1. That Infamous Boson: from Obscure Curiosity to Holy Grail
Higgs Hunting 2011
John Ellis
CERN & King’s College London

2. Outline The context The proposal The baptism
A phenomenological profile
The quest for the Holy Grail
When is a Higgs not a Higgs?
The LHC has shown there is new physics beyond Higgs

3. Life before Higgs Guage theories irrelevant?
Early steps towards quantization (Feynman 1963, deWitt, Faddeev & Popov 1967)
But require massless gauge bosons?
Spontaneous breaking of global symmetries (Nambu 1960, Goldstone 1961)
Later shown renormalizable (Lee, Symanzik 1969)
But require massless Goldstone boson?
Could two wrongs make a right?

4. The Curses of the Massless Bosons
Counter-example in theory of superconductivity (Anderson 1963)
Speculated that the Goldstone and Yang-Mills zero-mass problems could cancel (also Klein & Lee 1964)
But this was thought impossible in relativistic theory because no time-like vector (Gilbert 1964)

5. The Seminal Papers

6. The Englert-Brout-Higgs Mechanism
Vacuum expectation value of scalar field
Englert & Brout: June 26th 1964
First Higgs paper: July 27th 1964
Pointed out loophole in argument of Gilbert if gauge theory described in Coulomb gauge
Accepted by Physics Letters
Second Higgs paper with explicit example sent on July 31st 1964 to Physics Letters, rejected!
Revised version (Aug. 31st 1964) accepted by PRL
Guralnik, Hagen & Kibble (Oct. 12th 1964)

7. The Englert-Brout-Higgs Mechanism
Guralnik, Hagen & Kibble

8. The Higgs boson Higgs pointed out a massive scalar boson
“… an essential feature of [this] type of theory … is the prediction of incomplete multiplets of vector and scalar bosons”
Englert, Brout, Guralnik, Hagen & Kibble did not comment on its existence

9. Spontaneous breaking of symmetry
Nambu, EB, GHK and Higgs
Spontaneous breaking of symmetry

10. Consecration & Baptism
Incorporation in electroweak theory (Weinberg 1967, Salam 1968)
Renormalizability breakthrough (‘t Hooft 1970, 1971)
Comprehensive & influential review by Ben Lee (Batavia Conference, 1972)
Refers repeatedly to ‘Higgs’ fields, scalars
Ben Lee the Baptist

11. Ben Lee the Baptist

12. Early Phenomenological Bounds
Emission from stars:
MH > 0.7 me (Sato & Sato, 1975)
Neutron-electron scattering:
MH > 0.7 MeV (Rafelski, Muller, Soff & Greiner; Watson & Sundaresan; Adler, Dashen & Treiman; 1974)
Neutron-nucleus scattering:
MH > 13 MeV (Barbieri & Ericson, 1975)
Nuclear 0+ – 0+ transitions:
MH > 18 MeV (Kohler, Watson & Becker, 1974)

13. A Phenomenological Profile of the Higgs Boson
Neutral currents (1973)
Charm (1974)
Heavy lepton τ (1975)
Much attention to search for W±, Z0
For us, the Big Issue: is there a Higgs boson?
Previously ~ 10 papers on Higgs bosons
MH > 18 MeV
First attempt at systematic survey

14. A Phenomenological Profile of the Higgs Boson
Higgs decay modes and searches in 1975: 14

15. Higgs Boson on the Experimental Agenda
Searches at LEP:
(EG, Yellow report 76-18)
e+e-  Z + H
(EGN 76, Ioffe & Khoze 78,
Lee, Quigg & Thacker 77)
Z  H + μ+μ-
(EG 76, Bjorken 1978)
MH > GeV

16. Higgs Boson on the LEP Agenda
e+e-  Z + H
Z  H + μ+μ-
JE & Gaillard, 1976

17. Higgs Boson on the LEP Agenda

18. Higgs Boson on the pp Agenda_
Higgs Boson on the pp Agenda
gg  H
W±/Z0 + H
Georgi, Glashow, Machacek & Nanopoulos, 1978
Glashow, Nanopoulos & Yildiz, 1978

19. Higgs Boson on the SSC Agenda

20. Higgs Boson on the LHC Agenda

21. Higgs Hunting goes Mainstream

22. Estimating the Mass of the Higgs
Electroweak radiative corrections are sensitive to massive particles:
Sensitivity to the top quark mass >> sensitivity to Higgs mass:
But LEP measurements gave an indication for a light Higgs even before the top discovery
JE, Fogli & Lisi, 1990/1

23. Estimating the Mass of the Higgs
First attempts in 1990, 1991:
Very difficult before the discovery of the top
JE, Fogli & Lisi, 1990/1

24. Estimating the Mass of the Higgs
After the discovery of the top:
Solid indications of a light Higgs
JE, Fogli & Lisi, 1994/5

