Electromagnetic interactions of heavy-ions give access to a unique programme of photon-induced studies in the QCD and QED sectors of the Standard Model. On the one hand, exclusive photoproduction of bottomonium ($\gamma\,A \rightarrow \Upsilon\,A$, where the nucleus $A$ remains intact) offers a useful means to constrain the small-$x$ nuclear gluon density. On the other, two-photon exclusive dilepton production ($\gamma\,\gamma\rightarrow l^{+}\,l^{-}$) allows us to study QED at field-strengths close or above the Schwinger limit ($E_{\rm crit}\approx$~1.5 $\times$10$^{16}$~V/cm). We present full simulation studies of exclusively photo-produced $\Upsilon$ and high-mass dileptons in Pb-Pb collisions at the LHC

d'Enterria, D

Kumar, V

Mohanty, A

CERN Document Server

Available on CMS information server CMS CR-2008/049The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report06 July 2008Exclusive Photoproduction of Upsilon and leptonpairs in Pb-Pb at 5.5 TeVD. d’Enterria, V. Kumar, and A. MohantyAbstractElectromagnetic interactions of heavy-ions give access to a unique programme of photon-inducedstudies in the QCD and QED sectors of the Standard Model. On the one hand, exclusive photopro-duction of bottomonium (γ A → ΥA, where the nucleus A remains intact) offers a useful meansto constrain the small-x nuclear gluon density. On the other, two-photon exclusive dilepton produc-tion (γ γ → l+ l−) allows us to study QED at field-strengths close or above the Schwinger limit(Ecrit ≈ 1.5 ×1016 V/cm). We present full simulation studies of exclusively photo-produced Υ andhigh-mass dileptons in Pb-Pb collisions at the LHC.Presented at Workshop on High-energy photon collisions at the LHC,April 2008,CERN Geneve,Switzerland1 IntroductionAll charges accelerated at very high energies generate electromagnetic elds which, in the equivalent photon ap-proximation [1], can be considered as (quasi-real) photon beams. A signicant fraction of the p-p [2] and Pb-Pb [3]collisions at the LHC will involve photon interactions at TeV energies giving access to a unique programme of γ-induced studies. In the heavy-ion running mode, the strong electromagnetic eld due to the coherent action of theZ = 82 TeV-energy proton charges generates a ux of photons with energies and intensities much larger than thoseof equivalent e-p collisions at HERA. Lead beams at 2.75 TeV have Lorentz factors γ = 2930 leading to maxi-mum photon energies ωmax ≈ γ/R∼ 80 GeV. These photons can then collide either with the other incoming 2.75TeV/nucleon nucleus at center-of-mass energies up to W maxγPb ≈ 1 TeV/nucleon, or they can interact with anothersimilarly radiated photon leading to photon-photon collisions at W maxγγ ≈ 160 GeV.Pb*Pbγ QQ...Pb*PbPbPb*Pb*-l...PbPbPb*+lγγFigure 1: Diagrams for exclusive photo-production of (i) diffractive quarkonium in γA collisions (top), and (ii)dileptons in two-photon interactions (bottom) in ultra-peripheral Pb-Pb at the LHC. The dashed vertical linesseparate soft Pb-Pb photon exchanges from the harder γ-induced interaction.We discuss here the possibilities of the CMS experiment to study (i) the low-x regime of the nuclear parton dis-tribution functions (PDFs) via exclusive quarkonium photoproduction (Fig. 1, top), and (ii) QED in the non-perturbative (high-eld) regime via high-mass exclusive dilepton production (Fig. 1, bottom) in ultraperipheralcollisions (UPCs) of lead ions.2 Low-x nuclear PDFs via ϒ photoproduction in Pb-Pb at the LHCThe gluon density, xG(x,Q2), at small fractional momenta x = pparton/pproton . 