5 Searching for the Higgs boson took: Fifty years,thousands of people,and billions of dollars.Why all the fuss?
6 ImpactCMS and ATLAS submitted their papers to Physics Letters on July 31.As of today, each paper has been cited 128 times:SupersymmetryExotic BSM PhysicsNeutrino PhysicsDark MatterMuon g-2Vacuum 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 LHCIs it the Higgs boson?
8 One field to rule them all… Standard ModelMatter: quarks and leptonsSymmetries: U(1)Y, SU(2)L, SU(3)CLocal 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 fermionsGauge interactionFree gauge bosonsThe term is not gauge invariant → need massless gauge bosonsU(1)EM: photon is the gauge boson → electromagnetic interactionsSU(2)L: W+, W-, Z0 are the gauge bosons → weak interactionsSU(3)C: gluons (8 of them) are the gauge bosons → strong interactionsWorks 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* MechanismIntroduce a complex scalar field f and a massless gauge boson 𝐴 𝜇 :“vev”Gauge invariance:real scalar field 𝐻Expand 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 parameterInteraction: couples to particles according to their mass
12 Is this the only possibility? NO! Additional Higgs fields fiComposite Higgs: top-quark condensateTechnicolor: new gauge interactionsExtra dimensionsCritical to determine if the new particle is the SM Higgs boson
15 Bounds on the SM Higgs Mass: 1976 and A. Linde, JETP Lett. 23 (1976) 64Requiring 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 + XWas 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 fieldThis result ushered in the modern era of Higgs searches at LEP and FNALCabibbo, 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 GeVLine 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-strahlungDecaying to 𝑏 𝑏 or 𝜏 𝜏𝒎 𝑯 >𝟏𝟏𝟒.𝟒 GeV
20 Searching at the Tevatron (≤ 2010) Operating at FNAL from𝑝 𝑝 collider with 𝑠 up to 1.96 TeVDiscovery of the top quarkMeasurements of 𝑚 𝑡 and 𝑚 𝑊Search for the Higgs boson𝒎 𝑯 <𝟏𝟓𝟖 𝐨𝐫>𝟏𝟕𝟓 𝐆𝐞𝐕
23 Large Hadron Colliderproton-proton collider inside the 27km LEP tunnel: Construction: Operation:1232 superconducting dipole magnets with B > 8 TeslaWorld’s largest cryogenic plant2011: 7 TeV2012: 8 TeV
24 ~3000 scientist, engineers, and students working on each experiment ATLAS and CMS~3000 scientist, engineers, and students working on each experimentGiant multipurpose particle detectors designed to find or exclude the Higgs boson and signs of physics beyond the SMHumans
25 Standard Model @ CMS Top Cross Sections 𝑠 =7 TeVTop 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 FusionVector-Boson FusionHiggs-strahlungTop 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 massNarrow!Observed width dominated by detector resolution@High massHiggs becomes a broad resonance dominated by natural widthTheory input is criticalDet. 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 largeFermion decays (bb+tt) are accessibleNatural width is negligibleOnly region in mH where
45 Where do we stand?Observation in CMS and ATLAS of a new boson with a mass of roughly 125 GeV decaying to vector bosonsIt 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 experimentWhat to watch for in November:Is the SM Higgs boson excluded in tt?Is there growing or shrinking evidence in bb?
49 Pattern of couplingsOverall, 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
52 Does it have the right spin/parity? So far, not enough data to determine spin or parityDifficult 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!