Some recent results from several fixed-target dimuon production experiments
at Fermilab are presented. In particular, we discuss the use of Drell-Yan data
to determine the flavor structure of the nucleon sea, as well as to deduce the
energy-loss of partons traversing nuclear medium. Future dilepton experiments
at RHIC could shed more light on the flavor asymmetry and possible
charge-symmetry-violation of the nucleon sea. Clear evidence for scaling
violation in the Drell-Yan process could also be revealed at RHIC.Comment: 5 pages, talk presented at the RIKEN-BNL Workshop on 'Hard Parton
  Physics in Nucleus-Nucleus collisions, March 199

McGaughey, P. L.

Moss, J. M.

Peng, J. C.

English

arXiv

Some recent results from several fixed-target dimuon production experiments at Fermilab are presented. In particular, we discuss the use of Drell-Yan data to determine the flavor structure of the nucleon sea, as well as to deduce the energy-loss of partons traversing nuclear medium. Future dilepton experiments at RHIC could shed more light on the flavor asymmetry and possible charge-symmetry-violation of the nucleon sea. Clear evidence for scaling violation in the Drell-Yan process could also be revealed at RHIC

Peng, J.C.

McGaughey, P.L.

Moss, J.M.

UNT Digital Library

,. ,LA-UR-99-~774Title:Author(s):Submitted to:ILosAlamosNATIONAL LABORATORYDilepton Production at Ferrnilab and RHICJ. C. Peng, P. L. McGaughey, and J. M. MossRIKEAJ-BNL Worksho~ on Hard Parton_—— —Physics in Nucleus-N;cleus CollisionsBrookhaven National LaboratoryMarch 1-5, 1999RECEIVED— .—_—— —- ——— ——— ——Los Alamos National laboratory, an affirmative actionkqual opportunity empldyer, is operated by the University of California for the U.S. Department of Energyunder contract W-7405-ENG-36. By acceptance of thk article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free Iioense topublish or reproduce the pubikhed form of thk contribution, or to allow others to do so, for U.S. Government purposes. The Los Alamos National Laboratoryrequests that the pubtisher identify this article aa work performed under the auspices of the U.S. Department of Energy,FomrNo. SSSR5ST 262S 10/01DISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument..DILEPTON PRODUCTION AT FERMILAB AND RHICJ. C. Peng, P. L. McGaughey, and J. M. MossPhysics Division, Los Alamos National Laboratory, Los Alamos, NM 87545Some recent results from several fixed-target dlmuon production experiments at Fermilab are pre-sented. In particular, we discuss the use of Drell-Yan data to determine the flavor structure of thenucleon sea, as well as to deduce the energy-loss of partons traversing nuclear medium. Futuredilepton experiments at RHIC could shed more light on the flavor asymmetry and possible charge-symmetry-violation of the nucleon sea. Clear evidence for scaling violation in the Drell-Yan processcould also be revealed at RHIC.1. IntroductionThe first dilepton production experiment was performedat the AGS almost 30 years ago [1]. Over the years,hadron-induced dilepton production experiments haveled to the discoveries of various vector bosons (J/V, T,and ZO). They also provided important and often uniqueinformation on parton distributions in nucleons, nuclei,and mesons.A series of fixed-target dimuon production experiments(E772, E789, E866) have been carried out at Fermilab inthe last 10 years. Some highlights from these experimentsare presented here. In addition, the prospect for perform-ing several dilepton production experiments at RHIC isdiscussed.2. Scaling Violation in Drell-Yan ProcessIn the “Naive” Drell-Yan (DY) model, the differentialcross section, m3d2 ~/dxFdm, has an expression showingits scaling property as follows:M3 d2~ 87ra2 XI X2dikfdxF =——x9 $1+X2~a e~[qa(zl)qa(zz) + Qa(zl)qo(zz)]. (1)The right-hand side of Eq. (1) is only a function of ZI, X2and is independent of the beam energies. This scalingproperty no longer holds when QCD corrections to theDY are taken into account.While logarithmic scaling violation is well establishedin Deep-Inelastic Scattering (DIS) experiments, it is notwell confirmed in DY experiments at all. As an exam-ple, Figure 1 compares the NA3 data [2] at 400 GeVwith the E605 [3] and E772 [4] data at 800 GeV. Thesolid curve in Figure 1 corresponds to NLO calculationfor 800 GeV p + d (W = 38.9 GeV) and it describes theNA3/E605/E772 data well. No evidence for scaling vio-lation is seen. As discussed in a recent review [5], thereare mainly two reasons for