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Edmond Iancu

Publications and source records attributed to Edmond Iancu.

At least 55 records · Page 3Linked to original sources
arXiv2012

QCD in heavy ion collisions

These lectures provide a modern introduction to selected topics in the physics of ultrarelativistic heavy ion collisions which shed light on the fundamental theory of strong interactions, the Quantum Chromodynamics. The emphasis is on the partonic forms of QCD matter which exist in the early and intermediate stages of a collision -- the colour glass condensate, the glasma, and the quark-gluon plasma -- and on the effective theories that are used for their description. These theories provide qualitative and even quantitative insight into a wealth of remarkable phenomena observed in nucleus-nucleus or deuteron-nucleus collisions at RHIC and/or the LHC, like the suppression of particle production and of azimuthal correlations at forward rapidities, the energy and centrality dependence of the multiplicities, the ridge effect, the limiting fragmentation, the jet quenching, or the dijet asymmetry.

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arXiv2011

Interference Phenomena in Medium Induced Radiation

We consider the interference pattern for the medium-induced gluon radiation produced by a color singlet quark-antiquark antenna embedded in a QCD medium with size $L$ and `jet quenching' parameter $\hat q$. Within the BDMPS-Z regime, we demonstrate that, for a dipole opening angle $θ_{q\bar q} \ggθ_c\equiv {2}/{\sqrt{\hat q L^3}}$, the interference between the medium--induced gluon emissions by the quark and the antiquark is suppressed with respect to the direct emissions. This is so since direct emissions are delocalized throughout the medium and thus yield contributions proportional to $L$ while interference occurs only between emissions at early times, when both sources remain coherent. Thus, for $\tqq \ggθ_c$, the medium-induced radiation is the sum of the two spectra individually produced by the quark and the antiquark, without coherence effects like angular ordering. For $\tqq \llθ_c$, the medium--induced radiation vanishes.

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arXiv2011

Interference effects in medium-induced gluon radiation

As a step towards understanding the in-medium evolution of a hard jet, we consider the interference pattern for the medium induced gluon radiation produced by a color singlet quark-antiquark antenna embedded in a QCD medium with size L. We focus on the typical kinematics for medium-induced gluon radiation in the BDMPS-Z regime, that is, short formation times τ_f << L and relatively large emission angles θ>>θ_c = 2/\sqrt{\hat q L^3}, with \hat q the `jet quenching' parameter. We demonstrate that, for a dipole opening angle θ_{q\bar q} larger than θ_c, the interference between the medium-induced gluon emissions by the quark and the antiquark is parametrically suppressed with respect to the corresponding direct emissions. Physically, this is so since the direct emissions can be delocalized anywhere throughout the medium and thus yield contributions proportional to L. On the contrary, the interference occurs only between gluons emitted at very early times, within the characteristic time scales for quantum and color coherence between the two emitters, which in this regime are much smaller than L. This implies that, for θ_{q\bar q} >> θ_c, the medium-induced radiation by the dipole is simply the sum of the two BDMPS-Z spectra individually produced by the quark and the antiquark, without coherence effects like angular ordering. For θ_{q\bar q} << θ_c, the BDMPS-Z radiation by the dipole vanishes.

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arXiv2011

Multiparticle Dynamics in the LHC Era

Using the theory talks at ISMD2010 as a guidance, I present a personal review of our current understanding of multiparticle interactions in QCD. For more clarity, I separately consider hard, semi-hard, and soft interactions, and I devote most of the space to those phenomena for which progress has been recently made from first principles. Also, priority is given to processes which are directly relevant for QCD studies at the LHC, notably to forward particle production and ultrarelativistic heavy ion collisions.

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arXiv2010

Parton saturation at strong coupling from AdS/CFT

I describe the parton picture at strong coupling emerging from the gauge/gravity duality, with emphasis on the universality of the phenomenon of parton saturation. I discuss several consequences of this picture for the phenomenology of a strongly coupled quark-gluon plasma, which are potentially relevant for heavy ion collisions at RHIC and LHC.

