Forward physics with CMS

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by  Monika Grothe

CMS CR 2008/112
AvailableonCMSinformationserver
The Compact Muon Solenoid Experiment
Conference Report
Mailing address: CMS CERN, CH-1211 GENEVA 23, Switzerland
16December 2008
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n Forward physics with CMS
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MonikaGrothea)
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[ Abstract

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ForwardphysicswithCMSattheLHCcoversawiderangeofphysicssubjects,includingverylow-
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8 xBj QCD, underlyingeventand multipleinteractionscharacteristics, γ-mediatedprocesses, shower
9 developmentattheenergyscaleofprimarycosmicrayinteractionswiththeatmosphere,diffractionin
9 thepresenceofahardscaleandevenMSSMHiggsdiscoveryincentralexclusiveproduction.Selected
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feasibilitystudiestoillustratetheforwardphysicspotentialofCMSarepresented.
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Presentedat2008PhysicsatLHC,Split,Croatia,Sept29-Oct4,2008
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1 Introduction
Forwardphysicsat the LHC coversa wide rangeof diversephysicssubjectsthat havein commonthat particles
produced at small polar angles, θ, and hence large values of rapidity provide a defining characteristics. At the
Large-Hadron-Collider (LHC), where proton-proton collisions occur at center-of-mass energies of 14 TeV, the
maximalpossiblerapidityisy = ln √s ∼ 11.5. ThecentralcomponentsofCMSareoptimizedforefficient
max mπ
detection of processeswith large polar angles and hence high transverse momentum, p . They extenddown to
T
about|θ| = 1 fromthebeamaxisor|η| = 5,whereη = −ln[tan(θ/2)]isthepseudorapidity. Intheforward
◦
region,thecentralCMScomponentsarecomplementedbyseveralCMSandTOTEMsubdetectorswithcoverage
beyond |η| = 5. TOTEM is an approved experiment at the LHC for precision measurements of the pp elastic
and total cross sections. The combinedCMS and TOTEM apparatus comprisestwo suites of calorimeterswith
trackingdetectorsin frontplusnear-beamprotontaggers. TheCMS HadronForward(HF) calorimeterwith the
TOTEMtelescope T1in frontcoversthe region3 < |η| < 5, the CMS CASTORcalorimeterwith the TOTEM
telescopeT2infrontcovers5.2 < |η| < 6.6. TheCMSZDCcalorimetersareinstalledattheendofthestraight
LHC beam-linesection, at a distance of ±140m fromthe interactionpoint(IP).Near-beamprotontaggerswill
beinstalledbyTOTEMat±147mand±220mfromtheIP.ThekinematiccoverageofthecombinedCMSand
TOTEMapparatusisunprecedentedatahadroncollider.TheCMSandTOTEMcollaborationshavedescribedthe
considerablephysicspotentialofjointdatatakinginareporttotheLHCC[1]. Furthernear-beamprotontaggers
incombinationwithveryfasttimingdetectorstobeinstalledat±420mfromtheIP(FP420)areintheproposal
stageinCMS.FP420wouldgiveaccesstopossiblediscoveryprocessesinforwardphysicsattheLHC[2].
2 Physics with forward detectors
TheCMSHF,CASTORandZDCcalorimeters[3]andtheTOTEMT1andT2telescopes[4]areparticulatlysuited
forQCDstudiesatverylowvaluesofBjorken-x,whichhavethepotentialofdifferentiatingbetweenDGLAP-type
and BFKL-like parton evolutionand of identifyingeffects of parton saturation. Measurementsat the HERA ep
colliderhaveexploredlow-x dynamicsdowntovaluesofafew10 5. AttheLHCtheminimumaccessiblex
Bj −
decreasesbyafactor∼10foreach2unitsofrapidity.AprocesswithahardscaleofQ∼10GeVandwithinthe
acceptanceofT2/CASTOR(η =6)canoccuratxvaluesaslowas10 6.
