A precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c
proton beam incident on hydrogen and deuterium targets is reported. Over
140,000 Drell-Yan muon pairs with dimuon mass M_{mu+ mu-} >= 4.5 GeV/c^2 were
recorded. From these data, the ratio of anti-down (dbar) to anti-up (ubar)
quark distributions in the proton sea is determined over a wide range in
Bjorken-x. A strong x dependence is observed in the ratio dbar/ubar, showing
substantial enhancement of dbar with respect to ubar for x<0.2. This result is
in fair agreement with recent parton distribution parameterizations of the sea.
For x>0.2, the observed dbar/ubar ratio is much nearer unity than given by the
parameterizations.Comment: REVTeX, to be published in Phys. Rev. Let

A. Baldit

A. D. Martin

A. Signal

A. Szczurek

A. W. Thomas

B. A. Mueller

B. K. Park

C. A. Gagliardi

C. N. Brown

D. A. Ross

D. D. Koetke

D. F. Geesaman

D. K. Wise

D. M. Alde

D. M. Kaplan

D. M. Lee

E. A. Hawker

E. J. Eichten

E. M. Henley

G. Kyle

G. Moreno

G. Petitt

G. R. Young

G. T. Garvey

H. L. Lai

J. C. Peng

J. C. Webb

J. D. Bush

J. L. Willis

J. M. Moss

J. Selden

K. Gottfried

L. D. Isenhower

M. A. Vasiliev

M. Arneodo

M. E. Beddo

M. E. Sadler

M. J. Leitch

N. Makins

P. Amaudruz

P. E. Reimer

P. L. McGaughey

P. M. Nord

P. N. Kirk

P. W. Stankus

R. E. Tribble

R. S. Towell

S. B. Kaufman

S. D. Drell

S. D. Ellis

S. Kumano

T. A. Carey

T. C. Awes

T. H. Chang

T. N. Thompson

V. Papavassiliou

W-Y. P. Hwang

W. E. Cooper

W. E. Sondheim

W. M. Lee

X. C. He

Y. C. Wang

Z. F. Wang

Physical Review Letters

English

arXiv

Hawker, E. A.

Awes, T. C.

Beddo, M. E.

Brown, C. N.

Bush, J. D.

Carey, T. A.

Chang, T. H.

Cooper, W. E.

Gagliardi, C. A.

Garvey, G. T.

Geesaman, D. F.

He, X. C.

Isenhower, L. D.

Kaufman, S. B.

Kaplan, D. M.

Kirk, P. N.

Koetke, D. D.

Kyle, G.

Lee, D. M.

Lee, W. M.

Leitch, M. J.

Makins, N.

McGaughey, P. L.

Moss, J. M.

Mueller, B. A.

Nord, P. M.

Park, B. K.

Papavassiliou, V.

Peng, J. C.

Petitt, G.

Reimer, P. E.

Sadler, M. E.

Selden, J.

Stankus, P. W.

Sondheim, W. E.

Thompson, T. N.

Towell, R. S.

Tribble, R. E.

Vasiliev, M. A.

Wang, Y. C.

Wang, Z. F.

Webb, J. C.

Willis, J. L.

Wise, D. K.

Young, G. R.

OAKTrust Digital Repository (Texas A&M Univ)

