arXiv subjects

Nestor Armesto

Publications and source records attributed to Nestor Armesto.

30 records · Page 2Linked to original sources

arXiv2010

A unitarized model of inclusive and diffractive DIS with Q2-evolution

We discuss the interplay of low-x physics and QCD scaling violations by extending the unified approach describing inclusive structure functions and diffractive production in $γ* p$ interactions proposed in previous papers, to large values of Q2. We describe the procedure of extracting, from the non-perturbative model, initial conditions for the QCD evolution that respect unitarity. Assuming Regge factorization of the diffractive structure function, a similar procedure is proposed for the calculation of hard diffraction. The results are in good agreement with experimental data on the proton structure function $F_2$ and the most recent data on the reduced diffractive cross section, $x_P σ_r^{\D(3)}$. Predictions for both $F_2$ and $F_L$ are presented in a wide kinematical range and compared to calculations within high-energy QCD.

hep-ph↗

arXiv2009

Angular-ordered parton showers with medium-modified splitting functions

Modified Altarelli-Parisi splitting functions were recenty proposed to model multi-parton radiation in a dense medium and describe jet quenching, one of most striking features of heavy-ion collisions. We implement medium-modified splitting functions in the HERWIG parton shower algorithm, which satisfies the angular ordering prescription, and present a few parton-level results, such as transverse momentum, angle and energy-fraction distributions, which exhibit remarkable medium-induced effects. We also comment on the comparison with respect to the results yielded by other implementations of medium-modified splitting functions in the framework of virtuality-ordered parton cascades.

hep-ph↗

arXiv2009

Constraint fitting of experimental data with a jet quenching model embedded in a hydrodynamical bulk medium

We present a global fit to single- and double-inclusive suppression data of high-$p_T$ particles in central Au+Au collisions at top RHIC energy. We also include in this analysis data on heavy quarks via their D and B meson semi-leptonic decays (i.e. non-photonic electrons). The analysis is based on the parton quenching weights for medium-induced gluon radiation computed in the BDMPS approximation then embedded in a hydrodynamical description of the bulk medium. Our results indicate that values of the transport coefficient $\hat q$ more than four times larger than perturbative estimates are preferred by experimental data. This confirms previous calculations based on simpler implementations of the medium geometry or only the single-inclusive suppression. We also comment on the statistical compatibility of the heavy quark data within a radiative only energy loss scenario, and on the sensitivity of the results to nuclear modification of the parton distribution functions (PDFs) and to assumptions on the energy loss during times in the collision prior to the hydrodynamical behavior.}

hep-ph↗

arXiv2009

Monte Carlo for Jet Showers in the Medium

The most commonly employed formalisms of radiative energy loss have been derived in the high- energy approximation. In its present form, it is reliable only for the medium modifications of inclusive particle spectra. Modifications to this formalism are expected to be important for less inclusive measurements. This is especially relevant for reconstructed jets in heavy-ion collisions, which are becoming available only recently. We present some ideas to overcome this limitation. Specifically, we show an implementation of radiative energy loss within a jet parton shower. This implementation has been done within the PYTHIA Monte Carlo event generator. We present the publicly available routine Q-PYTHIA and discuss some of the obtained physics results.

hep-ph↗

arXiv2009

Q-PYTHIA - a Monte Carlo implementation for jet quenching

RHIC experiments have established jet quenching as a fundamental tool in the study of hot matter in heavy-ion collisions. The energy reached is enough for high-pT inclusive particle studies and one- or two-particle correlations up to pT= 10-20 GeV. The corresponding data are successfully described by a formalism based on radiative energy losses. This formalism needs, however, to be improved for more exclusive observables, in particular for the reconstructed jet properties. Jet reconstruction is a new branch in heavy-ion collisions with very high expectations as a tool for the characterization of the medium properties. We present here an implementation of the jet quenching phenomena based on a modification at the level of the splitting functions in parton showers in standard, general purpose, Monte Carlos as PYTHIA or HERWIG.

hep-ph↗

arXiv2008

Reaction-diffusion processes in zero transverse dimensions as toy models for high-energy QCD

We examine numerically different zero-dimensional reaction-diffusion processes as candidate toy models for high-energy QCD evolution. Of the models examined -- Reggeon Field Theory, Directed Percolation and Reversible Processes -- only the latter shows the behaviour commonly expected, namely an increase of the scattering amplitude with increasing rapidity. Further, we find that increasing recombination terms, quantum loops and the heuristic inclusion of a running of the couplings, generically slow down the evolution.

hep-ph↗

arXiv2006

Long Range Forward-Backward Correlations and the Color Glass Condensate

We discuss forward-backward correlations in the mutliplicity of produced particles in heavy ion collisions. We find the Color Glass Condensate generates distinctive predictions for the long range component of this correlation. In particular, we predict the growth of the long range correlation with the centrality of the collision. We argue that the correlation for mesons is stronger than that for baryons.

hep-ph↗

arXiv2006

Jet quenching at finite `t Hooft coupling and chemical potential from AdS/CFT

Following the nonperturbative prescription for the jet quenching parameter recently proposed by Liu, Rajagopal and Wiedemann, we compute the first correction in the inverse `t Hooft coupling corresponding to string alpha' corrections in the dual background. We also consider the introduction of a chemical potential for the U(1)^3 gauged R-symmetry. While the former mildly diminishes the jet quenching parameter -this suggesting a smooth interpolation between the strong coupling and perturbative results-, the latter generically increases its value. We comment on the extension of this setup to quarks of finite mass.

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

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↗