Search

From our previously obtained shear viscosity to entropy density ratio ($\eta/s$) in the framework of clustering of color sources (Color String Percolation Model: CSPM), we calculate the jet quenching parameter $\hat {q}$ and trace anomaly $\Delta = (\varepsilon -3\it p)/T^{4}$ as a function of temperature. It is shown that the scaled $\hat {q}/T^{3}$ is in agreement with the recent JET Collaboration estimates. The inverse of $\eta/s$ is found to represent $\Delta$. The results for $\Delta$ are in excellent agreement with Lattice Quantum Chromo Dynamics (LQCD) simulations. From the trace anomaly and energy density $\epsilon$, the equation of state is obtained as a function of temperature and compared with LQCD simulations. It is possible that there is a direct connection between the $\eta/s$ and $\Delta$. Thus the estimate of transport coefficient $\eta/s$ provides $\hat {q}$ and $\Delta$ as a function of temperature. Both $\Delta$ and $\eta/s$ describe the transition from a strongly coupled QGP to a weakly coupled QGP., Comment: 5 pages, 4 figures, Accepted for publication in Phys. Rev. C. arXiv admin note: text overlap with arXiv:1501.01524

A brief introduction of the relationship of string percolation to the Quantum Chromo Dynamics (QCD) phase diagram is presented. The behavior of the Polyakov loop close to the critical temperature is studied in terms of the color fields inside the clusters of overlapping strings, which are produced in high energy hadronic collisions. The non-Abelian nature of the color fields implies an enhancement of the transverse momentum and a suppression of the multiplicities relative to the non overlapping case. The prediction of this framework are compared with experimental results from the SPS, RHIC and LHC for $pp$ and AA collisions. Rapidity distributions, probability distributions of transverse momentum and multiplicities, Bose-Einstein correlations, elliptic flow and ridge structures are used to evaluate these comparison. The thermodynamical quantities, the temperature, and energy density derived from RHIC and LHC data and Color String Percolation Model (CSPM) are used to obtain the shear viscosity to entropy density ratio ($\eta/s$). It was observed that the inverse of ($\eta/s$) represents the trace anomaly $\Delta =(\varepsilon-3P)/T^{4}$. Thus the percolation approach within CSPM can be successfully used to describe the initial stages in high energy heavy ion collisions in the soft region in high energy heavy ion collisions. The thermodynamical quantities, temperature and the equation of state are in agreement with the lattice QCD calculations. Thus the clustering of color sources has a clear physical basis although it cannot be deduced directly from QCD., Comment: 107 pages, 37 figures

The presence of scaling variables in experimental observables provide very valuable indications of the dynamics underlying a given physical process. In the last years, the search for geometric scaling, that is the presence of a scaling variable which encodes all geometrical information of the collision as well as other external quantities as the total energy, has been very active. This is motivated, in part, for being one of the genuine predictions of the Color Glass Condensate formalism for saturation of partonic densities. Here we extend these previous findings to the case of experimental data on elliptic flow. We find an excellent scaling for all centralities and energies, from RHIC to LHC, with a simple generalization of the scaling previously found for other observables and systems. Interestingly the case of the photons, difficult to reconcile in most formalisms, nicely fit the scaling curve. We discuss the possible interpretations of this finding in terms of initial or final state effects., Comment: 6 pages, 4 figures, accepted for publication in Phys Rev C

We test the validity of the black disk limit in elastic scattering by studying the evolution of the dip in the scaling variable $\tau=-t_{D}\sigma^{tot}$, where $t_{D}$ is the transverse momentum squared at the dip and $\sigma_{tot}$ the total cross section. As $s\rightarrow \infty $ and $-t_{D} \rightarrow 0 $, $\tau$ may consistently be approaching the black disc value, $\tau \xrightarrow[ \sqrt{s}\rightarrow \infty ]{} \tau_{BD}=35.92$ GeV$^{2}$ mb., Comment: 6 pages, 2 figures

Ultra High Energy Cosmic Rays with energies above ~ 10^18 eV provide an unique window to study hadronic interactions at energies well above those achieved in the largest man-made accelerators. We argue that at those energies string percolation may occur and play an important role on the description of the induced Extensive Air Showers by enhancing strangeness and baryon production. This leads to a significant increase of the muon content of the cascade in agreement with recent data collected at UHECR experiments. In this work, the effects of string percolation in hadronic interactions are implemented in an EAS code and their impact on several shower observables is evaluated and discussed.

