Transcript The Hunt for the Higgs - University of Edinburgh

The Hunt for the Higgs
Nigel Glover
Institute for Particle Physics Phenomenology
Durham University
on the occasion of Professor Higgs’ 80th Birthday
The Higgs Boson
•
•
•
•
Why do we need it?
What is it?
Why haven’t we found it yet?
How are we going to find it?
Why do we need it?
The Standard Model of Particles
1.
Gauge Sector
– Strong Interactions
2004
–
2.
Electroweak Interactions
1979
1999
Flavour Sector
– Quark Mixing
1991
2007
2008
3.
Electroweak Symmetry
Breaking Sector
1997
2008
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?
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
What is it?
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
More symmetry breaking
Came to particle physics from condensed matter physics
above Tc
below Tc
Heisenberg ferromagnet
Theory has rotational invariance; ground state is not
invariant  Symmetry has been broken
Global symmetry breaking
Consider a model with a complex scalar
field φ
L = μφ* μφ – V(φ*φ)
with
V(φ*φ) = -μ2φ*φ+λ (φ*φ)2
• The global U(1) symmetry is broken by
a vacuum expectation value <φ> of the
φ-field given, at the classical level, by
the minimum of V.
• degeneracy of vacuum leads to
massless Nambu-Goldstone
oscillations
<>
Yoichiro Nambu
Jeffrey Goldstone
Gauge symmetry breaking
Consider the same model with gauge interactions
L = Dμφ* Dμφ – V(φ*φ) -1/4 FμνFμν
with
Dμ= μ+ieA μ,
φ=<φ>+h
Expanding φ around the true vacuum <φ> generates a
mass for the “photon” Aμ
<φ>
<φ>
M2 = e2<φ>2
Aμ
Aμ
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
The men behind gauge symmetry
breaking
Robert Brout
Peter Higgs
Francois Englert
1964 Physics Letters (15 September),
Physical Review
LettersSociety
(19 October)
1997 European
Physical
Prize
1964 Physical Review Letters (31 August)
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 (g12+g22) v2
So what is the Higgs boson?
The Higgs boson is the quantum
fluctuation of the Higgs field
<φ>
h
 produced by self interactions
Mh2 = λ <φ>2
<φ>
h
 In the Standard Model, Mh, is a
free parameter
Government policy!
Hmmm.
The Higgs boson
has no spin at all!
Mr Blair explains the Higgs boson to Professor Stirling
Properties of the Higgs boson
h
In the Standard Model, the Higgs boson couples to the
fermions – quarks and leptons

<φ>
h


φ = <φ>+h

 Higgs couplings are proportional to the fermion masses
 So it couples most strongly to the most massive particle
Dawn of the
Electroweak Standard Model
Papers with
Higgs in the
title
ICHEP
Fermilab
Higgs
Brout/Englert
Weinberg
Salam
‘t Hooft
Veltman
citations
Theoretical constraints on Mh
Radiative corrections change
the shape of the Higgs
potential at large and small
Higgs boson mass
 Triviality
Λ < v exp(4π2v2/3Mh2)
 Vacuum Stability
Λ < v exp(4π2Mh2/3yt4v2)
Unitarity
Higgs exchange needed to prevent unitarity violation in WW
scattering at high energies
 Mh < 780 GeV
 New phenomena required at the TeV scale
Why haven’t we found it?
…in more than 20 years of
experiments costing nearly £10B?
LHC
construction
Papers with
Higgs in the
title
LEP
construction
LEP
running
Tevatron II
running
citations
Peter Reid
Precision measurements
MZ = 91.1875 +/- 0.0021 GeV
• LEP operated at CERN
throughout the 1990’s
– 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
Indirect limits
• Making precise measurements
means sensitivity to quantum
fluctuations
W
W
W
H
top
Z
• The Higgs has a small but
measurable effect
H
top
Z
Indirect limits
• The net effect of the precision
measurements is to place a
limit on the Higgs boson mass
Less likely
• At 95% confidence
mH > 32 GeV
mH < 185 GeV
6
95% confidence
4
2
0
20
100
m H [GeV]
400
Direct searches at LEP
b
• With enough energy in a
collision, one could just
produce a Higgs boson
e-
e+
e-
Z
b
-
Z
+
Z
b
-
b
• But there is also background
e+
-
H
Z
+
-
Signal or Background?
eFixed
by
accelerator
e+
Z
b
-
b
Z
+
-
Identified
by
detector
A Higgs event?
Where is the Higgs?
September …… December 2000
Results from LEP
Most likely
6
4
95%
Ruled
Out
2
0
20
100
m H [GeV]
400
The TEVATRON at Fermilab
The current highest energy accelerator on earth
The Higgs signal at the TEVATRON
b
• Enough energy to produce a
Higgs boson
… and trigger on the b quarks
• But there is also background…
again
q
q-
q
q-
-
H
W
b
-
W
-
Z
b
-
b
W
-
-
Signal or Background?
Fixed
by
accelerator
in this case
proton and
antiproton
q
q-
Z
b
-
b
W
-
-
Identified
by
detector
Search update
• CDF and D0 have spent the last six years looking for the
Higgs
Best sensitivity in H -> WW* channel
Higgs search: Status March 2009
Tevatron starting to rule out some of the
possible Higgs boson mass range
How are we going to find it?
The right energy scale!
E
Quantum Gravity
MPl
• Unification of couplings?
αs
Grand unification?
Mgut
hierarchy
αw
αEM
E
SUSY?
susy
TeV
Mweak
LHC collisions
• Smallness of neutrino mass
EWSB
• Unitarity of WW scattering
Physics by scale
• Hierarchy problem?
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
The Large Hadron Collider at
CERN
World’s most
powerful particle
accelerator
CMS
Superconducting
magnets
– 8.3T at 1.9K
ALICE
ATLAS
LHCb
2 beams of protons
will collide 40
million times a
second
In construction since 1998
Due to start later this year
Finding the Higgs
Starting from this event…
• 800,000,000 protonproton interactions per
second
• ~100,000,000
electronic channels
• 0.0002 Higgs per
second
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!
The Higgs signal at the LHC
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 5σ
Depends on the number of
proton-proton collisions the LHC
can deliver. Maybe can do this
by 2012
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 haven’t 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 isn’t there, then theoretical framework of
Standard Model is in big trouble, and expect to see
other even more exciting new phenomena
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?
a lot of questions remain!
• What is the origin of the fermion mass?
• Why is the gauge structure SU(3)xSU(2)xU(1)?
GUT?
• Why are there three families?
• Why is the electroweak symmetry broken?
• Why are there 3+1 space-time dimensions?
• How is gravity involved?
Exciting times ahead!!
STRING
THEORY?
Peter Higgs by Ken Currie