arXiv subjects

A. Majumder

Publications and source records attributed to A. Majumder.

At least 55 records · Page 3Linked to original sources

arXiv2019

The JETSCAPE framework: p+p results

The JETSCAPE framework is a modular and versatile Monte Carlo software package for the simulation of high energy nuclear collisions. In this work we present a new tune of JETSCAPE, called PP19, and validate it by comparison to jet-based measurements in $p+p$ collisions, including inclusive single jet cross sections, jet shape observables, fragmentation functions, charged hadron cross sections, and dijet mass cross sections. These observables in $p+p$ collisions provide the baseline for their counterparts in nuclear collisions. Quantifying the level of agreement of JETSCAPE results with $p+p$ data is thus necessary for meaningful applications of JETSCAPE to A+A collisions. The calculations use the JETSCAPE PP19 tune, defined in this paper, based on version 1.0 of the JETSCAPE framework. For the observables discussed in this work calculations using JETSCAPE PP19 agree with data over a wide range of collision energies at a level comparable to standard Monte Carlo codes. These results demonstrate the physics capabilities of the JETSCAPE framework and provide benchmarks for JETSCAPE users.

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arXiv2019

The JETSCAPE framework

The JETSCAPE simulation framework is an overarching computational envelope for developing complete event generators for heavy-ion collisions. It allows for modular incorporation of a wide variety of existing and future software that simulates different aspects of a heavy-ion collision. The default JETSCAPE package contains both the framework, and an entire set of indigenous and third party routines that can be used to directly compare with experimental data. In this article, we outline the algorithmic design of the JETSCAPE framework, define the interfaces and describe the default modules required to carry out full simulations of heavy-ion collisions within this package. We begin with a description of the various physics elements required to simulate an entire event in a heavy-ion collision, and distribute these within a flowchart representing the event generator and statistical routines for comparison with data. This is followed by a description of the abstract class structure, with associated members and functions required for this flowchart to work. We then define the interface that will be required for external users of JETSCAPE to incorporate their code within this framework and to modify existing elements within the default distribution. We conclude with a discussion of some of the physics output for both $p$-$p$ and $A$-$A$ collisions from the default distribution, and an outlook towards future releases. In the appendix, we discuss various architectures on which this code can be run and outline our benchmarks on similar hardware.

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arXiv2019

Jet substructure modification in a QGP from a multi-scale description of jet evolution with JETSCAPE

The modification of jet substructure in relativistic heavy-ion collisions is studied using JETSCAPE, a publicly available software package containing a framework for Monte Carlo event generators. Multi-stage jet evolution in JETSCAPE provides an integrated description of jet quenching by combining multiple models, with each becoming active at a different stage of the parton shower evolution. Jet substructure modification due to different aspects of jet quenching is studied using jet shape and jet fragmentation observables. Various combinations of jet energy loss models are exploed, with medium background provided by (2 + 1)-D VISHNU with TRENTo+freestreaming initial conditions. Results reported here are from simulations performed within JETSCAPE framework.

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arXiv2019

Multi-stage jet evolution through QGP using the JETSCAPE framework: inclusive jets, correlations and leading hadrons

The JETSCAPE Collaboration has recently announced the first release of the JETSCAPE package that provides a modular, flexible, and extensible Monte Carlo event generator. This innovative framework makes it possible to perform a comprehensive study of multi-stage high-energy jet evolution in the Quark-Gluon Plasma. In this work, we illustrate the performance of the event generator for different algorithmic approaches to jet energy loss, and reproduce the measurements of several jet and hadron observables as well as correlations between the hard and soft sector. We also carry out direct comparisons between different approaches to energy loss to study their sensitivity to those observables.

