Publications (72)222.68 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: The case of gluon bremsstrahlung off a heavy quark in extended nuclear matter is revisited within the higher twist formalism. In particular, the inmedium modification of "semihard" heavy quarks is studied, where the momentum of the heavy quark is larger but comparable to the mass of the heavy quark ($p \gtrsim M$). In contrast to all prior calculations, where the gluon emission spectrum is entirely controlled by the transverse momentum diffusion parameter ($\hat q$), both for light and heavy quarks, in this work, we demonstrate that the gluon emission spectrum for a heavy quark (unlike that for flavors) is also sensitive to $\hat e$, which so far has been used to quantify the amount of lightcone drag experienced by a parton. This mass dependent effect, due to the nonlightlike momentum of a semihard heavyquark, leads to an additional energy loss term for heavyquarks, while resulting in a negligible modification of light flavor (and high energy heavy flavor) loss. This result can be used to estimate the value of this subleading nonperturbative jet transport parameter ($\hat e$) from heavy flavor suppression in ultrarelativistic heavyion collisions.  [Show abstract] [Hide abstract]
ABSTRACT: The past decade has seen huge advances in experimental measurements made in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and more recently at the Large Hadron Collider (LHC). These new data, in combination with theoretical advances from calculations made in a variety of frameworks, have led to a broad and deep knowledge of the properties of thermal QCD matter. Increasingly quantitative descriptions of the quarkgluon plasma (QGP) created in these collisions have established that the QGP is a strongly coupled liquid with the lowest value of specific viscosity ever measured. However, much remains to be learned about the precise nature of the initial state from which this liquid forms, how its properties vary across its phase diagram and how, at a microscopic level, the collective properties of this liquid emerge from the interactions among the individual quarks and gluons that must be visible if the liquid is probed with sufficiently high resolution. This white paper, prepared by the Hot QCD Writing Group as part of the U.S. Long Range Plan for Nuclear Physics, reviews the recent progress in the field of hot QCD and outlines the scientific opportunities in the next decade for resolving the outstanding issues in the field.  [Show abstract] [Hide abstract]
ABSTRACT: We study the effect of multiple scatterings on the propagation of hard partons and the production of jetbremsstrahlung photons inside a dense medium in the framework of deepinelastic scattering off a large nucleus. We include the momentum exchanges in both longitudinal and transverse directions between the hard partons and the constituents of the medium. Keeping up to the second order in a momentum gradient expansion, we derive the spectrum for the photon emission from a hard quark jet when traversing dense nuclear matter. Our calculation demonstrates that the photon bremsstrahlung process is influenced not only by the transverse momentum diffusion of the propagating hard parton, but also by the longitudinal drag and diffusion of the parton momentum. A notable outcome is that the longitudinal drag tends to reduce the amount of stimulated emission from the hard parton.  [Show abstract] [Hide abstract]
ABSTRACT: The case of heavy quark propagation in dense extended matter is studied in the multiple scattering formalism of the higher twist energy loss scheme. We consider the case of deep inelastic scattering off a large nucleus. The hard lepton scatters off a heavy quark fluctuation within one of the nucleons. This heavy quark then propagates through the dense medium, multiply scattering off the gluon field of the remaining nucleons in its path. We consider the fictitious process where a heavy quark propagates through the nucleus without radiation. Invoking SoftCollinear Effective Theory power counting arguments, we consider the case of a "semihard" heavyquark where the mass is of the order of the outgoing momentum and larger than the transverse momentum imparted per unit length due to scattering. In this limit, it is found that longitudinal momentum exchanges (quantified by the transport coefficient $\hat{e}$) have a comparable effect on the offshellness of the propagating quark, as the transverse momentum exchanges (quantified by $\hat{q}$) which constitute the leading cause of offshellness for propagating light quarks or gluons. Consequences of this new hierarchy for the propagation of the heavy quark in dense matter are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: In this review, intended for nonspecialists and beginners, we recount