Chapkin, M

Chung, S U

Gavillet, P

Obraztsov, V F

Ryabchikov, D I

Sokolov, A

Todorovova, S

CERN Document Server

Inclusive $J/psi$ Production in Two-Photon Collisions at LEP II with the DELPHI Detector

DELPHI Collaboration DELPHI 2002-083-CONF-617July 3, 2002Study of Inclusive J/ψ Productionin Two-Photon Collisions at LEP IIwith the DELPHI DetectorM. Chapkin, S.-U. Chung, P. Gavillet,V. Obraztsov, D. Ryabchikov, A. Sokolov, S. TodorovovaAbstractInclusive J/ψ production in photon-photon collisions has been observed at LEP IIbeam energies. A clear signal from the reaction γγ → J/ψ+X is seen. The numberof observed N(J/ψ → µ+µ−) events is 36 ± 7 for the integrated luminosity of 617pb−1, yielding a cross-section of σ(J/ψ+X) = 45.3±18.9 pb. Based on a study of theevent shapes of different types of γγ processes in the PYTHIA program, we concludethat (74 ± 22)% of the observed J/ψ events are due to the ‘resolved’ photons, thedominant contribution of which is most probably due to the gluon content of thephoton.PRELIMINARYContributed paper to the Summer 2002 conferences1 IntroductionAn important component of the e+e− collisions at LEP II energy is the two-photon fusionprocess. It has been pointed out that two-photon production of inclusive J/ψ’se+ + e− → e+ + e− + γ1+ γ2, (1)γ1+ γ2→ J/ψ +X (2)is a sensitive channel for investigating the gluon distribution in the photon [1].There are two important processes leading to inclusive J/ψ production. The corre-sponding typical diagrams are given in Fig. 1 a) and b). Less important diagrams are notconsidered here. The first process is described by the vector-meson dominance (VDM)model [2]γ1→ c+ c¯, γ2→ q + q¯ and γ1+ γ2→ J/ψ + q + q¯ (3)as shown in Fig. 1a). The vertices for γ1and γ2are connected by Pomeron exchange ordiffractive dissociation of photons. The final-state parton pairs c+ c¯ and q+ q¯ are both inthe state of JPC = 1−−, which means that the latter is dominated by the low-mass vectormesons ρ0, ω and φ but a more general inclusive hadronization of the partons may alsobe important.The second process is probably described by the color-octet model [3]. It proceedsthrough the so-called ‘resolved’ contribution of the photons, in which the intermediatephotons are ‘resolved’ into their constituent partons.γ1+ gγ→ c+ c¯, γ2+ gγ→ q + q¯, and γ1+ γ2→ J/ψ + q + q¯ (4)as shown in Fig. 1b).a) b)γγPomeronJ/ψc¯cq¯qρ0, ω, φγγgc¯cJ/ψq¯qFigure 1: Inclusive J/ψ production in γγ processes: a) through vector-meson dominance, b)via gluon content of the photon, i.e. ‘resolved’ contributions.It is seen that this process requires production of a ‘resolved’ gluon (gγ) from both photons.Thus, this production mechanism provides a sensitive probe of the gluon content of thephoton.The purpose of this letter is to report observation of inclusive J/ψ production fromthe two-photon fusion process, to give its production characteristics along with the cross-section and finally to assess the relative importance of the production processes discussedabove. Section 2 describes the selection process for the event sample collected for thisstudy. The measurement of inclusive J/ψ production in the µ+µ− channel and its inter-pretation in terms of resolved and diffractive processes is presented in section 3 followedby a summary and conclusions.12 Experimental ProcedureThe analysis presented here is based on the data taken with the DELPHI detector [4, 5]during the years 1996–2000 for which the center of mass energies√s for LEP rangedfrom 161 to 207 GeV, excluding the part of the data collected in the last period of 2000when one of the time projection chamber (TPC) sectors was not in operation. The totalintegrated luminosity used in the analysis is 617 pb−1.The charged particle tracks were measured in the 