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arXiv2015

Jets and Centrality in p(d)-A Collisions

The production of jets, and high-$p_{T}$ leading pions from jets, in d-Au collisions at the Relativistic Heavy-Ion Collider (RHIC) and p-Pb collisions at the Large Hadron Collider (LHC) are studied. Using a modified version of the event generator PYTHIA, in conjunction with a nuclear Glauber Monte-Carlo event generator, we demonstrate how events with a hard jet may be simulated, in such a way that the parton distribution function of the projectile nucleon is frozen during its interaction with the extended nucleus. Using our hybrid Monte-Carlo event generator, we demonstrate that the enhancement in $R_{pA}$ seen in peripheral events at RHIC and at LHC, as well as the depletion in central or semi-central events, is mainly due to "mis-binning" of central and semi-central events with a jet, as peripheral events. This occurs due to the reduction of soft particle production caused by a depletion of energy available in a nucleon (of the deuteron in the case of d-Au collisions), after the production of a hard jet. This represents a form of "color transparency" of the projectile nucleon, which has fluctuated to a state with fewer and harder partons, in events which lead to jet production. We conclude with discussions of the form of multi-parton correlations in a nucleon which may be responsible for such a startling effect.

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arXiv2014

Mass depletion: a new parameter for quantitative jet modification

We propose an extension to classify jet modification in heavy-ion collisions by including the jet mass along with its energy. The mass of a jet, as measured by jet reconstruction algorithms, is constrained by the jet's virtuality, which in turn has a considerable effect on such observables as the fragmentation function and jet shape observables. The leading parton, propagating through a dense medium, experiences substantial virtuality (or mass) depletion along with energy loss. Meaningful comparisons between surviving jets and jets produced in $p$-$p$ collisions require mass depletion to be taken into account. Using a vacuum event generator, we show the close relationship between the actual jet mass and that after applying a jet reconstruction algorithm. Using an in-medium event generator, we demonstrate the clear difference between the mass of a surviving parton exiting a dense medium and a parton with a similar energy formed in a hard scattering event. Effects of this difference on jet observables are discussed.

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arXiv2014

Possibility of a superfluid in high multiplicity p-p and p-Pb collisions

We consider the case where the saturated and unequilibriated initial stage of a high multiplicity p-p and p-Pb collision, may, due to high occupation number, display superfluid properties. The special case of an unequilibrated plasma of SU(2) gluons formed in the collision of two nuclei with deeply saturated gluon wave functions is considered. Color gauge symmetry is broken by the presence of a net non-vanishing, space-time dependent color expectation value. A two component scenario is envisioned, containing momentum states that are densely occupied below the saturation scale and a dilute normal fluid of excitations within this state. The spectrum of these excitations is shown to have an energy gap and thus it is hard to excite quanta out of the condensed state. Consequences for the stress energy tensor are outlined, in particular the possibility of a near invisid flow is discussed.

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arXiv2012

Bulk content delivery using co-operating end-nodes with upload/download limits

We study the problem of optimizing the cost of content delivery in a cooperative network of caches at end-nodes. The caches could be, for example, within the computers of users downloading videos from websites (such as Netflix, Blockbuster etc.), DVRs (such as TiVo, or cable boxes) used as part of video on demand services or public hot-spots (e.g. Wi-Fi access points with a cache) deployed over a city to serve content to mobile users. Each cache serves user requests locally over a medium that incurs no additional costs (i.e. WiFi, home LAN); if a request is not cached, it must be fetched from another cache or a central server. In our model, each cache has a tiered back-haul internet connection, with a usage cap (and fixed per-byte costs thereafter). Redirecting requests intended for the central server to other caches with unused back-haul capacity can bring down the network costs. Our goal is to develop a mechanism to optimally 1) place data into the caches and 2) route requests to caches to reduce the overall cost of content delivery. We develop a multi-criteria approximation based on a LP rounding procedure that with a small (constant factor) blow-up in storage and upload limits of each cache, gives a data placement that is within constant factor of the optimum. Further, to speed up the solution, we propose a technique to cluster caches into groups, solve the data placement problem within a group, and combine the results in the rounding phase to get the global solution.Based on extensive simulations, we show that our schemes perform very well in practice, giving costs within $5--15$% to the optimal, and reducing the network load at a central server by as much as 55% with only a marginal blow up in the limits. Also we demonstrate that our approach out-performs a non-cooperative caching mechanism by about 20%.

