Photonuclear vector meson production in ultra-peripheral Pb-Pb collisions studied by the ALICE experiment at the LHC
aa r X i v : . [ nu c l - e x ] M a r Photonuclear vector meson production inultra-peripheral Pb-Pb collisions studied by theALICE experiment at the LHC
Joakim Nystrand ∗ (for the ALICE Collaboration) Department of Physics and Technology, University of Bergen, Bergen, NorwayE-mail:
The strong electromagnetic fields surrounding the Pb-ions accelerated at the CERN Large HadronCollider (LHC) allow two-photon and photonuclear interactions to be studied in a so far unex-plored kinematic regime. Exclusive photoproduction of vector mesons can be studied in ultra-peripheral collisions, where the impact parameters are larger than the sum of the nuclear radii andhadronic interactions are strongly suppressed.During the heavy-ion runs at the LHC in 2010 and 2011, the ALICE collaboration used specialtriggers to select ultra-peripheral collisions. These triggers were based on the Muon spectrom-eter, the Time-of-Flight detector, the Silicon Pixel detector, and the VZERO scintillator array.Information from other detectors was also used in the analysis. The cross section for coherentphotoproduction of J / y mesons at forward rapidities will be presented. The result will be com-pared to model calculations and its implications for nuclear gluon shadowing will be discussed. Xth Quark Confinement and the Hadron Spectrum,October 8-12, 2012TUM Campus Garching, Munich, Germany ∗ Speaker. hotonuclear vector meson production in ultra-peripheral Pb-Pb collisions
Joakim Nystrand
1. Introduction
Collisions between heavy-ions at the CERN Large Hadron Collider (LHC) can be utilized tostudy particle production in photonuclear and two-photon interactions [1, 2]. These interactionsmay occur in ultra-peripheral collisions with impact parameters of several tens or even hundreds offemtometers, where the background from hadronic processes is negligible. In this talk, results oncoherent photoproduction of J / y vector mesons and two-photon production of m –pairs measuredby the ALICE Collaboration at the LHC will be presented [3].Following the first calculation more than 10 years ago [4], exclusive photoproduction of vectormesons in heavy-ion interactions has attracted an increased theoretical interest in recent years [5,6, 7, 8, 9]. The first results from the Brookhaven Relativistic Heavy-Ion Collider showed theexperimental feasibility of these studies [10, 11], and the increased energy at the LHC leads tosignificantly higher cross sections for heavy vector mesons. The higher energy also means thatpartons at lower Bjorken-x are probed.Exclusive photonuclear production of vector mesons implies that there is no net color transferbetween the photon and the nucleus. The interaction can proceed via the exchange of two gluons,as indicated by the Feynman diagram in Fig. 1 (left). The events considered here are thus charac-terized by a single J / y meson but no other particles being produced. The J / y is studied throughits dimuon decay. Another process with a similar topology is two-photon production of dileptonpairs, the Feynman diagram of which is also shown in Fig. 1 (right). Two-photon interactions andcoherent photoproduction are both associated with a low p T ( < ≈
100 MeV/c) of the final state.To take into account also the finite detector resolution, a cut of p T <
300 MeV/c is used to definecoherent interactions in the present analysis.The ALICE detector and the muon spectrometer are discussed in section 2. Section 3 describesthe data analysis and the cross section measurement. In section 4, finally, the cross section iscompared with model expectations.
2. The ALICE Experiment
The ALICE detector consists of a central barrel placed inside a large solenoid magnet coveringthe pseudorapidity range | h | < − . < h < − .
5, anda set of smaller detectors at forward rapidities. The current analysis is based on data from the muonspectrometer. In addition, the VZERO counters and Zero-Degree Calorimeters (ZDC) are used fortriggering and rejecting the contribution from hadronic interactions.The ALICE muon spectrometer consists of five tracking stations containing two planes ofcathode pad multi-wire proportional chambers. A ten interaction length thick absorber is placedbetween the primary vertex and the first tracking station. The third (middle) tracking station issituated inside a dipole magnet with a R Bdl = p and K weak decays.The VZERO counters are arrays of scintillator tiles situated on either side of the primary vertexat pseudorapidities 2 . < h < . hotonuclear vector meson production in ultra-peripheral Pb-Pb collisions Joakim Nystrand
PbPbPbPb g y J/ + m - mgg Pb PbPbPb
Figure 1:
Examples of Feynman diagrams for exclusive J / y production (left) and two-photon productionof dimuon pairs (right). − . < h < − . | h | > . × events collected with a special trig-ger for ultra-peripheral collisions in the forward region (FUPC) during the 2011 Pb-Pb run. Thepurpose of the FUPC trigger was to select two muons in an otherwise empty detector. It is basedon three requirements: a single muon trigger above a 1 GeV/c p T –threshold; at least one hit inVZERO-C; no hits in VZERO-A. The integrated luminosity for the data collected with this triggerin 2011 corresponds to about 55 m b − .
