Higgs Searches at LHC Marco Pieri, UCSD – San Diego Hadron Collider Physics Symposium 2005 4-9 July 2005, Les Diablerets, Switzerland • SM Higgs boson • MSSM Higgs bosons • Higgs bosons and SUSY particles • Measurement of Higgs boson parameters
Feb 05, 2016
Higgs Searches at LHC
Marco Pieri, UCSD – San Diego
Hadron Collider Physics Symposium 2005
4-9 July 2005, Les Diablerets, Switzerland
• SM Higgs boson• MSSM Higgs bosons• Higgs bosons and SUSY particles• Measurement of Higgs boson parameters
Higgs Searches at LHCHCP 4-9 July 2005 Les Diablerets
Marco Pieri - UCSD San Diego 2
Current status of Higgs Searches
INDIRECT CONSTRAINTS ON THE SM HIGGS BOSON Electroweak fits to all high Q2
measurements give: MH=129+74
-49 GeV MH<285 GeV @ 95% CL
The central value and the upper limit have increased during the last few years
DIRECT SEARCHES AT LEP GAVE NEGATIVE RESULTS SM Higgs
MH>114.1 GeV @95% CL MSSM neutral Higgs bosons
Mh, MA>92.9, 93.3 GeV @95% CL Charged Higgs Bosons
MH± >89.6 GeV @95% CL for BR(MH± → τν) =1 MH± >78.6 GeV @95% CL for any BR
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ATLAS and CMS are preparing for the search for the Higgs bosons of different models
Most of the studies presented are still carried out with fast simulation for the background. Full simulation has been used for the signal and for the estimation of the crucial aspects of the detectors
Studies with full simulation of signal and background are in progress
Most analyses shown in the following are optimized for the low luminosity phase
LHC operation Low luminosity phase:
ℒ ~ 2 x 1033 cm-2s-1
Int ℒ ~ 30 fb-1
High luminosity phase: ℒ ~ 1 x 1034 cm-2s-1
Int ℒ ~ 300 fb-1
Introduction
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SM Higgs production
NLO Cross sections M. Spira et al.
gg fusion
IVB fusion
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SM Higgs decays
When WW channel opens up pronounced dip in the ZZ BR
For very large mass the width of the Higgs boson becomes very large (ΓH >200 GeV for MH ≳ 700 GeV)
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Most important SM search channels
ProductionDECAY
Inclusive gg fusion IVB fusion
WH/ZH ttH
Hγγ YES YES YES YES YES
Hbb YES
Hττ YES
HWW* YES YES YES YES
HZZ*, Z ℓ+ℓ-, ℓ=e,μ YES YES
HZγ, Z ℓ+ℓ-, ℓ=e,μ very low σ
Low mass MH≲160 GeV
H → γγ and H → ZZ* → 4ℓ are the only channels with a very good mass resolution ~1%
Intermediate mass (160 GeV ≲MH≲700 GeV)
High mass (MH≳700 GeV)
inclusive H → WWinclusive H → ZZ
IVB fusion qqH → ZZ → ℓℓνν IVB fusion qqH → WW → ℓνjj
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H→ γγ
Sigma x BR ~90 fb for MH = 110-130 GeV
Irreducible backgrounds from gg→ γγ, qq → γγ, pp
→ γ jet → γγ jet Reducible background
from fake photons from jets and isolated π0 (isolation requirements)
Very good mass resolution ~1%
Vertex estimated from the underlying event and recoiling jet
H → γγ MH = 115 GeV
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H → ZZ* → 4ℓ
H → ZZ* → ℓ+ℓ-ℓ+ℓ- ℓ=e,μ Irreducible background:
ZZ production Reducible backgrounds
tt and Zbb Very good mass
resolution ~1%
In this channel (and in the H→ γγ) background can be easily estimated from data by fitting the sidebands
Above MH ~ 2MZ the two Z bosons are real and σxBR is larger
Golden channel for Higgs discovery at LHC
Branching ratio dip due to opening of WW channel
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H->bb, ttH production channel
Allows the measurement of the Higgs coupling to fermions
ttH → ℓνqqbbbb Most useful for very light Higgs:
MH ≲ 130 GeV Fully reconstruct the top decays
and estimate the right bb combination for the Higgs boson
Mass resolution not excellent > 10 %
b-tagging performances for CMS and ATLAS similar
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forward jets
Higgs decay products
IVB Fusion qqH (low mass H)
Tagging jets from qq are at high rapidity and large Δη
qqH → qqWW*, qqττ (also qqγγ) Much worse mass resolution or only
transverse mass measurable Background estimation from data
much more difficult
Background is highly reduced by tagging the two forward jets requiring low activity in the
central detector Signal to BG ratio is increased
reducing the effect of BG uncertainty
Proposed by Dockshitzer, Khoze, Trojan and Rainwater, Zeppenfeld et al.
