The Hunt for the Higgs

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The Hunt for the Higgs

• Nigel Glover
• Institute for Particle Physics Phenomenology
• Durham University
• on the occasion of Professor Higgs 80th Birthday

2
The Higgs Boson

• Why do we need it?
• What is it?
• Why havent we found it yet?
• How are we going to find it?

3
Why do we need it?
4
The Standard Model of Particles

• Gauge Sector
• Strong Interactions
• Electroweak Interactions
• Flavour Sector
• Quark Mixing
• Electroweak Symmetry Breaking Sector

2007
5
Force Carrying Quanta

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

• W,Z bosons (weak force)
• verified 1983
• MW, MZ 80 GeV/c2, 91 GeV/c2

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

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Why do we need the Higgs?

• Fundamental symmetries of nature require that
  all the elementary particles and force carriers
  are massless
• in an ideal world all elementary particles
  would be massless
• but in the real world the elementary particles
  have widely differing masses
• so the symmetry must be broken
• This is what the Higgs mechanism and electroweak
  symmetry breaking is all about

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What is it?
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What is symmetry breaking?

• Consider a smooth ball at the top of a very
  smooth symmetric hill
• The ball can roll in either direction
• there is a left-right symmetry
• But the ball can only fall in one direction
• the symmetry is broken

9
More symmetry breaking
Came to particle physics from condensed matter
physics
Heisenberg ferromagnet
Theory has rotational invariance ground state is
not invariant ? Symmetry has been broken
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Global symmetry breaking
lt??gt

• Consider a model with a complex scalar field f
• L ?µf ?µf V(ff)
• with
• V(ff) -µ2ff? (ff)2
• The global U(1) symmetry is broken by a vacuum
  expectation value ltfgt of the f-field given, at
  the classical level, by the minimum of V.
• degeneracy of vacuum leads to massless
  Nambu-Goldstone oscillations

Yoichiro Nambu
Jeffrey Goldstone
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Gauge symmetry breaking

• Consider the same model with gauge interactions
• L Dµf Dµf V(ff) -1/4 Fµ?Fµ?
• with
• Dµ ?µieA µ, fltfgth
• Expanding f around the true vacuum ltfgt generates
  a mass for the photon Aµ
• M2 e2ltfgt2

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Where did the Goldstone mode go?

• propagation of Goldstone mode corresponds to
  rotation of vacuum orientation
• equivalent to local gauge transformation and
  therefore unobservable
• violation of
• Goldstone Theorem
• produces extra longitudinal mode of massive
  gauge field

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The men behind gauge symmetry breaking
1964 Physics Letters (15 September),
Physical Review Letters (19 October) 1964
Physical Review Letters (31 August)
1997 European Physical Society Prize
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Higgs Mechanism in Particle Physics
SU(2)xU(1) Electroweak Standard Model relies on
spontaneous symmetry breaking

• Complex SU(2) doublet
• Higgs Field (four real scalars)
• Spontaneous symmetry breaking
• vacuum expectation value v
• three Goldstone bosons

Goldstone bosons give mass to W,Z MW2 ½ g22
v2 MZ2 ½ (g12g22) v2
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So what is the Higgs boson?

• The Higgs boson is the quantum fluctuation of the
  Higgs field
• produced by self interactions
• Mh2 ? ltfgt2
• In the Standard Model, Mh, is a free
  parameter

h
ltfgt
ltfgt
h
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Government policy!
Mr Blair explains the Higgs boson to Professor
Stirling
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Properties of the Higgs boson
h

• In the Standard Model, the Higgs boson couples
  to the fermions quarks and leptons
• Higgs couplings are proportional to the fermion
  masses
• So it couples most strongly to the most massive
  particle

?
ltfgt
h
?
?
f ltfgth
?
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Dawn of the Electroweak Standard Model
Papers with Higgs in the title
ICHEP Fermilab
t Hooft Veltman
Weinberg Salam
Higgs Brout/Englert
citations
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Theoretical constraints on Mh

• Radiative corrections change the shape of the
  Higgs potential at large and small Higgs boson
  mass
• Triviality
• ? lt v exp(4p2v2/3Mh2)
• Vacuum Stability
• ? lt v exp(4p2Mh2/3yt4v2)

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Unitarity

• Higgs exchange needed to prevent unitarity
  violation in WW scattering at high energies
• Mh lt 780 GeV
• New phenomena required at the TeV scale