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arXiv2012

Suppression of the high $p_T$ charged hadron $R_{AA}$ at the LHC

We present a parameter free postdiction of the high-$p_T$ charged-hadron nuclear modification factor ($R_{AA}$) in two centralities, measured by the CMS collaboration in $Pb$-$Pb$ collisions at the Large Hadron Collider (LHC). The evolution of the bulk medium is modeled using viscous fluid dynamics, with parameters adjusted to describe the soft hadron yields and elliptic flow. Assuming the dominance of radiative energy loss, we compute the medium modification of the $R_{AA}$ using a perturbative QCD based formalism, the higher twist scheme. The transverse momentum diffusion coefficient $\hat{q}$ is assumed to scale with the entropy density and normalized by fitting the $R_{AA}$ in the most central $Au$-$Au$ collisions at the Relativistic Heavy-Ion Collider (RHIC). This set up is validated in non-central $Au$-$Au$ collisions at RHIC and then extrapolated to $Pb$-$Pb$ collisions at the LHC, keeping the relation between $\hat{q}$ and entropy density unchanged. We obtain a satisfactory description of the CMS $R_{AA}$ over the $p_{T}$ range from 10-100 GeV.

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arXiv2010

The theory and phenomenology of perturbative QCD based jet quenching

The study of the structure of strongly interacting dense matter via hard jets is reviewed. High momentum partons produced in hard collisions produce a shower of gluons prior to undergoing the non-perturbative process of hadronization. In the presence of a dense medium this shower is modified due to scattering of the various partons off the constituents in the medium. The modified pattern of the final detected hadrons is then a probe of the structure of the medium as perceived by the jet. Starting from the factorization paradigm developed for the case of particle collisions, we review the basic underlying theory of medium induced gluon radiation based on perturbative Quantum Chromo Dynamics (pQCD) and current experimental results from Deep Inelastic Scattering on large nuclei and high energy heavy-ion collisions, emphasizing how these results constrain our understanding of energy loss. This review contains introductions to the theory of radiative energy loss, elastic energy loss, and the corresponding experimental observables and issues. We close with a discussion of important calculations and measurements that need to be carried out to complete the description of jet modification at high energies at future high energy colliders.

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arXiv2009

Momentum Broadening of a Fast Parton in a Perturbative Quark-Gluon Plasma

The average transverse momentum transfer per unit path length to a fast parton scattering elastically in a perturbative quark-gluon plasma is related to the radiative energy loss of the parton. We first calculate the momentum transfer coefficient $\hat q$ in terms of a classical Langevin problem and then define it quantum-mechanically through scattering matrix element. After treating the well known case of a quark-gluon plasma in equilibrium we consider an off-equilibrium unstable plasma. As a specific example, we treat the two-stream plasma with unstable modes of longitudinal chromoelectric field. In the presence of the instabilities, $\hat q$ is shown to exponentially grow in time.

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arXiv2009

Energy and momentum deposited into a QCD medium by a jet shower

Hard partons moving through a dense QCD medium lose energy by radiative emissions and elastic scatterings. Deposition of the radiative contribution into the medium requires rescattering of the radiated gluons. We compute the total energy loss and its deposition into the medium self-consistently within the same formalism, assuming perturbative interaction between probe and medium. The same transport coefficients that control energy loss of the hard parton determine how the energy is deposited into the medium; this allows a parameter free calculation of the latter once the former have been computed or extracted from experimental energy loss data. We compute them for a perturbative medium in hard thermal loop (HTL) approximation. Assuming that the deposited energy-momentum is equilibrated after a short relaxation time, we compute the medium's hydrodynamical response and obtain a conical pattern that is strongly enhanced by showering.

