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Electroweak Physics Lecture 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
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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) % 2 b-jets, 2 (e,μ) % 2 b-jets, missing energy 15% 2 jets, τ+τ− %
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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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Backgrounds: 4 jets Background from QCD, WW, ZZ
Problem: 4 jets give 3 possible mass combinations for mH However, mass ambiguities remain
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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> 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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Vector Boson Fusion: qqH(→)
[VBF] Jet f h Higgs Decay products Forward tagging jets Jet hadronic jets in forward-backward regions the forward jet tagging is a powerful background rejection tool hadronic activity suppressed in low η 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 H Phys. Rev. D59(1999) ATL-PHYS CMS NOTE 2003/033
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ATLAS & CMS Discovery Potential
CERN/LHCC ATLAS TDR 15 After detector calibration and LHC pilot run… …almost all the “allowed” mass range can be explored during the first year (10 fb-1) ...after 2 years (≈30 fb-1) 7σ significance over the whole mass spectrum, covered by more than one channel
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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
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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) τ polarisation asymmetry b and c quark final states ALR Tevatron + LEPII From Tevatron
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Goodbye Old Friend
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