The hadroproduction of lepton pairs with mass Q and finite transverse
momentum Q_T is described in perturbative QCD by the same partonic subprocesses
as prompt photon production. We demonstrate that, like prompt photon
production, lepton pair production is dominated by quark-gluon scattering in
the region Q_T>Q/2. This feature leads to sensitivity to the gluon density in
kinematical regimes accessible in collider and fixed target experiments, and it
provides a new independent method for constraining the gluon density.Comment: 5 pages, 7 figures, to appear in the Proceedings of the Fermilab
  Physics at Run II Workshop on ``QCD and Weak Boson Physics

Berger, E. L.

Klasen, M.

English

arXiv

UNT Digital Library

lnvitcd p:lptxprcsm[cd by E. L. Ekrger to bc publisbd ill thef?ocmlin$$ ofllz] XXXt/t Inrerna[iottof Co)lference 0,1High Energy F’Ay.sic.v(iCHEP 2000), O.wkcl. Jopon, Jul.v ?7 - t(U,$llSf 2, 2000.IANL-HEP-cP-00-098CONSTRAINTS ON THE PROTON’S GLUON DENSITY FROMLEPTON-PAIR PRODUCTIONEDMOND L. BERGERHigh Energy Physics Division, Argonne National Laborato~, Argonne, IL6O439, USAE-mail: berger@anLgovM. KLASENII. .fnstitut fir Tlworetische Physik, Universitiit Hamburg, D-22761 Hamburg, GermanyE-mail: michael.klasen@desy.deMsasive lepton-pair production, the Drell-Yan process, should be a good source of independent con-straints on the gluon density, free from the experimental and theoretical complications of photon iso-lation that beset studies of prompt photon production. We provide predictions for the spin-averagedand spin-dependent differential cross sections as a function of transverse momentum QT.1 IntroductionMassive lepton-pair production, hl + h2 +7* +X; ~“ + 1~,the Drell-Yan process, andprompt real photon production, hl + h2 +7 +X, are two of the most valuable probesof short-distance behavior in hadron reac-tions. They supply critical information onparton momentum densities and opportuni-ties for tests of perturbative quantum chro-modynamics (QCD). Spin-averaged partonmomentum densities may be extracted fromspin-averaged nucleon-nucleon reactions, andspin-dependent parton momentum densitiesfrom spin-dependent nucleon-nucleon reac-tions.The Drell-Ysn process has tended to bethought of primarily as a source of informa-tion on quark densities. Indeed, the mass andlongitudinal momentum (or rapidity) depen-dence of the cross section (integrated overthe transverse momentum QT of the pair)provide essential constraints on the antiquarkmomentum density, complementary to deep-inelastic lepton scattering from which onegains information of the sum of the quark andantiquark densities. Prompt real photon pro-duction, on the other hand, is a source of es-sential information on the giuon momentumdensity. At lowest order in perturbation the-ory, the reaction is dominated at large valuesof the transverse momentum ~ of the pro-duced photon by the QCD “Compton” sub-process, q + g + 7 + q. This dominance ispreserved at higher orders, indicating thatthe experimental inclusive cross section dif-ferential in ~ may be used to determine thedensity of gluons in the initial hadrons.In thk contribution, we summarize re-cent workl $2, in which we demonstrate thatthe QCD Compton subprocess, g+g + ~’ +qzJso dominates the Drell-Yan cross section inpolarized and unpolarized proton-proton re-actions for values of the transverse momen-tum Q~ of the pair that are larger thanroughly half of the pair mass Q, QT >Q/2. Dominance of the qg contribution inthe massive lepton-pair case is as strong ifnot stronger than it is in the prompt pho-ton case. Massive lepton-pair differentialcross sections are therefore an additional use-ful source of constraints on the the spin-averaged and spin-dependent gluon densities.Although the cross section is smaller thanthe prompt photon cross section, massive lep-ton pair production is cleaner theoreticallysince long-range fragmentation contributionsare absent as are the experimental and the-oretical complications associated with isola-tion of the real photon. As long QT is large, .DISCLAIMERThis 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 disciosed, 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 avaiiab[e originaldocument.>eFor PUMSJMLSuse.,lmillrl r,, r,r, ,,, ,!, r,,,!,,,, ‘Tpr, ,i, r,,,,,,,,,,,@l +T”xais= 1.8Tevwmm10Figure 1. Contributions from the partonic subpro-cessee 99 and q? to the invariant inclusive cross sec-tion Ed3u/dp3 as a function of QT for pp + Y*X ata = 1.8 TeV.the perturbative requirement of small cr,(Q~)can be satisfied without a large value of Q.We therefore explore and advocate the p~tential advantages of studies of #a/dQdQ~as a function of Q~ for modest values of Q,Q * 2GeV, below the range of the traditionalDrell-Yan region.2 Unpolarized Cross SectionsFor M + -Y*+ X at @ = 1.8 TeV and sev-eral values of the mass of the lepton-pair, wepresent in Fig. 1 the qtj and qg perturbativecontributions to the invariant inclusive crosssection l?&a/dp3 as a function of QT. Forsmall Q~, the q~ contribution exceeds thatof qg channel. However, as Q~ grows, theqg contribution becomes increasingly impor-tant aad accounts for 70 to 80 % of the rateonce QT = Q. (The q~ contribution begins to100pp+y”xa143.2m Gcv1 I r 1 t—%. . . .au. . . . .\ *‘,,‘$,w‘,$.,...