−
AnexampleisDrell-Yanproductionofe+e pairs,qq →γ⋆ →e+e ,whichprobesprimarilythequarkcontent
− −
of the proton. At values sufficiently low in x non-linear QCD effects of parton recombination and saturation
within the proton are expected to set in, taming the rapid rise of the proton gluon density observed by HERA
and preserving unitarity. Generator-levelstudies [1] indicate that saturation effects may manifest themselves as
production cross sections for Drell-Yan electrons within the T2/CASTOR acceptance that are sizably different
fromthosepredictedinmodelswithoutsaturation.
Anotherexampleofalow-xprocessisproductionofforwardjets, observableinHForCASTOR.TheCMSHF
calorimeteroffersexcellentjetreconstructioncapabilities. ForwardjetsinHFreachap resolutionof∼ 19%at
T
20GeV/c and∼ 10%at100GeV/c[5]. ParticularlysensitivetoBFKL-like QCD evolutiondynamicsaredijets
withlargerapidityseparation,whichenhancestheavailablephasespaceforBFKL-likepartonradiationbetween
thejets. Generator-levelstudiesfortheseso-calledMuller-Navelet-typedijetswith∆ηupto9arediscussedin[6].
Likewise dijets separated by a large rapidity gap are of interest since they indicate a process in which no color
flowoccursinthehardscatterbutwhere,contrarytothetraditionalpictureofsoftPomeronexchange,alsoahigh
transversemomentumtransferoccursacrossthegap.
Another area where the CMS forward calorimeters and TOTEM telescopes are essential tools is Monte Carlo
tuning. The hard scatter in hadron-hadron collisions takes place in a dynamic environment, refered to as the
“underlyingevent”(UE),whereadditionalsoftorhardinteractionsbetweenthepartonsandinitialandfinalstate
radiationoccur. The effectof the UE can only be describedby meansof tuning Monte Carlo multiplicitiesand
energyflow predictionsto data. As shown in [7], the forward detectors are sensitive to features of the UE that
centraldetectorinformationalonecannotconstrain.Highuncertaintiesarealsopresentinmodellingtheinteraction
ofprimarycosmicraysofPeVenergywiththeatmosphere.Theirrateofoccuranceperyearistoolowforreliable
quantitativeanalysis. Thecenter-of-massenergyinppcollisionsattheLHCcorrespondsto100PeVenergyina
fixedtargetcollision. Asdiscussedin[1],measurementsofenergyandparticleflowwithT2/CASTORandZDC
willbeabletoconstrainsignificantlytheMonteCarlomodelscurrentlyusedinthecosmicrayscommunity.
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3 Physics with a veto on the forward detetcors
Eventsofthetypepp → pXporpp → Xp,wherenocolorexchangetakesplacebetweentheproton(s)andthe
system X, can be causedby γ exchange,or by diffractiveinteractions. In bothcases, the absenceof color flow
betweentheproton(s)andthesystemX resultsinalargegapintherapiditydistributionofthehadronicfinalstate.
Inthefollowing,wediscuss,forseveralexemplaryprocesses,howtheforwarddetectorscanbeusedtodetectsuch
hadronicfinalstates.
Diffractioncanoccurwithahardscale. InthatcaseperturbativeQCD(pQCD)allowsthecrosssectionsforthese
processestobefactorizedintothatoneofthehardscatterandadiffractiveparticledistributionfunction(dPDF).
Indiffractivehadron-hadronscattering,rescatteringbetweenspectatorparticlesbreaksthefactorization. Theso-
called rapidity gap survival probability, < |S2| >, quantifies this effect [8] and can be measured by means of
the ratio of diffractiveto inclusiveprocesses with the same hard scale. At the Tevatron, the ratio is foundto be
O(1%) [9]. Theoretical expectations for < |S2| > the LHC vary from a fraction of a percent to as much as
O(25%)[10].
