Search for b b decay of Higgs associated with a vector boson at - - PowerPoint PPT Presentation

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Search for b b decay of Higgs associated with a vector boson at - - PowerPoint PPT Presentation

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Search for b b decay of Higgs associated with a vector boson at ATLAS Lei Zhang on behalf of ATLAS collaboration International Symposium on Higgs Physics, IHEP, Beijing, 12-16, Aug. 2013 1 Introduction Higgsb b is important for directly

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Search for bb decay of Higgs associated with a vector boson at ATLAS

Lei Zhang

  • n behalf of ATLAS collaboration

International Symposium on Higgs Physics, IHEP, Beijing, 12-16, Aug. 2013

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Introduction

  • Higgs→bb is important for directly testing the Higgs mechanism in the

quark sector.

  • Due to the large multi-jet background at LHC, the inclusive search of H-

>bb is almost impossible.

  • The Higgs associated production with W/Z boson is one of the most

promising channels for H->bb search.

  • Three distinct channels, i.g. 0 lepton, 1 lepton and 2 lepton, have been

explored mainly aiming to Z->vv, W->lv and Z->ll.

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Introduction

HCP2012: (ATLAS‐CONF‐2012‐161) Using combined 7TeV and partial 8TeV datasets, the observed (expected) limit is 1.8 (1.9) times the Standard Model prediction.

  • Higgs→bb is important for directly testing the Higgs mechanism in the

quark sector.

  • Due to the large multi-jet background at LHC, the inclusive search of H-

>bb is almost impossible.

  • The Higgs associated production with W/Z boson is one of the most

promising channels for H->bb search.

  • Three distinct channels, i.g. 0 lepton, 1 lepton and 2 lepton, have been

explored mainly aiming to Z->vv, W->lv and Z->ll.

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Introduction

HCP2012: (ATLAS‐CONF‐2012‐161) Using combined 7TeV and partial 8TeV datasets, the observed (expected) limit is 1.8 (1.9) times the Standard Model prediction.

  • Higgs→bb is important for directly testing the Higgs mechanism in the

quark sector.

  • Due to the large multi-jet background at LHC, the inclusive search of H-

>bb is almost impossible.

  • The Higgs associated production with W/Z boson is one of the most

promising channels for H->bb search.

  • Three distinct channels, i.g. 0 lepton, 1 lepton and 2 lepton, have been

explored mainly aiming to Z->vv, W->lv and Z->ll. In this talk, the latest ATLAS full run-I data result (ATLAS‐CONF- 2013‐079) will be presented.

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Event Selection

  • Common selections :

– At least two jets with PT

1>45GeV, PT 2 (or PT 3)>20GeV and |η|<2.5

– ΔR(jet, jet) cut has been optimized as a function of PT

V

0 lepton:

– Et

miss trigger

– Veto leptons – Et

miss (PT Z)>120 GeV

QCD rejection cuts: – Δφ(ET

miss, jets) > 1.5

– ΔφET

miss, PT miss ) < π/2

– ΔφET

miss, bb) > 2.8

1 lepton:

– 1-Lepton trigger+ET

miss trigger

– Exactly 1 lepton – MT

W<120GeV and ET miss>25GeV

QCD rejection cuts: – PT

W<160, MT W>40GeV

– PT

W>200, ET miss>50GeV

2 lepton:

– 1 and 2 -lepton trigger – Exactly 2 leptons – 81GeV < Mll < 99GeV – ET

miss<60GeV

– QCD negligible

Enter 0-lepton channel if the charged lepton is not identified Enter 1-lepton analysis if one charged lepton is not identified

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2-tag region plots 0-lepton: (mixture) Z+jets, top and W+jets.

2 jets 3 jets

Grean: W+jets Blue: Z+jets

The heavier coler, the heavier flavor jets. Orange: 𝐮𝐮

Dark Orange: Wt channel Yellow: s and t channel

Pink : Multi-jet

Control region(CR) following the same color convention.

Background composition:

post fit signal region (SR)

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1-lepton: top and W+jets. Some MJ at low PT

W bin.

