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A. H. Mueller

Publications and source records attributed to A. H. Mueller.

At least 55 records · Page 3Linked to original sources
arXiv2014

Parton energy loss in high energy hard forward processes in proton-nucleus collisions

We calculate the spectrum of energy loss differences due to gluon radiation in high energy hard forward processes in proton-nucleus collisions as compared to proton-proton collisions. We find that the nuclear induced energy loss scales linearly with the beam energy. We evaluate the spectrum and "typical" energy losses in a logarithmic and large $N_{c}$ approximation. The energy losses found appear large enough to be phenomenologically important.

hep-ph↗
arXiv2013

Radiative pT-broadening of high-energy quarks and gluons in QCD matter

We study radiative $p_{\perp}$-broadening of high energy quarks passing through hot and cold QCD matter. With $L$ the length of the matter and $l_{0}$ the size of constituents of the matter we find $\avg{p_{\perp}^{2}}$ has both double logarithmic terms, $\ln^{2}(L/l_{0})$, and single logarithmic terms, $\ln(L/l_{0})$, coming from gluon radiation induced by the matter. We use a (slight) extension of a formalism developed by B. Zakharov for studying energy loss, a formalism which, for much of our calculation, reduces to a simple dipole scattering analysis. We estimate the radiative contribution to be a sizable correction to the nonradiative value of $\avg{p_{\perp}^{2}}$. We also carry out a resummation of the double logarithmic terms that we find, and we briefly discuss running coupling effects which appear here in a rather unusual way.

hep-ph↗
arXiv2013

Sudakov Double Logarithms Resummation in Hard Processes in Small-x Saturation Formalism

In this manuscript, we present a complete study of the Sudakov double logarithms resummation for various hard processes in eA and pA collisions in the small-x saturation formalism. We first employ a couple of slightly different formalisms to perform the one-loop analysis of the Higgs boson production process in pA collisions, and demonstrate that Sudakov-type logarithms arise as the leading correction and that they can be systematically resummed in addition to the usual small-x resummation. We further study the Sudakov double logarithms for other processes such as heavy quark pair production and back-to-back dijet production in eA and pA collisions through detailed calculation of the corresponding one-loop diagrams. As the most important contribution from the one-loop correction, the Sudakov factor should play an important role in the phenomenological study of saturation physics in the pA programs at RHIC and the LHC.

hep-ph↗
arXiv2013

Predictions for $p+$Pb Collisions at sqrt s_NN = 5 TeV

Predictions for charged hadron, identified light hadron, quarkonium, photon, jet and gauge bosons in p+Pb collisions at sqrt s_NN = 5 TeV are compiled and compared. When test run data are available, they are compared to the model predictions.

hep-ph↗
arXiv2012

Sudakov Resummation in Small-x Saturation Formalism

Through an explicit calculation of massive scalar particle (e.g., Higgs boson) production in high energy pA collisions up to one-loop order, we demonstrate that the Sudakov-type logarithms in hard processes in small-x saturation formalism can be systematically separated from the typical small-x logarithms. The generic feature of the Sudakov logarithms and all order resummation is derived. We further comment on the phenomenological implications and extension to other hard processes in the small-x calculations.

hep-ph↗
arXiv2012

Jet evolution from weak to strong coupling

Recent studies, using the AdS/CFT correspondence, of the radiation produced by a decaying system or by an accelerated charge in the N=4 supersymmetric Yang-Mills theory, led to a striking result: the 'supergravity backreaction', which is supposed to describe the energy density at infinitely strong coupling, yields exactly the same result as at zero coupling, that is, it shows no trace of quantum broadening. We argue that this is not a real property of the radiation at strong coupling, but an artifact of the backreaction calculation, which is unable to faithfully capture the space-time distribution of the radiation. This becomes obvious in the case of a decaying system ('virtual photon'), for which the backreaction is tantamount to computing a three-point function in the conformal gauge theory, which is independent of the coupling since protected by symmetries. Whereas this non-renormalization property is specific to the conformal N=4 SYM theory, we argue that the failure of the three-point function to provide a local measurement is in fact generic: it holds in any field theory with non-trivial interactions. To properly study a localized distribution, one should rather compute a four-point function, as standard in deep inelastic scattering. We substantiate these considerations with studies of the radiation produced by the decay of a time-like photon at both weak and strong coupling. We show that by computing four-point functions, in perturbation theory at weak coupling and, respectively, from Witten diagrams at strong coupling, one can follow the quantum evolution and thus demonstrate the broadening of the energy distribution. This broadening is slow when the coupling is weak but it proceeds as fast as possible in the limit of a strong coupling.

hep-th↗
arXiv2012

pT-Broadening and production processes versus dipole/quadrupole amplitudes at next-to-leading order