qqH → qqγγ MH = 120 GeV
ATLAS
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ATLAS carried out a recent study in the qqH channels:
qqWW* → qq ℓνν ℓνν qqWW* → qq ℓνν jet jet
qqττ → qq ℓνν ℓνν qqττ → qq ℓν had ν with ℓ=e,μ
Main backgrounds Z+jets, tt and WW+jets
IVB fusion: qqH → qqττ
signal Wjj background
τ reconstruction:
τ decay products are highly boosted, assume that they are collinear
From module and direction of the measured missing Et derive the neutrinos momenta
Mass resolution ~10% at MH=120 GeV
ATLAS 30 fb-1
xτi=fraction of τ energy carried by visible decay products
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IVB fusion: qqH → qqWW*
H → WW → ℓνℓν or ℓνqq Trigger on the lepton(s) and on
missing Et
Main backgrounds tt and tW Higgs mass cannot be
reconstructed, only transverse mass
Difficult to estimate the BG from the sidebands (syst BG ~ 10%)
Plots for WW→ eμνν channel
One way to estimate the background: release lepton cuts use shape from MC
MH=120 GeV
MH=160 GeV
ATLAS
ATLAS ATLAS
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Results for low mass
ATLAS VBF channels improve a lot
discovery potential compared to previous results
No K-factors used, LO cross sections
With 30 fb-1 more than 5 sigma significance for MH>100 GeV
Higgs boson can be discovered in more than one channel, possible to measure its couplings
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For
low
mass
re
sult
s upd
ate
d
wit
h IV
B f
usi
on
Results for the whole mass range
All mass range accessible at 5σ significance with 10 fb-1
With a few fb-1 possible to discover the Higgs boson with mass between ~150 and ~500 GeV in the WW and ZZ channels
For mass larger than ~200 GeV use ZZ and WW leptonic decays For mass larger than ~700 GeV use qqH, H → ZZ → ℓℓνν and H → WW → ℓνqq
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MSSM Higgs Searches
Two Higgs doublets model 5 Higgs bosons: 2 Neutral scalars h,H 1 Neutral pseudo-scalar A 2 Charged scalars H±
In the Higgs sector all masses and couplings are determined by two independent parameters
Most common choice: tanβ – ratio of vacuum
expectation values of the two doublets
MA – mass of pseudo-scalar Higgs boson
In the MSSM: Mh ≲ 135 GeV
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Neutral MSSM Higgs bosons
Decoupling limit (MA≳200 GeV) h behaves like HSM
Standard model searches directly apply MH~MA~MH
±
MA=O(MZ) and large tanβ H behaves similarly to SM Higgs (SM searches apply)
In other cases for large tanβ and MA<200 GeV A → WW,ZZ never allowed at tree level, h,H→ WW,ZZ highly suppressed h,H,A almost exclusively decay into bb and ττ
Large MA small tanβ H,A decays almost 100% into tt for lower masses (200-300 GeV) also H → hh and A → Zh
If SUSY particles are light the Higgs bosons may decay into s-particles
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h,H production and decay
