1. Princeton Plasma Physics Laboratory Colloquium
Is it the Higgs Boson?
Jim Olsen
Princeton University
Princeton Plasma Physics Laboratory Colloquium
September 26, 2012

2. Many Princeton undergraduate students have worked on the CMS experiment over the years
Pierre Piroué

5. Searching for the Higgs boson took:
Fifty years,
thousands of people,
and billions of dollars.
Why all the fuss?

6. Impact
CMS and ATLAS submitted their papers to Physics Letters on July 31.
As of today, each paper has been cited 128 times:
Supersymmetry
Exotic BSM Physics
Neutrino Physics
Dark Matter
Muon g-2
Vacuum stability …
The precise nature of this new particle touches on all of these topics, and more. The LHC (and future iterations) may not be sufficient to answer all questions.

7. Outline Invention and early Higgs hunting
Discovery of a new boson at the LHC
Is it the Higgs boson?

8. One field to rule them all…
Standard Model
Matter: quarks and leptons
Symmetries: U(1)Y, SU(2)L, SU(3)C
Local gauge invariance: gauge bosons (force carriers)
Higgs field: spontaneous symmetry breaking and the Higgs boson

9. What’s the problem?
Applying local U(1) invariance, , to the Dirac Lagrangian:
Free fermions
Gauge interaction
Free gauge bosons
The term is not gauge invariant → need massless gauge bosons
U(1)EM: photon is the gauge boson → electromagnetic interactions
SU(2)L: W+, W-, Z0 are the gauge bosons → weak interactions
SU(3)C: gluons (8 of them) are the gauge bosons → strong interactions
Works for the EM and strong interactions, but W and Z bosons are massive (~100 GeV). Need a mechanism to give mass to gauge bosons.

10. Introduce a complex scalar field f and a massless gauge boson 𝐴 𝜇 :
The Higgs* Mechanism
Introduce a complex scalar field f and a massless gauge boson 𝐴 𝜇 :
“vev”
Gauge invariance:
real scalar field 𝐻
Expand around minimum:
Gauge boson mass!
Mass of a real scalar particle: Higgs boson!
* actually, the Nambu-Goldstone-Anderson-Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism

11. Properties of “the SM Higgs boson”
A single elementary scalar particle ( 𝐽 𝑃 = 0 + ) that gives mass (via the Higgs field) to the gauge bosons and the fermions (quarks and leptons)
Mass:
Because l is not predicted, the Higgs boson mass is a free parameter
Interaction: couples to particles according to their mass

12. Is this the only possibility? NO!
Additional Higgs fields fi
Composite Higgs: top-quark condensate
Technicolor: new gauge interactions
Extra dimensions
Critical to determine if the new particle is the SM Higgs boson

13. Higgs Hunting: 1975 – 2010

14. Higgs Phenomenology: 1975 125 GeV
Ellis, Gaillard, and Nanopoulos, Nucl. Phys. B106, 292
125 GeV

15. Bounds on the SM Higgs Mass: 1976
and A. Linde, JETP Lett. 23 (1976) 64
Requiring 𝑉 𝑣 <𝑉 0 gives:
𝑚 𝐻 > 𝑚 𝑊 2 + 𝑚 𝑍 4 −4 𝑚 𝑙 4 −12 𝑚 𝑞 𝜋 >7 GeV

16. “Discovery” of the Zeta(8.3) with the Crystal Ball detector
1984
“Discovery” of the Zeta(8.3) with the Crystal Ball detector
 → g + X
Was not confirmed in later runs.

17. The Role of the Top Quark
The SM Higgs boson couples to fermions according to their mass, 𝑚 𝑓 :
The top quark was discovered at Fermilab in 1995 with a mass near 173 GeV, clearly indicating it’s strong coupling to the Higgs field
This result ushered in the modern era of Higgs searches at LEP and FNAL
Cabibbo, Maiani, Parisi, Petronzio, Nucl. Phys. B158 (1979) 295

18. Searching at LEP Operating at CERN from 1989 - 2000
Electron-positron collider with 𝑠 up to 209 GeV
Line shape of the Z0 boson (number of light ns)
Precision electroweak measurements ( 𝑚 𝑊 )
Search for the Higgs boson

19. LEP Legacy Search strategy: 𝒎 𝑯 >𝟏𝟏𝟒.𝟒 GeV
Produced via Higgs-strahlung
Decaying to 𝑏 𝑏 or 𝜏 𝜏
𝒎 𝑯 >𝟏𝟏𝟒.𝟒 GeV

20. Searching at the Tevatron (≤ 2010)
Operating at FNAL from
𝑝 𝑝 collider with 𝑠 up to 1.96 TeV
Discovery of the top quark
Measurements of 𝑚 𝑡 and 𝑚 𝑊
Search for the Higgs boson
𝒎 𝑯 <𝟏𝟓𝟖 𝐨𝐫>𝟏𝟕𝟓 𝐆𝐞𝐕

21. At the dawn of the LHC era

22. Discovery of a new boson at the LHC

23. Large Hadron Collider
proton-proton collider inside the 27km LEP tunnel: Construction: Operation:
1232 superconducting dipole magnets with B > 8 Tesla
World’s largest cryogenic plant
2011: 7 TeV
2012: 8 TeV

24. ~3000 scientist, engineers, and students working on each experiment
ATLAS and CMS
~3000 scientist, engineers, and students working on each experiment
Giant multipurpose particle detectors designed to find or exclude the Higgs boson and signs of physics beyond the SM
Humans

25. Standard Model @ CMS Top Cross Sections
𝑠 =7 TeV
Top Cross Sections
~𝟕𝟎 papers published or in preparation on SM physics at 7 and 8 TeV. No deviations from predictions have been observed.

