Higgs, Less Higgs, Higgsless or more Higgs?

ITEP Winter SchoolOtradoe, February 2012

John Ellis, King’s College London & CERN

Plan of the Lectures

• The Higgs boson in the Standard Model

• Less Higgs or Higgsless?

• Higgs and Supersymmetry

Summary of the Standard Model

• Particles and SU(3) × SU(2) × U(1) quantum numbers:

• Lagrangian: gauge interactions

matter fermions

Yukawa interactions

Higgs potential

Status of the Standard Model

• Perfect agreement with all confirmed accelerator data

• Consistency with precision electroweak data (LEP et al) only if there is a Higgs boson

• Agreement seems to require a relatively light Higgs boson weighing < ~ 180 GeV

• Raises many unanswered questions:

mass? flavour? unification?

Precision Tests of the Standard Model

Lepton couplings Pulls in global fit

Open Questions beyond the Standard Model

• What is the origin of particle masses?due to a Higgs boson? + other physics?solution at energy < 1 TeV (1000 GeV)

• Why so many types of matter particles?matter-antimatter difference?

• Unification of the fundamental forces?at very high energy ~ 1016 GeV?probe directly via neutrino physics, indirectly via masses, couplings

• Quantum theory of gravity?(super)string theory: extra space-time dimensions?

Susy

Susy

Susy

•Why do Things Weigh?

•0

•Where •do •the •masses •come from•?

•Newton:• Weight proportional to Mass

•Einstein:• Energy related to Mass

•Neither explained origin of Mass

•Are masses due to Higgs boson?

• (the physicists’ Holy Grail)

Think of a Snowfield

Skier moves fast:

Like particle without mass

e.g., photon = particle of light

Snowshoer sinks into snow,

moves slower:

Like particle with mass

e.g., electron

Hiker sinks deep,

moves very slowly:

Particle with large mass

The LHC looks for

the snowflake:

the Higgs Boson

The Higgs Mechanism

• Postulated effective Higgs potential:

• Minimum energy at non-zero value:

• Non-zero masses:

• Components of Higgs field:

• π massless, σ massive:

Masses for Gauge Bosons

• Kinetic terms for SU(2) and U(1) gauge bosons:

where

• Kinetic term for Higgs field:

• Expanding around vacuum:

• Boson masses:

The Seminal Papers

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)

The Englert-Brout-Higgs Mechanism

Englert & Brout Guralnik, Hagen & Kibble

• •

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

• Discussed in detail by Higgs in 1966 paper

Nambu, EB, GHK and Higgs

Spontaneous breaking of symmetry

A Phenomenological Profile of the Higgs Boson

• Neutral currents (1973)

• Charm (1974)

• Heavy lepton τ (1975)

• 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

• Higgs decay modes and searches in 1975:

A Phenomenological Profile of the Higgs Boson

• Couplings proportional to mass:– Decays into heavier particles favoured

• But: important couplings through loops:– gluon + gluon → Higgs → γγ

Higgs Decay Branching Ratios

Constraints on Higgs Mass

• Electroweak observables sensitive via quantum loop corrections:

• Sensitivity to top, Higgs masses:

• Preferred Higgs mass: mH ~ 80 ± 30 GeV• Compare with lower limit from direct searches:

mH > 114 GeV

• No conflict!

The State of the Higgs in Mid-2011

• High-energy search:– Limit from LEP:

mH > 114.4 GeV

• High-precision electroweak data:– Sensitive to Higgs mass:

mH = 96+30–24 GeV

• Combined upper limit:

mH < 161 GeV, or 190 GeV including direct limit

• Exclusion from high-energy search at Tevatron:

mH < 158 GeV or > 173 GeV

Combining the Information from Previous Direct Searches and Indirect Data

mH = 125 ± 10 GeV Gfitter collaboration

Latest Higgs Searches @ Tevatron

Exclude (100,109); (156,177) GeV

Standard Model

prediction

Experimental

upper limit

Higgs Production at the LHC

A la recherche du Higgs perdu …

Higgs Hunting @ LHC: Status on Feb. 8th, 2012

Exclude 112.9 GeV to 115.5 GeV,

131 GeV to 238 GeV,

251 GeV to 466 GeVExclude 127 to 600 GeV

Has the Higgs been Discovered?Interesting hints around Mh = 125 GeV ?

CMS sees broad

enhancement

ATLAS prefers

125 GeV

ATLASSignals

• γγ: 2.8σ• ZZ: 2.1σ• WW: 1.4σ• Combined: 3.5σ

CMS Signals

Combined: 3.1 σ

Both compatible with mH = 125 GeV

Are ATLAS & CMS seeing the Same Thing?

Has the Higgs Boson been Discovered?

Evolution of signal from Dec. 2011 to Feb. 2012

Has the Higgs Boson been Discovered?

Unofficial blogger’s combination

NOT ENDORSED BY EXPERIMENTS

but he was right last time !

Combining the Information from Previous Direct Searches and Indirect Data

mH = 125 ± 10 GeVErler: arXiv:1201.0695

mH = 124.5 ± 0.8 GeV

Assuming the Standard Model

Do we already know the ‘Higgs’ has Spin Zero ?

• Decays into γγ, so cannot have spin 1

• 0 or 2?

• If it decays into ττ or b-bar: spin 0 or 1 or orbital angular momentum

• Can diagnose spin via angular correlations of leptons in WW, ZZ decays

The Spin of the Higgs Boson @ LHC

Higher mass: angular correlations in H → ZZ decays

Low mass: if H →γγ,It cannot have spin 1

Measuring Higgs Couplings @ LHC

Current LHC hint @ Mh = 125 GeV

For Mh = 120 GeV

Higgs Measurements @ LHC & ILC

STANDARD MODEL