arXiv:hep-ex/9803011v1  12 Mar 1998Measurement of the Light Antiquark Flavor Asymmetry in the Nucleon SeaE.A. Hawkerj, T.C. Awesi, M.E. Beddoh, C.N. Brownc, J.D. Busha, T.A. Careyf , T.H. Changh, W.E. Cooperc,C.A. Gagliardij, G.T. Garveyf , D.F. Geesamanb, X.C. Hed, L.D. Isenhowera, S.B. Kaufmanb, D.M. Kaplane,P.N. Kirkg, D.D. Koetkek, G. Kyleh, D.M. Leef , W.M. Leed, M.J. Leitchf , N. Makinsb∗, P.L. McGaugheyf ,J.M. Mossf , B.A. Muellerb, P.M. Nordk, B.K. Parkf , V. Papavassiliouh, J.C. Pengf , G. Petittd, P.E. Reimerf ,M.E. Sadlera, J. Seldenh, P.W. Stankusi, W.E. Sondheimf , T.N. Thompsonf , R.S. Towella†, R.E. Tribblej ,M.A. Vasilievj‡, Y.C. Wangg, Z.F. Wangg, J.C. Webbh, J.L. Willisa, D.K. Wisea, G.R. Youngi(FNAL E866/NuSea Collaboration)aAbilene Christian University, Abilene, TX 79699bArgonne National Laboratory, Argonne, IL 60439cFermi National Accelerator Laboratory, Batavia, IL 60510dGeorgia State University, Atlanta, GA 30303eIllinois Institute of Technology, Chicago, IL 60616fLos Alamos National Laboratory, Los Alamos, NM 87545gLouisiana State University, Baton Rouge, LA 70803hNew Mexico State University, Las Cruces, NM, 88003iOak Ridge National Laboratory, Oak Ridge, TN 37831jTexas A & M University, College Station, TX 77843kValparaiso University, Valparaiso, IN 46383(February 7, 2008)A precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incidenton hydrogen and deuterium targets is reported. Over 140,000 Drell-Yan muon pairs with dimuonmass Mµ+µ− ≥ 4.5 GeV/c2 were recorded. From these data, the ratio of anti-down (d¯) to anti-up(u¯) quark distributions in the proton sea is determined over a wide range in Bjorken-x. A strong xdependence is observed in the ratio d¯/u¯, showing substantial enhancement of d¯ with respect to u¯ forx < 0.2. This result is in fair agreement with recent parton distribution parameterizations of thesea. For x > 0.2, the observed d¯/u¯ ratio is much nearer unity than given by the parameterizations.13.85.Qk; 14.20.Dh; 24.85.+p; 14.65.BtNo known symmetry requires equality of the d¯ and u¯distributions in the proton. Until recently it had beengenerally assumed that d¯(x) = u¯(x) for lack of experi-mental evidence to the contrary. This assumption maybe evaluated by use of the expression∫10[F p2(x)− Fn2(x)]dxx=13−23∫10[d¯p(x) − u¯p(x)]dx.(1)Here F p2(x) and Fn2(x) are the proton and neutron in-elastic structure functions, and d¯p(x) and u¯p(x) are theanti-down and anti-up quark distributions in the protonsea as a function of Bjorken-x. Equation 1 requires theassumption of charge symmetry between the proton andneutron (i.e. up = dn, u¯p = d¯n, etc.). If the nucleon sea isflavor symmetric in the light quarks, the value of the inte-gral on the left is 1/3, a result referred to as the GottfriedSum Rule (GSR) [1]. In 1991 the New Muon Collabora-tion (NMC) at CERN presented evidence that the GSRis violated, based on deep inelastic muon scattering datafrom hydrogen (p) and deuterium (d). They reported afinal value of∫ 10[F p2(x) − Fn2 (x)]dxx= 0.235± 0.026 [2],which implies that∫ 10[d¯p(x) − u¯p(x)]dx = 0.147± 0.039, (2)a considerable excess of d¯p relative to u¯p. This result hasbeen adopted in the most current parameterizations ofthe parton distributions in the nucleon [3,4].Following publication of the NMC result, the use ofthe Drell-Yan process [5] was suggested [6] as a means bywhich the light antiquark content of the proton could bemore directly probed. This was first done by the FermilabE772 collaboration. They compared the production ofDrell-Yan muon pairs from isoscalar targets to that froma neutron rich target. This measurement sets constraintson the non-equality of u¯ and d¯ in the range 0.04 ≤ x ≤0.27 [7]. Later, the CERN experiment NA51 [8] carriedout a comparison of the Drell-Yan muon pair yield fromhydrogen and deuterium at a single value of x using a450 GeV/c proton beam and foundu¯pd¯p∣∣∣∣〈x〉=0.18= 0.51± 0.04± 0.05. (3)A recent review by Kumano [9] presents an extensive dis-cussion of the existing literature on the flavor asymmetryof the antiquark sea.1Fermilab Experiment 866 (E866) measured the Drell-Yan muon pair yield from 800 GeV/c proton bombard-ment of liquid deuterium and hydrogen targets. Fromthese data, d¯/u¯ and d¯ − u¯ in the proton over the range0.020 < x < 0.345 are extracted. A significant differencebetween the d¯ and u¯ distributions is found.E866 used a 3-dipole magnet spectrometer [10] em-ployed in previous experiments (E605, E772, and E789),modified by the addition of new drift chambers and ho-doscopes with larger acceptance at the first tracking sta-tion. Other improvements to the spectrometer includeda programmable trigger system [11] and a VME-baseddata acquisition system. An 800 GeV/c extracted pro-ton beam with up to 2 × 1012 protons per 20 s spillbombarded one of three identical 50.8 cm long cylindri-cal target flasks containing either liquid hydrogen, liq-uid deuterium or vacuum. After passing through thetarget, the remaining beam was intercepted by a cop-per beam dump located inside the second dipole magnet.The beam dump was followed by a 13.4 interaction lengthabsorber wall of copper, carbon and polyethylene whichblocked the entire aperture of the magnet. This absorberwall removed hadrons produced in the target and thedump, ensuring that only muons traversed the spectrom-eter’s detectors. The detection system consisted of fourtracking stations and a momentum analyzing magnet.The spectrometer’s acceptance as a function of pT , thetransverse momentum of the dimuon pair, was reasonableto 3.0 GeV/c, with some acceptance to 5.0 GeV/c.The targets alternated between hydrogen and deu-terium every five beam spills with a single spill collectedon the empty flask at each target change. The tar-gets were 3.54 and 8.14 g/cm2 thick, corresponding to7 and 15% of an interaction length for hydrogen anddeuterium, respectively. Beam intensity was monitoredby secondary-emission detectors, an ion chamber andquarter-wave RF cavities. Two four-element scintillatortelescopes viewing the targets at nearly 90◦ monitoredthe luminosity, beam duty factor and data acquisitionlive time. The trigger required a pair of triple hodoscopecoincidences having the topology of a muon pair from thetarget. Typically 70 triggers per second were recordedwith an electronic live time above 98%. An integratedflux of 1.3× 1017 protons was delivered.Over 330,000 Drell-Yan events were recorded, usingthree different spectrometer settings which focused low,intermediate and high mass muon pairs. The data col-lected with the low and intermediate mass settings havesystematic effects of a few percent which require addi-tional study. The data from the high mass setting arerelatively free from these effects due to the greatly re-duced rates in the tracking chambers. Therefore, thisLetter presents only the results from the high mass set-ting, with over 140,000 Drell-Yan events.In calculating the Drell-Yan yields, a small correction(averaging 0.2%) for random coincidences between two0.70.80.911.11.20 0.05 0.1 0.15 0.2 0.25 0.3 0.35CTEQ4MMRS(R2)“CTEQ4M (d_ - u_ = 0)”x2σpd/2σppFIG. 