Decoupling region
Large tanβ mainly
bb, ττ decays Large tanβ hbb, Hbb (and
Abb) production dominates
h,H decays h,H productiontanβ = 30
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Results from SM Higgs Searches
In a large part of the MSSM parameter space SM Higgs searches are effective to find the MSSM h boson
In the decoupling region if h observed hard to distinguish SM from MSSM
Search for H, A and H±
For large tanβ exploit the large cross section of Higgs boson production in association with a bb pair bbH,A → bbττ bbH,A → bbμμ bbH,A → bbbb (very
difficult)
B-tagging (+ τ id and missing Et for the τ channel) are the key issues
CMS 5σ discovery contours
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bbH,A → bbττ for MH ≲ 400 GeV
ττ → ℓνν ℓνν ττ → ℓνν had ν
Higher mass also add
ττ → had ν had ν b-tagging, τ id and
missing Et are the basic ingredients
bbH,A → bbττ
From the cross section measurement it is possible to extract the value of tanβ
tanβ uncertainty due to variation of SUSY parameters (MH
MAX scenario considered) in a range ±20% is 6%
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bbH,A → bbμμ
H,A→μμ low rate, BR(H→μμ) ~10-3
high efficiency precise mass measurement
(μμ mass resolution ~1%)
Main backgrounds:
Z/γ* → μμ tt → μμ X
Selection requires 2 muons, b-tagging and central jet veto
CMS 20 fb-1
CMS5σ discovery contours
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Results on H,A
5σ discovery regions in the MHMAX scenario
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Charged Higgs bosons H±
MH± <mt-mb
Mainly produced in top decays tt→tH±b
in the MSSM BR(H±→ τν)~100%
includes top decays
MH± >mt+mb Mainly produced in association with
a t quark (gb→tH±) BR(H±→ tb)~100% for small tanβ H±→ tb decay dominates but BR
(H±→ τν) still sizeable for large tanβ
T. Plehn et al.
Analyses are in progress for the mass region MH± ~ mtop
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MH± >mt+mb
gb → tH± with H± → τν and t → bqq Exploit helicity correlations Similar endpoint of MT at MW for the
background MT can also be used for Higgs mass
measurement (likelihood fit)
MH± <mt-mb
Main channel tt → bH±bW →
bτνbℓν ATLAS also considers tt → bτνbqq Use transverse MT mass built with τ
jet + missing Et
tt background has MT < MW
Main search channel H±→τν
ATLAS 10 fb-1
30 fb-1
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Discovery regions for Charged Higgs Bosons
ATLAS search in tt → bH±bW → bτνbqq improves the sensitivity in this region
5σ discovery regions in the MSSM tanβ – MA plane for MHMAX scenario
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MSSM scans
MHMAX scenario
MSUSY = 1 TeV maximal mh < 133 GeV
No-mixing scenario MSUSY = 1 TeV mh < 116 GeV
Gluophobic scenario suppressed coupling to gluons
(cancellation of top+stop loops) Small rate for : gg H MSUSY = 350 GeV, mh < 119 GeV
