8. Beyond SM

Question 1

State what is currently known about the Higgs

Answer 1

• Mass
• Width
• Charge (0)
• Spin (0) and not a vector as it can decay to two photons
• Parity (0)

Question 2

What is the coupling strength of the Higgs to a particle proportional to?

Answer 2

The particle mass

Question 3

Describe the easiest mechanism to detect the Higgs

Answer 3

QCD -> top -> Higgs
- Higgs produced in hadronic clutter. Hard to isolate the decay into hadrons due to this
- Easy to see the decays into non hadronic states such as 2 photons, W W* and Z Z*

Question 4

Describe which particles the Higgs cannot decay to and why

Answer 4

Cannot decay into the top and anti top
- Combined mass is greater than the Higgs
- Can decay into any other matching particle and anti particle pair

Question 5

Why is the Higgs decay into photons surprising

Answer 5

Higgs is electrically neutral and photons are massless
- Higgs coupling strength is proportional to the mass of the particle
- Uses the top triangle or W boson triangle inbetween

Question 6

Give some examples of important results involving the Higgs which haven’t been detected yet

Answer 6

The Higgs coupling to a top and anti top not through a loop
The Higgs decaying to a muon and anti muon and charm anti charm
Determination of the total width of the Higgs

Question 7

What is the Yukawa coupling for the top and how does it decay

Answer 7

Close to 1 Yukawa coupling
Decays weakly

Question 8

Describe the 4 processes for producing top quarks

Answer 8

1. Gluon-gluon fusion
2. T channel top production
3. Quark antiquark annihilation
4. Single top

Question 9

Why must the top quark decay weakly, and what is the most common decay process called?

Answer 9

Because the Strong and EM decays require top flavour conservation
- Most common process is the W emission

Question 10

In the top decay process, how might the W+ decay?

Answer 10

• Decay into a charged lepton and corresponding neutrino
• Decay into a q qbar

Question 11

What is the idea of the grand unification theory (GUT)?

Answer 11

To unify the electroweak with the strong interaction into a single gauge group and single theory

Question 12

What is the current hypothesis for GUT

Answer 12

The running of the 3 gauge coupling constants might converge towards unified values at very high energies (10^17 GeV)

Question 13

Why do the couplings “run” at higher energies?

Answer 13

Due to self interactions

Question 14

Describe the group for the GUT and the problems with its predictions

Answer 14

Based on the SU(5) group
- SU 1,2,3 are all subgroups of SU(5)
- 24 generators so the 12 bosons are joined by 12 more (X,Y bosons)
- Predicts the Weinberg angle as 3/8 before normalisation
- Predicts proton decay as u,d quarks aren’t stable and can decay into leptons. Not observed as of yet

Question 15

How many generators are in an SU(n) group?

Answer 15

n^2 - 1

Question 16

What is the approximate predicted lifetime of the proton?

Answer 16

Order 10^34 years

Question 17

What is the hierarchy problem?

Answer 17

In perturbation theory, once all loop diagrams are included, the mass of the Higgs is not undercontrol, and tends to an infinitely large value

Question 18

How is the hierarchy problem solved?

Answer 18

By postulating a new SuperSymmetry
- For every fermion species, there is a corresponding boson and vv
- Fermions and bosons make renormalised contributions to the Higgs mass of opposite sign

Question 19

How does SUSY affect space-time?

Answer 19

SUSY is a symmetry of s-t

Question 20

Describe the new family of particles postualted by SUSY

Answer 20

Boson partners of fermions have an “s” infront of their name to indicate “supersymmetric”
- squarks and sleptons - top squark/stop
Fermion partners of gauge gauge bosons are gauginos
- Photino, wino, zino, gluino
Fermion partners of the Higgs are Higgsinos

Question 21

Why has SUSY not been observed?

Answer 21

SUSY is spontaneously broken, so SUSY partners get masses much larger than their normal partners
- The very large masses given to the Higgs by loop diagrams are cancelled at high energies by the SUSY breaking scale
- This is imperfect at low energies so the SUSY breaking scale cant be much bigger than the Higgs mass otherwise the Higgs mass should be larger