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=√