High Energy Gluons Research Articles

The LPM effect in sequential bremsstrahlung: analytic results for sub-leading (single) logarithms

Consider the in-medium splitting g → gg of a very high-energy gluon traversing a QCD medium, accounting for the Landau-Pomeranchuk-Migdal (LPM) effect. It has been known for some time that soft radiative corrections to that splitting generate a double-log correction to the splitting rate, whose effects can be absorbed into running of the medium parameter hat{q} describing the rate of transverse momentum kicks to high-energy particles due to small-angle scattering from the medium. Less has been known about sub-leading, single logarithms in this context. In this paper, we find analytic formulas for those single logs (with various caveats and clarifications).

Properties of g→bb¯ at small opening angles in pp collisions with the ATLAS detector at s=13 TeV

The fragmentation of high-energy gluons at small opening angles is largely unconstrained by present measurements. Gluon splitting to $b$-quark pairs is a unique probe into the properties of gluon fragmentation because identified $b$-tagged jets provide a proxy for the quark daughters of the initial gluon. In this study, key differential distributions related to the $g\rightarrow b\bar{b}$ process are measured using 33 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data recorded by the ATLAS experiment at the LHC in 2016. Jets constructed from charged-particle tracks, clustered with the anti-$k_t$ jet algorithm with radius parameter $R = 0.2$, are used to probe angular scales below the $R=0.4$ jet radius. The observables are unfolded to particle level in order to facilitate direct comparisons with predictions from present and future simulations. Multiple significant differences are observed between the data and parton shower Monte Carlo predictions, providing input to improve these predictions of the main source of background events in analyses involving boosted Higgs bosons decaying into $b$-quarks.

Small-x resummation from HELL.

Small-x logarithmic enhancements arising from high-energy gluon emissions affect both the evolution of collinearly-factorized parton densities and partonic coefficient functions. With the higher collider energy reached by the LHC, the prospect of a future high-energy collider, and the recent deep-inelastic scattering (DIS) results at small-x from HERA, providing phenomenological tools for performing small-x resummation has become of great relevance. In this paper we discuss a framework to perform small-x resummation for both parton evolution and partonic coefficient functions and we describe its implementation in a computer code named High-Energy Large Logarithms (HELL). We present resummed and matched results for the DGLAP splitting functions and, as a proof of principle, for the massless structure functions in DIS. Furthermore, we discuss the uncertainty from subleading terms on our results.

Renormalization of the jet-quenching parameter

We study the radiative processes that affect the propagation of a high energy gluon in a dense medium, such as a quark-gluon plasma. In particular, we investigate the role of the large double logarithms corrections, $\sim\alpha_s \ln^2 L/\tau_0$, that were recently identified in the study of $p_\perp$-broadening by Liou, Mueller and Wu. We show that these large corrections can be reabsorbed in a renormalization of the jet quenching parameter controlling both momentum broadening and energy loss. We argue that the probabilistic description of these phenomena remains valid, in spite of the large non-locality in time of the radiative corrections. The renormalized jet-quenching parameter is enhanced compared to its standard perturbative estimate. As a particular consequence, the radiative energy loss scales with medium size $L$ as $L^{2+\gamma}$, with $\gamma=2\sqrt{\alpha_s N_c /\pi}$, as compared to the standard scaling in $L^2$.

Jets and flow within a pQCD-based partonic transport model

We present fully dynamic simulations of heavy ion collisions at RHIC energies within the perturbative QCD-based partonic transport model BAMPS, focusing on the simultaneous investigation of jet-quenching and elliptic flow. The model consistently features elastic and inelastic 2 ↔ 3 processes, the latter being based on the Gunion-Bertsch matrix element. We discuss first attempts to extend the model to include light quark degrees of freedom and study the energy loss of high energy gluons and quarks in a static partonic medium. The difference between gluons and quarks in inelastic processes is found to be weaker than expected from color factors, due to a self-quenching effect associated with a cut-off modeling the LPM effect.

Investigation of jet quenching and elliptic flow within a pQCD-based partonic transport model

The partonic transport model BAMPS (a Boltzmann approach to multiparton scatterings) is employed to investigate different aspects of heavy ion collisions within a common framework based on perturbative QCD. This report focuses on the joint investigation of the collective behavior of the created medium and the energy loss of high–pT gluons traversing this medium. To this end the elliptic flow and the nuclear modification factor of gluons in heavy ion collisions at 200 AGeV are simulated with BAMPS.The mechanism for the energy loss of high energy gluons within BAMPS is studied in detail. For this, purely elastic interactions are compared to radiative processes, gg → ggg, that are implemented based on the matrix element by Gunion and Bertsch. The latter are found to be the dominant source of energy loss within the framework employed in this work.

Relativistic shock waves and Mach cones in viscous gluon matter

To investigate the formation and the propagation of relativistic shock waves in viscous gluon matter we solve the relativistic Riemann problem using a microscopic parton cascade. We demonstrate the transition from ideal to viscous shock waves by varying the shear viscosity to entropy density ratio η/s. Furthermore we compare our results with those obtained by solving the relativistic causal dissipative fluid equations of Israel and Stewart (IS), in order to show the validity of the IS hydrodynamics. Employing the parton cascade we also investigate the formation of Mach shocks induced by a high-energy gluon traversing viscous gluon matter. For η/s = 0.08 a Mach cone structure is observed, whereas the signal smears out for η/s ≥ 0.32.

