The world’s longest-serving heavy-ion collider, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, started its latest run in December. In addition to further probing the quark–gluon plasma, the focus of RHIC Run 22 (the 3.8 km-circumference collider’s 22nd run in as many years) is on testing innovative accelerator techniques and detector technologies for the Electron–Ion Collider (EIC) due to enter operation at Brookhaven in the early 2030s.
The EIC, which will add an electron storage ring to RHIC, will collide 5–18 GeV electrons (and possibly positrons) with ion beams of up to 275 GeV per nucleon, targeting luminosities of 1034 cm–2 s–1 and a beam polarisation of up to 85%. This will enable researchers to go beyond the present one-dimensional picture of nuclei and nucleons: by correlating the longitudinal components of the quark and gluon momenta with their transverse momenta and spatial distribution inside the nucleon, the EIC will enable 3D “nuclear femtography”.
Preparations for the EIC rely on RHIC’s unique ability to collide polarised proton beams via the use of helical dipole magnets, which offers a directional frame of reference to study hadron collisions. The last time polarised protons were collided at RHIC was 2017. For Run 22, the accelerator team aims to accumulate proton–proton collisions at the highest possible polarisation, and also at the highest energies (255 GeV per beam). To ensure the EIC hadron beams are as tightly packed as possible, thus maximising the luminosity, the accelerator team will try a technique previously used at RHIC to accelerate larger particles, but which has never been used with protons before.
“We are going to split each proton bunch into two when they’re still at low energy in the Booster, and accelerate those as two separate bunches,” explains Run-22 coordinator Vincent Schoefer. “That splitting will alleviate some of the stress during low energy, and then we can merge the bunches back together to put very dense bunches into RHIC.” Such merging is challenging, he adds, because it takes around 300,000 turns in the Alternating Gradient Synchrotron (the link between the Booster and RHIC), during which the protons must be handled “very gently”.
To further reduce the spread of high-energy hadron beams, the team will explore several cooling strategies (a major challenge for high-energy hadron beams) for possible use at the EIC. One is coherent electron cooling, whereby electrons from a high-gain free-electron laser are used to attract the protons closer to a central position. In addition, the team plans to ramp up beams of helium-3 ions to develop methods for measuring the polarisation of particles other than protons. Measuring how particles in the beam scatter off a gas target is the established method, but ions such as helium-3 can complicate matters by breaking up when they strike the target. To accurately measure the polarisation of helium-3 and other beams at the EIC, it is necessary to identify when this breakup occurs. During Run 22 the RHIC team will test its ability to accurately characterise scattering products using unpolarised helium-3 beams to develop new polarimetry methods.
During the run, RHIC’s recently upgraded STAR detector will track particles emerging from collisions at a wider range of angles than ever before (covering a rapidity of –1.5 – 4.2). The upgrades include finer granulated sensors for the inner part of the time projection chamber, and two new forward-tracking detectors and electromagnetic and hadronic calorimetery at one end of the detector, which will allow better reconstruction of jets.
In addition to increasing the dataset for exploring colour-charge interactions, these upgrades will give physicists crucial information about the detector technologies and the behaviour of nucleon structure relevant to the EIC. RHIC’s other main detector, the upgraded PHENIX, is under construction and scheduled to enter operation during Run 23 next year.
“Our goal this run is basically doing EIC physics with proton–proton collisions,” says Elke-Caroline Aschenauer, who led the STAR upgrade project. “We have to verify that what you measure in electron–proton collisions at the EIC and in proton–proton events at RHIC is universal – meaning it doesn’t depend on which probe you use to measure it.”