Exploring the initial stage of high multiplicity proton-proton collisions by determining the initial temperature of the quark-gluon plasma

Abstract

We have analyzed identified particle transverse momentum spectra in high multiplicity events in p p collisions at LHC energies √ s = 0.9 – 13

TeV published by the CMS Collaboration using the color string percolation model (CSPM). In CSPM color strings are formed after the collision, which decay into new strings through color neutral q − ¯ q pairs production. With the increase in the p p collisions energy number of strings grow and randomly statistically overlap producing higher string tension of the composite strings. The net color in the overlap string area is a vector sum of the randomly oriented strings. The Schwinger color string breaking mechanism produces these color neutral q − ¯ q pairs at time ∼ 1

fm /c, which subsequently hadronize. The initial temperature is extracted both in low and high multiplicity events.The shear viscosity to entropy density ratios η / s are obtained as a function of temperature. For the higher multiplicity events at √ s = 7 and 13 TeV the initial temperature is above the universal hadronization temperature and is consistent with the creation of deconfined matter. The η / s is similar to that in Au + Au collisions at √ s N N = 200

GeV . The small value of η / s above the universal hadronization temperature suggested that the matter is a strongly coupled quark gluon plasma. In these small systems it can be argued that the thermalization is a consequence of the quantum tunneling through the event horizon introduced by the quarks confined in the colliding nucleons and their deceleration due to string formation, in analogy to the Hawking-Unruh radiation which provides a stochastic approach to equilibrium. The disk areas cluster on the nucleon transverse collision area. At the 2 D percolation threshold a macroscopic spanning cluster suddenly occurs at the temperature T i = T h , representing a small connected droplet of q − ¯ q pairs, the quark-gluon plasma (QGP). T h is the universal hadronization temperature ∼ 167.7

MeV . The collision energy dependent buildup of the 2D percolation clusters defines the temperature range 159 ± 9

MeV of the crossover transition between hadrons to the QGP in reasonable agreement with the lattice quantum chromodynamics (LQCD) pseudocritical temperature value of 155 ± 9

MeV . Color string percolation model is the new initial stage paradigm for the study of the high density matter produced in p p and A + A collisions. With CSPM we can directly explore the thermodynamics of the QGP above the universal hadronization temperature.