THE SEARCH FOR THE HIGGS BOSON

Aungshuman ZamanDepartment of Physics and Astronomy

Stony Brook UniversityOctober 11, 2010

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What Is This Talk All About

• Why is the search for the Higgs Boson important?– Gauge theory and standard model.

• How can we detect Higgs boson?– Direct and indirect search.

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How do we explain nature at its smallest scale?

Quantum Mechanics + Special Relativity (No Gravity)

QFT

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Gauge Symmetry

• We demand Lagrangian density is invariant under certain continuous local transformations--- Gauge Transformations.

• These symmetry transformations form groups.• This imposition of condition on the field

theories gives us the force carrying particles.

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Gauge Theories

• U(1) Elecromagnetismphoton

• SU(2) Weak Interaction, , Z

• SU(3) Strong Interaction8 gluons

Standard Model SU(3)×SU(2)×U(1) ?

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BUT… …

• There are problems with this picture

– 1. Weak force carriers are massive unlike the photon and gluon.

– 2. Leptons and quarks should not be massive.

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Here comes the Higgs

• Englert-Brout-Higgs-Guralnik-Hagen-Kibble (1963-64)

• SU(2) not an exact symmetry.• Introduce one extra scalar field--- HIGGS field with non-zero vacuum expectation value

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• The Mexican hat potential: The ground state lacks the symmetry of the whole system.

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Higgs Boson completes the SM picture

• The electroweak symmetry is spontaneously broken.• Electroweak gauge bosons acquire mass through the

“Higgs Mechanism.”• According to the simplest model, Higgs boson is a

scalar particle with couplings to other particle. This coupling is responsible for the mass of leptons and quarks.

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So search for the Higgs boson is very important for our

understanding of the universe.

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Experimental Search for Higgs

• Indirect: Precision Electroweak Constraints– Precision measurement of the W,Z and t masses

has been used to establish indirect limits on SM Higgs mass.

– (Fermilab, LEP and SLD) exclusion of a SM Higgs boson having a mass greater than 285 GeV/c2 at 95% CL. (2006)

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Experimental Search for Higgs

• Direct Search– LEP (1989-2000; electron-positron at 45-200 GeV)– Tevatron (proton-antiproton at 2 TeV)– LHC (proton-proton at 7-14 TeV; The discovery of

the Higgs particle was a primary motivation for the LHC.)

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Large Electron-Positron collider (LEP)

• LEP data sets the experimental lower bound for the mass of the SM Higgs boson at 114.4 GeV/c2 (95% CL)

• In 2000, data from LEP suggested inconclusively that the Higgs Particle of a mass around 115 GeV might have been observed.

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Important parameters

• Higgs cross section

• Higgs Branching Ratio

• Background

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Higgs Cross section (in pb)

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Higgs Decay Ratio

While searching for the Higgs particle in a given mass range, the decay modes are selected on the basis of branching ratio as well as the relative background for the process in that mass range.

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Low mass region: MH <135 Gev/c2

• pp H b (B0, B±, Λb, π0, π± )

• Higgs branching ratio (BR) is roughly 85%

• Background: p (q ) b

• Signal to Background ratio (S/B) is very poor!!

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So at Tevatron……

• Signal: () WH l ν

ZH l+ l - ZH ν

• Background: W+ ; e.g. W + Wb Wb

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At LHC… …• pp H γ γ• Branching Ratio ~ 10-4

• So we are throwing away 99.99% of the data.

Larger energy makes S/B even worseWHY??

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LHC

Two high energy photons set the Higgs process apart from the regular processes (q¯q →γ , gg → γ and quark bremsstralung). A bump in the di-photoninvariant mass spectrum.

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High mass region; MH>135 GeV/c2

Both Tevatron and LHC

• Easier, S/B comparatively good• Dominant channel: H WW(*)

• Background: pp WW(*) l ν l νWZ l ν l‘ ν’ZZ l ν l‘ ν’

• Angular correlation between final state leptons.

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Higgs mass range narrows down at Tevatron

In 2010, data from CDF and D0 experiments at the Tevatron exclude the Higgs boson in the range between 158 GeV/c2 and 175 GeV/c2 (95% CL)

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So, Where do we stand?

Status as of August 2010, to 95% confidence interval.

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Bibliography• Professor John Hobbs, Stony Brook University. • Professor Patrick Meade, Stony brook University.• Introduction to elementary particles, D. Griffiths• Tests of the Standard Electroweak Model at the Energy Frontier, John D.

Hobbs, Mark S. Neubauer and Scott Willenbrock • Precise predictions for Higgs cross sections at the Large Hadron Collider,

Robert Harlandera• Indirect limit on the standard model Higgs boson mass from the precision

Fermilab, LEP, and SLD data, J. H. Field• SEARCHES FOR THE HIGGS BOSON AT LHC, M. DELMASTRO, on behalf of the

ATLAS and CMS collaborations, European Laboratory for Particle Physics (CERN)• Wikipedia,• Scholarpedia