0.01 and low, yet perturbative, Q2 isa subject of intensive experimental and theoretical activity [4]. On the one hand, DGLAP analyses based on DISe-p data cannot reliably determine xG as it is only indirectly constrained by the log(Q2) dependence of the quarkdistributions (F2 scaling violations). On the other, there are well funded theoretical arguments [5] that support theinapplicability of DGLAP- or BFKL-type evolution equations at low enough values of x, due to the increasingimportance of gluon-gluon fusion processes, unaccounted for in such linear-QCD approaches. Alternatively, thelow-x parton saturation regime can be well described theoretically within the Color-Glass-Condensate [6] orblack-disk limit [7] framework.Our knowledge of the low-x gluon distribution in the nucleus is even more scarce [8]. Nuclear DIS data only coverthe range above x≈ 10−2 (Fig. 2), and gluon saturation effects are expected to be much larger in nuclei than in theproton due to their larger transverse parton density. Exclusive Q flQ photoproduction offers an attractive opportunityto constrain the low-x gluon density at moderate virtualities, since in such processes the gluon couples directly tothe c or b quarks (see Fig. 1, top) and the cross section is proportional to the gluon density squared (see [9] andrefs. therein). The mass of the Q flQ vector meson introduces a relatively large scale, amenable to a perturbativeQCD (pQCD) treatment.1x  -610 -510 -410 -310 -210 -110 1)2 (GeV2 Q-210-110110210310410)2 (GeV2 Q: QUltra-peripheral Q (|y| < 2.5)ΥLHC  (|y| < 2)ΨRHIC J/Nuclear DIS & DY data:NMC (DIS)SLAC-E139 (DIS)FNAL-E665 (DIS)EMC (DIS)FNAL-E772 (DY)perturbativenon-perturbativeFigure 2: Kinematic (x,Q2) plane probed in e-,γ-A collisions: existing nuclear DIS data compared to ultraperiph-eral Q flQ photoproduction ranges.At the LHC, γA → J/ψ(ϒ)A processes, probe the gluon distribution at values (Fig. 2) as low as x = M2V /W 2γA ≈10−2(10−4). Gluon saturation effects are expected to reveal themselves through strong suppression of hard exclu-sive diffraction yields relative to the leading-twist approximation [7]. While this suppression may be beyond thekinematics achievable for J/ψ photoproduction in UPCs at RHIC (x≈ 0.01 and Q2eff ≈M2V /4≈ 3 GeV2) [10], itcould be important in UPCs at the LHC [3].Table 1: Cross sections predicted by the STARLIGHT model [11] for exclusive heavy-quarkonium photoproductionin ultraperipheral Pb-Pb interactions at 5.5 TeV accompanied by neutron emission in single (Xn) or double (Xn|Xn)Pb breakup (top diagram in Fig. 1). (σXn includes σXn|Xn).Process σtot σXn σXn|XnγPb→ J/ψPb 32 mb 8.7 mb 2.5 mbγPb→ ϒ(1S)Pb 173 µb 78 µb 25 µbTable 1 lists the expected cross sections for J/ψ and ϒ photoproduction in Pb-Pb UPCs at the LHC, as given bythe STARLIGHT Monte Carlo [11]. The STARLIGHT model satisfactorily reproduces the existing UPC J/ψ [10]and low- [12] and high-mass [10] dielectron data at RHIC energies. The predicted total cross section for ϒ(1S)photoproduction is of the order of 150 µb in agreement with other calculations [13]. Inclusion of leading-twistshadowing effects in the nuclear PDFs reduces the yield by up to a factor of two, σϒ = 78 µb [13]. Even largerreductions are expected in calculations including gluon-saturation (CGC) effects [14, 15].The cross-sections are given also for the case where one or both nuclei are mutually Coulomb-excited  indicatedby the soft γ’s exchanged in the diagrams of Fig. 1. In the case of ϒ photoproduction about 50% of the UPCsinduce Giant-Dipole-Resonance (GDR) oscillations of the interacting nuclei which will subsequently deexcite vianeutron emission. The presence of very forward neutrons detectable in Zero-Degree-Calorimeters (ZDCs) [16], ina