1.2 T magnetic field by a set oftracking detectors including the TPC, the outer detector (OD) and the forward/backwardchambers FCA and FCB. The following criteria were applied:(a) particle momentum p > 200 MeV/c;(b) relative momentum error of a track ∆p/p < 100%;(c) impact parameter of a track, transverse to the beam axis ∆xy < 3 cm;(d) impact parameter of a track, along the beam axis ∆z < 7 cm;(e) polar angle of a track, with respect to the beam axis 10◦ < θ < 170◦;(f) track length ` > 30 cm.The neutral particles (γ, pi0, K0L, n) were selected by demanding that the calorimetricinformation, not associated with charged particle tracks, satisfy the following cuts:(g) E(neutral) > 0.2 GeV for the electromagnetic showers unambiguously identifiedas photons;(h) E(neutral) > 0.5 GeV for all the other showers;(i) polar angle of a neutral particle tracks, with respect to the beam axis 10◦ < θ <170◦.In order to ensure a very high trigger efficiency, the selected events were required tosatisfy at least one of the following sets of criteria:(j1) one or more charged particle tracks in the barrel region (40◦ < θ < 140◦) withpt > 1.2 GeV/c;(j2) one or more neutral particle tracks in the Forward Electromagnetic Calorimeter(FEMC) ( 10◦ < θ < 36◦ and 144◦ < θ < 170◦) with energy greater than 10 GeV;(j3) the sum of the number of charged particle tracks in the barrel with pt > 1 GeV/c, ofcharged particle tracks in the forward region (10◦ < θ < 40◦ or 140◦ < θ < 170◦)with pt > 2 GeV/c and of neutral particle track in the FEMC with E > 7 GeV isgreater than one;(j4) the sum of the number of charged particle tracks in the barrel with pt > 0.5 GeV/cof charged particle tracks in the forward region with pt > 1 GeV/c and of neutralparticle tracks in the FEMC with E > 5 GeV is greater than four.The trigger efficiency for the events which passed the above requirements is bigger than98%.The hadronic two-photon events are characterized by a low visible invariant mass.Consequently, the following additional cuts were applied:(k) the visible invariant mass, Wvis, calculated from the four-momentum vectors ofthe measured charged and neutral particle tracks, is less than 35 GeV/c2;(l) the number of charged particle tracks Nch satisfies 4 ≤ Nch ≤ 30;(m) the sum of the transverse energy components with respect to the beam directionfor all charged particle tracks (∑EvisT) is greater than 3 GeV.The comparison of the Wvis distributions—after the cuts on Nch and∑EvisT—both forthe data and the events simulated by PYTHIA (Fig. 2) shows that the cut Wvis ≤ 352GeV/c2 rejects the major part of the non-two-photon events.Wvis, GeV/c2N eventWvis,  GeV/c2Figure 2: Wvis distributions for the LEP II DELPHI data and for the simulated γγ → hadrons,e+e− → Z0γ, e+e− →W+W− and the sum of all above Monte-Carlo contributionsA total of 274 510 events remain in the data sample after applying all these cuts.The main background comes from the process e+e− → Zγ and amounts to ∼1.2% of theselected γγ events. The backgrounds from the e+e− → W+W− and other processes arenegligible, as seen in Fig. 2.J/ψ candidates have been selected using the µ+µ− decay channel. For the muon pairselection the following criteria were imposed:(n) at least two charged particle tracks with zero net charge should satisfy thestandard DELPHI muon-tagging algorithm [5] or be identified as muons by thehadronic calorimeter;(o) the tracks should not come from any reconstructed secondary vertex or be identi-fied as a kaon, proton or electron by the standard DELPHI identification packages;3 Inclusive J/ψ ProductionIn Fig. 3 we give the invariant mass distribution for pairs of identified µ+µ− selected asoutlined in the previous section. It is seen that the J/ψ is produced over little background.A least