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Gouranga C. Nayak

Publications and source records attributed to Gouranga C. Nayak.

54 records · Page 3Linked to original sources
arXiv2003

Screening and antiscreening in anisotropic QED and QCD plasmas

We use a transport-theory approach to construct the static propagator of a gauge boson in a plasma with a general axially- and reflection-symmetric momentum distribution. Non-zero magnetic screening is found if the distribution is anisotropic, confirming the results of a closed-time-path-integral approach. We find that the electric and magnetic screening effects depend on both the orientation of the momentum carried by the boson and the orientation of its polarization. In some orientations there can be antiscreening, reflecting the instabilities of such a medium. We present some fairly general conditions on the dependence of these effects on the anisotropy.

hep-ph↗
arXiv2003

Color Octet Contribution to High p_T J/ΨProduction in pp Collisions at \sqrt s = 500 and 200 GeV at RHIC

We compute \frac{dσ}{dp_T} of the J/ψproduction in pp collisions at RHIC at \sqrt s = 500 and 200 GeV by using both the color octet and singlet models in the framework of non-relativistic QCD. The J/ψwe compute here includes the direct J/ψfrom the partonic fusion processes and the J/ψcoming from the radiative decays of χ_J's both in the color octet and singlet channel. The high p_T J/ψproduction cross section is computed within the PHENIX detector acceptance ranges: -0.35 < η< 0.35 and 1.2 < \|η| < 2.4, the central electron and forward muon arms. It is found that the color octet contribution to J/ψproduction is dominant at RHIC energy in comparison to the color singlet contributions. We compare our results with the recent preliminary data obtained by PHENIX detector for the high p_T J/ψmeasurements. While the color singlet model fails to explain the data completely the color octet model is in agreement with the single data point above 2 GeV transverse momentum. A measurement of J/ψproduction at RHIC in the next run with better statistics will allow us to determine the validity of the color octet model of J/ψproduction at RHIC energies. This is very important because it is necessary to know the exact mechanism for J/ψproduction in pp collisions at RHIC if one is to make predictions of J/ψsuppression as a signature of quark-gluon plasma. These mechanisms also play an important role in determining the polarized spin structure function of the proton at RHIC.

hep-ph↗
arXiv2003

Interaction of a TeV Scale Black Hole with the Quark-Gluon Plasma at LHC

If the fundamental Planck scale is near a TeV, then parton collisions with high enough center-of-mass energy should produce black holes. The production rate for such black holes has been extensively studied for the case of a proton-proton collision at \sqrt s = 14 TeV and for a lead-lead collision at \sqrt s = 5.5 TeV at LHC. As the parton energy density is much higher at lead-lead collisions than in pp collisions at LHC, one natural question is whether the produced black holes will be able to absorb the partons formed in the lead-lead collisions and eventually `eat' the quark-gluon plasma formed at LHC. In this paper, we make a quantitative analysis of this possibility and find that since the energy density of partons formed in lead-lead collisions at LHC is about 500 GeV/fm^3, the rate of absorption for one of these black holes is much smaller than the rate of evaporation. Hence, we argue that black holes formed in such collisions will decay very quickly, and will not absorb very many nearby partons. More precisely, we show that for the black hole mass to increase via parton absorption at the LHC the typical energy density of quarks and gluons should be of the order of 10^{10} GeV/fm^3. As LHC will not be able to produce such a high energy density partonic system, the black hole will not be able to absorb a sufficient number of nearby partons before it decays. The typical life time of the black hole formed at LHC is found to be a small fraction of a fm/c.

hep-ph↗
arXiv2003

Minijet Initial Conditions For Non-Equilibrium Parton Evolution at RHIC and LHC

An important ingredient for the non-equilibrium evolution of partons at RHIC and LHC is to have some physically reasonable initial conditions for the single particle phase space distribution functions for the partons. We consider several plausible parametrizations of initial conditions for the single particle distribution function f_i(x,p) and fix the parameters by matching \int f(x,p)p^μd σ_μto the invariant momentum space semi-hard parton distributions obtained using perturbative QCD (pQCD), as well as fitting low order moments of the distribution function. We consider parametrizations of f_i(x,p) with both boost invariant and boost non-invariant assumptions. We determine the initial number density, energy density and the corresponding (effective) temperature of the minijet plasma at RHIC and LHC energies. For a boost non-invariant minijet phase-space distribution function we obtain ~ 30(140) /fm^3 as the initial number density, ~ 50(520) GeV/fm^3 as the initial energy density and ~ 520(930) MeV as the corresponding initial effective temperature at RHIC(LHC).

