ALICE closes in on parton energy loss

In a new publication submitted to the Journal of High Energy Physics, the ALICE collaboration has reported transverse momentum (p_T) spectra of charged hadrons in proton–proton (pp), proton–lead (pPb) and lead–lead (PbPb) collisions at an energy of 5.02 TeV per nucleon pair. The results shed further light on the dense quark-gluon plasma (QGP) thought to have existed shortly after the Big Bang.

At high transverse momentum, hadrons originate from the fragmentation of partons produced in hard-scattering processes. These processes are well understood in pp collisions and can be modelled using perturbative quantum chromodynamics.

In PbPb collisions, the spectra are modified by the energy loss that the partons suffer when propagating in the QGP. Proton–lead collisions serve as a baseline for initial-state effects such as the modification of the gluon density of the nucleons of colliding lead nuclei.

To characterise the change of spectra in nuclear collisions with respect to the expectation from pp collisions, the nuclear modification factors R_PbPb (R_pPb) are calculated by dividing the p_T spectra from PbPb (pPb) collisions by the spectra measured in pp collisions, scaled by the number of binary nucleon–nucleon collisions in the PbPb (pPb) collisions (see figure).

The nuclear modification factor in proton–lead collisions is consistent with unity at high transverse momentum. This shows that initial-state effects from the parton density in the lead nucleus are small and that the strong suppression observed in PbPb collisions is caused by final-state parton-energy loss in the QGP. The new results with higher statistics have much improved systematic uncertainties compared to the earlier publications based on Run 1 data. This is possible because of the improvements in the particle reconstruction and its description in Monte Carlo simulations, as well as data-driven corrections based on identified particles.

The suppression in PbPb collisions at 5.02 TeV is found to be similar to that at the collision energy of 2.76 TeV despite the harder spectrum at the higher energy, which indicates a stronger parton-energy loss and a larger energy density of the medium at the higher energy.

Theoretical models are able to describe the main features of the ALICE data; the improved precision of the measurements will allow researchers to constrain theoretical uncertainties further and to determine transport coefficients in the QGP. The upcoming PbPb run scheduled for November this year and the large pp reference sample collected at the end of 2017 will improve the statistical precision substantially and further extend the covered range of the transverse momentum.