Pb) instants before the collision. Heavy quarks are produced in the initial moments of the collision, and a quark-gluon plasma forms around them. The temperature falls rapidly, and the plasma disintegrates. Heavy quarks combine with other quarks to form various types of hadron, including ephemeral particles called D and B mesons. Eventually, the hadrons decay.

“Some decay pathways produce electrons, and it was precisely the anisotropy in the distribution of the resulting electrons’ trajectories that indicated the possible production of quark-gluon plasma. This is a signature associated with the production of the plasma,” Zanoli said.

“The distinctive feature of the experiment I studied is that based on the end result of the collision. It can thus be concluded that the heavy quarks were produced at the start of the process, not in later stages as occurs in other measurements involving light quarks.”

According to Zanoli, the system’s energy density was still extremely high when the production of heavy quarks occurred, and its evolution is an interesting tool to study the presence of quark-gluon plasma.

“These heavy quarks, which are produced before the plasma and traverse it, provide information on the plasma, rather like positrons passing through the human body in a PET scan,” he said. “If the particles studied had been produced at the end of the process, this analogy wouldn’t hold and we wouldn’t be able to describe the characteristics of the quark-gluon plasma on the basis of the end-result. But the heavy quarks were produced at the beginning, so they can be considered highly reliable markers.”

Primordial universe and astrophysical objects

A great deal of current research in particle physics focuses on quark-gluon plasma. There are two reasons for this. The first is that it is now possible to produce this plasma experimentally in colliders such as the LHC and RHIC. The other reason has to do with motivation: experiments involving quark-gluon plasma can afford insights into the nature of the primordial universe and processes involving exotic astrophysical objects such as neutron stars.

“Production of quark-gluon plasma in the lab has been made possible by the very high energy density achievable in today’s large colliders,” Zanoli said.

An energy level of 5 TeV is not particularly high for macroscopic objects containing enormous numbers of particles distributed in a large volume, but if 5 TeV is divided by the volume of a proton, the result is an energy density humanity has only now found ways to produce at the laboratory scale.

The article “Azimuthal anisotropy of heavy-flavor decay electrons in p-Pb collisions at √ sNN = 5.02 TeV” can be read at journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.072301.