‘- . . . . . . .l<y<l--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 2. Contributions from the partonic subpr~cesses qg and qq to the invariant inclusive cross sec-tion Ed3u/dp3 as a function of QT for w + -y*X atW = 200 GeV.be felt a second time at very large QT owingto the valence nature of the ~ density in theP.) Subprocesses other than those initiatedby the q~ and qg initial channels contributenegligibly.Prompt photons have been observed inFermilab Tevatron collider experiments withvalues of ~ extending to 100 GeV and be-yond. Lepton-pair cross sections are smallerowing to the factor Qem/(37rQ2 ) associatedwith the decay of the virtual photon to p~p–.It should be possible to examine massivelepton-pair cross sections in the same datasample out to QT of 30 GeV or more. Thestatistical limitation to QT of about 30 GeVmeans that the reach in zglti~~, the fractionallight-cone momentum carried by the incidentgluon, is limited presently to 2QT/@ N0.033, a factor of three less than that poten-tially accessible with prompt photons. It is ~IIvaluable nevertheless to investigate the gluondensity in the region XglUOn* 0.033, and less,with a process that has reduced experimentaland theoretical systematic uncertainties fromthose of the prompt photon case.In Fig. 10 of the first paperl, we showa comparison with data of our calculated in-variant inclusive cross section E&u/dp3 asa function of QT for p + P + y* + X, with-y* + I.#p”, at @ = 630 GeV, with 27nP <Q <2.5 GeV. The theoretical expectation isin good agreement with the data published bythe CERN UA1 collaboration. Dominanceof the qg component is evident over a largeinterval in QT. It would be valuable to makea similar comparison with Tevatron data.Results similar to those above are shownin Fig. 2forpp+. -y*+ Xat@ =200GeV appropriate for the RHIC collider atBrookhaven. The fraction of the cross sec-tion attributable to qg tiltiated subprocessesagain increases with QT, growing to 8070 forQT ~ Q. Predictions of spin-averaged andspin-dependent cross sections for the ener-gies of the RHIC collider may be found inthe second paper2. Adopting the nomirdvalue E#v/dp3 = 10-3pb/GeV2, we estab-lish that the massive lepton-pair cross sec-tion may be meaaured to QT =14 and 18.5GeV in pp + T* + X at & =200 and 500GeV, respectively, when 2< Q <3 GeV, zmdto QT =11.5 and 15 GeV when 5< Q <6 GeV. In terms of reach in Z91U0., thesevalues of QT may be converted to zg~tio~=XT = 2QT/fl = 0.14 and 0.075 at fi =200 and 500 GeV when 2< Q <3 GeV, andto xgl~o~ = 0.115 and 0.06 when 5 < Q <6 GeV. The smaller cross section in the caseof massive lepton-pair production means thatthe reach in x~ltio~ is restricted to a factor ofabout two to three less, depending on @and Q, than that potentially accessible withprompt photons in the same sample of data.In the first paperl, we compare our spin-averaged cross sections with fixed-target dataon massive lepton-pair production at largevalues of QT, and we establish that 6xed-order perturbative calculations, without re-summation, should be reliable for QT > Q/2.Although the qg Compton subprocess isdominant, one might question whether un-certainties associated with the quaxk densitycompromise the possib~lty to determine thegluon density. In this context, it is usefulto reca114that when the Compton subprocessis dominant, the spin-averaged cross sectionsfor prompt photon production and for lepton-pair production may be rewritten in a formin which the quark densities do not appearexplicitly, but, instead, dkectly in terms ofthe proton structure function Fz(z, p?) meu-sured in spin-averaged deep-inelastic lepton-proton scattering. An analogous statementapplies in the spin-dependent case where thelepton pair cross section may be expressedin terms of the gl (z, p; ) structure functionmeasured in spin-dependent deepinelaaticlepton-proton scattering.AcknowledgmentsWork at Argonne National Laboratory is supported by the U.S. Department of Energy,Division of High Energy Physics, under Con-tract W-31-109-ENG38. M.K. is supportedby DFG through grant KL 1266/1-1.References1. E. L. Berger, L. E. Gordon, andM. Klasen, Phys. Rev. D 58, 074012(1998) and Nuci. Phys. Proc. Suf)l)i.82, 179 (2000).2. E. L. Berger, L. E. Gordon, andM. Klssen, Ph~s. Rev. D 62, 014014(2000) and hepph/0001190.3. CERN UA1 Collaboration, C. Albajar etai, Ph~s. Lett. B209, 397 (1988).4. E. L. Berger and J.-W. Qiu, Phys. Rev.D 40,778 (1989) and D 40,3128 (1989).

Constraints on the Proton's gluon density from lepton-pair production.

The hadroproduction of lepton pairs with mass Q and finite transverse momentum Q{sub T} is described in perturbative QCD by the same partonic subprocesses as prompt photon production. The authors demonstrate that, like prompt photon production, lepton pair production is dominated by quark-gluon scattering in the region Q{sub T} &gt; Q/2. This feature leads to sensitivity to the gluon density in kinematical regimes accessible in collider and fixed target experiments, and it provides a new independent method for constraining the gluon density

Constraints on the gluon density from lepton pair production

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.344.435

Constraints on the Gluon Density from Lepton Pair Production

Abstract

Similar works

Full text

Available Versions

UNT Digital Library

UNT Digital Library