Coherent J/psi photoproduction in ultra-peripheral PbPb collisions at root s(NN)=2.76 TeV with the CMS experiment

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Physics

Letters

B

www.elsevier.com/locate/physletb 1 66 2 67 3 68 4 69 5 70 6 71 7 72 8 73 9 74 10 75 11 76 12 77 13 78 14 79 15 80 16 81 17 82 18 83 19 84 20 85 21 86 22 87 23 88 24 89 25 90 26 91 27 92 28 93 29 94 30 95 31 96 32 97 33 98 34 99 35 100 36 101 37 102 38 103 39 104 40 105 41 106 42 107 43 108 44 109 45 110 46 111 47 112 48 113 49 114 50 115 51 116 52 117 53 118 54 119 55 120 56 121 57 122 58 123 59 124 60 125 61 126 62 127 63 128 64 129 65 130

Coherent

J

/ψ

photoproduction

in

ultra-peripheral

PbPb

collisions

at

√

s

_{N N}

=

2

.

76

TeV with

the

CMS

experiment

.

The

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received23May2016

Receivedinrevisedform21June2017 Accepted3July2017 Availableonlinexxxx Editor:M.Doser Keywords: CMS Physics

Heavyioncollisions Ultra-peripheralcollisions UPC

J/Psi

The cross section for coherent J/ψ photoproduction accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, Xn0n, is measured with the CMS experiment in ultra-peripheral PbPb collisions at √sN N=2.76 TeV. The analysis is based on a

data sample corresponding to an integrated luminosity of 159 μb−1, collected during the 2011 PbPb run. The J/ψ mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is d

σ

coh

Xn0n/d y(J/ψ)=0.36 ±0.04 (stat)±

0.04 (syst) mb in the rapidity interval 1.8 <|y|<2.3. Using a model for the relative rate of coherent photoproduction processes, this Xn0nmeasurement gives a total coherent photoproduction cross section of d

σ

coh_{/d y(J/ψ)=1.82 ±0.22 (stat)±0.20 (syst)±0.19 (theo) mb. The data strongly disfavor the impulse} approximation model prediction, indicating that nuclear effects are needed to describe coherent J/ψ photoproduction in

γ

+Pb interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing.

©2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3_.

1. Introduction

Photon-inducedreactionsaredominantinUltra-Peripheral Col-lisions(UPC)ofheavyions,whichinvolveelectromagnetic interac-tionsatlargeimpactparametersofthecollidingnuclei.Becauseof theextremelyhighphoton fluxinultra-peripheral heavy-ion col-lisionswhich isproportionalto Z2_{,where Z isthecharge ofthe}

nucleus,photon–nucleuscollisionsattheLHCareabundant [1–3].

Furthermore,in UPCs the LHC can reach unprecedented

photon-leadandphoton–protoncenter-of-massenergies.

VectormesonphotoproductioninUPCshasreceivedrecent in-terest [3]. Exclusive J

/ψ

photoproduction off protons is defined bythe reaction

γ

+

p

→

J

/ψ

+

p, withthe characteristicfeatures that, apart from the vector meson in the final state, no other particles are produced andthe vector meson has a mean trans-verse momentum significantly lower than in inclusive reactions. Anothercharacteristicfeatureisthat inexclusivephotoproduction thequantumnumbersofthefinalstatecanbestudied unambigu-ously. The

γ

+

p

→

J

/ψ

+

p production process has been stud-iedbyH1andZEUS collaborationsattheelectron–protoncollider HERA[4–6],by theCDF collaboration in proton–antiproton colli-sionsattheTevatron[7],andbytheALICEandLHCbcollaborations

 _{E-mailaddress:cms-publication-committee-chair@cern.ch.}

attheLHC,inproton-lead[8]andproton–protoncollisions[9], re-spectively.Sincethecrosssectionofphotoproducedvectormesons suchasJ

/ψ

,

ψ(

2S

)

,and

ϒ(

nS

)

,inleadingorderperturbativeQCD, isproportionalto thegluondensitysquaredinthetarget [10,11], the study of such diffractive processes in high-energy collisions is expected to provide insights into the role played by gluons in hadronic matter. As an example, a J

/ψ

produced at rapidity

y is sensitive to the gluon distribution at x

= (

MJ/ψ

/

√

s

)

e±y _at

hardscales Q2

∼

M2_J/ψ

/

4,whereMJ/ψ istheJ

/ψ

mass,

√

s isthe

center-of-massenergyofthecollidingsystemandy istherapidity ofthe J

/ψ

[10,11].The relevantvaluesof x thatcanbe explored inthisanalysisareinthe10−2 _to10−5 _range.

In ultra-peripheral nucleus–nucleus collisions, vector mesons canbeproducedin

γ

+

A interactionsoffoneofthenuclei[12–20]. Such interactions are characterized by very low multiplicity, and indeed the majority of such events are exclusive, i.e.

γ

+

A

→

J

/ψ

+

A. The interaction that produces the vector mesonis clas-sifiedascoherentifthephotoninteractswiththewholenucleus, leavingthe nucleusintact. In incoherent interactions, thephoton interactswithasinglenucleon,andthenucleusbreaksapart.The

requirement of having coherent photoproduction constrains the

meantransversemomentumofthevectormesonstobeofthe or-der of pT

≈

60MeV for PbPbcollisions at

√

sN N

=

2

.

76 TeV[1–3].

Thisfollowsfromthefactthatthetransversemomentum distribu-tionisdrivenbythetargetformfactor.Becausethenucleonradius http://dx.doi.org/10.1016/j.physletb.2017.07.001

0370-2693/©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

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issmallerthan thatof thenuclei, themomentumtransfer tothe vector meson from incoherentphotoproduction is higher, of the orderof500 MeV at

√

sN N

=

2

.

76 TeV.Suchamomentumtransfer

causesthe target nucleustobreak up and,in mostcases,it pro-ducesneutronsatverysmallangleswithrespecttothePbbeams (forward neutrons). However, vectormesons produced coherently

can also be accompanied by forward neutrons. Owing to the

in-tense electromagnetic fields present in ultra-peripheral nucleus– nucleuscollisions,additionalindependentsoftelectromagnetic in-teractions can occur between the nuclei giving rise to forward neutrons. The emission of such neutrons is understood in terms of giant dipole resonances [21]. Neutron-differential studies are consideredasapromisingtooltodecouplelow-x andhigh-x con-tributionsinvectormesonphotoproduction,e.g.[22].

Ultimately, UPC studies at hadron colliders and similar mea-surements atthe proposed electron–ioncolliders [23,24] are ex-pectedtoreduceuncertaintiesinourknowledgeoftheinitialstate ofahigh-energynucleus–nucleuscollision,inparticular,regarding theintrinsicdistribution andfluctuationsofgluonsinthe nuclei. The uncertaintyover the initialstate iscurrentlyan impediment to measuringfundamental properties ofthe quark–gluonplasma, such asviscosity,to a highprecision [25].The largesttheoretical uncertaintycomes fromthegluon distributionfunction innuclei,

which at a given value of the Bjorken variable x may be

de-pleted(shadowing)orenhanced(anti-shadowing)withrespectto the scaled gluon distribution function in the proton. These par-ton distribution functions(PDFs) have been parameterized using global fitting techniques, such as EPS09 [26], that evolve quark, antiquark, and gluon distributions as a function of Q2_{. The}

fit-tingresultsfromEPS09havealargeuncertaintyforgluonPDFsfor

x

<

10−2 andlow Q2 due to thelack of experimental data.The datafromultra-peripheralcollisionsattheLHChavethepotential toprovide newconstraintstothe gluonPDFsin protonsand nu-clei. Recenttheoretical work has beencarried out to includethe studyofUPCvectormesonphotoproductioninglobalPDFfits[27, 28].

The STAR and PHENIX collaborations at RHIC have

stud-ied

ρ

0 _{and J}

_/ψ

_{photoproduction in ultra-peripheral AuAu}

col-lisions [29–31]. Although RHIC studies have demonstrated the feasibility of measuring these processes, it was not possible to significantly constrain the nuclear gluon PDFs. The J

/ψ

analysis was statistically limited [29], while for UPC

ρ

0 _{analyses a hard}

scale cannot be established to perform perturbative QCD calcu-lations. The production rate for UPC physics processes is much higherattheLHC.TheALICEcollaborationhasmeasuredcoherent photoproductionofJ

/ψ

mesonsinultra-peripheralPbPbcollisions at

√

sN N

=

2

.

76 TeV [32,33]. These datahave been used to

com-pute the nuclear suppression factor R

= (

GA

/

AGN

)

2, where GA

andGN are the gluondistributions in a nucleus( A

=

208 inthe

caseof the Pb nuclei) andin a free proton,respectively, obtain-ing R

=

0

.

