arXiv subjects

Pradip Roy

Publications and source records attributed to Pradip Roy.

At least 55 records · Page 3Linked to original sources

arXiv2014

Systematic study of Charmonium production in pp collisions at the LHC energies

We have performed a systematic study of $J/ψ$ and $ψ(2S)$ production in $p-p$ collisions at different LHC energies and at different rapidities using the leading order (LO) non-relativistic QCD (NRQCD) model of heavy quarkonium production. We have included the contributions from $χ_{cJ}$ ($J$ = 0, 1, 2) and $ψ(2S)$ decays to $J/ψ$. The calculated values have been compared with the available data from the four experiments at LHC namely, ALICE, ATLAS, CMS and LHCb. In case of ALICE, inclusive $J/ψ$ and $ψ(2S)$ cross-sections have been calculated by including the feed-down from $B$ meson using Fixed-Order Next-to-Leading Logarithm (FONLL) formalism. It is found that all the experimental cross-sections are well reproduced for $p_T >$ 4 GeV within the theoretical uncertainties arising due to the choice of the factorization scale. We also predict the transverse momentum distributions of $J/ψ$ and $ψ(2S)$ both for the direct and feed-down processes at the upcoming LHC energies of $\sqrt{s} =$ 5.1 TeV and 13 TeV for the year 2015.

hep-ph↗

arXiv2013

Wake in anisotropic Quark-Gluon Plasma

We calculate the wake in charge density and the wake potential due to the passage of a fast parton in an anisotropic quark gluon plasma(AQGP). For the sake of simplicity small $ξ$(anisotropic parameter) limit has been considered. When the velocity($v$) of the jet is parallel to the anisotropy direction(${\hat n}$) and remains below the phase velocity($v_p$), the wake in induced charge density shows a little oscillatory behavior in AQGP, contrary to the isotropic case. With the jet velocity greater than the phase velocity, the oscillatory behavior increases with $ξ$. Also for $v>v_p$ one observes a clear modification of the cone-like structure in presence of anisotropy. For the parallel direction in the backward region, the depth of the wake potential decreases with the increase of $ξ$ for $v v_p$ and $v<v_p$. In the perpendicular direction, the wake potential is symmetric in the forward and backward regions. With the increase of $ξ$, the depth of negative minimum is moving away from the origin irrespective of the jet velocity. On the other hand when the jet velocity is perpendicular to the anisotropy direction, we find significant changes in the case of both wake charge density and potential in comparison to the isotropic case.

hep-ph↗

arXiv2013

Wake potential in collisional anisotropic quark-gluon plasma

Within the framework of Boltzmann transport equation with a Bhatnagar-Gross-Krook (BGK) collisional kernel, we study the wake potential induced by fast partons traveling through the high-temperature QCD plasma which is anisotropic in momentum-space. We calculate the dielectric response function of a collisional anisotropic quark-gluon plasma (AQGP) for small $ξ$ (anisotropic parameter) limit. Using this, the wake potential for various combinations of the anisotropy parameter ($ξ$) and the collision rate ($ν$) is evaluated both for parallel and perpendicular directions of motion of the fast parton. It is seen that the inclusion of the collision modifies the wake potential and the amount as well as the nature of the potential depends on the combinations of $ξ$ and $ν$

hep-ph↗

arXiv2012

Gluon dissociation of $J/ψ$ in anisotropic {\em Quark-Gluon-Plasma}

We calculate the gluon dissociation cross-section in an anisotropic quark gluon plasma expected to be formed due relativistic nucleus-nucleus collisions. The initial rapid longitudinal expansion of the system leads to momentum space anisotropy due which the observables are affected in such a system. We show that the thermally weighted cross-section of gluon dissociation undergoes modification in anisotropic plasma leading to a decreased probability of $J/ψ$ suppression both for RHIC and LHC energies. The dependence of the cross section as well as the survival probability on the direction of propagation of the charmonium with respect to the anisotropy axis are presented. The sensitivity of the survival probability on the initial conditions have also been demonstrated.

hep-ph↗

arXiv2012

Effect of running coupling on photon emission from quark gluon plasma

We discuss the role of running coupling on the thermal photon yield from quark gluon plasma. It is shown that the photon production rate from the partonic phase is considerably enhanced when running coupling is considered with respect to a fixed value. However, we show by explicit evaluation that although this difference survives the space-time evolution the experimental data cannot distinguish between the two once the hard contribution, which is an essential component of photon production mechanism, is added.

