arXiv subjects

C. Pajares

Publications and source records attributed to C. Pajares.

At least 109 records · Page 6Linked to original sources

arXiv1999

Inplication of percolation of colour strings on multiplicities, correlations and the transverse momentum

In the colour string model the impact of string percolation on multiplicities, their long-range correlations and average transverse momentum is studied. The multiplicities are shown to be damped by a simple factor which follows from the percolation theory. A clear signature of the phase transition is found to be a behaviour of the correlations for intensive observables, such as average transverse momentum, which can be detected in the high-energy heavy ion collisions.

hep-ph↗

arXiv1999

Cumulative particle production as a rare event

The generalization of the Glauber formula for cumulative production events is derived. On its basis the multiplicity distribution in such events is related to the one in the minimum bias events. As compared to the rare events of type $C$, the formula involves a shift in the arguments determined by the multiplicity from a collision with a cluster of several nucleons.

hep-ph↗

arXiv1998

Impact Parameter Dependences of the Number of Interacting Nucleons and Mean Multiplicities in High Energy Heavy Ion Interactions

We consider the dependences of the average number of interacting nucleons in high energy heavy ion collisions on the impact parameter in two cases, when the colliding nuclei have equal atomic weights, and when one nucleus is significantly more heavy in comparison with the second one. We argue that in the case of trigger of some rare event (say, $J/ψ$, or $Υ$ production) the multiplicity of the secondaries can change several times for minimum bias sample, but it should be stable in the case of central events.

hep-ph↗

arXiv1998

Multiparticle Production and Percolation of Strings

It is shown that the multiplicity distribution associated to self-shadowing events satisfies an universal equation in terms of the minimum bias distribution. The number of elementary collisions, basic in the structure of any multiple scattering, is reduced if a collective effect like percolation of strings takes place. The main consequences of the percolation are briefly discussed.

hep-ph↗

arXiv1998

Signals of phase transition in atmospheric showers

We discuss collective effects like percolation, quark gluon plasma, or string fusion and their effect on the longitudinal development of high energy showers in air. It is shown that iron-air showers could develop more slowly than expected and produce the observed change in the slope of $X_max$ at the highest energies.

hep-ph↗

arXiv1997

Probabilistic versus Field-Theoretical Description of Heavy Flavour Production off Nuclei

The absorptive corrections resulting from the rescattering of a heavy flavour (the so-called nuclear absorption) are usually calculated with a probabilistic formula valid only in the low energy limit. We extend this formula to all energies using a quantum field theoretical approach. For charmonium and bottonium we find that the absorptive corrections in the rigorous treatment are very similar to the ones in the probabilistic approach. On the contrary, at sufficiently high energy, open charm and bottom are absorbed as much as charmonium and bottonium - in spite of the fact that their absorptive cross-sections are zero, and therefore they are not absorbed in the probabilistic model. At high enough energies there are also absorptive corrections due to the shadowing of the nucleus structure function, which are present for all systems including Drell-Yan pair production. These shadowing corrections cancel in the low energy limit.

hep-ph↗

arXiv1997

Fusion of strings vs. percolation and the transition to the quark-gluon plasma

In most of the models of hadronic collisions the number of exchanged colour strings grows with energy and atomic numbers of the projectile and target. At high string densities interaction between them should melt them into the quark-gluon plasma state. It is shown that under certain assumptions about the the string interaction, a phase transition to the quark gluon plasma indeed takes place in the system of many colour strings. It may be of the first or second order (percolation), depending on the particular mechanism of the interaction. The critical string density is about unity in both cases. The critical density may have been already reached in central Pb-Pb collisions at 158 A GeV.

hep-ph↗

arXiv1997

Nuclear Effects in Charmonium Production in QCD

It is shown that the nuclear shadowing of charmonium due to the modification of the nuclear parton distribution is similar in the factorization approach based on non relativistic QCD and in the color evaporation model. In the first model, a separate study of the color octet and color singlet contributions to the yields of the various charmonium states as well as the contributions of these states to the total $J/Ψ$ production is performed. It is found a clear $x_F$ dependence of these contributions which can reproduce experimental data for moderate $x_F$.

hep-ph↗

arXiv1997

Fast antibaryon production: A new collective effect in Pb-Pb collisions?

Recent experimental data on antibaryon production outside nucleon-nucleon kinematical limits have been obtained in Pb-Pb collisions at SPS energies. We present a possible explanation for this phenomenon based on the existence of collective effects. String Fusion Model results are compared to the data and predictions are also presented.

hep-ph↗

arXiv1997

Production Associated to Rare Events in High Energy Hadron-Hadron Collisions

At very high energy the same universal relation between the multiparticle or the transverse energy distribution associated to a rare event $C$, $P_C$ and the corresponding minimum bias distribution P, $P_C(ν)\equiv ν/<ν> P(ν)$, $ν\equiv n$ or $E_T$ works for nucleus-nucleus collisions as well as for hadron-hadron collisions. This suggests that asymptotically, all hadronic processes are similar.