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Why havent we found it?
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in more than 20 years of experiments costing
nearly 10B?
Papers with Higgs in the title
Tevatron II running
LEP running
citations
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Peter Reid
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Precision measurements

• LEP operated at CERN throughout the 1990s
• 3 light neutrinos
• precision weak interaction measurements
• established gauge theory of strong interaction
• Measuring the Z mass to this accuracy is like
  measuring your body weight with an error of 1
  gram
• the weight of a lungful of air

MZ 91.1875 /- 0.0021 GeV
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Indirect limits

• Making precise measurements means sensitivity to
  quantum fluctuations
• The Higgs has a small but measurable effect

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Indirect limits

• The net effect of the precision measurements is
  to place a limit on the Higgs boson mass
• At 95 confidence
• mH gt 32 GeV
• mH lt 185 GeV

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Direct searches at LEP

• With enough energy in a collision, one could just
  produce a Higgs boson
• But there is also background

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Signal or Background?
Identified by detector
Fixed by accelerator
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A Higgs event?
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Where is the Higgs?
September December 2000
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Results from LEP
It should be around here!
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The TEVATRON at Fermilab
The current highest energy accelerator on earth
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The Higgs signal at the TEVATRON

• Enough energy to produce a Higgs boson
• and trigger on the b quarks
• But there is also background again

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Signal or Background?
Fixed by accelerator in this case proton and
antiproton
Identified by detector
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Search update

• CDF and D0 have spent the last six years looking
  for the Higgs

Best sensitivity in H -gt WW channel
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Higgs search Status March 2009
Tevatron starting to rule out some of the
possible Higgs boson mass range
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How are we going to find it?
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The right energy scale!
E
MPl
Quantum Gravity

• Unification of couplings?
• Smallness of neutrino mass
• Unitarity of WW scattering
• Hierarchy problem?

LHC collisions
TeV
Mweak
EWSB
Physics by scale
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Electroweak Symmetry Breaking

• Standard Model (SM), SUSY, . . .
• Higgs mechanism, elementary scalar particle(s)
• Strong electroweak symmetry breaking
  (technicolour, .)
• new strong interaction, non-perturbative
  effects, resonances,
• Higgsless models in extra dimensions
• boundary conditions for SM gauge bosons and
  fermions on Planck and TeV branes in
  higher-dimensional space
• New phenomena required at the TeV scale

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The Large Hadron Collider at CERN
Worlds most powerful particle accelerator Superc
onducting magnets 8.3T at 1.9K 2 beams of
protons will collide 40 million times a second
In construction since 1998 Due to start later
this year
CMS
ALICE
ATLAS
LHCb
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Finding the Higgs

• 800,000,000 proton-proton interactions per second
• 100,000,000 electronic channels
• 0.0002 Higgs per second

Starting from this event
We look for this signature
Selectivity 1 in 1013 Like looking for 1
person in a thousand world populations Or for a
needle in 20 million haystacks!
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The Higgs signal at the LHC
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Higgs discovery

• Observability of the SM Higgs in CMS with 105
  pb-1.
• The ATLAS and CMS detectors can probe the entire
  mass range up to MH 1 TeV with a signal
  significance well above 5s
• Depends on the number of proton-proton
  collisions the LHC can deliver. Maybe can do
  this by 2012

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Summary Higgs Boson

• Why do we need it?
• to give masses to the fundamental particles
• What is the Higgs boson?
• a quantum fluctuation of the Higgs field
• Why havent we seen it?
• hints at LEP, but too few events
• looking now at the TEVATRON
• How are we going to find it?
• If its there, will definitely find at the LHC
  in 2011
• If it isnt there, then theoretical framework
  of Standard Model is in big trouble, and expect
  to see other even more exciting new phenomena

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Discovering the Higgs will be a massive step
forward

• BUT just a discovery will not be sufficient
• Is it a Higgs boson?
• What are its mass, spin and CP properties?
• What are its couplings to fermions and gauge
  bosons?
• Are they really proportional to the masses of the
  particles?
• What are its self-couplings?
• Are its properties compatible with the SM. . . ?
• How many Higgs bosons are there?

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a lot of questions remain!

• What is the origin of the fermion mass?
• Why is the gauge structure SU(3)xSU(2)xU(1)?
• Why are there three families?
• Why is the electroweak symmetry broken?
• Why are there 31 space-time dimensions?
• How is gravity involved?

GUT?
STRING THEORY?
Exciting times ahead!!
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Peter Higgs by Ken Currie