A partisan review of some of the most important $\gamma\gamma$ channels
accessible at LEP 2, with special stress on the measurement of the photon
structure function $F_{2}^{\gamma}$ and on associated problems with Monte Carlo
modelling.Comment: 7 pages including 5 figures. Invited talk given at the LEP2
  Phenomenology Workshop, Oxford, U.K., 14 April 199

Miller, D. J.

English

arXiv

A partisan review of some of the most important $\gamma\gamma$ channels accessible at LEP 2, with special stress on the measurement of the photon structure function $F_{2}^{\gamma}$ and on associated problems with Monte Carlo modelling

Miller, D.J.

CERN Document Server

UCL/HEP 97-06Questions in Two Photon Physics at LEP2;including data Monte-Carlo comparison z.D.J. MILLERUniversity College London, Gower Street, London WC1E 6BT, EnglandAbstract. A partisan review of some of the most important γγ channels accessibleat LEP 2, with special stress on the measurement of the photon structure functionFγ2 and on associated problems with Monte Carlo modelling.1. IntroductionttaguntaggedP   ~ 0Wθ2~Q2Figure 1. Variables inelectron-photon DIS.There is a long agenda of possible γγ topicsfor LEP2 to study. This talk dwells most uponthe measurement of F γ2 , with brief visits to someof the rest. I apologise in advance for OPAL-centricity.2. Extracting F γ2 (x;Q2)Figure 1 shows the Feynman graph from whichthe interesting variables are dened. The taggedlepton is either an electron or a positron, detectedin the forward or endcap detectors with ascattering angle t and energy Et.d2eγ!eXdxdQ2=22xQ4[(1 + (1− y)2)F γ2 (x;Q2)− y2F γL(x;Q2)]Q2 = −q2 ’ 2EbEt(1− cost)y = 1− (Et=Eb)cos2(t=2)x =Q2Q2 + P 2 +W 2’Q2Q2 +W 2where q is the four-momentum of the o-shell probe photon and Eb is the beam energy.Note that It is very dicult to measure FL because the rate is lower at high y where thisterm is most signicant, and there are serious backgrounds generated by beam-associated low energy electrons { at least in OPAL and DELPHI. (For y >> 0:5,Et << 0:5, yet most tagged studies require Et > 0:7Eb.) There is a suggestionthat ALEPH may be able to do it [1] but it is hard to be hopeful. Nothing haschanged since the 1986 Aachen green book [2].z Invited talk given at the LEP2 Phenomenology Workshop, Oxford, U.K., 14 April 1997205101520253035400 5 10 15 20 25 30 35 40W  [GeV]Wvis [GeV]Wvis without FR (θ > 200mrad)Wvis with FR (θ > 25mrad)(a)   HERWIG  05101520253035400 5 10 15 20 25 30 35 40W  [GeV]Wvis [GeV]Wvis without FR (θ > 200mrad)Wvis with FR (θ > 25mrad)(b)   F2GEN pointlike  00.511.522.533.54-6 -4 -2 0 2 4 6η=−ln(tan(θ//2))Energy flow   1/N dE/dη   [GeV]OPAL <Q2> = 13.0 GeV2Corrected DataF2GEN pointlikeF2GEN perimissHERWIGPYTHIAFigure 2. W − Wvis cor-relation [3] for various MonteCarlo models with and withoutthe included simulation of theOPAL forward region (FR) be-tween 25 <  < 200 mrad.Figure 3. Hadronic energy flow perevent as a function of pseudorapid-ity; data [4] compared with variousMonte Carlo models. The tag is al-ways at negative  and is not shown. The double tagged rate is low with existing luminometers, but all of theexperiments are now taking their very small angle taggers more seriously so weshould soon see measurements of P 2 dependence from 0 to ’ 0:5 GeV2. In singly tagged studies for F γ2 real progress has been made, but there are practicalproblems, mainly because the mass W of the γγ system is hard to measure (acompletely a dierent situation from HERA where the target proton has a knownmomentum). These problems will be the main focus of my talk.Our photons come from a very soft, spread-out spectrum. We are forced tomeasure the visible mass Wvis of the hadronic tracks and clusters. This is an ecientmeasurement in the barrel region ( > 300 mrad) but the eciency and accuracyboth fall o in the endcaps and luminometers. The two standard QCD Monte Carlos,HERWIG [5] and PYTHIA [6], both show a serious loss of correlation [7] betweenthe generated and the visible values at large W (e.g. section (a) of gure 2, wherethe open circles represent a measurement using only the barrel detectors). Thislack of correlation at large W corresponds to very poor reconstruction of x in theinteresting region where HERA sees rising values of the proton structure function asx decreases [8, 9]. There is great theoretical interest in knowing if the photon hasa similar behaviour to the proton at low x because of its hadron-like character, orwhether the photon’s direct coupling to quark pairs makes a visible dierence. Atrst