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Physics
Letters
B
www.elsevier.com/locate/physletb
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
Availableonline8July2017 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 ofthe interaction point and no neutronactivityon the otherside, Xn0n,is measured withthe
CMS experiment in ultra-peripheral PbPb collisions at√sN N=2.76 TeV. The analysis is based on a
data samplecorresponding to an integrated luminosity of 159 μb−1, collectedduring 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
cohXn0n/d y(J/ψ)=0.36±0.04(stat)± 0.04(syst) mb in therapidity interval1.8<|y|<2.3. Usingamodel for therelative rate ofcoherent photoproductionprocesses,thisXn0nmeasurementgivesatotalcoherentphotoproductioncrosssection
ofd
σ
coh/d y(J/ψ)=1.82±0.22(stat)±0.20(syst)±0.19(theo) mb.Thedatastronglydisfavor theimpulseapproximation model prediction, indicatingthat nuclear effects are neededto describe coherentJ/ψ photoproduction in
γ
+Pb interactions. The data are found to be consistent with the leading twist approximation,whichincludesnucleargluonshadowing.©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
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 reactionγ
+
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. Theγ
+
p→
J/ψ
+
p production process has been stud-iedbyH1andZEUS collaborationsattheelectron–protoncollider HERA[4–6],by theCDF collaboration in proton–antiproton colli-sionsattheTevatron[7],andbytheALICEandLHCbcollaborationsE-mailaddress:cms-publication-committee-chair@cern.ch.
attheLHC,inproton-lead[8]andproton–protoncollisions[9], re-spectively.Sincethecrosssectionofphotoproducedvectormesons suchasJ
/ψ
,ψ(
2S)
,andϒ(
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∼
M2J/ψ/
4,whereMJ/ψ istheJ/ψ
mass,√
s isthecenter-of-massenergyofthecollidingsystemandy istherapidity ofthe J
/ψ
[10,11].The relevantvaluesof x thatcanbe explored inthisanalysisareinthe10−2 to10−4 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.Becausethenucleonradiushttp://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.
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 AuAucol-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 UPCρ
0 analyses a hardscale 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+−00..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 directionfromthein-teraction point usingzero degree calorimeters.Using thistrigger, both coherent andincoherent J
/ψ
mesons andγ
+
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:Section
2
describestheCMS 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 ofthemuonsmatchedto 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 andγ
+
γ
→
μ
+μ
−events.TheUPCtriggerhasthefollowing require-ments:an energydepositconsistentwithatleastone neutronin eitheroftheZDCs;noactivityinatleastoneoftheBSC+
orBSC−
scintillators;thepresenceofatleastonesinglemuonwithouta pTthresholdrequirement,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 spot centre. The length of the pixel clustersmust be consistent
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
√
sNN
=
2.
76 TeV[40–42].Thisresolutionallowsagoodseparationbetweenevents 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),isasubsetofthetriggeredevents.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 regionis 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 degreeoffreedom lessthanthree,andatransverse(longitudinal)impact parameteroflessthan0.3(20) cm from themeasuredvertex. For this analysis, only events with dimuons having pT
<
1.
0 GeV, intherapidity 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
andγ
+
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
photoproductioninγ
+
p resultsfrom HERAtocompute thecoherentJ/ψ
crosssection inγ
+
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
, andγ
+
γ
→
μ
+μ
− events.The fitusesamaximumlikelihood algo-rithmthattakesunbinnedprojectionsofthedataininvariantmass andpTasinputs.TheshapesofthepTdistributionsforthesethreeprocessesaredeterminedfrom 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 thatoriginatesfromγ
+
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 therapid-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) forγ
+
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 moreeasilyobservedthan incoherentJ/ψ
events.Due to thesmallsample sizeofthisanalysis, thecoherentJ/ψ
cross sec-tionismeasuredbysummingupbothconfigurations.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.)
The combined acceptance ( A) and efficiency (
ε
) correction factor for J/ψ
events in the Xn0n break-up mode,(
Aε
)
J/ψ, is5
.
9±
0.
5(syst)%. The 8% systematic uncertainty on the correc-tions are described in Section 5. Two factors contribute to the(
Aε
)
J/ψ: 1) the product of acceptance multiplied by the offline reconstruction efficiency and 2) the trigger efficiency (ε
trig). Thefirsttermismeasuredtobe12
.
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-quirement,whiletheUPCdataareusedtocomputetheefficiency ofthemuon qualityrequirements. Theoffline selectiondiscussed aboveis applied,butthetriggerrequirementisnot demanded at thisstage of theefficiencycalculation. The UPCtrigger efficiencyTable 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%
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
ZDCε
pixel-trackε
BSCε
dimuon.Be-cause these trigger components are uncorrelated to each other they are measured separately. The
ε
dimuon term is measured tobe 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 computeε
dimuon are studiedcorre-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), andε
BSC is fullyefficient.Thesys-tematicuncertaintyfortheacceptanceandefficiencycorrectionis discussedinthefollowingsection.
