Program Head: John Arrington
The Relativistic Nuclear Collisions (RNC) program in the Nuclear Science Division at LBNL carries out experimental research in High Energy Nuclear Physics to elucidate the fundamental nature of Quantum Chromo-dynamics (QCD), which governs the behavior of the basic constituents of matter, quarks and gluons. This program involves probing the structure and interactions of these fundamental constituents both ordinary matter (protons, neutrons, and nuclei) and in dense, energetic states of matter associated with the early universe. In addition to carrying out experiments and analyzing data to address these fundamental questions about the nature of matter, we also develop the detectors and computational approaches needed to drive this program of cutting-edge research.
High Energy Heavy Ion Collisions
This program studies the nature of strongly interacting matter and exploration of the QCD phase diagram. Lattice QCD calculations indicate that at sufficiently high temperatures (~2 trillion degrees), nuclear matter dissolves into a plasma of quarks and gluons, called the quark-gluon plasma (QGP). The QGP filled the universe 10 microseconds after the Big Bang, and may exist today in the cores of neutron stars.
On Earth, the QGP can only be studied via the collision of heavy atomic nuclear at very high energy, and we perform such studies as members of the STAR and sPHENIX experiments at the Relativistic Heavy Ion Collider (RHIC) at BNL, and the ALICE experiment at the Large Hadron Collider (LHC) at CERN. The picture of the QGP that has emerged from these experimental studies is quite different from initial theoretical expectations of a rather featureless gas of non-interacting quarks and gluons. Rather, the QGP displays complex and fascinating collective behavior, whose basic constituents are “quasi-particles” that are quite different from bare quarks and gluons, and appear to interact with the lowest viscosity allowed by nature. Experimental study of the QGP has attracted attention from neighboring fields of physics, including condensed matter, plasma physics, and even string theory.
The RNC has played a central role in the STAR experiment throughout its history, leading fundamental discoveries about the QGP. Most significantly, the collective dynamical behavior of the QGP and the interaction and modification of hard QCD jets in the QGP, or “jet quenching”, as predicted by our theory colleagues in the NSD. More recently, members of the RNC group have turned their attention to questions about the behavior of heavy quarks in the QGP, as well as to an experimental search for the QGP critical point via a beam energy scan (BES). The RNC recently joined the new sPHENIX experiment in pursuit of next-generation heavy quark measurements needed to characterize the microscopic picture of the QGP.
The RNC also plays a key role in A Large Ion Collider Experiment (ALICE) at the LHC. The main aim of ALICE is to characterize the physical properties of the QGP created with heavy ion collisions at the LHC but to also study QCD in smaller systems such as proton-proton, proton-lead, Oxygen-Oxygen and possibly more-to-come (e.g. Argon-Argon) in future LHC runs. RNC scientists working within ALICE Collaboration are focused on two directions: 1) unraveling the microscopic picture of jet quenching within QGP using jets and heavy quarks and 2) ultra-peripheral collisions (UPCs) allowing to study structure of nuclei and properties of hadrons. Performing experiments at the LHC is an ideal opportunity for detailed studies using these rare probes thanks to the high energy (currently highest achieved in a laboratory) of the collisions between ions or other species achieved at CERN. LHC’s Run 3 began in 2022 and members of the RNC play an active role in the collection and analysis of the new, high precision, data that ultimately will allow us to perform state-of-the-art measurements providing new, otherwise unreachable, insights into the properties of QGP.
Quark-gluon Structure of Matter
The RNC also studies the quark-gluon structure of the conventional matter – protons, neutrons, and nuclei – that make up more than 99% of the visible mass in the universe. This program is focused on key measurements that provide information on the spatial, momentum, spin, and flavor structure of quarks in protons and neutrons, as well as studies of the modification of proton and neutron structure in dense nuclei.
The program began with studies of the spin structure of protons in polarized proton-proton collisions at RHIC, aimed at understanding the origin of the spin of the proton after earlier measurements showed that only a fraction was carried by the spin of its constituent quarks. There is now a much broader program making use of the precise, high-luminosity measurements that can be performed at Jefferson Lab (JLab). Current focus areas include using the “mirror nuclei” 3H and 3He to better understand the structure of neutrons, and understanding the nature of extremely energetic configurations inside of nuclei and the potential impact of these “short-range correlations” on the quark structure of protons and neutrons in nuclei.
A profound, long-standing problem in QCD is the nature of the spin of the nucleon. The quark and anti-quark spins are known to carry only a small fraction of the proton spin, whereas the distribution of the remaining fraction amongst gluon spin and orbital angular momenta is not well understood. RHIC is not only the world’s first and most flexible heavy ion collider, but is also the world’s only polarized proton collider, enabling unique studies of proton spin structure. Members of the RNC are focusing on world-leading measurements of nucleon spin structure and spin in QCD, through measurements with STAR of jets and other probes.