sight the HERWIG and PYTHIA predictions suggest that these measurementsmight be impossible at LEP.But there is hope. Making use of the sampled hadronic energy in the endcaps andluminometers, the observed energy flows seen in gure 3 are by no means as strongly3forward as HERWIG or PYTHIA would suggest. In fact, the experimental pointshave some of the character of the extreme-case Monte Carlo sample from the purelypointlike F2GEN generator [10]. And note from section (b) of gure 2 that usingthis pointlike F2GEN sample with hadronic energy from the forward region detectorsalmost completely restores the correlation between Wvisible and true W . HERWIGand PYTHIA are clearly not yet fully suitable to be used as unfolding Monte Carloprogrammes for the extraction of F γ2 . They must have something missing, possiblyin the way they treat hard sub-processes like photon-gluon fusion. Interestingly, thePHOJET [11] Monte Carlo shows signs of lying closer to the OPAL data, in a stillunpublished study with 1:5 < Q2 < 6 GeV2, but the present version of PHOJET isnot properly set up for virtual photons.The working group must look at what needs to be done to produce a Monte Carlogenerator that is suitable for unfolding. This generator should be driven by physicsfrom outside the γγ eld but must match all aspects of the γγ data. The group isalso in a position to study the eects of the particular unfolding package that is usedto go from the observed x distribution to the true distribution. There are now twounfolding packages in use, based on very dierent statistical methods, and the newALEPH results use both of them: SVD [12] and RUN [13]. My impression is thatthey agree reasonably well when the statistics are good.The question list for the group is probably longer than the answer list will be: How can the QCD Monte Carlo programmes be improved? What will the remaining systematics then be? Can we ever see if there is a low x rise in the structure function like that observedfor the proton at HERA? How should the charm threshold be treated: can realistic and consistentkinematics and phase-space be used in the Monte Carlo models; should QCDevolution be included, or just the Quark parton Model; what value should betaken for the eective charm mass? Will we be able to t for MS from the high Q2 evolution? Will double tags work?3. Inclusive studiesEver since γγ physics started at PEP and PETRA there have been attempts to testthe predictions of QCD by studying jets at high pT , mostly with untagged events.And every experiment has developed its own tools, its own set of variables and itsown cuts [14]. Now OPAL has introduced an analysis [15] based very closely on whatis done in photoproduction at HERA, and there is some hope that the other LEPexperiments will follow suit. They use a development of the xγ variable which is anestimator of the fraction of the target photon’s momentum carried by the hard partonwhich produces identied jets with high ET ,xγ =Pjets Ej  pz;jPhadronsEi  pz;iwhere pz;i is the momentum of the ith hadron projected along the beam direction.The  ambiguity arises because the untagged initial state is intrinsically symmetric,unlike the situation in ep, and either photon may be the target. Three main categoriesof events with high ET jets are expected: direct, singly resolved or doubly resolved { as4shown in Figure 4 [16]. Using the PYTHIA Monte Carlo, OPAL shows that the directsample should be very cleanly separated from the resolved samples by requiring bothx+γ and x−γ to be greater than 0.8. They conrm this separation in the experimentaldata for two jet events with ET > 3 GeV by computing an eective parton scatteringangle  in the dijet C. of M. and showing that the \direct" (xγ > 0:8) sample hasthe expected rather flat distribution , while the \resolved" sample (x+γ or x−γ less than0.8) is much more forward-backward peaked, as predicted on a parton level by lowestorder QCD (and as seen in analyses of photoproduction at HERA [17]). This is a neweld which has opened up at LEP 2 where the background in the untagged channelfrom Z0 ! hadrons is greatly reduced compare with LEP 1.etc.  etc.a)     directb)     singlyresolvedc)     doublyresolvedqqFigure 4. Feynman graphsfor direct and resolved pro-cesses.A development of this analysis will beto study the jet structure in tagged eventsand to see how it varies as a function of theQ2 of the probe photon. It is not obviouswhat the equivalent variable should be in thiscase to ET in the untagged case, since thetransverse energy