61+₋0_0..₀₄05 for x

∼

10−3 [34]. Theseresults haveprovided evidence that the nuclear gluon density is below that expected for a simple superposition of protons and neutrons in the nu-cleus [32,33].Models that neglect nuclear gluon shadowing such as starlight [35] and the impulse approximation [19], or mod-elsthat maximizethegluon shadowing,such asEPS08[36],have beenruledoutbythesemeasurements.

ThisLetterreportsthestudyofthecoherentJ

/ψ

photoproduc-tioncross section measured inultra-peripheral PbPb collisions at

√

sN N

=

2

.

76 TeV, as well as the dependence of this cross

sec-tion on the associated production of forward or backward

neu-trons, i.e. ontheso-calledneutron break-up moderatios [18]. To focus on events withlow backgrounds, following the experience at RHIC [30], the UPC trigger selected events with at least one neutronineithertheforwardorbackward directionfromthe

in-teraction point usingzero degree calorimeters.Using thistrigger, both coherent andincoherent J

/ψ

mesons and

γ

+

γ

→

μ

+

μ

−

events in conjunction with at leastone neutron can be studied. Thisdatasampleisthenusedtomeasurethecrosssectionfor co-herent J

/ψ

photoproductionaccompaniedbyatleastoneneutron from soft independent processes. The J

/ψ

candidates are recon-structedthroughthedimuondecaychannelintherapidityinterval 1

.

8

<

|

y

|

<

2

.

3, adding a new rapidity range to recent measure-mentsofcoherentJ

/ψ

photoproductionattheLHC[32,33].

Thispaperisorganizedasfollows:Section2describestheCMS detector, Section 3 reports on the event selection and analysis strategy, Section4describesthesignalextractionandcorrections, Section 5 summarizestheuncertainties ofthe measurement,and Section 6discusses theresults.Finally,inSection 7thesummary isgiven.

2. TheCMSdetector

The central feature of the CMS apparatus is a superconduct-ing solenoid of6 m internal diameter,providing a magnetic field of 3.8 T. Withinthesolenoidvolume are a siliconpixelandstrip tracker,aleadtungstatecrystalelectromagneticcalorimeter(ECAL), andabrassandscintillatorhadron calorimeter(HCAL),each com-posedofabarrelandtwoendcapsections.Thesilicontracker mea-sureschargedparticleswithin thepseudorapidityrange

|

η

|

<

2

.

5. It consists of 1440silicon pixel and15 148 silicon strip detector modules andis located in the 3.8 T field of the superconducting solenoid. Thepseudorapiditycoverage fortheECALandHCAL de-tectors is

|

η

|

<

3

.

0.Muons aremeasured using theCMSdetector inthepseudorapidity range

|

η

|

<

2

.

4. Themuondetectionplanes aremadeusingthreetechnologies:drifttubes,cathodestrip cham-bers, andresistiveplatechambers.The pT ofthemuonsmatched

to reconstructedtracksismeasured witharesolutionbetter than 1.5%[37].TheHadronicForward(HF)calorimeters(3

.

0

<

|

η

|

<

5

.

2) complementthe coverage providedby thebarrelandendcap de-tectors. The beamscintillator counters(BSCs)areplastic scintilla-torsthatpartiallycoverthefaceoftheHFcalorimeters.Theyhave a pseudorapidity rangebetween3.9 and4.4, witha time resolu-tionof3ns.Thezerodegreecalorimeters(ZDCs)aretwo ˇCerenkov calorimeterscomposedofalternatinglayersoftungstenandquartz fibers, situated in betweenthe two protonbeam lines. They are sensitivetoneutronsandphotonswith

|

η

|

>

8

.

3.TheHF,BSCand ZDC systems each consist of two detectors at either side of the interaction point:HF±,BSC±,ZDC±,respectively. Amoredetailed description oftheCMSdetector,together witha definitionofthe coordinate systemusedandtherelevant kinematicvariables, can befoundin[38].

3. EventselectionandMonteCarlosamples

Thisanalysisusesthedatasamplecollected withtheCMS de-tector in the2011PbPb run, which corresponds toan integrated luminosity of159 μb−1 [39].The eventsare selected witha

ded-icated trigger designed to record UPC J

/ψ

vector mesons and

γ

+

γ

→

μ

+

μ

−events.TheUPCtriggerhasthefollowing require-ments:an energydepositconsistentwithatleastone neutronin eitheroftheZDCs;noactivityinatleastoneoftheBSC

+

orBSC

−

scintillators;thepresenceofatleastonesinglemuonwithouta pT

thresholdrequirement,andatleastonetrackinthepixeldetector. Thefirstthreetriggerrequirementsareimplementedinhardware, while the last requirement is carried out by the software trig-ger.Torejectbeam-gas interactionsandsuppressnon-UPCevents the followingrequirementsare imposedoffline. The z positionof

the primary vertex is required to be within 25 cmof the beam

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withtracksoriginatingfromthisvertex.Thisrequirementremoves beam-background eventsthatproduceelongated pixelclusters.In addition, eventsare rejectedif the time difference between two hitsfromtheBSCsisabove20nswithrespecttothemeanflight timebetweenthem(73ns).Thisrequirementremovesbeam-halo events,whilekeepingalltheultra-peripheralPbPbevents.

As mentioned above, one of the UPC trigger requirements is the presence of atleast one neutron. The eventsstudied inthis analysisare classifiedby the patternof neutron deposition

mea-sured in the ZDCs [40–42]. The ZDC energy spectrum shows a

clear one neutron peak and the detectors have an energy

res-olution of about 20% for single neutrons in PbPb collisions at

√

_s

NN

=

2

.

76 TeV[40–42].Thisresolutionallowsagoodseparation

betweenevents withzero, one, or multiple neutrons in a given ZDC detector. A given event is considered to have no neutrons in the ZDC if the calorimeter energy is less than 420 GeV, one

neutronif the energy liesbetween 420 GeVand 1600GeV, and

morethanone neutroniftheenergyisabove 1600GeV. The co-herent J

/ψ

crosssection is measured forthe casewhen theJ

/ψ

mesonsare accompanied by atleast one neutron onone side of the interaction point and no neutron activity on the other side ( Xn0n). The Xn0n break-up mode, which is conventionally

writ-tenasPb

+

Pb

→

Pb

+

Pb

+

J

/ψ

( Xn0n),isasubsetofthetriggered

events.This break-up mode is well suited for rejecting non-UPC backgroundduetoitsasymmetricconfiguration[43].

Apartfromthe Xn0n break-up mode,the UPCtriggeralso

se-lectsthe XnXn,1n0n,and1n1nbreak-up modes.The XnXn mode

requires that both ZDCs record at least one neutron. The 1n0n

mode requiresthat one ofthe ZDCs detects exactly one neutron withnoneutronactivity onthe other ZDCside. Finally,the 1n1n

moderequiresbothZDCstohaveexactlyoneneutron.

InadditiontotheZDCrequirement,twoselectionsareapplied to reject non-UPCevents. First,only events withexactly two re-constructed tracks are kept. Second, the HF cell withthe largest energydepositisrequiredtohaveanenergybelow3.85 GeV.This requirement, which is determined studying events triggered on emptybunches,ensuresthat theHFenergyisconsistentwiththe presenceofphoton-inducedprocesseswhichleaveverylowsignal inboththeHF

+

andHF

−

detectors.

Inthis analysis, both muons have to satisfy the quality crite-riadescribed below, andmust lie within thephase space region 1

.

2

<

|

η

|

<

2

.

4 and 1

.

2

<

pT

<

1

.

8 GeV. This phase space region

is chosen to ensure good statisticalprecision on the data-driven measurement of the single-muon efficiency (see Section 4). The CMScollaborationhasdevelopedseveraltypesofmuon identifica-tion[37].Inthisanalysis, alltracksinthesilicontrackerthat are identifiedasmuons, basedoninformationofthemuondetectors, are used. The algorithm extrapolates each reconstructed silicon trackoutward toits most probablelocation within each detector ofinterest(ECAL,HCAL,muonsystem).Thisprocedureenablesthe identificationofsinglemuonswithverylowtransversemomenta. Toreduceadditionalmuonsorchargedparticletracksthatcanbe misidentifiedasmuons andtoensure good-quality reconstructed tracks, the single muons are required to pass the following cri-teria: more than 4 hits in the tracker, at least one of which is requiredto be ina pixel layer, a trackfit witha

χ

2 _{per degree}

offreedom lessthanthree,andatransverse(longitudinal)impact parameteroflessthan0.3(20) cm from themeasuredvertex. For this analysis, only events with dimuons having pT

<

1

.

0 GeV, in

therapidity interval 1

.

8

<

|

y

|

<

2

.

3, are considered. The dimuon candidates are required to be within the invariant mass region 2

.

6

<

m

(

μ

+

μ

−

)

<

3

.