Inhardsinglediffraction(SD)atLHCenergies,thecomplicationarisesthattherapiditygapisgenerallyboosted
to large values of rapidity. A gap within the acceptance of the HF and CASTOR calorimeterslimits the values
of ξ, the fractionalmomentumloss of the diffractivelyscattered proton, to ξ < 0.01. Two processeshave been
studied in detail [11, 12], SD dijet production, pp → pjjX, sensitive to the dPDF gluon content, and SD W
production,pp→pWX (W →µν),sensitivetothedPDFquarkcontent. Bothstudiesassumesingle-interaction
data,i.e.absenceofeventpile-up.Theeventselectionisbasedonthetwodimensionaldistributionsoftowerswith
above-noiseactivityonthegapside,a. inHFversusCASTOR,orb. inahigh-ηsliceversusalow-ηsliceofHF
towers.Thegapsideisdefinedasthesidewiththelowerhadronicactivity,adefinitionthatcanbeequallyapplied
tonon-diffractiveevents.
Aftereventselection,thetwo-dimensionalmultiplicitydistributionsontheselectedgapsideshowclearlyvisible
differencesbetweenthePyhtia-onlyprediction(nohardSD)andthePythiaplusPomwig(hardSD)one.Assuming
<|S2|>≈5%inPomwig,aparticularlyclearexcessispresentinthe[0,0]bins. ForSDW production,O(100)
eventsper100pb 1 areexpectedin[n(CASTOR),n(HF)]=[0,0],witharatioofSDtonon-diffractiveeventsof
−
∼20.EvenmorefavorableisthesituationforSDdijetproduction,whereO(300)eventsper10pb 1areexpected,
−
witharatioof∼ 30. Dissociationofthediffractivelyscatteredprotonwouldleadtoasignalenhancementinthe
[0,0]binof∼30%forbothchannels. ThestudiesassumethatCASTORwillbeavailableonlyononesideinthe
firstphaseofCMSdatataking.AsecondCASTORintheoppositehemisphereandtheuseofT1,T2willimprove
the observable excess further. A method to establish that the observed population of the [0,0] bins is indeed
indicativeofthe presenceof SD eventsinthe datais describedin [12]. Themethodis basedonthe observation
thatthesizeoftheSDsignalinthe[0,0]binscanbecontrolledinapredictablewaywhenthecutsforenhancing
theSDsignalaremodified.Observationofsignalsinthe[0,0]binsofthesizegivenabovewouldalreadyexclude
valuesof<|S2|>atthelowendofthespectrumoftheoreticalpredictions.
Exclusive dimuon and dielectron production with no significant additional activity in the CMS detector occurs
with high cross section in γ-mediated processes at the LHC, either as the pure QED process γγ → ll or in Υ
photoproduction. Theeventselectioninbothcases[13]isbasedonrequiringthatoutsideofthetwoleptons,no
othersignificantactivityisvisiblewithinthecentralCMSdetector,neitherinthecalorimeternorinthetracking
system.
ForthepureQEDprocess,in100pb 1ofsingleinteractiondata,O(700)eventsinthedimuonchannelsandO(70)
−
inthedielectronchannelcanbeselected.Eventsinwhichoneoftheprotonsdissociatesarethedominantsourceof
backgroundandarecomparableinstatisticstothesignal. Thisbackgroundcanbesignificantlyreducedbymeans
of a veto on activity in CASTOR and ZDC, by 2/3 in a configurationwith a ZDC on each side anda CASTOR
ononlyonesideoftheIP.Thetheoreticallyverypreciselyknowncrosssectionofthis(almost)pureQEDprocess
isanidealcalibrationchannel. With100pb 1 ofsingle-interactiondata,anabsoluteluminositycalibrationwith
−
O(5%) precisionis feasible. Futhermore, exclusivedimuonproductionis an ideal alignmentchannelwith high
statisticsfortheproposedprotontaggersat420mfromtheIP.
UpsilonphotoproductioncanconstrainQCDmodelsofdiffraction.AssumingtheSTARLIGHTMonteCarlocross
sectionprediction,the1S,2Sand3Sresonanceswillbeclearlyvisiblein100pb 1ofsingleinteractiondata[13].
−
By means of p2(Υ) as estimator of the transfered four-momentumsquared, t, at the proton vertex, it might be
T
possible to measure the t dependenceof the cross section. This dependenceis sensitive to the two-dimensional
gluondistributionoftheprotonandwouldgiveaccesstothegeneralizedpartondistributionfunction(GPD)ofthe
proton.
