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Peter Levai

Publications and source records attributed to Peter Levai.

At least 19 recordsLinked to original sources
arXiv2014

Initial-State Bremsstrahlung versus Final-State Hydrodynamic Sources of Azimuthal Harmonics in p+A at RHIC and LHC

Recent pT<2~GeV azimuthal correlation data from the Beam Energy Scan (BES) and D+Au runs at RHIC/BNL and, especially, the surprising similarity of azimuthal $v_n\{2m\}(p_T)$ ``transeverse flow'' harmonics in $p+Pb$ and $Pb+Pb$ at LHC have challenged the uniqueness of local equilibrium ``perfect fluid'' interpretations of those data. We report results at QM14 on azimuthal harmonics associated with initial-state non-abelian ``wave interference'' effects predicted by perturbative QCD gluon bremsstrahlung and sourced by Color Scintillation Arrays (CSA) of color antennas. CSA are naturally identified with multiple projectile and target beam jets produced in inelastic p+A reactions. We find a remarkable similarity between azimuthal harmonics sourced by initial state CSA and those predicted with final state perfect fluid models of high energy p+A reactions. The question of which mechanism dominates in $p+A$ and $A+A$ remains open at this time.

hep-ph↗
arXiv2011

Jet energy loss in heavy ion collisions from RHIC to LHC energies

The suppression of hadron production originated from the induced jet energy loss is one of the most accepted and well understood phenomena in heavy ion collisions, which indicates the formation of color deconfined matter consists of quarks, antiquarks and gluons. This phenomena has been seen at RHIC energies and now the first LHC results display a very similar effect. In fact, the suppression is so close to each other at 200 AGeV and 2.76 ATeV, that it is interesting to investigate if such a suppression pattern can exist at all. We use the Gyulassy-Levai-Vitev description of induced jet energy loss combined with different nuclear shadowing functions and describe the experimental data. We claim that a consistent picture can be obtained for the produced hot matter with a weak nuclear shadowing. The interplay between nuclear shadowing and jet energy loss playes a crucial role in the understanding of the experimental data.

hep-ph↗
arXiv2011

Feeding of the elliptic flow by hard partons

We propose that in nuclear collisions at the LHC the elliptic flow may get a contribution from leading hard and semihard partons which deposit energy and momentum into the hydrodynamic bulk medium. The crucial effect is that these partons induce wakes which interact and merge if they come together. The contribution to the integrated elliptic flow is estimated with the help of a toy model to about 25% of the observed value and shows strong event-by-event fluctuations.

nucl-th↗
arXiv2009

Strange and charm quark-pair production in strong non-Abelian field

We have investigated strange and charm quark-pair production in the early stage of heavy ion collisions. Our kinetic model is based on a Wigner function method for fermion-pair production in strong non-Abelian fields. To describe the overlap of two colliding heavy ions we have applied the time-dependent color field with a pulse-like shape. The calculations have been performed in an SU(2)-color model with finite current quark masses. For strange quark-pair production the obtained results are close to the Schwinger limit, as we expected. For charm quark the large inverse temporal width of the field pulse, instead of the large charm quark mass, determines the efficiency of the quark-pair production. Thus we do not observe the expected suppression of charm quark-pair production connecting to the usual Schwinger-formalism, but our calculation results in a relatively large charm quark yield. This effect appears in Abelian models as well, demonstrating that particle-pair production for fast varying non-Abelian gluon field strongly deviates from the Schwinger limit for charm quark. We display our results on number densities for light, strange, charm quark-pairs, and different suppression factors as the function of characteristic time of acting chromo-electric field.

hep-ph↗
arXiv2008

Transverse and longitudinal momentum spectra of fermions produced in strong SU(2) fields

We study the transverse and longitudinal momentum spectra of fermions produced in a strong, time-dependent non-Abelian SU(2) field. Different time-dependent field strengths are introduced. The momentum spectra are calculated for the produced fermion pairs in a kinetic model. The obtained spectra are similar to the Abelian case, and they display exponential or polynomial behaviour at high p_T, depending on the given time dependence. We investigated different color initial conditions and discuss the recognized scaling properties for both Abelian and SU(2) cases.

hep-ph↗
arXiv2007

Intrinsic transverse momentum in di-hadron correlations

We study di-hadron correlations in proton-proton collisions at $\sqrt{s} = 200$ GeV and interpret experimental data in terms of a fragmentation width and a momentum imbalance. A fragmentation width of $580 \pm 50$ GeV/c is obtained, and the measured momentum imbalance gives an `intrinsic' transverse momentum width of partons in the proton of $2.6 \pm 0.2$ GeV/c. Consequences to heavy ion collisions are discussed.

hep-ph↗
arXiv2002

Decisive Role of Fragmentation Functions in Hard Hadron Production

It is demonstrated that the fragmentation functions at large momentum fraction play a key role in hard hadron production from relativistic proton-proton collisions. We find that this region of the fragmentation functions is not strongly constrained by the electron-positron data. This freedom can be used (together with the transverse momentum distribution of partons) to reproduce hard pion-to-proton ratio data in relativistic proton-proton collisions.

hep-ph↗
arXiv2002

Differences in high p_t meson production between CERN SPS and RHIC heavy ion collisions