We study the evolution of the particle density, dn/d\eta at fixed \eta with the beam rapidity Y in the framework of string percolation model. Our main results are: (i) The width of the "plateau" increases proportionally to Y, (ii) limiting fragmentation is violated, and (iii) the particle density, reduces to a step function., Comment: 8 pages, 2 figures, to appear in Nuclear Physics A. Minor changes are done

Using the Dirac procedure to treat constraints dynamical sistems applied to gravitation, as described in the context of Teleparallel Equivalent of General Relativity (TEGR), we investigate, from the first class constraints, the gauge transformations in the fundamental field: the components of tetrads. We have shown that there is no an isotropy in physical space with respect to gauge transformations, i.e., given an arbitrary gravitational field, coming from a gauge transformation in the internal space, physical space reacts differently in the spatial and temporal components. By making an appropriate choice, we have found a gauge transformation for the components of tetrad field that allows a direct analogy with the gauge transformations of the Yang-Mills theory. In addition, to the asymptotically flat case in which the algebra index is fixed, we recover the transformations of Electromagnetism. Moreover, still considering the asymptotically flat case, the dependence of the gauge transformation parameter in the space-time variables is periodic, just like in the Electromagnetism. Furthermore, the gravitoelectric and gravitomagnetic fields as have recently been defined, make sense since they allow a direct relation with the momenta, which is analogous to what occurs in other gauge theories., Comment: 13 pages

We argue that there are two mechanisms responsable for the growth of high energy cross-sections - \sigma_{tot} and \sigma_{el}, say - in pp collisions. One is by the increase of matter \emph{density}, resulting in the black disk saturation. The other is by exclusively radial expansion affecting the periphery of the overlap area. Within this simple model we can reproduce all available data in pp from ISR to LHC. In order to achieve a fast growth in the very high energy cosmic ray energies, we propose a fast black disk saturation followed by the dominance of geometric scaling (GS).

The Color String Percolation Model (CSPM) is used to determine the shear viscosity to entropy ratio ($\eta/s$) of the Quark-Gluon Plasma (QGP) produced in Au-Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at RHIC and Pb-Pb at $\sqrt{s_{NN}}$ = 2.76 TeV at LHC. The relativistic kinetic theory relation for $\eta/s$ is evaluated using CSPM values for the temperature and the mean free path of the QGP constituents. The experimental charged hadron transverse momentum spectrum is used to determine the percolation density parameter $\xi$ in Au-Au collisions (STAR). For Pb-Pb at $\sqrt{s_{NN}}$ = 2.76 TeV $\xi$ values are obtained from the extrapolation at RHIC energy. The value of $\eta/s$ is 0.204$\pm$0.020 and 0.262$\pm$0.026 at the CSPM initial temperatures of 193.6$\pm$3 MeV (RHIC) and 262.2 $\pm$13 MeV (LHC) respectively. These values are 2.5 and 3.3 times the AdS/CFT conjectured lower bound $1/4\pi$. We compare the CSPM $\eta/s$ analytic expression with weak coupling (wQGP) and strong coupling (sQGP) calculations. This indicates that the QGP is a strongly coupled fluid in the phase transition region., Comment: 4 pages, 3 figures Accepted for publication in European Physical Journal C (Particles & Fields)

We use geometric scaling invariant quantities to measure the approach, or not, of the imaginary and real parts of the elastic scattering amplitude, to the black disk limit, in $pp$ collisions at very high energy., Comment: 11 pages, 4 figures

The percolation of strings gives a good description of the RHIC experimental data on the elliptic flow, v2 and predicted a rise on the integrated v2 of the order of 25% at LHC such as it has been experimentally obtained. We show that the dependence of v2 on pT for RHIC and LHC energies is approximately the same as it has been observed, for all the centralities. We show the results for different particles and the dependence of v2 on the centralities and rapidity. Our results are compatible with an small value of the ratio eta/s in the whole energy range such as it was expected in the percolation framework., Comment: 20 pages, 12 figures

The similarities between string percolation and Glasma results are emphasized, special attention being paid to rapidity long range correlations, ridge structure and elliptic flow. As the string density of high multiplicity pp collisions at LHC energies has similar value as the corresponding to Au-Au semi-central collisions at RHIC we also expect in pp collisions long rapidity correlations and ridge structure, extended more than 8 units in rapidity., Comment: 3 pages, 3 figures, conference Quark Confinement and the hadron spectrum IX

We compare string percolation phenomenology to Glasma results on particle rapidity densities, effective string or flux tube intrinsic correlations, the ridge phenomena and long range forward-backward correlations. Effective strings may be a tool to extend the Glasma to the low density QCD regime. A good example is given by the minimum of the negative binomial distribution parameter k expected to occur at low energy/centrality., Comment: Submitted to Phys. Lett B

We discuss the elliptic flow dependence on pseudorapidity and number of participating nucleons in the framework of string percolation, and argue that the geometry of the initial overlap region of interaction, projected in the impact parameter plane, determines the experimentally measured azimuthal asymmetries. We found good agreement with data., Comment: 7 pages, 2 figures

Based on the non-linear logistic equation we study, in a qualitative and semi-quantitative way, the evolution with energy and saturation of the elastic differential cross-section in $pp(\bar{p}p)$ collisions at high energy. Geometrical scaling occurs at the black disk limit, and scaling develops first for small values of the scaling variable $|t|\sigma_{tot.}$. Our prediction for $d \sigma / \ d t$ at LHC, with two zeros and a minimum at large $|t|$ differs, as far as we know, from all existing ones., Comment: Latex, 7 pages, 3 figures

Saturation models -- colour glass condensate and string percolation -- impose a strict relation between the average transverse momentum, , and the rapidity particle densities, dn/dy. By combining this relation with an appropriate evolution equation for dn/dy, and imposing energy-momentum conservation, we obtain a fair description of data, for generic AB collisions (hadron-hadron, hadron-nucleus and nucleus-nucleus) at all rapidities and (high) energies. Predictions are given for the LHC., Comment: 16 pages, 4 figures, 2 tables

The collective interactions of many partons in the first stage of the collisions is the usual accepted explanation of the sizable elliptical flow. The clustering of color sources provides a framework of partonic interactions. In this scheme, we show a reasonable agreement with RHIC data for pT<1.5 GeV/c in both the dependence of v2 transverse momentum and in the shape of the nuclear modified factor on the azimuthal angle for different centralities. We show the predictions at LHC energies for Pb-Pb. In the case of proton-proton collisions a sizable v2 is obtained at this energy., Comment: To appear in Journal of Physics G

In nuclear collisions, a sizable fraction of the available energy is carried away by baryons. As the baryon number is conserved, the net-baryon $B-\bar{B}$ retains information on the energy-momentum carried by the incoming nuclei. A simple and consistent model for net-baryon production in high energy proton-proton and nucleus-nucleus collisions is presented. The basic ingredients of the model are valence string formation based on standard PDFs with QCD evolution and string fragmentation via the Schwinger mechanism. The results of the model are presented and compared with data at different centre-of-mass energies and centralities, as well as with existing models. These results show that a good description of the main features of net-baryon data is possible in the framework of a simplistic model, with the advantage of making the fundamental production mechanisms manifest., Comment: 9 pages, 12 figures; in fig. 11 a) the vertical scale was corrected

We argue that string percolation is in the origin of i) an approximately flat rapidity distribution and of ii) an approximately constant forward-backward correlation parameter $b$ over a substantial fraction of the available rapidity. Predictions are given for $pp$ collisions at LHC, $\sqrt{s}=14$TeV and $\sqrt{s}=5.5$TeV., Comment: 9 pages, 4 figures (eps), latex

We show that several effects considered nuclear effects are not nuclear in the sense that they do not only occur in nucleus-nucleus and hadron-nucleus collisions but, as well, they are present in hadron-hadron (proton-proton) collisions. The matter creation mechanism in hh, hA and AA collisions is always the same. The pT suppression of particles produced in large multiplicity events compared to low multiplicity events, the elliptic flow and the Cronin effect are predicted to occur in pp collisions at LHC energies as a consequence of the obtained high density partonic medium.

We present our results on transverse momentum fluctuations, multiplicity fluctuations and transverse momentum distributions for baryons and mesons in the framework of the clustering of color sources. We determine under what conditions the initial state configurations can lead to color connection, and more specifically, if variations of the initial state can lead to a transition from disconnected to connected color clusters, modifying the number of effective sources. We find that beyond a critical point, one has a condensate, containing interacting and hence color-connected sources. This point thus specifies the onset of color deconfinement. We show that the transverse momentum and multiplicity distributions are related to each other in a defined way. We obtain a non-monotonic dependence of the $p_T$ and multiplicity fluctuations with the number of participants. We present our results for the fluctuations and the transverse momentum distributions at RHIC energies compared to the existing experimental data and our predictions for LHC energies., Comment: 4 pages, 7 figures. Contribution to QM2008, to be published in Indian Journal of Physics

