5. Unified Electroweak Theory

Question 1

What 4 things does the unified electroweak theory explain?

Answer 1

1. How the photon remains massless
2. The inverse square form of Coulomb’s law
3. The W Z bosons acquiring mass from the Higgs
4. The limitation of the theory to 1fm

Question 2

What is helicity?

Answer 2

The projection of a particles spin onto its momentum direction

Question 3

What do positive and negative helicity states correspond to?

Answer 3

Positive are right handed
Negative are left handed

Question 4

Is helicity Lorentz invariant, and a conserved quantity?

Answer 4

It is conserved
It is only Lorentz invariant for massless particles

Question 5

Is chirality Lorentz invariant, and a conserved quantity?

Answer 5

Chirality is only conserved for massless particles
It is Lorentz invariant

Question 6

How are chirality and helicity related

Answer 6

They are equivalent for massless particles
They differ by an amount proportional to the particle’s mass for massive particles

Question 7

Which components do the EM and strong force couple to for fermions?

Answer 7

They couple to both left and right handed components with equal probability

Question 8

Which components does the weak interaction couple to for fermions?

Answer 8

Only left handed components (or RH of antiparticles)
- This is responsible for the parity violation

Question 9

When trying to unify the electroweak theory, what were the 3 obstacles that had to be overcome?

Answer 9

1. The large difference in strength between the weak and EM interactions
   - Mediated by large bosons
2. Why is the photon massless, but the W and Z so massive
   - Couplings to Higgs
3. Why does EM have vector couplings but the weak have V-A couplings

Question 10

What are radiative corrections?

Answer 10

When the theoretical predictions differ from experimental results
- Often work to the lowest order which neglects virtual boson and fermion loops

Question 11

Describe what is meant by the energy frontier when considering particle physics research

Answer 11

Trying to build the most energetic particle accelerator to reach the highest possible masses for new particles

Question 12

Describe what is meant by the precision frontier when considering particle physics research

Answer 12

Try and detect unobserved particles by using very precise measurements e.g. loop diagrams