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What is the Higgs Boson?

Jason Nielsen

SCIPP / UC Santa Cruz

VERTEX 2004

June 25, 2007

And how do we search for it?

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Challenge of Particle Physics

Particle collisions at the energy frontier enable us to pursue these and other questions about nature

• Unification of the basic forces and the origin of mass for thefundamental particles

• Unexpected new physics or extra dimensions not included in Standard Model

• Unknown new physics(forces or particles)hinted at by cosmology

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Fundamental Particles & Forces

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Force Carrier Quanta

Gauge symmetry is fundamental to electrodynamics• when extended to electroweak theory, requires massless W,Z• how to accomodate their large masses?

Photon (electromagnetic)• verified 1922• mass of photon = 0

W,Z bosons (weak force)• verified 1983• mW, mZ: 80 GeV/c2, 91 GeV/c2

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Higgs Mechanism in Field Theory

Additional fields with constructed potential• just like gravitational field, electric field

Electroweak “Standard Model” relies on broken symmetry

Introduction of a pervasive Higgs field• Rotationally symmetric potential• But the stable minimum breaks the symmetry!

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Spontaneous Symmetry BreakingCame to particle physics from condensed matter physics

Theory has rotational invariance; ground state is not invariant Symmetry has been broken by external factor

Pencil on point Heisenberg ferromagnet

above Tc

below Tc

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Higgs Mechanism in Field Theory

Goldstone bosons give mass to W±,Z• One physical scalar boson: Higgs boson whose mass is unknown

Discovery of the Higgs boson would help verify this approachOtherwise, much head-scratching and new theories!

Spontaneous symmetry breaking• Lost degree of freedom -> Goldstone bosons

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Why is the Top Quark So Massive?

Note: Higgs couplings explain fundamental fermion mass but not everyday mass!

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mas

s (G

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Schwinger (1957): a coupling produces effective mass termsthrough the action of the vacuum fluctuations (Higgs boson)

Top quark most affected by this “Higgs field molasses”

Interaction with Higgs quantumdefines mass of fermions

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So What IS the Higgs boson?

Higgs boson is a physical condensate of the pervasive postulated Higgs field

Similar to photon, except Higgs boson is not a force carrier

What kinds of particles do it couple to?• Its couplings are proportional to the fermion masses• So it couples most strongly to the most massive particles

This makes it clear how to search for it, if it exists…

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Wringing Out the Higgs Condensate

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H

Z

Z*

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e-

But Higgs boson is fleeting:decays immediately to characteristic “final state”

Physical Higgs bosons can be produced, given enough energy

(Here ECM > mH + mZ)

That’s where the collider comes in

That’s where the detector comes in

H

Z

bb

qq

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Recent Physics Results

mH < 182 GeV/c2 at 95% CL (including previous searches)

Updated winter 2007 with newTevatron mW=80.4±0.04 GeV

Effects of the Higgs boson are felt via loop interactions

Precision measurementsare sensitive to the Higgs mass

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How does the Higgs Boson Decay?

Notice coupling to massive particles (bb, tt, WW, ZZ)

For low mass Higgs, expect decay to b quark pairs;For very high mass Higgs expect decay to ZZ

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Rare Higgs Decays (?)(Claus Grupen)

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Identifying b Quarks from Higgs

proton-antiprotonInteraction point

B hadron

B hadrons have lifetimes of 1.5 ps: find the decay vertex!

Fit tracks together to form secondary vertex• measure flight distance of B hadron• typical flight distance is 0.5 cm from interaction point• close, precise measurement provided by silicon is crucial

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One Provocative Candidate Event

3 NN b-tagged jets

ECM=206.7 GeV

HZ bbbb selection

Reconstructed mH = 110 ± 3 GeV/c2

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Bumps in the Mass Spectrum

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Decay products of the Higgs boson form a mass resonance- similar to resonances from past discoveries of new particles

Strategy for identifying Higgs boson production:1. Excess of events in Wbb signature (or other signature)2. Higgs decay products form a invariant mass peak

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Tevatron Cross Section HierarchyIn proton-antiproton collisions at s = 1.96 TeV:

b-jet pairs from QCD

high-energy leptons

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Particle productionrates vary widely:the Higgs is the“needle in the haystack!”

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What kind of unit is a “barn?”

Photo: Reidar Hahn, Fermilab

the term “barn” wasn't officially declassified until 1948

Apparently there was also a unit called the “shed”: 10- 48 cm2

This summer CDF will have collected 3 giga-sheds of data!

Manhattan Project physicistsgave the name to the typical nuclear cross-sectiondefined as 10-24 cm2

Practically “as big as a barn”where (sub)-nuclear processes are concerned

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bb Dijet Invariant Mass Distribution

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Large Hadron Collider at CERN

Italy

p p14 TeV

Next generation collider: startup scheduled for 2008

ATLASCMS

Luminosity target: 1034cm-2 s-1

Increased production of heavy particles like Higgs, top quark

More particles at higher energyrequires new detector designand technology

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Higgs Decay to Photons

Rare decay in SM

LHC detectors have been optimized to find this peak!

H t

t

g

g

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Higgs Decay to ZZ

Requires precise measurement of muon curvature

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ATLAS Experiment at LHC

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ATLAS Experiment at LHC

ATLAS collaborator

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ATLAS DETECTORNov. 2005

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Installation of inner detector end-cap

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Prospects for SM Higgs at LHC

Should discover SM Higgsregardless of mass value

Low-mass Higgs channels:• H ! (sm =1.5 GeV/c2)• W,Z boson fusion to Higgs:

then HWW or H tt

• ttH: top quark again!

High-mass Higgs channels:

• golden mode 4e/ opens >2mZ

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Identifying Particle Signatures

ATLAS trigger system can identify specific signatures online

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“Hunt for Higgs” WWW Site

http://www.sciencemuseum.org.uk/antenna/bigbang/huntforhiggs/index.asp

Let’s have a look together at the “Hunt for Higgs”

One of the best I’ve seen at describing what really happens

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Future of the Higgs Search

• Tevatron experiments still searching

• LHC turns on in 2008– Commissioning and calibrating detectors

• Understand non-Higgs backgrounds

• Find the Higgs boson peak above the bkgd!

• My guess is that it will take a few years to

collect enough events to convince ourselves

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