by Kai Schweda. Published: 12 July 2018

On June 12, Kai Schweda, of GSI, gave a LHC seminar on “closing in on parton energy loss with charged particle spectra in Xe-Xe and Pb-Pb collisions with ALICE” at CERN. Here is a summary of his talk.

Fig.1: Comparison of the nuclear modification factor in Xe-Xe and Pb-Pb collisions integrated over identical regions of pTas a function of charged particle multiplicity density.


In a new manuscript submitted to Physics Letters B, the ALICE collaboration reports on transverse momentum (pT) spectra of charged hadrons in xenon-xenon (Xe-Xe) collisions at an energy of 5.44 TeV per nucleon-nucleon pair. These results improve our understanding of the hot and dense quark-gluon plasma (QGP), a state of matter that permeated our universe 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 modeled using perturbative quantum chromodynamics. In collisions of heavy nuclei, the spectra are modified by the energy loss that a parton suffers when propagating in the QGP. The energy loss features a decisive dependence on the path length that the partons travel inside the QGP and the underlying mechanism. For example, in a static medium, it is linear for elastic collisions with the constituents of the QGP, while it is quadratic in case of induced gluon-radiation processes and a sufficiently thin medium. 

In October 2017, the LHC collided Xe nuclei for about six hours and ALICE recorded 1.1 million collisions. This gives unique access to the path length dependence of parton energy loss at similar collision geometry shapes since the radius of the Xe nucleus is some 20% smaller than that of lead which is usually accelerated at LHC: the isotope of Xe that was used has 129 nucleons, whereas Pb has 208 nucleons.

In order to characterize the change of spectra in nuclear collisions with respect to the expectation from pp collisions, the nuclear modification factor RXeXe(RPbPb) is calculated by dividing the pspectra from Xe-Xe (Pb-Pb) collisions by the spectra from pp collisions at the same energy, scaled by the number of binary nucleon–nucleon collisions in Xe-Xe (Pb-Pb) collisions. Since a pp reference at 5.44 TeV is not yet measured, it was interpolated from existing data at 5 and 7 TeV. Improvements in the particle reconstruction and its description in Monte Carlo simulations, as well as data-driven corrections based on identified-particle yields, led to a reduction of systematic uncertainties of a factor of about two.

A comparison of the nuclear modification factor in Xe–Xe and Pb–Pb collisions integrated over identical regions in pTas a function of charged particle multiplicity density is shown in Fig. 1. Remarkable agreement is found between Xe–Xe collisions and Pb–Pb collisions at a similar energy (5.02 TeV), as well as at 2.76 TeV, at charged particle multiplicity densities above dNch/dη > 400. This similarity is qualitatively consistent with the expected quadratic path length dependence of medium-induced radiative energy loss. At lower multiplicity densities, the values on the nuclear modification factor still agree, however within rather large uncertainties. 

Fig.2: Nuclear modification factor in Xe-Xe collisions and comparison to model predictions.


State-of-the-art models calculations are able to describe the main features of the ALICE data, see Fig. 2. However, the improved precision of the experimental data is yet to be met by models. Additionally, the now accessible path length dependence poses a serious challenge to all models and will guide the path to understand parton energy loss at a quantitative level.

Further reading: