arXiv subjects

Rupa Chatterjee

Publications and source records attributed to Rupa Chatterjee.

At least 37 records · Page 2Linked to original sources

arXiv2018

Transport Dynamics of Parton Interactions in pp Collisions at LHC Energies

We investigate the transport dynamics of partons in proton-proton collisions at the Large Hadron Collider using a Boltzmann transport approach, the parton cascade model. The calculations include semi-hard pQCD interaction of partons populating the nucleons and provide a space-time description of the collision in terms of cascading partons undergoing scatterings and fragmentations. Parton production and number of collisions rise rapidly with increase in center of mass energy of the collision. For a given center of mass energy, the number of parton interactions is seen to rise stronger than linear with decreasing impact parameter before saturating for very central collisions. The strangeness enhance factor $γ_s$ for the semi-hard processes is found to rise rapidly and saturate towards the highest collision energies. Overall, our study indicates a significant amount of partonic interactions in proton-proton collisions, which supports the observation of fluid-like behavior for high multiplicity proton-proton collisions observed in the experiments.

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arXiv2017

Production of Charm Quarks in a Parton Cascade Model for Relativistic Heavy Ion Collisions at $\sqrt{s_{\textrm {NN}}}$= 200 GeV

We study the production and dynamics of heavy quarks in the parton cascade model for relativistic heavy ion collisions. The model is motivated by the QCD parton picture and describes the dynamics of an ultra-relativistic heavy-ion collision in terms of cascading partons which undergo scattering and multiplication while propagating. We focus on the dynamics of charm quark production and evolution in p+p and Au+Au collisions for several different interaction scenarios, i.e. collisions only between primary partons without radiation of gluons, multiple collisions without radiation of gluons and multiple collisions with radiation of gluons, allowing us to isolate the contributions of parton rescattering and radiation to charm production. We also discuss results of an eikonal approximation of the collision which provides a valuable comparison with mini-jet calculations and clearly brings out the importance of multiple collisions.

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arXiv2017

Spectra and elliptic flow of thermal photons from full overlap U+U collisions at RHIC

We calculate $p_T$ spectra and elliptic flow for tip-tip and body-body configurations of full overlap uranium-uranium (U+U) collisions using a hydrodynamic model with smooth initial density distribution and compare the results with those obtained from Au+Au collisions at RHIC. Production of thermal photons is seen to be significantly larger for tip-tip collisions compared to body-body collisions of uranium nuclei in the region $p_T >$ 1 GeV. The thermal photon spectrum from body-body collisions is found to be close to the spectrum from most central Au+Au collisions at RHIC. The elliptic flow parameter calculated for body-body collisions is found to be large and comparable to the $v_2(p_T)$ for mid-central collisions of Au nuclei. On the other hand, as expected, the $v_2(p_T)$ is close to zero for tip-tip collisions. The qualitative nature of the photon spectra and elliptic flow for the two different orientations of uranium nuclei is found to be independent of the initial parameters of the model calculation. We show that the photon results from fully overlapping U+U collisions are complementary to the results from Au+Au collisions at RHIC.

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arXiv2017

Anisotropic flow of thermal photons at RHIC and LHC

We calculate elliptic and triangular flow parameters of thermal photons using an event-by-event hydrodynamic model with fluctuating initial conditions at 200A GeV Au+Au collisions at RHIC and at 2.76A TeV Pb+Pb collisions at the LHC for three different centrality bins. The photon elliptic flow shows strong centrality dependence where $v_2(p_T)$ increases towards peripheral collisions both at RHIC and at the LHC energies. However, the triangular flow parameter does not show significant dependence to the collision centrality. The elliptic as well as the triangular flow parameters found to underestimate the PHENIX data at RHIC by a large margin for all three centrality bins. We calculate $p_T$ spectrum and anisotropic flow of thermal photons from 200A GeV Cu+Cu collisions at RHIC for 0--20\% centrality bin and compare with the results with those from Au+Au collisions. The production of thermal photon is found to decrease significantly for Cu+Cu collisions compared to Au+Au collisions. However, the effect of initial state fluctuation is found to be more pronounced for anisotropic flow resulting in larger $v_2$ and $v_3$ for Cu+Cu collisions. We study the correlation between the anisotropic flow parameters and the corresponding initial spatial anisotropies from their event by event distributions at RHIC and at the LHC energies. The linear correlation between $v_2$ and $ε_2$ is found be stronger compared to the correlation between $v_3$ and $ε_3$. In addition, the correlation coefficient is found to be larger at LHC than at RHIC.

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arXiv2016

Initial Temperature and Extent of Chemical Equilibration of Partons in Relativistic Collision of Heavy Nuclei

We emphasize that a knowledge of energy and entropy densities of quark gluon plasma - a thermalized de-confined matter, formed in relativistic heavy ion collisions fixes the formation temperature and the product of gluon fugacity and formation time uniquely, {\em provided} we know the relative fugacities of quarks and gluons. This also provides that a smaller formation time would imply larger fugacities for partons. Next we explore the limits of chemical equilibration of partons during the initial stages in relativistic collision of heavy nuclei. The experimentally measured rapidity densities of transverse energy and charged particle multiplicity at RHIC and LHC energies are used to estimate the energy and number densities with the assumption of formation of a thermally equilibrated quark gluon plasma which may be chemically equilibrated to the same or differing extents for quarks and gluons. The estimates are found to be very sensitive to the correction factor used for the Björken energy density for identifying it with the initial energy density. The extent of chemical equilibration near the end of the QGP phase is inferred by solving master equations by including the processes $gg \leftrightarrow ggg$ and $gg \leftrightarrow q\overline{q}$ along with expansion and cooling of the plasma. The possible consequences for invariant mass distribution of intermediate mass dileptons radiated from the plasma are discussed which could distinguish between different scenarios.

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arXiv2016

Specific Heat of Matter Formed in Relativistic Nuclear Collisions

We report the excitation energy dependence of specific heat (\cv) of hadronic matter at freeze-out in Au+Au and Cu+Cu collisions at the Relativistic Heavy Ion Collider energies by analyzing the published data on event-by-event mean transverse momentum (\meanpt) distributions. The \meanpt~distributions in finite \pt~ranges are converted to distributions of effective temperatures, and dynamical fluctuations in temperature are extracted by subtracting widths of the corresponding mixed event distributions. The heat capacity per particle at the kinetic freeze-out surface is presented as a function of collision energy, which shows a sharp rise in \cv~below \sNN~=~62.4~GeV. We employ the Hadron Resonance Gas (HRG) model to estimate \cv~at the chemical and kinetic freeze-out surfaces. The experimental results are compared to the HRG and other theoretical model calculations. HRG results show good agreement with data. Model predictions for \cv~at the Large Hadron Collider energy are presented.

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arXiv2015

Characterization of relativistic heavy-ion collisions at the Large Hadron Collider through temperature fluctuations

We propose to characterize heavy-ion collisions at ultra-relativistic energies by using fluctuations of energy density and temperature. Temperature fluctuations on an event-by-event basis have been studied both in terms of global temperature of the event, and locally by constructing fluctuation maps in small phase space bins in each event. Global temperature fluctuations provide an estimation of the specific heat of the system. Local temperature fluctuations of the event may be ascribed to the remnants of initial energy density fluctuations. Together these two observables give an insight into the system created in heavy-ion collisions and its evolution. Event-by-event hydrodynamic calculations indeed provide adequate theoretical basis for understanding the origin of the fluctuations. We demonstrate the feasibility of studying global and local temperature fluctuations at the Large Hadron Collider energy by the use of AMPT event generator.

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arXiv2014

Thermal photon $v_3$ at LHC from fluctuating initial conditions

We calculate the triangular flow parameter $v_3$ of thermal photons for 0--40\% central collisions of Pb nuclei at LHC using an event-by-event hydrodynamic model with fluctuating initial conditions. Thermal photon $v_3$ with respect to the the participant plane angle is found to be positive and significant compared to the elliptic flow parameter $v_2$ of thermal photons. In addition, photon $v_3$ as a function of $p_T$ shows similar qualitative nature to photon $v_2$ in the region $1< p_T <6$ GeV/$c$. We argue that while $v_3$ originates from $ε_3$ deformations of the initial state density distribution, fast buildup of radial flow due to fluctuations is the main driving mechanism for the observed large value.

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arXiv2014

Zeroing in on the initial state -- tomography using bulk, jets and photons

One of the unsolved problems in the current 'standard model' of heavy ion physics is the apparent rapid thermalization of QCD matter in the pre-equilibrium stage. While it is challenging to probe this mechanism directly, there are now several observables available which allow tomographic imaging of the initial state geometry, which is expected to carry remnant information of the equilibration mechanism. On the fluid dynamics side, scaled fluctuations in the momentum space anisotropy parameters v_n image the initial eccentricity fluctuations epsilon_n almost directly with only a weak dependence on the details of the fluid dynamical evolution. From a different direction, due to the strong non-linear dependence of their emission rates on temperature, thermal photons and their v_n are very sensitive to the initial state graininess. Finally, the v_2 and v_3 of high P_T hadrons coming from hard processes reflect the attenuation pattern of partons propagating through the inhomogeneous matter density after some fluid dynamical evolution. Combining information from all these channels does not yet lead to a fully consistent picture, however intriguing trends pointing towards non-trivial initial state dynamics emerge.

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arXiv2014

Maps of the Little Bangs Through Energy Density and Temperature Fluctuations

In this letter we propose for the first time to map the heavy-ion collisions at ultra-relativistic energies, similar to the maps of the cosmic microwave background radiation, using fluctuations of energy density and temperature in small phase space bins. We study the evolution of fluctuations at each stage of the collision using an event-by-event hydrodynamic framework. We demonstrate the feasibility of making fluctuation maps from experimental data and its usefulness in extracting considerable information regarding the early stages of the collision and its evolution.

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arXiv2014

Triangular flow of thermal photons from an event-by-event hydrodynamic model for 2.76A TeV Pb+Pb collisions at LHC

We calculate the triangular flow parameter $v_3$ of thermal photons from an event-by-event ideal hydrodynamic model for $0--40\%$ central collisions of Pb nuclei at $\sqrt{s_{NN}}$=2.76 TeV at LHC. $v_3$ determined with respect to the participant plane (PP) is found to be non-zero, positive and its $p_T$ dependence is qualitatively similar to the elliptic flow parameter $v_2$(PP) of thermal photons in the range $1 \le p_T \le 6$ GeV/$c$. In the range $p_T \, \le $ 3 GeV/$c$, $v_3$(PP) is found to be about $50--75\%$ of $v_2$(PP) and for $p_T \, >$ 3 GeV/$c$ the two anisotropy parameters become comparable. The local fluctuations in the initial density distribution as well as the initial global geometry of the produced matter in the event-by-event hydrodynamic framework are responsible for this substantial value of $v_3({\rm PP})$. However, as expected, the triangular flow parameter calculated with respect to the reaction plane $v_3$(RP) is found to be close to zero. We show that $v_3$(PP) strongly depends on the value of the fluctuation size scale $σ$ especially in the higher $p_T \, (\ge 3 {\rm GeV}/c)$ region where a larger value of $σ$ results in a smaller $v_3({\rm PP})$. In addition, the $v_3{\rm (PP)}$ is found to increase with the assumed formation time of the thermalized system.

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arXiv2013

Elliptic flow of thermal photons from event-by-event hydrodynamic model

Elliptic flow of direct photons in relativistic heavy ion collisions is believed to be dominated by contribution from thermal radiation of quark gluon plasma up to $p_T$ $\sim 5$ GeV/$c$, although other sources start outshining the thermal contribution at already smaller values of $p_T$ in the direct photon spectrum. The elliptic flow of thermal photons from ideal hydrodynamics considering a smooth initial density distribution under-predicts the PHENIX direct photon data from 200A GeV Au+Au collisions at RHIC by a large margin in the range $1 < p_T < 5$ GeV/$c$. However, a significant enhancement of thermal photon production due to fluctuations in the initial QCD matter density distributions is expected. We show that such fluctuations result in substantially larger photon elliptic flow for $p_T > 2.5 $ GeV/$c$ compared to a smooth initial-state-averaged density profile. The results from event-by-event hydrodynamics are found to be sensitive to the fluctuation size parameter. However, the effects of initial state fluctuations are insufficient to account for the discrepancy to the PHENIX data for direct photon elliptic flow. Furthermore, the photon $v_2$ is reduced even more when we include the NLO pQCD prompt photon component. We also calculate the spectra and elliptic flow of thermal photons for 2.76A TeV Pb+Pb collisions at LHC and for the 0--40% centrality bin. Thermal photons from event-by-event hydrodynamics along with prompt photons from NLO pQCD calculations explain the ALICE preliminary direct photon data well in the region $p_T \ge 2.5$ GeV/c. Similar to RHIC, the elliptic flow results at LHC are again found to be much smaller than the ALICE preliminary $v_2$ data.

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arXiv2012

Influence of initial state fluctuations on the production of thermal photons

Inhomogeneities in the initial QCD matter density distribution increase the production of thermal photons significantly compared to a smooth initial-state-averaged profile in the region $p_T > 1$ GeV/$c$ in an ideal hydrodynamic calculation. This relative enhancement is more pronounced for peripheral collisions, for smaller size systems as well as for lower beam energies. A suitably normalized ratio of central-to-peripheral yield of thermal photons reduce the uncertainties in the hydrodynamical initial conditions and can be a useful parameter to study the density fluctuations and their size. The fluctuations in the initial density distribution also lead to a larger elliptic flow of thermal photons for $p_T >$ 2.0 GeV/$c$ compared to the flow from a smooth profile.

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arXiv2012

Centrality and initial formation time dependence of the emission of thermal photons from fluctuating initial conditions at RHIC and LHC

Event-by-event fluctuating initial conditions (IC) in the ideal hydrodynamic calculation are known to enhance the production of thermal photons significantly compared to a smooth initial state averaged profile in the range $p_T >$ 1 GeV/$c$ for 200A GeV Au+Au collisions at RHIC and 2.76A TeV Pb+Pb collisions at LHC. The 'hotspots' or the over-dense regions in the fluctuating IC produce more high $p_T$ photons compared to the smooth IC due to the strong temperature dependent emission of the thermal radiation. This enhancement is expected to be more pronounced for peripheral collisions, for lower beam energies, and for larger values of plasma formation time. A suitably normalized ratio of central to peripheral yield of thermal photons ($R_{cp}^γ$) is a potential probe to study the density fluctuations and their size in the initial conditions.

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arXiv2012

Collision centrality and $τ_0$ dependence of the emission of thermal photons from fluctuating initial state in ideal hydrodynamic calculation

Fluctuations in the initial QCD matter density distribution are found to enhance the production of thermal photons significantly in the range 2 \leq pT \leq 4 GeV/c compared to a smooth initial state averaged profile in ideal hydrodynamic calculation for 200 AGeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and 2.76 ATeV Pb+Pb collisions at the Large Hadron Collider (LHC). The thermal emission of photons is strongly dependent on the initial temperature of the system where the presence of 'hotspots' in the initial state translates into enhanced production of photons compared to a smooth profile. The effect of fluctuations in the initial state is found to be stronger for peripheral collisions and for lower beam energies. The pT spectra are found to be quite sensitive to the value of the initial formation time of the plasma which is not known unambiguously and which may vary with collision centralities at a particular beam energy. Increase in the value of the formation time lowers the production of thermal photons compared to the results from a shorter formation time. However, the relative enhancement from fluctuating initial tates (compared to a smooth initial state) is found to be stronger for the larger values of formation time. The pT spectra alone are found to be insufficient to quantify the fluctuations in the initial density distribution due to the uncertainties in the initial conditions. A suitably normalized ratio of central-to-peripheral yield as a function of collision centrality and pT can be a useful measure of the fluctuation size scale.

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arXiv2011

Thermal photons from fluctuating initial conditions

Event-by-event fluctuations of initial QCD-matter density produced in heavy-ion collisions at RHIC enhance the production of thermal photons significantly in the region $2 \le p_T \le 4$ GeV/$c$ compared to a smooth initial-state averaged profile in the ideal hydrodynamic calculation. This enhancement is a an early time effect due to the presence of hotspots or over-dense regions in the fluctuating initial state. The effect of fluctuations is found to be stronger in peripheral than in central collisions.

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arXiv2011

Enhancement of thermal photon production in event-by-event hydrodynamics

Thermal photon emission is widely believed to reflect properties of the earliest, hottest evolution stage of the medium created in ultra-relativistic heavy-ion collisions. Previous computations of photon emission have been carried out using a hydrodynamical medium description with smooth, averaged initial conditions. Recently, more sophisticated hydrodynamical models which calculate observables by averaging over many evolutions with event-by-event fluctuating initial conditions (IC) have been developed. Given their direct connection to the early time dynamics, thermal photon emission appears an ideal observable to probe fluctuations in the medium initial state. In this work, we demonstrate that including fluctuations in the IC may lead to an enhancement of the thermal photon yield of about a factor of 2 in the region $2 < p_T < 4$ GeV/$c$ (where thermal photon production dominates the direct photon yield) compared to a scenario using smooth, averaged IC. Consequently, a much better agreement with PHENIX data is found. This can be understood in terms of the strong temperature dependence of thermal photon production, translating into a sensitivity to the presence of 'hotspots' in an event and thus establishing thermal photons as a suitable probe to characterize IC fluctuations.

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arXiv2011

Dynamics of Quark Gluon Plasma and Interference of Thermal Photons

The quantum statistical interference between identical particles emitted from a completely chaotic source is expected to provide valuable input for the space time description of the system. Intensity interferometry of thermal photons produced in heavy ion collisions is a very promising tool to explore the structure and dynamics of the collision fireball. Thermal photons having $K_T \, \le \, 2$ GeV/$c$ get competing contribution from both hadronic and quark matter phases and this competition gives rise to a rich structure in the outward correlation function, owing to the interference between the photons from the two sources. The temporal separation between the two sources provides the lifetime of the system and the correlation results are found to be sensitive to quark hadron phase transition temperature and the formation time of the plasma. The outward correlation function strongly depends on the equation of state of the strongly interacting matter and is seen to clearly distinguish between the lattice based and bag model equations of state.

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