hep-ph↗

arXiv2012

Dijet induced collective modes in an anisotropic quark-gluon plasma

We discuss the collective modes due to the propagation of two oppositely moving relativistic jets (dijet) in an anisotropic quark-gluon plasma(AQGP) and compare the results with the case of single jet propagation. For the sake of simplicity, assuming a tsunami-like initial jet distribution, we observe that the dispersion relations for both the stable and unstable modes are altered significantly due to the passage of dijet in comparison with the case of single jet propagation. It has been further demonstrated that the growth rate of instability, due to introduction of dijet in the system, increases compared to the case of single jet case. As in the case of single jet propagation, the instability always grows when the jet velocity is perpendicular to the wave vector. We, thus, argue that the introduction of dijet in the AQGP, in general, leads to faster isotropization than single jet propagation.

hep-ph↗

arXiv2012

Collective modes of an anisotropic quark-gluon plasma induced by relativistic jets

We discuss the characteristics of collective modes induced by relativistic jets in an anisotropic quark-gluon plasma(AQGP). Assuming a tsunami-like initial jet distribution, it is found that the dispersion relations for both the stable and unstable modes are modified substantially due to the passage of jet compared to the case when there is no jet. It has also been shown that the growth rate of instability first increases compared to the no jet case and then completely turned into damping except the case when the jet velocity is perpendicular to the wave vector in which case the instability always grows. Thus, the introduction of the jet in the AQGP, in general, might to faster isotropization for the special case when the wave vector is parallel to the anisotropy axis.

hep-ph↗

arXiv2011

Nuclear modification factor in an anisotropic {\em Quark-Gluon-Plasma}

We calculate the nuclear modification factor ($R_{AA}$) of light hadrons by taking into account the initial state momentum anisotropy of the quark gluon plasma (QGP) expected to be formed in relativistic heavy ion collisions. Such an anisotropy can result from the initial rapid longitudinal expansion of the matter. A phenomenological model for the space time evolution of the anisotropic QGP is used to obtain the time dependence of the anisotropy parameter $ξ$ and the hard momentum scale, $p_{\rm hard}$. The result is then compared with the PHENIX experimental data to constrain the isotropization time scale, $τ_{\rm iso}$ for fixed initial conditions (FIC). It is shown that the extracted value of $τ_{\rm iso}$ lies in the range $0.5 \leq τ_{\rm iso} \leq 1.5$. However, using fixed final multiplicity (FFM) condition does not lead to any firm conclusion about the extraction of the isotropization time. The present calculation is also extended to contrast with the recent measurement of nuclear modification factor by ALICE collaboration at $\sqrt{s}=2.76$ TeV. It is argued that in the present approach, the extraction of $τ_{\rm iso}$ at this energy is uncertain and, therfore, refinement of the model is necessary. The sensitivity of the results on the initial conditions has been discussed. We also present the nuclear modification factor at LHC energies with $\sqrt{s} = 5.5$ TeV.

hep-ph↗

arXiv2011

Energy loss due to field fluctuations in a two-stream QCD plasma

We derive the expression for the collisional energy loss in two stream plasma induced by the fluctuating chromoelectric field. It is revealed that the main contribution here comes from the unstable modes which grow exponentially with time. A strong direction dependence of the energy loss has also been demonstrated.

hep-ph↗

arXiv2011

Radiative energy loss in an anisotropic Quark-Gluon-Plasma

We calculate radiative energy loss of heavy and light quarks in anisotropic medium (static) in first order opacity expansion. Such an anisotropy can result from the initial rapid longitudinal expansion of the matter, created in relativistic heavy ion collisions. Significant dependency of the energy loss on the anisotropy parameter ($ξ$) and the direction of propagation of the partons with respect to the anisotropy axis is found. It is shown that the introduction of early time momentum-space anisotropy can enhance the fractional energy loss in the direction of the anisotropy, whereas it decreases when the parton propagates perpendicular to the direction of the anisotropy.

nucl-th↗

arXiv2011

Effect of running coupling on photons from jet - plasma interaction in relativistic heavy ion collisions

We discuss the role of collisional energy loss on high $p_T$ photon data measured by PHENIX collaboration by calculating photon yield in jet-plasma interaction. The phase space distribution of the participating jet is dynamically evolved by solving Fokker-Planck equation. We treat the strong coupling constant ($α_s$) as function of momentum and temperature while calculating the drag and diffusion coefficients. It is observed that the quenching factor is substantially modified as compared to the case when $α_s$ is taken as constant. It is shown that the data is reasonably well reproduced when contributions from all the relevant sources are taken into account. Predictions at higher beam energies relevant for LHC experiment have been made.

hep-ph↗

arXiv2010

Photons from jet - plasma interaction in relativistic heavy ion collisions

We expose the role of collisional energy loss on high $p_T$ photon data measured by PHENIX collaboration by calculating photon yield in jet plasma interaction. The phase space distribution of the participating jet is dynamically evolved by solving Fokker-Planck equation. It is shown that the data is reasonably well reproduced when contributions from all the relevant sources are taken into account. Predictions at higher beam energies relevant for LHC experiment have been made.

hep-ph↗

arXiv2010

Jet-photons from an anisotropic {\em Quark-Gluon-Plasma}

We calculate the $p_T$ distributions of jet conversion photons from {\em Quark Gluon Plasma} with pre-equilibrium momentum-space anisotropy. A phenomenological model has been used for the time evolution of hard momentum scale $p_{\rm hard}(τ)$ and anisotropy parameter $ξ(τ)$. As a result of pre-equilibrium momentum-space anisotropy, we find significant modification of the jet conversion photon $p_T$ distribution. For example, with {\em fixed initial condition} (FIC) pre-equilibrium anisotropy, we predict significant enhancement of the jet-photon $p_T$ distribution in the entire region, whereas for pre-equilibrium anisotropy with {\em fixed final multiplicity} (FFM), suppression of the jet conversion photons $p_T$ distribution is observed. The results with FFM (as it is the most realistic situation) have been compared with high $p_T$ PHENIX photon data. It is found that the data is reproduced well if the isotropization time lies within $1.5$ fm/c.

hep-ph↗

arXiv2010

Rapidity distribution of photons from an anisotropic Quark-Gluon-Plasma

We calculate rapidity distribution of photons due to Compton and annihilation processes from {\em Quark Gluon Plasma} (QGP) with pre-equilibrium momentum-space anisotropy. We also include contributions from hadronic matter with late stage transverse expansion. A phenomenological model has been used for the time evolution of hard momentum scale $p_{\rm hard}(τ)$ and anisotropy parameter $ξ(τ)$. As a result of pre-equilibrium momentum-space anisotropy, we find significant modification of photons rapidity distribution. For example, with {\em fixed initial condition} (FIC) {\em free-streaming} ($δ=2$) interpolating model we observe significant enhancement of photon rapidity distribution at fixed $p_T$, where as for FIC {\em collisionally-broadened} ($δ=2/3$) interpolating model the yield increases till $y \sim 1$. Beyond that suppression is observed. With {\em fixed final multiplicity} (FFM) {\em free-streaming} interpolating model we predict enhancement of photon yield which is less than the case of FIC. Suppression is always observed for FFM {\em collisionally-broadened} interpolating model.

hep-ph↗

arXiv2010

$π$-$η$ mixing and charge symmetry violating NN potential in matter

We construct density dependent Class $III$ charge symmetry violating (CSV) potential due to mixing of $π$-$η$ meson with off-shell corrections. The density dependence enters through the non-vanishing $π$-$η$ mixing driven by both the neutron-proton mass difference and their asymmetric density distribution. The contribution of density dependent CSV potential is found to be appreciably larger than the contribution of vacuum CSV potential.

nucl-th↗

arXiv2010

Photons from anisotropic Quark-Gluon-Plasma

We calculate medium photons due to Compton and annihilation processes in an anisotropic media. The effects of time-dependent momentum-space anisotropy of {\em Quark-Gluon-Plasma} (QGP) on the medium photon production are discussed. Such an anisotropy can results from the initial rapid longitudinal expansion of the matter, created in relativistic heavy ion collisions. A phenomenological model for the time-dependence of the parton hard momentum scale, $p_{hard}(τ)$, and anisotropy parameter, $ξ(τ)$, has been used to describe the plasma space-time evolution. We find significant dependency of photon yield on the isotropization time ($τ_{iso}$). It is shown that the introduction of early time momentum-space anisotropy can enhance the photon production by a factor of $10 (1.5)$ (in the central rapidity region) for {\em free streaming} ({\em collisionally-broadened}) {\em interpolating} model if we assume fixed initial condition. On the other hand, enforcing the fixed final multiplicity significantly reduces the enhancement of medium photon production.

hep-ph↗

arXiv2009

Matter induced charge symmetry violating NN potential

We construct density dependent Class $III$ charge symmetry violating (CSV) potential due to mixing of $ρ$-$ω$ meson with off-shell corrections. Here in addition to the usual vacuum contribution, the matter induced mixing of $ρ$-$ω$ is also included. It is observed that the contribution of density dependent CSV potential is comparable to that of the vacuum contribution.

nucl-th↗

arXiv2009

Measuring the isotropization time of quark-gluon plasma from direct photons at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

We calculate transverse momentum distribution of direct photons from various sources by taking into account the initial state momentum anisotropy of {\em Quark-Gluon-Plasma} (QGP) and late stage transverse flow effects. The total photon yield, calculated for various combinations of initial conditions, transition temperatures, is then compared with the recent measurement of photon transverse momentum distribution by the PHENIX collaboration. It is shown that the presence of such an anisotropy can describe the PHENIX photon data better than the isotropic case. We show that the isotropization time thus extracted remains of the order of 1 fm/c and is almost insensitive to the initial conditions used.

hep-ph↗