26. Higgs Boson Production at the LHC
Gluon Fusion
Vector-Boson Fusion
Higgs-strahlung
Top Fusion (t t H)
LHC in 2012, at record luminosity (7 x 1033 cm-2s-1) and energy (8 TeV), is now producing SM Higgs bosons (MH = 125 GeV) at a rate ~𝟕𝟓𝟎/hr

27. What does a Higgs boson look like?
@Low mass
Narrow!
Observed width dominated by detector resolution
@High mass
Higgs becomes a broad resonance dominated by natural width
Theory input is critical
Det. Res. = 10-20%
(b b , tt, WW)
Det. Res. = 1-2%
(gg, ZZ)

28. How does it Decay (mH = 125 GeV) ?
Branching Fractions (%)
Cross sections are large
Fermion decays (bb+tt) are accessible
Natural width is negligible
Only region in mH where

29. CMS Discovery Potential
LHC Searches
CMS Discovery Potential tt WW
20% bb
10%
Detector Resolution gg 1% ZZ
Sensitivity

30. Compact Muon Solenoid (CMS)

31. Searching for H → gg

32. ~10 cm

33. Mass scale and resolution
Calibrated at the Z pole

34. Diphoton invariant mass
ATLAS
CMS
> 4s
> 4s

35. “Compelling Evidence”
Probability
Interpretation
“Evidence”
“Compelling Evidence”
“Observation”

36. Searching for H → ZZ → 4 leptons

37. Searching for H → WW → 2l2n
> 2s
> 2s

38. Adding* gg, ZZ, and WW (4+3+2=5)
> 5s
> 5s
*ASSUMING it is the SM Higgs!

39. Do CMS and ATLAS agree on the mass?
M CMS =125.3±0.4±0.5 GeV
M ATLAS =126.0±0.4±0.4 GeV

40. AP photo
“As a layman, I think we have it. But as a scientist, I have to say, `What do we have?’” – R. Heuer

41. N. Arkani-Hamed (SavasFest 2012)

42. Impact of a 125 GeV Higgs boson
Giudice and Strumia, Nucl. Phys. B858 (2012) 63

43. Impact of a 125 GeV Higgs boson
Vacuum Stability
“The vacuum is unstable but sufficiently long-lived, compared to the age of the universe.”
G. Isidori (Higgs Hunting 2012)

44. Is it the (SM) Higgs boson?

45. Where do we stand?
Observation in CMS and ATLAS of a new boson with a mass of roughly 125 GeV decaying to vector bosons
It is certainly looking and walking like the SM Higgs boson. Does it also quack like the SM Higgs boson? Some questions:
Does it couple to fermions?
Are the relative couplings consistent with prediction?
Is it spin 0 or 2? Is it a scalar or a pseudoscalar?
Does it decay to exotic final states?

46. Does it couple to fermions?
In the context of the SM Higgs boson phenomenology, we already have strong indirect evidence for a coupling to the top quark via the loop in the dominant production mechanism.

47. Final results from the Tevatron
2.5s (Global)
2.9s (bb)
Is the Tevatron seeing H → bb?

48. Search for tt and bb at CMS
CMS has better sensitivity for H → bb than any other experiment
What to watch for in November:
Is the SM Higgs boson excluded in tt?
Is there growing or shrinking evidence in bb?

49. Pattern of couplings
Overall, consistent with the SM expectation, but far from excluding other possibilities (and hint of something in gg)

50. Is “g” the same for W and Z?
𝑹 𝑾/𝒁 = 𝟎.𝟗 −𝟎.𝟔 +𝟏.𝟏

51. Global consistency?

52. Does it have the right spin/parity?
So far, not enough data to determine spin or parity
Difficult to separate 0 and 2, easier to check + vs. -
With the data expected by end of the year, maybe 3s

53. Does it decay to exotic final states?
Branching Fractions (%)
Until the bb channel is seen, constraints on the full width will be weak.

54. Would I buy SM Higgs boson stock? Probably!
Is it the SM Higgs boson?
Does it couple to fermions?
Maybe!
Are the relative couplings consistent with prediction?
Is it a spin 0 scalar?
Does it decay to exotic final states?
Would I buy SM Higgs boson stock? Probably!

55. 7 + 8 TeV: ~20fb-1