hep-ph↗

arXiv1997

Strangeness enhancement in the String Fusion Model Code

String Fusion Model Code results for Pb--Pb central collisions at SPS energies including the string fusion interaction mechanism are compared to the last experimental data. Predictions for RHIC energies are also presented. On the other hand, the evolution of the strangeness enhancement ratio $E_s=\frac{<Λ>+4 }{3<π^->}$ with the energy and with the atomic number of participant nuclei is discussed.

hep-ph↗

arXiv1997

Moment Analysis, Multiplicity Distributions and Correlations in High Energy Processes: Nucleus-Nucleus Collisions

Cumulant oscillations, or $H_q$ moment oscillations, appear if the KNO multiparticle distribution decreases at large z, $z\equiv n/ $, faster than the exponential, $exp(-D z^μ)$, with $μ> 1$. In nucleus-nucleus interactions this behaviour is related to the limitation in the average number of elementary central collisions (or average number of strings centrally produced), due to the finite number of nucleons involved. Colour deconfinement, via percolating string fusion, will drastically decrease the fraction of centrally produced strings and increase the cut-off parameter $μ$: Moment oscillations will be displaced to smaller q and the width of the KNO distribution and forward-backward particle correlations will become smaller.

hep-ph↗

arXiv1997

Multiplicity and Transverse Energy Distributions Associated to Rare Events in Nucleus-Nucleus Collisions

We show that in high energy nucleus-nucleus collisions the transverse energy or multiplicity distribution P_C, associated to the production of a rare, unabsorbed event C, is universally related to the standard or minimum bias distribution P by the equation $P_C(ν)={ν\over<ν>}P(ν)$, with $\sum P(ν)=1$ and $ν\equiv E_T$ or n. Deviations from this formula are discussed, in particular having in view the formation of the plasma of quarks and gluons. This possibility can be distinguished from absortion or interaction of comovers, looking at the curvature of the $J/Ψ$ over Drell-Yan pairs as a function of E_T.

hep-ph↗

arXiv1996

Hard Processes on Nuclear Targets in QCD

We calculate the A-dependence of open charm and beauty as well as Drell-Yan pair production cross sections on nuclear targets taking into account the difference of quark and gluon distributions in free and bound nucleons. The results are presented for both proton-nucleus and nucleus-nucleus collisions. For the case of heavy flavour production at comparatively low energies, if $σ\sim A^α$, the values of $α$ are slightly higher than unity. With increasing energy $α$ decreases in all cases and becomes smaller than unity. We also show $α$ as a function of Feynman-$x$ of the produced $Q\overline{Q}$ or $l^+l^-$ pair and obtain that it decreases significantly in the beam fragmentation region. The sensitivity of our results to the used set of parton distributions is also discussed.

hep-ph↗

arXiv1996

Percolation approach to Quark Gluon Plasma and $J/ψ$ suppression

It is shown that the critical threshold for percolation of the overlapping strings exchanged in heavy ion collisions can naturally explain the sharp strong suppresion of $J/ψ$ shown by the experimental data on central Pb--Pb collisions, which does not occur in central O--U and S--U collisions.

hep-ph↗

arXiv1996

Diffraction dissociation, an important background to photon-photon collisions via heavy ion beams at LHC

The cross sections for the production of hadrons in quasi-real photon-photon collisions in proton-proton and heavy ion interactions are compared with the corresponding cross sections for central diffraction and for photon-pomeron collisions. The signatures, heavy ions or protons with only slightly changed momenta together with two large rapidity gaps and a cluster of produced hadrons in the central region, are nearly identical in all three processes. Therefore, it will be rather difficult to distinguish the reactions experimentally. It is found, that central diffraction is the dominant process in collisions of protons, light and medium-heavy ions. The photon-pomeron and photon-photon processes have quite similar cross sections in collisions of heavy ions like lead.

hep-ph↗

arXiv1996

Fusion of strings as a hadronic accelerator

It is shown that the fusion of strings is a source of particle production in nucleus--nucleus collisions outside the kinematical limits of nucleon--nucleon collisions. The spectrum of different particles is compared with the high energy data on p--A collisions obtaining a reasonable agreement. Results for A--B collisions at $\sqrt{s}=19.4$ AGeV and $\sqrt{s}=200$ AGeV are given. It is shown that the fusion of strings can accelerate particles up to the highest energy detected in cosmic rays without help of any additional cosmic accelerator. Also the rise of the average shower depth of maximun for cosmic rays in the energy range between $10^{16}$ eV and $10^{19}$ eV can be explained by the same mechanism without requiring any change in the chemical composition.

hep-ph↗

arXiv1996

$D$,$D^(2010)$ and $D^{}_{2}(2460)$ charmed mesons production in hadron--hadron collisions

A resently developed generalization of the Quark--Gluon String Model to the case of bosonic resonances production in hadron--hadron collisions is used for the calculation of inclusive production of charmed mesons $D$, $D^*(2010)$ and $D^{*}_{2}(2460)$. A simple relation which determines the dependence of the charmed meson production cross sections on their spin J is obtained. It is shown that the theoretical predictions for the inclusive spectra and production cross sections of these charmed mesons are confirmed experimentally with a reasonable accuracy.

hep-ph↗