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N. Armesto

Publications and source records attributed to N. Armesto.

At least 55 records · Page 3Linked to original sources
arXiv2006

On correlations and discreteness in non-linear QCD evolution

We consider modifications of the standard non-linear QCD evolution in an attempt to account for some of the missing ingredients discussed recently, such as correlations, discreteness in gluon emission and Pomeron loops. The evolution is numerically performed using the Balitsky-Kovchegov equation on individual configurations defined by a given initial value of the saturation scale, for reduced rapidities $y=(α_s N_c/π) Y<10$. We consider the effects of averaging over configurations as a way to implement correlations, using three types of Gaussian averaging around a mean saturation scale. Further, we heuristically mimic discreteness in gluon emission by considering a modified evolution in which the tails of the gluon distributions are cut-off. The approach to scaling and the behavior of the saturation scale with rapidity in these modified evolutions are studied and compared with the standard mean-field results. For the large but finite values of rapidity explored, no strong quantitative difference in scaling for transverse momenta around the saturation scale is observed. At larger transverse momenta, the influence of the modifications in the evolution seems most noticeable in the first steps of the evolution. No influence on the rapidity behavior of the saturation scale due to the averaging procedure is found. In the cut-off evolution the rapidity evolution of the saturation scale is slowed down and strongly depends on the value of the cut-off. Our results stress the need to go beyond simple modifications of evolution by developing proper theoretical tools that implement such recently discussed ingredients.

hep-ph↗
arXiv2006

Heavy-quark energy loss at RHIC and LHC

The attenuation of heavy-flavored particles in nucleus-nucleus collisions tests the microscopic dynamics of medium-induced parton energy loss and, in particular, its expected dependence on the identity (color charge and mass) of the parent parton. We discuss the comparison of theoretical calculations with recent single-electron data from RHIC experiments. Then, we present predictions for the heavy-to-light ratios of D and B mesons at LHC energy.

hep-ph↗
arXiv2005

How sensitive are high-pt electron spectra at RHIC to heavy quark energy loss?

In nucleus-nucleus collisions, high-pt electron spectra depend on the medium modified fragmentation of their massive quark parents, thus giving novel access to the predicted mass hierarchy of parton energy loss. Here we calculate these spectra in a model, which supplements the perturbative QCD factorization formalism with parton energy loss. In general, we find - within large errors - rough agreement between theory and data on the single inclusive electron spectrum in pp, its nuclear modification factor, and its azimuthal anisotropy. However, the nuclear modification factor depends on the relative contribution of charm and bottom production, which we find to be affected by large perturbative uncertainties. In order for electron measurements to provide a significantly more stringent test of the expected mass hierarchy, one must then disentangle the b- and c-decay contributions, for instance by reconstructing the displaced decay vertices.

hep-ph↗
arXiv2005

Heavy-to-light ratios as a test of medium-induced energy loss at RHIC and the LHC

The ratio of nuclear modification factors of high-$p_T$ heavy-flavored mesons tolight-flavored hadrons (heavy-to-light ratio) is shown to be a sensitive tool to test medium-induced energy loss at RHIC and LHC energies. Heavy-to-light ratios of $D$ mesons at RHIC in the region $7<p_T<12$ GeV, and of $D$ and $B$ mesons at the LHC in the region $10<p_T<20$ GeV, are proposed for such a test. Finally, the different contributions to the nuclear modification factor for electrons at RHIC are analyzed. Preliminary PHENIX and STAR data are compatible with radiative energy loss provided the contribution of electrons from beauty decays is small compared to that from charm.

hep-ph↗
arXiv2005

Nuclear size and rapidity dependence of the saturation scale from QCD evolution and experimental data

The solutions of the Balitsky-Kovchegov evolution equations are studied numerically and compared with known analytical estimations. The rapidity and nuclear size dependences of the saturation scale are obtained for the cases of fixed and running coupling constant. These same dependences are studied in experimental data, on lepton-nucleus, deuteron-nucleus and nucleus-nucleus collisions, through geometric scaling and compared with the theoretical calculations.

hep-ph↗
arXiv2005

Flow effects on jet profiles and multiplicities

We study the effects of low-$p_T$ collective flow on radiative energy loss from high-$p_T$ partons traversing the QCD medium created in high-energy nucleus-nucleus collisions. We illustrate this idea through three examples. Due to longitudinal flow, jet profiles at the LHC present marked asymmetries in the $η\times ϕ$-plane, and widths in $η$ and $ϕ$ of particle distributions associated with a high-$p_T$ trigger at RHIC become different. Finally, transverse flow implies an increase of high-$p_T$ $v_2$ at RHIC.

hep-ph↗
arXiv2004

Numerical analysis of the Balitsky-Kovchegov equation with running coupling: dependence of the saturation scale on nuclear size and rapidity

We study the effects of including a running coupling constant in high-density QCD evolution. For fixed coupling constant, QCD evolution preserves the initial dependence of the saturation momentum $Q_s$ on the nuclear size $A$ and results in an exponential dependence on rapidity $Y$, $Q^2_s(Y) = Q^2_s(Y_0) \exp{[ \barα_s d (Y-Y_0) ]}$. For the running coupling case, we re-derive analytical estimates for the $A$- and $Y$-dependences of the saturation scale and test them numerically. The $A$-dependence of $Q_s$ vanishes $\propto 1/ \sqrt{Y}$ for large $A$ and $Y$. The $Y$-dependence is reduced to $Q_s^2(Y) \propto \exp{(Δ^\prime\sqrt{Y+X})}$ where we find numerically $Δ^\prime\simeq 3.2$. We study the behaviour of the gluon distribution at large transverse momentum, characterizing it by an anomalous dimension $1-γ$ which we define in a fixed region of small dipole sizes. In contrast to previous analytical work, we find a marked difference between the fixed coupling ($γ\simeq 0.65$) and running coupling ($γ\sim 0.85$) results. Our numerical findings show that both a scaling function depending only on the variable $r Q_s$ and the perturbative double-leading-logarithmic expression, provide equally good descriptions of the numerical solutions for very small $r$-values below the so-called scaling window.

