Electroweak Physics Lecture 6

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# Electroweak Physics Lecture 6 - PowerPoint PPT Presentation

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 PhysicsLecture 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
• 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 couples to every fermion in proportion to their mass

Higgs couples to every fermion in proportion to their mass

Higgs in the Lagrangian

Higgs couples to WW and ZZ

Higgs in the Lagrangian

Four boson coupling:

WWHH, ZZHH

Higgs couples to itself

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
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
• 2 jets and missing energy
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
• Background from QCD, WW, ZZ
• Problem: 4 jets give 3 possible mass combinations for mH
• However, mass ambiguities remain
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
• 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

Gluon-gluon fusion

Associated Production

Diffractive production

Higgs Production at the Tevatron

Jets produced far forward in the detector

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

No sign of a signal!

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?
• Maybe!
• Depends on Tevatron Luminosity
• And what the mass of the Higgs is…

Higgs Decay products

Forward tagging jets

f

h

Vector Boson Fusion: qqH(→)

[VBF]

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

Jet

Phys. Rev. D59(1999) 014037

ATL-PHYS-2003-004

CMS NOTE 2003/033

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
• 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
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!
• 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?
• 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

Extracted from σ(e+e−→ff)

Afb (e+e−→ℓℓ)

τ polarisation asymmetry

b and c quark final states

ALR

Tevatron + LEPII

From Tevatron