Higgs Searches at ATLAS Liron Barak (Weizmann Institute of Science) - - PowerPoint PPT Presentation

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Higgs Searches at ATLAS Liron Barak (Weizmann Institute of Science) - - PowerPoint PPT Presentation

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Higgs Searches at ATLAS Liron Barak (Weizmann Institute of Science) on behalf of the ATLAS collaboration Outline LHC and ATLAS SM Higgs Boson Production Decay modes Combination MSSM Higgs bosons Summary L. Barak

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Higgs Searches at ATLAS

Liron Barak (Weizmann Institute of Science)

  • n behalf of the ATLAS collaboration
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SLIDE 2
  • LHC and ATLAS
  • SM Higgs Boson

– Production – Decay modes – Combination

  • MSSM Higgs bosons
  • Summary

Outline

  • L. Barak

2 LISHEP, 6 July 2011

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LHC and ATLAS

  • In 2010:

– Proton proton collisions – ATLAS recorded 45 pb-1 of integrated luminosity at √s=7 TeV – Depending on data quality selections, 35 pb-1 or more used in analysis

  • In 2011:

– Running since early March with √s=7 TeV

– >1 fb-1 data has been recorded

– Hope for 4-5 fb-1 of integrated luminosity before the end of the year

  • Beyond:

– LHC will run in 2012 – Then, long shutdown and run at higher energy.

  • L. Barak

LISHEP, 6 July 2011 3

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SLIDE 4

gluon-fusion (dominant) VBF (less suppressed at higher MH) WH/ZH (contribute to H->gg, bb) ttH (contribute to H->gg, bb)

SM Higgs Boson - Production

  • L. Barak

LISHEP, 6 July 2011 4

Signal cross section and its uncertainties are given by “Handbook of LHC Higgs Cross sections:

  • 1. Inclusive Observables” arXiv: 1101.0593(hep-ph)
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SM Higgs Boson - Decay

Decay modes:

  • H -> WW

– Dominant in intermediate and high mass regions

  • H -> ZZ
  • H -> gg

– Low BR – Relevant in low mass region – Clean signature

  • H -> bb (not in this talk)

– Dominant in low mass region – Very challenging (high QCD background)

  • H -> tt (not in this talk)

– Low production rate – High Z -> tt background

The Challenge:

<1 detectable Higgs boson per 1012 collisions

  • L. Barak

LISHEP, 6 July 2011 5

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SLIDE 6

SM Higgs Boson

  • L. Barak

LISHEP, 6 July 2011 6

Decay modes:

  • H -> WW

– Dominant in intermediate and high mass region

  • H -> ZZ
  • H -> gg

– low BR – Important in low mass region – Clean signature

  • H -> bb (not in this talk)

– Dominant in low mass region – Very challenging (high QCD bg)

  • H -> tt (not in this talk)

– Low production rate – High Z -> tt background

The Challenge:

<1 detectable Higgs boson per 1012 collisions

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SLIDE 7

SM Higgs Boson

H -> gg (data 2011):

  • Mass range 110-140 GeV
  • Preselection cuts:

– 2 isolated photons with pT > 25 GeV and |h| < 2.37, fulfilling tight shower shape requirements – 1 g with pT > 40 GeV

– 100 < Mgg < 150 GeV

  • L. Barak

LISHEP, 6 July 2011 7

ATLAS-CONF-2011-085

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SLIDE 8

SM Higgs Boson

H -> gg (data 2011):

  • Main backgrounds:

– Reducible (g-jet and Jet-jet) – Irreducible (gg and Drell-Yan)

  • Strategy to estimate:

– Double sideband method – Electron photon fake rate from Z ee

  • Limit: The observed excluded s

ranges from 4.2 to 15.8 times the sSM at 95% CL (6-7 was expected)

  • L. Barak

LISHEP, 6 July 2011 8

ATLAS-CONF-2011-085

q q g g

q g g g p0 q g tight g ID isolated non isolated loose g ID A B D C 

* B C A D 

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SM Higgs Boson

H -> gg (data 2010):

  • L. Barak

LISHEP, 6 July 2011 9

arXiv 1106.2748v2

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SM Higgs Boson

H -> WW (data 2010):

  • L. Barak

LISHEP, 6 July 2011 10

ggF VBF

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SM Higgs Boson

H -> WW* -> lnln (l=e,m):

  • Mass range 120-200 GeV

Most sensitive to MH=2MW

  • 9 channels – (H+0j, 1j, 2j) * (ee, mm, em)

– Jet pT > 25 GeV and Jet |h| < 4.5

  • Preselection cuts:

