STAR collaboration

STAR collaboration. The STAR (Solenoidal Tracker at RHIC) collaboration is a large international scientific group operating a major particle detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York. Its primary mission is to study the properties of the quark–gluon plasma, a state of matter believed to have existed microseconds after the Big Bang, by colliding heavy ions such as gold and nuclei of lighter elements at near-light speeds. The collaboration comprises over 700 scientists, engineers, and students from more than 60 institutions across a dozen countries, making fundamental contributions to the field of high-energy nuclear physics.

Overview

The collaboration was formed in the early 1990s to design and construct the STAR detector, a sophisticated apparatus optimized for tracking the thousands of particles produced in each ultra-relativistic collision at RHIC. The experiment began taking data with the start of RHIC operations in 2000, ushering in a new era of research into quantum chromodynamics under extreme temperature and density conditions. Its research program has expanded over decades, encompassing not only heavy-ion collisions but also polarized proton collisions to probe the spin structure of the nucleon and studies of smaller collision systems to understand the onset of collective phenomena. The collaboration's work is integral to the broader scientific mission of the United States Department of Energy's Office of Science.

Scientific goals and discoveries

A central goal is the creation and characterization of the quark–gluon plasma, with STAR providing compelling evidence for its formation through observations of elliptic flow, jet quenching, and strange quark enhancement. The collaboration made the landmark observation of the "perfect fluid" nature of the quark–gluon plasma, demonstrating it behaves as a liquid with near-zero viscosity, a finding highlighted by the American Physical Society. Studies of J/ψ meson suppression and regeneration have provided insights into deconfinement and the screening of the strong interaction. In the spin physics program, measurements of W boson production in polarized proton collisions have yielded precise constraints on the contributions of different quark flavors to the proton's spin, challenging earlier models like the European Muon Collaboration's "spin crisis" findings.

Experimental setup and detector

The core of the experiment is the large, cylindrical Time Projection Chamber, which provides precise three-dimensional tracking and particle identification for charged particles over a wide acceptance. This is complemented by sub-detectors including the Barrel Electromagnetic Calorimeter for measuring photons and electrons, the Time-of-Flight system for particle identification, and the Heavy Flavor Tracker for detecting decays of particles containing charm quarks and bottom quarks. Major upgrades, such as the installation of the Event Plane Detector and the forward Roman Pots system, have been implemented through projects like the RHIC Physics Program and in preparation for the Electron-Ion Collider. The detector's design was heavily influenced by earlier experiments like the ALICE experiment at CERN and benefits from technologies developed for the Large Hadron Collider.

Organization and collaboration

The collaboration is governed by an elected Spokesperson and an Executive Committee, with scientific work organized into numerous working groups focusing on specific physics topics or detector subsystems. Member institutions span the globe, including major contributions from Lawrence Berkeley National Laboratory, University of California, Los Angeles, Institute of High Energy Physics (China), Joint Institute for Nuclear Research in Dubna, and Variable Energy Cyclotron Centre in Kolkata. Early leadership involved key figures from institutions like Yale University and Massachusetts Institute of Technology. The group operates under the umbrella of the RHIC Collaborations and maintains close ties with fellow experiments like PHENIX collaboration and the future sPHENIX detector.

Data analysis and publications

The collaboration employs sophisticated computing grids, such as the Open Science Grid, to process and distribute the petabytes of collision data collected over many runs. Physics results are published in high-impact journals including Physical Review Letters and Physical Review C, after a rigorous internal review process by the collaboration's publication committee. Notable publications include the first observations of antimatter helium-4 nuclei, the measurement of global polarization of Lambda baryons implying the most vortical fluid ever observed, and detailed studies of di-jet asymmetry. Data and derived results are often made publicly available through repositories like the HepData database, supporting the broader research efforts of the high-energy physics community.