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C. Pajares

Publications and source records attributed to C. Pajares.

At least 91 records · Page 5Linked to original sources
arXiv2004

Colour strings vs. hard pomeron in perturbative QCD

Average multiplicities and transverse momenta in AA collisions are studied in the soft and hard regions, in fusing string and perturbative QCD scenarios respectively. Striking similarities are found between the predictions of the two approaches. Multiplicities per string and average p_t^2 are found to, respectively, drop and rise with A in a very similar manner, so that their product is nearly a constant. In both approaches total multiplicities grow as A, that is as the number of participants. The high tail of the p_t distribution is found to behave as A^1.1 in the perturbative QCD scenario.

hep-ph↗
arXiv2003

Transverse momentum fluctuations and percolation of strings

The behaviour of the transverse momentum fluctuations with the centrality of the collision shown by the Relativistic Heavy Ion Collider data is naturally explained by the clustering of color sources. In this framework, elementary color sources --strings-- overlap forming clusters, so the number of effective sources is modified. These clusters decay into particles with mean transverse momentum that depends on the number of elementary sources that conform each cluster, and the area occupied by the cluster. The transverse momentum fluctuations in this approach correspond to the fluctuations of the transverse momentum of these clusters, and they behave essentially as the number of effective sources.

hep-ph↗
arXiv2003

Correlations between multiplicities and average transverse momentum in the percolating colour strings approach

Long range correlations multiplicity-multiplicity, $p_T^2$-multiplicity and $p^2 - p^2_T$ are studied in the percolating colour string picture under different assumptions of the dynamics of string interaction. It is found that the strength of these correlations is rather insensitive to these assumptions nor to the geometry of formed fused string clusters. Both multiplicity-multiplicity and $p_T^2$-multiplicity correlations are found to scale and depend only on the string density. The $p_T^2$-multiplicity correlations, which are absent in the independent string picture, are found to be of the order of 10% for central heavy ion collisions and can serve as a clear signature of string fusion. In contrast the $p^2_T - p^2_T$ correlations turned out to be inversely proportional to the number of strings and so very small for relaistic collisions.

hep-ph↗
arXiv2002

Centrality dependence of the multiplicity and transverse momentum distributions at RHIC and LHC and the percolation of strings

The dependence of the multiplicity and the transverse momentum distribution on the number of collisions are studied for central and peripheral Au-Au collisions at SPS, RHIC and LHC energies in the framework of percolation of strings. A scaling law relating the multiplicity to the mean transverse momentum is obtained. Our results are in overall agreement with the SPS and RHIC data, obtaining a suppression on $p_T$ distribution even for $p_T$ larger than 1 GeV/c.

hep-ph↗
arXiv2002

On the jet spectrum in nucleus-nucleus interactions

We derive the inclusive transverse spectrum of minijets in nuclear collisions at very high energies by assuming that the nuclear S-matrix factorizes into a product of elastic S-matrices for elementary partonic collisions. Interference effects and, in particular, the contribution of loop diagrams are fully taken into account in the derivation of the spectrum, which is shown to coincide with the result earlier obtained by superposing the elementary interactions incoherently. A quantitative analysis confirms that the deformation induced by multiple collsions is a large effect at RHIC and LHC energies, for transverse momenta ~ 20 GeV.

hep-ph↗
arXiv2002

Genuine Three-Body Bose-Einstein Correlations and Percolation of Strings

Recent data show a large difference of the genuine three-body Bose-Einstein correlations in S-Pb collisions and in Pb-Pb central collisions being close to zero in the first case and to one in the second one. These results, unexpected from conventional approaches, are naturally explained by the percolation of colour strings produced in the collisions and subsequent incoherent fragmentation of the formed clusters.

hep-ph↗
arXiv2002

Analysis of the first RHIC results in the String Fusion Model

First results from RHIC on charged multiplicities, evolution of multiplicities with centrality, particle ratios and transverse momentum distributions in central and minimum bias collisions, are analyzed in a string model which includes hard collisions, collectivity in the initial state considered as string fusion, and rescattering of the produced secondaries. Multiplicities and their evolution with centrality are successfully reproduced. Transverse momentum distributions in the model show a larger $p_T$-tail than experimental data, disagreement which grows with increasing centrality. Discrepancies with particle ratios appear and are examined comparing with previous features of the model at SPS.

hep-ph↗
arXiv2001

Cumulative particle production and percolation of strings

Calculations of the production rate of particles with $x>1$ in nuclear collisions due to the interaction of colour strings are presented. Momentum and colour sum rules are used to determine the fragmentation functions of fused strings. Mechanisms of the string interaction are considered with total and partial overlapping in the transverse plane. The results reveal strong dependence of the chosen mechanism. In the percolation scenario with partial overlapping the $x$-dependence of the production rates agrees well with the existing data. The magnitude of the rates for $π^+$ production is in agreement with experiment. However the rates for the protons are substantially below the data.

hep-ph↗
arXiv2001

$J/Ψ$ suppression at SPS, RHIC and LHC energies and the percolation of strings

We study the enhancement of $c \bar c$ pair production that takes place in central heavy ion collisions due to the formation of clusters of strings. These clusters produce heavy flavors more efficiently due to their higher color. We discuss the competition between this mechanism and the well-known screening of color charge, which, above a critical string density, reduces strongly the probability of binding these $c \bar c$ pairs to form $J/Ψ$ particles. The dependence of $J/Ψ$ suppression on the centrality shows a peak at both RHIC and LHC energies corresponding to the percolation critical density.

hep-ph↗
arXiv2001

Monte Carlo model for nuclear collisions from SPS to LHC energies

A Monte Carlo model to simulate nuclear collisions in the energy range going from SPS to LHC, is presented. The model includes in its initial stage both soft and semihard components, which lead to the formation of color strings. Collectivity is taken into account considering the possibility of strings in color representations higher than triplet or antitriplet, by means of string fusion. String breaking leads to the production of secondaries. At this point, the model can be used as initial condition for further evolution by a transport model. In order to tune the parameters and see the results in nucleus-nucleus collisions, a naif model for rescattering of secondaries is introduced. Results of the model are compared with experimental data, and predictions for RHIC and LHC are shown.

hep-ph↗
arXiv2000

Baryon and strangeness enhancement at SPS, RHIC and LHC energies in the String Fusion Model

Strangeness and baryon enhancement in heavy ion collisions are discussed in the framework of the String Fusion Model. The Monte Carlo version of this model is shown to reasonably reproduce three of the features that have been pointed out as evidences of the finding of the Quark Gluon Plasma. Namely hyperon/antihyperon enhancement, dependence of the slope of the $p_{T}$ distributions on the mass of the produced particles and on the centrality of the collision, are described. Predictions for RHIC and LHC energies are presented.

hep-ph↗
arXiv2000

On the forward-backward correlations in a two-stage scenario

It is demonstrated that in a two-stage scenario with elementary Poissonian emitters of particles (colour strings) arbitrarily distributed in their number and average multiplicities, the forward- backward correlations are completely determined by the final distribution of the forward particles. The observed linear form of the correlations then necessarily requires this distribution to have a negative binomial form. For emitters with a negative binomial distribution of the produced particles distributed so as to give the final distribution also of a negative binomial form, the forward-backward correlations have an essentially non-linear form, which disagrees with the experimental data.

hep-ph↗
arXiv2000

Transverse momentum distributions and their forward- backward correlations in the percolating colour string approach

The forward-backward correlations in the $p_T$ distributions, which present a clear signature of non-linear effects in particle production, are studied in the model of percolating colour strings. Quantitative predictions are given for these correlations at SPS, RHIC and LHC energies. Interaction of strings also naturally explains the flattening of $p_T$ distributions and increase of $ $ with energy and atomic number for nuclear collisions

hep-ph↗
arXiv2000

Chaotic sources and Percolation of strings

It is shown that different ways of interacting strings formed in high energy nucleus-nucleus collisions cause a different strength of the chaoticity parameter lambda of Bose-Einstein correlations. In particular, in the case of percolation of strings, lambda shows a peculiar dependence on the string density, very similar to the dependence of the fractional average cluster size. In both, the derivative on the string density is maximum at the critical point. The reasonable agreement with the existing experimental data indicates that percolation of strings can actually occurs.

hep-ph↗
arXiv2000

Consequences of parton's saturation and string's percolation on the developments of cosmic ray showers

At high gluon or string densities, gluons' saturation or the strong interaction among strings, either forming colour ropes or giving rise to string's percolation, induces a strong suppression in the particle multiplicities produced at high energy. This suppression implies important modifications on cosmic ray shower development. In particular, it is shown that it affects the depth of maximum, the elongation rate, and the behaviour of the number of muons at energies around 10^{17}-10^{18} eV. The existing cosmic ray data point out in the same direction.

astro-ph↗
arXiv2000

Central rapidity densities of charged particles at RHIC and LHC

Predictions on central rapidity densities of charged particles at energies of the Relativistic Heavy Ion Collider and the Large Hadron Collider, for central collisions between the largest nuclei that will be available at these accelerators, are reviewed. Differences among the results of the existing models are discussed in relation with their underlying physical basis and with the possibilities to discriminate them.

hep-ph↗