1. The ratio σpd/2σpp of Drell-Yan cross sections vs.x2. Shown are next-to-leading order calculations, weightedby acceptance, of the Drell-Yan cross section ratio using theCTEQ4M (solid) and MRS(R2) (dashed) parton distribu-tions. Also shown is a leading order calculation of the crosssection ratio using CTEQ4M (dotted). In the lower CTEQ4Mcurve d¯ − u¯ has been set to 0 as described in the text. Theerrors are statistical only. An additional 1% systematic un-certainty is common to all points.unrelated, oppositely charged muons was made. Thiscorrection was evaluated by studying the observed ratesof same charge muon pairs. A background rate of approx-imately 8% from the target flask and beam line windowswas measured using the evacuated flask and was sub-tracted. Given the identical geometry of the deuteriumand hydrogen targets and the slight difference in the av-erage interaction point due to beam attenuation in thetarget, acceptance differences between the two targetswere shown by a Monte Carlo simulation of the spec-trometer to be very small. The measured cross sectionratio was corrected for differences in beam attenuationin the targets, target density and a small hydrogen con-tamination in the deuterium target. Additionally, thehigh luminosity caused a small, rate dependent ineffi-ciency. This led to a correction of 1.2% in the cross sec-tion ratio. The systematic error in the ratio of yieldsfrom the two targets is dominated by the uncertaintiesin the rate dependence (±0.6%), hydrogen contaminationin the deuterium target (±0.2%) and beam attenuation(±0.2%). All other contributions, including the produc-tion of muon pairs from secondary hadron reinteraction,are small. The total systematic error in the cross sectionratio is less than ±1%.The resulting ratio of the Drell-Yan cross section pernucleon for p+ d to that for p+ p is shown in Fig. 1 andTable I as a function of x2 [12,13], the momentum frac-tion (Bjorken-x) of the target quark in the parton model.2TABLE I. Ratio of deuterium to hydrogen cross sectionsper nucleon vs. x2. The average of the kinematic variablesfor each bin is also tabulated. Note that 〈x1〉 = 〈xF 〉 + 〈x2〉.The errors are statistical only. An additional 1% systematicuncertainty is common to all points.〈pT 〉 〈Mµ+µ− 〉〈x2〉 〈xF〉 (GeV/c) (GeV/c2) σpd/2σpp0.036 0.537 0.92 5.5 1.039 ± 0.0170.057 0.441 1.03 6.5 1.079 ± 0.0130.082 0.369 1.13 7.4 1.113 ± 0.0150.106 0.294 1.18 7.9 1.133 ± 0.0200.132 0.244 1.21 8.5 1.196 ± 0.0290.156 0.220 1.21 9.3 1.124 ± 0.0350.182 0.192 1.20 9.9 1.091 ± 0.0430.207 0.166 1.19 10.6 1.098 ± 0.0550.231 0.134 1.18 11.1 1.055 ± 0.0670.264 0.095 1.18 11.8 0.967 ± 0.0670.312 0.044 1.12 12.8 0.881 ± 0.141(The Bjorken-x of the beam parton is denoted by x1.)Muon pairs with mass, Mµ+µ− , below 4.5 GeV/c2 or be-tween 9.0 and 10.7 GeV/c2 were removed to eliminatecontributions from the J/ψ and Υ resonance families.The data clearly show that the Drell-Yan cross sectionper nucleon for p + d exceeds p + p over an apprecia-ble range in x2. Figure 1 also shows the predictions fornext-to-leading order calculations [14] of the cross sectionratio, weighted by the E866 spectrometer’s acceptance,using the CTEQ4M [3] and MRS(R2) [4] parton distri-butions. The lower curve shows the predicted ratio for amodified CTEQ4M parton distribution which maintainsthe parameterization for d¯p+u¯p but sets d¯p−u¯p = 0. Thedata are in reasonable agreement with the unmodifiedCTEQ4M and the MRS(R2) predictions for x2 < 0.15.It is clear that d¯p 6= u¯p in this range. Above x2 = 0.15the data lie well below both parameterizations.The acceptance of the spectrometer was largest forxF = x1−x2 > 0. In this kinematic regime the Drell-Yancross section is dominated by the annihilation of a beamquark with a target antiquark. This fact, coupled withthe assumption of charge symmetry between the neutronand proton and the assumption that the deuteron partondistributions can be expressed as the sum of the protonand neutron distributions, yields