signicant fraction of UPC events provides a crucial tool to trigger on those collisions. In Section 4 we presentthe expected ϒ→ e+e−,µ+µ− reconstruction performances and yields for Pb-Pb at 5.5 TeV in CMS.3 High-field QED via exclusive lepton-pair productionIn proton-proton collisions, exclusive dilepton production pp → p l+ l−p has been proposed [17] as a standardbeam luminosity calibration process at the LHC, thanks to its precisely known (nearly [18]) pure QED cross sec-tion. Experimentally, such a process can be tagged with rapidity-gaps (exclusivity) conditions [19] or via forwardprotons [2], and has a clear signature in the exclusive back-to-back dileptons (|∆φ(l+ l−)|> 2.9) measured in CMS(or ATLAS) within |η|< 2.5.2Exclusive dileptons are also easily accessible in electromagnetic Pb-Pb collisions (bottom diagram of Fig. 1) sincethe continuum cross sections  thanks to the Z4 photon ux enhancement factor  are much larger than in p-p. As a matter of fact, the huge Z4 enhancement factor of equivalent photon uxes in the ion running mode atthe LHC, results in QED elds of the same order of magnitude or above the so-called Schwinger limit, Ecrit =m2c3/(e~)≈ 1.5×1016 V/cm, which sets the threshold for the generation of real e+e− pairs out of the vacuum byan (strong enough) electromagnetic eld. Under such conditions, Z αem ≈ 0.6, the l+l− production probabilitiesare close to one and the lowest order perturbative calculations of the cross sections violate unitarity. Detailedcalculations involving higher order processes, such as the exchange of multiple photons and the production ofmultiple pairs as shown in Fig. 3(b-d), have been carried out in the recent years [20]. Testing of QED in the non-perturbative regime will be thus possible via the measurement of exclusive dileptons in ultraperipheral nucleus-nucleus collisions at the LHC. A ∗ A ∗γγl+l−(a) (b)(c) (d) A ∗ A ∗γγl+l− A ∗ A ∗γγl+l− A ∗ A ∗γγl+l−γγl+l−Figure 3: Feynman diagrams for l+l− production in high-energy UPCs: (a) lowest order, (b) next to lowest orderthree-photon exchange, (c) four-photon exchange, (d) double l+l− pair production.Table 2 lists the expected cross sections for the lepton pair continuum in two ranges of invariant-mass ranges aspredicted by STARLIGHT. The dilepton continuum yields are 2 to 3 orders of magnitude larger than those due tothe exclusive quarkonia decays. However, the single leptons from the continuum are much more forward-peakedthan those from J/ψ or ϒ and fall much more often outside the CMS acceptance. The fraction of the continuumcross sections accompanied by nuclear breakup with neutron emission is expected to be the same as in the caseof quarkonia photoproduction, i.e. of the order of ∼ 50% for moderately high mass dileptons. Exclusive dileptonproduction in STARLIGHT is calculated combining the two equivalent (Weizs¤acker-Williams) photon uxes fromeach ion with the Breit-Wheeler formula for γγ → l+ l− (the leading order diagram of Fig. 3). The level ofagreement is good with other calculations [20] when the pair invariant mass is not too low. If strong QED eldsare accounted for, multiple pairs are also produced at lower masses. In the next section we present the expectedexclusive lepton pair contributions around the ϒ mass in the CMS detector.Table 2: Cross sections for dilepton production in two-photon collisions from ultraperipheral Pb-Pb interactions at5.5 TeV (bottom diagram in Fig. 1) according to STARLIGHT [11] for two different invariant mass regions.Process γγ→ e+e− γγ→ µ+µ−σ(M > 1.5 GeV/c2) 139 mb 45 mbσ(M > 6.0 GeV/c2) 2.8 mb 1.2 mb4 Expected UPC dilepton results in CMSIn [21] we present the