Is it the Higgs Boson?

Jim OlsenPrinceton University

Princeton Plasma Physics Laboratory ColloquiumSeptember 26, 2012

Is it the Higgs Boson?

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

Pierre Pirou

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

CMS and ATLAS submitted their papers to Physics Letters on July 31.As of today, each paper has been cited 128 times:ImpactDark MatterSupersymmetryMuon g-2Vacuum stabilityExotic BSM PhysicsNeutrino PhysicsThe 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. OutlineInvention and early Higgs huntingDiscovery of a new boson at the LHCIs it the Higgs boson?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 Whats the problem?Applying local U(1) invariance, , to the Dirac Lagrangian:

Free fermionsGauge interactionFree gauge bosonsThe 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.The Higgs* Mechanism* actually, the Nambu-Goldstone-Anderson-Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism

vev

Expand around minimum:

Gauge invariance:

Gauge boson mass!

Mass of a real scalar particle: Higgs boson!

Properties of the SM Higgs bosonMass:

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 condensateTechnicolor: new gauge interactionsExtra dimensionsCritical to determine if the new particle is the SM Higgs bosonHiggs Hunting: 1975 2010

Higgs Phenomenology: 1975Ellis, Gaillard, and Nanopoulos, Nucl. Phys. B106, 292125 GeV

Bounds on the SM Higgs Mass: 1976

and A. Linde, JETP Lett. 23 (1976) 64

Discovery of the Zeta(8.3) with the Crystal Ball detector g + XWas not confirmed in later runs.1984The Role of the Top Quark

The top quark was discovered at Fermilab in 1995 with a mass near 173 GeV, clearly indicating its 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

Searching at LEP

Operating at CERN from 1989 - 2000Line shape of the Z0 boson (number of light ns)Search for the Higgs bosonLEP Legacy

Search strategy:Produced via Higgs-strahlung

Operating at FNAL from 1985 - 2011Discovery of the top quarkSearch for the Higgs boson

At the dawn of the LHC era

Discovery of a new boson at the LHCLarge Hadron Collider

proton-proton collider inside the 27km LEP tunnel: Construction: 1998-2008 Operation: 2009 - 1232 superconducting dipole magnets with B > 8 TeslaWorlds largest cryogenic plant2011: 5fb-1 @ 7 TeV2012: >10fb-1 @ 8 TeVATLAS and CMS

Humans~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 SMStandard Model @ CMS

Top Cross SectionsHiggs Boson Production at the LHC

Gluon FusionVector-Boson FusionHiggs-strahlung

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. = 1-2%(gg, ZZ)

How does it Decay (mH = 125 GeV) ?

Branching Fractions (%)

Cross sections are largeFermion decays (bb+tt) are accessibleNatural width is negligibleOnly region in mH whereLHC Searches

CMS Discovery PotentialSensitivityDetector ResolutionWWggZZttbb1%10%20%Compact Muon Solenoid (CMS)Searching for H gg

~10 cmMass scale and resolution

Calibrated at the Z pole

Diphoton invariant mass

ATLASCMS> 4s> 4s

ProbabilityInterpretation

EvidenceCompelling Evidence

Observation

Searching for H ZZ 4 leptons

> 3s> 3sSearching for H WW 2l2n

> 2s> 2sAdding* gg, ZZ, and WW (4+3+2=5)

*ASSUMING it is the SM Higgs!> 5s> 5s

Do CMS and ATLAS agree on the mass?

As a layman, I think we have it. But as a scientist, I have to say, `What do we have? R. Heuer

AP photo

N. Arkani-Hamed (SavasFest 2012)Impact of a 125 GeV Higgs boson

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

Vacuum StabilityThe 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?Are the relative couplings consistent with prediction?Is it spin 0 or 2? Is it a scalar or a pseudoscalar?Does it couple to fermions?Does it decay to exotic final states?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?

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

What to watch for in November:Is the SM Higgs boson excluded in tt?Is there growing or shrinking evidence in bb?CMS has better sensitivity for H bb than any other experimentPattern 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?

Global consistency?

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 3sDoes it decay to exotic final states?Branching Fractions (%)

Until the bb channel is seen, constraints on the full width will be weak.Is it the SM Higgs boson?Are the relative couplings consistent with prediction?Is it a spin 0 scalar?Does it couple to fermions?Does it decay to exotic final states?Maybe!Maybe!Maybe!Maybe!Would I buy SM Higgs boson stock? Probably!

7 + 8 TeV: ~20fb-1