The many lives of RHIC

The proton–proton collider ISABELLE, at Brookhaven National Laboratory, was cancelled in July 1983 with its 3.8 km tunnel already constructed (CERN Courier July/August 2026 p22). Within the year, the nuclear-physics community had proposed reusing it for a quantum-chromodynamics (QCD) machine capable of producing the quark–gluon plasma (QGP) – a state in which quarks and gluons are no longer confined within protons and neutrons. Thus began, in 1991, the construction of the Relativistic Heavy Ion Collider (RHIC), which became the first collider designed specifically for heavy ions and the only one to collide polarised protons (see “Bird’s-eye” figure).

When RHIC’s first gold ions collided in 2000, the BRAHMS, PHENIX, PHOBOS and STAR experiments observed strong collective motion and a suppression of high-transverse-momentum particles, evidence for matter at record energy density. Because the suppression could also have originated in the structure of the gold nuclei themselves, the four experiments turned to deuteron–gold (d–Au) collisions, in which no hot matter was expected to be created but any effects from the gold nuclei would still be apparent. By examining different aspects of the events, they observed no suppression and proved that the effect seen in gold–gold (Au–Au) collisions was due to jet quenching – the loss of energy by fast quarks and gluons in the dense QGP medium. The complementarity of their results was made explicit in August 2003, when the cover of Physical Review Letters displayed a plot from each (see “Back-to-back” figure). In 2005, the experiments published white papers on their collected measurements, which revealed that the matter created at RHIC behaved not as the anticipated weakly interacting gas, but as a strongly interacting, nearly perfect liquid, flowing with almost no internal friction. These remain among the most highly cited papers in nuclear physics.

Snakes in the rings

The RHIC spin programme, which collided beams of spin-aligned protons, was started in 2001, in response to the so-called “proton-spin crisis” – the 1988 finding by the European Muon Collaboration that quarks could only account for a fraction of a proton’s spin. Polarised collisions were not part of RHIC’s original design, and much of the required technology had yet to be demonstrated. A high-intensity optically pumped polarised ion source (OPPIS) produced beams with up to 83% polarisation. At the same time, partial Siberian snake magnets kept the proton spins aligned as the beam gained energy in the Alternating Gradient Synchrotron (AGS) without elaborate resonance-by-resonance corrections. At RHIC, two full Siberian snakes in each ring allowed protons to reach 255 GeV with little polarisation loss. Spin rotators provided longitudinal polarisation at the experiments, while proton–carbon and polarised hydrogen-jet polarimeters monitored the beam polarisation (CERN Courier October 2005 p15).

After more than a decade of development of luminosity, polarisation and measurement techniques, the STAR and PHENIX collaborations decisively established that gluon polarisation contributes to the spin of the proton and, from parity-violating W decays, found an unexpected difference between the polarisations of the up and down antiquarks within a polarised proton. The observation of large transverse single-spin asymmetries at high energy and the exploration of their connection to the transverse-momentum-dependent structure of the proton will be a lasting legacy of RHIC. The latest datasets, taken with transverse polarisation, allow for the exploration of the three-dimensional spin-dependent structure of the proton, developing techniques that will be used at the future Electron–Ion Collider (EIC).

Stochastic cooling

Heavy ions in a stored beam scatter off each other, causing the beam size to grow and the collision rates to drop, so that the machine requires frequent refilling. To counteract this effect, stochastic cooling systems, which sense variations within the beam at one point of the ring and correct them with kicks at another, were installed in RHIC between 2006 and 2014, to act in both the transverse and the longitudinal directions. While stochastic cooling had previously been used at much lower energies for unbunched beams, RHIC was the first machine to apply it to bunched beams and the first hadron collider to cool both of its beams.

The system demonstrated its full capabilities during RHIC’s uranium–uranium (U–U) run in 2012 (CERN Courier October 2012 p17) and subsequently enabled a large increase in Au–Au luminosity, eventually reaching 44 times the design value for the average store luminosity (see “Cooler and brighter” figure). Stochastic cooling also allowed more ions to be stored. As a result, RHIC could operate with high luminosity near the “burn-off” limit, where almost all ions are lost through collisions with those in the opposing beam.

The high luminosity, together with the silicon-detector upgrades operating in PHENIX from 2011 and in STAR from 2014, enabled next-generation studies of the QGP created at RHIC, including precise observations that charm quarks flow along with the nearly perfect liquid, measurements of electromagnetic probes to constrain its temperature and space–time evolution, and measurements of quarkonia as a major probe of deconfinement. The control over the beams that cooling brought also underpinned many of the later runs with novel beam combinations.

New ions, new questions

RHIC was designed for flexibility. Its two superconducting rings operated almost independently, coupled only by two dipole magnets in each of the six interaction regions, which brought the beams into collision and separated them again. The ion sources and injector chain could deliver beams ranging from polarised protons to uranium ions and switch rapidly between species for asymmetric collisions. Over its lifetime, the machine collided 12 different pairs of species, five of them asymmetric and three involving at least one polarised beam, at 18 centre-of-mass energies, for a total of 33 distinct combinations of species, energy and collision configuration (see “Thirty-three ways” figure). The STAR detector also operated in fixed-target mode, with gold beams at 13 energies from 3.85 to 100 GeV/nucleon.

