Could the Higgs boson be invisible

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Could the Higgs boson be invisible?

• Pauline Gagnon
• Indiana University
• Why do we need the Higgs boson?
• Current status on Higgs searches
• Could it be invisible?
• Could we see it if its invisible?

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The Standard Model

• Theoretical model describing constituents of
  matter and their interactions
• All experimental observations corroborate the SM
  predictions to great precision
• Two central ideas
• 1. All matter is made of quarks and leptons
• 2. Forces between quarks and leptons are mediated
  by exchange particles bosons
• W?, Z0, photon, gluon

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(No Transcript)
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2. Fundamental interactions
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Where does the Higgs boson come into play?

• Electroweak theory predicts 4 massless bosons
• Peter Higgs invented a mechanism called
  electroweak symmetry breaking which turns the

• 4 massless bosons into
• 3 massive bosons W, W-, Z0
• 1 massless boson ?0

All fermions acquire mass by interacting with the
Higgs field analogous to drag force
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Where have we looked for the Higgs so far?

• CERN the European Laboratory for Particle
  physics near Geneva, Switzerland with LEP (Large
  Electron Positron Collider)
• Aleph, OPAL, L3 and Delphi
• Fermilab, since 2000 with the Tevatron
• D0
• CDF

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Higgs searches at CERN

• LEP I 1989-1994 ee- ? Z ? Z H0
• 17 million Z decays analyzed
• SM Higgs mass limit mH gt 65 GeV (95 CL)
• MSSM Higgs h0 and A0 mh,A gt 45 GeV (95 CL)
• LEP II 1995-2000 ee- ? Z ? Z H0
• ECM 135, 161, 171, 183, 189-209 GeV
• Integrated luminosity 2.46 fb-1 _at_ ECM gt189
• 0.55 fb-1 _at_ ECM
  gt206
• SM limit mH gt114.1 GeV (95 CL)
• MSSM limit mH gt91.1 GeV (95 CL)

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SM constraints on mH
Consistency check

Consistency
One sided limit (LEP SLD
Tevatron) mHlt 186 GeV _at_95 CL.
Renormalising to mHgt114 GeV (LEP SLD
Tevatron) mHlt 219 GeV _at_95 CL.
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Tests of the Standard Model - EPS 2005 - July 25
- Sijbrand de Jong
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Searches for alternative Higgs at LEP
Fermiophobic Higgs mh gt 109.7 GeV/c2
Invisible Higgs mh gt 114.4 GeV/c2
Yukawa production
Charged Higgs mH gt 78.6 GeV/c2
Decay-mode independent
Doubly charged Higgs mH gt 99.3 GeV/c2
Anomalous couplings
Flavour-independent Higgs
Low mA Higgs
2HDM limits
CP-violating Higgs
NMSSM
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Tevatron Higgs Searches Combined (Spring 2006)
95 CL Limits/SM
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Higgs prospects for discovery/exclusion 1998-1999
Higgs SUSY study / 2003 update
Tests of the Standard Model - EPS 2005 - July 25
- Sijbrand de Jong
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Tevatron Higgs prospects for discovery/exclusion
Prospects in time
mid 2009
End 2007
End 2006
Tests of the Standard Model - EPS 2005 - July 25
- Sijbrand de Jong
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LHC the Large Hadron Collider at CERNATLAS one
of 4 new detectors built for LHC
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ATLAS detector
3000 scientists 150 institutes 34 countries 144
ft long 72 ft diameter 2007-2017
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Upcoming milestones

• 500-piece jigsaw puzzle available end of April
  2006

• TRT will be lowered into place this summer

• Full detector completed by summer 2007

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IU contributions to ATLAS

• TRT Transition Radiation Tracker
• Half the TRT barrel modules were constructed here
  at IU from 1998-2004
• Studies on e/? separation using the TRT
• Modules Quality Control done at CERN
• Modules assembly on the detector
• Software development for track reconstruction and
  monitoring of the TRT detector
• Tier-2 GRID center
• Data analysis on invisible Higgs

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Quality control lab at CERN
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Full TRT Barrel after assembly
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Standard Model Higgs decay channels
Higgs decays to heaviest available fermions
until mH gt 2mW or 2mZ For low mH H ?
??- or H ? bb For mH gt 160 GeV H ? WW or
H ? ZZ
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Invisible Higgs decays

• No invisible Higgs decays in Standard Model
• But many other models predict invisible Higgs
  branching fraction with strong suppression of the
  main SM decay channels
• Many such models MSSM, extra dimensions, NMSSM,
  spontaneously breaking R-parity models etc.