5 GeV. No like-signdimuon pairs are found inthis region. Applying the muon quality requirements, after all otheranalysisselections,onlyrejectsonedimuoncandidateoutof 518events.

Inorderto computeacceptance andefficiencycorrectionsand forsignal extraction purposes,Monte Carlo(MC) samplesfor co-herent J

/ψ

, incoherent J

/ψ

and

γ

+

γ

events in the dimuon decaychannel are generated, usingthe starlight MC event gen-erator[15,35,44,45].TheseeventsareprocessedwiththefullCMS

simulation and reconstruction software. The starlight

genera-tor models two-photon andphoton–hadron interactions at ultra-relativistic energies. In the case of photon-nuclear reactions, it modelsbothcoherentandincoherenteventsusingthevector me-son dominance model.It usesthe Glauber approach for calculat-inghadron–nucleuscrosssectionsfromhadron–nucleonones,and makesuseofexclusiveJ

/ψ

photoproductionin

γ

+

p resultsfrom HERAtocompute thecoherentJ

/ψ

crosssection in

γ

+

Pb inter-actions[15].The starlight generatorisalsousedtosimulatethe variousbreak-upmodesforoneorbothPbnuclei,whichassumes thattheprobabilitiesforexchangeofmultiplephotonsinasingle eventfactorizeinimpactparameterspace[46].

4. Signalextractionandcorrections

AfterapplyingtheselectionsdescribedinSection3,thedimuon invariantmassandpTdistributionsaresimultaneouslyfittedin

or-der to extract the number ofcoherent J

/ψ

, incoherent J

/ψ

, and

γ

+

γ

→

μ

+

μ

− events.The fitusesamaximumlikelihood algo-rithmthattakesunbinnedprojectionsofthedataininvariantmass andpTasinputs.TheshapesofthepTdistributionsforthesethree

processesaredeterminedfrom starlight simulation.Theyieldfor each of these processes in the pT distribution is a free

parame-terofthe fit.Thedimuon invariant massdistributionofthe sum ofcoherentandincoherentJ

/ψ

eventsisdescribedwithaCrystal Ball function [47], which accountsfor the detector resolution as well asthe radiativetailfrominternalbremsstrahlung. A second-order polynomialaccountsfor theunderlying dimuoncontinuum thatoriginatesfrom

γ

+

γ

→

μ

+

μ

− events.The fithasninefree parameters: three for the yields of each of the processes, two for theshape ofthe Crystal Ballfunction tail,two for the mean and width of the Crystal Ball function, and two parameters for the shape of thesecond-order polynomial. The fitconstrains the numberofcoherentJ

/ψ

,incoherentJ

/ψ

,anddimuoncontinuum eventstobethesameintheinvariant massandpT distributions.

The projections of the Xn0n break-up data onto the dimuon

in-variant mass and pT axes are shown in Fig. 1. As discussed in

Section 1, the average pT distribution for the coherent events is

peakedatlower pT valuesthanthosefromincoherentevents.

Re-constructedcoherent J

/ψ

eventsaredominantfor pT

<

0

.

15GeV,

whereas reconstructed incoherent J

/ψ

events are dominant for

pT

>

0

.

15GeV. For events with pT

<

0

.

15 GeV and in the

rapid-ity interval 1

.

8

<

|

y

|

<

2

.

3, the fit yields 207

±

18(stat) for the coherentJ

/ψ

candidates, 75

±

13(stat) for incoherentJ

/ψ

events and75

±

13(stat) for

γ

+

γ

events.

Inaddition,thedatasampleisstudiedintermsofthefollowing twocases:(i)neutronsemittedinthesamerapidityhemisphereas the J

/ψ

,and(ii)neutrons emitted intheopposite rapidity hemi-spherethantheJ

/ψ

.ThenumberofcoherentJ

/ψ

eventsisfound tobe consistent,withinthestatisticalandsystematicuncertainty, between the two cases. This suggests that the emitted neutrons

and the photoproduced J

/ψ

events are independent processes,

within thecurrentuncertainty.Ontheotherhand,forincoherent J

/ψ

photoproductionmostofthe eventsare foundin the config-urationwhere theneutronsandthe J

/ψ

mesonsareproduced in thesamehemisphere.ThissuggeststhatinincoherentJ

/ψ

photo-productionthelow-x andhigh-x contributionsare decoupledand canbe moreeasily observedthanincoherentJ

/ψ

events.A sim-ilarqualitative behaviorisobservedinphotoproduced J

/ψ

events in

γ

+

p interactionsbyALICE[8]wheretheratioofnon-exclusive

1 66 2 67 3 68 4 69 5 70 6 71 7 72 8 73 9 74 10 75 11 76 12 77 13 78 14 79 15 80 16 81 17 82 18 83 19 84 20 85 21 86 22 87 23 88 24 89 25 90 26 91 27 92 28 93 29 94 30 95 31 96 32 97 33 98 34 99 35 100 36 101 37 102 38 103 39 104 40 105 41 106 42 107 43 108 44 109 45 110 46 111 47 112 48 113 49 114 50 115 51 116 52 117 53 118 54 119 55 120 56 121 57 122 58 123 59 124 60 125 61 126 62 127 63 128 64 129 65 130

Fig. 1. Resultsfromthesimultaneousfittodimuoninvariantmass(top)andpT

(bot-tom)distributionsfromopposite-signmuonpairswithpT<1.0GeV,1.8<|y| <2.3

and2.6<m(μ+μ−)<3.5 GeV fortheXn0nbreak-upmode,afterallselectionsare

applied.Intheleftpanelthegreencurverepresentstheγ+γcomponent (second-orderpolynomial)andtheblackcurvethesumoftheγ+γ,incoherentJ/ψ,and coherentJ/ψ components(seetextfordetails).Intherightpanelthegreen,red, andbluecurvesrepresentγ+γ,coherentJ/ψ,andincoherentJ/ψ components, respectively.Theblackcurverepresentsthesumoftheγ+γ,coherentJ/ψ,and incoherentJ/ψcomponents.Onlystatisticaluncertaintiesareshown.Thedataare notcorrectedbyacceptanceandefficiencies,andtheMCtemplatesarefoldedwith thedetectorresponsesimulation.(Forinterpretationofthereferencestocolorin thisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

toexclusiveJ

/ψ

eventsisfoundtobelargerathigh-x withrespect tothatatlow-x.Duetothesmallsamplesizeofthisanalysis,the coherentJ

/ψ

crosssectionismeasuredbysummingupboth con-figurations.

The combined acceptance ( A) and efficiency (

ε

) correction factor for J

/ψ

events in the Xn0n break-up mode,

(

A

ε

)

J/ψ, is

5

.

9

±

0

.

5(syst)%. The 8% systematic uncertainty on the correc-tions are described in Section 5. Two factors contribute to the

(

A

ε

)

J/ψ_{: 1) the product of acceptance multiplied by the offline}

reconstruction efficiency and 2) the trigger efficiency (

ε

trig). The

firsttermismeasuredtobe12

.

0

±

0

.

5(syst)%.Itisobtainedfrom both data and MC simulations. The starlight generator is used as an input to the full Geant4 [48] simulation of the CMS de-tector. This simulation is used to model the efficiency for all of theselectionsexcept theHF andthemuon quality requirements. Zerobias data are used to compute the efficiencyof the HF

re-Table 1

SummaryofsystematicuncertaintiesforcoherentJ/ψ

eventsintheXn0nconfiguration.

Source Uncertainty (1) Signal extraction 5% (2) Neutron tagging 6% (3) HF energy limit 2% (4) MC acceptance corrections 1% (5) ZDC efficiency estimation 3% (6) Tracking reconstruction 4% (7) Int. luminosity determination 5% (8) Branching fraction 0.55% (9) Two-photon e+e−background 2%

Total 11%

quirement,whiletheUPCdataareusedtocompute theefficiency ofthe muonquality requirements.The offlineselection discussed above isapplied,butthetrigger requirementisnotdemanded at this stage ofthe efficiencycalculation. TheUPC triggerefficiency

ε

trig for events passing the event selection is 49

.

5

±

3

.

5(syst)%.

This is computed by taking the product of the efficiencies of

the individual components:

ε

trig

=

ε

ZDC

ε

pixel-track

ε

BSC

ε

dimuon.

Be-cause these trigger components are uncorrelated to each other they are measured separately. The

ε

dimuon term is measured to

be 0

.

71

±

0

.

02(syst) from the analysisoftheUPCdata usingthe

“tag-and-probe” method [37] in which coherent J

/ψ

candidates

are reconstructed fora widerkinematic rangethan inthe analy-sis. Twodifferentmethodsto compute

ε

dimuon are studied

corre-spondingtotwodifferentbackgroundparametrizations.Sinceboth methods give consistent results within thestatistical uncertainty, the

ε

dimuon systematic uncertainty is found to be at the 2–3%

level.Theothercomponentsofthetriggerefficiencydonotrequire the reconstructionofcoherent J

/ψ

candidatesandthey are mea-sured separately using control triggers:

ε

ZDC

=

0

.

