Universality of the transverse momentum distributions in the framework of percolation of strings

Abstract

In the framework of percolation of color sources, the transverse momentum distribution in heavy ion and p + p collisions at all centralities and energies are shown to follow a universal behaviour. The width of the distribution depends on the fluctuations of the number of color sources per cluster. At low densities, there are only independent single color sources, no fluctuations occurs and the distribution is described by a single exponential. At very high densities, only one cluster of many color sources appears and therefore there are not fluctuations either and the hardness of the distribution is suppressed. The Cronin effect in this framework is due to a maximum of the fluctuations which decreases as the density increases. We obtain a good agreement with experimental data including the low p T behaviour and the spectra for different particles.We show that the transverse momentum and multiplicity distributions are related to each other in a defined way. This point is satisfied by the experimental data on p + p collisions at different energies.

Rapidity and centrality dependence in the percolating color strings scenario

“…The color string model with fusion [1], [2] and percolation [3]- [6] has produced results on multiplicities of secondaries which are in general agreement with the existing experimental data. The string fusion model predicted a strong reduction of multiplicities both at RHIC and LHC energies.…”

“…In the previous paper [6], in the framework of percolation of strings, a reasonable agreement with the data was obtained, describing A-A collisions as an exchange of clusters of overlapping strings. In percolation each cluster behaves like a new string with a larger tension, its value depending on the number of strings fused into the cluster and the cluster's transverse area.…”

“…In this way, the suppression of high p T at large density follows as a result of formation of large clusters of color strings. In [6] we assumed that the spectrum of a simple string was a single exponential in p 2 T . However, at low string density, as in pp collisions, when fusion of strings is insignificant, the experimental data clearly show a power-like tail for the p T distribution.…”

Density saturation and the decrease of the normalised width of the multiplicity distribution in high energy pp collisions

“…We assume for P (n; Xn 1 ) a Poisson distribution, this being consistent with the single string distribution of low energy e + e − annihilations. Note that, as emphasised in [11], an equation similar to (1), with the same function W (X), can be written for the inclusive p T distribution, f (p T ).…”

Universal geometrical scaling in pp, pA and AA collisions and saturation of gluons

“…In the color glass condensate model [15,16] the more the saturation momentum grows with the energy and the number of participants, the smaller the room for hard collisions. This also happens in the framework of percolation strings [17,18] where the area covered by strings increases with the size and energy of the nuclei leaving less room for hard scatterings. Let us mention that the scaling found is naturally incorporated in the colour glass condensate [15,16] where the p T spectrum depends only on p 2 T /Q 2 S .…”