a simple approximateform of the Drell-Yan cross section ratio,σpd2σpp∣∣∣∣x1≫x2≈12(1 + 14d1u1)(1 + 14d1u1d¯2u¯2)(1 +d¯2u¯2). (4)The subscripts 1 and 2 denote that the parton distribu-tions in the proton as functions of x1 and x2, respectively.In the case that d¯ = u¯, the ratio is 1. This equation il-lustrates the sensitivity of the Drell-Yan measurement tod¯/u¯ and is valid only for x1 ≫ x2. It does, however,imply an excess of d¯ with respect to u¯ for the data. Esti-0.40.60.811.21.41.61.822.20 0.05 0.1 0.15 0.2 0.25 0.3 0.35CTEQ4MNA 51xd_  / u_FIG. 2. The ratio of d¯/u¯ in the proton as a function of xextracted from the Fermilab E866 cross section ratio. Thecurve is from the CTEQ4M parton distributions. The errorbars indicate statistical errors only. An additional systematicuncertainty of ±0.032 is not shown. The result from NA51 isalso plotted as an open box.mates of the nuclear effects in deuterium are significantlyless than statistical errors shown in Fig. 1 [15].Some of the data, especially at higher x2, do not sat-isfy the x1 ≫ x2 criterion of Eq. 4. Consequently, d¯/u¯was extracted iteratively by calculating the leading orderDrell-Yan cross section ratio using a set of parton dis-tribution functions as input and adjusting d¯/u¯ until thecalculated cross section ratio agreed with the measuredvalue. In this procedure, the values for the d¯ + u¯, va-lence and heavy quark distributions given by the globalfits [e.g., CTEQ4M and MRS(R2)] were assumed to becorrect. In the beam proton, when x1 ≤ 0.345, the d¯/u¯distribution was assumed to be the same as in the targetproton. For x1 > 0.345, a constant value of 1 for d¯/u¯ wasused. Varying the high-x1 value of d¯/u¯ produced almostno change in the low x2 bins, and less than a 3% changein the highest x2 bin. This procedure was followed us-ing both the CTEQ4M and MRS(R2) parameterizationsand negligible differences were seen. The extracted d¯/u¯ratio is shown in Fig. 2 along with the prediction madeby CTEQ4M. A qualitative feature of the data, not seenin either parameterization, is the rapid decrease towardsunity of the d¯/u¯ ratio beyond x = 0.2. At x = 0.18, theextracted d¯/u¯ ratio is somewhat smaller than the valueobtained by NA51. Although the average value of Q2(M2µ+µ−) is different for the two data sets, the changein d¯/u¯ predicted by the parton distributions due to Q2evolution is small.To address the GSR violation observed by NMC, theextracted d¯/u¯ ratio is used together with the CTEQ4Mvalue of d¯ + u¯ to obtain d¯ − u¯. (Nearly identical results3-0.200.20.40.6Systematic Errord_  - u_(a)CTEQ4MMRS(R2)00.050.10.150 0.05 0.1 0.15 0.2 0.25 0.3 0.35Systematic Errorx∫ (d_ - u_) dx(b)CTEQ4MMRS(R2)FIG. 3. Fermilab E866 values for (a) d¯ − u¯ and (b)∫0.345x(d¯− u¯)dx′ in the proton versus x. The curves rep-resent the corresponding values obtained by the CTEQ4M(solid) and MRS(R2) (dashed) parameterizations. The barat 0.147 ± 0.039 on the left axis in (b) shows the result ob-tained by NMC for the integral from 0 to 1.are obtained if MRS(R2) is used instead of CTEQ4M.)Since each bin in x has a different average Q2, d¯− u¯ wasscaled to a common Q value of 7.35 GeV/c, the averagefor the entire data set. Based on this, the integral ofd¯ − u¯ between xmin and 0.345 is calculated. Both d¯ − u¯and∫0.345xmin(d¯− u¯)dx are shown in Fig. 3. The integralreaches a value of 0.068 ± 0.007(stat) ± 0.008(syst) atxmin = 0.02. This may be compared with the CTEQ4Mand MRS(R2) parameterizations which have values of0.076 and 0.100, respectively, for the integral over thesame region. Over the range 10−4 < x < 1, CTEQ4Mgives a value of 0.108 for the integral, and MRS(R2) gives0.160. Above x = 0.345, it is unlikely there