this. First, unlike the DIS,the DY cross section is a convolution of two structurefunctions. Scaling violation implies that the structurefunctions rise for z ~ 0.1 and drop for z ~ 0.1 as Q2increases. For proton-induced DY, one often involves abeam quark with xl > 0.1 and a target antiquark withX2 < 0.1. Hence the effects of scaling violation are par-tially cancelled. Second, unlike the DIS, the DY experi-ment can only probe relatively large Q2, namely, Q2 >16GeV2 for a mass cut of 4 GeV. This makes it more diffi-cult to observe the logarithmic variation of the structurefunctions in DY experiments.Possible indications of scaling violation in DY pro-cess have been reported in two pion-induced experiments,E326 [6] at Fermilab and NA1O [7] at CERN. E326 col-laboration compared their 225 GeV m– + W DY crosssections against calculations with and without scaling vi-olation. They observed better agreement when scalingviolation is included. This analysis is subject to the un-certainties associated with the pion structure functions,as well as the nuclear effects of the W target. The NA1Ocollaboration measured r– + W DY cross sections atthree beam energies, namely, 140, 194, and 286 GeV. Bychecking the ratios of the cross sections at three differentenergies, N-AIO largely avoids the uncertain y of the pionstructure functions. However, the relatively small spanin @, together with the complication of nuclear effects,make the NA1O result less than conclusive.RHIC provides an interesting opportunity for unam-biguously establishing scaling violation in the DY pro-cess. Figure 1 shows the predictions for p + d at W =500 GeV. The scaling-violation accounts for a factor oftwo drop in the DY cross sections when G is increasedfrom 38.9 GeV to 500 GeV. It appears quite feasible toestablish scaling violation in DY with future dilepton pro-duction experiments at RHIC,3. Flavor-Asymmetry and Charge-Symmetry-Violation of the Nucleon SeaThe DY process complements DIS as a tool to probeparton distributions in nucleons and nuclei. This is wellillustrated in the recent progress in the study of flavor-asymmetry in the nucleon sea.Until recently, it had been assumed that the distri-butions of c and d quarks in the proton were identical.While the equality of ii and d is not required by anyknown symmetry, it is a plausible assumption for seaquarks generated by gluon splitting. As the masses of1.pA +p+p-X‘“’~X107Xlo”Xlo’Xlo’I0.080 .090.1 0.2 0.3 0,4 0.5 0.6+FIG. 1. Comparison of DY cross section data with NLOcalculations using MRST [15] structure functions. N“ote that7 = Zlzz. The E772 [4], E605 [3], and NA3 [2] data pointsare shown as circles, squares, and triangles, respectively. Thesolid curve corresponds to fixed-target p+d collision at 800GeV, while the dotted curve is for p+d collision at W = 500GeV.the up and down quarks are small compared to the con-finement scale, nearly equal number of up and down seaquarks should result.The assumption of ii(z) = ~(z) can be tested by mea-surements of the Gottfried integral [8], defined as1IG = /[ F:(Z, Q2) – F;($, Q2)] /3 dz =owhere F: and F2n are the proton and neutron structurefunctions measured in DIS experiments. The second stepin Eq. 2 follows from the assumption of charge symmetry.Under the assumption of a symmetric sea, tiP = &, theGottfried Sum Rule (GSR) [8], ~G = 1/3, is obtained.The most accurate test of the GSR was reported by theNew Muon Collaboration (NMC) [9], which measured F;and F; over the region 0.004 ~ x ~ 0.8. They deter-mined the Gottfried integral to be 0.235 + 0.026, signif-icantly below 1/3. This result implies that the integralof d – ii is nonzero. However, the x-dependence of ~ – iiremained unspecified.The proton-induced DY process provides an indepen-dent means to probe the flavor asymmetry of the nucleonsea [10]. An important advantage of the Drell-Yan pro-cess is that the x-dependence of ~/ti can be determined.,0.8I0.6 I Io.4~’’’’’’’’’’ ”’’’””I,.,0 0.05 0.1 0.15 0.2 0.25 0~3 OIx5FIG. 2. The ratio of d/ii in the proton as a function of xextracted from the Fermilab E866 cross section ratio. Thecurves are from various pa ton distributions. The error barsindicate statistical errors only. An additional systematic un-certainty of +0.032 is not shown. Also shown is the resultfrom NA51, plotted as an open box, The open circles cor-respond to d/ii values extracted with the assumption of theCSV effect reported in ref. [10].The NA51 [11] and the E866 [12] collaborations havecompared the proton-induced