possibilities of the CMS detector to carry out the measurement of exclusive ϒ→ l+ l− andcontinuum l+ l− pairs produced in ultra-peripheral PbPb collisions at √sNN = 5.5 TeV, tagged with forward neutrondetection in the ZDCs [16]. The CMS electromagnetic calorimeter ECAL and the muon spectrometers, coveringthe pseudorapidity ranges |η|< 3. and |η|< 2.5 respectively, are used to measure the nal leptons. Two dedicatedUPC level-1 triggers have been dened which require respectively hits above ∼2.5 GeV in the ECAL and muonchambers, no activity in one of the hadron-forward (HF) calorimeters (i.e. no particle produced above threshold3 y-5 -4 -3 -2 -1 0 1 2 3 4 5/dy e+e-→ΥdN02004006008001000  e+e-→ Υinputreco y-5 -4 -3 -2 -1 0 1 2 3 4 5/dy-µ+µ →ΥdN0200400600800100012001400160018002000-µ+µ → ΥinputrecoFigure 4: Generated and reconstructed rapidity spectra of the UPCs ϒ measured in the e+e− (left) and µ+µ− (right)channels in CMS.within 3 < |η|< 5 as expected in exclusive events) and a neutron signal in one or both of the ZDCs. Signals aregenerated with the STARLIGHT Monte Carlo according to the cross sections quoted in Tables 1 and 2. The geo-metrical acceptance, detector response and resolutions, and reconstruction efciency losses, have been determinedfor the dimuon and dielectron channels. In the case of the ϒ signal, the rapidity distributions in both analysesare complementary, with the dimuons peaked around |y| = 2 and the dielectrons at |y| < 1 (see Fig. 4). The pTresolution (not shown here) is good enough to separate the coherent (peaked at very low pT ≈MV /γ≈ 30 MeV/c)from the incoherent (pT & 0.3 GeV/c) [22] components.Figure 5 shows the reconstructed dN/dMl+ l− around the ϒ mass (only the ground-state, ϒ(1S), of the bottomo-nium family was generated). In the real data, any residual combinatorial background can be removed by directlysubtracting the like-sign (l+l+ or l−l−) from the unlike-sign pairs. The signal over continuum is around unityfor both decay modes. The combined reconstructed mass spectra are tted to a Gaussian (for the ϒ peak) plus anexponential for the underlying lepton pair contribution. The reconstructed bottomonium peak positions and widthsare: Mµ+µ− = 9.52 GeV/c2(σµ+µ− = 0.090 GeV/c2) and Me+e− = 9.34 GeV/c2(σe+e− = 0.154 GeV/c2), very close tothe nominal Mϒ = 9.46 GeV/c2mass. In the dimuon channel, the good mass resolution of our measurement wouldallow for a clean separation of the ϒ′ (10.02 GeV/c2) and ϒ′′ (10.36 GeV/c2) which are also vector mesons and canbe photoproduced (but were not included in the current simulation).The total ϒ production yields are obtained from an integration within 3σ of the corresponding peak maxima.The nal ϒ acceptance and efciencies are εrec ×A × εyield−extract = 21% for the e+e− analysis and 19% for theµ+µ− one. The total expected number of ϒ events, normalised to the nominal PbPb integrated luminosity ofR Ldt = 0.5 nb−1, are 220 ± 15 (stat) and 180 ± 14 (stat) in the e+e− and µ+µ− channels, respectively, with a ∼ 10%systematic uncertainty. With such a statistics, detailed pT ,y studies can be carried out, that will help constrain thelow-x gluon density in the nucleus.In the dilepton continuum sector, the same analysis would yield a few thousand e+e− and µ+µ− pairs in the massrange above Ml+ l− ≈ 6 MeV/c2 that will allow us to test the predictions of QED models including non-perturbativecorrections.5 SummaryExclusive quarkonia and dilepton production in ultra-peripheral Pb-Pb collisions at 5.5 TeV provide useful toolsto carry out various studies of the QCD and QED sectors of the Standard Model. High-energy ϒ photoproductionprovides a particularly