The photoproduction of vector mesons in ultra-peripheral collisions (UPC) is a powerful tool to probe the nuclear gluon distribution (Pb-Pb collisions) and the gluon structure function in the proton (p-Pb collisions). The

first measurements of coherent photoproduced J/ψ and ψ(2S) in Pb-Pb collisions at √sNN = 2.76TeV, performed with the ALICE detector, are reported and compared to STARLIGHT and QCD based models, in order to investigate nuclear gluon shadowing. The first results of the measurement of exclusive J/ψ photoproduction off protons in p-Pb collisions at √sNN = 5.02TeV performed by the ALICE Collaboration are also mentioned

De Gruttola, D.

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The photoproduction of vector mesons in ultra-peripheral colli- sions (UPC) is a powerful tool to probe the nuclear gluon distribution (Pb-Pb collisions) and the gluon structure function in the proton (p-Pb collisions). The first measurements of coherent photoproduced J /ψ and ψ (2S) in Pb-Pb collisions at √ s NN =2 . 76 TeV, performed with the ALICE detector, are reported and compared to STARLIGHT and QCD based models, in order to investigate nuclear gluon shad- owing. The first results of the measurement of exclusive J /ψ photoproduction off protons in p-Pb collisions at √ s NN =5 . 02 TeV performed by the ALICE Collabora- tion are also mentione

De Gruttola, D

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Physics of ultra-peripheral collisions with ALICE at the LHC