DY dimuon yields from hy-drogen and deuterium targets, and they deduced the ra-tios of ~/ii as shown in Figure 2. For x < 0.15, d/iiincreases linearly with x and is in good agreement withthe CTEQ4M [13] and MRS(R2) [14] parameterizations.However, a distinct feature of the data, not seen in eitherparameterization, is the rapid decrease towards unity of~/ti beyond X2 = 0.2. The E866 data clearly affect thecurrent parameterization of the nucleon sea. The mostrecent structure functions of Martin et al. [15] (MRST) in-cluded the E866 data in its global fit and its parametriza-tion of ~/ii is shown in Figure 2.Many papers have considered virtual mesons as the ori-gin for the observed d/ii asymmetry (see recent review ofKumano [16]). Here the n+ (Ju) cloud, dominant in theprocess, p + r+n, leads to an excess of ~ sea. Compari-son of the E866 data with various theoretical models hasalso been made [17].It should be emphasized that the extraction of ~/iivalues from the NA51 and E866 DY experiments re-quired the assumption of charge symmetry, namely, & =tiP, tin = &, etc. Evidence for a surprisingly large charge-symmetry-violation (CSV) effect was recently reportedby Boros et al. [18,19] based on an analysis of F2 struc-2.,. .I U(p+p + w+ x)/u(p+p + w- x)8420s “2 = 500 GeV....’“....,..,, . ..”.... . .....MRS SO’I I r 1 1-0.6 –0.4 –0.2 o 0.2 0.4XFFIG. 3. Predictions of a(p +p ~ iV+z)/a(p +p ~ W-z)as a function of xp at & = 500 GeV. The dashed curve cor-responds to the d/ti symmetric MRS SO’ structure functions,while the solid and dotted curves are for the d/ti asymmetricstructure function MRST and MRS(R2), respectively.ture functions determined from muon and neutrino DISexperiments. A large asymmetry, d~ (z) % 1.25tiP(z) for0.008< x <0.1, is apparently needed to bring the muonand neutrino DIS data into agreement. How would thisfinding, if confirmed by further studies, affect the E866analysis of the flavor asymmetry? First, CSV alone couldnot account for the E866 data. In fact, an even largeramount of flavor asymmetry is required to compensatethe possible CSV effect [19]. This is illustrated in Figure2, where the open circles correspond to the d/ii values onewould have obtained if the CSV effect reported by Boroset al. is assumed. Second, there has been no indicationof CSV for z >0.1. Thus the large d/ii asymmetry fromE866 for z >0.1 is not affected.To disentangle the ~/ii asymmetry from the possibleCSV effect, one could consider W boson production, ageneralized DY process, in p + p collision at RHIC. Aninteresting quantity to be measured is the ratio of thep+p -+ W++ z and p+p + W-+ z cross sections [20].It can be shown that this ratio is very sensitive to d/ti.An important feature of the W production asymmetryin p + p collision is that it is completely free from theassumption of charge symmetry. Figure 3 shows the pre-dictions for p+ p collision at @ = 500 GeV. The dashedcurve corresponds to the d/ii symmetric MRS SO’ [21]structure functions, while the solid and dotted curves arefor the ~/ti asymmetric structure function MRST andMRS(R2), respectively. Figure 3 clearly shows that W1.11 “’’’”lI110.9nf%3__Y10 102Mass NumberFIG. 4. Ratios of heavy-nucleus to deuterium integratedyields per nucleon for 800 GeV proton production of dimuonsfrom the Drell-Yan process and from decays of the J/@, ~’,Y(lS), and T(2S + 3S) states [5]. The short dash and longdash curves represent the approximate nuclear dependencefor the b~ and cc states, A0g6 and A0g2, respectively.asymmetry measurements at RHIC could provide an in-dependent determination of ZIG.4. Nuclear Medium Effects of Dilepton Produc-tionFrom a high-statistics measurement of dilepton produc-tion in 800 GeV proton-nucleus interaction, the target-mass dependence of DY, J/Y!, Q!, and T productionshave been determined in E772 [22-24]. As shown in Fig-ure 4, different nuclear dependence are observed for dif-ferent dilepton processes. While the DY process showsalmost no nuclear dependence, pronounced nuclear ef-fects are seen for the production of heavy quarkoniumstates. E772 found that J/V and ~’ have similar nucleardependence. The nuclear dependence for ‘Y, T’ and T“are less than that observed for the J/Ii? and ~’. Withinstatistics, the various T resonances also have very similarnuclear dependence.Although the integrated DY yields in E772 show lit-tle nuclear dependence, it is instructive to examine theDY nuclear dependence on various kinematic variables.Using the simple A“ expression to fit the DY nucleardependence, the values of a are shown in Figure 5 as afunction of q (x2), &f, xF, and p~. Several features areobserved:1. A suppression of the DY yields from heavy nuclear3,,a.0.61.04t t096~0.1 0.2 6 8 10 12 141.04 ~1.02 1FIG. 5.Drell-Yan1 1 10.0 0.3 0.6 1.0 2.0 3.0x, PTNuclear dependence coefficient a for 800 GeV p+Aprocess versus various kinematic variables [22].1.2 r , ,+(a) ———+11.11’Trlnoo.