The effects of pions for vacuum polarization in background magnetic fields are considered. The effects of quark condensates is also briefly addresses. Although these effects are out of the measurement accuracy of laboratory experiments they may be relevant for gamma-ray burst propagation. In particular, for emissions from the center of the galaxy, we show that the mixing between the neutral pion and photons results in a deviation of the gamma-ray spectrum from the standard power-law in the TeV range., Comment: 6 pages, 3 figures; Based in arXiv:0707.4200 with a new very short discussion in qq condensates. To appear in the Proceedings of Scadron'70

We argue that the increase of the ratio baryon/meson due to the presence of strong colour fields and percolation in ultra-high energy hadronic collisions, helps to explain some of the global features of ultra-high energy cosmic ray cascades at E>10^18 eV and, in particular the observed excess in the number of muons with respect to current models of hadronic interactions. A reasonable agreement with the small value and slope of the average depth of shower maximum Xmax vs shower energy -- as seen in data collected at the Pierre Auger Observatory -- can be obtained with a fast increase of the p-Air production cross-section compatible with the Froissart bound., Comment: 12 pages, 3 Figs

Proceedings of the 12th International Conference on Elastic and Diffractive Scattering (Blois Workshop) - Forward Physics and QCD, Comment: Proceedings of the 12th International Conference on Elastic and Diffractive Scattering (Blois Workshop) - Forward Physics and QCD, 549 pages replaced to include list of conveners

In the framework of percolation of strings, the transverse momentum distributions in AA and hh collisions at all centralities and energies follow a universal behavior. The width of these distributions is related to the width of the distribution of the size of the clusters formed from the overlapping of the produced strings. The difference between the distributions for baryons and mesons originates in the fragmentation of clusters of several strings which enhance the particles with higher number of constituents. The results agree with SPS and RHIC data. The predictions for LHC show differences for baryons compared with RHIC. At LHC energies we obtain also a high pt suppression for pp high multiplicity events compared with pp minimum bias., Comment: Accepted for publication in Eur.Phys.J.C

It is well known that, in nuclear collisions, a sizable fraction of the available energy is carried away by baryons. As the baryon number is conserved, the net-baryon $B-\bar{B}$ retains information on the energy-momentum carried by the incoming nuclei. A simple but consistent model for net-baryon production in high energy hadron-hadron, hadron-nucleus and nucleus-nucleus collisions is presented. The basic ingredients of the model are valence string formation based on standard PDFs with QCD evolution and string fragmentation via the Schwinger mechanism. The results of the model are presented and compared with both data and existing models. These results show that a good description of the main features of net-baryon data is possible on the framework of a simplistic model, with the advantage of making the fundamental production mechanisms manifest.

We discuss forward-bacward rapidity correlations in the general situation of asymmetrical collisions, asymmetric rapidity windows, higher rapidities and higher energy. We give predictions for RHIC and LHC., Comment: 6 pages, 3 figures

We use a simple Colour Glass Condensate/String Percolation Model argument to show the existence, due to energy conservation, of bounds to the violation of Feynman scaling and limiting fragmentation., Comment: 5 pages, 1 figure. Final version

We compute QCD corrections to QED calculations for vacuum polarization in background magnetic fields. Formally, the diagram for virtual $e\bar{e}$ loops is identical to the one for virtual $q\bar{q}$ loops. However due to confinement, or to the growth of $\alpha_s$ as $p^2$ decreases, a direct calculation of the diagram is not allowed. At large $p^2$ we consider the virtual $q\bar{q}$ diagram, in the intermediate region we discuss the role of the contribution of quark condensates $\left< q\bar{q}\right>$ and at the low-energy limit we consider the $\pi^0$, as well as charged pion $\pi^+\pi^-$ loops. Although these effects seem to be out of the measurement accuracy of photon-photon laboratory experiments they may be relevant for $\gamma$-ray burst propagation. In particular, for emissions from the center of the galaxy (8.5 kpc), we show that the mixing between the neutral pseudo-scalar pion $\pi_0$ and photons renders a deviation from the power-law spectrum in the $TeV$ range. As for scalar quark condensates $\left< q\bar{q} right>$ and virtual $q\bar{q}$ loops are relevant only for very high radiation density $\sim 300 MeV/fm^3$ and very strong magnetic fields of order $\sim 10^{14} T$., Comment: 15 pages, 4 figures; Final version

We present predictions, from a simple model, for multiplicity distributions in rapidity for Pb-Pb and p-Pb central collisions at the LHC., Comment: 2 pages, 2 figures, Contributed to Workshop on Heavy Ion Collisions at the LHC: Last Call for Predictions, Geneva, Switzerland, 14 May - 8 Jun 2007