2-tag region plots 0-lepton: (mixture) Z+jets, top and W+jets.

Background composition:

post fit signal region (SR)

2 jets 3 jets

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1-lepton: top and W+jets. Some MJ at low PT

W bin.

2-lepton: Z+jets some top at low PT

Z regions.

2-tag region plots 0-lepton: (mixture) Z+jets, top and W+jets.

Background composition:

post fit signal region (SR)

2 jets 3 jets

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Important corrections:

  • tt sample (POWHEG+PYTHIA):

– Top pT correction applied at the level of generated top quarks.

  • V+jets sample (LO SHERPA):

– Δφ(jet, jet) correction applied due to NLO effect (arXiv: 1207.5030v1) . – The PT

V modeling greatly improved after correction.

Background modeling (I)

No Δφ corr. With Δφ corr. The most sensitive region

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Multijet: (Data driven method)

  • Source: 0 lepton(mis-measured jets); 1 / 2 lepton (the jets faking leptons )
  • The amount: 0 lepton ( 1% ); 1 lepton (15% - <1%); 2-lepton ( negligible).

Non-multijet backgrounds: (Mbb shape from MC)

  • Fixed to MC: Diboson, single-top, V+light-jets
  • Float in fit: V+cl, V+bl, V+bb/cc, and tt

Background modeling (II)

2jets, 1-tags 3jets, 1-tags 2jets, 2-tags 3jets, 2-tags Top eμ 3 PT

V bins

0-lepton CR CR SR SR

  • 5 PT

V bins

1-lepton CR CR SR SR

  • 5 PT

V bins

2-lepton CR CR SR SR CR

Control Region(CR) : Normalization Signal Region (SR) : Shape

Inputs to Global fit

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Global fit Model: the idea

eμ, 2-tag 2lep,2tag 1lep,2tag 0lep,2tag

  • 2lep. 1tag
  • 1lep. 1tag
  • Adjust the normalization by simultaneous fit

– Normalization floated: V+cl, V+bl/bb/cc(HF) and 𝐮𝐮 – Background scale factors have been correlated among regions.

Illustrative plots

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Global fit Model: the idea

eμ, 2-tag 2lep,2tag 1lep,2tag 0lep,2tag

𝐮𝐮

  • 2lep. 1tag
  • 1lep. 1tag

Illustrative plots

  • Adjust the normalization by simultaneous fit

– Normalization floated: V+cl, V+bl/bb/cc(HF) and 𝐮𝐮 – Background scale factors have been correlated among regions.

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Global fit Model: the idea

eμ, 2-tag 2lep,2tag 1lep,2tag 0lep,2tag

𝐮𝐮

  • 2lep. 1tag
  • 1lep. 1tag

Illustrative plots

Z+cc/bb/bl

  • Adjust the normalization by simultaneous fit

– Normalization floated: V+cl, V+bl/bb/cc(HF) and 𝐮𝐮 – Background scale factors have been correlated among regions.

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Global fit Model: the idea

eμ, 2-tag 2lep,2tag 1lep,2tag 0lep,2tag

𝐮𝐮

  • 2lep. 1tag
  • 1lep. 1tag

Illustrative plots

W+cc/bb/bl

  • Adjust the normalization by simultaneous fit

– Normalization floated: V+cl, V+bl/bb/cc(HF) and 𝐮𝐮 – Background scale factors have been correlated among regions.

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Z+cc/bb/bl

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Global fit Model: the idea

eμ, 2-tag 2lep,2tag 1lep,2tag 0lep,2tag

𝐮𝐮

  • 2lep. 1tag
  • 1lep. 1tag

Z+cl W+cl

Illustrative plots

  • Adjust the normalization by simultaneous fit

– Normalization floated: V+cl, V+bl/bb/cc(HF) and 𝐮𝐮 – Background scale factors have been correlated among regions.