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arXiv2009

CCFM Evolution with Unitarity Corrections

We considerably extend our previous analysis of the implementation of an absorptive boundary condition, which mimics saturation effects, on the linear CCFM evolution. We present detailed results for the evolution of the unintegrated gluon density in the presence of saturation and extract the energy dependence of the emerging saturation momentum. We show that CCFM and BFKL evolution lead to almost identical predictions after including the effects of gluon saturation and of the running of the coupling. We moreover elucidate several important and subtle aspects of the CCFM formalism, such as its relation to BFKL, the structure of the angular ordered cascade, and the derivation of more inclusive versions of CCFM. We also propose non--leading modifications of the standard CCFM evolution which may play an important role for phenomenological studies.

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arXiv2009

BFKL and CCFM evolutions with saturation boundary

We perform numerical studies of the BFKL and CCFM equations for the unintegrated gluon distribution supplemented with an absorptive boundary which mimics saturation. For the BFKL equation, this procedure yields the same results for the saturation momentum and the gluon distribution above saturation as the non-linear BK equation, for both fixed and running coupling, and for all the considered energies. This similarity goes beyond expectations based on the correspondence with statistical physics, which hold only for fixed coupling and asymptotically high energies. For the CCFM equation, whose non-linear generalization is not known, our method provides the first study of the approach towards saturation. We find that, in the running-coupling case, the CCFM and BFKL predictions for the energy dependence of the saturation momentum are identical within our numerical accuracy. A similar saturation boundary could be easily implemented in the CCFM-based Monte Carlo event generators, so like CASCADE.

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arXiv2009

Color Glass Condensate and its relation to HERA physics

I give a brief overview of the effective theory for the Color Glass Condensate, which is the high-density gluonic matter which controls high-energy scattering in QCD in the vicinity of the unitarity limit. I concentrate on fundamental phenomena, like gluon saturation, unitarization, and geometric scaling, and the way how these are encoded in the formalism. I emphasize the importance of the next-to-leading order corrections, especially the running of the coupling, for both conceptual and phenomenological issues. I survey the implications of the CGC theory for the HERA physics and its phenomenological applications based on saturation models.

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arXiv2008

Partons and jets in a strongly-coupled plasma from AdS/CFT

We give a pedagogical review of recent progress towards understanding the response of a strongly coupled plasma at finite temperature to a hard probe. The plasma is that of the N=4 supersymmetric Yang-Mills theory and the hard probe is a virtual photon, or, more precisely, an R-current. Via the gauge/gravity duality, the problem of the current interacting with the plasma is mapped onto the gravitational interaction between a Maxwell field and a black hole embedded in the AdS_5 x S^5 geometry. The physical interpretation of the AdS/CFT results can be then reconstructed with the help of the ultraviolet/infrared correspondence. We thus deduce that, for sufficiently high energy, the photon (or any other hard probe: a quark, a gluon, or a meson) disappears into the plasma via a universal mechanism, which is quasi-democratic parton branching: the current develops a parton cascade such that, at any step in the branching process, the energy is almost equally divided among the daughter partons. The branching rate is controlled by the plasma which acts on the colored partons with a constant force ~ T^2. When reinterpreted in the plasma infinite momentum frame, the same AdS/CFT results suggest a parton picture for the plasma structure functions, in which all the partons have fallen at very small values of Bjorken's x. For a time-like current in the vacuum, quasi-democratic branching implies that there should be no jets in electron-positron annihilation at strong coupling, but only a spatially isotropic distribution of hadronic matter.

hep-ph↗
arXiv2008

Partons and jets at strong coupling from AdS/CFT

Calculations using the AdS/CFT correspondence can be used to unveil the short-distance structure of a strongly coupled plasma, as it would be seen by a `hard probe'. The results of these calculations admit a natural physical interpretation in terms of parton evolution in the plasma: via successive branchings, essentially all partons cascade down to very small values of the longitudinal momentum fraction x and to transverse momenta smaller than the saturation momentum Q_s\sim T/x. This scale Q_s controls the energy loss and the transverse momentum broadening of an energetic jet propagating through the plasma. This picture has some striking consequences, like the absence of jets in electron-proton annihilation at strong coupling, of the absence of particle production at forward and backward rapidities in hadron-hadron collisions, which look very different from the corresponding predictions of perturbative QCD and also from the known experimental situation.

