Fri. Dec 10 Phy107 Lecture 37

Fundamental Matter Particles

Three generations ofleptons and quarks

All feel theweak force.

Charged leptons feelweak & EM forces

Quarks feelweak, EM, & strong

Wed. Dec 15 Phy107 Lecture 39

Lepton decay• Weak interaction is responsible for particle decay

e—

νe

Generation I

Electron is stable

µ—

νµ

Generation II

Emit W-

2x10-6 seconds

τ—

ντ

Generation III

Emit W-

3x10-13 seconds

Charge

-1

0

Wed. Dec 15 Phy107 Lecture 39

And also quark decay• Quarks have color charge, electric charge, and weak charge

— other interactions swamp the weak interaction

• But similar to leptons, quarks can change their flavor(decay) via the weak force, by emitting a W particle.

u

d

Generation I

c

s

Generation II

Emit W+

2x10-12 seconds

t

b

Generation III

Emit W+

10-23 seconds

Charge

+2/3

-1/3

Wed. Dec 15 Phy107 Lecture 39

Flavor change between generations• But for quarks, not limited to within a generation

• Similar to leptons, quarks can change their flavor (decay)via the weak force, by emitting a W particle.

u

d

Generation I

c

s

Generation II

Emit W-

t

b

Generation III

Emit W-

10-12 seconds

Charge

+2/3

-1/3

Fri. Dec 10 Phy107 Lecture 37

Particle mass in the SM

• The standard model has great successes,but in its first incarnation all particles wererequired to have zero mass.

• Otherwise the mathematicsgives non-physical results.

• Clearly a major problem,since many particles do have mass.

Fri. Dec 10 Phy107 Lecture 37

Mass• Here’s the experimental

masses of SM particles.

• Original SM gives zeromass for all particles.

• But can give particlesmass by coupling to a newfield, the Higgs field.

• Higgs boson is an(unobserved) excitationof the Higgs field.

Fri. Dec 10 Phy107 Lecture 37

What is mass?

• Think of inertial mass:– inertial mass is a particle’s

resistance to changes in velocity.

• When you apply the same force to particles,the smaller the mass, the larger theacceleration.

• What is the origin of mass?

Fri. Dec 10 Phy107 Lecture 37

Mass in the SM• In the standard model (SM),

particles have mass because they interactwith something that pervades the universe.

This something is theHiggs field

Particles ‘hit’ the Higgsfield when you try toaccelerate them

Mass =(chance of hit) x (Higgs density)

Coupling constant

Fri. Dec 10 Phy107 Lecture 37

Mass and the Higgs fieldImagine a party in a room packed full

of people.Nobody is moving around much, just

standing and talking.

Now a popular person enters the room,attracting a cluster of hangers-on

that impede her motionAs she moves she attracts the people

she comes close to- the ones shehas left return to their even spacing.

Her motion is impeded - she hasbecome more massive.

Fri. Dec 10 Phy107 Lecture 37

• In this example, the popular person plays the roleof an electron, or a W boson, or any particle.

• The people in the room represent the Higgs field.

• The interaction with the Higgs field gives theparticle its mass

• A particularly unpopular person could movethrough the room quite easily, with almost nomass. This is the analog of a low-mass particle.

• In present theories, the ‘popularity’ of eachparticle is an input parameter.

Fri. Dec 10 Phy107 Lecture 37

The Higgs bosonThe Higgs boson is a quantum

excitation of the Higgs field.

In analogy, suppose an interestingrumor is shouted in thru the door.

The people get quite excited.

They cluster to pass on the rumor, and thecluster propagates thru the room.

Since the information is carried by clustersof people, and since it was clusteringwhich gave extra mass to the popular

person, the clusters also have mass.

Fri. Dec 10 Phy107 Lecture 37

• The Higgs boson is predicted to be just such aclustering in the Higgs field.

• We will find it much easier to believe that the fieldexists, and that the mechanism for giving otherparticles mass is true, if we actually see the Higgsparticle itself.

Fri. Dec 10 Phy107 Lecture 37

How can we ‘see’ the Higgs?

• The Higgs boson needs to be createdin order to see it.

• Einstein showed that energy and mass areequivalent, through E = mc2.

• This requires an energy of at least the rest massenergy of the Higgs particle.

• The Higgs mass is unknown, but presentexperiments suggestmHiggs > 100 GevmHiggs < 200 Gev

Fri. Dec 10 Phy107 Lecture 37

Symmetry breaking again

• The theory says that the Higgs field has a‘vacuum expectation value’.

• That is, the Higgs field is always nonzero.• This is different than other fields we have

talked about.• At high energies, this is not true,

and all particles are massless.

Fri. Dec 10 Phy107 Lecture 37

Beyond the standard model?

• Standard model has been enormously successful.• Consistent picture of particles and their

interactions.• Predictive power with unusual accuracy.• But…

– SM w/ Higgs mechanism has 19 input parameters.– No suggestion of why there are 3 generations of

leptons/quarks.– No explanation of left-right asymmetries.– Quarks and leptons are ‘unrelated’ fundamental

particles