Through the systematic inspection of graphs in the framework of lightcone perturbation theory, we demonstrate that an identity between the evolution of pT-broadening amplitudes with the energy and the evolution of forward scattering amplitudes of color dipoles off nuclei holds at next-to-leading order accuracy. In the general case, the relation is not a graph-by-graph correspondence, neither does it hold strictly speaking for definite values of the momenta: Instead, it relates classes of graphs of similar topologies, and in some cases, the matching requires an analytical continuation in the appropriate longitudinal momentum variable. We check that the same kind of relation is also true at next-to-leading order between amplitudes for the production of dijets and quadrupole forward amplitudes.

hep-ph↗
arXiv2011

Gluon saturation effects on the color singlet J/Psi production in high energy dA and AA collisions

We derive the formulae for the cross section of J/Psi production in high energy pA and AA collisions taking into account the gluon saturation/color glass condensate effects. We then perform the numerical calculations of the corresponding nuclear modification factors and find a good agreement between our calculations and the experimental data on J/Psi production in dA collisions. We also observe that cold nuclear modification effects alone cannot describe the data on J/Psi production in AA collisions. Additional final state suppression (at RHIC) and enhancement (at LHC) mechanisms are required to explain the experimental observations.

hep-ph↗
arXiv2011

On the small-x evolution of the color quadrupole and the Weizsäcker-Williams gluon distribution

Color quadrupoles have been found to be important in the proper description of observables sensitive to the small-x regime in nuclei as well as in the operator definition of the Weizsäcker-Williams gluon distribution. In this paper, we derive the small-x evolution equation of the quadrupole and the Weizsäcker-Williams gluon distribution without taking large N_c limit and study the properties of the equation in both dilute and saturation regime. We find that the quadrupole evolution follows the BFKL evolution in the dilute regime and then saturates in the dense region due to nonlinear terms. This leads us to conclude that the Weizsäcker-Williams gluon distribution should obey the same geometrical behavior as the dipole gluon distribution as found in the inclusive DIS measurement.

hep-ph↗
arXiv2011

Radiation by a heavy quark in N=4 SYM at strong coupling

Using the AdS/CFT correspondence in the supergravity approximation, we compute the energy density radiated by a heavy quark undergoing some arbitrary motion in the vacuum of the strongly coupled N=4 supersymmetric Yang-Mills theory. We find that this energy is fully generated via backreaction from the near-boundary endpoint of the dual string attached to the heavy quark. Because of that, the energy distribution shows the same space-time localization as the classical radiation that would be produced by the heavy quark at weak coupling. We believe that this and some other unnatural features of our result (like its anisotropy and the presence of regions with negative energy density) are artifacts of the supergravity approximation, which will be corrected after including string fluctuations. For the case where the quark trajectory is bounded, we also compute the radiated power, by integrating the energy density over the surface of a sphere at infinity. For sufficiently large times, we find agreement with a previous calculation by Mikhailov [hep-th/0305196].

hep-th↗
arXiv2010

Aspects of the UV/IR correspondence : energy broadening and string fluctuations

We show that a source which radiates in the vacuum of the strongly coupled N=4 SYM theory produces an energy distribution which, in the supergravity approximation, has the same space-time pattern as the corresponding classical distribution: the radiation propagates at the speed of light without broadening. We illustrate this on the basis of several examples: a small perturbation propagating down a steady string, a massless particle falling into AdS_5, and the decay of a time-like wave-packet. A similar observation was made in Phys. Rev. D81 (2010) 126001 for the case of a rotating string. In all these cases, the absence of broadening is related to the fact that the energy backreaction on the boundary arises exclusively from the bulk perturbation at, or near, the boundary. This is so since bulk sources which propagate in AdS_5 at the speed of light do not generate any energy on the boundary. We interpret these features as an artifact of the supergravity approximation, which fails to encode quantum mechanical fluctuations that should be present even in the strong coupling limit. We argue that such fluctuations should enter the dual string theory as longitudinal string fluctuations, which are not suppressed at strong coupling. We heuristically estimate the effects of such fluctuations and argue that they restore the broadening of the radiation, in agreement with expectations from both quantum mechanics and the ultraviolet/infrared correspondence.

hep-th↗
arXiv2010

Correlations in impact-parameter space in a hierarchical saturation model for QCD at high energy

In order to get an estimate of the homogeneity of the distribution of matter in a fast hadron or nucleus, we compute the correlations of the saturation scales Q_s between different points in impact-parameter space, in some specific saturation models. We find that these correlations are quite strong: The saturation scale is nearly uniform in domains whose sizes scale like exp[const*ln^2(1/α_s^2)]/Q_s, which means that the density of gluons should not fluctuate significantly over regions of that typical size. We expect these conclusions as well as the explicit analytical expressions we obtain for the correlations to be true also for full QCD in appropriate limits.

