Higgs boson production in association with a photon in Vector Boson Fusion at LHC

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Higgs boson production in association with a
photonin Vector Boson Fusion at LHC

• Emidio Gabrielli
• Helsinki Institute of Physics

work made in coll. with F.Maltoni, B. Mele,
M.Moretti, F.Piccinini, R.Pittau
(to
appear soon)
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Higgs boson searches at LHC

• Higgs boson search is one of main tasks of LHC
• range of mass is not predicted, varying from LEP
  
• limit of 114.5 GeV up to theoretical upper
  bound
• L 800 GeV
• SM precision tests favor a light Higgs
  boson,with
• mass below 200 GeV
• discovery of a light Higgs boson is challenging
  at
• LHC due to the huge background (bckg) in bb
• discovery channels of Higgs boson at LHC
  strongly
• depend on the actual Higgs mass

ss
-
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Higgs boson production mechanisms
inclusive x-sections in pb

• gluon-gluon fusion, main mechanism in the whole
  range
• x-section of order of O(10) pb for mH lt 200 GeV
• Vector boson fusion is also sizeable
• t-t interesting at low Higgs masses

-
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Higgs boson decay modes
BR
Width
-

• b-b dominates from H mass up to m(H) lt 130 GeV
• WW dominates above m(H) gt 130 GeV
• rare channels are relevant for discovery of
  light Higgs

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Higgs discovery channels

• QCD and EW production mechanisms have
• large x-sections
• the QCD bckg is challenging
• one needs to isolate phase space where
• signal/background is enhanced
• strategy search strongly depends on Higgs
• mass range

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keep in mind that interesting cross
sections, after optimization of cuts for
enhancing signal/bckg BRs of final states,
are of the order of few fb
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(CMS Physics TDR, Vol. II, CERN/LHCC 2006-021)
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(CMS Physics TDR, Vol. II, CERN/LHCC 2006-021)
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• Mass range 115-130 GeV
• H ? g g
• Mass range 130-150 U 180- L GeV
• H ? Z Z ? 4l (lm,e) golden channel
• Mass range 150-170 GeV
• H ? WW ? 2l 2n (lm,e) silver channel

for m(H)165 GeV, only 1fb-1 necessary for Higgs
discovery
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other production mechanisms of Higgs boson in
association with
-
-
tt H where H ? bb or H ? gg qq H where H ?WW
? mn qq
-
-
-
the Htt mode suffers problem of too many jets
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Higgs boson decay in bb(bar)

• not a discovery channel, but very important for
• measuring the Yukawa Hbb coupling at LHC
• up to now only Htt and HW 2j have been
  considered.
• bckg can be reduced approximatively at the
• level of the signal
• optimized x-sections of signal are of the order
  of few fb. Htt has been disfavored by recent
  analyses

-
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inclusive cross sections of HW jj in
fb Rainwater (2001)
13.9 11.2 8.6 5.1 2.5
in principle, HW jj might allow to measure Hbb
coupling for mH120 GeV. accurate studies of
background and parton shower effects are missing.
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mH 120 GeV
a
b
c
S/B 1/10
1/7 1/5
Rainwater (2001)
a
b
c
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-
-
Htt and H ? bb
Drollinger, Muller, Denegri (2001)

• signature suitable for low Higgs mass,
• moderate x-section
• 3 final states considered fully leptonic,
  semileptonic,
• fully hadronic

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-
Htt L30 fb-1 mH115 GeV
Drollinger, Muller, Denegri (2001)
recent CMS analysis disfavors these
expectations difficult to measure (CMS Physics
TDR, Vol. II, CERN/LHCC 2006-021)

• too many jets in the event
• major bckg from ttbb , Ztt, ttNjets and QCD
  multijets
• major problem, normalization of the bckg from
  data
• many sources of uncertainties MC predictions,
  Jet energy scale
• systematics might kill the signal !

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expected range of combined significance (dileptons
semileptonicall-hadrons) L60fb-1, mH120 GeV
-
-
Htt
Htt
left ? loose working pointsright ? tight
working points tight? stronger b-tagging cuts
(CMS Physics TDR, Vol. II, CERN/LHCC 2006-021)

• dots marking the currently estimated value with
  no x-section uncertainty (dBdBsyst)

• the star corresponds to what one would obtain
  for 1 and 4 uncertainties on tt Nj and ttbb
  background respectively

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A promising channel to measure bbH coupling at
LHC
E.G., F. Maltoni, B. Mele, M. Moretti,
F.Piccinini, R.Pittau
(to appear soon)
pp ? H g j j
-
bb
same mechanism of H production by VV fusion
emission of a g
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signal at partonic level
qq ? qq H g
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main advantages