91

±

0

.

03(syst),

ε

pixel-track

=

0

.

76

±

0

.

03(syst), and

ε

BSC is fullyefficient.The

sys-tematicuncertaintyfortheacceptanceandefficiencycorrectionis discussedinthefollowingsection.

5. Systematicuncertaintiesandcross-checks

ThesystematicuncertaintiesaresummarizedinTable 1andcan be divided into three groups. The first group corresponds to the systematicuncertaintydueto thesignal extraction(5%).The sec-ond group correspondsto theacceptance timesefficiency correc-tion (8%after combiningthe uncertainties onthe neutron detec-tionefficiency,HFenergyrequirement, MCcorrection,ZDCtrigger efficiency,andJ

/ψ

reconstructionefficiency).Thethirdgroup cor-responds tothe uncertaintyinthe luminositydetermination (5%) andinthebranchingratio(0.55%).Theindividualuncertaintiesare

summarizedbelow.

1. Theuncertaintyinthesignal extractionisfoundtobe 5%.To estimatethisuncertainty,thefittingfunctionsusedtodescribe theinvariantmassdistributionoftheJ

/ψ

andthecontinuum arechangedtoaGaussianorLandaudistribution,respectively. Alsothemassregionusedforthesignalextractionischanged to 2

.

4

<

m

(

μ

+

μ

−

)

<

8

.

0 GeV. The systematic uncertainty is providedbythemaximumvariationoftheresults.

2. Theuncertaintyintheneutrondetectionefficiencyisfoundto be6%. Thisuncertaintyismainlyduetothepresenceof low-frequencynoiseinthereadoutandisestimatedbycomparing resultsfromtwodifferentreconstruction algorithms.Foreach event the ZDC signal is recorded in 10 time slices of 25 ns each.The standardreconstructionmethodusesthe difference between thesignal in the main time slice andthe following

1 66 2 67 3 68 4 69 5 70 6 71 7 72 8 73 9 74 10 75 11 76 12 77 13 78 14 79 15 80 16 81 17 82 18 83 19 84 20 85 21 86 22 87 23 88 24 89 25 90 26 91 27 92 28 93 29 94 30 95 31 96 32 97 33 98 34 99 35 100 36 101 37 102 38 103 39 104 40 105 41 106 42 107 43 108 44 109 45 110 46 111 47 112 48 113 49 114 50 115 51 116 52 117 53 118 54 119 55 120 56 121 57 122 58 123 59 124 60 125 61 126 62 127 63 128 64 129 65 130

one. This differentiationsuppresses the low-frequency noise. The alternative method estimates the noise from time slices beforethemainsignal.

3. The uncertainty associated with the HF energy requirement is found to be 2%. To estimate this uncertainty, the HF en-ergy limit is decreased from 3.85to 2.95 GeV, changing the limit fromkeeping99% oftheelectronicnoise eventsto95%. Also, thedefinitionoftheHFenergyrequirementisvaried by using the signal from groups of calorimeter cells known as towers,insteadoftheindividual cells.The

η

symmetryofthe calorimeters is checked by defining separate limits for HF

+

andHF

−

forbothindividual cells andtowers. Theanalysisis repeatedforeach caseandtheroot-mean-squareofthe final numberofsignalcandidatesisusedtoestimatethesystematic uncertaintyassociatedwiththisrequirement.

4. TheuncertaintyintheMC acceptancecorrectionsisfoundto be1%.ThisisestimatedbyvaryingthepTandrapidityshapes

(

±

30% away from the mean distribution) used to produce

thesecorrections.AsshowninSection4, starlight reproduces verywellthepT shapeforthevariousprocesses.Theshapeof

the pT distributions reflects the nuclear densitydistribution,

whichhaslittleuncertainty.

5. TheuncertaintyfortheZDCcomponentoftheUPCtriggeris foundtobe3%.Thisisestimatedbyusingdedicated monitor-ingtriggers.

6. TheuncertaintyfortheJ

/ψ

reconstructionefficiencyisfound tobe4%.Thisiscomputedusingthetrackreconstruction effi-ciencyuncertaintythatisfoundtobe1–2%[49].

7. Theuncertaintyoftheintegratedluminosity determinationis estimatedtobe5%,basedontheanalysisofdatafromvander Meerscans[50].Thisuncertaintyalsocoversthepossible mul-tipleinteractionsinthesamebunchcrossingoriginatingfrom electromagneticdissociation(EMD)processeswhichcould af-fecttheexclusivityrequirement.

8. Theuncertainty in thebranching fraction forJ

/ψ

decayinto muonsis0.55%[51].

9. A contamination from an electromagnetic e+e− pair could causeapossiblelossofevents,whereoneoftheelectronshits theBSCscintillatorandthusvetoestheevent.Usingacontrol data sample wherenovetoatthetrigger levelisapplied, an upperlimitonsuchaninefficiencyisfoundbytheALICE col-laboration tobesmallerthan2% inthecoherentJ

/ψ

analysis, atforwardrapidity[32].Sincenodatasample,witha compa-rableluminositytotheoneusedinthisanalysis,existwithout a vetoontheBSC,andinorderto beconservative,a 2% sys-tematicuncertaintyisassignedduetopossiblecontamination fromtwo-photone+e− background.

Theseindividual systematicuncertainties areaddedin quadra-tureresultinginatotalsystematicuncertaintyof11% forthe co-herentJ

/ψ

crosssectioninthe Xn0nconfiguration.

Asanadditionalcross-checkoftheoverall analysis,the

γ

+

γ

process is studied. As discussed in Section 4, the resulting yield of

γ

+

γ

events in the 2

.

6

<

m

(

μ

+

μ

−

)

<

3

.

5 GeV mass interval isNγ_X+γ

n0n

=

75

.

2

±

12

.

7(stat)

±

8

.

3(syst),whilethemeasuredcross

sectionis44

.

2

±

1

.

8(stat)

±

0

.

40(syst) μb.Thisresultisconsistent withtheQEDcalculationprovidedbythe starlight MCattheone standarddeviationlevel.The

γ

+

γ

→

μ

+

μ

− crosssection inthe

dimuon mass range 4 to 8 GeV (not shown) is also found to be

inagreement with the starlight predictionwithin one standard deviation, when considering the statistical andsystematic uncer-tainties.

6. Resultsandcomparisontotheoreticalmodelson photonuclearinteractions

Forthe Xn0nbreak-upmode,thecoherentJ

/ψ

crosssectionin

thedimuondecaychannelisgivenby

d

σ

coh Xn0n d y

(

J

/ψ )

=

Ncoh_X n0n

B

(

J

/ψ

→

μ

+

μ

−

)

L

int



y

(

A

ε

)

J/ψ (1)

where

B(

J

/ψ

→

μ

+

μ

−

)

=

5

.

96

±

0

.

03(syst)% is the branching fraction of J

/ψ

to dimuons [51], Ncoh

Xn0n is the coherent J

/ψ

yield of prompt J

/ψ

candidates for pT

<

0

.

15GeV,

L

int

=

159

±

8(syst) μb−1isanintegratedluminosity,



y

=

1 istherapiditybin width,and

(

A

ε

)

J/ψ

=

5

.

9

±

0

.

5(stat)%isthecombinedacceptance times efficiency correction factor as discussed in Section 4. The coherentJ

/ψ

yieldofpromptJ

/ψ

candidatesisgivenby

Ncoh_X

n0n

=

Nyield

1

+

fD

(2)

where Nyield is the coherent J

/ψ

yield asextracted from the fit

shown in Fig. 1, and fD is the fraction of J

/ψ

mesons coming

from coherent

ψ(

2S

)

→

J

/ψ

+

anything. As mentioned in Sec-tion 4, Nyield

=

207

±

18(stat) for coherent J

/ψ

candidates with

pT

<

0

.

15GeV in the rapidity interval 1

.

8

<

|

y

|

<

2

.

3. There are

not enough data to perform a coherent

ψ(

2S

)

analysis, so the feed-down correction hasto rely on MC simulations. In order to calculate fD,coherent

ψ(

2S

)

eventsaresimulatedusing starlight,

while pythia isusedtosimulatethe

ψ(

2S

)

decayintotheJ

/ψ

[32, 33] obtaining fD

=

0

.

018

±

0

.

011(theo). The theoretical

uncer-taintyof60%in fD isobtainedfrom[32,33].Theresultingcoherent

J

/ψ

yield for prompt J

/ψ

candidates is Ncoh_Xn0n

=

203

±

18(stat). Thus, thecoherentJ

/ψ

photoproductioncross section forprompt J

/ψ

mesons in the Xn0n break-up mode is d

σ

_Xcoh_n0n

/

d y

(

J

/ψ)

=

0

.

36

±

0

.

