Parton Energy Loss Limits and Shadowing in Drell-Yan Dimuon Production

Abstract

A precise measurement of the ratios of the Drell-Yan cross section per nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is reported. The behavior of the Drell-Yan ratios at small target parton momentum fraction is well described by an existing fit to the shadowing observed in deep-inelastic scattering. The cross section ratios as a function of the incident parton momentum fraction set tight limits on the energy loss of quarks passing through a cold nucleus. The Drell-Yan process, where a beam quark (antiquark) fuses with a target antiquark (quark) producing a muon pair, can be used to study the interactions of fast partons penetrating through cold nuclei. Only initial state interactions are important in Drell-Yan since the dimuon in the final state does not interact strongly with the partons in the medium. This makes Drell-Yan scattering an ideal tool to study energy loss of fast quarks in nuclear matter by comparing the observed yields from a range of nuclear targets. The dynamics of fast parton energy loss in nuclear matter is the subject of considerable theoretical interest [1][2][3][4] and has significant implications for the physics of relativisitic heavy ion collisions.Drell-Yan scattering is closely related to deep-inelastic scattering (DIS) of leptons, but unlike DIS it can be used specifically to probe antiquark contributions in target parton distributions. When DIS on nuclei occurs at x < 0.08, where x is the parton momentum fraction, the cross section per nucleon decreases with increasing nuclear number A due to shadowing [5,6].

Parton energy loss and modified beam quark distribution functions in Drell–Yan process in collisions

“…Shown in Fig. 8 are the calculated DY cross section ratios as a function of x ′ for different nuclei (Be, F e and W ) with (solid) and without (dashed) the effect of initial beam parton energy loss via multiple interaction, as compared to the Fermilab E866 experimental data [32]. In our numerical evaluation of the effect of initial beam parton energy loss, the quark transport parameterq 0 at the center of a large nucleus is the only free parameter and is fixed toq 0 = 0.024 ± 0.008 GeV 2 /fm from the phenomenological study of parton energy loss and modified fragmentation function in nuclear DIS by the HERMES experiment [15].…”

“…One, however, often uses p + B with a light nucleus target B as an approximate of the p + p reference data (modulo isospin corrections) as in the Fermilab E866 [32] experiment. Under this approximation, the measured nuclear modification can be compared to…”

“…We will use the EPS08 parameterization [43] of the nuclear parton distribution functions (nPDF) for the target nucleus in the calculation of DY spectra in addition to the effect of parton energy loss and medium modification of the beam parton distributions. We will then compare the calculated DY spectra to the data from the Fermilab Experiment 866 (E866) [32]. We will compare the effect of nuclear shadowing of target partons and the beam parton energy loss and investigate in which kinetic region the effect of beam parton energy loss dominate.…”

“…Since dilepton pairs in the final state do not have strong interaction with the target nucleus, only initial state interactions are important in DY process. Therefore, it is an ideal tool to study fast parton propagation and energy loss in cold nuclear medium [31][32][33][34][35][36][37]. The higher-twist approach has also been applied to the study of nuclear effects in the DY process in p + A collisions [38,39].…”

Improved measurement of thed¯/ūasymmetry in the nucleon sea

“…However, it is important to estimate the magnitude of these corrections. The nuclear dependence of proton-induced Drell-Yan dimuon production at 800 GeV/c has been determined by FNAL E866/NuSea [41] and by FNAL E772 [42]. These experiments measured the relative Drell-Yan cross sections per nucleon on a range of nuclear targets.…”