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arXiv2009

The in-medium scale evolution in jet modification

The in-medium modification of the scale dependence of the fragmentation function in dense matter, brought about by higher twist corrections to the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equations, is derived. A phenomenologically motivated resummation is outlined which incorporates the next-to-leading twist single gluon emission kernel along with the vacuum emission kernel and provides an in-medium virtuality evolution of the final fragmentation function of a hard jet propagating through dense matter. The concept of a fragmentation function is generalized to include a dependence on distance traveled in the medium. Following this, numerical implementations are carried out and compared to experimental results on the single inclusive suppression observed in Deep-Inelastic scattering (DIS) off a large nucleus.

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arXiv2009

Baryon-strangeness correlations: a diagnostic of strongly interacting matter

The correlation between baryon number and strangeness elucidates the nature of strongly interacting matter, such as that formed transiently in high-energy nuclear collisions. This diagnostic can be extracted theoretically from lattice QCD calculations and experimentally from event-by-event fluctuations. The analysis of present lattice results above the critical temperature severely limits the presence of q-qbar bound states, thus supporting a picture of independent (quasi)quarks.

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arXiv2008

Elastic energy loss and longitudinal straggling of a hard jet

The elastic energy loss encountered by jets produced in deep-inelastic scattering (DIS) off a large nucleus is studied in the collinear limit. In close analogy to the case of (non-radiative) transverse momentum broadening, which is dependent on the medium transport coefficient $\hat{q}$, a class of medium enhanced higher twist operators which contribute to the non-radiative loss of the forward light-cone momentum of the jet ($q^-$) are identified and the leading correction in the limit of asymptotically high $q^-$ is isolated. Based on these operator products, a new transport coefficient $\hat{e}$ is motivated which quantifies the energy loss per unit length encountered by the hard jet. These operator products are then computed, explicitly, in the case of a similar hard jet traversing a deconfined quark-gluon-plasma (QGP) in the hard-thermal-loop (HTL) approximation. This is followed by an evaluation of sub-leading contributions which are suppressed by the light-cone momentum $q^-$, which yields the longitudinal "straggling" i.e., a slight change in light cone momentum due to the Brownian propagation through a medium with a fluctuating color field.

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arXiv2008

Characterizations of the medium in jet quenching calculations

The modification of hard jets in dense matter has so far been described by four different formalisms based on perturbative QCD (pQCD). In these proceedings, we compare the various approximations made in these different schemes, especially those regarding the structure of the medium through which jets propagate. Following this, we highlight some of the major differences in the various physical processes contained in the different approaches.

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arXiv2008

Systematic Comparison of Jet Energy-Loss Schemes in a 3D hydrodynamic medium

We utilize a 3D hydrodynamic model to provide the medium evolution for a systematic comparison of jet energy-loss calculations in the BDMPS/ASW, HT and AMY approaches. We find that the parameters of all three calculations can be adjusted to provide a good description of inclusive data on $R_{AA}$ versus transverse momentum. However, we do observe slight differences in their predictions for the azimuthal angular dependence of $R_{AA}$ vs. $p_T$. We also note that the value of the transport coefficient $\hat{q}$ needed in the three approaches to describe the data differs significantly.

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arXiv2008

Modification of the dihadron fragmentation function in nuclear matter

The medium modification of dihadron fragmentation functions in the deeply inelastic scattering (DIS) off a large nucleus is studied within the framework of the higher-twist expansion in the collinear factorization formalism. It is demonstrated that the modification due to multiple parton scattering in the nuclear medium is similar to that of the single hadron fragmentation function. However, the conditional distribution of the associated hadron given by the ratio of dihadron to single hadron fragmentation function shows only slight modification. The final results depend modestly on the nuclear density distribution. Comparisons with the experimental results on two hadron correlations as obtained by the HERMES collaboration at DESY are presented.

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