– 2 opposite-sign hard and isolated leptons – MET > 30 GeV

– Mll > 15 GeV, |Mll-MZ| > 10 GeV (for ll=ee, mm)

  • Topological selection (optimized

according to mass regions and channels):

– b jet veto – Dfll < 1.3, 1.8 – Mll < 50, 65, 80 GeV and |PT

ll| > 30 GeV

– Transverse mass 0.75*MH<MT<MH

  • L. Barak

LISHEP, 6 July 2011 11

ATLAS-CONF-2011-005

H+0j

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SM Higgs Boson

H -> WW* -> lnln (l=e,m):

  • Main backgrounds:

– WW (Mll and Dfll)

– Z+jets (ABCD - Mll and MET) – Top (jet veto, reverse the b jet veto) – W+jets (anti isolation)

  • L. Barak

LISHEP, 6 July 2011 12

ATLAS-CONF-2011-005

  • Data driven method

– Define control regions (sample enriched in particular backgrounds) – Subtract the contamination of other backgrounds in control regions – Define “comparable” variables – Propagate estimation from control regions to signal regions (using scales from data/MC)

NWW

SR =αWW × (NWW CR − b top ·Ntop CR − ...)

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SM Higgs Boson

H -> WW* -> lnln (l=e,m):

  • Exclusion limit
  • A 95% CL upper limit on

s x BR(H->WW*) : 54 pb @ MH=120 GeV 11 pb @ MH=160 GeV 71 pb @ MH=200 GeV

  • 1.2 x sSM excluded @

MH = 160 GeV (2.4 was expected)

  • L. Barak

LISHEP, 6 July 2011 13

ATLAS-CONF-2011-005

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SLIDE 14

SM Higgs Boson

H -> WW* -> lnln (l=e,m):

  • L. Barak

LISHEP, 6 July 2011 14

arXiv 1106.2748v2

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SM Higgs Boson

H -> WW* -> lnqq (l=e,m):

  • Mass range 220-600 GeV

Models: SM and 4th generation of fermions

  • 2 channels – H+0j, 1j

– Jet pT > 30 GeV and Jet |h| < 4.5

  • Preselection cuts:

– 1 muon or electron (fullfill tight selection criteria) with pT > 30 GeV

– 0 additional muon or electron (fullfill medium selection criteria) with pT > 20 GeV

– MET > 30 GeV (against QCD) – 2 or 3 jets with pT > 30 GeV and |h| < 4.5

– 2 jets with 71 < Mjj < 91 GeV and |h| < 2.8 – b jet veto (against top)

  • L. Barak

LISHEP, 6 July 2011 15

ATLAS-CONF-2011-052

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SM Higgs Boson

H -> WW* -> lnqq (l=e,m):

  • Main backgrounds:

– W/Z+jets (Data) – Multi-jet (Data) – Top (MC) – Diboson (MC)

  • Strategy to estimate:

– Data driven method:

  • Estimation using a fit to ET

miss

distribution

  • Shape from MC (W/Z+jets) or

from an anti isolated region (multi-jet)

  • Mass reconstruction

– Using Mln = MW constraint – Smallest |Pz

n|

  • L. Barak

LISHEP, 6 July 2011 16

ATLAS-CONF-2011-052

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SM Higgs Boson

H -> WW* -> lnqq (l=e,m):

  • Exclusion limit
  • Exponential fit to the

background to set limits with a profile likelihood (systematic sources as nuisance parameters)

  • At MH = 320 GeV, the 95% CL

upper limit is approximately 7.13 x sSM (15.3 was expected)

  • For the SM4 prediction,

no value of MH is excluded.

  • L. Barak

LISHEP, 6 July 2011 17

ATLAS-CONF-2011-052

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SLIDE 18

SM Higgs Boson

H -> WW* -> lnqq (l=e,m):

  • L. Barak

LISHEP, 6 July 2011 18

arXiv 1106.2748v2

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SLIDE 19

SM Higgs Boson

H -> ZZ (data 2010):

  • L. Barak

LISHEP, 6 July 2011 19

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SM Higgs Boson

H -> ZZ* -> llll (l=e,m):

  • Mass range 130-600 GeV

Fully reconstructed final state

  • Preselection cuts:

– >1 quadruplet of 2 pairs of same-flavour opposite charge leptons:

  • >2 leptons in the quadruplet with

pT > 20 GeV

  • Zl1l2 – the dilepton pair closest to

the Z boson mass |Ml1l2-MZ| < DM12 (12-15 GeV)

  • Zl3l4 – the sub leading

(Ml3l4 > 15-60 GeV)