hep-ph↗
arXiv2002

Graviton and Radion Production at LHC: From pp and PbPb Collisions

If the Planck scale is around a TeV in theories with large extra dimension, then gravitons should be produced in high energy hadronic collisions at the LHC. In this paper we compute the direct graviton production cross section in a pp and PbPb collision at LHC at \sqrt s^{NN}= 14 TeV and 5.5 TeV respectively. The graviton production cross section in a lead-lead collision is enhanced in comparison to a pp collision at LHC depending upon the minimum transverse energy of the jet E_{T,jet}^{min} above which the graviton production cross section is computed. For two extra dimensions and above E_{T,jet}^{min} = 100, 500, 1000, 1500 GeV the ratio of graviton production cross sections in a PbPb collision to that in a pp collision is found to be 2220, 400, 43, 2. For four extra dimensions this ratio is found to be 470, 100, 10, 0.5. In the Randall-Sundrum model the radion production cross section is also found to be enhanced in a lead-lead collision over that in a pp collision at LHC. The ratio of the radion production cross sections in a PbPb collision to that in a pp collision at LHC is found to be 1650, 460, 180, 67, 23, 1 for radion of masses 100, 500, 1000, 1500, 2000, 3000 GeV respectively.

hep-ph↗
arXiv2002

Black Hole Production at LHC: String Balls and Black Holes from pp and Lead-lead Collisions

If the fundamental planck scale is near a TeV, then parton collisions with high enough center-of-mass energy should produce black holes. The production rate for such black holes at LHC has been extensively studied for the case of a proton-proton collision. In this paper, we extend this analysis to a lead-lead collision at LHC. We find that the cross section for small black holes which may in principle be produced in such a collision is either enhanced or suppressed, depending upon the black hole mass. For example, for black holes with a mass around 3 TeV we find that the differential black hole production cross section, dσ/dM, in a typical lead-lead collision is up to 90 times larger than that for black holes produced in a typical proton-proton collision. We also discuss the cross-sections for `string ball' production in these collisions. For string balls of mass about 1 (2) TeV, we find that the differential production cross section in a typical lead-lead collision may be enhanced by a factor up to 3300 (850) times that of a proton-proton collision at LHC.

hep-ph↗
arXiv2002

Infrared Behaviour of The Gluon Propagator in Non-Equilibrium Situations

The infrared behaviour of the medium modified gluon propagator in non-equilibrium situations is studied in the covariant gauge using the Schwinger-Keldysh closed-time path formalism. It is shown that the magnetic screening mass is non-zero at the one loop level whenever the initial gluon distribution function is non isotropic with the assumption that the distribution function of the gluon is not divergent at zero transverse momentum. For isotropic gluon distribution functions, such as those describing local equilibrium, the magnetic mass at one loop level is zero which is consistent with finite temperature field theory results. Assuming that a reasonable initial gluon distribution function can be obtained from a perturbative QCD calculation of minijets, we determine these out of equilibrium values for the initial magnetic and Debye screening masses at energy densities appropriate to RHIC and LHC. We also compare the magnetic masses obtained here with those obtained using finite temperature lattice QCD methods at similar temperatures at RHIC and LHC.

hep-ph↗
arXiv2002

Non-Zero Magnetic Screening Mass in QED and QCD at One Loop Level in Non-Equilibrium

Using the Schwinger-Keldysh closed time path integral formalism we show that the magnetic screening mass in QED and QCD at one loop level is non-zero as long as the single particle distribution function f(\vec{k}) is non-isotropic, {i.e.} it depends on the direction of the momentum. For isotropic distribution functions such as those corresponding to thermal equilibrium the magnetic screening mass at one loop level is found to be zero which is consistent with finite temperature field theory. The non-zero magnetic screening mass in non-isotropic non-equlibrium situations has fundamental importance in that it acts as a natural cut-off to remove infrared divergences in the magnetic sector. Thus it allows one to avoid infrared problems which previously made it difficult to use a transport theory approach using perturbative QCD or QED scattering kernels to study the thermalization of a QED or QCD plasma.

hep-ph↗
arXiv2002

Closed-Time Path Integral Formalism and Medium Effects of Non-Equilibrium QCD Matter

We apply the closed-time path integral formalism to study the medium effects of non-equilibrium gluon matter. We derive the medium modified resummed gluon propagator to the one loop level in non-equilibrium in the covariant gauge. The gluon propagator we derive can be used to remove the infrared divergences in the secondary parton collisions to study thermalization of minijet parton plasma at RHIC and LHC.

hep-ph↗
arXiv2002

Parton Production Via Vacuum Polarization

We discuss the production mechanism of partons via vacuum polarization during the very early, gluon dominated phase of an ultrarelativistic heavy-ion collision in the framework of the background field method of quantum chromodynamics.

hep-ph↗
arXiv2001

Partonic Scattering Cross Sections in the QCD Medium

A medium modified gluon propagator is used to evaluate the scattering cross section for the process gg-->gg in the QCD medium by performing an explicit sum over the polarizations of the gluons. We incorporate a magnetic sreening mass from a non-perturbative study. It is shown that the medium modified cross section is finite, divergence free, and is independent of any ad-hoc momentum transfer cut-off parameters. The medium modified finite cross sections are necessary for a realistic investigation of the production and equilibration of the minijet plasma expected at RHIC and LHC.

