LHC Prospects on Standard Model Higgs

LHC Prospects on LHC Prospects on Standard Model HiggsStandard Model Higgs

Riccardo Ranieri

INFN and Università degli Studi di FirenzeINFN and Università degli Studi di Firenze

on behalf of ATLAS and CMS Collaborationson behalf of ATLAS and CMS Collaborations

ICHEP’04ICHEP’04

3232ndnd International Conference on High Energy International Conference on High Energy PhysicsPhysics

Beijing, China 16-22 August 2004Beijing, China 16-22 August 2004

ICHEP’04Beijing, 16-22 August 2004

LHC Prospects on Standard Model Higgs

Riccardo Ranieri 2

ATLAS & CMS at LHCATLAS & CMS at LHC– Detectors optimised for Higgs boson

and SUSY searches very high energy: LHCLHC pp √s=14 TeVpp √s=14 TeV

– inelastic cross section: σσpppp=55 mb=55 mb

– interaction rate: 40 MHz40 MHz high luminosity: (2x)10(2x)1033 33 cmcm-2-2ss-1-1101034 34 cmcm--

22ss-1-1

– per year: 20 fb20 fb-1-1100 fb100 fb-1-1

BIG detectors– CMS: 15 m x 21.5 mCMS: 15 m x 21.5 m– ATLAS: 25 mATLAS: 25 m x 46 mx 46 m

CMS = Compact Muon CMS = Compact Muon SolenoidSolenoid

LHC = LHC = Large Large Hadron Hadron ColliderCollider

ATLAS = A Toroidal LHC ATLAS = A Toroidal LHC ApparatuSApparatuS

first collisions in Summer first collisions in Summer 20072007

http://www.cern.ch/lhc/



Riccardo Ranieri 3

Where will we be at LHC Where will we be at LHC startup?startup?

mH>114.4 GeV/c2 @ 95% CL

¿ Higgs boson signal mmHH=115 GeV/c=115 GeV/c22 from LEP2 data ?

CERN-EP/2003-011

LHWG Note/2002-01

Searches at TeVatron from now to 2007…

Design Projection: Design Projection: 4.4 fb4.4 fb-1-1

mmHH<130 GeV/c<130 GeV/c22 covered to exclusion covered to exclusion

33σσ evidence up to m evidence up to mHH=125 GeV/c=125 GeV/c22

Base Projection:Base Projection: 2.2 fb2.2 fb-1-1

mmHH<125 GeV/c<125 GeV/c22 covered to exclusion covered to exclusionFERMILAB-PUB-03/320-E



Riccardo Ranieri 4

Associated Associated productionproduction ttHttH and and bbHbbH

– high-pT lepton, top reconstruction,b-tag

-- --

SM Higgs Production at SM Higgs Production at LHCLHC

Gluon FusionGluon Fusion– the highest

cross section

Vector Boson FusionVector Boson Fusion– two high-pT

forward jets

Associated ProductionAssociated Production WHWH and and ZHZH

– one or two high-pT leptons useful for the trigger



Riccardo Ranieri 5

SM Higgs DecaysSM Higgs Decays

“light” Higgs

2mZ

– decays into Vector bosons W and Z

» “golden” channels

– two-photon decays» extremely

“clean” but rare and difficult to detect

LEP excluded

– hadronic and decays are favourite

» …but difficult to select



Riccardo Ranieri 6

Low Mass Higgs: HLow Mass Higgs: H→→This decay is very rare (Br≈10-3)

σ(pp→H115)xBr(H→)=76 fb (NLO)

S/B≈1/20– good resolution mass peakmass peak– Electromagnetic Calorimetres crucialcrucial

for H→: σσ(m(m)/m)/m≈1%≈1% needed

motivation for LAr (ATLAS) and PbWO4 (CMS) calorimetres

– high granularity– response uniformity

CERN/LHCC 96-40 ATLAS TDR 1

CERN/LHCC 96-41 ATLAS TDR 2

CERN/LHCC 97-33 CMS TDR 4

– 3 main background processes:» irreducibile: gg/qq→

81 pb81 pb

+jet (with “real” or “fake” second photon)9x109x104 4 pbpb

» hadronic QCD jets (π0 decays)

101088pbpb

» ATLAS reach with 10 fb10 fb-1-1 and mmHH=115 GeV/c=115 GeV/c22:» Signal Significance: SS//√B√B=2.0=2.0 (K-factors not included)



Riccardo Ranieri 7

Low Mass Higgs: ttH(Low Mass Higgs: ttH(→→bb)bb)This is the favourite decay σ(pp→H115)xBr(H→bb)=28 pb

S/B<10-7

tagging the top quarks helps a lot– t→bW(→μν)– t→bW(→jj)

“crowded” final state– 6 jets (4 of them are b-jets) 6 jets (4 of them are b-jets)