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Minimal Super Symmetric Model

• BR (H ??0?0) in the M2-? plane
• ?0 neutralino
• (lighest SUSY particle)
• M2 gaugino mass
• ? Higgs doublet mixing

BR (H ??0?0) 60

BR (H ??0?0) 20
Excluded by dark matter searches
Boudjema, Bélanger, Godbole hep-ph/0206311
Excluded by chargino searches at LEP
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Models with extra dimensions
Antoniadis, Tuckmantel, Zwirner, Nucl. Phys., B
707 (2005) 215-232

• H ? ? G
• G Goldstino
• M auxiliary mass parameter
• Lepton number is conserved

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Spontaneously Broken R-Parity
Hirsch, Romão, Valle, Moral, hep-ph/0407269

• H ? JJ
• J Goldstone boson (Majoron)
• RJb BR(H?JJ)
• BR(H?bb)
• ? reduced coupling to Z
• h Yukawa coupling
• Lepton number is not conserved and neutrinos get
  masses

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SM Higgs production mechanisms
q
dominated by gluon fusion qq ?qqH increases at
large mH ttH and WH,ZH less background
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Cross-sections and trigger for invisible Higgs
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Rapidity ?

• A convenient way to determine where a track goes
  is to use rapidity

? - ln tan (?/2)
? 0
? ?
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Missing transverse momentum
momentum is not balanced in the Z-direction

• But momentum has to be balanced in the transverse
  plane
• pT missing -? pT visible

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Vector boson fusion
q

• Decay characteristics
• two forward jets
• large pTmiss in central rapidity region
• Main backgrounds
• QCD 2 jets
• 2 jets Z, Z ? ??
• 2 jets W, W ? l?

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Main selection cuts efficiency
? in pb-1
No trigger for this channel yet
for 10 fb-1 S/?B 10
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The ATLAS 3-level trigger system
hardware
2.5 ms
software
10 ms
sec
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Trigger efficiency for VBF, H?invisible

• Currently, in trigger menu, we only trigger on
  central jets
• Level 1 ETMISS gt 60 GeV, PTjet gt 60 GeV,
  ?jetlt3.2
• Level 3 ETMISS gt 70 GeV, PTjet gt 70 GeV,
  ?jetlt3.2
• Trigger extension needed for VBF H?invisible with
  ?jet up to 4.9

Ongoing work
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2nd production mode ttH channel

• signal
• t? bl?, t ? bqq, H ?inv.
• main backgrounds
• tt largest background
• ttZ, Z???
• ttW, W?l?
• bb Z/? with Z/? ? ll
• bbW, W?l?
• QCD Zincl. , Wincl.

• selection (rejection)
• only 1 lepton (bb Z/?)
• 2 b-jets (Zincl. and Wincl. )
• t ? jjb, mjjmW,mjjbmt
• large mT and ETmiss (tt)

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Transverse mass

• Use only quantities measured in the transverse
  plane to get transverse mass
• mT2 (? ETi)2 - (? pTi)2

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Effect of mT cut on tt background
ttH signal ?0.5 pb-1
tt bgnd ?490 pb-1
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tt background composition

• bgnd tt events
• 70 tt ? bl? b??
• signal ttH
• 88 ttH ?bl? bqq H

bl?b??
bl?bqq
must remove b?? events
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Use inter-jet separation Rjj
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Selected events for ttH for 10 fb-1 data
S/?B 2.0 (1.6)
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Last mode Associated production. Two possible
channels HZ and WZ
W or Z
q
W or Z
H
q
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Event selection for associate production

• WH channel
• large missing pT
• one prompt lepton
• large transverse mass
• Main backgrounds
• WZ l? ??
• W incl., W l?
• tt bb, b cl?

