Drell-Yan Cross-section Ratios Research Articles

Nuclear parton densities and their uncertainties at the next-to-leading order

Considering the nuclear reactions which involve specific features at certain regions of the $x$-Bjorken variable which was first observed by the EMC group, we indicate that the parton distributions in nuclei are not simply as the parton densities in nucleons. In addition to the most commonly analyzed data sets for deep-inelastic scattering of charged leptons off nuclei, we also analyze the Drell-Yan dilepton production. We investigate parametrizations of nuclear parton distributions at the next-to-leading order (NLO) of ${\ensuremath{\alpha}}_{s}$. Finally, optimum nuclear structure functions are determined by a ${\ensuremath{\chi}}^{2}$ analysis of experimental data for the nuclear structure function ${F}_{2}^{A}$/${F}_{2}^{{A}^{\ensuremath{'}}}$ and Drell-Yan cross-section ratios. The related uncertainties are estimated by the Hessian method. Our results are in good agreement with the available experimental data and better than the results of some other fitting parametrization methods.

Initial-state quark energy loss from Drell–Yan production in proton-proton and proton-nucleus collisions

Drell-Yan production cross sections were measured by the NA3 and E866 experiments, in p-Pt and pp collisions, as a function of the dimuon mass and x F . We compare these measurements to next-to-leading order Drell-Yan calculations, made with the CTEQ6M parton densities modified (or not) by nuclear effects, using the EPS09 parameterization. The analysis of the data allows us to evaluate the initial-state quark energy loss. Drell-Yan measurements are ideally suited to isolate the initial-state parton energy loss, given the absence of final-state effects on the produced dimuon. Our study shows that these data indicate negligible quark energy loss and allow us to derive rather strict upper limits. For completeness, our study has been repeated using the less accurate measurements of Drell-Yan cross section ratios between heavy and light nuclear targets, provided by the E772 and E866 experiments. Our results provide an additional constraint on the models trying to explain quarkonium production in proton-nucleus collisions, as a function of quarkonium rapidity and collision energy, where initial- and final-state energy loss has frequently been assumed to play an important role, convoluted with several other complex mechanisms, including final-state quarkonium breakup, formation time effects, etc.

Uncertainty due to quark energy loss in determining the nuclear sea quark distribution from nuclear Drell-Yan data

By means of two different parametrizations of quark energy loss and the nuclear parton distributions determined only with lepton-nuclear deep inelastic scattering experimental data, a leading order phenomenological analysis is performed on the nuclear Drell-Yan differential cross section ratios for E772 experimental data. Uncertainty due to the quark energy loss effect is quantified in determining the nuclear sea quark distribution by using nuclear Drell-Yan data. It is found that the quark energy loss effect on nuclear Drell-Yan cross section ratios becomes greater with the increase of quark momentum fraction in the target nuclei. The uncertainty from the quark energy loss becomes bigger as the nucleus A becomes heavier. The Drell-Yan data on proton incident middle and heavy nuclei versus deuterium would result in an overestimate for nuclear modification in the sea quark distribution with neglecting the quark energy loss.

Determination of nuclear parton distribution functions and their uncertainties at next-to-leading order

Nuclear parton distribution functions (NPDFs) are determined by global analyses of experimental data on structure-function ratios ${F}_{2}^{A}/{F}_{2}^{{A}^{'}}$ and Drell-Yan cross-section ratios ${\ensuremath{\sigma}}_{\mathit{DY}}^{A}/{\ensuremath{\sigma}}_{\mathit{DY}}^{{A}^{'}}$. The analyses are done in the leading order (LO) and next-to-leading order (NLO) of running coupling constant ${\ensuremath{\alpha}}_{s}$. Uncertainties of the NPDFs are estimated in both LO and NLO for finding possible NLO improvement. Valence-quark distributions are well determined, and antiquark distributions are also determined at $x<0.1$. However, the antiquark distributions have large uncertainties at $x>0.2$. Gluon modifications cannot be fixed at this stage. Although the advantage of the NLO analysis, in comparison with the LO one, is generally the sensitivity to the gluon distributions, gluon uncertainties are almost the same in the LO and NLO. It is because current scaling-violation data are not accurate enough to determine precise nuclear gluon distributions. Modifications of the PDFs in the deuteron are also discussed by including data on the proton-deuteron ratio ${F}_{2}^{D}/{F}_{2}^{p}$ in the analysis. A code is provided for calculating the NPDFs and their uncertainties at given $x$ and ${Q}^{2}$ in the LO and NLO.

