Transcript Higgs at LHC

Higgs at LHC
WIN 07, Kolkata
Lorenzo Feligioni, CPPM
01/17/2006
Thanks to: L. Carminati, J.B. Devivie, L. Fayard, A. Nikitenko, M. Schumacher, G. Unal, L. Vacavant
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Outline
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Introduction
Higgs Phenomenology at the LHC
Experimental Setting
Higgs Searches
– Benchmark Analyses:
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H
ttH(Hbb)
VBF
HWW*
HZZ*
• Higgs properties
– Mass, coupling and spin
N.B. Only Standard Model Higgs discussed
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SM Higgs at the TeV scale
• Many theoretical arguments predict a Higgs mass at the
Tev scale:
• WW scattering violates unitarity if only Z/ are exchanged
– For Higgs to be able to restore it at any s: GFm2H ~< O(1)
• Triviality bound:
– Scalar sector is a 4 theory
• Energy cut off C where
SM is not trivial
 C~1016(3)GeV  mH<200(1000) GeV
• Vacuum stability bound:
– Fermionic contributions
could led to negative
self coupling for too small 
• Vacuum not a minimum anymore
 C~103(16)GeV  mH > 70 (130) GeV
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Higgs current limits
M. Gruenewald et. al
• Electroweak precision measurements
– SM Higgs field contributes
to radiative corrections for
many EW quantities
– Fits of SLC, LEP and Tevatron
EW measurements constrain mH
• Latest results Jan 2007
after CDF mW measurement
– mH=80+36-26 GeV
• Direct Searches
– LEP mH>114.4 GeV @95% CL
– Tevatron combined results
Tevatron limits
SM Higgs cross section
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Higgs @ LHC: production
• Gluon Gluon fusion:
– Dominant production mode
– NLO correction important
g
(A.Djouadi)
H
g
• K = 1.7
• Main contribution is gluon radiation
– many events with at least one jet
– NNLO cross section known
• Sig(NNLO)/Sig(NLO) = 1.3
• Vector Boson Fusion:
– small K factor ~ 1.1
• Small jet multiplicity in final state
q
H
q
– No color exchange between quarks
• large energetic jets at small pT
• Low hadronic activity in central
region from hard event
– a part from Higgs decay
• Production with Gauge boson:
– Known NNLO for QCD and EW
corrections
• Production with heavy quarks:
W
q
Typical uncertainties
on cross-sections
W
H
q
g
– More complicated final state
– More than 10 diagrams, known at NLO
g
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•
•
•
t
gg
VBF
WH,ZH
ttH
10-20 % (NNLO)
~ 5%
(NLO)
~< 5% (NNLO)
10-20 % (NLO)
H
t
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Higgs Decay
• Light Higgs (110<mH(GeV)<130)
– Dominant mode is Hbb (75-50%)
– H and cc with 3--7%
– Higgs decay to  through
loop of massive particle
~ few permil
– HVV(*) rises close
to 130 GeV
• Intermediate Higgs
(130<mH(GeV)<180)
– HVV(*) most important
decay mode
• HZZ(*) decreases when
2 on shell W bosons can be produced
• Heavy Higgs (180<mH(GeV)<1000)
– HVV
– For mH~400 GeV the decay in two top quarks also plays a role
• All BR calculated at NLO, error within few %
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Experimental Setting
LHC:
– Proton-Proton collisions @ 14
TeV
– First run in 2007 at 900 GeV
– First run @ 14 TeV in 2008
– Luminosity:
• Low luminosity ~1033cm-2s-1
 ~ 30 fb-1 between 2008 and
2010/2011
• High Luminosity ~1034cm-2s-1
 ~300 fb-1 by 2014/2015
– Pile-up:
• ~ 2 (low luminosity) to 20
(high luminosity) pp
interactions (“minimum bias”)
per bunch crossing (every 25
ns)
– Trigger to go from 40 MHz
interaction rate to ~200Hz to
disk for offline analysis
CMS and ATLAS:
Powerful e////b identification
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Photon ID: eff~80%, R(jet)~103
Electron ID: eff~80%, R(jet)~105
b ID: eff~60%, R(light jet)~100
hadrons: eff~50% R(jet)~100
Good energy measurement of e/ and 
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~1-2 % for pT(e)~25-50 GeV
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Benchmark Analyses
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• H:
H
Irreducible background:
– NLO BR(mH=120 GeV)
~97.5 pb
– inclusive analysis:
• two high pT photons
– m2=E1E2(1-cos12)
• Low mass: intrinsic width negligible
– Recovery of conversion
• ~30% conversion in the tracker:
– possibly reconstructed