squares fit to theM(µ+µ−) distribution with a Gaussian for the signal and a secondorder polynomial for the background gives the following results:J/ψ mass: M = 3119±8 MeV/c2J/ψ width: σ(obs) = 35±7 MeV/c23Figure 3: M(µ+µ−) distribution from the LEP II DELPHI data. The solid curve correspondsto a Gaussian fit over a second order polynomial background.The observed width of the peak is consistent within errors with the invariant massresolution of the pair of charged particle tracks in the mass region around 3GeV/c2. Thenumber of observed events from the fit is N(J/ψ) = 36 ± 7. If we take the L3 result[6]on beauty cross-section from γγ events and the PDG value[7] for the branching ratioof beauty hadrons to J/ψ, the expected number of J/ψ → µ+µ− from beauty hadronsis 2.1 ± 0.6. The backgrounds from the processes e+ + e− → Z + γ → J/ψ + X andγ + γ → χc2 → J/ψ + pi+ + pi− + pi0 are less than 0.20 and 0.30 events, respectively.According to our selection criteria the system X contains at least two charged particletracks, hence we do not consider such sources of J/ψ production as γ+γ → χc2 → J/ψ+γ.We checked that in the four-prong events with J/ψ → µ+µ− candidates there is no photonconversions.We used the PYTHIA 6.156 generator [8] to estimate the efficiency. The generatedevents were passed through the simulation package of the DELPHI detector [9] and thenprocessed with the same reconstruction and analysis programs as the real data. There isa substantial fraction of PYTHIA events where J/ψ’s are produced just as a simple fusionof two photons because there is not enough phase space to produce additional particles.We do not use such events. The process where both photons are VDM photons we willcall as ‘diffractive’ and the process without VDM photons we will call as ‘resolved’.The characteristics of the J/ψ production are shown on Fig.4 to Fig.8. For all thesefigures, the background was estimated from the analysis of the M(µ+µ−) spectrum forevery bin of a given distribution and was statistically subtracted.Fig. 4 shows the p2T(J/ψ) distribution. As expected, the PYTHIA prediction for thep2T(J/ψ) distribution is more sharply peaked near zero for the diffractive MC events (seeFig.1 (a) ) than for the ‘resolved’ MC events (see Fig.1 (b) ). We fitted the experimental4Figure 4: p2T(J/ψ) distribution from the LEP II DELPHI data, shown as points with error bars.The histogram is a composite of the renormalized ‘resolved’ and ‘diffractive’ processes fromPYTHIA (see text).p2T(J/ψ) distribution as a function of the two categories of MC PYTHIA eventsdNdp2T= f · dNdp2T∣∣∣∣∣Diffractive+ (1− f) · dNdp2T∣∣∣∣∣Resolved(5)which gives f = (26±22)% (PYTHIA distributions in Fig 4 are normalized to the data).The PYTHIA study tells us that the experimental efficiencies are very different for thetwo categories:(diffractive) = (0.98± 0.04)%(resolved) = (3.87± 0.09)% (6)According to PYTHIA, about one-half of all the γγ events with J/ψ → µ+µ− are producedwith the charged particle tracks at polar angles below 10 degrees, so that they are invisibleto the DELPHI detector. The individual efficiencies as a function of p2Tare given in Fig. 5.Some insight may be gained into these efficiencies if they are broken down into productsof two factors, as follows:(diffractive) = γ γ(diffractive)× J/ψ→µ+µ−(diffractive)(resolved) = γ γ(resolved)× J/ψ→µ+µ−(resolved)(7)where γ γ is the efficiency for the process γ γ → J/ψ + X and J/ψ→µ+µ− is that forJ/ψ → µ+µ−. As expected, the latter is relatively process-independentJ/ψ→µ+µ−(diffractive) = (37.0± 1.5)%J/ψ→µ+µ−(resolved) = (32.1± 0.7)%(8)It is clear, therefore, that the difference in efficiency in (6) is mostly due to γ γ—this ishighly process-dependent and hence model-dependent.5The overall experimental efficiency