arXiv subjects

K. J. Eskola

Publications and source records attributed to K. J. Eskola.

At least 73 records · Page 4Linked to original sources

arXiv2002

A perturbative QCD analysis of charged-particle distributions in hadronic and nuclear collisions

We compute the distributions of charged particles at large transverse momenta in $p\bar p(p)$, $pA$ and $AA$ collisions in the framework of perturbative QCD, by using collinear factorization and the modern PDFs and fragmentation functions. At the highest cms-energies the shape of the spectra measured in $p\bar p(p)$ collisions at large $q_T$ can be well explained. The difference between the data and the lowest-order computation is quantified in terms of a constant $K$-factor for each energy. The $K$-factor is found to systematically decrease with growing $\sqrt s$. Also a lower limit for the partonic transverse momentum, $p_0$, is extracted for each $\sqrt s$ based on the comparison with the measurements. A systematic increase of $p_0$ as a function of $\sqrt s$ is found. Nuclear effects in the charged-particle spectra in $pA$ and $AA$ collisions at RHIC and LHC are studied in the framework of collinear factorization by applying the EKS98 nuclear corrections to the parton distributions. The nuclear effects are shown to mostly enhance the computed spectra. A comparison with the recent PHENIX data from central and peripheral Au+Au collisions at RHIC is done.

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arXiv2002

Rapidity dependence of particle production in ultrarelativistic nuclear collisions

We compute the rapidity dependence of particle and transverse energy production in ultrarelativistic heavy ion collisions at various beam energies and atomic numbers using the perturbative QCD + saturation model. The distribution is a broad gaussian near $y=0$ but the rapid increase of particle production with the beam energy will via energy conservation strongly constrain the rapidity distribution at large $y$.

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arXiv2002

Constraints for nuclear gluon shadowing from DIS data

The $Q^2$ dependence of the ratios of the cross sections of deep inelastic lepton--nucleus scattering is studied in the framework of leading twist, lowest order perturbative QCD. The $\log Q^2$ slope of the ratio $F_2^{\rm Sn}/F_2^{\rm C}$ is computed by using the DGLAP evolution equations, and shown to be sensitive to the nuclear gluon distribution functions. Four different parametrizations for the nuclear effects of parton distributions are studied. We show that the NMC data on the $Q^2$ dependence of $F_2^{\rm Sn}/F_2^{\rm C}$ rule out the case where nuclear shadowing (suppression) of gluons at $x\sim 0.01$ is much larger than the shadowing observed in the ratio $F_2^A/F_2^{\rm D}$. We also show that the possible nonlinear correction terms due to gluon fusion in the evolution equations do not change this conclusion. Some consequences for computation of RHIC multiplicities, which probe the region $x\gsim0.01$, are also discussed.

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arXiv2001

Initial state of the QGP from perturbative QCD + saturation

The production of the initial state of the QGP in very high-energy $AA$ collisions is discussed within the framework of perturbative QCD and saturation. The next-to-leading order computation of the transverse energy of minijets is reviewed. Saturation of parton production, conjectured to occur at a dynamically determinable perturbative scale, leads to estimates of the initial densities. The final state multiplicities are predicted by assuming an isentropic hydrodynamical further evolution. Comparison with RHIC data is shown.

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arXiv2001

Nuclear parton distributions in the DGLAP approach

Determination of the nuclear parton distributions within the framework of perturbative QCD, the DGLAP equations in particular, is discussed. Scale and flavour dependent nuclear effects in the parton distributions are compared with the scale and flavour independent parametrizations of HIJING and of the Hard Probe Collaboration. A comparison with the data from deep inelastic lepton-nucleus scattering and the Drell-Yan process in proton-nucleus collisions is shown.

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arXiv2001

Effects of shadowing on Drell-Yan dilepton production in high energy nuclear collisions

We compute cross sections for the Drell-Yan process in nuclear collisions at next-to-leading order (NLO) in α_s. The effects of shadowing on the normalization and on the mass and rapidity dependence of these cross sections are presented. An estimate of higher order corrections is obtained from next-to-next-to-leading order (NNLO) calculation of the rapidity-integrated mass distribution. Variations in these predictions resulting from choices of parton distribution sets are discussed. Numerical results for mass distributions at NLO are presented for RHIC and LHC energies, using appropriate rapidity intervals. The shadowing factors in the dilepton mass range 2 < M < 10 GeV are predicted to be substantial, typically 0.5 - 0.7 at LHC, 0.7 - 0.9 at RHIC, and approximately independent of the choice of parton distribution sets and the order of calculation.

