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Carlos A. Salgado

Publications and source records attributed to Carlos A. Salgado.

At least 127 records · Page 7Linked to original sources
arXiv2007

Hard QCD probes to quark-gluon plasma

Completely unexplored regimes of QCD, dominated by high-density/temperature effects, are available in heavy ion experiments at collider energies. The successful RHIC program shows how relevant the high transverse momentum part of the spectrum is for the characterization of the properties of the created medium. It points, as well, to interesting properties of the nuclear wave function at small fraction of momentum x, probably dominated by saturated color fields. In both domains, the imminent LHC program will provide a phase space enlarged by orders of magnitude with respect to those studied at RHIC. I will review the present status of hard probes in heavy ion collisions as well as the expectations for the LHC.

hep-ph↗
arXiv2007

A global reanalysis of nuclear parton distribution functions

We determine the nuclear modifications of parton distribution functions of bound protons at scales $Q^2\ge 1.69$ GeV$^2$ and momentum fractions $10^{-5}\le x\le 1$ in a global analysis which utilizes nuclear hard process data, sum rules and leading-order DGLAP scale evolution. The main improvements over our earlier work {\em EKS98} are the automated $χ^2$ minimization, simplified and better controllable fit functions, and most importantly, the possibility for error estimates. The resulting 16-parameter fit to the N=514 datapoints is good, $χ^2/{\rm d.o.f}=0.82$. Within the error estimates obtained, the old {\em EKS98} parametrization is found to be fully consistent with the present analysis, with no essential difference in terms of $χ^2$ either. We also determine separate uncertainty bands for the nuclear gluon and sea quark modifications in the large-$x$ region where they are not stringently constrained by the available data. Comparison with other global analyses is shown and uncertainties demonstrated. Finally, we show that RHIC-BRAHMS data for inclusive hadron production in d+Au collisions lend support for a stronger gluon shadowing at $x<0.01$ and also that fairly large changes in the gluon modifications do not rapidly deteriorate the goodness of the overall fits, as long as the initial gluon modifications in the region $x\sim 0.02-0.04$ remain small.

hep-ph↗
arXiv2006

Heavy ions theory review

Some of the new developments in the theory of heavy ion collisions are reviewed. Much of the last progress have been triggered by the high energies available at RHIC. In the near future, the LHC will extend the energy reach in heavy ions by a factor thirty and give access to new QCD regimes characterized by large densities and temperatures and corresponding modified evolution equations.

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arXiv2006

Suppression of heavy flavors at RHIC & LHC

Some of the open questions on jet quenching are expected to be clarified by measuring heavy-flavored mesons at high transverse momentum. The formalism based on radiative in-medium energy-loss, which describes other high-pt results at RHIC, gives definite predictions for the suppression of charm and beauty quarks. However, the uncertainties from both contributions to the observed electrons make the interpretation of the data difficult due to the absence of a well calibrated benchmark. We review the present situation as well as the consequences for the future LHC. We also comment on the use of heavy flavored jet angular correlations as an additional tool to study the underline dynamics of jet quenching.

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arXiv2006

Jet Shapes in Opaque Media

We present general arguments, based on medium-induced radiative energy loss, which reproduce the non-gaussian shapes of away-side di-jet azimuthal correlations found in nucleus-nucleus collisions at RHIC. A rather simple generalization of the Sudakov form factors to opaque media allowing an effective description of the experimental data is proposed.

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arXiv2005

RHIC results from LHC perspectives

RHIC data opens new ways of characterizing the medium created in a heavy ion collision by measuring particles with high transverse momentum. In addition to the observation of the predicted jet quenching, the new data on particle correlations indicate that a strong modification of the jet-like shapes is driven by the dynamical properties of the medium. I will review the lessons we are learning from RHIC and outline some new directions which could become of primary importance at the LHC.

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arXiv2005

Medium-Modified Jet Shapes in Heavy-Ion Collisions

Jet quenching has been established as one of the main tools to study the properties of the medium produced in heavy ion collisions. Most of the experimental effort has been, up to now, on the measurements of inclusive particle suppression. This observable suffers, however, of limitations due to different trigger-bias effects. The study of jets (or particle correlations) in a medium is the most promising way out for a better characterization of the medium properties. I will present how these more differential measurements can be used to study not only the density of the medium (the traditional parameter fixed by jet quenching measurements) but also more dynamical quantities as flow fields.

hep-ph↗
arXiv2005

Jet quenching

High-pt particles produced in nucleus-nucleus collisions constitute a powerful tool to study the medium properties. The energy loss resulting from the propagation of these particles in the produced medium translates into a suppression of the high-pt yields. These effects are usually associated to medium-induced gluon radiation which, in turn, predicts a broadening of the jet-like signals. Both the energy loss and the jet broadening are expected to increase proportionally to the medium density. In the more realistic case of a dynamically expanding medium, the gluon radiation becomes anisotropic due to the presence of a preferred direction in the transverse plane with respect to the axis of propagation. This anisotropy translates into deformed jet-shapes which provide new posibilities to study these flows by high-pt measurements.