25. Precision Tests of the Standard Model
Lepton couplings
Pulls in global fit

26. The State of the Higgs: Pre-EPS
High-energy search:
Limit from LEP:
mH > GeV
High-precision electroweak data:
Sensitive to Higgs mass:
mH = 96+30–24 GeV
Combined upper limit:
mH < 157 GeV, or 186 GeV including direct limit
Exclusion from high-energy search at Tevatron:
mH < 158 GeV or > 173 GeV

27. Higgs Search @ Tevatron
Tevatron excludes Higgs between 158 & 173 GeV 27

28. First Higgs Searches @ LHC
No exclusion yet, but significant contribution to global fit 28

29. Information from Direct Higgs Searches
Gfitter collaboration

30. Combining the Information from Direct Searches and Indirect Data
mH = GeV
Gfitter collaboration

31. The Stakes in the Higgs Search
How is particle symmetry broken?
Is there an elementary scalar field?
What is the fate of the Standard Model?
Did mass appear when the Universe was a picosecond old?
Did Higgs help create the matter in the Universe?
Did a related inflaton make the Universe so big and old?
Why is there so little dark energy?

32. Theoretical Constraints on Higgs Mass
Large → large self-coupling → blow up at low energy scale Λ due to
renormalization
Small: renormalization
due to t quark drives
quartic coupling < 0
at some scale Λ
→ vacuum unstable
Bounds on Higgs mass depend on Λ

33. What is the probable fate of the SM?
Confidence Levels (CL) without/with Tevatron exclusion
Confidence Levels (CL) for different fates
Blow-up excluded
at 96% CL
CL as
function of
instability
scale 
Espinosa, JE, Giudice, Hoecker, Riotto, arXiv 33

34. The LHC will Tell the Fate of the SM
Examples with LHC measurement of mH = 120 or 115 GeV
Espinosa, JE, Giudice, Hoecker, Riotto

35. How to Stabilize a Light Higgs Boson?
Top quark destabilizes potential: introduce introduce stop-like scalar:
Can delay collapse of potential:
But new coupling must be
fine-tuned to avoid blow-up:
Stabilize with new fermions:
just like Higgsinos
Very like Supersymmetry!

36. Supersymmetry Search at LHC
95% CL region
68% CL region
MasterCode collaboration:
χ2 = 30.2, fit probability = 11.6% 36

37. CMSSM HiggsMass Full LHC 2010 data CMSSM
O.Buchmueller, JE et al: in preparation

38. Latest Higgs Searches @ Tevatron
Exclude (100,109); (156,177) GeV 38

39. Latest Higgs Searches @ LHC
Exclude (149,206); (300,440)
Exclude (155,190); (295,450)
Excess of (120, 140) GeV? 39

40. Blogger’s LHC Higgs Combination
“Nonsense” (Bill Murray)
Is the much-discussed (130, 140) large enough to be likely to be a Standard Model Higgs?
Watch out for an ~ 120 GeV!

41. Plots we would like to see?
p0 for bkgrd only
CLs for signal
What would Higgs
signal look like?
p0 expected if
also signal
Best fit + bands for σ/σSM 41

42. What if the Higgs is not a Higgs?
Tree-level Higgs couplings ~ masses
Coefficient ~ 1/v
Couplings ~ dilaton of scale invariance
Broken by Higgs mass term –μ2, anomalies
Cannot remove μ2 (Coleman-Weinberg)
Anomalies give couplings to γγ, gg
Generalize to pseudo-dilaton of new (nearly) conformal strongly-interacting sector
Couplings ~ m/V (V > v?), additions to anomalies

43. A Phenomenological Profile of a Pseudo-Dilaton
New strongly-interacting sector at scale ~ V
Pseudo-dilaton only particle with mass << V
Universal suppression of couplings to Standard Model particles ~ v/V
Possible enhancement of coupling to gg
Possible suppression of coupling to γγ
Modified self-couplings: effective potential
~ χ4 [ln(χ/V) – ¼] + anomalous dimensions
Pseudo-baryons as dark matter?

44. There must be New Physics beyond the Higgs Boson
potential
collapses
Higgs
coupling
blows up
Higgs coupling less
than in Standard Model

45. Conversation with Mrs Thatcher: 1982
What do you do?
Wouldn’t it be better if they
found what
you predicted?
Think of things for the experiments to look for, and hope they find something different
Then we would not learn anything! 45

46. Supersymmetry? Would unify matter particles and force particles
Related particles spinning at different rates
½ /
Higgs - Electron - Photon - Gravitino – Graviton
Many phenomenological motivations
Would help fix particle masses
Would help unify forces
Predicts light Higgs boson
Could fix discrepancy in gμ - 2
Could provide dark matter for the
astrophysicists and cosmologists

47. Imagine a Room …
What lies Beyond?
… Open
The Door

48. What lies Beyond?