Simple formula for high-energy gluon bremsstrahlung in a finite, expanding medium

Previous authors have considered the problem of the medium effects on single gluon bremsstrahlung associated with producing a high-energy particle in a finite, time-dependent QCD plasma. Working to leading logarithmic order, I show that the result for the bremsstrahlung gluon spectrum can be cast into a remarkably simple form in the general case. I similarly analyze the process of pair production. Also, I comment on the radius of convergence of the opacity expansion in cases where the leading-log approximation holds, showing that the opacity expansion does not converge when the thickness of the plasma is greater than roughly the bremsstrahlung formation time.

Gluon Scattering in = 4 Super Yang-Mills at Finite Temperature

We extend the AdS/CFT prescription of Alday and Maldacena to finite temperature T , defining an amplitude for gluon scattering in N = 4 Super Yang-Mills at strong coupling from string theory. It is defined by a lightlike “Wilson loop” living at the horizon of the T-dual to the black hole in AdS space. Unlike the zero temperature case, this is different from the Wilson loop contour defined at the boundary of the AdS black hole metric. Thus at nonzero T there is no relation between gluon scattering amplitudes and the Wilson loop. We calculate a gauge theory observable that can be interpreted as the amplitude at strong coupling for forward scattering of a low energy gluon (E < T ) off a high energy gluon (E T ) in both cutoff and generalized dimensional regularization. The generalized dimensional regularization is defined in string theory as an IR modified dimensional reduction. For this calculation, the corresponding usual Wilson loop of the same boundary shape was argued to be related to the jet quenching parameter of the finite temperature N = 4 SYM plasma, while the gluon scattering amplitude is related to the viscosity coefficient.

THE TWO TYPES OF CHERENKOV GLUONS AT LHC ENERGIES

Beside comparatively low energy Cherenkov gluons observed at RHIC, there could be high energy gluons at LHC, related to the high energy region of positive real part of the forward scattering amplitude. In both cases they give rise to particles emitted along some cone. The characteristics of the cones produced by these two types of gluons are different. Therefore different experiments are needed to detect them. The cosmic ray event which initiated this idea is described in detail.

CHERENKOV GLUONS (PREDICTIONS AND PROPOSALS)

The coherent hadron production analogous to Cherenkov radiation of photons gives rise to the ring-like events. Being projected on the ring diameter they produce the two-bump structure recently observed for the away-side jets at RHIC. The position of the peaks and their height determine such properties of the hadronic medium as its nuclear refractive index, the parton density, the free path length and the energy loss of Cherenkov gluons. Cherenkov gluons may be responsible for the asymmetry of dilepton mass spectra near ρ-meson observed in experiment. Beside comparatively low energy gluons observed at RHIC, there could be high energy gluons at LHC, related to the high energy region of positive real part of the forward scattering amplitude and possessing different characteristics. This would allow to scan (x, Q2)-plane determining the parton densities in its various regions.

Cherenkov gluons at RHIC and LHC

The coherent hadron production analogous to Cherenkov radiation of photons gives rise to the ring-like events. Being projected on the ring diameter they produce the two-bump structure recently observed for the away-side jets at RHIC. The position of the peaks and their height determine such properties of the hadronic medium as its nuclear index of refraction, the parton density, the free-path length and the energy loss of Cherenkov gluons. Beside comparatively low-energy gluons observed at RHIC, there could be high-energy gluons at LHC, related to the high-energy region of a positive real part of the forward scattering amplitude and possessing different characteristics.

Nonequilibrium QCD: Interplay of hard and soft dynamics in high-energy multigluon beams

A quantum-kinetic formulation of the dynamical evolution of a high-energy nonequilibrium gluon system at finite density is developed to study the interplay between quantum fluctuations of high-momentum (hard) gluons and the low-momentum (soft) mean color field that is induced by the collective motion of the hard particles. From the exact field equations of motion of QCD, a self-consistent set of approximate quantum-kinetic equations are derived by separating hard and soft dynamics and choosing a convenient axial-type gauge. This set of master equations describes the momentum space evolution of the individual hard quanta, the space-time development of the ensemble of hard gluons, and the generation of the soft mean field by the current of the hard particles. The quantum-kinetic equations are approximately solved to order ${g}^{2}(1+g\ifmmode \bar{A}\else \={A}\fi{})$ for a specific example, namely, the scenario of a high-energy gluon beam along the light cone, demonstrating the practical applicability of the approach.

On the sign of the isoscalar-meson mixing amplitude in QCD

The sign and magnitude of the amplitude β for isoscalar\(\bar qq\) quark annihilation into two gluons followed by\(\bar qq\) creation into an isoscalar meson dominates particle nonet mixing. The consequent theory of particle mixing and also its phenomenology uniformly predict that the pseudoscalar and vector isoscalar amplitudes βp and βV are positive in sign. We present simple arguments to show that the positive sign of βp may readily result by any mechanism such as gluon confinement and/or asymptotic freedom which will suppress contributions from high-energy gluon intermediate states. This also explains why the corresponding βpp parameter for the parapositronium state in QED is negative in contrast.