De-confinement and clustering of color sources in nuclear collisions

Abstract

A brief introduction of the relationship of string percolation to the Quantum Chromo Dynamics (QCD) phase diagram is presented. The behavior of the Polyakov loop close to the critical temperature is studied in terms of the color fields inside the clusters of overlapping strings, which are produced in high energy hadronic collisions. The non-Abelian nature of the color fields implies an enhancement of the transverse momentum and a suppression of the multiplicities relative to the non overlapping case. The prediction of this framework are compared with experimental results from the SPS, RHIC and LHC for pp and AA collisions. Rapidity distributions, probability distributions of transverse momentum and multiplicities, Bose-Einstein correlations, elliptic flow and ridge structures are used to evaluate these comparison.The thermodynamical quantities, the temperature, and energy density derived from RHIC and LHC data and Color String Percolation Model (CSPM) are used to obtain the shear viscosity to entropy density ratio (η/s). It was observed that the inverse of (η/s) represents the trace anomaly ∆ = (ε − 3P )/T 4 . Thus the percolation approach within CSPM can be successfully used to describe the initial stages in high energy heavy ion collisions * Corresponding author

Indication of change of phase in high-multiplicity proton-proton events at LHC in string percolation model

“…In this scenario of the QGP study, an unexpected feature, namely the "ridge" in the long range near side angular correlation, in distinct class of "high multiplicity" events of proton-proton collisions [10] at √ s = 7 TeV at LHC has triggered revival of an old school of thought [11][12][13][14] of the possibility of formation of a collective medium in pp collisions also. Several of subsequent theoretical and phenomenological studies [15][16][17][18][19][20][21][22][23], in different approaches, endorse the possibility, indicating the need for further investigations in understanding the high-multiplicity pp events vis-a-vis the QGP.In this article, we address the issue of collectivity in high multiplicity pp events in the framework of the String Percolation Model, that has successfully explained the collectivity and the change of phase in nucleus-nucleus collisions [24][25][26]. Also SPM describes the centre-of-mass energy dependence of mid-rapidity multiplicity [25] and the pseudorapidity distributions [26] of produced charged particles in pp collisions, for the entire range of energy, available so far.…”

“…The cluster of percolated color strings is considered to be equivalent to the de-confined partonic state of matter [24]. In fact, there has been considerable progress in the SPM in establishing connection [24] between the percolation phase transition of color strings and the QCD phase transition in heavy-ion collisions.One of the cardinal parameters in SPM is the transverse impact parameter density of strings, ζ t . For pp collisions, [24] one can write ζ t ≡ ( r0 Rp ) 2N s where r 0 is ≃ 0.25 fm the single string transverse size, R p ≃ 1 fm is the proton transverse size andN s is the average number of single strings.…”

“…In fact, there has been considerable progress in the SPM in establishing connection [24] between the percolation phase transition of color strings and the QCD phase transition in heavy-ion collisions.…”

“…In this article, we address the issue of collectivity in high multiplicity pp events in the framework of the String Percolation Model, that has successfully explained the collectivity and the change of phase in nucleus-nucleus collisions [24][25][26]. Also SPM describes the centre-of-mass energy dependence of mid-rapidity multiplicity [25] and the pseudorapidity distributions [26] of produced charged particles in pp collisions, for the entire range of energy, available so far.…”

“…At a certain critical density, the strings start percolating through one another, forming a macroscopic cluster of strings -a geometrical phase transition takes place. The cluster of percolated color strings is considered to be equivalent to the de-confined partonic state of matter [24]. In fact, there has been considerable progress in the SPM in establishing connection [24] between the percolation phase transition of color strings and the QCD phase transition in heavy-ion collisions.…”