Electroweak Physics Lecture 6 Direct and Indirect Searches

Electroweak Physics Lecture 6 • Direct and Indirect Searches for the Higgs 1

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 2

Higgs in the Lagrangian Higgs couples to every fermion in proportion to their mass 3

Higgs in the Lagrangian Higgs couples to WW and ZZ 4

Higgs in the Lagrangian Four boson coupling: WWHH, ZZHH Higgs couples to itself 5

Higgs Decay Modes 6

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 7

LEPII • 1996 to 2000: LEPII e+e− collisions at √s 161 to 209 Ge. V 8

e+e−→W+W− 10

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) 50% 5% • 2 b-jets, missing energy 15% • 2 jets, τ+τ− 7% 11

4 -jet Aleph Higgs Event 12

Higgs Event at L 3 • 2 jets and missing energy 13

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, m. H test Q as a function for different m. H • L includes information about many properties of the event 14

Backgrounds: 4 jets • Background from QCD, WW, ZZ • Problem: 4 jets give 3 possible mass combinations for m. H • However, mass ambiguities remain 15

Higgs Significance • − 2 ln. Q>0 more likely to be background only • − 2 ln. Q<0 more likely to be background+signal • At m. H=115 Ge. V, more likely to be signal+background than just background • Hint of a Higgs signal right at the end of the kinematic limit! 16

The Higgs Candidate Events 17

Reconstructed Higgs Mass • But remember the mass doesn’t contain all the information! • No unambiguous measurement of a signal → set a limit • m. H> 114. 4 Ge. V/c² at 95% CL 18

• If at first you don’t succeed… 19

Higgs Production at the Tevatron Jets produced far forward in the detector Gluon-gluon fusion Associated Production Diffractive production 20

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 21

Search for the Higgs at CDF • H→WW→ℓνℓν : 2 charged leptons and missing-ET No sign of a signal! 22

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! 23

All Results from the Tevatron 24

Can Tevatron Find the Higgs? • Maybe! – Depends on Tevatron Luminosity – And what the mass of the Higgs is… 25

Higgs Production at LHC 26

Higgs Signals at LHC 27

Vector Boson Fusion: qq. H(→ ) Jet [VBF] Forward tagging jets Jet Higgs Decay products f h • 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 Phys. Rev. D 59(1999) 014037 ATL-PHYS-2003 -004 CMS NOTE 2003/033 – S/√B≥ 5 in m. H=120÷ 140 Ge. V/c 2 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 28

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 29

Higgs Searches Summary • No sure sign, yet. • Best limit is from LEPII: m. H>114. 4 Ge. V/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? 30

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 31

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 32

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 1 Te. V • 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… 33

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 34

Goodbye Old Friend 35

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