3. Data analysis
The offline event selection was done in such a way as to maximize the yield of exclusively pro-duced muon pairs from J / y decay and two-photon interactions, while minimizing the backgroundfrom hadronic collisions and beam gas interactions. Only events with two oppositely chargedtracks in the muon spectrometer were considered. The analysis was restricted to J / y rapiditiesbetween − . < y < − . − . < h , < − . p T dependent cut on thedistance of closest approach of the tracks from the primary vertex position in the transverse planewas applied, and at least one of the muon track candidates had to match a trigger track above the1 GeV/c threshold. More details about the analysis cuts are available in [3].Exclusive photoproduction of vector mesons normally leaves the nuclei intact. But the strongfields associated with heavy-ions at high energies make exchange of multiple photons possible.These additional photons have low energy but may lead to nuclear break up, followed by emissionof one or a few neutrons in the forward region. The energy deposit in the ZDCs was therefore notrequired to be zero but only to be less than 6 TeV. This cut reduces the hadronic background athigher J / y transverse momentum but does not remove any events with p T < . J / y candidates in the invariant mass range 2 . < hotonuclear vector meson production in ultra-peripheral Pb-Pb collisions Joakim Nystrand ) (GeV/c - m + m M ) D i m uon c and i da t e s / ( M e V / c ALICE-3.6 DataSum y Coherent J/ y Incoherent J/’ decays y from y J/ - m + m fi gg = 2.76 TeV NN s y Pb+Pb+J/ fi Pb+Pb Figure 2: Invariant mass distribution (left) for dimuons surviving the analysis cuts. The curve shows thesum of a Crystal Ball function and an exponential fitted to the data, as described in the text. Transverese mo-mentum distribution (right) for J / y candidates with 2 . < m inv < . . The histograms are explainedin the text. m inv < . . The number of J / y s was extracted by fitting the invariant mass distributionin Fig. 2 to the sum of a Crystal Ball function, representing the signal, and an exponential, rep-resenting the background from two-photon interactions. This gave an extracted number of J / y s N yield = ± ( stat ) ± ( syst ) . The systematic error on the yield was obtained by varying theCrystal Ball tail parameters.The transverse momentum distribution of J / y candidates is shown in Fig 2 (right). The his-tograms show the expected contribution from coherent J / y production, incoherent J / y produc-tion, J / y from the decay y ′ → J / y + X , and two-photon production of dimuon pairs. As can beseen, the signal region p T < J / y production as well as feed down from y ′ decay.These contributions can be expressed as fractions of the number of coherent J / y s ( N cohJ / y ): f I = N incohJ / y N cohJ / y , f D = N FD N cohJ / y (3.1)The method used to estimate these fractions and the associated uncertainties is explained indetail in [3]. The fraction of J / y s from feed-down was obtained from the calculated y ′ photo-production cross section from the models in [4] and [6]. The acceptance and efficiency for recon-structing a J / y was obtained by simulating the decay y ′ → J / y + X with Pythia and passing theevents through the ALICE Geant detector simulation package. The result was f D = . ± . p T distribution in Figure 2 to Monte Carlo templates representing the four contributions mentionedabove. The relative normalization for coherent and incoherent production was left free in the fit,while the contribution from feed-down from photoproduced y ′ was constrained from the estimate f D = . ± . 06. The two-photon contribution was obtained from the fit of the dimuon continuumoutside the J / y peak. This gave the result f I = . + . − . .The contribution from hadronic J / y production was estimated from the measured yield above p T > J / y yield in p-p collisions to the 20% most peripheral, hadronic Pb-Pb4 hotonuclear vector meson production in ultra-peripheral Pb-Pb collisions Joakim Nystrand collisions. Both estimates showed that the contribution from hadronic production is negligible inthe p T < J / y s is thus related to the extracted yield by N cohJ / y = N yield + f I + f D . (3.2)The resulting number of coherent J / y s is N cohJ / y = ± ( stat ) + − ( syst ) .During the 2011 Pb-Pb run, the VZERO detector had a threshold corresponding to an energydeposit above that from a minimum ionizing particle. This made it difficult to accurately simulatethe VZERO-C trigger efficiency for low multiplicity events. To avoid these uncertainties, the J / y cross section was obtained by using the number of reconstructed gg → m + m − events. Two photonproduction of dimuon pairs is a standard QED process which has been proposed earlier as a lumi-nosity monitor [12]. The J / y cross section can then be written in a way that is independent of thetrigger efficiency and the integrated luminosityd s coh J / y d y = BR ( J / y → m + m − ) · N coh J / y N gg · ( Acc × e ) gg ( Acc × e ) J / y · s gg D y . (3.3)Here, ( Acc × e ) gg and ( Acc × e ) J / y are the acceptance and efficiency of the muon spectrometer fortwo-photon and coherent J / y events, respectively. BR ( J / y → m + m − ) = J / y and D y = s gg is calculated using STARLIGHT for a dimuon within − . < y < − . − . < h , < − . 