• trigger on gamma at high pT
• the large gluon component in QCD bckg is
• less active than Hjj in radiating a
• central photon
• dynamical effects suppress radiation of
• central photon with respect to signal
• moderate x-section
• provide a new independent test of Hbb
• and HWW couplings

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x-axis
Center of mass Scattering Angle
Pseudo-rapidity
P
q
h -Logtan(q/2)
z-axis
Proton
Proton
Azimuthal scattering angle
Transverse xy-plane
y-axis
pT
Transverse momentum pTpsin(q)
f
x-axis
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signal cross section pp?H g jj
minimal set of kinematical cuts

• Higgs BR to bb(bar)?HDECAY

s (H g jj ) 3 x s (HWjj)

• full tree-level matrix element for EW
  process to pp? H g jj computed with ALPGEN
  alpha(S)?0. Results checked by MADEVENT

• PDF set is CTEQ5L

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Higgs production in V V fusion signal (no
photon)
the basic partonic process is qq ? qq H
Mangano, Moretti, Piccinini, Pittau, Polosa (2003)
-

• characteristic of the signal pp? H(?bb) 2j
• two Jets with large invariant mass
• widely separated in rapidity (forward/backward)
• typical transverse momentum of jets pT(j) 40
  GeV
• Higgs decay products lying at intermediate
  rapidity

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Higgs boson production via VV fusion (background)

• the signal has large x-section, BUT it is
  difficult to
• measure due to the huge QCD background

a set of t,u-channel diagrams
s-channel diagrams (suppressed at LHC)
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how the emission of a photon affects the
background

• one would expect by naive QED rescaling

• if so, then there would be no advantage in
• considering a photon emission

• QED naive rescaling holds for inclusive
  processes
• but not always when restricted regions of
  phase space
• are considered !

• the requirement of centrality dramatically
  increases
• S/B ratio, while signal cross section roughly
  follows
• QED naive rescaling

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• cuts made at partonic level (we considered also
  
• partonic shower effects)

• optimized sub-set of kinematical cuts enhancing
  S/B

• m(jj) gt 800 GeV
• mH(1-10) lt m(bb) lt mH(110)
• pT (g) gt 20 GeV

leading contribution to bckg of VVB fusion
-
g g ? bb g g g q ? bb g q q q ? bb
q q
-
-
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bckg(g)/bckg 1/3000
signal(g)/signal 1/100
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main bckg to pp ? H g j j
bckg is less active by requiring a central photon
dynamical effect destructive interference for
gamma at large angles a) b) and c) d)
dominant effect, but suppressed by the b-quark
electric charge
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what happens if the radiation of a photon from
b-coupling is switched off
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no destructive interference at large angle, due
to the charged gauge boson

• different angular photon distribution
• with respect to bckg
• sensitive to HWW and Hbb couplings

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other advantages of the requirement of a central
photon

• suppress contaminations from g g ? H
• depletes the HZZ amplitudes with respect to WWH

-
a measure of bb g jj could lead to a combined
determination of bbH and WWH coupl.
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basic cuts to analyze relevant distributions
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the most relevant distributions
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optimized cuts (following the distributions)
we used L 100 fb-1 e(b) 60
?eff. b-tag e(bb) 70 ?eff. bb reconstraction
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pp ? H g j j
L100 fb-1
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Reducible backgrounds

• pp ? g 4 jets where two among the light
• jets are faked-tagged as
  
• b-jets
• pp ? bb 3 jets where one of the light jets
• is misdentified as a
  photon
• pp ? 5 jets where one of the light jets is
• misidentified as a photon
• and two light jets are fake-
• tagged as b-jets

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number of events (mH120 GeV)
efake ? for mistagging light-jet as a b-jet
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Parton shower effects and central jet veto

• no color is exchanged in the signal between up
  and
• down fermionic lines

• on the contrary, bckg is characterized by the
• presence of t-channel virtual gluons

• higher order QCD radiation would be much more
• relevant for the bckg than for signal

• invariant mass and rapidity separation between
  the
• tagging jets are expected to decrease in the
  bckg
• events with respect to partonic configurations.

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• A veto on additional jet activity in the central
  rapidity region could be very
• effective in suppressing the bckg more than
• the signal
• bckg events could be lowered by a factor
• of about four with respect to the the
• partonic estimates with a central jet
• veto strategy
• a more refined analysis will be necessary for
  quantitative statements

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Conclusions

• The measure of Hbb coupling is challenging at
  the LHC
• we propose a promising channel
• pp ? H jj g (VV fusion )
• main advantages with respect to Hjj
• - less active bckg after requiring a
  central photon
• - trigger on photon
• after suitable cuts ? signf 3
• (for L100 fb-1 , mH 120 GeV)
• viable channel to measure Hbb, HWW
  couplings