hep-ph↗
arXiv2004

Jet quenching

Jet quenching studies play a prominent role in our current understanding of ultra-relativistic heavy ion collisions. In this review I first present the available formalism to compute medium-induced gluon radiation. Then I discuss its effect on single particle spectra, with dedicated attention to the case of the radiating parton being a massive quark. Next I examine more differential observables like jet shapes and multiplicities, and the consequences of flow on radiative energy loss. I conclude with some remarks.

hep-ph↗
arXiv2004

Review of Monte Carlo methods for particle multiplicity evaluation

I present a brief review of the existing models for particle multiplicity evaluation in heavy ion collisions which are at our disposal in the form of Monte Carlo simulators. Models are classified according to the physical mechanisms with which they try to describe the different stages of a high-energy collision between heavy nuclei. A comparison of predictions, as available at the beginning of year 2000, for multiplicities in central AuAu collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and PbPb collisions at the CERN Large Hadron Collider (LHC) is provided.

hep-ph↗
arXiv2004

Nuclear Shadowing and Diffraction

The relation between diffraction in lepton-proton collisions and shadowing of nuclear structure functions which arises from Gribov inelastic shadowing, is described. A model realizing such relation, which produces a parameter-free description of experimental data on nuclear structure functions at small $x$, is presented. The application to the description of multiplicities in nuclear collisions is discussed and related to other approaches.

hep-ph↗
arXiv2004

Collective behavior in nuclear interactions and shower development

The mechanism of hadronic interactions at very high energies is still unclear. Available accelerator data constrain weakly the forward rapidity region which determines the development of atmospheric showers. This ignorance is one of the main sources of uncertainty in the determination of the energy and composition of the primary in hadron-induced atmospheric showers. In this paper we examine the effect on the shower development of two kinds of collective effects in high-energy hadronic interactions which modify the production of secondary particles. The first mechanism, modeled as string fusion, affects strongly the central rapidity region but only slightly the forward region and is shown to have very little effect on the shower development. The second mechanism implies a very strong stopping; it affects modestly the profile of shower maximum but broadens considerably the number distribution of muons at ground. For the latter mechanism, the development of air showers is faster mimicking a heavier projectile. On the other hand, the number of muons at ground is lowered, resembling a shower generated by a lighter primary.

astro-ph↗
arXiv2004

Gluon Radiation off Massive Quarks in a QCD Medium

Medium-induced gluon radiation from massless and massive quarks is treated in the same formalism. The dead cone which regulates gluon radiation from massive quarks in the vacuum at small angles, is filled in the medium but constitutes a small fraction of the available phase space. Our study indicates that the energy loss for charmed hadrons at RHIC should be smaller than for light hadrons, but still sizable.

hep-ph↗
arXiv2003

Introduction to Low x Physics and Saturation

The idea of saturation of parton densities in small x physics is briefly introduced. Some aspects of saturation are described, mainly focusing on the status of our knowledge on the non-linear equations describing the high parton density regime. Implications of saturation ideas on the description of nuclear collisions at the Relativistic Heavy Ion Collider are discussed.

hep-ph↗
arXiv2003

Hard probes in heavy ion collisions at the LHC: PDFs, shadowing and $pA$ collisions

This manuscript is the outcome of the subgroup ``PDFs, shadowing and $pA$ collisions'' from the CERN workshop ``Hard Probes in Heavy Ion Collisions at the LHC''. In addition to the experimental parameters for $pA$ collisions at the LHC, the issues discussed are factorization in nuclear collisions, nuclear parton distributions (nPDFs), hard probes as the benchmark tests of factorization in $pA$ collisions at the LHC, and semi-hard probes as observables with potentially large nuclear effects. Also, novel QCD phenomena in $pA$ collisions at the LHC are considered. The importance of the $pA$ program at the LHC is emphasized.

hep-ph↗
arXiv2003

Nuclear Structure Functions at Small x from Inelastic Shadowing and Diffraction

A model for shadowing in nuclear structure functions at small $x$ and small or moderate $Q^2$ is presented using the relation with diffraction that arises from Gribov's reggeon calculus. A reasonable description of the experimental data is obtained with no fitted parameters. A comparison with predictions from other models is performed.

hep-ph↗
arXiv2003

Nuclear Structure Functions at Small x in Multiple Scattering Approaches

A simple model for nuclear structure functions in the region of small $x$ and small and moderate $Q^2$, is presented. It is a parameter-free extension, in the Glauber-Gribov approach to nuclear collisions, of a saturation model for the nucleon. A reasonable agreement with experimental data on ratios of nuclear structure functions is obtained. The unintegrated gluon distribution and the behavior of the saturation scale which result from this model are discussed.

hep-ph↗
arXiv2003

Nuclear Structure Functions at Small x from Inelastic Shadowing and Diffraction

Nuclear structure functions at small x and small or moderate $Q^2$ are studied using the relation with diffraction on nucleons which arises from Gribov's Reggeon Calculus. A reasonable description of experimental data is obtained with no fitted parameters. A comparison with other models and predictions for future lepton-ion colliders are provided. Consequences for the reduction of multiplicities in nucleus-nucleus collisions at energies of RHIC and LHC are examined.

hep-ph↗