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Z+cc/bb/bl W+cc/bb/bl

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  • Modeling:

– Mbb shape – PT

V and Top pt

– Jet multiplicity

  • Experimental:

– JER/JES – B-tagging – Lepton ID – ET

miss

Global fit Model:

Constrain Systematic Uncertainties

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  • Modeling:

– Mbb shape – PT

V and Top pt

– Jet multiplicity

Migration between channels through lepton sysetmatics

  • Experimental:

– JER/JES – B-tagging – Lepton ID – ET

miss

For each channel For each bin For each PT

V bin

Global fit Model:

Constrain Systematic Uncertainties

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  • VZ(bb): a similar signature with 5 times larger

cross section than VH(bb)

  • Diboson fit: a validation of the Higgs analysis.
  • The obs. (exp.) significance of VZ is 4.8 (5.1) σ.
  • μVZ = 𝝉𝐧𝐟𝐛𝐭(𝐖𝐚)

𝝉𝐓𝐍(𝐖𝐚)

= 0.9±0.2.

Fitted results: Diboson

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For mH = 125 GeV :

  • 7TeV:

– 2 sigma deficit w.r.t. SM expectation.

  • 8TeV:

– Consistent with both S+B and B only hypothesis in 1σ.

  • Combined fit result:

Results still dominated by statistical uncertainties.

Fitted results:

μHiggs for each year and channel

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  • 1 sigma excess in 8TeV data at mH=125GeV, as well as higher

masses.

  • Deficit in 7TeV data which makes the combined results a small

excess at mH=125 GeV.

  • At mH=125GeV, The observed (expected) upper limit at 95 C.L. is

1.4(1.3) times SM prediction.

Fitted results: Upper limit

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Summary

  • A search for H->bb has been performed via the Higgs associated

production with a vector boson by using full ATLAS run-I data.

  • Diboson(VZ) result is consistent with the SM expectation, with 4.8𝜏

significance.

  • For mH=125 GeV, VH→bb cross section combined fit results is:
  • The observed (expected) 95% C.L. limit for mH=125 GeV is 1.4 (1.3)

times the SM expectation. Respect to the previous analysis, a 35% improvement in significance on top of luminosity due to the

  • ptimization and reduced systematics.

With run-I data, we are close to the critical region, and it will be very exciting to look the first bunch of data at 2015.

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Th Than ank k yo you

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backup

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Trigger and Evt. selection

Zero lepton: ET

miss trigger

One lepton: 1-Lepton trigger + ET

miss trigger (for

muon channel) Two lepton: 1-Lepton trigger + 2-lepton trigger

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Xsec*Br and acceptance

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0-lepton: data-MC plots

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1-lepton: data-MC plots

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2-lepton: data-MC plots

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Background modeling (II):

V+jets before Δφ(jet, jet) correction

  • V+jets: modeled by Leading order(LO) generator.
  • Mismodeling found in Δφ(jet, jet) and PT

V distribution.

  • Interprated as Next Leading Order(NLO) effect--arxiv:

1207.5030v1

Δφ(jet, jet) correction has been applied

Δφ(jet, jet) PT

V

High PTV region is the most sensitive region.

1 lepton, 2 jets, 0 tag

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Background modeling (II):

V+jets after Δφ(jet, jet) correction

  • After Δφ(jet, jet) correction:

– The modeling of the PT

W distribution greatly improved.

– This correction has been applied in all channels and all regions.

  • Z+jets : similar correction applied
  • The similar reweighting has been performed in Z+jets processes.

Δφ(jet, jet) PT

V

1 lepton, 2 jets, 0 tag

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Δφ(jet, jet) mismodeling: NLO effect

arXiv: 1207.5030v1

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  • Trigger: neglable except the 0-lepton (120-160GeV) which is about 5%.
  • JES: from 4% to 1% depending on the jet pT.
  • B-tag: 2-3% over most of the jet pT range. Due to the sample dependence,

a 2% and 5% extra uncertainties have been applied for b and c jet, respectively.

  • Lumi : 2.8% for 2012, 1.8% for 2011.

Modeling Sys. summary

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Fitted BKG scale factor

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Separated limit: 7/8 TeV

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ATLAS Higgs mu summary

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Mbb resolution: Bukin fit

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