04(stat)

±

0

.

04(syst) mb. Althoughthed

σ

coh

Xn0n

/

d y

(

J

/ψ)

measurementisinterestinginits

ownright[18,22],itisalsorelevanttocompareourresultstothe theoretical predictionsandrecentresultsfromtheALICE collabo-ration [32,33] that are available for the total coherent J

/ψ

cross section.AsmentionedinSection3,oneoftheUPCtrigger require-ments is the presence of at least one forward neutron. For this reasonitisnotpossibletoscalethemeasuredcoherentJ

/ψ

cross section inthe Xn0n break-up modeto thetotal crosssection

us-ing ourowndata.However,asmentionedinSection 3, starlight can simulatecoherent vector meson photoproductionin the

var-ious break-up modes for one or both Pb nuclei. The starlight

MC generator is found to give a good description of the break-upratiosoncoherent

ρ

0 _{photoproductionmeasuredbySTAR[29]}

andALICE[46]. Itis alsofound togive a gooddescriptionofthe fractionofcoherentJ

/ψ

eventswithnoneutronemittedwith re-spect to the total number of coherent J

/ψ

events, measured by ALICE [33]. Moreover, starlight gives a good description of the

break-up ratios measured in this analysis. We measure the

ra-tios of the coherent J

/ψ

cross section in two different break-up modes( XnXnand1n1n)tothatofthe Xn0nmode forJ

/ψ

events

with pT

<

0

.

15GeV and in the rapidity interval 1

.

8

<

|

y

|

<

2

.

3.

Themeasuredbreak-upratiosare0

.

36

±

0

.

04(stat) forXnXn

/

Xn0n

and0

.

03

±

0

.

01(stat) for 1n1n

/

Xn0n,whilethe starlight

predic-tion is 0

.

37

±

0

.

04(theo) and 0

.

020

±

0

.

002(theo), respectively. Theseratios arealso compatiblewiththeextractedJ

/ψ

yield for eachbreak-upconfiguration,determinedwiththesignalextraction procedure described inSection 4.Only statisticaluncertainties in themeasuredbreak-upratiosaregivensincethesedominateover thesystematicuncertainties.Thefeed-downcorrectionfrom

ψ(

2S

)

decaysisnotappliedfortheseratiossincethiscontributionis ex-pectedtocanceloutintheratio.The10%uncertaintyquotedinthe

1 66 2 67 3 68 4 69 5 70 6 71 7 72 8 73 9 74 10 75 11 76 12 77 13 78 14 79 15 80 16 81 17 82 18 83 19 84 20 85 21 86 22 87 23 88 24 89 25 90 26 91 27 92 28 93 29 94 30 95 31 96 32 97 33 98 34 99 35 100 36 101 37 102 38 103 39 104 40 105 41 106 42 107 43 108 44 109 45 110 46 111 47 112 48 113 49 114 50 115 51 116 52 117 53 118 54 119 55 120 56 121 57 122 58 123 59 124 60 125 61 126 62 127 63 128 64 129 65 130

Fig. 2. DifferentialcrosssectionversusrapidityforcoherentJ/ψproductionin ultra-peripheralPbPbcollisionsat√sN N=2.76 TeV,measuredbyALICE[32,33]andCMS (seetextfordetails).Theverticalerrorbarsincludethestatisticalandsystematic uncertaintiesaddedinquadrature,andthehorizontalbarsrepresenttherangeof themeasurementsiny.Alsotheimpulseapproximationandtheleadingtwist ap-proximationcalculationsareshown(seetextfordetails).

starlightpredictionforthebreak-upmodescalingfactorsisbased onrecentresultsonUPC

ρ

0 _{photoproductionfromtheALICE}

col-laboration[46].Notethattheneutronbreak-uptheoretical descrip-tionisindependentofwhetheraJ

/ψ

ora

ρ

0 _{isproduced[45,46].}

Thescalingfactorbetweenthe Xn0nbreak-upmodeandthetotal

crosssection is 5

.

1

±

0

.

5(theo).After applyingthisscaling factor we obtain the total coherent J

/ψ

photoproduction cross section d

σ

coh

_/

_{d y}

₍

_J

_/ψ)

₌

₁

_.

₈₂

_±

₀

_.

_22(stat)

_±

₀

_.

_20(syst)

_±

₀

_.

_{19(theo) mb.}

In Fig. 2, the coherent J

/ψ

photoproduction cross section is compared to recent ALICE measurements [32,33], to calculations by Guzeyet al. [19,34]based onthe impulseapproximation, and toresultsobtainedusingtheleading twistapproximation(see

be-low).ThedatafromALICEandCMSshowa steadydecreasewith

rapidity.

The leading twist approximation prediction is obtained from Ref. [19] and is in good agreement with the data. It is a cal-culation at the partonic level that uses a diffractive proton PDF as an input, following the leading twist approximation which is basedona generalizationoftheGribov–Glauber nuclear shadow-ingapproach[52].Thetheoreticaluncertaintybandfortheleading twist approximation resultshown in Fig. 2is 12% andis dueto theuncertaintyinthestrengthofthegluon recombination mech-anism. Thisuncertainty isuncorrelatedwiththe photon flux un-certainty.The nuclear gluon distributionuncertainty is largest at mid-rapidity where x

∼

10−3 _{in the nuclear gluon distribution.}

At forwardrapidity,integratingover all possibleemitted neutron configurations,thereisatwo-foldambiguityaboutthephoton di-rection.In thisregion, the measurements are mostly sensitive to

x

∼

10−2 [32].

The dataare alsocompared tothe impulse approximation re-sultthat uses datafromexclusive J

/ψ

photoproductionin

γ

+

p interactions toestimatethe coherentJ

/ψ

crosssection in

γ

+

Pb collisions. Theimpulse approximation calculationneglects all nu-cleareffectssuchastheexpectedmodificationofthegluondensity intheleadnucleicomparedtothatoftheproton.Thiscalculation

overpredictstheCMSmeasurement bymore than3standard

de-viationsinthe rapidity interval 1

.

8

<

|

y

|

<

2

.

3, whenaddingthe experimentalandtheoreticaluncertaintiesinquadrature.

The cross section for vector meson photoproduction in ultra-peripheralPbPbcollisionsisgivenbythesumoftwocrosssection terms, since photons can be emitted by either of the colliding Pb nuclei.Each termisthe product ofthreequantities: the pho-ton flux, theintegral over squarednuclear formfactor FA

(

t

)

and

the forward differential cross section d

σ

/

dt

(

t

=

0

)

of

γ

+

p

→

J

/ψ

+

p,wheret isthemomentumtransferfromthetargetnucleus squared. The FA

(

t

)

istheFouriertransformofthematterdensity

ρ

(

t

)

,whiletheelementarycrosssectiond

σ

/

dt hasbeenmeasured byvariouscollaborations[5–9],asdescribedinSection1.The im-pulseapproximationresultshowninFig. 2isperformedbyGuzey et al. using the methods they describe in Ref. [34] witha pQCD motivatedparametrization[53]ofexclusiveJ

/ψ

datain

γ

+

p in-teractionswhich incorporatesvery recentLHC results[8,9]. Thus, intheimpulseapproximationthereisanexperimentaluncertainty associated to fitting the measured elementarycross section data totheparametrization[53]andthisuncertaintyisatthe4%level forthe relevantphoton–proton center-of-massenergies discussed in this analysis. In addition, there are two theoretical uncertain-tiesintheimpulseapproximationcalculation.Thefirsttheoretical uncertainty is due to the matter density distribution and is es-timated to be 5% based on studies ofseveral matter distribution densities [34]. The second theoretical uncertainty is due to the uncertainty inthephoton flux andisestimated tobe 5%. Thisis dominatedbythetreatmentofthephotonfluxfactorforthecase when the PbPb collisions take place at small impact parameters

∼

2RA.Thesetwouncertaintiesarecorrelatedandsotobe

conser-vativethecombinedtheoreticaluncertaintyistakentobe 10%. The data are also consistent with the central value of the

EPS09 global fit from2009 (not shown), which has large

uncer-tainties [26].Other calculationsofthecoherentJ

/ψ

cross section are not considered because the theoretical uncertainties are not available.

7. Summary

The coherent J

/ψ

photoproduction cross section in

ultra-peripheral PbPb collisions at

√

sN N

=

2

.

76 TeV, in conjunction

with at least one neutron on one side of the interaction point

and no neutron activity on the other side, is measured to be

d

σ

coh

Xn0n

/

d y

(

J

/ψ)

=

0

.

36

±

0

.

04(stat)

±

0

.

04(syst) mb in the

rapid-ity interval 1

.

8

<

|

y

|

<

2

.

3. This measurement is extrapolated to thetotalcoherentJ

/ψ

crosssection,resultingind

σ

coh

_/

_{d y}

₍

_J

_/ψ)

₌

1

.