prod
Tevatron results
• Irreducible background:
– Diphoton background:
• now computed at NLO
• Computation agrees with
Tevatron data
Reducible background:
/jet
• Reducible Background:
0 fragmentation function in quark and gluon jet
(from Kniehl et al, NPB582(2000)514)
– Large cross section
• Isolation criteria:
• Need good 0 rejection
• 0 tend not to take all
parton energy
+[…] jet/jet
Jets in /jet events initiated by quarks  higher fake rate
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H: ATLAS
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Transverse momentum cuts
Jet rejection ~5000 (PT> 25 GeV)
– pT(1)>40 GeV, pT(2)>25 GeV
– ||<2.5
– No photons in calorimeter cracks
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Photon identification cuts
Photon reconstruction and calibration
– Converted photon  bigger cluster
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Photons direction corrected for PV

Low luminosity:
•  from strips and middle calorimeter
• Zv measure from ID (z=40 m)

High luminosity:
Good angular resolution
Strip Cal =0.0031
• Photons direction obtained with
calorimeter information only
• crucial role for fine  segmented strips
layer
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Invariant mass distribution of the two
photons is reconstructed
Improve the discovery potential using
the shape of kinematical variables
Overall vertex resolution from calorimeter
alone at high luminosity: = 1.6 cm
– Likelihood ratio method based on PT
and cosq* (well predicted in NLO
calculations) of signal and background
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H: CMS
• Cut Based analysis:
- pT(1)>40 GeV
- pT(2)>25 GeV
- ||<2.5
- Shower shape and
isolation
CMS Note 2006 112
barrel with large R9
barrel with small R9
endcaps with large R9
endcaps with small R9
- Suppress jet bkgrd
- Invariant mass for
different categories based
on:
• R9=E(3x3)/Esc
•  regions
• Optimized analysis:
- NN used, inputs:
• kinematics
•  isolation as input
Not isolated photons from jets
Isolated photons
Signal region
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H @ LHC with 30 fb-1
ATLAS
Significance (m=130GeV)for 30fb-1
CMS (all using NLO rates)
TDR
new “cut” new “optimized”
7.5
6.0
8.2
ATLAS (stat. errors only)
TDR(LO) new NLO “cut” +likelihood
3.9
6.3
8.7
CMS Note 2006 112
Inclusive cut-based
analyses less powerful but
less relying on MC information
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ttH(Hbb)
• ttH(bb): potential discovery channel for light higgs
– All hadronic final state has higher branching fraction:
• more difficult trigger
ttH(Hbb) semileptonic
– Both experiments looked at the semileptonic final state
• isolated lepton
• Missing energy
• ≥ 6 jets, ≥ 4 jets b-tag
– need large b-tagging efficiency: signal (4b)
– In general high jet multiplicity:
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Large contribution of ISR/FSR
 difficult to simulate
 Reducible Background:
– tt(+jj)
• Larger background
• b-tagging must be optimized to have
strong light jet rejection
ttbb Production diagrams via QCD
– WWbbjj, Wjjjjjj
• discriminated by reconstructing tt
pairs
 Irreducible background:
–
ttbb (EW/QCD)
• slight differences in kinematic properties w.r.t ttH
– could be discriminated using likelihood functions
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ttH(Hbb): ATLAS
• After a first optimistic
evaluation on the TDR:
Semileptonic top
b-tagging
b = 60 %
Rc = 10
Rl = 100
– Updating pdf: 20% smaller
cross sections
– Matrix element based Monte
Carlo for ttbb background
 higher jet multiplicity
• 2 likelihood functions
used:
– Fast simulation analysis
– pairing likelihood used to
associate b quarks from top
decay
– Second likelihood to
discriminate ttbb
• ttH more energetic
• different in angular
distributions for b coming
from Higgs decay
mH = 120 GeV, L = 30 fb-1
S/B = 2.8
LO cross sections
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ttH(Hbb): CMS
• 3 final states considered
• ttH semileptonic:
Semileptonic top
Cms note 2006 119
– Low energetic jets accepted if
associated with tracks from PV
b-tagging
b = 54%
Rc = 6.2
Rl = 62.5
• 3 Likelihoods
• Constrained fit on W masses
• b-tagging used to associate b
to top and higgs
• Kinematic function discriminate
higher energetic Higgs products
• Dilepton analysis
– counting experiment
• 2 opposite charged leptons (e,)
• substantial missing energy (2 ’s)
• All hadronic
– minimize 2 mass
– Using centrality variable
CMS summary
S/√B
s-lep 
s-lep e
dilep
All had
1.8
1.6
1.4
2.4
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Things to do with ttH
• ttH(WW): direct
measurement of top Yukawa
coupling
– ggH more sensitive to new
physics
• ATLAS: Pre-selection
– 2 or 3 leptons
– ≥ 6 or 4 jets, 2 b-jets
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•
ttH(H)
CMS TDR 2006:
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
ttH final state for 100 fb-1:
~7 signal events, 3.5 sigmas