is1=f(diffractive)+1− f(resolved)(9)which gives  = (2.19+1.27−0.59)%. Under the assumption that PYTHIA captures the kine-matical features of the resolved and diffractive processes—but not their absolute cross-sections—the cross-section for inclusive J/ψ production isσ = N(J/ψ) · (Br · L · )−1 = 45.3± 18.9 pb (10)where Br = (5.88± 0.10)% is the branching ratio for J/ψ → µ+µ− [7] and L = 617 pb−1is the total integrated luminosity. The quoted error reflects only that associated with thevisible part of the spectra; in particular, it does not include the uncertainties inherent tothe PYTHIA program.Figure 5: Efficiencies for resolved and diffractive processes as functions of p2T.Because of the model-dependent aspect of this analysis (see, for example, the effi-ciencies given in (6)), it is of interest to quote the ‘visible’ cross-section. Substitut-ing J/ψ→µ+µ−(diffractive) and J/ψ→µ+µ−(resolved) for (diffractive) and (resolved)—respectively—in (9), the ‘visible’ cross-section can be calculated; it isσvis = 2.99± 0.58(stat)± 0.10(syst) pb (11)The main source of systematic uncertainty comes from the determination of the relativefractions of resolved and diffractive events which have different efficiencies (8). Followingthe same argument, we also give the ‘visible’ production rate 〈n〉 for J/ψ production〈n〉 = N(J/ψ) ·(Nt ·Br · J/ψ→µ+µ−)−1= (6.7± 1.3(stat)± 0.2(syst))× 10−3 (12)where Nt is the data sample for the γγ selection as given in the previous section, i.e.Nt = 274 510.6The rapidity distribution for the J/ψ is shown in Fig. 6. The PYTHIA MC events havebeen combined using the same fraction f found in (5) and then normalized to the observednumber of events in 0 < |y| < 2.0. The same techniques have been used to compare theexperimental distributions of M(J/ψ + X), M(X), the charged and total multiplicities[Nch(X) and Ntot(X)], in Fig.7 a, b, c, and d. There is fair agreement within statisticsbetween the shapes of our measured distributions and the PYTHIA predictions (usingthe best fit as found in (5) for the relative content of diffractive and resolved events andrenormalizing the PYTHIA prediction to the number of observed events).Figure 6: |y| distribution for the J/ψ from the LEP II DELPHI data, shown as points with errorbars. The histogram is a composite of the renormalized ‘resolved’ and ‘diffractive’ processesfrom PYTHIA (see text).The acceptance-corrected distributions in cos θ where θ is the helicity angle of µ+ inthe rest frame of J/ψ → µ+µ− are shown in Figs.8 (a–c), along with the results of a fit tothe form (1 + a cos2 θ). The fitted parameters are a = −0.93± 0.57 for the total sample(a), a = −1.76± 0.53 for p2T(J/ψ) < 1.0 (GeV/c2) (b) and a = 0.71± 1.33 for p2T(J/ψ) > 1.0(GeV/c)2 (c). These results indicate that the J/ψ’s are produced with little polarization athigh p2T(J/ψ), where the main contribution comes from the resolved processes.7Figure 7: Visible distributions of M(J/ψ + X) (a), M(X) (b), charged (c) and total (d) mul-tiplicities of the X system. Each histogram is a composite of the renormalized ‘resolved’ and‘diffractive’ processes from PYTHIA (see text).Figure 8: Acceptance-corrected distributions in cos θ where θ is the helicity angle of µ+ in therest frame of J/ψ → µ+µ−. The figures (a–c) correspond to the total sample (a), the subsampleswith p2T(J/ψ) < 1.0 (GeV/c)2 (b) and p2T(J/ψ) > 1.0 (GeV/c)2 (c).84 ConclusionsWe have studied the inclusive J/ψ production from γγ collisions. The data have been takenby the DELPHI collaboration during the LEP II phase, i.e.