Niess, V.

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Exclusive dimuon measurements with LHCbV. NiessTo cite this version:V. Niess. Exclusive dimuon measurements with LHCb. Physics at LHC 2011, Jun 2011,Perouse, Italy. 4 p., 2012. <in2p3-00678377>HAL Id: in2p3-00678377http://hal.in2p3.fr/in2p3-00678377Submitted on 12 Mar 2012HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Exclusive dimuon measurements with LHCbValentin NiessLaboratoire de Physique Corpusculaire de Clermont-Ferrand,Universite´ Blaise Pascal (UMR 6533 CNRS-IN2P3)24 Avenue des landais, F-63177 Aubie`re Cedex, FRANCEOn Behalf of the LHCb Collaboration.1 IntroductionExclusive particle production in proton-proton collisions is an elastic process in whichthe protons remain intact, p+p→ p+X+p, and the additional particles are createdthrough photon and/or gluon propagators. In the case of purely photon propagators,this is an electromagnetic process which can be theoretically calculated with highaccuracy. When gluons are involved, these processes constitute an important testingground for QCD, since the object that couples to the proton must be colourless. In thispaper, we summarise preliminary cross-section measurements of exclusive productionprocesses involving dimuon final states, with the 2010 data collected by the LHCbdetector. More in depth details are provided in [1]. The processes we consideredare the resonant Central Exclusive Production (CEP) of J/Ψ and Ψ(2S) and thenon-resonant diphoton production, resulting in a dimuon continuum. The additionof a photon in the final state further allows the cross-section for exclusive χc to bemeasured and the contributions coming from χc0, χc1, and χc2 to be estimated.2 PrerequisitesThe LHCb detector is a single arm forward spectrometer [2]. It has two key featureswhich makes it efficient for the study of CEP: (1) LHCb is fully instrumented in theforward region, between pseudorapidities, η, of 1.9 and 4.9 and (2) LHCb is ableto trigger on low momentum µ, down to P > 3 GeV/c and transverse momentumPT > 0.4 GeV/c. For this analysis we made particular use of the following sub-detectors. The tracking system consists of the vertex locator (VELO), 42 siliconstrip detectors placed along the beamline, a Silicon Tracker and an Outer Trackerof straw tubes. The VELO surrounds the interaction region allowing for partialreconstruction of Backward tracks (−4.0 < η < −1.5) and Forward tracks down toη = 1.0. The calorimeter system includes a scintillating pad detector (SPD), providing1the multiplicity in charged tracks at the first trigger level. The muon detectors consistof five stations located within 20 interaction lengths of absorbers.Dedicated Monte Carlo generators have been used to produce exclusive signalevents which are then passed through the full LHCb detector simulation. The dipho-ton produced dimuons have been generated using LPAIR [3]. Two generators havebeen used to make samples of exclusive J/Ψ and Ψ(2S): StarLight [4] and SuperChic[5].The production of χc by double pomeron fusion has been performed with SuperChic.In addition diffractive events were generated using POMWIG [6] and DPEMC [7].3 Selection of exclusive eventsThe final state protons are only marginally deflected in CEP. They remain in thebeam-pipe and so are undetected in LHCb. Consequently, the experimental signatureis a dimuon only final state with possibly a single photon from a χc. However, becauseLHCb is not hermetic, there is sizable backgrounds from non-elastic processes wherethe companion inelastic products are outside of the detector acceptance. Much of thework involved in this analysis concerned the evaluation of this inelastic background,which was taken from the data insomuch as is possible. The experimental study wasrestricted to clear single elastic processes, requiring only one interaction per bunch-crossing. The corresponding effective integrated luminosity was of ∼3 pb−1, insteadof 36 pb−1 for the full 2010 LHCb data set.CEP involves colourless propagators which can be signed by the lack of QCDcorrelated particle production. Experimentally it results in a pseudorapidity gap. Byrequiring that the dimuon triggered events have no Backward tracks in the VELOone indeed sees the emergence of a clear peak of events with only 2 forward tracks.Those are candidates for CEP.The resonant J/Ψ and Ψ(2S) are selected by requiring a dimuon invariant masswithin 65 MeV/c2 of the PDG value [8]. The J/Ψ suffers from feed down fromΨ(2S) → J/Ψπ+π− and χc → J/Ψγ with companion products of the J/Ψ outsideof the detector acceptance. These feed downs were estimated from simulation andnormalised to the observed number of Ψ(2S) and χc in the data. Since inelastic eventstend to have an average PT higher than elastic ones, the respective fractions of elasticand inelastic exclusive J/Ψ were estimated from a fit of the PT spectrum. The shapeof the signal was taken from simulation and the shape of the background from thedata sidebands, considering events with more than 2 forward tracks. It was cross-checked that the latter background shape little depends on the number of additionalforward tracks in the event. For the Ψ(2S) we assumed the same contamination ininelastic events than for the J/Ψ. In addition, in