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arXiv2001

Heavy ion collision multiplicities and gluon distribution functions

Atomic number ($A$) and energy ($\roots$) scaling exponents of multiplicity and transverse energy in heavy ion collisions are analytically derived in the perturbative QCD + saturation model. The exponents depend on the small-$x$ behaviour of gluon distribution functions at an $x$-dependent scale. The relation between initial state and final state saturation is also discussed.

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arXiv2001

Multiplicities and Transverse Energies in Central AA Collisions at RHIC and LHC from pQCD, Saturation and Hydrodynamics

We compute the particle multiplicities and transverse energies at central and nearly central AA collisions at RHIC and LHC. The initial state is computed from perturbative QCD supplemented by the conjecture of saturation of produced partons. The expansion stage is described in terms of hydrodynamics assuming longitudinal boost invariance and azimuthal symmetry. Transverse flow effects, a realistic list of hadrons and resonance decays are included. Comparison with the data of the multiplicities at $\sqrt s=56$ AGeV and 130 AGeV from RHIC is done and predictions for the full RHIC energy and LHC energy are made for the multiplicities and transverse energies. The reduction from the initially released minijet transverse energy to the $E_T$ in the final state is less than in the one-dimensional case but still dramatic: a factor of 2.7 at RHIC, and 3.6 at the LHC.

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arXiv2001

Centrality dependence of multiplicity, transverse energy, and elliptic flow from hydrodynamics

The centrality dependence of the charged multiplicity, transverse energy, and elliptic flow coefficient is studied in a hydrodynamic model, using a variety of different initializations which model the initial energy or entropy production process as a hard or soft process, respectively. While the charged multiplicity depends strongly on the chosen initialization, the p_t-integrated elliptic flow for charged particles as a function of charged particle multiplicity and the p_t-differential elliptic flow for charged particles in minimum bias events turn out to be almost independent of the initial energy density profile.

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arXiv2001

Obtaining the nuclear gluon distribution from heavy quark decays to lepton pairs in p$A$ collisions

We have studied how lepton pairs from decays of heavy-flavoured mesons produced in p$A$ collisions can be used to determine the modifications of the gluon distribution in the nucleus. Since heavy quark production is dominated by the $gg$ channel, the ratio of correlated lepton pair cross sections from $D\bar D$ and $B\bar B$ decays in p$A$ and pp collisions directly reflects the ratio $R_g^A \equiv f_g^A/f_g^p$. We have numerically calculated the lepton pair cross sections from these decays in pp and p$A$ collisions at SPS, RHIC and LHC energies. We find that ratio of the p$A$ to pp cross sections agrees quite well with the input $R_g^A.$ Thus, sufficiently accurate measurements could be used to determine the nuclear modification of the gluon distribution over a greater range of $x$ and $Q^2$ than presently available, putting strong constraints on models.

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arXiv2000

Transverse energy from minijets in ultrarelativistic nuclear collisions: a next-to-leading order analysis

We compute in next-to-leading order (NLO) perturbative QCD the amount of transverse energy produced into a rapidity region $ΔY$ of a nuclear collision from partons created in the few-GeV subcollisions. The NLO formulation assumes collinear factorization and is based on the subtraction method. We first study the results as a function of the minimum transverse momentum scale and define and determine the associated $K$-factors. The dependence of the NLO results on the scale choice and on the size of $ΔY$ is also studied. The calculations are performed for GRV94 and CTEQ5 sets of parton distributions. Also the effect of nuclear shadowing to the NLO results is investigated. The main results are that the NLO results are stable relative to the leading-order (LO) ones even in the few-GeV domain and that there exists a new kinematical region not present in the LO, which is responsible for the magnitude of the $K$-factors. The dependence on the size of $ΔY$ is found to be practically linear within two central units of rapidity, and the scale dependence present at RHIC energies is seen to vanish at the LHC energies. Finally, the effect of NLO corrections to the determination of transverse energies and multiplicities in nuclear collisions is discussed within the framework of parton saturation.

