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

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

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

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.

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

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

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.

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

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

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

Higgs Hunting: 1975 – 2010

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

Bounds on the SM Higgs Mass: 1976 and A. Linde, JETP Lett. 23 (1976) 64 Requiring 𝑉 𝑣 <𝑉 0 gives: 𝑚 𝐻 > 3 2 𝑚 𝑊 2 + 𝑚 𝑍 4 −4 𝑚 𝑙 4 −12 𝑚 𝑞 4 16 𝜋 2 1 2 >7 GeV

“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.

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

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

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

Searching at the Tevatron (≤ 2010) Operating at FNAL from 1985 - 2011 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

At the dawn of the LHC era

Discovery of a new boson at the LHC

Large Hadron Collider proton-proton collider inside the 27km LEP tunnel: Construction: 1998-2008 Operation: 2009 - 1232 superconducting dipole magnets with B > 8 Tesla World’s largest cryogenic plant 2011: 5fb-1 @ 7 TeV 2012: >10fb-1 @ 8 TeV

~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

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.

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

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)

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

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

Compact Muon Solenoid (CMS)

Searching for H → gg

~10 cm

Mass scale and resolution Calibrated at the Z pole

Diphoton invariant mass ATLAS CMS > 4s > 4s

“Compelling Evidence” Probability Interpretation “Evidence” “Compelling Evidence” “Observation”

Searching for H → ZZ → 4 leptons

Searching for H → WW → 2l2n > 2s > 2s

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

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

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

N. Arkani-Hamed (SavasFest 2012)

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

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)

Is it the (SM) Higgs boson?

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?

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.

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

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?

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

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

Global consistency?

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

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

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!

7 + 8 TeV: ~20fb-1