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arXiv2005

Testing the Color Charge and Mass Dependence of Parton Energy Loss with Heavy-to-light Ratios at RHIC and LHC

The ratio of nuclear modification factors of high-pT heavy-flavored mesons to light-flavored hadrons (``heavy-to-light ratio'') in nucleus-nucleus collisions tests the partonic mechanism expected to underlie jet quenching. Heavy-to-light ratios are mainly sensitive to the mass and color-charge dependences of medium-induced parton energy loss. Here, we assess the potential for identifying these two effects in D and B meson production at RHIC and at the LHC. To this end, we supplement the perturbative QCD factorized formalism for leading hadron production with radiative parton energy loss. For D meson spectra at high but experimentally accessible transverse momentum (10 < pT < 20 GeV) in Pb-Pb collisions at the LHC, we find that charm quarks behave essentially like light quarks. However, since light-flavored hadron yields are dominated by gluon parents, the heavy-to-light ratio of D mesons is a sensitive probe of the color charge dependence of parton energy loss. In contrast, due to the larger b quark mass, the medium modification of B mesons in the same kinematical regime provides a sensitive test of the mass dependence of parton energy loss. At RHIC energies, the strategies for identifying and disentangling the color charge and mass dependence of parton energy loss are more involved because of the smaller kinematical range accessible. We argue that at RHIC, the kinematical regime best suited for such an analysis of D mesons is 7 < pT < 12 GeV, whereas the study of lower transverse momenta is further complicated due to the known dominant contribution of additional, particle species dependent, non-perturbative effects.

hep-ph↗
arXiv2004

Low-pT Collective Flow Induces High-pT Jet Quenching

Data on low-pT hadronic spectra are widely regarded as evidence of a hydrodynamic expansion in nucleus-nucleus collisions. In this interpretation, different hadron species emerge from a common medium that has built up a strong collective velocity field. Here, we show that the existence of a collective flow field implies characteristic modifications of high-pT parton fragmentation. We generalize the formalism of parton energy loss to the case of flow-induced, oriented momentum transfer. We also discuss how to embed this calculation in hydrodynamic simulations. Flow effects are found to result generically in characteristic asymmetries in the eta-phi-plane of jet energy distributions and of multiplicity distributions associated to high-pT trigger particles. But collective flow also contributes to the medium-induced suppression of single inclusive high-pT hadron spectra. In particular, we find that low-pT elliptic flow can induce a sizeable additional contribution to the high-pT azimuthal asymmetry by selective elimination of those hard partons which propagate with significant inclination against the flow field. This reduces at least partially the recently observed problem that models of parton energy loss tend to underpredict the large azimuthal asymmetry v2 of high-pT hadronic spectra in semi-peripheral Au+Au collisions.

hep-ph↗
arXiv2004

Relating high-energy lepton-hadron, proton-nucleus and nucleus-nucleus collisions through geometric scaling

A characteristic feature of small-x lepton-proton data from HERA is geometric scaling -- the fact that over a wide range of x and Q^2 all data can be described by a single variable $Q^2/Q_{sat}^2(x)$, with all x-dependence encoded in the so-called saturation momentum $Q_{sat}(x)$. Here, we observe that the same scaling ansatz accounts for nuclear photoabsorption cross sections and favors the nuclear dependence $Q_{sat,A}^2\propto A^αQ_{sat}^2$, $α\simeq 4/9$. We then make the empirical finding that the same A-dependence accounts for the centrality evolution of the multiplicities measured in Au+Au collisions at RHIC. It also allows to parametrize the high-p_t particle suppression in d+Au collisions at forward rapidities. If these geometric scaling properties have a common dynamical origin, then this A-dependence of $Q_{sat,A}^2$ should emerge as a consequence of the underlying dynamical model.

hep-ph↗
arXiv2004

Measuring The Collective Flow With Jets

In nucleus--nucleus collisions, high-pT partons interact with a dense medium, which possesses strong collective flow components. Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow. Both components cannot be disentangled on the basis of leading hadron spectra, but the measurement of particle production associated to high-pT trigger particles, jet-like correlations and jets, allows for their independent characterization. In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the $η\times ϕ$-plane. We argue that data on the medium-induced broadening of jet-like particle correlations in Au+Au collisions at RHIC provide a first evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.

