The Hunt for the HiggsThe Hunt for the Higgs

Nigel GloverNigel GloverInstitute for Particle Physics PhenomenologyInstitute for Particle Physics Phenomenology

Durham UniversityDurham University

on the occasion of Professor Higgs’ 80th Birthdayon the occasion of Professor Higgs’ 80th Birthday

The Higgs BosonThe Higgs Boson

• Why do we need it?Why do we need it?

• What is it?What is it?

• Why haven’t we found it yet?Why haven’t we found it yet?

• How are we going to find it?How are we going to find it?

Why do we need it?Why do we need it?

The Standard Model of ParticlesThe Standard Model of Particles

1. Gauge Sector– Strong Interactions

– Electroweak Interactions

2. Flavour Sector– Quark Mixing

3. Electroweak Symmetry Breaking Sector

2004

1979 1999

2007

1997 2008

1991

2008

Force Carrying QuantaForce Carrying Quanta

Gauge symmetry is fundamental to electrodynamics when extended to electroweak theory, requires massless W,Z

how can we accommodate 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

Why do we need the Higgs?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 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 it?

What is symmetry breaking?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 breakingMore symmetry breaking

above Tc

below Tc

Came to particle physics from condensed matter physics

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

Heisenberg ferromagnet

Global symmetry breakingGlobal symmetry breaking

Consider a model with a complex scalar field φ

LL = = μμφφ** μμφφ – V( – V(φφ**φφ) )

with

V(V(φφ**φφ) = -) = -μμ22φφ**φφ++λλ ( (φφ**φφ))22

• 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 breakingGauge symmetry breaking

Consider the same model with gauge interactions

LL = D = Dμμφφ** D Dμμφφ – V( – V(φφ**φφ) -1/4 F) -1/4 FμνμνFFμνμν

with

DDμμ= = μμ+ieA +ieA μμ, , φφ=<=<φφ>+h>+h

Expanding φ around the true vacuum <φ> generates a mass for the “photon” Aμ

MM22 = e = e22<<φφ>>22

<φ>

AμAμ

<φ>

Where did the Goldstone mode go?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 The men behind gauge symmetry breakingbreaking

Peter Peter HiggsHiggs

Francois EnglertFrancois EnglertRobert BroutRobert Brout

1964 Physics Letters (15 September),1964 Physics Letters (15 September), Physical Review Letters (19 October)Physical Review Letters (19 October)1964 Physical Review Letters (31 August)1964 Physical Review Letters (31 August)

1997 European Physical Society Prize1997 European Physical Society Prize

Higgs Mechanism in Particle PhysicsHiggs Mechanism in Particle Physics

Goldstone bosons give mass to W±,Z

MW2 = ½ g2

2 v2 MZ2 = ½ (g1

2+g22) v2

Complex SU(2) doublet Higgs Field (four real scalars)

Spontaneous symmetry breaking vacuum expectation value v three Goldstone bosons

SU(2)xU(1) Electroweak “Standard Model” relies on spontaneous symmetry breaking

So what is the Higgs boson?

The Higgs boson is the quantum fluctuation of the Higgs field

produced by self interactions

Mh2

= λ <φ>2

In the Standard Model, Mh, is a free parameter

<φ>

<φ>

h

h

Government policy!Government policy!

Mr Blair explains the Higgs boson to Professor Mr Blair explains the Higgs boson to Professor StirlingStirling

Hmmm. Hmmm. The Higgs The Higgs boson boson has no spin at has no spin at all!all!

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

Properties of the Higgs bosonProperties of the Higgs bosonh

h

<φ>

φ = <φ>+h

Dawn of the Dawn of the Electroweak Standard ModelElectroweak Standard Model

citations

Papers withHiggs in the title

ICHEPFermilab

WeinbergSalam

HiggsBrout/Englert

‘t HooftVeltman

Theoretical constraints on MTheoretical constraints on Mhh

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(4π2Mh2/3yt

4v2)

UnitarityUnitarity

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?Why haven’t we found it?

LHC construction

LEP construction

LEP running

Tevatron II running

Papers withHiggs in the title

citations

……in more than 20 years of in more than 20 years of experiments costing nearly experiments costing nearly £10B£10B??

Peter Reid

Precision measurementsPrecision measurements

• 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

MMZZ = 91.1875 +/- 0.0021 GeV = 91.1875 +/- 0.0021 GeV

Indirect limitsIndirect limits

• Making precise measurements means sensitivity to quantum fluctuations

• The Higgs has a small but measurable effect

WW

W

H

Z ZH

t op

t op

6

4

2

02 0 10 0 4 00

Less

like

lym [GeV ] H

Indirect limitsIndirect limits

• The net effect of the precision measurements is to place a limit on the Higgs boson mass

• At 95% confidence

mH > 32 GeV

mH < 185 GeV

95%95% confidenceconfidence

Direct searches at LEPDirect searches at LEP

• With enough energy in a collision, one could just produce a Higgs boson

• But there is also background

e-

e+Z Z

H

b

b-

+

-

b

b-

-

Ze

+Ze+

-

Signal or Background?Signal or Background?

b

b-

-

Ze

+Ze+

-

Fixed by

accelerator

Identified by detector

A Higgs event?A Higgs event?

Where is the Higgs?Where is the Higgs?

September …… December 2000

6

4

2

02 0 10 0 4 00

Mos

t lik

ely

m [GeV ] H

Results from LEPResults from LEP

95%95%Ruled Ruled OutOut It should be

It should be

around here!

around here!

The TEVATRON at FermilabThe TEVATRON at Fermilab

The current highest energy accelerator on earthThe current highest energy accelerator on earth

• Enough energy to produce a Higgs boson

… and trigger on the b quarks

• But there is also background… again

b

b-

-

-

Z

W

q

q-

q

qW

H

b

b-

-- W

-

The Higgs signal at the TEVATRONThe Higgs signal at the TEVATRON

b

b-

-

-

Z

W

q

q-

Signal or Background?Signal or Background?

Fixed by

accelerator

in this case proton and antiproton

Identified by detector

Search updateSearch update

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

Best sensitivity in H -> WW* channelBest sensitivity in H -> WW* channel

Higgs search: Status March 2009Higgs search: Status March 2009

Tevatron starting to rule out some of the possible Higgs boson mass range

How are we going to find it?How are we going to find it?

The right energy scale!The right energy scale!

• Unification of couplings?

• Smallness of neutrino mass

• Unitarity of WW scattering

• Hierarchy problem?

E

MPl

TeV Mweak

Quantum Gravity

MgutGrand unification?

LHC collisions

EWSB

hie

rarch

y

Physics by scale

susySUSY?

αs

αw

αEM

E

Electroweak Symmetry BreakingElectroweak 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

World’s most powerful particle accelerator

Superconducting magnets

– 8.3T at 1.9K

2 beams of protons will collide 40 million times a second

In construction since 1998Due to start later this year

The Large Hadron Collider at The Large Hadron Collider at CERNCERN

CMS

ATLASLHCb

ALICE

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!

Finding the HiggsFinding the Higgs

• 800,000,000 proton-proton interactions per second

• ~100,000,000 electronic channels

• 0.0002 Higgs per second

The Higgs signal at the LHCThe Higgs signal at the LHC

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

Higgs discoveryHiggs discovery

Summary – Higgs BosonSummary – 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 eventslooking 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 Discovering the Higgs will be a massive step forwardmassive 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!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 3+1 space-time dimensions?

• How is gravity involved?

GUT?

STRING THEORY?

Exciting times ahead!!

Peter Higgs by Ken CurriePeter Higgs by Ken Currie