The ability to collide nuclei of varying sizes and shapes has opened a new window on their structure. Starting with the U–U run, the near-perfect liquidity of the QGP has been used with increasing precision to take snapshots of nuclear shape. The method works because collective flow encodes the geometry of the initial overlap region in the momenta of the emitted particles, allowing the shapes of the initial nuclei to be inferred. Comparisons of p–Au, d–Au and ³He–Au collisions extended this to small systems, providing evidence of fluid behaviour in small QGP droplets, while the isobar run took it to high precision by comparing ⁹⁶Ru–⁹⁶Ru with ⁹⁶Zr–⁹⁶Zr.

A quarter century of RHIC beam operations ended on 6 February 2026 with a replay of the 2021 O–O collisions, the only ion species run in common with the LHC

That two-month-long run used stochastic cooling and levelling to alternate between the two isobars daily, with 20-hour stores at constant luminosity. Beyond the shape differences of the two nuclei, the uniform data-taking conditions allowed for subtle measurements of charge versus baryon transport. These provide indications that topological configurations called baryon junctions, rather than valence quarks, carry baryon number in high-energy collisions. A quarter century of RHIC beam operations (see “Long runs” figure) ended on 6 February 2026 with a replay of the 2021 O–O collisions, the only ion species run in common with the LHC (CERN Courier May/June 2026 p10).

The phase diagram

Colliders normally aim for the highest possible collision energy. RHIC also pushed the other way, far below its nominal injection energy. Operating there is challenging, since larger beam sizes, increased superconducting magnet field errors, and stronger space-charge and intrabeam-scattering effects all reduce collision rates. The effort called for extensive optimisation of machine and beam parameters. To increase the luminosity at the lowest energies, the new Low Energy RHIC electron Cooler (LEReC) was built, in which a beam of electrons travelling alongside the ions draws heat from them.

This was the world’s first electron cooler based on an electron beam accelerated by radio-frequency (RF) systems. Earlier coolers relied on electrostatic acceleration to produce the very small momentum spread required for effective cooling. RF acceleration can reach much higher energies, but produces a large momentum spread, requiring multiple RF correction stages before the beam can be used for cooling. By overcoming this limitation, the RHIC electron cooler established a technology critical for the EIC.

The QCD phase diagram, when viewed on the axes of baryon chemical potential and temperature, is potentially quite rich. At the highest RHIC energy, where the baryon density is small, lattice QCD calculations suggest that the phase transition from QGP to bound hadronic matter is a smooth crossover (CERN Courier September/October 2026 p41). At high baryon density, instead, effective models of QCD indicate that the transition should be first order. If a crossover and a first-order transition line both exist, they meet at a QCD critical point. Its signature would be a non-monotonic variation with collision energy of the event-by-event fluctuations in the number of net protons produced. The second phase of RHIC’s beam energy scan, enabled at its lowest energies by electron cooling, searched for it in collisions from 7.7 to 19.6 GeV, as well as in fixed-target collisions from 3 to 13.7 GeV. RHIC’s Beam Energy Scan has provided the world’s most systematic exploration of the QCD phase diagram at finite baryon density, including measurements of fluctuations, correlations and collective observables sensitive to the possible critical point and to the nature of the transition. Beyond the search for the critical point, the broad, high-statistics dataset has enabled precision studies of how the fluid responds to baryon doping – the excess of baryons over antibaryons that grows as the collision energy falls – and under what conditions fluidity breaks down.

Enter sPHENIX

The sPHENIX detector, which took over PHENIX’s hall in 2023 with a superconducting solenoid, latest-generation tracking detectors and calorimeters for measuring jets, required the highest possible luminosity in a limited vertex range of ±10 cm. To meet these requirements, a full crossing angle of 1 mrad was implemented, and the machine protection system was upgraded to safeguard the new silicon detectors close to the beam pipe.

In the 2025 run, sPHENIX took more than 200 PB of data, more than all previous RHIC raw datasets combined. High luminosity was also delivered to STAR, with the crossing angle switched between 0 and 1 mrad during the store (see “Double exposure” figure). While sPHENIX is still in the calibration phase for the bulk of its data, it has made standard-candle measurements of transverse energy and multiplicity in Au–Au collisions and has submitted for publication measurements of isolated photons and – for the first time at RHIC – fully corrected dijet momentum-imbalance measurements in p–p collisions. sPHENIX was designed to perform the full suite of jet-related measurements that have developed in the decades since the original jet-quenching discovery, such as fully reconstructed jets, jet substructure and jets containing heavy flavour. The statistical precision of the sPHENIX dataset will allow jet properties to be compared between RHIC and the LHC as a function of energy scale, to tease out the emergence of near-perfect fluidity from the interactions of quarks and gluons.

RHIC has combined sustained collider innovation with a constantly evolving scientific programme

RHIC has combined sustained collider innovation with a constantly evolving scientific programme. It leaves a unique legacy in relativistic heavy-ion and polarised-proton physics, accelerator science and technology, as well as detector and computing development. Its impact extends from fundamental research and broader applications to the future EIC, which will collide polarised electrons with polarised protons and with nuclei to map the gluons inside them. The machine will build directly on RHIC by reusing one of its superconducting rings, its tunnel and the hadron injector complex. Moreover, while beam operations have ended, the rich RHIC dataset will continue to be analysed for at least another decade.