5. Systematicuncertaintiesandcross-checks
Thesystematicuncertaintiesaresummarizedin
Table 1
andcan 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 one. This differentiationsuppresses the low-frequency noise. The alternative method estimatesthe noise fromtime 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.Theshapeofthe 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 isNγX+γn0n
=
75.
2±
12.
7(stat)±
8.
3(syst),whilethemeasuredcrosssectionis44
.
2±
1.
8(stat)±
0.
40(syst) μb.Thisresultisconsistent withtheQEDcalculationprovidedbythe starlight MCattheone standarddeviationlevel.Theγ
+
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
FortheXn0nbreak-upmode,thecoherentJ
/ψ
crosssectioninthedimuondecaychannelisgivenby
d
σ
coh Xn0n d y(
J/ψ )
=
NcohX n0nB
(
J/ψ
→
μ
+μ
−)
L
inty
(
Aε)
J/ψ (1)where
B(
J/ψ
→
μ
+μ
−)
=
5.
96±
0.
03(syst)% is the branching fraction of J/ψ
to dimuons [51], NcohXn0n 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(
Aε
)
J/ψ=
5.
9±
0.
5(stat)%isthecombinedacceptance times efficiency correction factor as discussed in Section 4. The coherentJ/ψ
yieldofpromptJ/ψ
candidatesisgivenbyNcohX
n0n
=
Nyield
1
+
fD(2)
where Nyield is the coherent J
/ψ
yield asextracted from the fitshown in Fig. 1, and fD is the fraction of J
/ψ
mesons coming from coherentψ(
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 arenot enough data to perform a coherent
ψ(
2S)
analysis, so the feed-down correction hasto rely on MC simulations. In order to calculate fD,coherentψ(
2S)
eventsaresimulatedusing starlight, while pythia isusedtosimulatetheψ(
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 NcohXn0n
=
203±
18(stat).Thus, thecoherentJ
/ψ
photoproductioncross section forprompt J/ψ
mesons in the Xn0n break-up mode is dσ
Xcohn0n/
d y(
J/ψ)
=
0
.
36±
0.
04(stat)±
0.
04(syst) mb. Althoughthedσ
cohXn0n
/
d y(
J/ψ)
measurementisinterestinginitsownright[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 crosssectionus-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
eventswith pT
<
0.
15GeV and in the rapidity interval 1.
8<
|
y|
<
2.
3.Themeasuredbreak-upratiosare0
.
36±
0.
04(stat) forXnXn/
Xn0nand0
.
03±
0.
01(stat) for 1n1n/
Xn0n,whilethe starlightpredic-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-downcorrectionfromψ(
2S)
decaysisnotappliedfortheseratiossincethiscontributionis ex-pectedtocanceloutintheratio.The10%uncertaintyquotedinthe starlightpredictionforthebreak-upmodescalingfactorsisbased onrecentresultsonUPC
ρ
0 photoproductionfromtheALICEcol-laboration[46].Notethattheneutronbreak-uptheoretical descrip-tionisindependentofwhetheraJ
/ψ
oraρ
0isproduced[45,46].Thescalingfactorbetweenthe Xn0nbreak-upmodeandthetotal
Fig. 2. DifferentialcrosssectionversusrapidityforcoherentJ/ψproductionin ultra-peripheralPbPbcollisionsat√sN N=2.76 TeV,measuredbyALICE[32,33]andCMS (seetextfordetails).Theverticalerrorbarsincludethestatisticalandsystematic uncertaintiesaddedinquadrature,andthehorizontalbarsrepresenttherangeof themeasurementsiny.Alsotheimpulseapproximationandtheleadingtwist ap-proximationcalculationsareshown(seetextfordetails).
we obtain the total coherent J
/ψ
photoproduction cross section dσ
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
photoproductioninγ
+
p interactions toestimatethe coherentJ/ψ
crosssection inγ
+
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 Pbnuclei. Eachtermis theproduct ofthree quantities: the pho-ton flux,the integral oversquared nuclearform factor FA
(
t)
and the forward differential cross section dσ
/
dt(
t=
0)
ofγ
+
p→
J/ψ
+
p,wheret isthemomentumtransferfromthetargetnucleussquared. The FA
(
t)
istheFouriertransformofthematterdensityρ
(
t)
,whiletheelementarycrosssectiondσ
/
dt hasbeenmeasured byvariouscollaborations[5–9],asdescribedinSection1.The im-pulseapproximationresultshowninFig. 2
isperformedbyGuzey et al. using the methods they describe in Ref. [34] witha pQCD motivatedparametrization[53]ofexclusiveJ/ψ
datainγ
+
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 dσ
cohXn0n
/
d y(
J/ψ)
=
0.
36±
0.
04(stat)±
0.
04(syst) mb in therapid-ity interval 1
.
8<
|
y|
<
2.
3. This measurement is extrapolated to thetotalcoherentJ/ψ
crosssection,resultingindσ
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 andQ2valuesareneededtodescribethedata.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). AcknowledgementsWecongratulateourcolleaguesintheCERNaccelerator 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 personneloftheWorldwideLHCComputingGridfordeliveringso effectively thecomputinginfrastructure essentialto our analyses. Finally, we acknowledge the enduring support for the construc-tion andoperationofthe 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
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