Researchers from ELTE at the STAR experiment

The RHIC complex at the Brookhaven National Laboratory (left); the STAR experimental apparatus and a specific nuclear collision superimposed (right).

As recently reported by the Brookhaven National Laboratory, the protons accelerated at RHIC in 2024 will be polarized, meaning their internal spin will favor a particular direction. By observing how certain particles appear during collisions relative to the direction of the proton’s spin, researchers can determine how the proton’s internal components – quarks and gluons – contribute to its overall spin. This will help them better understand the fundamental building blocks of our universe and the forces at work within them.

Proton-proton collisions also provide important comparative data for collisions of larger nuclei. At RHIC, researchers also study collisions of gold nuclei to better understand the properties of quark matter. Collisions of nuclei containing many protons and neutrons, like gold, recreate conditions of the early universe, "liberating" the quarks and gluons that make up protons and neutrons from the nuclei. These experiments aim to study the quark-gluon plasma (QGP) that forms and the strong interaction, the most powerful force in nature, which holds quarks together and, indirectly, protons and neutrons.

Previously, the STAR experiment has collected ample proton-proton data in many runs, but this year's data collection is the first to use the new detector developments at the planned collision energy of 200 billion electron volts (GeV). The newly installed detectors can track particles that fly out perpendicular to the beam direction and are very close to the collision point, as well as those appearing in the so-called forward direction, close to the beam pipe. The capabilities of these new detector elements allow STAR physicists to draw more precise conclusions about the properties of quark-gluon plasma and gain insights into the details of proton spin.

Researchers from ELTE, led by Máté Csanád, participated in the first simulations for utilizing the Event Plane Detector.

Due to RHIC's developments, the STAR experiment will record more data in 2024 (and later in 2025) than ever before, laying the groundwork for future research at accelerators, particularly the Electron-Ion Collider (EiC), soon to be built at RHIC's location.

The electronics of the particle detectors will be able to read out approximately 5000 collisions per second – more than double the previous rate. This generates a huge amount of data, making its processing a challenging task. To facilitate data processing and storage, traditional hardware readout-triggering devices (known as triggers) will still be used to select which events need to be recorded and which are less interesting. However, machine learning and other artificial intelligence tools will also play an important role in data analysis this time.

During the proton-proton collisions in 2024, the particles will be polarized perpendicular to the beam direction. Asymmetries in particle production relative to the proton spin axis can provide insights into how the properties of the proton depend on the internal dynamics of its constituents. The internal motion of quarks and gluons, for example, can significantly contribute to the proton's spin.

ELTE researchers inside the accelerator next to the STAR experiment detectors (left) and during the STAR data collection (right).

The data collection of large experiments like STAR is usually led by participating researchers, who are responsible for communicating with the accelerator operation, continuously monitoring the experimental apparatus (detectors), operating low and high voltage systems, starting and stopping data collection, and handling the superconducting magnets with several megawatts of power. Since the experiments run continuously, researchers work in three shifts daily, forming teams of four. Members of the ELTE STAR-Hungary research group (Máté Csanád, Dániel Kincses, Márton Nagy, and Sneha Bhosale) participated in the measurements in person during the summer of 2024 – the ELTE team managed the STAR data collection every day from 7:30 AM to 4:30 PM for a week. Additionally, the ELTE researchers are involved in data analysis, particularly for femtoscopic measurements. Máté Csanád also directed the experiment's data archiving (HEPdata) and is currently a member of the committee coordinating the collaboration’s invited talks.

The following videos show event reconstruction recordings of collisions captured by the STAR at the RHIC collider:

The research group operates within the Thematic Excellence Program's Astro- and Particle Physics Thematic Area, and participation in the STAR experiment is currently supported by the NKFIH OTKA OTKA K-138136 and PD-146589 projects.