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

2. 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

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

4. Higgs in the Lagrangian
Higgs couples to WW and ZZ

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

6. Higgs Decay Modes

7. 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

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

9. e+e−  W+W−  − q q

10. e+e−→W+W−

11. 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) %
2 b-jets, missing energy 15%
2 jets, τ+τ− %

12. 4-jet Aleph Higgs Event

13. Higgs Event at L3
2 jets and missing energy

14. 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

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

16. 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!

17. The Higgs Candidate Events

18. Reconstructed Higgs Mass
But remember the mass doesn’t contain all the information!
No unambiguous measurement of a signal → set a limit
mH> GeV/c² at 95% CL

19. If at first you don’t succeed…

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

21. 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

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

23. 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!

24. All Results from the Tevatron

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

26. Higgs Production at LHC

27. Higgs Signals at LHC

28. 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)
ATL-PHYS
CMS NOTE 2003/033

29. ATLAS & CMS Discovery Potential
CERN/LHCC 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

30. 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?

31. 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

32. 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

33. 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…

34. 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

35. Goodbye Old Friend