Fundamental Matter Particles

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Fri. Dec 10 Phy107 Lecture 37 Fundamental Matter Particles Three generations of leptons and quarks All feel the weak force. Charged leptons feel weak & EM forces Quarks feel weak, EM, & strong

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Fundamental Matter Particles
All feel the weak force.
Charged leptons feel weak & EM forces
Quarks feel weak, EM, & strong
Wed. Dec 15 Phy107 Lecture 39
Lepton decay • Weak interaction is responsible for particle decay
e—
νe
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
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
Particle mass in the SM
• The standard model has great successes, but in its first incarnation all particles were required to have zero mass.
• Otherwise the mathematics gives 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 zero mass for all particles.
• But can give particles mass by coupling to a new field, the Higgs field.
• Higgs boson is an (unobserved) excitation of 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 the acceleration.
• 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 interact with something that pervades the universe.
This something is the Higgs field
Particles ‘hit’ the Higgs field when you try to accelerate them
Mass = (chance of hit) x (Higgs density)
Coupling constant
Fri. Dec 10 Phy107 Lecture 37
Mass and the Higgs field Imagine 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 motion As she moves she attracts the people
she comes close to- the ones she has left return to their even spacing.
Her motion is impeded - she has become more massive.
Fri. Dec 10 Phy107 Lecture 37
• In this example, the popular person plays the role of 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 the particle its mass
• A particularly unpopular person could move through the room quite easily, with almost no mass. This is the analog of a low-mass particle.
• In present theories, the ‘popularity’ of each particle is an input parameter.
Fri. Dec 10 Phy107 Lecture 37
The Higgs boson The Higgs boson is a quantum
excitation of the Higgs field.
In analogy, suppose an interesting rumor is shouted in thru the door.
The people get quite excited.
They cluster to pass on the rumor, and the cluster propagates thru the room.
Since the information is carried by clusters of people, and since it was clustering which 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 a clustering in the Higgs field.
• We will find it much easier to believe that the field exists, and that the mechanism for giving other particles mass is true, if we actually see the Higgs particle itself.
Fri. Dec 10 Phy107 Lecture 37
How can we ‘see’ the Higgs?
• The Higgs boson needs to be created in order to see it.
• Einstein showed that energy and mass are equivalent, through E = mc2.
• This requires an energy of at least the rest mass energy of the Higgs particle.
• The Higgs mass is unknown, but present experiments suggest mHiggs > 100 Gev mHiggs < 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