V.J. Kolhinen

Scale evolution of nuclear parton distributions

Using the NMC and E665 nuclear structure function ratios F2A/F2D and F2A/F2C from deep inelastic lepton-nucleus collisions, and the E772 Drell-Yan dilepton cross sections from proton-nucleus collisions, and incorporating baryon number and momentum sum rules, we determine nuclear parton distributions at an initial scale Q02. With these distributions, we study QCD scale evolution of nuclear parton densities. The emphasis is on small values of x, especially on scale dependence of nuclear shadowing. As the main result, we show that a consistent picture can be obtained within the leading twist DGLAP evolution, and in particular, that the calculated Q2 dependence of F2Sn/F2C agrees very well with the recent NMC data.

Obtaining the nuclear gluon distribution from heavy quark decays to lepton pairs in pA collisions

We have studied how lepton pairs from decays of heavy-flavoured mesons produced in pA collisions can be used to determine the modifications of the gluon distribution in the nucleus. Since heavyquark production is dominated by the gg channel, the ratio of correlated lepton pair cross sections from DD-bar and BB-bar decays in pA and pp collisions directly reflects the ratio R{sub g}A= f{sub g}A/f{sub g}p. We have numerically calculated the lepton pair cross sections from these decays in pp and pA collisions at SPS, RHIC and LHC energies. We find that ratio of the pA to pp cross sections agrees quite well with the input R{sub 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 Q2 than presently available, putting strong constraints on models.

CONSTRAINTS FOR NUCLEAR GLUON SHADOWING FROM DIS DATA

Abstract 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 Sn / F 2 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 Sn / F 2 C rule out the case where nuclear shadowing (suppression) of gluons at x ∼0.01 is much larger than the shadowing observed in the ratio F 2 A / F 2 D . We also show that the possible non-linear 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 ≳0.01, are also discussed.

Nuclear parton distributions — a DGLAP analysis

Abstract Nuclear parton distributions f A ( x, Q 2 ) are studied within a framework of the DGLAP evolution. Measurements of F A 2 / F D 2 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 Sn 2 / F C 2 agrees very well with the recent NMC data, and that the ratios R A f = 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 A f ( x , Q 2 ) for all parton flavours f in A > 2, and at 10 −6 ≤ x ≤ 1 and 2.25 GeV 2 ≤ Q 2 ≤ 10 4 GeV 2 .

Nuclear Parton Distributions in the DGLAP Approach

Determination of the nuclear parton distributions within the framework of perturbative QCD, the DGLAP equations in particulars, 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.

Enhancement of charm quark production due to nonlinear corrections to the dglap equations

We have studied how parton distributions based on the inclusion of nonlinear scale evolution and constraints from HERA data affect charm production in pp collisions at center-of-mass energies of 5.5, 8.8 and 14 TeV. We find that, while the resulting enhancement can be substantial, it is very sensitive to the charm quark mass and the scale entering the parton densities and the strong coupling constant.

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.

D-meson enhancement in pp collisions at the LHC due to nonlinear gluon evolution

When nonlinear effects on the gluon evolution are included with constraints from HERA, the gluon distribution in the free proton is enhanced at low momentum fractions, x {approx}< 0.01, and low scales, Q{sup 2} {approx}< 10 GeV{sup 2}, relative to standard, DGLAP-evolved, gluon distributions. Consequently, such gluon distributions can enhance charm production in pp collisions at center of mass energy 14 TeV by up to a factor of five at midrapidity, y {approx} 0, and transverse momentum p{sub T} {yields} 0 in the most optimistic case. We show that most of this enhancement survives hadronization into D mesons. Assuming the same enhancement at leading and next-to-leading order, we show that the D enhancement may be measured by D{sup 0} reconstruction in the K{sup -}{pi}{sup +} decay channel with the ALICE detector.

DGLAP analyses of nPDF: constraints from data

We explain how the constraints from present experimental data can be used to obtain the nPDF in the framework of LO DGLAP evolution. We will also compare the only two available sets of this type and comment on the important information that neutrino factories could provide.

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 αs. 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 distributions 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.