A report from the ALICE experiment
Jets are the most abundant high‑energy objects produced in collisions at the LHC, and often contaminate searches for new physics. In heavy‑ion collisions, however, these collimated showers of hadrons are not a background but one of the main tools to probe the deconfined state of strongly interacting matter known as the quark‑gluon plasma.
There are many open questions about the structure of the quark‑gluon plasma: What are the relevant degrees of freedom? How do high‑energy quarks and gluons interact with the hot QCD medium? Do factorisation and universality hold in this extreme environment? To answer these questions, experiments study how jets are modified in heavy‑ion collisions, where, unlike in proton‑proton collisions, they may interact with the constituents of the quark‑gluon plasma. Since jet production and interactions can be computed in perturbative QCD, comparing theoretical calculations to measurements can provide insight to the properties of the quark‑gluon plasma.
In this spirit, the ALICE collaboration has measured the inclusive jet production yield in both Pb‑Pb and proton–proton (pp) collisions at a centre‑of‑mass energy of 5.02 TeV. Jets were reconstructed from a combination of information from the ALICE tracking detectors and electromagnetic calorimeter for a variety of jet radii R. The detectors’ excellent performance with soft tracks was exploited to allow the measurements to cover the lowest jet transverse momentum (pT,jet) region measured at the LHC, where jet modification effects are predicted to be strongest. The measured jet yields in Pb‑Pb collisions exhibit strong suppression compared to pp collisions, consistent with theoretical expectations that jets lose energy as they propagate through the quark‑gluon plasma (figure 1). For relatively narrow R = 0.2 jets, the data show stronger suppression at lower pT, jet than at higher pT,jet, suggesting that lower pT,jet jets lose a larger fraction of their energy. Additionally, the data show no significant R dependence of the suppression within the uncertainties of the measurement, which places constraints on the angular distribution of the “lost” energy.
Several theoretical models, spanning a range of physics approximations from jet‑medium weak‑coupling to strong‑coupling, were compared to the data. The models are able to generally describe the trends of the data, but several models exhibit hints of disagreement with the measurements. These data complement existing jet measurements from ATLAS and CMS, and take advantage of ALICE’s high‑precision tracking system to provide additional constraints on jet‑quenching models in heavy‑ion collisions at low pT. Moreover, these measurements can be used in combination with other jet observables to extract properties of the medium such as the transverse momentum diffusion parameter, which describes the angular broadening of jets as they traverse the quark–gluon plasma, as a function of the medium temperature and the jet pT.
The “reference” measurements in pp collisions contain important QCD physics themselves. This new set of measurements was performed systematically from R = 0.1 to R = 0.6, in order to span from small R, where hadronisation effects are large, to large R, where underlying event effects are large. These data can be used to constrain the perturbative structure of the inclusive jet cross section, as well as hadronisation and underlying event effects, which are of broad interest to the high‑energy physics community.
Going forward, ALICE is actively working to further constrain theoretical predictions in both pp and Pb‑Pb collisions by exploring complementary jet measurements, including jet substructure, heavy‑flavour jets, and more. With a nearly 10 times larger Pb‑Pb data sample collected in 2018, upcoming analyses of the data will be important for connecting observed jet modifications to properties of the quark‑gluon plasma.
ALICE Collab. 2020 Phys. Rev. C 101 034911.