82

±

0

.

22(stat)

±

0

.

20(syst)

±

0

.

19(theo) mb in the measured rapidity interval. Theseresults complementrecentmeasurements on coherent J

/ψ

photoproduction in ultra-peripheral PbPb colli-sions at

√

sN N

=

2

.

76 TeV by theALICEcollaboration. An impulse

approximation model prediction isstrongly disfavored, indicating thatnucleareffectsexpectedtobepresentatlowx andQ2_values

areneededtodescribethedata.Thepredictiongivenbythe lead-ingtwistapproximation,whichincludesnucleargluonshadowing, is consistent withthedata. Inaddition, weobserve that, in con-trast tocoherent J

/ψ

events,thevast majorityofincoherentJ

/ψ

candidatesareintheconfigurationwhentheJ

/ψ

andtheemitted neutronsareinthesamerapidityhemisphere(high-x component). ThisisqualitativeagreementwiththerecentphotoproductedJ

/ψ

analysisoffprotonsbytheALICEcollaboration.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator

depart-ments for the excellent performance of the LHC and thank the

technical andadministrativestaffs atCERNand atother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentersand

1 66 2 67 3 68 4 69 5 70 6 71 7 72 8 73 9 74 10 75 11 76 12 77 13 78 14 79 15 80 16 81 17 82 18 83 19 84 20 85 21 86 22 87 23 88 24 89 25 90 26 91 27 92 28 93 29 94 30 95 31 96 32 97 33 98 34 99 35 100 36 101 37 102 38 103 39 104 40 105 41 106 42 107 43 108 44 109 45 110 46 111 47 112 48 113 49 114 50 115 51 116 52 117 53 118 54 119 55 120 56 121 57 122 58 123 59 124 60 125 61 126 62 127 63 128 64 129 65 130

personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythecomputinginfrastructure essential toour analyses. Finally, we acknowledge the enduring support for the construc-tionandoperationofthe LHCandtheCMSdetectorprovided by thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS

and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil);

MES(Bulgaria);CERN;CAS,MOST,andNSFC(China);COLCIENCIAS (Colombia);MSESandCSF(Croatia);RPF(Cyprus);MoER,ERCIUT andERDF(Estonia); AcademyofFinland,MEC, andHIP (Finland);

CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany);

GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India);

IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic of Korea); LAS (Lithuania); MOE and UM (Malaysia); BUAP,

CIN-VESTAV,CONACYT,LNS, SEP,andUASLP-FAI(Mexico); MBIE(New

Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portu-gal);JINR(Dubna);MON,RosAtom,RASandRFBR(Russia);MESTD (Serbia);SEIDIandCPAN(Spain);SwissFundingAgencies

(Switzer-land); MST (Taipei); ThEPCenter, IPST, STAR and NSTDA

(Thai-land);TUBITAKandTAEK(Turkey);NASUandSFFR(Ukraine);STFC (UnitedKingdom);DOEandNSF(USA).

Individuals have received support from the Marie-Curie

pro-gramandtheEuropeanResearchCouncilandEPLANET(European

Union);the Leventis Foundation; the A.P. SloanFoundation; the AlexandervonHumboldt Foundation;theBelgianFederal Science

Policy Office; the Fonds pour la Formation à la Recherche dans

l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap

voor Innovatie door Wetenschap en Technologie (IWT-Belgium);

theMinistryofEducation, YouthandSports (MEYS)ofthe Czech Republic;theCouncilofScienceandIndustrialResearch,India;the HOMING PLUSprogram ofthe Foundation forPolish Science,

co-financed from European Union, Regional Development Fund; the

MobilityPlusprogramoftheMinistryofScienceandHigher Edu-cation(Poland);theOPUSprogramoftheNationalScienceCenter (Poland);the Thalis andAristeia programs cofinancedby EU-ESF andtheGreek NSRF;the NationalPrioritiesResearch Programby

Qatar National Research Fund; the Programa Clarín-COFUND del

Principado de Asturias; the Rachadapisek Sompot Fund for Post-doctoralFellowship,ChulalongkornUniversity(Thailand);the

Chu-lalongkorn Academic into Its 2nd Century Project Advancement

Project(Thailand);andtheWelchFoundation,contractC-1845. References

[1] C.A.Bertulani,S.R.Klein,J.Nystrand,Physicsofultra-peripheralnuclear col-lisions, Annu. Rev.Nucl. Part. Sci.55 (2005) 271,http://dx.doi.org/10.1146/ annurev.nucl.55.090704.151526,arXiv:nucl-ex/0502005.

[2] A.J.Baltz,ThephysicsofultraperipheralcollisionsattheLHC,Phys.Rep.458 (2008)1,http://dx.doi.org/10.1016/j.physrep.2007.12.001,arXiv:0706.3356. [3] J.G. Contreras, J.D.Tapia Takaki, Ultra-peripheral heavy-ioncollisionsat the

LHC, Int. J. Mod. Phys. A 30 (2015) 1542012, http://dx.doi.org/10.1142/ S0217751X15420129.

[4] C.Alexa, et al., H1,Elastic and proton-dissociativephotoproduction of J/ψ

mesonsatHERA,Eur.Phys.J.C73(2013)2466,http://dx.doi.org/10.1140/epjc/ s10052-013-2466-y,arXiv:1304.5162.

[5] A.Aktas,etal.,H1,ElasticJ/ψ productionatHERA,Eur.Phys.J.C46(2006) 585,http://dx.doi.org/10.1140/epjc/s2006-02519-5,arXiv:hep-ex/0510016. [6] S.Chekanov,etal.,ZEUS,ExclusivephotoproductionofJ/ψmesonsatHERA,

Eur. Phys. J. C 24 (2002) 345,http://dx.doi.org/10.1007/s10052-002-0953-7, arXiv:hep-ex/0201043.

[7] T.Aaltonen,etal.,CDF,Observationofexclusivecharmoniumproductionand γ γ→μ+μ− inpp collisions¯ at√s=1.96 TeV,Phys.Rev.Lett.102(2009) 242001,http://dx.doi.org/10.1103/PhysRevLett.102.242001,arXiv:0902.1271. [8] ALICE Collaboration, Exclusive J/ψ photoproduction off protons in

ultra-peripheralp-Pbcollisions at √sNN=5.02 TeV, Phys. Rev.Lett. 113(2014)

232504,http://dx.doi.org/10.1103/PhysRevLett.113.232504,arXiv:1406.7819. [9] LHCbCollaboration,UpdatedmeasurementsofexclusiveJ/ψ andψ(2S)

pro-ductioncross-sectionsinppcollisions at √s=7 TeV, J.Phys. G41(2014) 055002,http://dx.doi.org/10.1088/0954-3899/41/5/055002,arXiv:1401.3288. [10] M.G.Ryskin,DiffractiveJ/ψelectroproductioninLLAQCD,Z.Phys.C57(1993)

89,http://dx.doi.org/10.1007/BF01555742.

[11] S.J.Brodsky,L.Frankfurt,J.F.Gunion,A.H.Mueller,M.Strikman,Diffractive lep-toproduction ofvectormesonsinQCD,Phys.Rev.D50(1994)3134,http:// dx.doi.org/10.1103/PhysRevD.50.3134,arXiv:hep-ph/9402283.

[12] A.Adeluyi,C.A.Bertulani,Constraininggluonshadowingusing photoproduc-tioninultraperipheralpAandAAcollisions,Phys.Rev.C85(2012)044904,

http://dx.doi.org/10.1103/PhysRevC.85.044904,arXiv:1201.0146.

[13] A.Adeluyi,T.Nguyen,CoherentphotoproductionofJ/ψ andϒmesonsin ul-traperipheralpPbandPbPbcollisionsattheCERNLHC,Phys.Rev.C87(2013) 027901,http://dx.doi.org/10.1103/PhysRevC.87.027901,arXiv:1302.4288. [14] A.Cisek,W.Schafer,A.Szczurek,Exclusivecoherentproductionofheavyvector

mesonsinnucleus–nucleuscollisionsatLHC,Phys.Rev.C86(2012)014905,

http://dx.doi.org/10.1103/PhysRevC.86.014905,arXiv:1204.5381.

[15] S.Klein,J.Nystrand,Exclusivevectormesonproductioninrelativisticheavyion collisions,Phys.Rev.C60(1999)014903,http://dx.doi.org/10.1103/PhysRevC. 60.014903,arXiv:hep-ph/9902259.

