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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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