hep-ph↗
arXiv2001

Leading-Order Gluon-Pair Production from a Space-Time Dependent Chromofield

We describe gluon-pair production from a space-time dependent chromofield via vacuum polarization within the framework of the background field method of QCD. The processes we consider are first order in the action. We derive the corresponding source terms for gluon pair production for situations that can be described by a 1+1 dimensional approach. Especially, we observe that within the range of applicability of our approach the principal contribution to gluon production is included. Gluon production from a space-time dependent chromofield will play an important role in the production and evolution of the quark-gluon plasma in ultra relativistic heavy-ion collisions at RHIC and LHC.

hep-ph↗
arXiv2000

Color Current Induced by Gluon in Background Field Method of QCD

By using the background field method of QCD in a path integral approach, we derive the equation of motion for the classical chromofield and for the gluon in a system containing the gluon and the classical chromofield simultaneously. This inhomogeneous field equation contains a current term, which is the expectation value of a composite operator including linear, square and cubic terms of the gluon field. We also derive identities which the current should obey from the gauge invariance. We calculate the current at the leading order where the current induced by the gluon is opposite in sign to that induced by the quark. This is just the feature of the non-Abelian gauge field theory which has asymptotic freedom. Physically, the induced current can be treated as the 'displacement' current in the polarized vacuum, and its effect is equivalent to redefining the field and the coupling constant.

hep-th↗
arXiv2000

Bottom and Charm Production at LHC and RHIC

We study $b\bar{b}$ and $c\bar{c}$ production and the influence of nuclear shadowing at LHC and RHIC energies. We find a significant reduction in the production cross section of both charm and bottom at RHIC and LHC. Bound states such as $Υ$ and $J/Ψ$ are suppressed by this reduction in the charm production cross sections. Therefore, $J/Ψ$ suppression may not be useful as a signature for the quark gluon plasma.

hep-ph↗
arXiv2000

Equilibration of the Gluon-Minijet Plasma at RHIC and LHC

We study the production and equilibration of the gluon-minijet plasma expected to be formed in the central region of ultrarelativistic heavy-ion collisions at the BNL-RHIC and the CERN-LHC by solving a self-consistent relativistic transport equation. We compute the minijet production within perturbative QCD. Subsequent collisions among the semi-hard partons are treated by considering the elastic gg -> gg processes with screening of the long wavelength modes taken into account. We determine the time $τ_{eq}$ where close to ideal hydrodynamic flow sets in, and find rather similar numbers for central heavy-ion collisions at BNL-RHIC and CERN-LHC energies. The number densities, energy densities and temperatures of the minijet plasma are found to be different at RHIC and LHC, e.g. $T(τ_{eq})\sim220$ MeV and 380 MeV, respectively.

hep-ph↗
arXiv2000

Gluon Pair Production From Space-Time Dependent Chromofield

We compute the probabilty for the processes A -> q \bar{q}, gg via vacuum polarization in the presence of a classical space-time dependent non-abelian field A by applying the background field method of QCD. The processes we consider are leading order in gA and are simillar to A -> e^+e^- in QED. Gluons are treated like a matter field and gauge transformations of the quantum gluonic field are different from those of the classical chromofield. To obtain the correct physical polarization of gluons we deduct the corresponding ghost contributions. We find that the expression for the probability of the leading order process A -> gg is transverse with respect to the momentum of the field. We observe that the contributions from higher order processes to gluon pair production need to be added to the this leading order process. Our result presented here is a part of the expression for the total probability for gluon pair production from a space-time dependent chromfield. The result for q \bar{q} pair production is similar to that of the e^+e^- pair production in QED. Quark and gluon production from a space-time dependent chromofield will play an important role in the study of the production and equilibration of the quark-gluon plasma in ultra relativistic heavy-ion collisions.

hep-th↗
arXiv2000

Production and Equilibration of the Quark-Gluon Plasma with Chromoelectric Field and Minijets

Production and equilibration of quark-gluon plasma are studied within the color flux-tube model, at the RHIC and LHC energies. Non-Abelian relativistic transport equations for quarks, antiquarks and gluons, are solved in the extended phase space which includes coordinates, momenta and color. Before the chromoelectric field is formed, hard and semihard partons are produced via minijets which provide the initial conditions necessary to solve the transport equations. The model predicts that in spite of the vast difference between the RHIC and LHC incident energies, once the local equilibrium is reached, the energy densities, the number densities and the temperatures at the two machines may not be very different from each other. The minijet input significantly alters the evolution of the deconfined matter, unless the color field is too strong. For the input parameters used here the equilibration time is estimated to be $\sim 1$ fm at RHIC and $\sim 0.5$ fm at LHC, measured from the instant when the two colliding nuclei have just passed through each other. The temperature at equilibration is found to be $\sim 250$ MeV at RHIC and $\sim 300$ MeV at LHC.

hep-ph↗