High multiplicityppevents andJ/ψproduction at energies available at the CERN Large Hadron Collider

Abstract

We discuss the dependence of J/ψ production on the charged particle multiplicity in proton-proton collisions at LHC energies. We show that, in the framework of parton saturation or string interaction models, the hard J/ψ production exhibits a significant growth with the multiplicity, which is stronger than linear in the high density domain. This departure from linearity, that should affect any hard observable, applies for high multiplicity proton-proton collisions in the central rapidity region and is a consequence of the parton saturation or the strong interaction among colour ropes that take place at LHC energies. Our assumption, the existence of coherence effects present in proton-proton collisions at high energy, can also be checked by studying the particular shape of the probability distribution associated to the J/ψ production.RHIC [1,2,3,4] and LHC [5,6,7] data on heavy-ion collisions have shown several important features which indicate the formation of a high density partonic medium with characteristic properties as the low shear viscosity and high opacity. Since the energy density achieved in high multiplicity events produced in pp collisions at LHC is comparable to the reached density in CuCu central collisions or AuAu peripheral collisions at √ s N N = 200 GeV, it is pertinent to wonder about the possibility to obtain a similar high density medium which would be reflected in experimental observables, similar to heavy-ion collisions. An illustration of this would be the predicted ridge structure [8,9] observed by CMS collaboration [10] in pp collisions. Also, the eventuality that other observables, as long range rapidity correlations [11,12,13], energy loss [14], or the elliptic flow [15,16], are measurable in pp collisions, has been reckoned with in different frameworks.We address here to the J/ψ production in high multiplicity pp collisions. We will show that the rise of J/ψ production in the highest multiplicity events observed by the ALICE collaboration [17] can be naturally explained as a consequence of string interaction or parton saturation. This feature is particularly important, since, due to the absence of nuclear effects in this case, it can be use as a baseline in order to disentangle different mechanism that are expected in heavy-ion collisions, as the J/ψ suppression due to sequential dissociation [18] or the J/ψ enhancement due to the recombination of uncorrelated c andc quarks [19].

Measurement of J/ψ production in p + p collisions at s=500 GeV at STAR experiment

“…They agree quite well with data for both p T bins. The percolation model [9] also agrees well with data for p T > 0 GeV/c. However, it is worth noting that the percolation model differs significantly from PYTHIA at higher event activity bins, which calls for more measurements.…”

J/ψ production in p+p at $\sqrt{s}=500\,{\rm{GeV}}$ collisions and Au+Au collisions at $\sqrt{{s}_{NN}}=200\,{\rm{GeV}}$ at the STAR experiment

“…PYHTIA8 with a default setting, including the Multiple-Parton Interaction (MPI) contributions to particle production, can describe the rising trend and p T dependence in data. Percolation model [7] also qualitatively reproduces the trend in data. |y| < 1.…”

Measurement of charm and beauty production at central rapidity versus charged-particle multiplicity in proton-proton collisions at s = 7 $$ \sqrt{s}=7 $$ TeV

“…This suggests that heavy-flavour relative yields enhancement is not significantly influenced by hadronisation, but more likely directly related to the cc and bb production processes. The heavy-flavour relative yield enhancement as a function of the charged-particle multiplicity is qualitatively described by: (a) PYTHIA 8.157 calculations including the MPI contributions to particle production [31], (b) percolation model estimates of the influence of colour charge exchanges during the interaction [41,73], (c) predictions by the EPOS 3 event generator which provides a description of the initial conditions followed by a hydrodynamical evolution [71,72]. How--29 -JHEP09 (2015)148 ever, the PYTHIA 8.157 [31] event generator seems to under-estimate the increase of heavy flavour yields with the charged-particle multiplicity at high multiplicities.…”