Studies of the Quark Gluon Plasma at STAR and CMS at UC Davis

Quantum Chromodynamics (QCD) is the theoretical framework that describes the strong nuclear force. The fundamental particles that interact via this force are the quarks and gluons that build up protons and neutrons, which in turn build up the nuclei that make up all atoms. This project aims to study the nature of matter at very high temperatures (about a million times hotter than the sun’s core), where nuclear matter reaches a phase called the Quark-Gluon Plasma (QGP).

These extremely high temperatures are reached at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), which have produced striking evidence for QGP formation. At RHIC, the UC Davis Nuclear Physics Group (UCD NPG) will study the thermodynamic properties of the transition in the high-density regime, taking advantage of the versatility of the collider to access different regions of the QCD phase diagram, with particular focus on the existence of a critical point.

The UCD NPG will study both fixed-target collisions and beam-beam collisions to scan areas of the phase diagram where indications of the critical point have been found. At the LHC, the UCD NPG will use heavy-quark bound states to study the properties of the QGP at the highest available temperatures, made even higher by the upgrade of the LHC energy to 5.5 TeV per nucleon pair.

Heavy quarkonium states, such as bottom-antibottom or charm-anticharm, are sensitive to the modification of the QCD potential expected in the QGP phase. This award will support studies of the pattern of modifications for the bottom and charm quark states that allow a characterization of the temperature of the medium and of the changes to the QCD potential in the QGP phase.

The UCD NPG will promote science education by reaching out to the broadest possible audiences through the NSF-funded giant-screen film “Secrets of the Universe”, narrated and starring Co-Pi Calderon de la Barca, and which features the students and work from this group. Through public screenings of the film with live Q&A sessions, the UCD NPG group will educate the public about the science done at the LHC and will encourage children, particularly girls and underrepresented kids, to consider pursuing careers in STEM.

The UCD NPG will train and mentor REU and UCD undergraduate students throughout the project. The UCD NPG, in its pivotal role pertaining to the RHIC fixed-target program, will conduct a study of collisions that mimic cosmic radiation. Such studies have a direct impact on the NASA space-radiation protection program aimed at understanding and mitigating the effects of cosmic radiation during lengthy space missions.

The scope of this project involves studies at RHIC and at the LHC. At the energy range covered by RHIC and the fixed-target program of the STAR experiment, from 3 – 200 GeV, the UCD NPG will study the phase diagram of QCD matter. Analyses of hadron spectra and baryon-number fluctuations will be used to characterize the QCD environment at each measured energy.

These studies will enable searches for the conjectured critical point of the phase diagram near the low-end of the energy range scanned by RHIC. This part of the project will advance our knowledge of the existence of such a critical point and its location in the phase diagram of the strong interaction. At the top energy available at the LHC, the UCD NPG will study heavy-quarkonium states with the CMS experiment.

The energy upgrade at the LHC, enabling higher energies and luminosities, will allow studies of events with two quarkonia mesons in the same collision. Events where an Upsilon (bottomonium) and J/psi (charmonium) are simultaneously produced will provide a new window to study the QGP, and how the QCD potential affects the formation of these states in the high-temperature, deconfined phase of the strong interaction.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.