Quarkonium production and suppression in Pb+Pb and p+A collisions at SPS energies

We present the results on charmonium production as measured by experiment NA50 at the CERN-SPS in p+A and Pb+Pb collisions. The J/ ψ / Drell-Yan and ψ ′ / Drell-Yan cross-section ratios as a function of centrality are obtained from Pb+Pb data samples collected at 158 GeV per nucleon in the 1998 run and, under improved experimental conditions, in 2000. The recent very high precision NA50 proton-nucleus results allow to determine the J/ ψ nuclear absorption only from proton induced reactions, and to calculate the expected J/ ψ yield in Pb+Pb collisions at 158 GeV. The J/ ψ expected production obtained in this way is compared to the NA50 results in Pb+Pb collisions, as well as to the NA38 results for S+U reactions.

Nuclear modification of valence-quark distributions and its effects on NuTeV [formula omitted] anomaly

We investigated a nuclear modification difference between up- and down-valence quark distributions by analyzing structure function F_2 and Drell-Yan cross-section ratios. Although nuclear modifications of the valence-quark distributions themselves are rather well determined, it is difficult to find their difference from the present data. We estimated such an effect on the NuTeV sin^2 theta_W value and its uncertainty by the Hessian method. At this stage, it is not large enough to explain the whole NuTeV anomaly. However, the modification difference cannot be precisely determined, so that further studies are needed.

Nuclear corrections of parton distribution functions

We report global analysis results of experimental data for nuclear structure-function ratios F_2^A/F_2^{A'} and proton-nucleus Drell-Yan cross-section ratios sigma_{DY}^{pA}/sigma_{DY}^{pA'} in order to determine optimum parton distribution functions (PDFs) in nuclei. An important point of this analysis is to show uncertainties of the distributions by the Hessian method. The results indicate that the uncertainties are large for gluon distributions in the whole x region and for antiquark distributions at x>0.2. We provide a code for calculating any nuclear PDFs at given x and Q^2 for general users. They can be used for calculating high-energy nuclear reactions including neutrino-nucleus interactions, which are discussed at this workshop.

Nuclear parton distribution functions and their uncertainties

We analyze experimental data of nuclear structure-function ratios ${F}_{2}^{A}∕{F}_{2}^{{A}^{\ensuremath{'}}}$ and Drell-Yan cross section ratios for obtaining optimum parton distribution functions (PDFs) in nuclei. Then, uncertainties of the nuclear PDFs are estimated by the Hessian method. Valence-quark distributions are determined by the ${F}_{2}$ data at large $x$; however, the small-$x$ part is not obvious from the data. On the other hand, the antiquark distributions are determined well at $x\ensuremath{\sim}0.01$ from the ${F}_{2}$ data and at $x\ensuremath{\sim}0.1$ by the Drell-Yan data; however, the large-$x$ behavior is not clear. Gluon distributions cannot be fixed by the present data and they have large uncertainties in the whole $x$ region. Parametrization results are shown in comparison with the data. We provide a useful code for calculating nuclear PDFs at given $x$ and ${Q}^{2}$.

Energy Loss of Fast Quarks in Nuclear Drell–Yan DimuonProduction

The energy loss effect in nuclear matter, which is anothernuclear effect apart from the nuclear effects on the partondistribution as in the deep inelastic scattering process, can bemeasured best by the nuclear dependence of the high energy nuclearDrell–Yan process. By means of the quark energy lossparametrization given in literature and the nuclear partondistribution extracted only with lepton–nucleus deep inelasticscattering experimental data, measured Drell–Yan production crosssections are analysed for 800-GeV protons incident on a variety ofnuclear targets from FNAL E866. The average energy loss of quarksare given by fitting the Fe/Be and W/Be Drell–Yan cross sectionratios versus the incident parton momentum fraction.

High-Energy Proton-Induced Dimuon Production from Nuclei

In the framework of the Glauber model, taking into account theenergy loss of the beam proton through the nucleus, we analyse the measuredDrell-Yan production cross sections for an 800 GeV proton beam incident onBe, Fe and W nuclear targets. We have found that thenuclear Drell-Yan cross section ratios are suppressed due to theenergy loss in the initial state. The calculated results ofthe energy loss are in very good agreement with the Fermilab experiment 866.

D¯/ūasymmetry and the origin of the nucleon sea

The Drell-Yan cross section ratios, {sigma}(p+d)/{sigma}(p+p), measured in Fermilab E866, have led to the first determination of {bar d}(x)/{bar u}(x), {bar d}(x){minus}{bar u}(x), and the integral of {bar d}(x){minus}{bar u}(x) for the proton over the range 0.02{le}x{le}0.345. The E866 results are compared with predictions based on parton distribution functions and various theoretical models. The relationship between the E866 results and the NMC measurement of the Gottfried integral is discussed. The agreement between the E866 results and models employing virtual mesons indicates that these non-perturbative processes play an important role in the origin of the {bar d},{bar u} asymmetry in the nucleon sea. {copyright} {ital 1998} {ital The American Physical Society}

What can Drell-Yan data tell us about the EMC effect and nucleon parton distributions?

We show that knowledge of thex1 andx2 dependence of the ratio of Drell-Yan cross-sections measured on heavy nuclei and deuterium can give us information about the origin of the EMC effect.