useful means to constrain the poorly known low-x gluon distribution of the nucleus ina clean environment. Exclusive dileptons allow us to test QED in the non-perturbative (high eld-strength)regime. We have presented the perspectives of the CMS experiment in 5.5 TeV Pb-Pb collisions at the LHC. Inthe absence of strong non-linear QCD effects, around 400 photo-produced ϒ will be reconstructed in the CMSacceptance with nominal integrated luminosities. The same analysis cuts would yield a few thousand e+e− andµ+µ− pairs in the mass range above Ml+ l− ≈ 6 MeV/c2.4)2 (GeV/ce+e-m7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12-1 )2entries (60 MeV/c050100150200250300-1PbPb UPC - 5.5 TeV - 0.5 nb)-e+ e→ (Υ → Pb γ-e+ e→ γ γ[STARLIGHT model. Full CMS sim+reco])2 (GeV/c-µ+µm7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12-1 )2entries (60 MeV/c050100150200250-1PbPb UPC - 5.5 TeV - 0.5 nb)-µ+µ → (Υ → Pb γ-µ+µ → γ γ[STARLIGHT model. Full CMS sim+reco]Figure 5: Expected e+e− (top) and µ+µ− (bottom) invariant mass distributions from γPb→ ϒPb (followed byϒ→ l+ l−) and γγ→ l+ l− continuum in UPC Pb-Pb at √sNN = 5.5 TeV.AcknowledgmentsSpecial thanks are due to Joakim Nystrand for providing the STARLIGHT Monte Carlo predictions and for usefuldiscussions, as well as to Mark Strikman for informative exchanges and to Aurelien Hees for his valuable par-ticipation in the rst phase of these studies. Dde is supported by the 6th EU Framework Programme contractMEIF-CT-2005-025073.References[1] C. von Weizs¤acker Z. Physik 88 612 (1934); E. Fermi Nuovo Cimento 2 143 (1924).[2] J. Favereau et al., CP3-08-04 (June 2008), to be submitted to Eur. Phys. J. C[3] A. Baltz, Phys. Rept. 458, 1 (2008).[4] See e.g. D. d’Enterria, Eur. Phys. J. A 31 (2007) 816.[5] L. Gribov, E. M. Levin and M. G. Ryskin, Phys. Repts. 100 1 (1983);A. H. Mueller and J. w. Qiu, Nucl. Phys. B 268 427 (1986)[6] See e.g. F. Gelis, T. Lappi and R. Venugopalan, Int. J. Mod. Phys. E 16, 2595 (2007); J. Jalilian-Marian andY. V. Kovchegov, Prog. Part. Nucl. Phys. 56 104 (2006).[7] L. Frankfurt, M. Strikman and C. Weiss, Ann. Rev. Nucl. Part. Sci. 55 403 (2005)[8] N. Armesto, J. Phys. G32, R367 (2006)[9] A. D. Martin et al., arXiv:0709.4406 (2007).[10] D. d’Enterria [PHENIX Collaboration], Proceeds. Quark Matter’05, arXiv:nucl-ex/0601001.[11] S. R. Klein, J. Nystrand, Phys. Rev. C60, 014903 (1999); J. Nystrand Nucl. Phys. A752 470c (2005); A. Baltz,S. Klein, J. Nystrand, Phys. Rev. Lett. 89, 012301 (2002).[12] J. Adams et al. [STAR Collaboration], Phys. Rev. C 70, 031902 (2004)[13] L. Frankfurt, V. Guzey, M. Strikman and M. Zhalov, JHEP 0308 043 (2003)[14] V. P. Goncalves and M. V. T. Machado, arXiv:0706.2810 (2007).[15] Yu. P. Ivanov, B. Z. Kopeliovich and I. Schmidt, arXiv:0706.1532 (2007).[16] O. A. Grachov et al. [CMS Collaboration], AIP Conf. Proc. 867 258 (2006)[17] D. Bocian and K. Piotrzkowski, Acta Phys. Polon. B 35 2417 (2004) and refs. therein.[18] See B. Pire, these proceedings.5[19] J. Hollar, S. Ovyn and X. Rouby [CMS Collaboration], CMS PAS-DIF-07-001; see also J. Hollar, theseproceeds.[20] See G. Baur and C. Guclu, these proceedings, and refs. therein.[21] D. d’Enterria (ed.) et al. [CMS Collaboration], J. Phys. G. 34 2307. (2007).[22] M. Strikman, M. Tverskoy and M. Zhalov, Phys. Lett. B626 72 (2005)6

Exclusive Photoproduction of $\Upsilon$ and lepton pairs in Pb-Pb at 5.5 TeV

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Exclusive Photoproduction of Υ\UpsilonΥ and lepton pairs in Pb-Pb at 5.5 TeV

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Exclusive Photoproduction of $\Upsilon$ and lepton pairs in Pb-Pb at 5.5 TeV