[16] T.Lappi,H.Mantysaari,J/ψproductioninultraperipheralPb+Pbandp+Pb col-lisionsatenergiesavailableattheCERNLargeHadronCollider,Phys.Rev.C87 (2013)032201,http://dx.doi.org/10.1103/PhysRevC.87.032201,arXiv:1301.4095. [17] V.P.Goncalves,M.V.T.Machado,Vectormesonproductionincoherenthadronic interactions:anupdateonpredictionsforRHICandLHC,Phys.Rev.C84(2011) 011902,http://dx.doi.org/10.1103/PhysRevC.84.011902,arXiv:1106.3036. [18] V. Rebyakova,M. Strikman,M. Zhalov, Coherent ρ0 _{and J/ψ}

photoproduc-tioninultraperipheral processeswith electromagneticdissociationofheavy ionsatRHICandLHC,Phys.Lett.B710(2012)647,http://dx.doi.org/10.1016/ j.physletb.2012.03.041,arXiv:1109.0737.

[19] V. Guzey, M. Strikman, M. Zhalov, Disentangling coherent and incoherent quasielasticJ/ψ photoproductiononnucleibyneutrontaggingin ultraperiph-eralioncollisionsattheLHC,Eur.Phys.J.C74(2014)2942,http://dx.doi.org/ 10.1140/epjc/s10052-014-2942-z,arXiv:1312.6486.

[20] Yu.P.Ivanov,B.Z.Kopeliovich, I.Schmidt,Vectormeson productionin ultra-peripheral collisions at LHC, in: Workshop on Heavy Ion Collisions at the LHC: Last Call for Predictions, Geneva, Switzerland, May 14–June 8,2007, 2007, https://inspirehep.net/record/752847/files/arXiv:0706.1532.pdf, arXiv: 0706.1532.

[21] B.L.Berman,S.C.Fultz,Measurementsofthegiantdipoleresonancewith mo-noenergeticphotons,Rev.Mod.Phys.47(1975)713,http://dx.doi.org/10.1103/ RevModPhys.47.713.

[22] V.Guzey,E.Kryshen,M.Zhalov,Coherentphotoproductionofvectormesons inultraperipheral heavyioncollisions:updateforrun 2at theCERNLarge Hadron Collider, Phys. Rev.C 93 (2016) 055206, http://dx.doi.org/10.1103/ PhysRevC.93.055206,arXiv:1602.01456.

[23] A.Accardi,etal.,Electronioncollider:thenextQCDfrontier–understanding thegluethatbindsusall,arXiv:1212.1701,2012.

[24] J.L.AbelleiraFernandez,etal.,ALargeHadronElectronCollideratCERN: re-portonthephysicsanddesignconceptsformachineanddetector,J.Phys.G39 (2012) 075001, http://dx.doi.org/10.1088/0954-3899/39/7/075001, arXiv:1206. 2913.

[25] Y.Akiba,etal.,ThehotQCDwhitepaper:exploringthephasesofQCDatRHIC andtheLHC,arXiv:1502.02730,2015.

[26] K.J.Eskola,H.Paukkunen,C.A.Salgado,EPS09:anewgenerationofNLOand LOnuclearpartondistributionfunctions,J.HighEnergyPhys.04(2009)065,

http://dx.doi.org/10.1088/1126-6708/2009/04/065,arXiv:0902.4154.

[27] S.P.Jones,A.D.Martin,M.G.Ryskin,T.Teubner,ExclusiveJ/ψandϒ photopro-ductionandthelowx gluon,J.Phys.G43(2016)035002,http://dx.doi.org/10. 1088/0954-3899/43/3/035002,arXiv:1507.06942.

[28] J.Rojo,etal.,ThePDF4LHCreportonPDFsandLHCdata:resultsfromrunIand preparationforrunII,J.Phys.G42(2015)103103,http://dx.doi.org/10.1088/ 0954-3899/42/10/103103,arXiv:1507.00556.

[29] B.I.Abelev,etal.,STAR,ρ0_{photoproductioninultraperipheralrelativisticheavy}

ioncollisionsat√sN N=200 GeV,Phys.Rev.C77(2008)034910,http://dx.doi. org/10.1103/PhysRevC.77.034910,arXiv:0712.3320.

[30] S.Afanasiev,etal.,PHENIX,PhotoproductionofJ/ψandofhighmasse+e−in ultra-peripheralAu+Aucollisionsat√sN N=200 GeV,Phys.Lett.B679(2009) 321,http://dx.doi.org/10.1016/j.physletb.2009.07.061,arXiv:0903.2041. [31] G.Agakishiev,etal.,STAR,ρ0_{photoproductioninAuAucollisionsat}√_s

N N= 62.4 GeVwiththeSTARdetector,Phys. Rev.C85(2012)014910,http://dx. doi.org/10.1103/PhysRevC.85.014910,arXiv:1107.4630.

[32] ALICECollaboration,CoherentJ/ψ photoproductioninultra-peripheralPb–Pb collisionsat√sN N=2.76 TeV,Phys.Lett.B718(2013)1273,http://dx.doi.org/ 10.1016/j.physletb.2012.11.059,arXiv:1209.3715.

[33] ALICE Collaboration, Charmonium and e+e− pair photoproduction at mid-rapidityinultra-peripheralPb–Pbcollisionsat√sNN=2.76TeV,Eur.Phys.J.C

73(2013)2617,http://dx.doi.org/10.1140/epjc/s10052-013-2617-1,arXiv:1305. 1467.

[34] V.Guzey,E.Kryshen,M.Strikman,M.Zhalov,Evidencefornucleargluon shad-owing from theALICE measurementsofPbPbultraperipheral exclusiveJ/ψ

production,Phys.Lett.B726(2013)290,http://dx.doi.org/10.1016/j.physletb. 2013.08.043,arXiv:1305.1724.

[35] S.R.Klein,J.Nystrand,J.Seger,Y.Gorbunov,J.Butterworth,STARlight:aMonte Carlo simulation program for ultra-peripheral collisions ofrelativistic ions,

1 66 2 67 3 68 4 69 5 70 6 71 7 72 8 73 9 74 10 75 11 76 12 77 13 78 14 79 15 80 16 81 17 82 18 83 19 84 20 85 21 86 22 87 23 88 24 89 25 90 26 91 27 92 28 93 29 94 30 95 31 96 32 97 33 98 34 99 35 100 36 101 37 102 38 103 39 104 40 105 41 106 42 107 43 108 44 109 45 110 46 111 47 112 48 113 49 114 50 115 51 116 52 117 53 118 54 119 55 120 56 121 57 122 58 123 59 124 60 125 61 126 62 127 63 128 64 129 65 130

Comput.Phys.Commun.212(2017)258,http://dx.doi.org/10.1016/j.cpc.2016. 10.016,arXiv:1607.03838.

[36] K.J.Eskola, H.Paukkunen,C.A.Salgado, Animprovedglobalanalysisof nu-clearpartondistributionfunctionsincludingRHICdata,J.HighEnergyPhys. 07 (2008)102,http://dx.doi.org/10.1088/1126-6708/2008/07/102, arXiv:0802. 0139.

[37] CMSCollaboration,PerformanceofCMSmuonreconstructioninppcollision eventsat√s=7 TeV,J.Instrum.7(2012)P10002,http://dx.doi.org/10.1088/ 1748-0221/7/10/P10002,arXiv:1206.4071.

[38] CMSCollaboration,TheCMSexperimentattheCERNLHC,J.Instrum.3(2008) S08004,http://dx.doi.org/10.1088/1748-0221/3/08/S08004.

[39] CMS Collaboration, Absolute Calibration ofthe Luminosity Measurementat CMS: Winter 2012 Update, CMSPhysics Analysis Summary CMS-PAS-SMP-12-008,2012,http://cdsweb.cern.ch/record/1434360.

[40] CMSCollaboration,StatusofzerodegreecalorimeterforCMSexperiment,AIP Conf.Proc.867(2006)258,http://dx.doi.org/10.1063/1.2396962,arXiv:nucl-ex/ 0608052.

[41] CMSCollaboration,Measurementofthepseudorapidityandcentrality depen-denceofthetransverseenergydensityinPbPbcollisionsat√sN N=2.76 TeV, Phys.Rev.Lett.109(2012)152303,http://dx.doi.org/10.1103/PhysRevLett.109. 152303,arXiv:1205.2488.

[42] CMSCollaboration,Studiesofazimuthaldihadroncorrelationsinultra-central PbPbcollisionsat√sN N=2.76 TeV,J.HighEnergyPhys.02(2014)088,http:// dx.doi.org/10.1007/JHEP02(2014)088,arXiv:1312.1845.

[43] M.Chiu, A.Denisov,E.Garcia,J.Katzy,S.N.White, Measurementofmutual Coulomb dissociation in √sN N=130 GeVAu+Au collisions at RHIC, Phys. Rev.Lett.89(2002)012302,http://dx.doi.org/10.1103/PhysRevLett.89.012302, arXiv:nucl-ex/0109018.

[44] STARLIGHTMonteCarlogenerator,Onlinedocumentationavailableat http:// starlight.hepforge.org,2015.

