Higgs boson photoproduction at the LHC M.B. Gay Ducati and G.G. Silveira HighEnergyPhysicsPhenomenologyGroup,UFRGS, CaixaPostal15051,CEP91501-970-PortoAlegre,RS,Brazil. 1 1 0 Abstract. We present the current development of the photoproduction approach for the Higgs 2 boson with its application to pp and pA collisions at the LHC. We perform a different analysis fortheGapSurvivalProbability,whereweconsideraprobabilityof3%andalsoamoreoptimistic n value of 10% based on the HERA data for dijet production.As a result, the cross section for the a J exclusiveHiggsbosonproductionisabout2fband6fbin ppcollisionsand617and2056fbfor pPbcollisions,consideringthegapsurvivalfactorof3%and10%,respectively. 8 2 Keywords: Higgsboson,DoublePomeronExchange,UltraperipheralCollisions PACS: 12.38.Bx,12.40.Nn,13.85.Hd,14.80.Bn ] h p - INTRODUCTION p e h As an alternativeway to study the Higgs boson production at the LHC, the Central Ex- [ clusive Diffractive (CED) production has been currently analyzed as a new framework 1 for particle production [1]. Indeed, the Double Pomeron Exchange (DPE) and the two- v 4 photon process offer the opportunity to study the exclusive Higgs boson production in 0 protonand nuclei collisions.Oneway to increasethesecross sections isto considernu- 6 cleicollisions,especiallyforthetwo-photonprocess,wherethephotonfluxisenhanced 5 . byafactorofZ in pAcollisions,andZ2 inAAones.Forinstance,thepredictedcrosssec- 1 tionfortheHiggsbosonproductioninPbPbcollisionsis18pb[2],whichisenhancedby 0 1 five orders if compared to the predictions for pp collisions (0.18 fb). However, in DPE 1 thisenhancementissmaller,showinganincreasingfrom3fbfor ppcollisionsto100fb : v inAuAuones[3].Inthissense,wecomputethecrosssectionswiththephotoproduction i X mechanism for the CED Higgs boson production in hadron-hadron and hadron-nucleus r collisionsat theLHC. a PHOTOPRODUCTION MECHANISM Considering the g p interaction in high-energy collisions, we compute the cross section for the CED Higgs boson production by DPE in the g p subprocess [4], which is one of the possible subprocess in Ultraperipheral Collisions (UPC) [5]. The photon fluctuates into a quark-antiquark pair, and then the interaction occurs between the proton and this pairby theexchangeofgluonsinthet-channel. Thediagram at partoniclevelis taken intoaccount inorder to computethescattering amplitude, where a quasi-real photon interacts with a quark nto the proton. The imag- inary part of the scattering amplitude is computed by the use of the Cutkosky Rules ImA= 1 d(PS) A A , with A and A being the amplitudes in the left- and the right- 2 3 L R L R R COLORDIPOLE q γ χ χ γ L R q¯ kµ SCREENING kµ H GLUON rµ p f (x,k2) p g FIGURE1. DiagramthatrepresentsthephotoproductionmechanismfortheHiggsbosonproduction. hand sides of the central line that splits the diagram in Fig.1 in two pieces, and d(PS) 3 is the volumeelement of the three-body phase space. This integration results in the fol- lowingamplitude s M2 a dk2 ImAT =−6 p Hv Ns F Tgg (k2,Q2) k2 , (1) c Z whereF T istheimpactfactorfortheg -g transition gg F Tgg (k2,Q2)=4paa s(cid:229) e2q 1dt dr k2[tQ22+r ((11−rt ))2+][rk22t+(1(1−t )r )2], (2) q Z0 − − Q2 is the virtuality of the initial photon, v = 246 GeV is the vacuum expectation value of the Electroweak Theory, e is the charge of the quark in the dipole, a and a q s are the electromagnetic and strong coupling constants, respectively, t is the Feynman parameter, and k is the transverse momentum of the gluons. In this calculation, we introducetheSudakovparametrization km =r qm +b pm +km , withqm =qm Q2pm . ′ ′ − s Inordertoincludeallthepartoniccontentoftheproton,on⊥ehastoreplacethecontri- butionofthegqcouplingbytheunintegratedpartondistributionfunction f (x,k2,m 2)= g K¶ G(x,k2)/¶ lnk2, where the function G(x,k2) is the integrated gluon distribution function, and the multiplicative factor K = 1.2 takes into account the non-diagonality of the distribution [6]. In this work we apply the MSTW2008LO parametrization for suchdistributionfunction[7]. To obtain the event rate, one have to integrate the amplitudesquared given by Eq.