+ additional ISR/FSR jets+ additional ISR/FSR jets» 4 b-tagged jets needed to

reduce combinatorics

– 1 isolated lepton1 isolated lepton» it’s the key for trigger

optimised analysis– pz from W-mass constraints

– likelihood pairing of jets

----

-

ATL-PHYS-2003-024

ν

»Final result for Final result for likelihood analysis likelihood analysis ((mmHH=115 GeV/c=115 GeV/c22):):

»30 fb-1: S/√B=3.4»10 fb10 fb-1-1: S/√B=2.0: S/√B=2.0



Riccardo Ranieri 8

Jet

Vector Boson Fusion: Vector Boson Fusion: qqH(→qqH(→) )

hadronic jets in forward-backward regions– the forward jet tagging is a powerful

background rejection tool hadronic activity suppressed in low low ηη region

– emitted vector bosons are colour-singlets Search for →→ℓℓννℓ’ℓ’νν’’, , ℓℓνν+jet+jet final states

– S/√B≥5S/√B≥5 in mmHH=120=120÷÷140 GeV/c140 GeV/c22 range with 40 40 fbfb-1-1

» S/√B≈2.5S/√B≈2.5 in one LHC year in one LHC year» this process offers the possibility for a direct

measurement of Yukawa coupling H

Phys. Rev. D59(1999) 014037

ATL-PHYS-2003-004

CMS NOTE 2003/033

Jet

Higgs Decay products

Forward tagging jets

[VBF]



Riccardo Ranieri 9

Light Higgs SummaryLight Higgs Summary

Detector/performance requirements:Detector/performance requirements: HH→→ response uniformityresponse uniformity of electromagnetic

calorimeter HH→→bbbb b-taggingb-tagging capability of tracking system qqH(qqH(→→)) efficient jet reconstructionjet reconstruction in 3<|η|<5 aallll good understandingunderstanding (<10%) of the

backgroundbackground

Three complementary channelsThree complementary channels

Each channel contributes with ~22σσ to the total significance

Observation of all channels is very important to extract a Observation of all channels is very important to extract a convincing signal in the first year(s)convincing signal in the first year(s)

--



Riccardo Ranieri 10

High Mass Higgs: HHigh Mass Higgs: H→→WWWW(*)(*)

• VBF qqH→qqWW– Di-lepton final state

WW→ℓνℓν (ℓ=e,μ) b-jet and veto in the central

region to reject tt background

Drell-Yan background ee, μμ discarded with lepton cuts:

– Mℓℓ<75 GeV/c2; pTmiss>30

GeV/c

– Main decay channel around mH=170 GeV/c2

Isolated leptons WW(*)→ℓνℓν (ℓ=e,μ)Missing transverse energy ET

miss

Dangerous background from top decays

– central jet vetoWW spin correlations for the signal

– small ℓ+ℓ- opening angles

CMS NOTE 2003/033 ATL-PHYS-2003-005

10 fb-1 are more than enough if mH is around 170 GeV/c2

-



Riccardo Ranieri 11

– The “goldengolden” channel well defined peaks ZZ→→μμ++μμ--

– mmHH>2m>2mZZ: real Z’s

main backgrounds– reducible: tt, Zbb

» μ isolation

» Z reconstruction (mZ)

– irreducible: ZZ» qq production mechanism

dominates softer muons Luminosity required for a 55σσ

discovery:– 1010-30 fb-30 fb-1-1 if m if mHH>2m>2mZZ

» 2-3 LHC years

– up to 100 fbup to 100 fb-1-1 if m if mHH<2m<2mZZ

» only one reconstructed Z» high luminosity runs

High mass Higgs: High mass Higgs: HH→→ZZZZ(*)(*)→→44μμ

CMS AN 2003-005

CMS AN 2003-007

- -

-



Riccardo Ranieri 12

ATLAS & CMS Discovery ATLAS & CMS Discovery PotentialPotential

After detector calibration and LHC pilot run…

– …almost all the “allowed” mass range can be explored during the first year first year (10 fb-1)

– ...after 2 years 2 years (≈30 fb-

1) 77σσ significance over the whole mass spectrum, covered by more than onemore than one channel

» LEP excess is near…

CMS NOTE 2003/033CERN/LHCC 99-15 ATLAS TDR 15



Riccardo Ranieri 13

ConclusionConclusion– LHC has potential for Standard Model Higgs

boson discovery already in the first yearalready in the first year (months?) of operation 1 LHC day at 1033 cm-2s-1 ≡ 10 years at previous

machines

– The ATLASATLAS and CMSCMS detectors are designeddesigned for Higgs boson search

no surpriseno surprise that they can cover the full spectrum of Standard Model HiggsHiggs masses within 1 year1 year of start of physics collisions

(however at the beginning a lot of time will be needed to reach the desired performance, optimise physics selection and measure backgrounds)

…first collisions are near…

LHC Prospects on Standard Model Higgs

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