Anomalous baryon production in p+p and Pb+Pb collisions at the LHC

by Dr Vasile Topor Pop (McGill University)

Tuesday 31 May 2011, 15:00 → 16:00 Europe/Rome

LAE meeting room (INFN-LNL)

With the HIJING/B¯B v2.0 event generator, we explore the phenomenological consequences of the suppression of perturbative quantum chromodynamics (pQCD) mini-jet production, of the enhanced ”in medium” strong longitudinal color field (SCF) and of (gluonic) baryon-anti-baryon junctions (J¯J loops). This analysis focused on p + p collisions at centre of mass energy ps = 0.900, 2.36 and 7 TeV and on Pb+Pb collisions at psNN = 2.76 TeV, where recent data have been reported by LHC Collaborations (ALICE, ATLAS, CMS). For Pb+Pb collisions nuclear effects like shadowing and parton energy loss (”jet quenching”) are included. We show that combined effects of hard and soft sources of multiparticle production can reproduce the available p + p data in the range 0.02 < ps < 7 TeV assuming a dependence of the effective string tension, , on energy (s) = 0 (s/s0)0.06 GeV/fm. This exponent is half of that predicted by the color glass saturation (CGC) model, where gluon fusion mechanisms are assumed. The large (strange)baryon-to-mesons ratios measured at the Tevatron and the LHC are well described. A significant enhancement of these ratios is predicted up to the highest LHC energy (14 TeV). Data on charged hadron multiplicity and charged hadron nuclear modification factor RAA(pT ) in central (0-5%) Pb+Pb collisions from the ALICE experiment at the LHC are used to constrain the main parameters of the ”jet quenching” phenomena (energy loss and mean free path of initial partonparton interactions). Predictions for the energy and centrality dependence of rapidity densities (2dNch/dy)/Npart) and especially the hadron flavor dependence (mesons and baryons) of the nuclear modification factor RAA(pT ) are discussed. By studying baryon/meson ratios, we show that the jet quenching in central collisions suppresses the hard pQCD component of the particle spectra, thereby exposing novel components of baryon dynamics that we attribute to (gluonic) baryon-antibaryon junctions (J¯J loops) and to SCF effects. We predict that a baryon/mesons anomaly at intermediate transverse momentum values (pT < 10 GeV/c) will persist at LHC energies, with a moderate centrality dependence.