Origin of the local minimum in normalized mean transverse momentum versus multiplicity

Determine the physical origin of the local minimum observed around normalized multiplicity Nch/Nch0 ≈ 1.05 in the dependence of the normalized mean transverse momentum (pT_mean/pT_mean0) on normalized charged-particle multiplicity (Nch/Nch0) for charged particles within |eta| < 0.5 in Pb–Pb collisions at sqrt(sNN) = 5.02 TeV, using spectra extrapolated to the full pT range.

Background

The analysis measures the normalized average transverse momentum of charged particles as a function of normalized charged-particle multiplicity in ultra-central Pb–Pb collisions at sqrt(sNN)=5.02 TeV, within |eta|<0.5 and with spectra extrapolated to the full pT range. The observable shows a weak decline toward a local minimum near Nch/Nch0 ≈ 1.05, followed by a steep rise at higher multiplicities.

While the Trajectum hydrodynamic model qualitatively reproduces a minimum and the Gardim et al. model does not, the simplified fitting framework used to extract the speed of sound does not capture this feature, leading the authors to restrict fits to Nch/Nch0 > 1.14. Despite model comparisons, the specific physical mechanism producing the observed local minimum remains unidentified.