both cases the selection of elasticevents was purified by requiring a dimuon PT < 0.9 GeV/c, resulting in a ∼70%purity.2χc candidates were selected from exclusive J/Ψ with a single additional γ. Feeddown from Ψ(2S) decaying to χcγ, J/Ψη and J/Ψπ0π0 was estimated from simulation,normalised to the number of Ψ(2S) in the data. The fraction of inelastic events islarger than previously with. The purity is of ∼40%, estimated from a fit of thePT spectrum of the J/Ψ as for the CEP J/Ψ. Finally the three χc resonances aredisentangled by a fit of the invariant mass spectrum with shapes taken according tothe simulation.Dimuons from exclusive diphoton are selected in the continuum by vetoing onthe dimuon invariant mass of resonances. Furthermore, in order to nullify the back-ground from miss-identified pions and kaons decaying in flight, a dimuon invariantmass greater than 2.5 GeV/c2 was required. The fraction of inelastic events wasdetermined from the PT spectrum as for the J/Ψ. In this case a theoretical predic-tion for the background is provided by LPAIR and POMWIG. Good agreement wasfound with the background shape estimated from the data, which provides an overallcross-check of the background extraction procedure from the data. Further requiringPT < 100 MeV/c we have an almost 100% pure selection.4 Cross-section determinationThe cross-section σ, was calculated from the number, N , of selected events havingcorrected for the total efficiency, ǫ, and purity, p, and dividing by the effective luminos-ity, L, via σ = (pN)/(ǫL). The preliminary results are compiled in Table 1 togetherwith their detailed error budget. The total efficiency varies from 71% for Ψ’s downto 25% for diphotons. It was taken from the simulation with systematics based onMonte-Carlo to data comparisons. It includes triggering, track reconstruction, muonsand photon identification as well as selection efficiencies. Note that the acceptance isnot included in the efficiency, hence the numbers we quote are cross-section times thebranching ratio into the final state of interest, with all final state particles betweenpseudorapidities of 2 and 4.5. The effective luminosity determination in this analysiscurrently has a large uncertainty. The uncertainty on the luminosity for this analysiswas estimated as 10% with and additional uncertainty of 10% on the calculation ofthe number of interactions per crossing, µ, resulting in a total uncertainty of 20%.5 ConclusionLHCb has observed the first clear signals for exclusive J/Ψ, Ψ(2S) and χc mesons inp-p collisions. The preliminary measured cross-section are consistent with previousresults [9, 10, 11] and theoretical estimates considering uncertainties. In addition, wewere able to separate the individual χc components and found out that the χc0, χc1and χc2 resonances appear to be produced roughly equally.3In the future it is foreseen to measure the diphoton cross section down to a dimuoninvariant mass of 1 GeV/c2. This channel is actually a candidate for high precisionluminosity measurements at the LHC, due to both the accurate theoretical predictionand clean experimental measurement.References[1] R. Aaij, et al. (LHCb Collaboration), LHCB-CONF-2011-022 (2011).[2] A. A. Alves Jr., et al. (LHCb Collaboration), JINST 3, S08005 (2008).[3] J. Vermaseren, Nucl. Phys. B229, 347 (1983).[4] S. Klein and J. Nystrand, Phys. Rev. C60, 014903 (1999).[5] L. A. Harland-Lang, V. A. Khoze, M. G. Ryskin and W. J. Stirling, arXiv:hep-ph/0909.4748 (2009).[6] B. E. Cox and J. R. Forshaw, Phys. Comm. 144, 104 (2002) [arXiv:hep-ph/0010303].[7] M. Boonekamp and T. Kues, arXiv:hep-ph/031227 (2004).[8] K. Nakamura, et al. (Particle Data Group), J. Phys. G37, 075021 (2010).[9] T. Aaltonen, et al. (CDF Collaboration), Phys. Rev. Lett. 102, 242001 (2009).[10] G. Abbiendi, et al. (ZEUS Collaboration), Z. Phys. C75, 215 (1997); I. Abbt, etal. (ZEUS Collaboration), JHEP 1005, 085 (2010).[11] A. Adloff, et al. (H1 Collaboration), Phys. Lett. B483, 23 (2000); A. Aktas, etal. (H1 Collaboration), Phys. Lett. B568, 205 (2003).Efficiency # Events Purity Leff (pb−1) σ(η ∈ [2; 4.5])× BR (pb)J/Ψ 0.71± 0.07 1468± 38 0.71± 0.03 3.1± 0.6 474± 12± 51± 92Ψ(2S) 0.71± 0.07 40± 6 0.67± 0.03 3.1± 0.6 12.2± 1.8± 1.3± 2.4χc0 0.34± 0.06 25± 6 0.39± 0.13 3.1± 0.6 9.3± 2.2± 3.5± 1.8χc1 0.43± 0.05 56± 18 0.39± 0.13 3.1± 0.6 16.4± 5.3± 5.8± 3.2χc2 0.44± 0.04 99± 29 0.39± 0.13 3.1± 0.6 28.0± 5.4± 9.7± 5.4di-γ 0.25± 0.03 40± 6 0.97± 0.01 2.3± 0.5 67± 10± 7± 15Table 1: Summary of numbers in preliminary cross-section calculation. The sourcesof uncertainty quoted for σ × BR are statistical, systematic and luminosity.4

Exclusive dimuon measurements with LHCb

We report on studies of exclusive dimuon production, using LHCb experimental data. Muon pairs can be exclusively produced by two-photon fusion (a QED process ideally suited to obtaining a precise integrated luminosity measure), or by resonance production. We show results for exclusive dimuon production, and first observations of the exclusive $J/\psi, \psi(2S)$ and $\chi_c$ states, obtained with 37 pb$^{–1}$ of data at $\sqrt{s}$ = 7 TeV. We compare our cross-section measurements to predictions

Exclusive dimuon measurements with LHCb

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