The medium dependent finite width is introduced into an exactly solvable model with the general mass-volume spectrum of the QGP bags. The model allows us to estimate the minimal value of the QGP bags' width from the lattice QCD data. The large width of the QGP bags not only explains the observed deficit in the number of hadronic resonances comparing to the Hagedorn mass spectrum, but also clarifies the reason why the heavy QGP bags cannot be directly observed as metastable states in a hadronic phase. 25.75.Nq Keywords: Hagedorn spectrum, finite width of quark-gluon bags, subthreshold suppression of bags

The ALICE Collaboration at the LHC has measured the J/ψ and ψ′ photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at .The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 μb−1. The cross section for coherent and incoherent J/ψ production in the rapidity interval −0.9

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Abstract:We have studied the transverse-momentum (p T ) dependence of the inclusive J/ψ production in p-Pb collisions at √ s NN = 5.02 TeV, in three center-of-mass rapidity (y cms ) regions, down to zero p T . Results in the forward and backward rapidity ranges (2.03 < y cms < 3.53 and −4.46 < y cms < −2.96) are obtained by studying the J/ψ decay to µ + µ − , while the mid-rapidity region (−1.37 < y cms < 0.43) is investigated by measuring the e + e − decay channel. The p T dependence of the J/ψ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/ψ mean p T values. Forward and mid-rapidity results show a suppression of the J/ψ yield, with respect to pp collisions, which decreases with increasing p T . At backward rapidity no significant J/ψ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψ production in Pb-Pb collisions are also discussed. JHEP06(2015)055The suppression of charmonia, bound states of c andc quarks, and in particular of the J/ψ state, has long been proposed as a signature for the formation of a plasma of quarks and gluons (QGP) [1] in ultrarelativistic nucleus-nucleus collisions. However, it was soon realized that charmonium production can also be modified by nuclear effects not necessarily related to QGP formation [2]. These so-called cold nuclear matter (CNM) effects can be investigated by studying charmonium production in proton-nucleus (p-A) collisions as confirmed by the analysis of results obtained by several fixed-target (SPS [3, 4], HERA [5] and Tevatron [6]) and collider (RHIC [7] and LHC [8, 9]) experiments.Theoretical models have studied the production of charmonium in p-A collisions and the effects of the surrounding cold nuclear medium by introducing various mechanisms which include nuclear shadowing, gluon saturation, energy loss and nuclear absorption. Models [10][11][12] inspired by Quantum ChromoDynamics (QCD) describe charmonium production as a two-step process, with the cc pair created in a hard parton scattering, followed by its evolution into a bound state with specific quantum numbers. The pair creation is sensitive to the Parton Distribution Functions (PDFs) in both colliding partners and, at high energy, occurs mainly via gluon fusion. Although PDFs are known to be modified in a nuclear environment, information on the dependence of such modifications on the fraction x (Bjorken-x) of the nucleon momentum carried by the gluons and on the four-momentum squared Q 2 transferred in the scattering is still limited [13][14][15]. Charmonium production measurements can therefore provide insight into the so-called nuclear shadowing, i.e., on how the nucleon gluon PDFs are modified in a nucleus.Modifications of the initial state of the ...

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Intensive radiation of magnetic bremsstrahlung type (synchrotron radiation) resulting from the interaction of escaping quarks with the collective confining colour field is discussed as a new possible mechanism of observed direct photon anisotropyThe mighty wealth of experimental data on relativistic heavy ion collisions collected in the different experiments in recent years (even before putting LHC in operation) is reasonably well described (but less well understood) in the framework of approach based on the relativistic hydrodynamic equations [1,2]. In particular, a (nearly) perfect hydrodynamics has successfully predicted an existence of radial and elliptic flows, their dependence on centrality, mass, beam energy and transverse momentum. Crucial moment of this approach is that the respective liquid possesses rather special transport properties. Indeed, the ratio of its shear viscosity coefficient η to the entropy density s, i.e. η/s, develops very small magnitude. Obviously, any microscopic interpretation of new experimental data at this energy scale should take into account this novel theoretical background but also to answer the most exciting question what is that fluid entity.Measuring the photon radiation in ultrarelativistic collisions of heavy nuclei has been suggested as one of the most indicative signals of producing new state of matter many years ago [3,4]. In this context the recent measurements by the PHENIX Collaboration which show the azimuthal anisotropy of produced direct photons very close to the hadron one [5] are rather exciting. This result appears to be in a serious contradiction with expected dominance of photon production from quark gluon plasma at an early stage of ion collision at the top RHIC (Brookhaven) and now available LHC (CERN) energies. The observed temperature of "anomalous" photon radiation (about T ave ≃ 220 Mev) is in accordance with the PHENIX Collaboration measurements [6] at the energy √ s = 200 GeV of heavy ion collisions. This temperature magnitude being considered as a result of averaging over the entire evolution of the matter created in nuclear collisions is noticeably higher than the phase transition temperature (this statement is wandering over the all phenomenological papers albeit we understand the lattice QCD declares 1) e-mail: snigirev@lav01.sinp.msu.ru the presence of a cross-over only [7]) and obviously supports the scenario of photon radiation from quark gluon plasma. Forming a gluon condensate which radiates the photons at the early stage of collisions is considered [8] as another alternative explanation of high photon source temperature measured.However, in both these scenarios the photon azimuthal anisotropy is declared to be small [9] and insufficient to explain the experimental data mentioned. For the time being this new result of the PHENIX Collaboration promoted great interest in both experimental and theoretical studies and several phenomenological suggestions [10,11,12,13,14] are under discussion to understand an origin of this exciting observation...

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