The study of lead–ion collisions at the LHC is a window into the quark–gluon plasma (QGP), a hot and dense phase of deconfined quarks and gluons. An important effect in heavy-ion collisions is jet quenching – the suppression of particle production at large transverse momenta (pT) due to energy loss in the QGP. This suppression is quantified by the nuclear-modification factor RAA, which is the ratio of particle production rate in Pb–Pb collisions to that in proton–proton collisions, scaled for the number of binary nucleon–nucleon collisions. A measured nuclear modification factor of unity would indicate the absence of final-state effects such as jet quenching.
Previous measurements of peripheral collisions revealed less suppression than seen in head-on collisions, but RAA remained significantly below unity. This observation indicates the formation of a dense and strongly interacting system – but it also poses a puzzle. In p–Pb collisions, no suppression has been observed, even though the energy densities are similar to those in peripheral Pb–Pb collisions.
The ALICE collaboration has recently put jet quenching to the test experimentally by performing a rigorous measurement of RAA in narrow centrality bins. The results (figure 1, left) show that the trend of a gradual reduction in the suppression of high-pT particle production as one moves from the most central collisions (corresponding to the 0% centrality percentile) to those with a greater impact parameter does not continue above a centrality of 75%. Instead, the data show a dramatically different behaviour: increasingly strong suppression for the most peripheral collisions. The change at 75% centrality shows that the suppression mechanism for peripheral collisions is fundamentally different from that observed in central collisions, where the suppression can be explained by parton energy loss in the QGP.
In a single Pb–Pb collision several nucleons collide. It has recently been suggested that the alignment of each nucleon collision plays an important role: if the nucleons are aligned, a single collision produces more particles, which results in a correlation between particle production at low pT, which is used to determine the centrality, and at high pT, where RAA is measured. The suppression in the peripheral events can be modelled with a simple PYTHIA- based model that does not implement jet-quenching effects, but incorporates the biases originating from the alignment of the nucleons, yielding qualitative agreement above 75% centrality (figure 1, right).
These results demonstrate that with the correct treatment of biases from the parton–parton interactions the observed suppression in Pb–Pb collisions is consistent with results from p–Pb collisions at similar multiplicities – an important new insight into the nuclear modification factor in small systems.
ALICE Collaboration 2018 arXiv 1805.05212 (submitted to Phys. Lett. B).