Small α scenario coupling to b and τ suppressed
for large tanβ, MA 150-500 GeV MSUSY = 800 GeV mh < 123 GeV
ATLAS studied the 4 benchmarks With 30 fb-1 h or H can be seen in
VBF channels in almost all parameter space
ATLAS preliminary
4 CP conserving benchmarks suggested by Carena et al.
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Results of MSSM scans
Smaller region covered by the ττ channel in the small α scenario (reduced coupling)
Covered by increased coupling to gauge bosons
ATLAS preliminary
covered by h→WW h→γγ (enhanced branching ratio to gauge bosons)
5σ discovery regions
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Higgs Bosons visibility in the MSSM
All the plane is covered but there is a large area where only h can be seen
4 Higgs observable
3 Higgs observable
2 Higgs observable
1 Higgs observable
5σ discovery regions in the MSSM tanβ – MA plane for MH
MAX scenario
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MSSM Higgs bosons and SUSY particles
If SUSY particles are heavier than the Higgs bosons Higgs bosons could be produced in gauginos decays:
χ2→ h,H,A χ1
χ1± → H±χ1
Different cascades possible involving heavier gauginos
Search for h,H → bb Neutralinos and charginos would
be copiously produced in the decays of squarks and gluinos
Possible to observe SUSY → h,H,A with h,H,A → bb If SUSY particles are lighter than Higgs bosons we could have a rich variety of decays,
some scenarios have been investigated: H,A → χ2 χ2
using χ2 → ℓ+ℓ- χ1 decay (4ℓ + missing Et events) h → χ1 χ1
invisible Higgs decays
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from chargino searches
Invisible decays of the Higgs Boson
IVB fusion is the most sensitive process
Trigger on forward jets + missing ET
Selection: forward jet tagging, central jet-
veto, M(jet jet) lepton veto, missing Et
Δφ jet-jet small
If we do not require gaugino mass unification and M1<<M2 Mχ can be rather small and BR(h → χχ) can be very large
ATLAS - Accessible region for 95% CL exclusion
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After discovering the Higgs bosons we should measure their parameters Studies for high luminosity (Int L = 300 fb-1)
SM Higgs boson mass direct reconstruction: 4ℓ, γγ, bb likelyhood fit WW
Measurement of Higgs bosons parameters
SM Higgs boson width
from ZZ → 4ℓ
ATLAS INT L = 300 fb-1
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Measurement of Higgs couplings
2ZZHZH
2WWHWH
2tttHttH
2ZZF
2wWFVBF
2tggHggH
g
g
g
gg
g
From σ x BR measurements in all channels where the Higgs boson can be observed:
H
2b
b
H
2
H
t(t)W(W)
H
2Z
Z
H
2W
W
g bb)BR(H
g )BR(H
gg )BR(H
g ZZ)BR(H
g WW)BR(H
2
Production cross section
Decay BR
SM framework
D. Zeppenfeld et al.
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Other studies
Many other scenarios have been studied
CP Violating MSSM Investigated by ATLAS, reduced discovery potential for small Higgs
boson masses
Strongly interacting Higgs Sector: VLVL scattering If no Higgs boson is found at LHC
Radions (Randall Sundrum model) φ → hh
...
See Atlas Physics TDR and the soon coming CMS Physics TDR for details
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Conclusions
ATLAS and CMS have studied the prospects of Higgs boson discovery for SM and MSSM
SM Higgs boson can be discovered with 5 sigma with 10 fb-1 at low luminosity in the whole mass range
At least 1 MSSM Higgs boson can be found for all investigated benchmarks In some regions difficult to discriminate between SM and MSSM
WW and ZZ fusion process is very important both for SM and MSSM
Two years from the beginning of LHC, must continue to prepare the actual analyses based on data with minimal use of MC information study of all the possible control samples needed to verify the
performances of the detector Studies with full simulation of signal and all the backgrounds
are in progress We are getting ready to find the Higgs boson(s) at LHC
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EXTRA
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H → ZZ → 4ℓ
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Higgs boson width
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Above MH~700 GeV the width of the Higgs boson becomes very large (>200 GeV), need higher rate
Use IVB fusion H → ZZ → ℓℓνν and
H → WW → ℓνjj
High mass search
ATLAS 100 fb-1
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MSSM h,H decays
Decoupling region
Large tanβ
bb, ττ decays
Small tanβ H decays into tt when allowed
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MSSM Production processes
Large tanβ hbb, Hbb and
Abb production dominates
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bbH,A->bbbb
CMS new analysis Investigation of feasibility of
the 4b channel S/B ~5%, large effects of
systematic error on BG estimation
Seems extremely difficult to control the BG with the needed precision
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Hadronic cannel: H±→tb
CMS repeated the study with NLO cross section calculation Old results showed some sensitivity New results by indicate that with the current analysis the very small
expected signal is washed out by systematic errors on the background estimation
Effect of systematic error on BG
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Discovery regions for MSSM Higgs bosons
ATLAS MHMAX scenario 300 fb-1
1 boson
2 bosons
3 bosons
All 4 bosons
h only
h,H,A,H+-
h,H,A
h,H,A
Excluded by LEP
Similar results in the other 3 benchmarks
5σ discovery regions
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Measurement of Higgs boson couplings