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DOI 10.1393/ncc/i2015-15002-9Colloquia: IFAE 2014IL NUOVO CIMENTO 38 C (2015) 2Physics of ultra-peripheral collisions with ALICE at the LHCD. De Gruttola(∗) on behalf of the ALICE CollaborationCentro Fermi Roma and Salerno INFN, Italyreceived 7 January 2015Summary. — The photoproduction of vector mesons in ultra-peripheral colli-sions (UPC) is a powerful tool to probe the nuclear gluon distribution (Pb-Pbcollisions) and the gluon structure function in the proton (p-Pb collisions). Thefirst measurements of coherent photoproduced J/ψ and ψ(2S) in Pb-Pb collisions at√sNN = 2.76TeV, performed with the ALICE detector, are reported and comparedto STARLIGHT and QCD based models, in order to investigate nuclear gluon shad-owing. The first results of the measurement of exclusive J/ψ photoproduction offprotons in p-Pb collisions at√sNN = 5.02TeV performed by the ALICE Collabora-tion are also mentioned.PACS 13.40.-f – Electromagnetic processes and properties.PACS 25.20.Lj – Photoproduction reactions.1. – Ultra-peripheral collisionsUltra-peripheral collisions may occur when two accelerated particles are separated byimpact parameters larger than the sum of their radii: in this case hadronic interactionsare strongly suppressed. Photonuclear processes can be studied in Pb-Pb and p-Pbcollisions at the LHC at unprecedentedly high energies. The physics of ultra-peripheralcollisions is reviewed in [1, 2].2. – Physics motivationsExclusive photoproduction of vector mesons in heavy-ion interactions, where a vectormeson but no other particles are produced in the event, is of particular interest since itis expected to probe the nuclear gluon distribution [3], for which there is considerableuncertainty in the low-x region [4].(∗) Contact email: daniele.de.gruttola@cern.chc© CERN on behalf of the ALICE CollaborationCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 D. DE GRUTTOLA on behalf of the ALICE COLLABORATION2.1. Pb-Pb collisions. – Exclusive photoproduction can be either coherent, if thephoton couples coherently to all nucleons, or incoherent, if the photon couples to asingle nucleon. Coherent production is characterized by low vector meson transversemomentum (〈pT 〉 ∼ 60MeV/c) and the target nucleus normally does not break up.Incoherent production, corresponding to quasi-elastic scattering off a single nucleon, ischaracterized by a higher transverse momentum (〈pT 〉 ∼ 500MeV/c) and the targetnucleus normally breaks up; except for single nucleons or nuclear fragments in the veryforward region no other particles are produced. In what follows, results on coherentJ/ψ production in the forward region reconstructed in the dimuon decay channel andcoherent J/ψ production at central rapidities in both leptonic decays are presented.2.2. p-Pb collisions. – In this case, the J/ψ photoproduction is dominated by theγp process and this gives the possibility to study the gluon PDF in the proton downto x ∼ 10−5. Considering the different rapidity ranges available for the measurement,ALICE can provide the cross section as a function of the γp centre of mass energy,ranging from 21GeV up to 1160GeV.3. – Results and comparison with modelsThe analysis is based on a sample of events collected during the 2011 Pb-Pb run andthe 2013 p-Pb run, selected with dedicated triggers, set up to select events containingtwo tracks in an otherwise empty detector, in order to select exclusive J/ψ photopro-duction events. The ALICE detector [5] consists of a central barrel placed inside a largesolenoid magnet (B = 0.5T), covering the pseudorapidity region |η| < 0.9 and a muonspectrometer covering the range −4.0 < η < −2.5. The detectors used in the analysis (fortriggering, tracking, offline selection and PID purposes) are the Silicon Pixel Detector(SPD), the Time Projection Chamber (TPC), the Time-of-Flight detector (TOF), theVZERO scintillator counters, the Electromagnetic Calorimeter (EMCAL), the ForwardMultiplicity Detector (FMD), a Muon Tracker and a Muon Trigger system and finallytwo sets of hadronic Zero Degree Calorimeters (ZDC), used to detect neutrons emittedin the very forward region, such as neutrons produced by electromagnetic dissociation.3.1. Measured cross sections and comparison with theoretical calculations. – The in-variant mass and pT distributions for the forward region are shown in fig. 1. The peakat the J/ψ mass is clearly visible and has been fitted to an exponential function to de-scribe the background plus a Crystal Ball parameterization to decribe the J/ψ. Theclear peak at low values in the transverse momentum distribution of the J/ψ candidatesis mainly due to coherent photoproduction. To estimate the fraction of incoherent eventin the selected sample, a fit of the pT distribution was performed, taking into accountcontributions from coherent and incoherent J/ψ production, as well as from the di-leptoncontinuum, the feed-down from ψ′ and, for the mid-rapidity region, the contaminationby the hadronic events. The shapes for the first five fitting functions (Monte Carlo templates) were provided by STARLIGHT events folded with the detector simulation, whilethe last one is extracted from data at higher centralities.As expected, at low transverse momentum the coherent production dominates. Theyield of coherent J/ψ is obtained by a cut on di-lepton momentum and a ZDC cuttuned to select events with less than 6 emitted neutrons. The momentum cut isbelow 300MeV/c at forward rapidity and below 200 (300)MeV/c at mid-rapidity forthe di-muon (di-electron) channel.PHYSICS OF ULTRA-PERIPHERAL COLLISIONS WITH ALICE AT THE LHC 3Fig. 1. – Left: invariant mass spectrum of forward (−3.6 < y < −2.6) coherent J/ψ → μ+μ−candidates with pT < 300MeV/c and less than 6 neutrons in the ZDC. Right: J/ψ pT dis-tribution fitted to a simulation template as described in the text. The same distributions forthe mid-rapidity region, for the dielectron and dimuon channels are not shown here and can befound in [7].The results for the forward and mid rapidity regions have been recently publishedrespectively in [6] and [7]. The coherent differential cross sections per unit rapidity are1.00± 0.18 (stat) +0.24−0.26 (syst) mb in the rapidity interval −3.6 < y < −2.6 and 2.38+0.34−0.24(stat + syst) mb at | y |< 0.9.The results at forward and central rapidities are compared to theoretical predictionsin fig. 2. The predictions can be divided into three categories: models that includeno nuclear effects (AB-MSTW08), models that use a Glauber approach to calculate thenumber of nucleons contributing to the scattering (STARLIGHT, GM, CSS and LM) andpartonic models, where the cross section is proportion al to the nuclear gluon distributionsquared (AB-EPS08, AB-EPS09, AB-HKN07, and RSZ-LTA). A detailed description ofall these models can be found in [7]. The coherent J/ψ cross section is found to be in goodagreement with the model which incorporates the nuclear gluon shadowing according tothe EPS09 parameterization (AB-EPS09). At the beginning of 2013, ALICE collected asample of p-Pb collisions at√sNN = 5.02TeV provided by LHC. The preliminary resultFig. 2. – Comparison among the published values of the cross section at forward rapidity, atcentral rapidities and several theoretical models.4 D. DE GRUTTOLA on behalf of the ALICE COLLABORATIONat forward rapidity is consistent with a power law dependence of the J/ψ photoproductioncross section in γ p energies from about 20 to 700GeV, or equivalently, from Bjorken-xbetween ∼ 2×10−2 to ∼ 2×10−5, with an exponent compatible with H1 and ZEUS, thusindicating no significant change in the gluon density behaviour of the proton betweenHERA [8,9] and LHC energies.REFERENCES[1] Baltz A. J. et al., Phys. Rep., 458 (2008) 1.[2] Bertulani C. A., Klein S. R. and Nystrand J., Annu. Rev. Nucl. Part. Sci., 55 (2005)271.[3] Rebyakova V., Strikman M. and Zhalov M., Phys. Lett. B, 710 (2012) 647.[4] Eskola K. J., Paukkunen H. and Salgado C. A., JHEP, 04 (2009) 065.[5] Aamodt K. et al. (ALICE Collaboration), JINST, 3 (2008) S08002.[6] Abelev B. et al. (ALICE Collaboration), Phys. Lett. B, 718 (2013) 1273.[7] Abelev B. et al. (ALICE Collaboration), Eur. Phys. J. C, 73 (2013) 2617.[8] Aktas A. et al., Eur. Phys. J. C, 46 (2006) 585.[9] Chekanov S. et al., Nucl. Phys. B, 695 (2004) 3.

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Physics of ultra-peripheral collisions with ALICE at the LHC

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