  • DR (l,l’) > 0.1 for all leptons

– Suppress leptons from b-hadrons with impact parameter significance requirements on the 2 least energetic leptons

  • L. Barak

LISHEP, 6 July 2011 20

ATLAS-CONF-2011-048

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H -> ZZ* -> llll (l=e,m):

  • Backgrounds

– ZZ (irreducible) – Z+QQ (reducible)

  • Limit:

– At MH < 130 GeV – too low BR – At MH ~ 160 GeV – 2 on shell W – At MH > 180 GeV – 2 on shell Z – At MH = 200 GeV, the 95% CL upper limit on s is approximately 24 x sSM (25 was expected)

SM Higgs Boson

  • L. Barak

LISHEP, 6 July 2011 21

ATLAS-CONF-2011-048

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SLIDE 22

SM Higgs Boson

H -> ZZ* -> llll (l=e,m):

  • L. Barak

LISHEP, 6 July 2011 22

arXiv 1106.2748v2

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SM Higgs Boson

H -> ZZ* -> llqq (l=e,m):

  • Mass range 200-600 GeV

2 on shell Z – suppress background

  • Preselection cuts:

– 2 same flavour leptons (muon

  • pposite sign)

– 76 < Mll < 106 GeV – MET < 50 GeV

– ≥ 2 jets with 70 < Mjj < 105 GeV

– For MH ≥ 360 GeV:

  • Jet pT > 50 GeV
  • Dfjj < p/2 and Dfll < p/2

– Constraining the dijet to the Z mass

  • Backgrounds:

– Diboson (irreducible – MC) – Z+jets (MC – after verifying in control region) – Top pair (reverse MET and Mll ) – Multijet (data)

  • L. Barak

LISHEP, 6 July 2011 23

ATLAS-CONF-2011-026

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SM Higgs Boson

H -> ZZ* -> llqq (l=e,m):

  • L. Barak

LISHEP, 6 July 2011 24

arXiv 1106.2748v2

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SM Higgs Boson

H -> ZZ* -> llnn (l=e,m):

  • Preselection cuts:

– b jet veto – For MH < 280 GeV:

  • MET > 66 GeV
  • Dfll < 2.64
  • Dfll > 1 (MH ≤ 260 GeV)

– For MH ≥ 280GeV:

  • MET > 82 GeV
  • Dfll < 2.25

– Looking for the transverse mass

  • Backgrounds:

– Top pair and W+jets (control region)

  • Significant contribution from

H‐>WW‐>lνlν , but orthogonal selection, no overlapping events

  • L. Barak

LISHEP, 6 July 2011 25

ATLAS-CONF-2011-026

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SM Higgs Boson

H -> ZZ* -> llqq/llnn (l=e,m):

  • L. Barak

LISHEP, 6 July 2011 26

ATLAS-CONF-2011-026

Upper limit on the s of between 3.5 and 39.0 x sSM (6.5 – 25 was expected)

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SM Higgs Boson

H -> ZZ* -> llqq/llnn (l=e,m):

  • L. Barak

LISHEP, 6 July 2011 27

arXiv 1106.2748v2

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SM Higgs Boson

Combination (data 2010):

  • The expected and observed cross

section limits, normalized to the sSM, as a function of the Higgs boson mass for the individual search channels.

  • The visually most apparent

difference between expected and

  • bserved limit is in the

H -> WW -> lνqq channel, which has a deficit approaching one sigma both at 320 and 480 GeV.

  • These results use the profile

likelihood method with a power constraint (PCL).

  • The theory uncertainties:

– Gluon-Fusion: 15-20% – VBF: 3-9%

  • L. Barak

LISHEP, 6 July 2011 28

arXiv 1106.2748v2

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SM Higgs Boson

Combination (data 2010):

  • The expected and observed upper limits on the total s

divided by the expected SM Higgs boson s.

  • Highest sensitivity is in the mass range 160-170 GeV
  • L. Barak

LISHEP, 6 July 2011 29

arXiv 1106.2748v2

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SM Higgs Boson

Combination (data 2010):

  • The expected and observed upper limits on the total s divided by the

expected 4SM Higgs boson s.

