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Register a new userElectromagnetic probes of a pure-glue initial state in nucleus-nucleus collisions at energies available at the CERN Large Hadron Collider
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Partonic matter produced in the early stage of ultrarelativistic nucleus-nucleus collisions is assumed to be composed mainly of gluons, and quarks and antiquarks are produced at later times. To study the implications of such a scenario, the dynamical evolution of a chemically nonequilibrated system is described by the ideal (2+1)-dimensional hydrodynamics with a time dependent (anti)quark fugacity. The equation of state interpolates linearly between the lattice data for the pure gluonic matter and the lattice data for the chemically equilibrated quark-gluon plasma. The spectra and elliptic flows of thermal dileptons and photons are calculated for central Pb+Pb collisions at the CERN Large Hadron Collider energy of $sqrt{s_{_{rm NN}}} = 2.76$ TeV. We test the sensitivity of the results to the choice of equilibration times, including also the case where the complete chemical equilibrium of partons is reached already at the initial stage. It is shown that a suppression of quarks at early times leads to a significant reduction of the yield of the thermal dileptons, but only to a rather modest suppression of the $p_T$-distribution of direct photons. It is demonstrated that an enhancement of photon and dilepton elliptic flows might serve as a promising signature of the pure-glue initial state.
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Collinear factorized perturbative QCD model predictions are compared for p+Pb at 4.4A TeV to test nuclear shadowing of parton distribution at the Large Hadron Collider (LHC). The nuclear modification factor (NMF), R_{pPb}(y=0,p_T<20 GeV/c) = dn_{p Pb} /(N_{coll}(b)dn_{pp}), is computed with electron-nucleus (e+A) global fit with different nuclear shadow distributions and compared to fixed Q^2 shadow ansatz used in Monte Carlo Heavy Ion Jet Interacting Generator (HIJING) type models. Due to rapid DGLAP reduction of shadowing with increasing Q^2 used in e+A global fit, our results confirm that no significant initial state suppression is expected (R_{pPb} (p_T) = 1 pm 0.1) in the p_T range 5 to 20 GeV/ c. In contrast, the fixed Q^2 shadowing models assumed in HIJING type models predict in the above p_T range a sizable suppression, R_{pPb} (p_T) = 0.6-0.7 at mid-pseudorapidity that is similar to the color glass condensate (CGC) model predictions. For central (N_{coll} = 12) p+ Pb collisions and at forward pseudorapidity (eta = 6) the HIJING type models predict smaller values of nuclear modification factors (R_{pPb}(p_T)) than in minimum bias events at mid-pseudorapidity (eta = 0). Observation of R_{pPb}(p_T= 5-20 GeV/c) less than 0.6 for minimum bias p+A collisions would pose a serious difficulty for separating initial from final state interactions in Pb+Pb collisions at LHC energies.
We study the diffusion of charm and beauty in the early stage of high energy nuclear collisions at RHIC and LHC energies, considering the interaction of these heavy quarks with the evolving Glasma by means of the Wong equations. In comparison with previous works, we add the longitudinal expansion as well as we estimate the effect of energy loss due to gluon radiation. We find that heavy quarks diffuse in the strong transverse color fields in the very early stage (0.2-0.3 fm/c) and this leads to a suppression at low $p_T$ and enhancement at intermediate low $p_T$. The shape of the observed nuclear suppression factor obtained within our calculations is in qualitative agreement with the experimental results of the same quantity for $D-$mesons in proton-nucleus collisions. We compute the nuclear suppression factor in nucleus-nucleus collisions as well, for both charm and beauty, finding a substantial impact of the evolving Glasma phase on these, suggesting that initialization of heavy quarks spectra in the quark-gluon plasma phase should not neglect the early evolution in the strong gluon fields.
We study the effect of strong longitudinal color fields (SCF) in p+p reactions up to Large Hadron Collider energies in the framework of the HIJING/BBbar v2.0 model that combines (collinear factorized) pQCD multiple minijet production with soft longitudinal string excitation and hadronization. The default vacuum string tension, kappa0 = 1 GeV/fm, is replaced by an effective power law energy dependent string tension, that increases monotonically with center-of-mass energy. The exponent 0.06 is found sufficient to reproduce well the energy dependence of multiparticle observables in RHIC, Tevatron, as well as recent LHC data. This exponent is found to be only half of that predicted by the Color Glass Saturation model, lambda(CGC)=0.115, where gluon fusion multiparticle production mechanisms are assumed. In HIJING/BBbar v2.0, the rapid growth of central-rapidity density with energy is due to the interplay of copious minijet production and increasing SCF contributions. The large (strange)baryon-to-meson ratios measured at Tevatron energies are well described. A significant enhancement of these ratios is predicted up to the highest LHC energy (14 TeV). The effect of JJbar loops and SCF on baryon-anti-baryon asymmetry and its relation to baryon number transport is also discussed.
Photoproduction of heavy quarks in ultraperipheral collisions can help elucidate important features of the physics of heavy quarks in Quantum Chromodynamics (QCD). Due to the dependence on parton distributions it can also potentially offer some constraining ability in the determination of nuclear parton distributions. In the present study we consider next-to-leading order (NLO) photoproduction of heavy quarks in ultraperipheral proton-proton (pp), proton-nucleus (pA), and nucleus-nucleus (AA) collisions at the CERN Large Hadron Collider (LHC). Total cross sections and rapidity distributions are considered and the influence of nuclear modifications of parton distributions on these quantities are explored for pA and AA collisions. We find that photoproduction of heavy quarks in PbPb collisions exhibit significant sensitivity to nuclear effects, and in conjunction with photoproduction in pPb collisions, affords good constraining potential for gluon shadowing determination.
We discuss the prospects of using jets as precision probes in electron-nucleus collisions at the future Electron-Ion Collider. Jets produced in deep-inelastic scattering can be calibrated by a measurement of the scattered electron. Such electron-jet tag and probe measurements call for an approach that is orthogonal to most HERA jet measurements as well as previous studies of jets at the future EIC. We present observables such as the electron-jet momentum balance, azimuthal correlations and jet substructure, which can provide constraints on the parton transport coefficient in nuclei. We compare simulations and analytical calculations and provide estimates of the expected medium effects. Implications for detector design at the future EIC are discussed.
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