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arXiv2000

Constraints for the nuclear sea quark distributions from the Drell-Yan process at the SPS

Nuclear modifications to the Drell-Yan dilepton production cross sections in p+A and A+A collisions in the leading twist approximation are caused by nuclear effects in the parton distributions of bound nucleons. For non-isoscalar nuclei, isospin corrections must also be considered. We calculate these effects for p+A and Pb+Pb collisions at CERN SPS energies. Our goal is to place constraints on nuclear effects in sea quark distributions in the region x > 0.2. We show that the net nuclear corrections remain small for p+A collisions at E_lab=450 GeV. However, in Pb+Pb collisions at E_lab=158 AGeV, effects of > 20% are predicted at large M. The data collected by the NA50 collaboration could thus be used to constrain the nuclear effects in the sea quark distributions in the region of the EMC effect, x > 0.3.

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arXiv2000

Centrality dependence of multiplicities in ultrarelativistic nuclear collisions

We compute the centrality dependence of multiplicities of particles produced in ultrarelativistic nuclear collisions at various energies and atomic numbers. The computation is carried out in perturbative QCD with saturated densities of produced gluons and by including effects of nuclear geometry. Numbers are given for Au+Au collisions at RHIC energies.

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arXiv2000

Production of transverse energy from minijets in next-to-leading order perturbative QCD

We compute in next-to-leading order (NLO) perturbative QCD the transverse energy carried into the central rapidity unit of hadron or nuclear collisions by the partons freed in the few-GeV subcollisions. The formulation is based on a rapidity window and a measurement function of a new type. The behaviour of the NLO results as a function of the minimum transverse momentum and as a function of the scale choice is studied. The NLO results are found to be stable relative to the leading-order ones even in the few-GeV domain.

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arXiv1999

High Energy Nuclear Collisions

Highlights of the results from ultrarelativistic heavy ion collisions at CERN-SPS are reviewed. In particular, I discuss how the experimental results indicate that a collective strongly interacting system has been produced, and what are the implications towards the Quark Gluon Plasma. The physical ideas behind measuring certain observables are introduced. The future program of high energy nuclear collisions at BNL-RHIC and CERN-LHC/ALICE is also briefly discussed.

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arXiv1999

Scaling of transverse energies and multiplicities with atomic number and energy in ultrarelativistic nuclear collisions

We compute how the initial energy density and produced gluon, quark and antiquark numbers scale with atomic number and beam energy in ultrarelativistic heavy ion collisions. The computation is based on the argument that the effect of all momentum scales can be estimated by performing the computation at one transverse momentum scale, the saturation momentum. The initial numbers are converted to final ones by assuming kinetic thermalisation and adiabatic expansion. The main emphasis of the study is at LHC and RHIC energies but it is observed that even at SPS energies this approach leads to results which are not unreasonable: what is usually described as a completely soft nonperturbative process can also be described in terms of gluons and quarks. The key element is the use of the saturation scale.

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arXiv1999

Nuclear Parton Distributions - a DGLAP Analysis

Nuclear parton distributions $f_A(x,Q^2)$ are studied within a framework of the DGLAP evolution. Measurements of $F_2^A/F_2^D$ in deep inelastic $lA$ collisions, and Drell--Yan dilepton cross sections measured in $pA$ collisions are used as constraints. Also conservation of momentum and baryon number is required. It is shown that the calculated $Q^2$ evolution of $F_2^{\rm Sn}/F_2^{\rm C}$ agrees very well with the recent NMC data, and that the ratios $R_f^A=f_A/f$ are only moderately sensitive to the choice of a specific modern set of free parton distributions. For general use, we offer a numerical parametrization of $R_f^A(x,Q^2)$ for all parton flavours $f$ in $A>2$, and at $10^{-6}\le x \le 1$ and $2.25 {\rm GeV}^2\le Q^2\le 10^4 {\rm GeV}^2$.

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arXiv1999

Lower limits for E_T and N_ch from pQCD & hydrodynamics at the central rapidity unit in central Au-Au collisions at RHIC

Final state average transverse energy and charged particle multiplicity at the central rapidity unit of central Au-Au collisions at RHIC are studied within a framework of lowest order perturbative QCD and Bjorken's hydrodynamical picture. In particular, effects of initial minijet production and the $pdV$ work during the subsequent evolution of the QGP are investigated. Nuclear shadowing effects are included in a consistent manner, and the dependence of the results on the minijet transverse momentum cut-off parameter $p_0$ is shown.

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