in tagged events containsthe hadronic recoil against the tag. It hasbeen suggested that it might be possible touse normal jet-nding algorithms in the C.of M. system of the visible hadrons, but weknow that missed forward energy will alwaysmake this a poor approximation to the truehadronic rest frame. Another possibility is touse the distribution of the Et;out variable (seegure 1 of [18]) instead of ET . A preliminarystudy by Rooke [19] has found a much largersignal in OPAL for two jet events than thepredictions of HERWIG or PYTHIA.One of the worst measured quantities in γγ physics is the total cross section γγfor W < 5 GeV. L3 [20] is now producing LEP2 results at higher W , up to ’ 70GeV. We should all try to match them. Here again, the big problem will be incorrecting believably for the lost hadrons going into the forward region and even downthe beampipe { a signicant number of diractive events at high W may give no signalat all in our detectors. L3 may not yet have understood the full extent of this problem(but see \Utopia", below).4. ResonancesThere is no doubt that we should continue to study charmonium resonances at LEP2.L3 has already had some success at LEP1 with c [21] and with c [22]. It is hardwork because there are lots of decay modes, but adding many channels together cangive respectable peaks. Hundreds of events should be collected in the end for eachof the charmonium resonances, enough to give worthwhile measurements of their γγpartial widths.It is not so clear how far we should pursue the study of non-charmonium resonancesin the 1 to 2 GeV region. Close and colleagues [23] have an important list of glueball5and hybrid candidates whose \stickiness" S has to be checked, where for resonance R,S =Γ(J= ! γR)=phasespaceΓRγγ=phasespace:(S ’ 1 for the f02 (1270), a clear qq state which is prominent in γγ ! +−, butS ’ 25 for (1440), a well known glueball candidate which is barely visible in γγ).The integrated luminosity at LEP 2 is never expected to exceed 500pb−1, whereasCleo II already has in excess of 3fb−1 and the new beauty factories BaBar and Belleshould each get more than 10fb−1. Beauty factories have almost as large a productioncross section for low mass resonances as LEP, and the nal states are easier to measurebecause they are not so strongly boosted along the beam direction. LEP triggers arealso heavily biased against low transverse momentum tracks and low multiplicities.Nevertheless, L3 is beginning to get into resonance studies. It will be hard for theworking group to judge how much should be done on resonances at LEP 2 because noone can tell how much eort will eventually be available for γγ resonance studies at thebeauty factories. [Note added later. In my judgement, the only two γγ ! resonanceresults at Photon ’97 which had real physics impact both came from Cleo II, withlarge statistics [24, 25]. On one of the two topics (Q2 dependence of 0 production)there was also an L3 paper [26] which did not add much at this stage, though withthe full LEP 2 statistics it will eventually give a useful check].5. The biggest exclusive cross-sections at LEP; γγ !vector meson pairs.10-210-111010 21 10 10 2σtot(γp)σ(γp → ρp)σ(γp → ωp)σ(γp → φp)σ(γp → J/ψp)ZEUS publ.H1 publ.ZEUS prel.H1 prel.fixed targetWγp (GeV)cross section (µb)Figure 5. The γp total cross section andthe cross sections for photoproductionof single vector-mesons, including recentHERA data.Figure 5 shows that the crosssection for γp ! p at HERAremains at about 1/12 of the totalγp cross section from about 10GeV to the highest visible energy.The same kind of behaviour isto be expected for the equivalent\elastic" channel γγ ! 00.No one has ever measured thischannel for  masses beyondabout 2.5 GeV because the processbecomes almost totally diractiveand the pions from high energy0 decays go strongly forward.These events must be coming ata rate of 1 every 20 seconds orso, but it is a real challenge tothe LEP experiments to devisetriggers which will identify them,together with related channels like!,  and the semi-inclusivediractive channels with a  plusa low mass forward jet. There willbe good old-fashioned soft stronginteraction physics to be done with such data. But there is even more interest in thesemi-hard processes where one or both of the vector mesons is a J= . Notice that6the γp! J= p cross section rises in Figure 5 much more rapidly than the total crosssection. QCD has predictions to make about such processes [27], and the nal statewill be much more triggerable and measurable than  if