Higgs Searches at LHC

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



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



SM Higgs production

NLO Cross sections M. Spira et al.

gg fusion

IVB fusion



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)



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



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



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



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



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



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



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



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



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



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



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



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



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



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%



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



Results on H,A

5σ discovery regions in the MHMAX scenario



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



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



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



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.



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



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



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



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



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



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.



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



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



EXTRA



H → ZZ → 4ℓ



Higgs boson width



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



MSSM h,H decays

Decoupling region

Large tanβ

bb, ττ decays

Small tanβ H decays into tt when allowed



MSSM Production processes

Large tanβ hbb, Hbb and

Abb production dominates



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



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



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



Measurement of Higgs boson couplings

Higgs Searches at LHC

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