[45] A.J.Baltz,Y.Gorbunov,S.R.Klein,J.Nystrand,Two-photoninteractionswith nu-clearbreakupinrelativisticheavyioncollisions,Phys.Rev.C80(2009)044902,

http://dx.doi.org/10.1103/PhysRevC.80.044902,arXiv:0907.1214.

[46] ALICECollaboration,Coherent ρ0 _{photoproductioninultra-peripheral Pb–Pb}

collisionsat√sN N=2.76 TeV,J.HighEnergyPhys.09(2015)095,http://dx. doi.org/10.1007/JHEP09(2015)095,arXiv:1503.09177.

[47] M.J.Oreglia,AStudyoftheReactionsψ→γ γψ,Ph.D.thesis,Stanford Univer-sity,1980,http://www.slac.stanford.edu/pubs/slacreports/slac-r-236.html,SLAC ReportSLAC-R-236,seeAppendixD.

[48] S. Agostinelli, et al., GEANT4, GEANT4—a simulation toolkit, Nucl. Instrum. MethodsA506(2003)250,http://dx.doi.org/10.1016/S0168-9002(03)01368-8. [49] CMS Collaboration, Measurement of Tracking Efficiency, Technical Report

CMS-PAS-TRK-10-002,CERN,Geneva,2010,https://cds.cern.ch/record/1279139, 2010.

[50] S.vanderMeer,Calibration ofthe EffectiveBeamHeight intheISR,CERN TechnicalReportCERN-ISR-PO-68-31,1968,http://cds.cern.ch/record/296752. [51] ParticleDataGroup,K.A.Olive,etal.,Reviewofparticlephysics,Chin.Phys.C

38(2014)090001,http://dx.doi.org/10.1088/1674-1137/38/9/090001. [52] V.N. Gribov, Glauber corrections and the interaction between high-energy

hadronsandnuclei,Sov.Phys.JETP29(1969)483,Zh.Eksp.Teor.Fiz.56(1969) 892.

[53] V. Guzey, M. Zhalov, Exclusive J/ψ production in ultraperipheral collisions atthe LHC:constrainsonthe gluondistributionsintheprotonand nuclei, J. HighEnergyPhys.10(2013)207,http://dx.doi.org/10.1007/JHEP10(2013)207, arXiv:1307.4526.

TheCMSCollaboration

V. Khachatryan,

A.M. Sirunyan,

A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam,

E. Asilar,

T. Bergauer,

J. Brandstetter,

E. Brondolin,

M. Dragicevic,

J. Erö,

M. Flechl,

M. Friedl,

R. Frühwirth

1

,

V.M. Ghete,

C. Hartl,

N. Hörmann,

J. Hrubec,

M. Jeitler

1

,

A. König,

M. Krammer

1

,

I. Krätschmer,

D. Liko,

T. Matsushita,

I. Mikulec,

D. Rabady,

N. Rad,

B. Rahbaran,

H. Rohringer,

J. Schieck

1

,

R. Schöfbeck,

J. Strauss,

W. Treberer-Treberspurg,

W. Waltenberger,

C.-E. Wulz

1

InstitutfürHochenergiephysikderOeAW,Wien,Austria

V. Mossolov,

N. Shumeiko,

J. Suarez Gonzalez

NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt,

T. Cornelis,

E.A. De Wolf,

X. Janssen,

A. Knutsson,

J. Lauwers,

S. Luyckx,

M. Van De Klundert,

H. Van Haevermaet,

P. Van Mechelen,

N. Van Remortel,

A. Van Spilbeeck

UniversiteitAntwerpen,Antwerpen,Belgium

S. Abu Zeid,

F. Blekman,

J. D’Hondt,

N. Daci,

I. De Bruyn,

K. Deroover,

N. Heracleous,

J. Keaveney,

S. Lowette,

S. Moortgat,

L. Moreels,

A. Olbrechts,

Q. Python,

D. Strom,

S. Tavernier,

W. Van Doninck,

P. Van Mulders,

G.P. Van Onsem,

I. Van Parijs

VrijeUniversiteitBrussel,Brussel,Belgium

H. Brun,

C. Caillol,

B. Clerbaux,

G. De Lentdecker,

G. Fasanella,

L. Favart,

R. Goldouzian,

A. Grebenyuk,

G. Karapostoli,

T. Lenzi,

A. Léonard,

T. Maerschalk,

A. Marinov,

L. Perniè,

A. Randle-conde,

T. Seva,

C. Vander Velde,

P. Vanlaer,

R. Yonamine,

F. Zenoni,

F. Zhang

2

UniversitéLibredeBruxelles,Bruxelles,Belgium

L. Benucci,

A. Cimmino,

S. Crucy,

D. Dobur,

A. Fagot,

G. Garcia,

M. Gul,

J. Mccartin,

A.A. Ocampo Rios,

D. Poyraz,

D. Ryckbosch,

S. Salva,

M. Sigamani,

M. Tytgat,

W. Van Driessche,

E. Yazgan,

N. Zaganidis

1 66 2 67 3 68 4 69 5 70 6 71 7 72 8 73 9 74 10 75 11 76 12 77 13 78 14 79 15 80 16 81 17 82 18 83 19 84 20 85 21 86 22 87 23 88 24 89 25 90 26 91 27 92 28 93 29 94 30 95 31 96 32 97 33 98 34 99 35 100 36 101 37 102 38 103 39 104 40 105 41 106 42 107 43 108 44 109 45 110 46 111 47 112 48 113 49 114 50 115 51 116 52 117 53 118 54 119 55 120 56 121 57 122 58 123 59 124 60 125 61 126 62 127 63 128 64 129 65 130

S. Basegmez,

C. Beluffi

3

,

O. Bondu,

S. Brochet,

G. Bruno,

A. Caudron,

L. Ceard,

S. De Visscher,

C. Delaere,

M. Delcourt,

D. Favart,

L. Forthomme,

A. Giammanco,

A. Jafari,

P. Jez,

M. Komm,

V. Lemaitre,

A. Mertens,

M. Musich,

C. Nuttens,

L. Perrini,

K. Piotrzkowski,

L. Quertenmont,

M. Selvaggi,

M. Vidal Marono

UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

N. Beliy,

G.H. Hammad

UniversitédeMons,Mons,Belgium

W.L. Aldá Júnior,

F.L. Alves,

G.A. Alves,

L. Brito,

M. Correa Martins Junior,

M. Hamer,

C. Hensel,

A. Moraes,

M.E. Pol,

P. Rebello Teles

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

E. Belchior Batista Das Chagas,

W. Carvalho,

J. Chinellato

4

,

A. Custódio,

E.M. Da Costa,

D. De Jesus Damiao,

C. De Oliveira Martins,

S. Fonseca De Souza,

L.M. Huertas Guativa,

H. Malbouisson,

D. Matos Figueiredo,

C. Mora Herrera,

L. Mundim,

H. Nogima,

W.L. Prado Da Silva,

A. Santoro,

A. Sznajder,

E.J. Tonelli Manganote

4

,

A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

S. Ahuja

a

,

C.A. Bernardes

b

,

A. De Souza Santos

b

,

S. Dogra

a

,

T.R. Fernandez Perez Tomei

a

,

E.M. Gregores

b

,

P.G. Mercadante

b

,

C.S. Moon

a

,

5

,

S.F. Novaes

a

,

Sandra S. Padula

a

,

D. Romero Abad

b

,

J.C. Ruiz Vargas

a_{UniversidadeEstadualPaulista,SãoPaulo,Brazil} b_{UniversidadeFederaldoABC,SãoPaulo,Brazil}

A. Aleksandrov,

R. Hadjiiska,

P. Iaydjiev,

M. Rodozov,

S. Stoykova,

G. Sultanov,

M. Vutova

InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria

A. Dimitrov,

I. Glushkov,

L. Litov,

B. Pavlov,

P. Petkov

UniversityofSofia,Sofia,Bulgaria

W. Fang

6

BeihangUniversity,Beijing,China

M. Ahmad,

J.G. Bian,

G.M. Chen,

H.S. Chen,

M. Chen,

T. Cheng,

R. Du,

C.H. Jiang,

D. Leggat,

R. Plestina

7

,

F. Romeo,

S.M. Shaheen,

A. Spiezia,

J. Tao,

C. Wang,

Z. Wang,

H. Zhang

InstituteofHighEnergyPhysics,Beijing,China

C. Asawatangtrakuldee,

Y. Ban,

Q. Li,

S. Liu,

Y. Mao,

S.J. Qian,

D. Wang,

Z. Xu

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,

A. Cabrera,

L.F. Chaparro Sierra,

C. Florez,

J.P. Gomez,

B. Gomez Moreno,

J.C. Sanabria

UniversidaddeLosAndes,Bogota,Colombia

N. Godinovic,

D. Lelas,

I. Puljak,

P.M. Ribeiro Cipriano

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic,

M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,

K. Kadija,

J. Luetic,

S. Micanovic,

L. Sudic