H ?0 ?0
WH
W l ?
l

?0
?

?0
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Associated vector boson WH
W incl.

• WH selection
• 1 lepton pTmiss gt 100 GeV
• large transverse mass mT
• main backgrounds
• WZ W?l?, Z???
• W inclusive
• tt, t ? bl?

large pTmiss ? off-shell Wincl.
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Z0H0? (ll- invisible) analysis strategy

• Trigger 1 or 2 prompt leptons
• Preselection
• large missing pT
• 2 leptons of opposite sign and same flavour
• loose cut on Z mass
• Final selection
• Use 10 discriminative variables in a
  multivariate analysis to extract signal

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Very large background is rejected after the
simple pre-selection cuts
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ATLAS reconstruction chain
MC generation
Simulation
Digitization
Fast reconstruction
Reconstruction
Create Analysis Object Data (AOD)
Analysis
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Solution add a filter
Filter goal emulate preselection cuts after MC
generation
MC generation
Filter
Simulation
Digitization
Fast reconstruction
Real data
Reconstruction
Create Analysis Object Data
Analysis
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Two handles in filter

• missing pT ? (invisible pT)
• Sum over invisible particles Higgs, neutrinos
  and particles falling outside the detector
  acceptance region, i.e.? gt 5.0
• Leptons from a Z
• Two leptons in the tracker acceptance region
• One or both leptons must pass the trigger
  requirements
• Two leptons of same flavor but opposite sign
• The reconstructed mass must be close to mZ

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Filter has looser cuts

• Filter cuts
• 2 leptons with ?lt 2.7
• Trigger requirements
• - 1 e with pT gt 18 GeV or
• - 2 e with pT gt 23 GeV or
• - 1 µ with pT gt 8 GeV or
• - 2 µ with pT gt 13 GeV
• missing pT gt 50 GeV
• mZ 25 GeV

• Preselection cuts
• 2 leptons with ?lt 2.5
• Trigger requirements
• - 1 e with pT gt 20 GeV or
• - 2 e with pT gt 25 GeV or
• - 1 µ with pT gt 10 GeV or
• - 2 µ with pT gt 15 GeV
• missing pT gt 90 GeV
• mZ 20 GeV

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Filter requirement 1 no event loss
Conclusion no significant event loss at the
filter level w.r.t. ATLAS fast simulation
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Filter requirement 2 large sample reduction
From 8360 CPU years down to 7 CPU years
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Cut flow on fast simulation after filter
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Cut flow on full reconstruction
Fast simulation reconstruction problems
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We need large MC sample to go further

• In ATLAS, all MC samples used for analyses must
  be approved
• MC samples produced centrally on the grid
• Our filter has been approved and our signal
  background samples will soon be centrally
  produced
• Next step multivariate analysis with fully
  reconstructed events (maximum likelihood, neural
  network or decision tree)

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Discriminating variables after preselection
(based on fast simulation)
pT lepton 1
pT lepton 2
Cos between leptons (2D)
Cos between leptons (3D)
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Discriminating variables after preselection
(based on fast simulation)
transverse mass
missing pT
Cos pTmiss-pT lepton 1
mZ
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Interpreting results w.r.t SM predictions

• ?2 ? ? x BR(H?invisible)
• ?SM x BRSM(H?bb)

?2 is a scaling factor, the ratio of the
production cross-section times the branching
fraction for Higgs to invisible of a certain
model to the prediction of the Standard Model
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Analysis potential _at_ 10 fb-1 S/?B 4.0 (2.6
with syst.)

• LHC luminosity
• 10 fb-1 expected in 2007
• 100 fb-1 per year the following years

Discovery potential Not much in 2007 ?2 0.5 in
2008 ?2 0.3 in 2010
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Summary

• LHC is due to start in 2007
• Last chance for exclusion or evidence at the
  Tevatron for mH lt 130 GeV
• LHC reach and luminosity should cover any
  possible Higgs decay mode
• Even being invisible wont be an excuse for the
  Higgs not to be seen!