hep-ph↗
arXiv2003

Medium-Induced Gluon Radiation off Massive Quarks Fills the Dead Cone

We calculate the transverse momentum dependence of the medium-induced gluon energy distribution radiated off massive quarks in spatially extended QCD matter. In the absence of a medium, the distribution shows a characteristic mass-dependent depletion of the gluon radiation for angles smaller than m/E, the so-called dead cone effect. Medium-modifications of this spectrum are calculated as a function of quark mass, initial quark energy, in-medium pathlength and density. Generically, medium-induced gluon radiation is found to fill the dead cone, but it is reduced at large gluon energies compared to the radiation off light quarks. We quantify the resulting mass-dependence for momentum-averaged quantities (gluon energy distribution and average parton energy loss), compare it to simple approximation schemes and discuss its observable consequences for nucleus-nucleus collisions at RHIC and LHC. In particular, our analysis does not favor the complete disappearance of energy loss effects from leading open charm spectra at RHIC.

hep-ph↗
arXiv2003

Medium Modification of Jet Shapes and Jet Multiplicities

Medium-induced parton energy loss is widely considered to underly the suppression of high-pt leading hadron spectra in 200 GeV/A Au+Au collisions at RHIC. Its description implies a characteristic kt-broadening of the subleading hadronic fragments associated to the hard parton. However, this latter effect is more difficult to measure and remained elusive so far. Here, we discuss how it affects genuine jet observables which are accessible at LHC and possibly at RHIC. We find that the kt-broadening of jet multiplicity distributions provides a very sensitive probe of the properties of dense QCD matter, whereas the sensitivity of jet energy distributions is much weaker. In particular, the sensitive kinematic range of jet multiplicity distributions is almost unaffected by the high multiplicity background.

hep-ph↗
arXiv2003

Energy Dependence of the Cronin Effect from Non-Linear QCD Evolution

The non-linear evolution of dense partonic systems has been suggested as one of the novel physics mechanisms relevant to the dynamics of hadron-nucleus and nucleus-nucleus collisions at collider energies. Here we study to what extent the description of Cronin enhancement in the framework of this non-linear evolution is consistent with the recent observation in 200 AGeV d--Au collisions at the Relativistic Heavy Ion Collider. We solve the Balitsky-Kovchegov evolution equation numerically for several initial conditions encoding Cronin enhancement. We find that the properly normalized nuclear gluon distribution is suppressed at all momenta relative to that of a single nucleon. Calculating the resulting spectrum of produced gluons in p-A and A-A collisions, we establish that the nonlinear QCD evolution is unable to generate a Cronin type enhancement, and that it quickly erases any such enhancement which may be present at lower energies.

hep-ph↗
arXiv2003

Calculating Quenching Weights

We calculate the probability (``quenching weight'') that a hard parton radiates an additional energy fraction due to scattering in spatially extended QCD matter. This study is based on an exact treatment of finite in-medium path length, it includes the case of a dynamically expanding medium, and it extends to the angular dependence of the medium-induced gluon radiation pattern. All calculations are done in the multiple soft scattering approximation (Baier-Dokshitzer-Mueller-Peigné-Schiff--Zakharov ``BDMPS-Z''-formalism) and in the single hard scattering approximation (N=1 opacity approximation). By comparison, we establish a simple relation between transport coefficient, Debye screening mass and opacity, for which both approximations lead to comparable results. Together with this paper, a CPU-inexpensive numerical subroutine for calculating quenching weights is provided electronically. To illustrate its applications, we discuss the suppression of hadronic transverse momentum spectra in nucleus-nucleus collisions. Remarkably, the kinematic constraint resulting from finite in-medium path length reduces significantly the transverse momentum dependence of the nuclear modification factor, thus leading to consistency with the data measured at the Relativistic Heavy Ion Collider (RHIC).

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arXiv2002

Determining the BDMPS transport coefficient via medium-modified fragmentation functions

In nucleus-nucleus collisions at RHIC and LHC, partons produced at high transverse momentum can undergo multiple scattering within the collision region prior to fragmenting into hadrons. We have studied the resulting medium-modified fragmentation function based on a calculation of the BDMPS-Z medium-induced gluon radiation for a dense, expanding medium of small finite extension. Here we explain how the BDMPS transport coefficient which measures the energy density attained in the collision, can be extracted from the observed modification of high-pt hadroproduction. We also comment on the significant remaining uncertainties in extracting the transport coefficient from data.

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