Higgs boson photoproduction at the LHC

0.04 MB·English

by  M. B. Gay Ducati

Higgs boson photoproduction at the LHC
M.B. Gay Ducati and G.G. Silveira
HighEnergyPhysicsPhenomenologyGroup,UFRGS,
CaixaPostal15051,CEP91501-970-PortoAlegre,RS,Brazil.
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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.
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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

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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. The GSP is a fundamental aspect to be determined with the
futuredatafromtheLHC,playinganimportantroleforreliablepredictionsofdiffractive
processes. Therefore, the photoproduction mechanism offers a new approach for the
Higgs boson production, showing a cross section competitive with other production
mechanisms.
ACKNOWLEDGMENTS
Thisworkwas partiallysupportedbyCNPq.
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