arXiv subjects

Yasushi Nara

Publications and source records attributed to Yasushi Nara.

86 records · Page 5Linked to original sources

arXiv2003

The Cronin Effect, Quantum Evolution and the Color Glass Condensate

We show that the numerical solution of the classical SU(3) Yang-Mills equations of motion in the McLerran-Venugopalan model for gluon production in central heavy ion collisions leads to a suppresion at low $p_t$ and an enhancement at the intermediate $p_t$ region as compared to peripheral heavy ion and pp collisions at the same energy. Our results are compared to previous, Color Glass Condensate inspired calculations of gluon production in heavy ion collisions. We revisit the predictions of the Color Glass Condensate model for $pA$ ($dA$) collisions in Leading Order and show that quantum evolution--in particular the phenomenon of geometric scaling and change of anomalous dimensions--preserves the Cronin enhancement of $pA$ cross section (when normalized to the leading twist term) in the Leading Order approximation even though the $p_t$ spectrum can change. We comment on the case when gluon radiation is included.

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arXiv2003

Pseudorapidity dependence of parton energy loss in relativistic heavy ion collisions

We analyze the recent data from the BRAHMS Collaboration on the pseudorapidity dependence of nuclear modification factors in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV by using the full three dimensional hydrodynamic simulations for the density effects on parton energy loss. We first compute the transverse spectra at $η=0$ and 2.2, and next take a ratio $R_η=R_{AA}(η=2.2)/R_{AA}(η=0)$, where $R_{AA}$ is a nuclear modification factor. It is shown that hydrodynamic components account for $R_η\simeq 1$ at low $p_\mathrm{T}$ and that quenched pQCD components lead $R_η < 1$ at high $p_\mathrm{T}$ which are consistent with the data. Strong suppression at $η=2.2$ is compatible with the parton energy loss in the final state.

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arXiv2003

Back-to-back correlations of high p_T hadrons in relativistic heavy ion collisions

We investigate the suppression factor and the azimuthal correlation function for high $p_T$ hadrons in central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV by using a dynamical model in which hydrodynamics is combined with explicitly traveling jets. We study the effects of parton energy loss in a hot medium, intrinsic $k_T$ of partons in a nucleus, and $p_{\perp}$ broadening of jets on the back-to-back correlations of high $p_T$ hadrons. Parton energy loss is found to be a dominant effect on the reduction of the away-side peaks in the correlation function.

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arXiv2003

Classical Gluodynamics of High Energy Nuclear Collisions: an Erratum and an Update

We comment on the relation of our previous work on the classical gluodynamics of high energy nuclear collisions to recent work by Lappi (hep-ph/0303076). While our results for the non-perturbative number liberation coefficient agree, those for the energy disagree by a factor of 2. This discrepancy can be traced to an overall normalization error in our non-perturbative formula for the energy. When corrected for, all previous results are in excellent agreement with those of Lappi. The implications of the results of these two independent computations for RHIC phenomenology are noted.

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arXiv2003

Hadrons as Signature of Black Hole Production at the LHC

In models with several large extra dimensions and fundamental Planck scale of the order of 1 TeV, black holes can be produced in large numbers at LHC energies. We compute the charged hadron spectra obtained from the decay of black holes created in p-p and Pb+Pb collisions at LHC. We show that hadrons from black hole decay dominate at transverse momenta p_T>30-100 GeV/c compared to usual QCD processes and black hole production signals are easy to identify in hadron transverse momentum spectra. Furthermore we show that a measurement of the charged hadron spectra probes Planck scales up to ~5 TeV for any number of extra dimensions.

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arXiv2002

Energy loss of partons traversing a QGP fluid

We analyse the azimuthal correlation function for high p_T charged hadrons in Au+Au collisions at the RHIC energy. By using a dynamical model in which hydrodynamics is combined with explicit propagation of high p_T partons, we study the effect of the intrinsic transverse momentum of initial partons on the azimuthal back-to-back correlations.

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arXiv2002

Elliptic Flow from Color Glass Condensate

We show that an observable fraction of the measured elliptic flow may originate in classical gluon fields at the initial stage of a peripheral high-energy nuclear collision. This mechanism complements the contribution of late stage mechanisms, such as those described by hydrodynamics, to the observed elliptic flow.