(1) overthemomentaoftheparticlesinthefinalstate,includingtheprescriptionfor f .The g resultforcentral rapidityreads 2 2 ds K a 4 M2 m 2 dk2 dyHdt(cid:12)yH,t=0 =Sg2ap28N8LpO5Bs NcHv! "Zk20 k2 f˜g(x,k2,m 2)F gTg (k2,Q2)# , (3) (cid:12) (cid:12) whereK istakento1.5,whichcorrespondstotheenhancementofthegg H cross NL(cid:12)O section at NLO accuracy [8], and B = 5.5 GeV 2 is the slope of the gluon-pr→oton form − factor. The function f˜(x,k2,m 2) = T(k2,m 2)G(x,k2) is the modified unintegrated gluondistributionfunctionthatincludestheSudakovformfactorT computedatLeading p LogarithmAccuracy (LLA). Regarding the phenomenological aspects introduced in this result, the Rapidity Gap SurvivalProbability(GSP)dependsparticularlyoftheprocessunderconsideration.The GSP for the g p process is not computed yet, and we use the one of 3% predicted for the Pomeron-Pomeron mechanism. However, we expect a higher survivalfactor for the g p interaction, since the large distances between the two colliding hadrons in UPC should decrease the probability of interaction between secondary particles. Analyzing theresultsforcentraldijetproductionatHERA,onefindsthatthesurvivalprobabilityis about10% [9], and we makepredictionswiththisprobabilityfor theCED Higgsboson photoproduction. ULTRAPERIPHERAL COLLISIONS The initial photon is emitted from one relativistic source object, which can be a proton oranucleus.Particularly,anucleushasZ protons,whichenhancesthephotonfluxin pA andAAcollisions.Infact,consideringtheluminosityandpile-upeffectsinthecollisions at the LHC, the pA collisionsmay offer the best experimental condition if compared to pp and AA collisions [2]. Additionally, in the photoproduction mechanism, we neglect the contribution from AA collisions, since the shadowing effects present in the nuclear PDF will decrease the production cross section by a factor of 0.2-0.3. The production crosssectionin UPCis givenby s had =2 w w max ddwnis g p, (4) Z min where w min = M2/2x√sNN and w max = Q2gL2b L2, and s g p is given by Eq.(3). The functions dni/dw are the photon fluxes foqr protons and nuclei, which can be found in Ref.[10].Inthissense,thephotonvirtualityisdecomposedintoQ2= w 2/g 2b 2 q2 L L − − ≤ R 2, with g =(1 b 2) 1/2 =√s/2m , which is restricted by the coherence condition −i L − L − p inUPC, dependingoftheradiusofthesourceobject. RESULTS The hadronic cross section is computed for pp, pPb, pAu, pAr, and pO collisions at the LHC. Actually, collisions involving gold nucleus are not going to be measured in the LHC, however we include such prediction to compare with previous results [3]. The Tab.1 shows the kinematics introduced in this calculation, and the predicted cross sections for the CED Higgs boson photoproduction for the two possibilities of the survivalfactor. Asonecansee,theproductioncrosssectionissignificantlyenhancedin pAcollisions takingnucleus withhigh Z. Theseresultsare higherthan the ones obtainedfor thetwo- photon and for the DPE mechanism. In the case of pp collisions, the cross section is TABLE 1. The predicted cross sections for the CED Higgs boson photoproduction at the LHC for M =120 H GeV,andthekinematicsparameters.Thecrosssectionis shownforthetwopossibilitiesoftheGSP:3%and10%. √sNN (TeV) gL R(fm) s had (fb) S2 3% 10% gap pp 14.0 7460 0.7 1.77 5.92 pO 9.90 5314 3.0 2.31 7.70 pAr 9.40 5045 4.1 21.56 71.87 pAu 8.86 4755 7.0 768. 2562. pPb 8.80 4724 7.1 617. 2056. similartothatoftheDPEmechanism,butoneorderhigherthanthatforthetwo-photon mechanism.ConsideringtheAA run that willoccur in theend of 2010,new data can be availablefornucleicollisionsinthenextyear. CONCLUSIONS In this work we applied the photoproduction mechanism for the CED Higgs boson production to pp and pA collisions in the LHC. The results show an enhanced cross sectionsforcollisionsinvolvingAuandPbnucleus,whichopenanewwaytodetectthe Higgs boson in the LHC. 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