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?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?

The Standard Model of Particles 1.Gauge Sector –Strong Interactions –Electroweak Interactions 2.Flavour Sector –Quark Mixing 3.Electroweak Symmetry Breaking Sector

Force 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 M W, M Z : 80 GeV/c 2, 91 GeV/c 2

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 real 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 above T c below T c 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 breaking Consider a model with a complex scalar field φ L =  μ φ*  μ φ – V(φ*φ) with V(φ*φ) = -μ 2 φ*φ+λ (φ*φ) 2 The global U(1) symmetry is broken by a vacuum expectation value 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 = D μ φ* D μ φ – V(φ*φ) -1/4 F μν F μν with D μ =  μ +ieA μ, φ= +h Expanding φ around the true vacuum generates a mass for the “photon” A μ M 2 = e 2 2 <φ><φ> AμAμ AμAμ <φ><φ>

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 Peter Higgs Francois Englert Robert Brout 1964 Physics Letters (15 September), Physical Review Letters (19 October) Physical Review Letters (19 October) 1964 Physical Review Letters (31 August) 1997 European Physical Society Prize

Higgs Mechanism in Particle Physics Goldstone bosons give mass to W ±,Z M W 2 = ½ g 2 2 v 2 M Z 2 = ½ (g 1 2 +g 2 2 ) v 2 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 M h 2 = λ 2  In the Standard Model, M h, is a free parameter <φ><φ> <φ><φ> h h

Government policy! Mr Blair explains the Higgs boson to Professor Stirling Hmmm. The Higgs boson has no spin at 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 boson h h     <φ><φ> φ = +h

Dawn of the Electroweak Standard Model citations Papers with Higgs in the title ICHEP Fermilab Weinberg Salam Higgs Brout/Englert ‘t Hooft Veltman

Theoretical constraints on M h Radiative corrections change the shape of the Higgs potential at large and small Higgs boson mass  Triviality Λ < v exp(4π 2 v 2 /3M h 2 )  Vacuum Stability Λ < v exp(4π 2 M h 2 /3y t 4 v 2 )

Unitarity Higgs exchange needed to prevent unitarity violation in WW scattering at high energies  M h < 780 GeV  New phenomena required at the TeV scale

Why haven’t we found it?

LHC construction LEP construction LEP running Tevatron II running Papers with Higgs in the title citations …in more than 20 years of experiments costing nearly £10B?

Peter Reid

Precision 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 M Z = / GeV

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

Indirect limits The net effect of the precision measurements is to place a limit on the Higgs boson mass At 95% confidence m H > 32 GeV m H < 185 GeV 95% confidence

Direct searches at LEP With enough energy in a collision, one could just produce a Higgs boson But there is also background

Signal or Background? Fixed by accelerator Identified by detector

A Higgs event?

Where is the Higgs? September …… December 2000

Results from LEP 95% Ruled Out It should be around here!

The TEVATRON at Fermilab The 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 The Higgs signal at the TEVATRON

Signal or Background? Fixed by accelerator in this case proton and antiproton 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! Unification of couplings? Smallness of neutrino mass Unitarity of WW scattering Hierarchy problem? E M Pl TeVTeV M weak Quantum Gravity M gut Grand unification? LHC collisions EWSB hierarchy Physics by scale susy SUSY? αsαs αwαw α EM E

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

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 1998 Due to start later this year The Large Hadron Collider at CERN CMS ATLAS LHCb ALICE

Starting from this event… We look for this “signature” Selectivity: 1 in Like looking for 1 person in a thousand world populations Or for a needle in 20 million haystacks! Finding the Higgs 800,000,000 proton- proton interactions per second ~100,000,000 electronic channels Higgs per second

The Higgs signal at the LHC

Observability of the SM Higgs in CMS with 10 5 pb -1. The ATLAS and CMS detectors can probe the entire mass range up to M H ~ 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 discovery

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 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)? 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 Currie