The Higgs Boson

What it is and how to find it

Roger BarlowManchester University

Slide 2/26

Particle Physics: the Goal

To deduce the laws of physics using the minimum number of arbitrary assumptions

"What really interests me is whether God had any choice in the creation of the world." --Albert Einstein

Slide 3/26

Elementary Particles: (1) The electron

Known for 100 years

• Very common

• Very light: mass of 9.109 10-31 kg

• Very small (pointlike?)

• Described by Quantum Mechanics. Wave function (r,t), a solution of the Schrödinger Equation –(ħ2/2m)2 =E

e-

Slide 4/26

(2) The photonArgument:Wave function has an arbitrary phaseConstant change of phase: ei does not change physics

It would be ‘nice’ if variable change of phase: ei(r) did not change physics…but terms mess up Schrödinger Equation

Modify S.E. new term –(ħ2/2m)(-ieA)2 =E And if ei then A A+(1/e) (Gauge Transformation)

A(r) describes another particle: Gauge Boson. Spin 1, interacts with electron, has zero mass (no A2 term)… the photon

Hence electromagnetism,Maxwell’s Equations,Etc Everything predicted except the actual value of e

Slide 5/26

(3) The positron

Relativity: Schrödinger Equation replaced by Dirac Equation

-iħ.(-ieA)+m=E is not just one complex function but 4.Extra components describe spin (up/down) and

particle/antiparticleAntiparticle has opposite charge Many more processes possible

QuantumElectroDynamicsQED

e-

e+

Slide 6/26

(4) The quark

quark - like an electron (has charge, spin ½, has antiparticle)

But also has an extra (triple) quantum number. Called ‘colour’ – red (1,0,0), green (0,1,0), blue (0,0,1)

Needed because of the Pauli Exclusion Principle in particles such as the ++ ,

made of 3 otherwise identical quarks.

Slide 7/26

Argue: the choice of red-green-blue axes arbitrary. Physics should not change if we switch around

Or even if we rotate the axes in r-g-b space. Rotation matrix R…

Even if R varies with position+time… extra R terms in equations.

Need extra function in equation with appropriate gauge transformation

New massless particle – Another Gauge Bosonthe gluonSimilar to QED but more complicated due to matrix

structure:QuantumChromoDynamics - QCD.Arbitrary constant is much larger than e. Strong force.

(5) The gluon

Slide 8/26

Pause for breath

Understand Electromagnetism and the Strong (nuclear) force, apart from a few arbitrary(?) constants. And technical details of calculations

That’s everything except gravity and beta decay. Not a ‘Theory of Everything’ but a ‘Theory of quite a lot’

Can’t do gravity…. But should manage beta decay

Slide 9/26

Beta decay as it ought to be… np e- du e- Quarks in protons/neutrons/nuclei are in two

‘flavours’: u and d. (Different charges and masses)

u and d are two states of the same fundamental entity - the quark

e and are two states of the same fundamental entity – the lepton

(Weak) isospin up or down.Run gauge theory argument again for up-down…

predicts Gauge Bosons W+, W0, W-

d

u

eW-

Slide 10/26

Slight(?) problem

Gauge Bosons have got to be massless.* Or the Gauge Invariance of the equations breaks down.

• Photons• Gluons • The W bosons They exist alright – but have masses ~80 GeV.

Theory stuck here for some time

* Mass: The minimum energy needed to create a particle

Slide 11/26

The Higgs Field

Suppose there is a field called H(r,t) that interacts with the electron, quark, W etc

OK, why not

Suppose that the lowest-energy stats is not H(r,t)=0 but H(r,t)=V

Seriously weird

Slide 12/26

Masses that are not masses

1. As a W propagates through space and time, it interacts with this nonzero Higgs field…

2. Which gives it an energy….

3. Even if it has no kinetic or potential energy…

4. Which means it has, to all intents and purposes, a mass. Without breaking gauge invariance

Happens to quarks and leptons too

Slide 13/26

The Standard Model

• Quarks and Leptons (x3 ‘generations’)

• Gauge Symmetries for the Weak, Strong and EM force

• Higgs mechanism giving masses to the W bosons

• Also mixing/unifying Weak and EM forces

• Also explains weak decays between generations (with a few more parameters)

Slide 14/26

Is the Standard Model true?

Yes!Predicts W/Z mass ratioPredicts cross sections and

branching ratios in many many particle decays

Accounts for parity violationAccounts for CP violation in

K and B sectorsNo experimental results in

disagreement

No!Does not predict quark and

lepton massesOr coupling constants…

28 free parameters altogether

Or why there are 3 generations

Or why there is parity violation

Higgs is an ad-hoc addition

Slide 15/26

Testing Higgs: from field to particle

• Quantum excitations of the H field are H particles(Same as any particle, though usually about 0)

• The Higgs coupling of any particle is proportional to its mass.

(actually the other way round…)H is best made by massive particlesH will decay to the heaviest allowed particles

H?Higgsness

Slide 16/26

Is the Higgs true?

• Probably not – it’s a very arbitrary kludge

• Many alternative theories have been proposed that are more elegant/beautiful/natural

• All have very similar effects until you get to high (TeV) energies

Slide 17/26

First Attempt: LEP

e-

e+

Z*

H

q

b

Z

b

q

Collide electrons and positrons at energies of 200 GeV

Slide 18/26

Saw some events, but..

Consistent with background

MH>114 GeV

Slide 19/26

Second Attempt: the LHC

Proton proton collisions at 14 TeV

Start operation next year

Slide 20/26

Experiments: ATLAS and CMS

Slide 21/26

Common features

Tracking• Magnetic Field• Measure charged particle

tracks with drift chambers or Silicon

• Curvature gives momentumCalorimetry• Material so Neutral particles

interact• Measure total energy by

scintillator etc Muon detection• Muons get through the

calorimeter

Slide 22/26

Looking for signals

Decay depends on MH

Plots shows signal if MH fairly large

Smaller values more difficult

Slide 23/26

Handling the data

• Collision rate 40 MHz

• Several events/collision

• Each event gives massive amount of data

• Massive data stream. >10 TB/y

• Tiny number of interesting events

Handled by Grid of computers all over Europe - and the world

10,000+ CPUs

Slide 24/26

Third Attempt: the ILCElectron positron

collisions at 1 TeV

Still at the design stage

Straight (not circular)

Chicago? Japan??

38 km? $6Bn?

Start 2015+?

Slide 25/26

Why?• LHC is a proton-proton

collider• Protons are made of

quarks• LHC is actually a quark-

quark collider• Quarks share proton

energy in a random way

?

junk

junk

7 TeV

• A 14 TeV proton-proton collision gives a whole spectrum of energies for quark-quark collisions

• And the unused energy appears as background particles

7 TeV

7 TeV

500 GeV 500 GeV1 TeV

Precision measurements

Exploration

Slide 26/26

The Future

• LHC will start next year• First serious data 2008+• Interesting results 2-3 years? after that• Should find Higgs - probably not quite as expected• Other new particles/new effects predicted by speculative

models (SUSY? GUTs?)• Exploration will be followed by precision measurements

at the ILC• Build Beyond the Standard Model theory with fewer

arbitrary parameters • Understand the universe we live in a little bit better