We present a prediction for Forward-Backward rapidity correlations at the LHC from a two step scenario model for particle production, Comment: 2 pages, 1 figure, Contributed to Workshop on Heavy Ion Collisions at the LHC: Last Call for Predictions, Geneva, Switzerland, 14 May - 8 Jun 2007

We explore the relationship between the Glasma framework and the String Percolation Model by introducing a simple model for the rapidity distributions in nucleus-nucleus collisions. The model is solved for both symmetrical and asymmetrical collisions. The phenomenon of limiting fragmentation is briefly discussed., Comment: 13 pages, 2 figures; final version

In the framework of Quark-Gluon String Model we calculate the inclusive spectra of secondaries produced in heavy ion collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. We demonstrate that the mechanism of secondary production changed drastically in the energy interval $\sqrt{s} = 20 - 60$ GeV and that is in agreement with qualitative estimates of Glauber-Gribov theory. The results of numerical calculations at intermediate energies are in reasonable agreement with the data. At RHIC energies numerically large inelastic screening corrections should be accounted for in calculations., Comment: 17 pages, 7 figures

We argue that two-step models, like String and Glasma models, with creation first of sources extended in rapidity that after locally decay into particles, lead to long range forward-backward correlations due to fluctuations in the number or the colour of the sources, and to a similar expression for the {\it F-B} correlation parameter $b$. In the simplest String percolation scenario, $b$ increases with centrality or the number of participating nucleons. However, asymptotically it decreases with energy. This is different from the Glasma model where $b$ increases with both, centrality and energy., Comment: 7 pages, 3 eps figures

A numerical estimative error of the magnitude of the vacuum polarization effect due to electron-positron by a factor of $10^3$ in [PRL97(2006)100403; 269901(E)] invalidates the study carried in this work in its present context., Comment: Withdrawn: see abstract

This paper has been withdrawn by the authors, due to issues concerning the use of unpublished experimental data., Comment: This paper has been withdrawn

In this work we argue that, in the interpretation of the energy dependence of the depth of the shower maximum and of the muon content in high energy cosmic ray showers ($E > 10^{17}$ eV), other variables besides the composition may play an important role, in particular those characterising the first (high energy) hadronic collisions. The role of the inelasticity, of the nature of the leading particle, and of the particle multiplicity are discussed. A consistent interpretation of existing data within a string percolation model implemented in a hybrid, one dimensional simulation method is given., Comment: 15 pages, 6 eps figures

We argue that clustering of color sources, leading to the percolation transition, may be the way to achieve deconfinement in heavy ion collisions. The critical density for percolation is related to the effective critical temperature of the thermal bath asociated to the presence of strong color fields inside the percolating cluster. We find that the temperature is rapidity, centrality and energy dependent. We emphasize the similarities of percolation of strings with color glass condensate., Comment: LaTeX, 9 pages

We show that in the presence of a slowly rotating strong transverse magnetic field there is an infinite spectrum of harmonic wave functions $A_n$ due to the first order QED correction (in $\alpha^2$) given by the Euler-Heisenberg Lagrangian. The frequency shifts are integer multiples $\pm \omega_0 n$ of the magnetic field angular frequency rotation $\omega_0=2\pi\nu_m$ and the several modes $n$ are coupled to the nearest harmonics $n\pm 1$. This is a new effect due to QED vacuum fluctuations, not exploit before, that can explain, both qualitatively and quantatively, the recent experimental results of the PVLAS collaboration without the need of a low mass intermediate particle, hence dismiss the recent claim of the discovery of the axion. [Erratum: The PVLAS signal cannot be explained by this mechanism.], Comment: 4 pages, 1 figure; includes Erratum: The PVLAS signal cannot be explained by this mechanism

The observed limiting fragmentation of charged particle distributions in heavy ion collisions is difficult to explain as it does not apply to the proton spectrum itself. On the other hand, string percolation provides a mechanism to regenerate fast particles, eventually compensating the rapidity shift (energy loss) of the nucleons. However a delicate energy-momentum compensation is required, and in our framework we see no reason for limiting fragmentation to be exact. A prediction, based on percolation arguments, is given for the charged particle density in the full rapidity interval at LHC energy $(\sqrt s =5500 GeV)$., Comment: 9 pages, 2 figures (2 eps files), latex

This paper has been withdrawn due to an error in figure 5., Comment: This paper has been withdrawn