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arXiv2007

Liouville field theory for gluon saturation in QCD at high energy

We argue that quantum Liouville field theory supplemented with a suitable source term is the effective theory which describes the short-range correlations of the gluon saturation momentum in the two-dimensional impact-parameter space, at sufficiently high energy and for a large number of colors. This is motivated by recent developments concerning the stochastic aspects of the high-energy evolution in QCD, together with the manifest scale invariance of the respective evolution equations and general considerations on the uncertainty principle. The source term explicitly breaks down the conformal symmetry of the (pure) Liouville action, thus introducing a physical mass scale in the problem which is identified with the average saturation momentum. We construct this source term for the case of a homogeneous distribution and show that this leads to an interesting theory: the relevant correlation functions are ultraviolet finite (and not just renormalizable) when computed in perturbation theory, due to mutual cancellations of the tadpole divergences. Possible generalizations to inhomogeneous source terms are briefly discussed.

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arXiv2006

In the Shadow of the Color Glass

I give a brief overview of recent theoretical progress within perturbative QCD concerning the high-energy dynamics in the vicinity of the unitarity limit. Special attention is payed to the most recent developments concerning the relation between high-energy QCD and statistical physics, the role of the pomeron loops, and the transition from geometric scaling to diffusive scaling with increasing energy.

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arXiv2005

From statistical physics to high energy QCD

I discuss recent progress in understanding the high-energy evolution in QCD, which points towards a remarkable correspondence with the reaction-diffusion problem of statistical physics.

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arXiv2005

Fluctuating pulled fronts & Pomerons

I give a physical discussion of the influence of particle number fluctuations on the high energy evolution in QCD. I emphasize the event-by-event description and the correspondence with the problem of `fluctuating pulled fronts' in statistical physics. I show that the correlations generated by fluctuations reduce the phase-space for BFKL evolution up to saturation. Because of that, the evolution `slows down', and the rate for the energy increase of the saturation momentum is considerably decreased. Also, the stochastic aspects inherent in fluctuations lead to the breakdown of geometric scaling and of the BFKL approximation. Finally, I explain the diagrammatic interpretation of the particle number fluctuations as initiators of the Pomeron loops.

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arXiv2004

The QCD S-matrix in the high-energy limit

I discuss elastic dipole-dipole scattering in QCD at high energies, with emphasis on the relation between Mueller's dipole picture and the Color Glass Condensate, and on the importance of rare fluctuations for the high-energy limit of the S-matrix.

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arXiv2004

Cronin effect and high-p_T suppression from the Color Glass Condensate

I give a pedagogical survey of the nuclear collective effects associated with gluon saturation and their impact on particle production in high-energy proton (or deuteron)-nucleus collisions at RHIC. At central rapidity, the theory predicts a Cronin peak due to independent multiple scattering off the valence quarks in the nucleus. At forward rapidities, the peak flattens out and disappears very fast, because of the correlations induced through quantum evolution in the nuclear gluon distribution at small x. Also, the ratio R_{pA} between the particle yield in proton-nucleus and proton-proton collisions is rapidly suppressed when increasing the rapidity, because of saturation effects which slow down the evolution of the nucleus compared to that of the proton. This behaviour could be responsible for a similar trend observed in the deuteron-nucleus collisions at RHIC.

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arXiv2003

Renormalization of Φ-derivable approximations in scalar field theories

We discuss the renormalization of Φ-derivable approximations for scalar field theories. In such approximations, the self-energy is obtained as the solution of a self-consistent equation which effectively resums infinite subsets of diagrams of perturbation theory. We show that a consistent renormalization can be carried out, and we provide an explicit construction of the counterterms needed to eliminate the subdivergences. The counterterms are calculated from the solution of an auxiliary gap equation which determines the leading asymptotic part of the self-energy. This auxiliary gap equation may be chosen as the gap equation of the massless theory at zero temperature. We verify explicitly that the counterterms determined at zero temperature are sufficient to eliminate the divergences which occur in finite temperature calculations.

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