Measurement of charged particles spectra in ultra-central ultra-relativistic heavy ion collisions in the ATLAS experiment

Ultra-relativistic heavy-ion collisions provide a unique opportunity to study Quantum Chromo-dynamics (QCD) under extreme conditions of temperature and energy density. The resulting quark-gluon plasma (QGP) can be characterized through the analysis of final-state particles. At high collision centrality, variations in the initial system size are minimized, allowing changes in track multiplicity to be primarily attributed to fluctuations in initial energy density and temperature.

This analysis investigates potential modifications to the transverse momentum spectrum of charged particles in the high transverse momentum range, attributed to interactions with the QGP. Using data from the 1% most central Pb–Pb collisions recorded by ATLAS at centre-of-mass energy of 5.02 TeV, events were divided into bins based on track multiplicity. The resulting transverse momentum spectra spectra were compared using distribution ratios and high pT track yields as a function of track multiplicity.

No statistically significant modification of the transverse momentum spectrum was observed across multiplicity bins. Furthermore, the data do not support the predicted proportionality between transverse momemntum and track multiplicity derived from the Linear Boltzmann Transport model.