hep-ph↗
arXiv2009

Light-like mesons and deep inelastic scattering in finite-temperature AdS/CFT with flavor

We use the holographic dual of a finite-temperature, strongly-coupled, gauge theory with a small number of flavors of massive fundamental quarks to study meson excitations and deep inelastic scattering (DIS) in the low-temperature phase, where the mesons are stable. We show that a high-energy flavor current with nearly light-like kinematics disappears into the plasma by resonantly producing mesons in highly excited states. This mechanism generates the same DIS structure functions as in the high temperature phase, where mesons are unstable and the current disappears through medium-induced parton branching. To establish this picture, we derive analytic results for the meson spectrum, which are exact in the case of light-like mesons and which corroborate and complete previous, mostly numerical, studies in the literature. We find that the meson levels are very finely spaced near the light-cone, so that the current can always decay, without a fine-tuning of its kinematics.

hep-th↗
arXiv2009

A lattice test of strong coupling behaviour in QCD at finite temperature

We propose a set of lattice measurements which could test whether the deconfined, quark-gluon plasma, phase of QCD shows strong coupling aspects at temperatures a few times the critical temperature for deconfinement, in the region where the conformal anomaly becomes unimportant. The measurements refer to twist-two operators which are not protected by symmetries and which in a strong-coupling scenario would develop large, negative, anomalous dimensions, resulting in a strong suppression of the respective lattice expectation values in the continuum limit. Special emphasis is put on the respective operator with lowest spin (the spin-2 operator orthogonal to the energy-momentum tensor within the renormalization flow) and on the case of quenched QCD, where this operator is known for arbitrary values of the coupling: this is the quark energy-momentum tensor. The proposed lattice measurements could also test whether the plasma constituents are pointlike (as expected at weak coupling), or not.

hep-ph↗
arXiv2009

Stochastic trailing string and Langevin dynamics from AdS/CFT

Using the gauge/string duality, we derive a set of Langevin equations describing the dynamics of a relativistic heavy quark moving with constant average speed through the strongly-coupled N=4 SYM plasma at finite temperature. We show that the stochasticity arises at the string world-sheet horizon, and thus is causally disconnected from the black hole horizon in the space-time metric. This hints at the non-thermal nature of the fluctuations, as further supported by the fact that the noise term and the drag force in the Langevin equations do not obey the Einstein relation. We propose a physical picture for the dynamics of the heavy quark in which dissipation and fluctuations are interpreted as medium-induced radiation and the associated quantum-mechanical fluctuations. This picture provides the right parametric estimates for the drag force and the (longitudinal and transverse) momentum broadening coefficients.

hep-th↗
arXiv2008

Deep inelastic and dipole scattering on finite length hot $\mathcal{N}=4$ SYM matter

Deep inelastic scattering of $\mathcal{R}$-currents and the scattering of a small dipole on finite length hot $\mathcal{N}=4$ SYM matter are discussed. In each case we find the scale when scattering becomes strong is determined by a saturation momentum $Q^2_s \sim LT^3/x$ where $L$ is the length of the matter. For $\mathcal{R}$-currents we analyze the operator product expansion. For infinite length matter the series generated by the OPE is not Borel summable but we are able to determine the exponential part of the tunneling amplitude determining $F_2$ when $\frac{Q^2}{Q^2_s}\gg 1$ from the position of the singularity closest to the origin on the real axis of the Borel plane. In finite length matter the OPE series is not convergent but it is Borel summable. When a small dipole, and the string connecting the ends of the dipole, pass through hot matter there is an induced motion of the string in the $5^{th}$ dimension. When $T^4 L \cosh η$, with the $η$ the rapidity of the string, is large enough the string would normally break into several parts after leaving the medium, however, this cannot happen in the classical approximation in which we work.

hep-th↗
arXiv2008

Comparing energy loss and $p_{\perp}$-broadening in perturbative QCD with strong coupling $\mathcal{N}=4$ SYM theory

We compare medium induced energy loss and $p_{\perp}$-broadening in perturbative QCD with that of the trailing string picture of SYM theory. We consider finite and infinite extent matter as well as relativistic heavy quarks which correspond to those being produced in the medium or external to it. When expressed in terms of the appropriate saturation momentum, we find identical parametric forms for energy loss in perturbative QCD and SYM theory. We find simple correspondences between $p_{\perp}$-broadening in QCD and in SYM theory although $p_{\perp}$-broadening is radiation dominated in SYM theory and multiple scattering dominated in perturbative QCD.

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