Diffraction at HERA and implications for Tevatron and LHC

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rXiv:0706.3484v4 [hep-ph] 21 Jul 2007DIFFRACTIONATHERAANDIMPLICATIONSFORTEVATRONANDLHC

L.SCHOEFFELCEASaclay,DAPNIA-SPP,91191Gif-sur-YvetteCedex,France

WedescribenewQCDfitstodiffractivecrosssectionsmeasuredatHERAandweusethepartondensitiesderivedfromthesefitstopredicttheshapeofthedijetmassfractionindoublePomeronexchangeattheTevatron.WediscusstheexistenceofexclusiveeventsinthisdijetchannelandsomeprospectsaregivenfortheLHC.

1DiffractionatHERA

OneofthemostimportantexperimentalresultsfromtheDESYepcolliderHERAistheob-

servationofasignificantfraction(around15%)ofdiffractiveeventsindeepinelasticscattering

(DIS)withlargerapiditygapbetweenthescatteredproton,whichremainsintact,andtherest

ofthefinalsystem1.InthestandardQCDdescriptionofDIS,sucheventsarenotexpectedin

suchanabundancesincelargegapsareexponentiallysuppressedduetocolorstringsformedbe-

tweentheprotonremnantandscatteredpartons.Fordiffractiveevents,however,acolorneutral

clusterofpartonsfragmentsindependentlyofthescatteredproton.Thetheoreticaldescription

ofdiffractiveeventsisarealchallengesinceitmustcombineperturbativeQCDeffectofhard

scatteringwithnonperturbativephenomenonofrapiditygapformation.

Therearevariousinterpretationsofthisphenomenon,butaveryappealingonereliesupona

partonicinterpretationofthestructureofthePomeron2.Itisdefinedinthepresenceofahard

scale,thephotonvirtualityQ2orjettransversemomentum,whichallowstoapplyperturbative

QCD.Softdiffraction,whensuchascaleismissing,isoutsidethescopeofthemodelbutcanbe

describedinthecontextofReggepolephenomenology.Thisphenomenology,however,turnsout

tobequiteusefulinthedescriptionofasoftpartofharddiffraction,responsiblefortherapidity

gapformation.Infact,itispossibletonicelydescribethediffractivecross-sectiondatafrom

HERAbyaQCDDGLAPevolutionofpartondistributionsinthePomeroncombinedwitha

ReggeparametrisationoffluxfactorsdescribingtheprobabilityoffindingaPomeronstateinthe

proton1.ItfollowsexactlythesameprocedurethanforstandardDISexceptthatthediffractive

distributionsarerelatedtothePomeron,whosefluxfactorisfactorisedandparametrisedasa

functionofxlP,themomentumfractionlostbytheproton.

Setsofdiffractivepartondistributionfunctions(dPDFs)areshowninfigure1.Inthe

infinitemomentumframe,thedPDFshaveaninterpretationofconditionalprobabilitiestofind

apartonintheprotonwiththemomentumfractionx=βxlP,whereβdenotesthefractionof

theparticularpartoninthePomeron.Thegluonsdominatethediffractiveexchangeandcarry

approximately70%ofthemomentum.Whilethequarkdensitiesarefoundtoberelatively

closeforH1andZEUSexperiments,thegluondensitydiffersbymorethanafactor2.New

preliminarydatafromZEUSreducethisdiscrepancy.H1 data (H1RAP)ZEUS data (ZEUSMX)All data (4 combined sets)

00.10.2z S(z)SingletQ2=3 GeV2

00.51

z G(z)GluonQ2=3 GeV2

00.10.2Q2=8.5 GeV2

00.51Q2=8.5 GeV2

00.10.2Q2=20 GeV2

00.51Q2=20 GeV2

00.10.2Q2=90 GeV2

00.51Q2=90 GeV2

00.10.2

10-210-11zQ2=800 GeV2

00.51

10-210-11Q2=800 GeV2

z

Figure1:SingletandgluondistributionsofthePomeronasafunctionofz,thefractionalmomentumofthePomeroncarriedbythestruckparton,derivedfromQCDfitsonH1RAPdataalone,ZEUSMXdataaloneorthefourdatasetstogether.ThepartondensitiesarenormalisedtorepresentxlPtimesthetruepartondensitiesmultipliedbythefluxfactoratxlP=0.003.

Inthefollowing,wewillusetheQCDfitstotheH1datatocomparewiththedijetmass

fractionsmeasuredbytheCDFcollaborationindoublePomeronexchangeandwediscussthe

possibleevidenceforexclusiveeventsinthiscontext.

2DiffractionatTevatronandLHC

ThedifferencebetweendiffractionatHERAandattheTevatronisthatdiffractionatthe

Tevatroncanoccurnotonlyoneitherpor¯psideasatHERA,butalsoonbothsides.Theformer

caseiscalledsinglediffractionwhereastheotheronedoublePomeronexchange.Inthesame

wayaswehavedefinedthekinematicalvariablesxlPandβatHERA,wedefineξ1,2astheproton

fractionalmomentumloss(orasthepor¯pmomentumfractioncarriedbythePomeron),and

β1,2,thefractionofthePomeronmomentumcarriedbytheinteractingparton.Theproduced

diffractivemassisequaltoM2=sξ1forsinglediffractiveeventsandtoM2=sξ1ξ2fordouble

Pomeronexchange,where√gk

k1

2xgx 1

2

pp

gk

k1

2xgx 1

2p

pP

P

gk

k1

2xgx 1

2p

pP

P"Standard""Exclusive "

"Inclusive"H , QQ H , QQ

H , QQ , γγ, γγ

, γγ

Figure2:Schemeofnondiffractive,inclusivedoublePomeronexchangeandexclusiveeventsattheTevatronorLHC

interactionandthetotalavailableenergyisusedtoproducetheheavyobjectandthePomeron

remnants.Theremaybeathirdclassofprocessesdisplayedintheupperrightfigure,namely

theexclusivediffractiveproduction.Exclusiveeventsallowaprecisereconstructionofthemass

andkinematicalpropertiesofthecentralobjectusingthecentraldetectororevenmoreprecisely

usingveryforwarddetectorsinstalledfardownstreamfromtheinteractionpoint3.Asmentioned

above,themassoftheproducedobjectcanbecomputedusingromanpotdetectorsandtagged

protons,M=√