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arXiv2002

Gluon production in the Color Glass Condensate model of collisions of ultrarelativistic finite nuclei

We extend previous work on high energy nuclear collisions in the Color Glass Condensate model to study collisions of finite ultrarelativistic nuclei. The changes implemented include a) imposition of color neutrality at the nucleon level and b) realistic nuclear matter distributions of finite nuclei. The saturation scale characterizing the fields of color charge is explicitly position dependent, $Λ_s=Λ_s(x_T)$. We compute gluon distributions both before and after the collisions. The gluon distribution in the nuclear wavefunction before the collision is significantly suppressed below the saturation scale when compared to the simple McLerran-Venugopalan model prediction, while the behavior at large momentum $p_T\gg Λ_s$ remains unchanged. We study the centrality dependence of produced gluons and compare it to the centrality dependence of charged hadrons exhibited by the RHIC data. We demonstrate the geometrical scaling property of the initial gluon transverse momentum distributions for different centralities. Classical Yang-Mills results for $p_T < Λ_s$ are simply matched to perturbative QCD computations for $p_T > Λ_s$-the resulting energy per particle is significantly lower than the purely classical estimates. Our results for nuclear collisions can be used as initial conditions for quantitative studies of the further evolution and possible equilibration of hot and dense gluonic matter produced in heavy ion collisions. Finally, we study $pA$ collisions within the classical framework. Our results agree well with previously derived analytical results in the appropriate kinematical regions.

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arXiv2002

Energy loss in high energy heavy ion collisions from the Hydro+Jet model

We investigate the effect of energy loss of jets in high energy heavy ion collisions by using a full three-dimensional space-time evolution of a fluid combined with (mini-)jets that are explicitly evolved in space-time. In order to fit the pi^0 data for the Au+Au collisions at sqrt(s_{NN}) = 130 GeV, the space-time averaged energy loss dE/dx(tau <= 3 fm/c) = 0.36 GeV/fm is extracted within the model. It is found that most energy loss occurs at the very early time less than 2 fm/c in the QGP phase and that energy loss in the mixed phase is negligible within our parameterization for jet energy loss. This is a consequence of strong expansion of the system.

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arXiv2002

Elliptic flow of colored glass in high energy heavy ion collisions

We compute the elliptic flow generated by classical gluon fields in a high energy nuclear collision. The classical gluon fields are described by a typical momentum scale, the saturation scale Lambda_s, which is, for RHIC energies, of the order of 1-2 GeV. A significant elliptic flow is generated only over time scales on the order of the system size R. The flow is dominated by soft modes $p_{T} \sim Lambda_s / 4$ which linearize at very late times $tau \sim R \gg 1/Lambda_s$. We discuss the implications of our result for the theoretical interpretation of the RHIC data.

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arXiv2001

Coherent gluon production in very high energy heavy ion collisions

The early stages of a relativistic heavy-ion collision are examined in the framework of an effective classical SU(3) Yang-Mills theory in the transverse plane. We compute the initial energy and number distributions, per unit rapidity, at mid-rapidity, of gluons produced in high energy heavy ion collisions. We discuss the phenomenological implications of our results in light of the recent RHIC data.

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arXiv2001

The effect of finite-range interactions in classical transport theory

The effect of scattering with non-zero impact parameters between consituents in relativistic heavy ion collisions is investigated. In solving the relativistic Boltzmann equation, the characteristic range of the collision kernel is varied from approximately one fm to zero while leaving the mean-free path unchanged. Modifying this range is shown to significantly affect spectra and flow observables. The finite range is shown to provide effective viscosities, shear, bulk viscosity and heat conductivity, with the viscous coefficients being proportional to the square of the interaction range.

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arXiv2000

Baryon distribution for high energy heavy ion collisions in a parton cascade model

The baryon distribution is studied by using a parton cascade model which is based on pQCD incorporating hard partonic scattering and dynamical hadronization scheme. In order to study baryon distribution,baryonic cluster formation is newly implemented as well as hadronic higher resonance states from parton/beam cluster decay. The net baryon number and charged hadron distributions are calculated with different $K$-factors in which parameters are fixed by elementary $p\bar p$ data at $E_{c.m.}=200$ GeV. It is found that baryon stopping behavior at SPS and RHIC energies are not consequence of hard parton scattering but soft processes.

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arXiv1998

A Parton-Hadron Cascade Approach in High-energy Nuclear Collisions

A parton-hadron cascade model which is the extension of hadronic cascade model incorporating hard partonic scattering based on HIJING is presented to describe the space-time evolution of parton/hadron system produced by ultra-relativistic nuclear collisions. Hadron yield, baryon stopping and transverse momentum distribution are calculated and compared with HIJING and VNI.

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