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Electroweak Physics Lecture 6. Direct and Indirect Searches for the Higgs. What Does On-Shell and Off-Shell Mean?. Q is the four-momentum of the boson Momentum transferred between the interacting fermions Q² = M²+ p·p If Q²~M², the boson is said to be on shell - PowerPoint PPT Presentation
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Electroweak PhysicsLecture 6
• Direct and Indirect Searches for the Higgs
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What Does On-Shell and Off-Shell Mean?
• Q is the four-momentum of the boson• Momentum transferred between the interacting fermions
• Q² = M²+p·p• If Q²~M², the boson is said to be on shell• If Q²<M² or Q²>M², the boson is said to be off shell
• If something is off shell we often say it is virtual• The more off-shell, the more the virtuality of the
boson
• This effect is only possible because of the Heisenberg Uncertainty Principle:
• ΔmΔt ≤2π
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Higgs in the Lagrangian
Higgs couples to every fermion in proportion to
their mass
Higgs couples to every fermion in proportion to
their mass
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Higgs in the Lagrangian
Higgs couples to WW and ZZ
2 Wgm
Zgm
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Higgs in the Lagrangian
Four boson coupling:WWHH, ZZHH
Higgs couples to itself
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Higgs Decay Modes
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Higgs Production
• Higgs production requires high energy →→ colliders
• We’ll discuss Higgs production and signal at:• LEPII• Tevatron• LHC
• Finally, indirect information on the Higgs
• Always talk about the SM Higgs, no BSM Higgs
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LEPII• 1996 to 2000: LEPII e+e− collisions at √s 161 to
209 GeV
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e+e− W+W− − q q
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e+e−→W+W−
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Higgs Production at e+e−• Higgs production at LEPII was mainly through the
Higgstralung process– An off-shell Z boson radiates a Higgs
• The maximum Higgs mass that can be produced is √s−MZ
• Higgs decays to two b-quarks: H→bb or H→τ+τ−
Signal:
• 4 jets (2 b, 2 others) 50%
• 2 b-jets, 2 (e,μ) 5%
• 2 b-jets, missing energy 15%
• 2 jets, τ+τ− 7%
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4-jet Aleph Higgs Event
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Higgs Event at L3
• 2 jets and missing energy
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Higgs Searches at LEPII
• Use most powerful method to separate signal & background– Lb: likelihood events are due to backgrounds
– Ls+b: likelihood event are due to background + Higgs signal with a given mass, mH
• L includes information about many properties of the event
test Q as a function for different mH
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• Background from QCD, WW, ZZ• Problem: 4 jets give 3 possible mass combinations
for mH
• However, mass ambiguities remain
Backgrounds: 4 jets
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Higgs Significance
• −2lnQ>0 more likely to be background only
• −2lnQ<0 more likely to be background+signal
• At mH=115 GeV, more likely to be signal+background than just background
• Hint of a Higgs signal right at the end of the kinematic limit!
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The Higgs Candidate Events
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Reconstructed Higgs Mass
• But remember the mass doesn’t contain all the information!• No unambiguous measurement of a signal → set a limit
• mH> 114.4 GeV/c² at 95% CL
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• If at first you don’t succeed…
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Higgs Production at the Tevatron
Gluon-gluon fusion
Associated Production
Diffractive production
Jets produced far
forward in the detector
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Higgs at the Tevatron• Three main search channels:
– Single Higgs production decays as: H→WW*– Associated Higgs production and H→bb or H→WW*
• No searches for diffractive Higgs (yet)– Would require far forward detectors to find the jets
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Search for the Higgs at CDF• H→WW→ℓνℓν :
2 charged leptons and missing-ET
No sign of a signal!
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Search for the Higgs at DØ
• HW→bbℓν – 2 tagged b-jets– 1 charged lepton
– Missing ET
• Try to fit data to different Higgs masses
No sign of a signal!
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All Results from the Tevatron
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Can Tevatron Find the Higgs?• Maybe!
– Depends on Tevatron Luminosity– And what the mass of the Higgs is…
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Higgs Production at LHC
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Higgs Signals at LHC
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Jet
Vector Boson Fusion: 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 →ℓνℓ’ν’, ℓν+jet final states– S/√B≥5 in mH=120÷140 GeV/c2 range with 40 fb-1
» S/√B≈2.5 in one LHC year» this process offers the possibility for a direct
measurement of Yukawa coupling HPhys. Rev. D59(1999) 014037
ATL-PHYS-2003-004
CMS NOTE 2003/033
Jet
Higgs Decay
products
Forward tagging
jets
[VBF]
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ATLAS & CMS Discovery Potential
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
CERN/LHCC 99-15 ATLAS TDR 15
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Higgs Searches Summary
• No sure sign, yet.
• Best limit is from LEPII: mH>114.4 GeV/c²
• Tevatron has some hope of finding a light Higgs
• If we believe in the Standard Model, LHC will find the Higgs
• What do we already know about the Higgs?
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Indirect Constraints on the Higgs Mass
• Almost every EWK variable we’ve talked about depends on the top quark mass, and the Higgs-mass:
• A, α, β different for different processes• But the functional dependence is the same
2top ln HC m m
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The Blue Band Plot Explained!
• Constraints from all the EWK measurements…
• Minimum value of the χ² is the best value for the Higgs in the SM
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What if there is no Higgs?
• Without new physics (including Higgs), the cross section of the WL WL→ WL WL violates unitarity when Q² exceeds about 1TeV
• Unitarity means the probability for the event happen is less than one
• So what ever might exist will appear eventually in the WL WL→ WL WL channel…
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The Wonderful World of the Electroweak
Extracted from σ(e+e−→ff)
Afb (e+e−→ℓℓ)
AL
R
τ polarisation asymmetry
b and c quark final states
From Tevatron
Tevatron + LEPII
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Goodbye Old Friend
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