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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

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Electroweak physics lecture 6
Electroweak PhysicsLecture 6

  • Direct and Indirect Searches for the Higgs

What does on shell and off shell mean
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

  • IfQ²~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

  • Higgs in the lagrangian
    Higgs in the Lagrangian

    Higgs couples to every fermion in proportion to their mass

    Higgs couples to every fermion in proportion to their mass

    Higgs in the lagrangian1
    Higgs in the Lagrangian

    Higgs couples to WW and ZZ

    Higgs in the lagrangian2
    Higgs in the Lagrangian

    Four boson coupling:


    Higgs couples to itself

    Higgs production
    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

  • Lepii

    • 1996 to 2000: LEPII e+e− collisions at √s 161 to 209 GeV

    E e w w q q
    e+e− W+W−  − q q

    E e w w

    Higgs production at e e
    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%

    Higgs event at l3
    Higgs Event at L3

    • 2 jets and missing energy

    Higgs searches at lepii
    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

    Backgrounds 4 jets
    Backgrounds: 4 jets

    • Background from QCD, WW, ZZ

    • Problem: 4 jets give 3 possible mass combinations for mH

    • However, mass ambiguities remain

    Higgs significance
    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!

    Reconstructed higgs mass
    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

    Higgs production at the tevatron

    Gluon-gluon fusion

    Associated Production

    Diffractive production

    Higgs Production at the Tevatron

    Jets produced far forward in the detector

    Higgs at the tevatron
    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

    Search for the higgs at cdf
    Search for the Higgs at CDF

    • H→WW→ℓνℓν : 2 charged leptons and missing-ET

    No sign of a signal!

    Search for the higgs at d
    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!

    Can tevatron find the higgs
    Can Tevatron Find the Higgs?

    • Maybe!

      • Depends on Tevatron Luminosity

      • And what the mass of the Higgs is…

    Vector boson fusion qqh

    Higgs Decay products

    Forward tagging jets



    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 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) 014037


    CMS NOTE 2003/033

    Atlas cms discovery potential

    CERN/LHCC 99-15 ATLAS TDR 15

    ATLAS & CMS Discovery Potential

    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

    Higgs searches summary
    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?

    Indirect constraints on the higgs mass
    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

    The blue band plot explained
    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

    What if there is no higgs
    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…

    The wonderful world of the electroweak
    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


    Tevatron + LEPII

    From Tevatron