√s of the LEP machine rangedfrom 161 to 207 GeV. A clear signal from the reaction γγ → J/ψ +X is seen.The inclusive cross-section is estimated to be σ(J/ψ+X) = 45.3±18.9 pb. Based on astudy of the p2T distribution of different types of γγ processes in the PYTHIA program, weconclude that some (74±22 )% of the observed J/ψ events are due to the ‘resolved’ photons,the dominant contribution of which is derived from a gluon content of the photon [3].The distributions in p2T(J/ψ), y and cos θ (for µ+ in the rest frame of J/ψ → µ+µ−) arepresented. In addition, a study is given of the characteristics of the system X. All distri-butions appear to be well reproduced within statistics by the renormalized combinationof the fitted ‘resolved’ and ‘diffractive’ contributions.AcknowledgementsWe are greatly indebted to our technical collaborators, to the members of the CERN-SLDivision for the excellent performance of the LEP collider, and to the funding agenciesfor their support in building and operating the DELPHI detector.We acknowledge in particular the support ofAustrian Federal Ministry of Education, Science and Culture, GZ 616.364/2-III/2a/98,FNRS–FWO, Flanders Institute to encourage scientific and technological research in theindustry (IWT), Belgium,FINEP, CNPq, CAPES, FUJB and FAPERJ, Brazil,Czech Ministry of Industry and Trade, GA CR 202/99/1362,Commission of the European Communities (DG XII),Direction des Sciences de la Matie`re, CEA, France,Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie, Germany,General Secretariat for Research and Technology, Greece,National Science Foundation (NWO) and Foundation for Research on Matter (FOM),The Netherlands,Norwegian Research Council,State Committee for Scientific Research, Poland, SPUB-M/CERN/PO3/DZ296/2000 andSPUB-M/CERN/PO3/DZ297/2000JNICT–Junta Nacional de Investigac¸a˜o Cient´ıfica e Tecnolo´gica, Portugal,Vedecka grantova agentura MS SR, Slovakia, Nr. 95/5195/134,Ministry of Science and Technology of the Republic of Slovenia,CICYT, Spain, AEN99-0950 and AEN99-0761,The Swedish Natural Science Research Council,Particle Physics and Astronomy Research Council, UK,Department of Energy, USA, DE–FG02–94ER40817.We are indebted to Dr. T. Sjo¨strand for his help with the diagrams included in thispaper and for his comments on PYTHIA. We thank M. Klasen for his help with thediagrams important for inclusive J/ψ production. S.-U. Chung is grateful for the warmhospitality extended to him by the CERN staff during his sabbatical year in the EPDivision.9References[1] ‘Physics at LEP2,’ edited by G. Altarelli, T. Sjo¨strand and F. Zwirner,CERN96-01 (Vol. 1), Feb 1996.See Section 7 (heavy-quark physics), p. 330, in the chapter on γ γ physics.[2] J.J. Sakurai and D. Schildknecht, Phys. Lett. B40 (1979) 121;I. F. Ginzburg and V.G. Serbo, Phys. Lett. B109 (1982) 231.[3] M. Klasen et al., Nucl. Phys. B609 (2001) 518;M.Klasen et al., hep-ph/0112259;R. M. Godbole et al., Phys. Rev. D65 (2002) 074003;E. L. Berger and D. Jones, Phys. Rev. D23 (1981) 1521;H. Jung, G. A. Schuler and J. Terro´n, Int. J. Mod. Phys. A7 (1992) 7955B. Naroska, Nucl. Phys. B (Proc. Suppl.) 82 (2000) 187.[4] P. Aarnio et al., DELPHI Collab., Nucl. Inst. Meth. A303 (1991) 233.[5] P. Abreu et al., DELPHI Collab., Nucl. Inst. Meth. A378 (1996) 57.[6] R. Acciarri et al., L3 Collab., Phys. Lett. B503 (2001) 10.[7] Particle Date Group, Eur. Phys. J. C15, (2000) 1.[8] T. Sjo¨strand et al, Comput. Phys. Comm. 135 (2001) 238.[9] Delphi notes DELPHI 89-15 PROG 130 and DELPHI 89-68 PROG 143.10

Inclusive $J\psi$ Production in Two-Photon Collisions at LEP II with the DELPHI Detector

Inclusive $J/psi$ Production in Two-Photon Collisions at LEP II with the DELPHI Detector

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Inclusive J/psiJ/psiJ/psi Production in Two-Photon Collisions at LEP II with the DELPHI Detector

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Inclusive $J/psi$ Production in Two-Photon Collisions at LEP II with the DELPHI Detector