the current status of the theory of jet modification in dense matter. We commence with an outline of the "traditional" observables which may be calculated without recourse to event generators. These include single and double hadron suppression, nuclear modification factor versus reaction plane etc. All of these measurements are used to justify both the required underlying physical picture of jet modification as well as the final obtained values of jet transport coefficients. This is followed by a review of the more modern observables which have arisen with the ability to reconstruct full jets, and the challenges faced therein. This is followed by a preview of upcoming theoretical developments in the field and an outlook on how the interface between these developments, phenomenological improvements, and upcoming data will allow us to quantitatively determine properties of the medium which effect the modification of hard jets.  [Show abstract] [Hide abstract]
ABSTRACT: In this review, intended for nonspecialists and beginners, we recount the current status of the theory of jet modification in dense matter. We commence with an outline of the "traditional" observables which may be calculated without recourse to event generators. These include single and double hadron suppression, nuclear modification factor versus reaction plane etc. All of these measurements are used to justify both the required underlying physical picture of jet modification as well as the final obtained values of jet transport coefficients. This is followed by a review of the more modern observables which have arisen with the ability to reconstruct full jets, and the challenges faced therein. This is followed by a preview of upcoming theoretical developments in the field and an outlook on how the interface between these developments, phenomenological improvements, and upcoming data will allow us to quantitatively determine properties of the medium which effect the modification of hard jets.  [Show abstract] [Hide abstract]
ABSTRACT: Within five different approaches to parton propagation and energy loss in dense matter, a phenomenological study of experimental data on suppression of large $p_T$ single inclusive hadrons in heavyion collisions at both RHIC and LHC was carried out. The evolution of bulk medium used in the study for parton propagation was given by 2+1D or 3+1D hydrodynamic models which are also constrained by experimental data on bulk hadron spectra. Values for the jet transport parameter $\hat q$ at the center of the most central heavyion collisions are extracted or calculated within each model, with parameters for the medium properties that are constrained by experimental data on the hadron suppression factor $R_{AA}$. For a quark with initial energy of 10 GeV we find that $\hat q\approx 1.1 \pm 0.3$ GeV$^2$/fm at an initial time $\tau_0=0.6$ fm/$c$ in Au+Au collisions at $\sqrt{s}=200$ GeV/n and $\hat q\approx 1.9 \pm 0.7 $ GeV$^2$/fm in Pb+Pb collisions at $\sqrt{s}=2.76 $ TeV/n. Compared to earlier studies, these represent significant convergence on values of the extracted jet transport parameter, reflecting recent advances in theory and the availability of new experiment data from the LHC.  [Show abstract] [Hide abstract]
ABSTRACT: The propagation of a virtual quark in a thermal medium is considered. The nonperturbative jet transport coefficient $\hat{q}$ is estimated in quark less SU(2) lattice gauge theory. The light like correlator which defines $\hat{q}$, defined in the regime where the jet has small virtuality compared to its energy, is analytically related to a series of local operators in the deep Euclidean region, where the jet's virtuality is of the same order as its energy. It is demonstrated that in this region, for temperatures in the range of T=400600 MeV, and for jet energies above 20 GeV, the leading term in the series is dominant over the nexttoleading term and thus yields an estimate of the value of $\hat{q}$. In these proceedings we discuss the details of the numerical calculation.  [Show abstract] [Hide abstract]
ABSTRACT: We outline a novel approach to develop an inmedium shower MonteCarlo event generator based on the highertwist formalism of jet modification. By undoing one of the lightcone integrals which sets the corresponding lightcone momentum to be equal in the amplitude and the complex conjugate, we introduce an uncertainty in the smaller lightcone momentum component. This allows for the generalization of the standard analytic formalism to a Wigner transform like formalism, where the nonconjugate large lightcone momentum and position are retained for each parton. Jets are generated eventbyevent by simulating this Wigner transform kernel. Simple results for longitudinal distributions of partons and hadrons from jets propagating through a hot brick of strongly interacting matter are presented. Values of the transport coefficient $\hat{q}$ are dialed to match phenomenologically relevant cases.  [Show abstract] [Hide abstract]