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4 Physics with near-beam proton taggers
TheLHCbeamlinewithitsmagnetsisessentiallyaspectrometerinwhichprotonsslightlyoffthebeammomentum
arebentsufficientlytobedetectablebymeansofdetectorsinsertedintothebeam-pipe. Athighluminosityatthe
LHC,protontaggingistheonlymeansofdetectingdiffractiveandγ mediatedprocessesbecauseareasoflowor
nohadronicactivityinthedetectorarefilledinbyparticlesfromoverlaidpile-upevents.
TheTOTEMprotontaggersat±220matnominalLHCopticshaveacceptanceforscatteredprotonsfromtheIP
for 0.02 < ξ < 0.2. Smaller values of ξ, 0.002 < ξ < 0.02, can be achievedwith protontaggersat ±420 m.
TheFP420proposal[2]foreseesemploying3-DSilicon,anextremelyradiationhardnovelSilicontechnology,for
theprotontaggers,andadditionalfasttimingCherenkovdetectorsfortherejectionofprotonsfrompile-upevents.
Theproposalis currentlyunderconsiderationin CMS. If approved,installation couldproceedin 2010,after the
LHCstart-up.
Forward proton tagging capabilities enhance the physics potential of CMS. They would render possible a pre-
cise measurement of the mass and quantum numbers of the Higgs boson should it be discovered by traditional
searches. TheyalsoaugmenttheCMSdiscoveryreachforHiggsproductionintheminimalsupersymmetricex-
tension(MSSM)oftheStandardModel(SM)andforphysicsbeyondtheSMinγpandγγ interactions. Acase
inpointisthecentralexclusiveproduction(CEP)process[14],pp → p+φ+p,wheretheplussigndenotesthe
absenceof hadronicactivity betweenthe outgoingprotons, which survivethe interactionintact, andthe state φ.
Thefinalstateconsistssolelyofthescatteredprotons,whichmaybedetectedintheforwardprotontaggers,and
the decay productsof φ which can be detected in the central CMS detector. Selection rules force the produced
stateφtohaveJCP =n++withn=0,2,...Thisprocessoffershenceanexperimentallyverycleanlaboratoryfor
thediscoveryofanyparticlewiththesequantumnumbersthatcouplesstronglytogluons. Additionaladvantages
arethepossibilitytodeterminethemassofthestateφwithexcellentresolutionfromthescatteredprotonsalone,
independentofitsdecayproducts,andthepossibility,uniqueattheLHC,todeterminethequantumnumbersofφ
directlyfromtheazimuthalasymmetrybetweenthescatteredprotons.Forwardprotontaggingwillalsogiveaccess
toarichQCDprogramonharddiffractionathighluminositiesandtoprecisionstudiesofγpandγγ interactions
atcenter-of-massenergiesneverreachedbefore.
References
[1] CMScollab.,TOTEMcollab.,CERN-LHCC-2006-039/G-124(2006)
[2] FP420collab.,arXiv:0806.0302,[hep-ex]
[3] CMScollab.,CERN-LHCC-2006-001(2006)
[4] TOTEMcollab.,CERN-LHCC-2004-002(2004)
[5] CMScollab.,CMSPASFWD-08-001(2008)
[6] CMScollab.,CMSCR-2008/097(2008)
[7] K.Borras,ProceedingsofDIS2007
[8] J.D.Bjorken,Phys.Rev.D47(1993)101;A.B.Kaidalovetal.,Eur.Phys.J.C21(2001)521
[9] CDFcollab.,Phys.Rev.Lett.78(1997)2698;D0collab.,Phys.Lett.B574(2003)169
[10] V. A. Khoze, A. D. Martin, M. G. Ryskin, arXiv:0802.0177[hep-ph]; M. B. G. Ducati, M. M. Machado,
M.V.T.Machado,arXiv:0706.3468[hep-ph]
[11] CMScollab.,CMSPASDIF-07-002(2008)
[12] CMScollab.,CMSPASFWD-08-002(2008)
[13] CMScollab.,CMSPASDIF-07-001(2008)
[14] V.A.Khoze,A.D.Martin,M.G.Ryskin.,Eur.Phys.JC19(2001)477,erratumC20(2001)599
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