t-tbar events with radiated
photons
W+4j+photons.
Background:

• Statistical uncertainty on Yukawa
coupling of 15% to 30% for 130
GeV ≤ mH ≤ 200 GeV
Atlas-sn-2002-019
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VBF
• Signal:
– qqHqqWW
– qqHqq
– Forward tagging jets:
• energetic jets at high 
– No color flow between initial partons
• No jet radiation
• Central Jet Veto
• Backgrounds:
Forward jets

– QCD Z/ + jet
• Big cross section at LHC
Higgs
• Central jet veto reduce this
background by 70%
– Veto can be checked in
Zee() events (no signal)
– top quark:
• Presence of b-quark can mimic
signal even at LO
• In tracker fiducial region b-tag
veto
Decay
Rainwater et al,hep-ph/9605444
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VBF: H
• Analysis:
CMS note 2006 88
L = 30 fb-1NLO cross sections
– 2 tagging jets
– Higgs decay products in central
region between tagging jets
• Jet veto
– dilepton or lepton-hadron final
state for  decays
– Use missing transverse
momentum + collinear
approximation of  decays to
reconstruct invariant mass of 
pair
– Resolution limited by missing ET
~10 GeV-13 GeV
ATLAS full simulation
• Low Luminosity Analysis
– Jet veto sensitive to pile-up
effects
– Sensitivity can be added by
looking at HWW
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HWW* 
• Important channel for
2mW <mH<2mZ
– HWW BR ~ 95%
– Exclusive VBF also sensitive in lower
mass regions
• Inclusive HWW*:
– Using dilepton final state
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NLO BR~2.4 pb
– no mass reconstruction available for
the presence of neutrinos
– Counting experiment
• need to determine shape of
background
– Lepton anti-correlated