ABSTRACT: The formalism of jet modification in the higher twist approach is modified to describe a hard parton propagating through a hot thermalized medium. The leading order contribution to the transverse momentum broadening of a high energy (near onshell) quark in a thermal medium is calculated. This involves a factorization of the perturbative process of scattering of the quark from the nonperturbative transport coefficient. An operator product expansion of the nonperturbative operator product which represents $\hat{q}$ is carried out and related via dispersion relations to the expectation of local operators. These local operators are then evaluated in quenched SU(2) lattice gauge theory.  [Show abstract] [Hide abstract]
ABSTRACT: The effect of multiple scatterings on the propagation of hard partonic jets in a dense nuclear medium is studied in the framework of deepinelastic scattering (DIS) off a large nucleus. Power counting arguments based on the Glauber improved SoftCollinearEffectiveTheory are used to identify the class of leading power corrections to the process of a single parton traversing the extended medium without emission. It turns out that the effect of longitudinal drag and diffusion (often referred to as straggling) is as important as transverse scattering, when relying solely on power counting arguments. With the inclusion of momentum exchanges in both transverse and longitudinal directions between the traversing hard parton and the constituents of the medium, we derive a differential equation for the time (or distance) evolution of the hard parton momentum distribution. Keeping up to the second order in a momentum gradient expansion, this equation describes inmedium evolution of hard jets which experience longitudinal drag and diffusion plus the transverse broadening caused by multiple scatterings from the medium.  [Show abstract] [Hide abstract]
ABSTRACT: We present a framework where first principles calculations of jet modification may be carried out in a nonperturbative thermal environment. As an example of this approach, we compute the leading order contribution to the transverse momentum broadening of a high energy (near onshell) quark in a thermal medium. This involves a factorization of a nonperturbative operator product from the perturbative process of scattering of the quark. An operator product expansion of the nonperturbative operator product is carried out and related via dispersion relations to the expectation of local operators. These local operators are then evaluated in quenched SU(2) lattice gauge theory.  [Show abstract] [Hide abstract]
ABSTRACT: We review the currently available formalisms for radiative energy loss of a highmomentum parton in a dense strongly interacting medium. The underlying theoretical framework of the four commonly used formalisms is discussed and the differences and commonalities between the formalisms are highlighted. A quantitative comparison of the single gluon emission spectra as well as the energy loss distributions is given for a model system consisting of a uniform medium with a fixed length of L=2 fm and L=5 fm (the `Brick'). Sizable quantitative differences are found. The largest differences can be attributed to specific approximations that are made in the calculation of the radiation spectrum.  [Show abstract] [Hide abstract]
ABSTRACT: We review the currently available formalisms for radiative energy loss of a highmomentum parton in a dense strongly interacting medium. The underlying theoretical framework of the four commonly used formalisms is discussed and the differences and commonalities between the formalisms are highlighted. A quantitative comparison of the single gluon emission spectra as well as the energy loss distributions is given for a model system consisting of a uniform medium with a fixed length of L=2 fm and L=5 fm (the `Brick'). Sizable quantitative differences are found. The largest differences can be attributed to specific approximations that are made in the calculation of the radiation spectrum.  [Show abstract] [Hide abstract]
ABSTRACT: We present a successful description of the medium modification of light and heavy flavor jets within a perturbativeQCDbased approach. Only the couplings involving hard partons are assumed to be weak. The effect of the medium on a hard parton, per unit time, is encoded in terms of three nonperturbative, related transport coefficients which describe the transverse momentum squared gained, the elastic energy loss, and diffusion in elastic energy transfer. Scaling the transport coefficients with the temperature of the medium, we achieve a good description of the centrality dependence of the suppression and the azimuthal anisotropy of leading hadrons. Imposing additional constraints based on leading order (LO) hard thermal loop (HTL) effective theory leads to a worsening of the fit, implying the necessity of computing transport coefficients beyond LO HTL.  [Show abstract] [Hide abstract]