are signif-icant contributions to the d¯ − u¯ integral since the seais relatively small in this region. Both CTEQ4M andMRS(R2) find this region contributes less than 0.002 tothe total integral. It is clear, however, that significantcontributions to the integral arise in the unmeasured re-gion below x = 0.02.Such a large d¯/u¯ asymmetry cannot arise from pertur-bative effects [16]. Most parameterizations of the partondistribution functions (e.g. CTEQ, MRS) simply assumea shape for d¯−u¯ that accommodates the NMC and NA51results. It has been suggested [17,18] that including theeffects of virtual mesons can account for the observedasymmetry and this appears [19–22] to be at least qual-itatively correct.In summary, this Letter reports a measurement of theDrell-Yan cross section ratio per nucleon of p+d to p+p.From this measurement the asymmetry of the light quarksea in the proton is extracted as a function of x. A featureof the present result is the reduction in d¯/u¯ for x > 0.2.The current data are in qualitative agreement with NA51,but with a smaller d¯/u¯ ratio at the single value of x whichthey measured. Over the range of x covered in this exper-iment (0.020 < x < 0.345), the integral of d¯ − u¯ reachesa value of 0.068 ± 0.007(stat) ± 0.008(syst). However,contributions to the integral arise from the unmeasuredregion below x = 0.020.We would like to thank the Fermilab Particle Physics,Beams and Computing Divisions for their assistance inperforming this experiment. This work was supported inpart by the U.S. Department of Energy.∗ Present address: University of Illinois, Urbana, IL 61801.† Also with University of Texas, Austin, TX 78712.‡ On leave from Kurchatov Institute, Moscow 123182, Rus-sia.[1] K. Gottfried, Phys. Rev. Lett. 18, 1174 (1967).[2] P. Amaudruz et al., Phys. Rev. Lett. 66 2712 (1991); M.Arneodo et al., Phys. Rev. D 50, R1 (1994).[3] H.L. Lai et al., Phys. Rev. D 55, 1280 (1997).[4] A.D. Martin, R.G.Roberts and W.J. Stirling, Phys. Lett.B 387, 419 (1996).[5] S.D. Drell and T.M. Yan, Ann. Phys. (NY) 66, 578(1971).[6] S.D. Ellis and W.J. Stirling, Phys. Lett. B 256, 258(1991).[7] P.L. McGaughey et al., Phys. Rev. Lett. 69, 1726 (1992).[8] A. Baldit et al., Phys. Lett. B 332, 244 (1994).[9] S. Kumano, Phys. Reports. (to be published).[10] G. Moreno et al., Phys. Rev. D 43, 2815 (1991).[11] C.A. Gagliardi et al., Nucl. Instrum. Methods (submit-ted).[12] We define xF = (x1 − x2) = 2pl/√s, where pl is thelongitudinal momenta of the muon pair in the center ofmass and s is the total center of mass energy squared.[13] Distributions of kinematic variables within bins may beobtained by contacting C.N. Brown at chuckb@fnal.gov.[14] The code necessary to calculate the next-to-leading ordercross section ratio was provided by W.K. Tung of CTEQ.[15] D.M. Alde et al., Phys. Rev. Lett. 64, 2479 (1990).[16] D.A. Ross and C.T. Sachrajda, Nucl. Phys. B 149, 497(1979).[17] A.W. Thomas, Phys. Lett. B 126, 97 (1983); A. Signal,A.W. Schreiber, and A.W. Thomas, Mod. Phys. Lett. A6, 271 (1991).[18] E.M. Henley and G.A. Miller, Phys. Lett. B 251, 453(1990).[19] W-Y.P. Hwang, J. Speth and G.E. Brown, Z. Phys. A339, 383 (1991).[20] S. Kumano, Phys. Rev. D 43, 3067 (1991); 43, 59 (1991);S. Kumano and J.T. Londergan, ibid. 44, 717 (1991)[21] E.J. Eichten, I. Hinchcliff, and C. Quigg, Phys. Rev. D45, 2269 (1992); 47, R747 (1993).[22] A. Szczurek, M. Ericson, H. Holtzman, and J. Speth,Nucl. Phys. A 596, 397 (1996).4

Measurement of the Light Antiquark Flavor Asymmetry in the Nucleon Sea

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Measurement of the Light Antiquark Flavor Asymmetry in the Nucleon Sea

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