“o~ (b)J1.11‘:Wf :0.09 0.27 0.46 0.650.87:0.8 I 1 I , I 0.7-0.2 0 0.2 0.4 06 0.8XFFIG. 6. Nuclear dependence coefficient a for ~theDrell-Yanprocess [22] versus ZF for (a) X2 <0.075, right, scale, and (b)Z2 z 0,075, left scale. The thin solid lines sho’.v a = 1. Thedashed line is a linear least-squares fit to the lower points.Also shown is the mean value of xl for (b).40- + J/+0.027 ((A/2 )”3- 1)~10MASS NUMB~RFIG. 7. The change of mean p: for nuclear target,A@) = (p:)(A) – (p~) (D), for 800 GeV p+A Drell-Yan pro-cess and J/V and T(l S) productions. The solid curves arebest fits to the A-dependence of A(p~). The J/V and T(lS)productions have identical curves for the A-dependence fits.2.3.targets is seen at small X2. This is consistent withthe shadowing effect observed in DIS. In fact, E772provides the only experimental evidence for shad-owing in hadronic reactions. The reach of small X2in E772 is limited by the mass cut (M ~ 4 GeV)and by the relatively small center-of-mass energy(recall that ZIZ2 = Lf2/s). p-A collisions at RHICclearly offer the exciting opportunity to extend thestudy of shadowing to much smaller x.Q(xF ) shows an interesting trend, namely, it de-cre=es as xF incre=es. It is tempting to at-tribute this behavior to initial-state energy-loss ef-fect. However, there is a strong correlation betweenXF and X2 (XF = xl – Z2), and it is essential to sep-arate the xF energy-loss effect from the X2 shadow-ing effect. Figure 6 shows a versus xF for two binsof X2, one in the shadowing region (Z2 ~ 0.075)and one outside of it (X2 ~ 0.075). There is no dis-cernible xF dependence for a once one stays out-side of the shadowing region. Therefore, the ap-parent suppression at large xF in Figure 5 reflectsthe shadowing effect at small X2 rather than theenergy-loss effect.a (pt ) shows an enhancement at large pt. This isreminiscent of the Cronin Effect [25] where thebroadening in pt distribution is attributed to mul-tiple parton-nucleon scattering. It is instructive*to compare the pt broadening for DY process andquarkonium production. Figure 7 shows A(p~), thedifference of mean p: between p-A and P-D inter-actions, as a function of A for DY, J/~, and T(lS)productions at 800 GeV. The DY and T data arefrom E772 [26], while the l/V results are fromE789 [27], E771 [28], and preliminary E866 anal-ysis [29]. More details on this analysis will be pre-sented elsewhere [26]. Figure 7 shows that @) iswell described by the simple expression a+bA1i3. Italso shows that the pt broadening for J/@ is verysimilar to T, but significantly larger (by a factorof 5) than the DY. A factor of 9/4 could be at-tributed to the color factor of the initial gluon inthe quarkonium production versus the quark in theDY process. The remaining difference could comefrom the final-state multiple scattering effect whichis absent in the DY process.Baier et al. [30] have recently derived a relation-ship between the partonic energy-loss due to gluonbremsstrahlung and the mean p: broadening accumu-lated via multiple parton-nucleon scattering:–dE/dz = ; cY~A(P;). (3)This non-intuitive result states that the total energy lossis proportional to square of the path length traversed bythe incident partons. From Figure 7 and Eq. 3, we deducethat the mean total energy loss, AE, for the p+W DYprocess is x 0.6 GeV. Such an energy-loss is too smallto cause any discernible effect in the xF (or Z1 ) nucleardependence. As shown in Figure 6, the dashed curvecorresponds to AE = 2.0 + 1.7 GeV (for p+ W), andthe E772 data are consistent with Eq. 3. A much moresensitive test for Eq. 3 could be done at RHIC, wherethe energy-loss effect is expected to be much enhancedin A-A collision [31].[1] J.H. Christenson et al., Phys. Rev. Lett. 25,1523 (1970).[2] J. Badier et al., Z. Phys. C 26,489 (1984),[3] G. Moreno et al., Phys. Rev, D 43,2815 (1991).[4] P.L. 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Dilepton Production at Fermilab and RHIC

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Dilepton Production at Fermilab and RHIC

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