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 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 Decay Modes

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−

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%

4-jet Aleph Higgs Event

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!

The Higgs Candidate Events

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

If at first you don’t succeed…

Higgs Production at the Tevatron Gluon-gluon fusion Associated Production Diffractive production 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!

All Results from the Tevatron

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

Higgs Production at LHC

Higgs Signals at LHC

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) 014037 ATL-PHYS-2003-004 CMS NOTE 2003/033

ATLAS & CMS Discovery Potential CERN/LHCC 99-15 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

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 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

Goodbye Old Friend