In this talk we present a perturbative QCD improved parton model calculation for light meson production in high energy heavy ion collisions. In order to describe the experimental data properly, one needs to augment the standard pQCD model by the transverse momentum distribution of partons ("intrinsic k_T"). Proton-nucleus data indicate the presence of nuclear shadowing and multiscattering effects. Further corrections are needed in nucleus-nucleus collisions to explain the observed reduction of the cross section. We introduce the idea of proton dissociation and compare our calculations with the SPS and RHIC experimental data.

nucl-th↗
arXiv2001

The role of jet quenching in the p-bar >= pi- anomaly at RHIC

Preliminary PHENIX data on $Au+Au$ reactions at $\sqrt{s}= 130$ AGeV suggest that $\bar{p}$ yields may exceed $π^-$ at high $p_{\rm T} > 2$ GeV/c. We propose that jet quenching in central collisions suppresses the hard PQCD component of the spectra in central $A+A$ reactions, thereby exposing a novel non-perturbative component of baryon dynamics. We suggest that baryon junctions provide a possible explanation of the anomalous component. We predict that the $\bar{p} \geq π^-$ anomaly is limited to a finite $p_{\rm T}$ window and decreases with increasing impact parameter.

hep-ph↗
arXiv2000

Effects of Dense Matter on Hadron Production in Heavy-Ion Collisions

The intrinsic transverse momentum distribution of partons in the nucleon can be used to explain a large amount of high-$p_T$ hadron and photon production data in high-energy nucleon-nucleon collisions at energies $\sqrt{s} \approx 20$ to 1800 GeV. However, proton-nucleus experiments at energies $\approx 30$ GeV show an extra enhancement (Cronin effect) in the yield of photons and mesons compared to a simple extrapolation of the proton-proton data. This enhancement is due to the effect of the dense hadronic matter encountered by the projectile proton in the nuclear environment. We discuss the origin and the properties of the nuclear enhancement.

nucl-th↗
arXiv1999

Phi, Omega and Rho production from deconfined matter in relativistic heavy ion collisions at CERN SPS

We investigate the production of the Phi meson and the Omega baryon which interact weakly with hot hadronic matter, and thus their spectra reflect the early stage of the heavy ion collisions. Our analysis shows that the hadronization temperature, T_had, and the transverse flow, v_T, of the initial deconfined phase are strongly correlated: T_had + a * (v_T)**2 = 0.25 GeV, where a=0.37 GeV in the Pb+Pb collision at 158 AGeV/c. When choosing appropriate initial values of T_had and v_T from the temperature region T_had=175 +- 15 MeV, the measured Rho meson spectra was reproduced surprisingly well by the MICOR model. We have found weak influence of final state hadronic interactions on the transverse hadron spectra at m_T - m_i > 0.3 GeV.

hep-ph↗
arXiv1999

Jet Quenching in Thin Quark-Gluon Plasmas I: Formalism

The modification and amplification of the gluon angular distribution produced along with hard jets in nuclear collisions is computed. We consider the limit of a thin quark-gluon plasma, where the number of rescatterings of the jet and gluons is small. The focus is on jet quenching associated with the formation of highly off-shell partons in hard scattering events involving nuclei. The interference between the initial hard radiation amplitude, the multiple induced Gunion-Bertsch radiation amplitudes, and gluon rescattering amplitudes leads to an angular distribution that differs considerably from both the standard DGLAP evolution and from the classical limit parton cascading. The cases of a single and double rescattering are considered in detail, and a systematic method to compute all matrix elements for the general case is developed. A simple power law scaling of the angular distribution with increasing number of rescatterings is found and used for estimates of the fractional energy loss as a function of the plasma thickness.

hep-ph↗
arXiv1999

Jet quenching in thin plasmas

We investigate the energy loss of quarks and gluons produced in hard processes resulting from final state rescatterings in a finite quark-gluon plasma. The angular distribution of the soft gluon bremsstrahlung induced by n_s=1 rescatterings in the plasma is computed in the Gyulassy-Wang model. Special focus is on how the interference between the initial hard radiation amplitude, the multiple induced Gunion-Bertsch radiation amplitudes, and gluon rescattering amplitudes modifies the classical parton cascade results.

hep-ph↗
arXiv1999

Hard Photons and Neutral Pions from RHIC

In order to fix the parameters for predictions of hard photon and pion production in $Au+Au$ collisions at $\sqrt{s}=200$ GeV, proton-proton and proton-nucleus data are analyzed in perturbative QCD in the energy range $\sqrt{s} \approx 20-60$ GeV and a prediction at RHIC energy is given.

hep-ph↗
arXiv1999

Saturating Cronin effect in ultrarelativistic proton-nucleus collisions

Pion and photon production cross sections are analyzed in proton-proton and proton-nucleus collisions at energies 20 GeV < s^1/2 < 60 GeV. We separate the proton-proton and nuclear contributions to transverse-momentum broadening and suggest a new mechanism for the nuclear enhancement in the high transverse-momentum region.

nucl-th↗
arXiv1998

High p_T Phenomena in Heavy Ion Collisions at s^1/2=20 and 200 AGeV

Jet quenching as observed via high p_T hadron production is studied in the HIJING1.35 model. We test the model on recent WA98 data on 160 AGeV Pb+Pb -> pi0 up to 4 GeV/c. The results depend sensitively on the model of the Cronin effect. At (RHIC) collider energies (s^1/2 > 200 AGeV), on the other hand, semi-hard hadron production becomes insensitive to the above model and is thus expected to be a cleaner probe of jet quenching signature associated with non-Abelian radiative energy loss.

hep-ph↗