Analyzing the large amount of data accumulated by the STAR and ALICE detectors is a formidable challenge and RNC provides significant computing capabilities for both experiments. The ALICE-USA Computing project provides grid computing resources for the ALICE experiment. Data from the STAR detector is stored and processed at the LBL ITD Lawrencium cluster and at NERSC. NERSC supercomputer facility resources supplement both STAR and ALICE computing resources, and are used for R&D projects and embedding simulation campaigns for ALICE and STAR experiments respectively.
Grid technologies for highly distributed computing play a key role in efficient usage of these resources, as does the high capacity HPSS mass storage system at NERSC. As a result of the R&D work of the computing group, opportunistic use of large amounts of compute resources at the Lawrencium cluster became available for the ALICE grid. With the deployment of the new Perlmutter machine at NERSC we also leverage allocation of GPU resources for the ALICE group, which is vital for the emerging ML/AI based research.
Members of the RNC are also devising novel techniques for theoretical understanding and data analysis of physical phenomena at colliders (e.g. RHIC, LHC, EIC) using Machine Learning. The aim is to design and apply methodologies which are able to benefit from the complete information obtained in measurements (as opposed to traditional analyses of single observables and binned distributions) and divine from this human-understandable knowledge about QCD. With this approach the scientists are able to propose a new generation of measurements and/or advise new directions in theoretical approaches.
Concurrently to these efforts members of the RNC are interested and pursue directions that enable studies of QCD using quantum computing. While the QC is in its early stages it is a rapidly developing subfield with transformative potential. Using quantum computers to simulate quantum processes may indeed overcome some shortcomings found in the classical techniques when calculating most interesting processes for the group (e.g. as jet quenching).
Instrumentation Enabling Cutting-Edge Research
The RNC is active in tracking detectors based on next generation Monolithic Active Pixel Sensors (MAPS), including a revolutionary mechanical support structure that was developed by RNC. The RNC made significant contributions to the upgrade of the Inner Tracking System (ITS2) utilizing the next-generation fast MAPS to the ALICE detector, which will enable the experiment to fully exploit the higher luminosities of the LHC in future runs. We also recently completed the construction and installation of the Micro-VerTeX detector (MVTX) using the same sensors for sPHENIX. Both are aimed for precision heavy flavor physics in QGP at RHIC and LHC, respectively
While much of the RNC instrumentation work is focused on MAPS detectors, we have constructed other critical detectors supporting the national heavy ion program. For STAR, we constructed the STAR central detector (the Time Projection Chamber, or TPC). The RNC, together with collaborators at other US institutions, led the construction and commissioning of the ALICE EMCal, a large electromagnetic calorimeter that enables ALICE to carry out unique measurements of jets and other hard probes in nuclear collisions.
We also carry out R&D on novel instrumentation with a wide range of potential applications in nuclear physics and beyond. Collaborating with physics and engineering divisions, we develop novel sensors around tight integration of radiation conversion materials, CMOS pixel and data handling ASICs, and mechanical support based on precision carbon composite parts. We aim to track particles with high spatial and timing resolution while minimizing the amount of material the particles interact with. Such detectors need to be able to survive high radiation as well as cryogenic environments.
Looking Ahead: Electron-Ion Collider
In addition to pursuing existing programs at RHIC, ALICE, and JLab, the RNC has been involved in the development of the physics case and detector design for the future Electron-Ion Collider (EIC). This is planned to be the next new nuclear physics facility built in the US and brings together both physics and detector technologies from the heavy-ion and hadronic physics communities. We have been involved in evaluating several important physics channels, developing the needs for tracking and far-forward detector systems, and have played several leadership roles at all stages of the project’s development. We were a founding member of the silicon consortium that developed a tracking/vertexing system based on the ITS3 sensors being developed at CERN, and look to play a significant role in building this system for the ePIC collaboration.
While the EIC is a primary focus of our program, RNC scientists are engaged in EIC-synergic future opportunities at the LHC. Members of the RNC are contributing to the development of the physics program and the detector design for the proposed Forward Calorimeter (FoCal) for ALICE that will enable the study of nucleon and nuclei structure at unprecedented small scales of momentum distribution of partons. Moreover, we are engaging in the design of the future ALICE 3 experiment at CERN that will take full advantage of the high-luminosity LHC runs in 2030’s. Also in these activities we draw from the experience and knowledge of the current ALICE setup, the future silicon trackers design (e.g. ITS3 in ALICE), and the work on the design of EPIC detector at the EIC.