5. The cross section is calculated for two intervals in invariant mass on each sideof the J / y peak (2 . < m inv < . and 3 . < m inv < . ). The number of eventsfound in each interval is 43 and 15, for the low and high mass interval, respectively. This togetherwith the cross sections, s gg = m b (low m inv ) and 3.7 m b (high m inv ), gave a total cross sectiond s coh J / y d y = . ± . ( stat ) + . − . ( syst ) mb . The systematic error is dominated by the uncertainty in the two-photon cross secion. Although thisis a standard QED process, the fact that the coupling to the nuclei is not small ( Z √ a rather than √ a ) and that the photon emitting nuclei are extended objects with an internal structure introducesa significant uncertainty in the cross section. Based on calculations [13] and constraints from datafrom RHIC [11, 14], this uncertainty is estimated to be 20%. Other important contributions to thesystematic error are the coherent signal extraction and the reconstruction efficiency [3]. 4. Comparison with models The measured cross section can be compared with the models mentioned earlier [4, 5, 6, 7, 8,9]. The models by Lappi and Mäntysaari [5], Cisek, Schäfer and Szczurek [8], and Goncalves andMachado [9] are based on the Color Dipole Model (CDM). The model by Klein and Nystrand[4],which is incorporated in the STARLIGHT Monte Carlo, uses data from exclusive vector mesonproduction at HERA as input to a Glauber calculation of the cross section for nuclear targets. The5 hotonuclear vector meson production in ultra-peripheral Pb-Pb collisions Joakim Nystrand y -4 -2 0 2 4 / d y ( m b ) s d RSZ-LTASTARLIGHTGMAB-EPS09AB-MSTW08AB-EPS08AB-HKN07CSS ALICE a) = 2.76 TeV NN s y Pb+Pb+J/ fi Pb+Pb ALICEAB-MSTW08STARLIGHTGMCSSRSZ-LTAAB-EPS08AB-EPS09AB-HKN07 = 2.76 TeV NN s y Pb+Pb+J/ fi Pb+Pb /dy (-3.6 < y < -2.6) (mb) s d b) Figure 3: The cross section from ALICE compared with d s / dy from models (left), and a comparison ofthe cross section integrated over − . < y < − . models by Rebyakova, Strikman and Zhalov [6], and Adeluyi and Bertulani [7], calculate the crosssection directly from the nuclear gluon distribution with the forward scattering amplitude beingproportional to the gluon distribution squared. Rebyakova, Strikman and Zhalov calculate themodifications to the nuclear gluon distrbution in the leading twist approximation, while Adeluyiand Bertulani use some of the standard parameterizations (HKN07, EPS09, and EPS08). Adeluyiand Bertulani also calculate the cross section by scaling the g + p → J / y + p cross section withthe number of nucleons assuming no nuclear effects (MSTW08).The results are shown in Fig. 3. One can see that models which are based on the color dipolemodel generally give a higher cross section than those which calculate the cross section directlyfrom the gluon distribution. This is true at the forward rapidities studied here, but is emphasizedeven more at mid-rapidity. The MSTW08 scaling of the cross section without nuclear effects andSTARLIGHT deviate by about 3 standard deviations from the measured value and are disfavored.Best agreement is found for models which include nuclear gluon shadowing consistent with theEPS09 or EPS08 parameterizations. The calculation by Lappi and Mäntysaari is in the same rangeas the other models using the color dipole model, but since their result was not available when theALICE paper was released it is not included in the figure. 5. Conclusions and outlook The cross section for exclusive J / y production in Pb-Pb collisions at the LHC has been mea-sured by the ALICE collaboration. The results show that the cross section cannot be undestoodfrom a simple scaling of the nucleon cross section neglecting nuclear effects. Best agreement isseen with models which include nuclear gluon shadowing. The cross section for exclusive J / y production at mid-rapidity is being studied in ALICE and final results will be published shortly. References [1] A. J. Baltz et al. Phys. Rept. (2008) 1.[2] C. A. Bertulani, S. R. Klein and J. Nystrand, Ann. Rev. Nucl. Part. Sci. (2005) 271. hotonuclear vector meson production in ultra-peripheral Pb-Pb collisions Joakim Nystrand[3] B. Abelev et al. [ALICE Collaboration], Phys. Lett. B (2013) 1273.[4] S. R. Klein and J. Nystrand, Phys. Rev. C , 014903 (1999).[5] T. Lappi and H. Mäntysaari, arXiv:1301.4095 [hep-ph].[6] V. Rebyakova, M. Strikman and M. Zhalov, Phys. Lett. B (2012) 647.[7] A. Adeluyi and C. A. Bertulani, Phys. Rev. C (2012) 044904.[8] A. Cisek, W. Schäfer and A. Szczurek, Phys. Rev. C (2012) 014905.[9] V. P. Goncalves and M. V. T. Machado, Phys. Rev. C (2011) 011902.[10] C. Adler et al. [STAR Collaboration], Phys. Rev. Lett. (2002) 272302.[11] S. Afanasiev et al. [PHENIX Collaboration], Phys. Lett. B (2009) 321.[12] G. Baur, K. Hencken, D. Trautmann, S. Sadovsky and Y. Kharlov, Phys. Rept. (2002) 359.[13] A. J. Baltz, Phys. Rev. C (2009) 034901.[14] J. Adams et al. [STAR Collaboration], Phys. Rev. C (2004) 031902.(2004) 031902.