ABSTRACT: Finite temperature lattice simulations of quantum chromodynamics (QCD) are sensitive to the hadronic mass spectrum for temperatures below the "critical" temperature T(c) ≈ 160 MeV. We show that a recent precision determination of the QCD trace anomaly shows evidence for the existence of a large number of hadron states beyond those known from experiment. The lattice results are well represented by an exponentially growing mass spectrum up to a temperature T=155 MeV. Using simple parametrizations of the hadron mass spectrum we show how one may estimate the total spectral weight in these yet undermined states.  [Show abstract] [Hide abstract]
ABSTRACT: We discuss various mechanisms for the creation of an asymmetric charge fluctuation with respect to the reaction plane among hadrons emitted in relativistic heavyion collisions. We show that such mechanisms exist in both, the hadronic gas and the partonic phases of QCD. The mechanisms considered here all require the presence of a strong magnetic field (the ``chiral magnetic effect''), but they do not involve parity or chargeparity violations. We analyze how a transient local electric current fluctuation generated by the chiral magnetic effect can dynamically evolve into an asymmetric charge distribution among the finalstate hadrons in momentum space. We estimate the magnitude of the eventbyevent fluctuations of the finalstate charge asymmetry due to the partonic and hadronic mechanisms.  [Show abstract] [Hide abstract]
ABSTRACT: 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 nonperturbative 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 heavyion 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.  [Show abstract] [Hide abstract]
ABSTRACT: We discuss several mechanisms for the creation of a charge asymmetry with respect to the reaction plane among hadrons emitted in relativistic heavyion collisions. We show that such mechanisms exist in both, the hadron gas and the quarkgluon plasma phase. All such mechanisms have in common that they require the presence of a strong magnetic field (the ``chiral magnetic effect''), but they do not involve P or PC symmetry violations. We analyze how a transient local electric current generated by the chiral magnetic effect can dynamically evolve into a charge asymmetry of the finalstate hadron distribution in momentum space. We estimate the magnitude of the eventbyevent fluctuations of the finalstate charge asymmetry due to the partonic and hadronic mechanisms. )  [Show abstract] [Hide abstract]
ABSTRACT: The energy loss of hard jets produced in the DeepInelastic scattering (DIS) off a large nucleus is considered in the collinear limit. In particular, the single gluon emission cross section due to multiple scattering in the medium is calculated. Calculations are carried out in the highertwist scheme, which is extended to include contributions from multiple transverse scatterings on both the produced quark and the radiated gluon. The leading length enhanced parts of these power suppressed contributions are resummed. Various interferences between such diagrams lead to the LandauPomeranchukMigdal (LPM) effect. We resum the corrections from an arbitrary number of scatterings and isolate the leading contributions which are suppressed by one extra power of the hard scale $Q^{2}$. All powers of the emitted gluon forward momentum fraction $y$ are retained. We compare our results with the previous calculation of single scattering per emission in the highertwist scheme as well as with multiple scattering resummations in other schemes. It is found that the leading ($1/Q^2$) contribution to the double differential gluon production cross section, in this approach, is equivalent to that obtained from the single scattering calculation once the transverse momentum of the final quark is integrated out. We comment on the generalization of this formalism to MonteCarlo routines.
Publication Stats
2k  Citations  
222.68  Total Impact Points  
Top Journals
Institutions

20122014

Wayne State University
 Department of Physics and Astronomy
Detroit, Michigan, United States


20092012

The Ohio State University
 Department of Physics
Columbus, Ohio, United States


20052008

Duke University
 Department of Physics
Durham, NC, United States


20032006

Lawrence Berkeley National Laboratory
 Nuclear Science Division
Berkeley, CA, United States


2004

University of California, Berkeley
Berkeley, California, United States


19992002

McGill University
 Department of Physics
Montréal, Quebec, Canada