Higgs Spin 0
e
W+
+
W-
e-
CMS analysis:
- ETmiss > 50 GeV
- jet veto in  < 2.5
- 30 <pT l max<55 GeV
- pT l min > 25 GeV
- 12 < mll < 40 GeV
NLO signal and background
• W+, W- opposite spin
• Lepton tend to be close
• Backgrounds:
– tt, tWb : rejected by vetoing the
presence of jets
– WW,WZ,ZZ: rejected by cutting on
kinematic variables
• BR~12.2 pb
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HWW*
• Background normalization
– Control region can be
defined in order to get
from data
CMS note 2006 47
Signal region
Bkgd region
normalization
• lower systematics
• top background:
– same lepton and MET
– two b-tagged jets
• WW background
– same lepton cuts
• only e, pairs to suppress Drell-Yan
•
– ll<140, mll>60 GeV
WZ background
– same final state
– additional lepton
• For 1,2 and 10 fb-1 syst err~19,16 and 11%
• mH=165 GeV, 5 discovery in less than 1 fb-1
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H4 leptons
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Golden channel for early
discovery at high masses
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BEFORE
NLO cross sections
Rely on good e/ identification and
energy resolution
Look for:
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two opposite sign pair of leptons
Same flavors
Coming from the PV
Compatible with Z mass
Irreducible background:
•
ZZ(*) -> 4l
• q qbar annihilation known at
NLO
• add ~20% to account for
ggZZ
AFTER
Reducible backgrounds:
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CMS
Zbb4 leptons
tt4 leptons
Reduced by isolation cuts +
anti impact parameters
Typical rejection ~ 100 for Zbb
After all selections ZZ continuum
remains as the dominant or sole
background
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H4 leptons
• Background shape from data to
reduce luminosity and
acceptance uncertainties
• R = ZZ4l / Z2l
HZZe+e-+-
CMS note 2006 136
– Theoretical systematics on R from
2% to 8% (varying with m4l)
– Statistical precision very high due to
large number of Z2l events
– Normalisation from
sidebands
HZZe+e-e+e-
CMS note 2006 115
• scaling background to what
is expected outside the
signal region
• Total systematic ~ few %
• Clean channel but low statistics
• especially < 130 GeV and ~170
GeV
• Discovery possible with 10 fb-1
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LHC Summary
Notice: CMS NLO cross sections, LO for ATLAS
ATLAS: new sensitivity study ongoing (fullsim, new MC generators)
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Higgs properties
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Higgs mass measurement
• Relatively easy from H or 4leptons.
• The channel H can also contribute at low luminosity
(H) only directly accessible for m>200 GeV
HWW no mass peak
High mass region: larger
Width, weaker statistical power
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Higgs spin, CP
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Observation of ggH or H
excludes spin 1
For MH>200 GeV, study spin/CP
from HZZ4l
Exclusion can be deduced from 
and  distributions
Z polarization
plane angle
Atlas-sn-2003-025
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Couplings
• Concentrate on “low” MH
• Series of assumption needed:
• Assume spin 0:
Atlas note phys-2003-030
• allow to use angular distribution
on HWW (most precise measure)
• measure .BR in different
channels:
.BR = (NS+B - <NB>)/L
• Uncertainties:
 Selection efficiencies
 Background subtraction
 Luminosity
• Second step: assume only
one Higgs boson
•BR(Hx)/BR(HWW) = x/W
•Reduced number of fitted
parameters
 smaller errors
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Couplings
• Third step, more assumptions:
Syst uncertainties from exp+theory
-No new particles in loop
-Light mass H not accessible
-Absolute scale not measurable,
measure gx/gW
• Express all rates and BR as a
function of 5 couplings:
gW,gZ,gtop,gb,g
Examples:
 (VBF): aWF.gW2+aZF.gZ2
 BR(): (b1.gW2 – b2.gtop2)/H
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Conclusions
• Early discoveries (~10 fb-1) could be possible
for HVV at large mass
• Low mass region more challenging:
– optimized H analyses predict discovery after 3
years
• other channels can be exploited: ttH, VBF
• Good understanding of detector needed to
assess performances and understand
background shapes
• Less than one year to first collisions!
– now time to focus to get ready for data
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Backup
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ttH(Hbb): ATLAS
• After a first optimistic
evaluation on the TDR:
– Updating pdf: 20% smaller
cross sections
– Matrix element based Monte
Carlo for ttbb background
 higher jet multiplicity
b-tagging working point
b = 60 %
Rc = 10
Rl = 100
• 2 likelihood functions
used:
– pairing likelihood used to
associate b quarks from top
decay
– Second likelihood to
discriminate ttbb
• ttH more energetic
• different in angular
distributions for b coming
from Higgs decay
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ttH(Hbb): CMS
• 3 final states considered
• ttH semileptonic:
– Low energetic jets accepted if
associated with tracks from PV
• 3 Likelihoods
• Constrained fit on W masses
• b-tagging used to associate b
to top and higgs
• Kinematic function discriminate
higher energetic Higgs products
b-tagging
working point
b = 54%
Rc = 6.2
Rl = 62.5
• Dilepton analysis
– counting experiment
• 2 opposite charged leptons (e,)
• substantial missing energy (2 ’s)
• All hadronic
– minimize 2 mass
– Using centrality variable
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ttH(Hbb): CMS
• 3 final states considered
• ttH semileptonic:
– Low energetic jets accepted if
associated with tracks from PV
• 3 Likelihoods
• Constrained fit on W masses
• b-tagging used to associate b
to top and higgs
• Kinematic function discriminate
higher energetic Higgs products
b-tagging
working point
b = 54%
Rc = 6.2
Rl = 62.5
• Dilepton analysis
– counting experiment
• 2 opposite charged leptons (e,)
• substantial missing energy (2 ’s)
• All hadronic
– minimize 2 mass
– Using centrality variable
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ttH(Hbb): ATLAS
• After a first optimistic
evaluation:
– Updating pdf: 20% smaller
cross sections
– Matrix element based Monte
Carlo for ttbb background
 higher jet multiplicity
b-tagging working point
b = 60 %
Rc = 10
Rl = 100
• 2 likelihood functions
used:
– pairing likelihood used to
associate b quarks from top
decay
– Second likelihood to
discriminate ttbb
• ttH more energetic
• different in angular
distributions for b coming
from Higgs decay
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