it contains a J= ! l+l−decay. The γγ ! J=  channel is well worth further study.6. UtopiaA flight of fancy: if LEP runs to the year 2000, if we had an extra $2M to spend andif we could nd 6 strong uncommitted groups to work flat out for four years. Then letus build a 0 double-tag spectrometer in an unused LEP straight section. We woulduse the Novosibirsk technique [28], with a simple central detector and magnetic bendsclose to the mini-beta quadrupoles so that the momenta of the two tagged electronscan be measured with very high precision by a series of small position sensitivedetectors placed along the outgoing beamlines. Each detector sees the image of thecollision point formed by electrons scattered close to 0 in a small range of momentaand angles, focussed by the quadrupole and dispersed by the bending magnet. Thesimplest such set-up would measure the total cross section with small errors up toWγγ ’ 100 GeV. Adding better large-angle taggers would give Fγ2 (x;Q2) without thecurrent uncertainties in unfolding x. More elaborate forward hadron tracking wouldallow γγ !  to be done properly. It would be a coherent programme and not tooexpensive, but somehow I do not see it happening.7. Conclusions and AcknowledgementsThis review is meant to be provocative rather than even-handed, and to start theworking group o with a list of issues to discuss. It is a good time to have a UKworkshop session on the problems of γγ physics at LEP because a signicant numberof the most active protagonists are based in this country.The organisers of the workshop are to be congratulated on putting together a verysound programme of work to be done in many elds of LEP 2 physics. I look forwardto IOP 1/2 day meetings over the next few years in which the LEP 2 experimentalresults are reported and discussed.This paper owes a great deal to the scientic advice and practical help of Dr. JanA. Lauber.References[1] Terry Sloan, Lancaster/ALEPH, private communication.[2] D. J. Miller. Proceedings of ECFA workshop on Physics at LEP200, Aachen 1986, eds A. Bo¨hmand W. Hoogland; CERN 87-08; ECFA 87/108; 207[3] J. A. Lauber for the OPAL collaboration. Proceedings of ICHEP ’96, Warsaw, eds Z. Ajduk andA. K. Wroblewski, World Scientic, Singapore (1996) 725[4] K. Ackersta et al. The OPAL collaboration. Z. Phys. C74(1997)33[5] G. Marchesini and B. R. 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Proceedings of the Lepton Photon Symposium, Cornell University, August 1993,eds P. Drell and D. Rubin, AIP Conference Proceedings No. 302; 654[15] R. Buergin for the OPAL collaboration. Proceedings of Photon ’97, Egmond aan See,Netherlands, 10-15 May 1997, ed A. Buijs, World Scientic, Singapore.http://www.fys.ruu.nl/ buijs/proceedings/program.html[16] C. H. Llewellyn-Smith. Phys. Lett. B79(1978)83[17] M. Derrick et al. The ZEUS collaboration. Phys. Lett. B322(1994)287 M. Derrick et al. TheZEUS collaboration. Phys. Lett. B384(1996)401[18] S. Cartwright, M. H. Seymour et al. Two photon working group report. These Proceedings.[19] A. M. Rooke. Proceedings of Photon ’97, Egmond aan See, Netherlands, 10-15 May 1997, edA. Buijs, World Scientic, Singapore.[20] W. VanRossum for the L3 collaboration. Proceedings of Photon ’97, Egmond aan See,Netherlands, 10-15 May 1997, ed A. Buijs, World Scientic, Singapore.http://www.fys.ruu.nl/ buijs/proceedings/program.html[21] O. Adriani et al (the L3 collaboration).CERN PPE 93-173[22] A. Buijs for the L3 collaboration. Proceedings of Photon ’95, Sheeld, England 8-13 April 1995,eds Miller, Cartwright and Khoze; World Scientic, Singapore (1995) 232.[23] F. E. Close et al. References in the two photon working group report [18]. These proceedings.[24] H. Paar for the Cleo II collaboration. Proceedings of Photon ’97, Egmond aan See, Netherlands,10-15 May 1997, ed A. Buijs, World Scientic, Singapore.http://www.fys.ruu.nl/ buijs/proceedings/program.html[25] V. Savinov for the Cleo II collaboration. Proceedings of Photon ’97, Egmond aan See,Netherlands, 10-15 May 1997, ed A. Buijs, World Scientic, Singapore.http://www.fys.ruu.nl/ buijs/proceedings/program.html[26] S. Braccini for the L3 collaboration. Proceedings of Photon ’97, Egmond aan See, Netherlands,10-15 May 1997, ed A. Buijs, World Scientic, Singapore.http://www.fys.ruu.nl/ buijs/proceedings/program.html[27] Physics at LEP2 CERN 96-01 (1996) 318[28] L. 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Two Photon Physics at LEP2; including data Monte-Carlo comparison

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