Medium Modification of Photon-Tagged and Inclusive Jets in High-Multiplicity Proton-Proton Collisions

Abstract

We study modification of the photon-tagged and inclusive jets in pp collisions at √ s = 7 TeV due to mini-quark-gluon plasma which can be produced in high multiplicity events. We show that for underlying events with dN ch /dη ∼ 20 − 60 the medium effects lead to a considerable modification of the photon-tagged and inclusive jet fragmentation functions. For inclusive jets the magnitude of the effect is surprisingly large. The effect is quite strong even for typical underlying events. We find that the spectrum of charged hadrons is suppressed by ∼ 35 − 40% at pT ∼ 5 − 10 GeV.The experiments at RHIC and LHC have provided strong evidence for the formation in AA collisions of a hot and dense QCD matter in the form of quark-gluon plasma (QGP). However, so far we have no compelling evidence for the formation of the QGP in pp collisions. It is widely believed that over the energy range covered by the accelerator experiments in the typical inelastic minimum bias pp collisions the collectivity in the final states is unimportant due to small energy density. Nevertheless, in rare high-multiplicity (HM) pp events the energy density may be comparable to that in AA collisions at RHIC and LHC energies. And if the thermalization time, τ 0 , is small enough, say τ 0 ∼ < 0.5 fm, the QGP should be formed quite likely to AA collisions.One can expect that collectivity in HM pp collisions may be observed via the hydrodynamic flow effects [1, 2] similar to that in AA collisions. The hydrodynamical simulation of the flow effects in pp collisions with the initial conditions from the IPGlasma model [3] has been performed in [4]. It was observed that for small-size QGP the theoretical uncertainties are large as compared to the QGP in AA collisions. This may make it difficult to extract information on the mini-QGP via measurements of the azimuthal flow coefficients.Although today we do not have clear evidence for the formation of the QGP in pp collisions, there are some indications in its favor. It is possible that observed by the CMS collaboration ridge correlation structure in HM pp events [5] is due to the transverse flow of the mini-QGP. But alternative explanation of this effect in the color glass condensate picture seems also possible [6]. In [7], employing Van Hove's idea [8] that phase transition should lead to anomalous behavior of p T as a function multiplicity, it has been argued that the pp data on p T signal possible plasma formation in the domain dN ch /dη ∼ 6 − 24.An unambiguous proof of formation of a dense QCD matter in pp collisions would be observation of jet modification (quenching) similar to that observed in AA collisions. It is important that conditions for the QGP production are better in events with jets, because multiplicity of soft offjet particles (so-called underlying events (UEs)) is enhanced by a factor of 2 − 3 [9]. In AA collisions jet quenching leads to suppression of highp T spectra characterized by the nuclear modification factor R AA defined as the ratio of the particle yield in the AA collision to the b...

Medium-Modification of Photon-Tagged Jets in AA Collisions

“…And its effect on the theoretical I AA can be imitated by some change in α s . However, contrary to the medium effects in pp collisions on the high-p T spectra, the photon-tagged jet FF, at least in principle, enables us [41] to study the effect of mini-QGP by measuring its variation with the multiplicity of soft off-jet particles (the so-called underlying events, see Ref. [42] for a review).…”

“…[42] for a review). But this requires measurements of the photon-tagged jet FF for very high underlying event multiplicities dN UE ch /dη ∼ 40 [41]. Such multiplicities are too high to be measured at RHIC energies in the γ+jet events.…”

Monte Carlo Glauber wounded nucleon model with meson cloud

“…The observations of the ridge effect in p+Pb collisions at √ s = 5.02 TeV [77][78][79] and in high multiplicity pp events at √ s = 7 TeV [80] support this idea. The estimates from the observed charged multiplicities show that for the typical pP b and pp events at the LHC energies the initial temperature of the mini QGP at the proper time τ ∼ 0.5 fm may be ∼ 250 MeV [81,82], which is well above the deconfinement temperature. In the scenario with the mini QGP formation the ridge effect in pP b and pp collisions may be connected with the hydrodynamic expansion of the azimuthally asymmetric initial plasma fireball [83][84][85].…”

Effect of a meson cloud on the jet nuclear modification factor in pA collisions

“…In principle the suppression of the jet R pP b in the central events might arise from the final state interaction effects in the small-size quark-gluon plasma, if it is formed in pP b collisions [3]. However, the fact the minimum bias jet R pP b observed in [2] is approximately consistent with unity, says that the potential effect of the plasma mini-fireball is small (or it is well compensated by the medium effects in pp collisions [4] due to modification of the denominator of (1) [3]). In [5] it was proposed that the ATLAS data [2] can be explained by the initial state correlations of the hard and soft partons in the wave function of the projectile proton.…”