Physics 249 Lecture 41, Dec 14th 2012
Reading: Chapter 12
HW 11: due Today
Reminder: Final is Dec 21st 10:05-12:05
Will distribute grade information after last HW is graded including an approximate curve. Probably Dec 17th.
1) The strong force.
The strong force only occurs for the quarks, which carry color charge. The force is mediated by the gluon. The gluons have color change. The strong force will again conserve all the charges.
The strong force also has approximately the same basic constant determining the probability.
However, the fact that the force carrier can interact with itself change changes the nature of the interaction essentially making strong interaction very high probability and short range.
At high energies you can still set up a situation in which there is only time for one interaction to take place and extrapolating experimental results to the highest energies (in this case energies at the beginning of the Universe) you find approximately the same fundamental constant governs the strong force.
2) Standard model of particle physics.
Two classes of particles, quarks and leptons, with three generations of each set of particles.
Three forces, electromagnetic, weak and strong.
a) Standard model particles
Leptons: electron, electron neutrino and antiparticle versions positron and anti electron and anti electron neutrino
Charges: q=+_1,0, weak electron lepton number = +-1
Antiparticles are identical in most properties such as mass but opposite in charge. This includes electric change and weak charge.
Interact electromagnetically (electron only) and weakly. Interaction means that they carry the charge of this force and can interact via 3 prong vertices with the force carriers of these forces.
Quarks, u and d quarks and antiparticle versions
Charges: q=+-2/3,-+1/3, weak quark flavor u and d, color red, green and blue
Antiparticles have anti-charges such as anti-color charge, anti-red, anti-green anti-blue.
Interact electromagnetically, strongly and weakly
Three generations of particles need to be introduced to explain the observed particles.
Muon and tau versions for the letpons.
c,s and t,b versions for the quarks.
Second and third generation charged leptons can decay via lepton flavor conserving decays involving the W boson.
Single second and third generation quarks can only decay via weak quark flavor changing decays involving the W boson. Higher generation particles don’t typically exist in nature they are produced in particle anti particle pairs to conserve lepton and weak charge
b) The standard model forces
Three forces. Introduced to explain why we saw interactions with substantially different properties. For instance, their substantial differences in strength or probability and range.
The electromagnetic force carried by the photon.
The weak force carried by the W and Z boson
The strong force carried by the gluon.
c) Unification. The three forces can be shown to characterized by the same fundamental constant. In the case of the weak force the essential element that makes it different is the mass of the force carriers. The clear question is why the W and Z bosons have mass while the photon and gluons do not. A further question is why do any of the particles have mass.
If you introduce one more field and associated particle, a spin 0 boson known as the Higgs boson, the particles would interact with it and acquire mass. In the full derivation you find one force carrying particle remains massless, the photon, and three are massive, the W+, W- and Z bosons. This theory also predicted the masses of the W and Z bosons(before the particles were directly discovered and their masses measured) and also predicts the Higgs boson (discovered just this year 50 years after the theory was developed).
3) Mysteries beyond the standard model
Mysteries solved.
a) Dual particle-wave nature of both matter particles and force carrying particles.
- They are solutions of the same type of wave equation that come from quantizing the energy momentum relationship
b) Particles and antiparticles
- These wave equation predict both particles and antiparticles.
c) The intrinsic quantum property of spin
- The wave equations also predict spin up and down particles solutions.
d) Why the weak force is so weak
- This is expected to happen using the same functional form for the probability of interactions if the W and Z bosons are massive.
e) The properties of the strong force including why it is strong and short range.
- This is expected to be true if the strong mediator, the gluon, carries color charge
f) The origin of mass. (partially solved)
- The interactions with the Higgs field will give the W and Z bosons and the fermions mass.
g) Unification of the three forces. (partially solved)
- The EM and weak force have the same coupling constant and the strong force very nearly has the same force constant at high energy.
Mysteries to be solved
a)The Higgs particle - This is the subject of my current research. The particle we have observed has the expected properties of the Higgs boson to the best ability of our experiments to measure but we have yet to measure all the properties precisely or in some cases at all.
b) Neutrinos have mass but it is almost immeasurably small. In original SM they are massless. Also why is the scale for neutrino masses so different from the other fermions.
c) Dark matter. Observations of galactic rotation curves, the expansion of the Universe and gravitational lensing indicate that there is some sort of particle out there that doesn’t interact via the strong or electromagnetic forces. This matter would not radiate so we call it dark matter.
d) Why is everything in the Universe made of matter? The processes we know result in equal amounts of matter and antimatter. A part of my research is investigating matter antimatter conversion processes.
e) Dark energy. The universal expansion rate is bigger than expected given the amount of matter and dark matter. Something is pushing it apart. We call this dark energy.
There are a number of other problems as well that involve the theory, results of calculations, and complexity of the standard model.
a) We would like the forces to unify at high energy. If you try to calculate the strengths of the interaction at very high energies, like those at the big bang, you find that they get very close to each other but don’t match up exactly.
b) Some diagrams in the SM have divergent contributions at high energy. Probabilities greater than 1 (100%). WW scattering is the main example. (I am researching this)
c) Why three generations, why all the masses we observe including such a large range, why the coupling constants we measure and why in general so complex?
d) Why do some of the values we measure have exactly the values we observe? To be precisely the values we observe there has to be a near complete cancelation between some contributions to the observed processes from interfering processes. This is known variously as the Hierarchy, naturalness and/or fine tuning problem.
e) Can we unify gravity with the other forces using a similar framework.
f) Why do we have to introduce the complex and in many cases counterintuitive frameworks of quantum mechanics and relativity to accurately describe nature. Is there something simpler?