  • Excluded for a Higgs mass in range 140 – 185 GeV

(135-210 GeV was expected)

  • L. Barak

LISHEP, 6 July 2011 30

arXiv 1106.2748v2

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MSSM Higgs Bosons

  • MSSM – Minimal

Supersymmetric Standard Model

  • At tree level, MSSM Higgs sector

described by two parameters:

– MA = mass of CP odd Higgs – tanβ = ratio of the ‘vev’ of 2 Higgs doublets

  • MSSM Higgs: h, H, A, H±

– Neutral Higgs bosons – h (CP even), H (CP even), A (CP odd) – Charged Higgs bosons - H ±

  • On top of the SM channels, one

could also observe:

– H→ μμ, ττ – H+→ τν, cs (MH+<Mt) – H+ →c+c0

  • σMSSM ~ (tanβ)2 x σSM
  • L. Barak

LISHEP, 6 July 2011 31

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MSSM Higgs Bosons

H -> tt -> lnthad (l=e,m):

  • Mass range 90-300 GeV
  • Preselection cuts:

– Ne+Nm = 1

– pTe > 20 GeV, pTm > 15 GeV

– Nt =1, pTt,vis > 20 GeV

– Qt·Qe/m = -1

– MET > 20 GeV

– MT < 30 GeV

  • Backgrounds

– Z -> tt (Validate with Z->mm data by embedding t's in the place of m) – QCD and W+jets (same sign control sample) – Z -> ee,mm (MC) – Diboson (MC) – Top (MC)

  • L. Barak

LISHEP, 6 July 2011 32

ATLAS-CONF-2011-024

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MSSM Higgs Bosons

H -> tt -> em+4n:

  • Mass range 90-300 GeV
  • Preselection cuts:

– Ne= 1 – Nm = 1

– pTe > 20 GeV, pTm > 15 GeV – Qm·Qe= -1 – HT < 120 GeV (against TOP)

  • Backgrounds

– Z -> tt (embedding method for validation) – QCD (ABCD method) – W+jets (MC) – Z -> ee, mm (MC) – Diboson (MC) – Top (MC)

  • L. Barak

LISHEP, 6 July 2011 33

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MSSM Higgs Bosons

H -> tt -> lnthad (l=e,m):

  • Exclusion limit
  • Low Higgs mass:

≥ tan b ~ 30

  • High tan b:

200 ≤ MA ≤ 300 GeV

  • L. Barak

LISHEP, 6 July 2011 34

ATLAS-CONF-2011-024

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MSSM Higgs Bosons

Charged Higgs: H+ -> tn (tan b > 1):

  • t -> nln (leptonic tau)

– W -> ln (di-lepton) or W -> qq (single lepton) – Discriminating variables:

  • COS q* and MT

H / MT2 H

  • t -> nqq (hadronic tau)

– W -> ln / qq – Estimating backgrounds from data

  • Fakes from electrons
  • Fakes from muons
  • Fakes from jets
  • QCD
  • L. Barak

LISHEP, 6 July 2011 35

ATLAS-CONF-2011-018, 051

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MSSM Higgs Bosons

Charged Higgs: H+ -> tn (tan b > 1):

  • t -> nln (leptonic tau)

– W -> ln (di-lepton) or W -> qq (single lepton) – Discriminating variables:

  • COS q* and MT

H / MT2 H

  • t -> nqq (hadronic tau)

– W -> ln / qq – Estimating backgrounds from data

  • Fakes from electrons
  • Fakes from muons
  • Fakes from jets
  • QCD
  • L. Barak

LISHEP, 6 July 2011 36

ATLAS-CONF-2011-018, 051

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SLIDE 37

MSSM Higgs Bosons

Charged Higgs: H+ -> tn (tan b > 1):

  • t -> nln (leptonic tau)

– W -> ln (di-lepton) or W -> qq (single lepton) – Discriminating variables:

  • COS q* and MT

H / MT2 H

  • t -> nqq (hadronic tau)

– W -> ln / qq – Estimating backgrounds from data

  • Fakes from electrons
  • Fakes from muons
  • Fakes from jets
  • QCD
  • L. Barak

LISHEP, 6 July 2011 37

ATLAS-CONF-2011-018, 051

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Summary

  • With ~36 pb-1, ATLAS excluded the

Higgs4SM at 140 < MH < 185 GeV

  • ATLAS also put an upper limit of 2.3xsSM

in 160-170 GeV

  • With 1 fb-1 of data, ATLAS expects to

achieve a combined SM-like Higgs boson exclusion sensitivity in the range 130 < MH < 460 GeV

  • L. Barak

LISHEP, 6 July 2011 38

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Summary

  • L. Barak

LISHEP, 6 July 2011 39

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Next year presentation

  • L. Barak

LISHEP, 6 July 2011 40 LISHEP, 6 July 2011

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Next year presentation

  • L. Barak

LISHEP, 6 July 2011 41 LISHEP, 6 July 2011

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Next year presentation

  • L. Barak

LISHEP, 6 July 2011 42 LISHEP, 6 July 2011