8. Beyond SM Flashcards
State what is currently known about the Higgs
- Mass
- Width
- Charge (0)
- Spin (0) and not a vector as it can decay to two photons
- Parity (0)
What is the coupling strength of the Higgs to a particle proportional to?
The particle mass
Describe the easiest mechanism to detect the Higgs
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*
Describe which particles the Higgs cannot decay to and why
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
Why is the Higgs decay into photons surprising
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
Give some examples of important results involving the Higgs which haven’t been detected yet
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
What is the Yukawa coupling for the top and how does it decay
Close to 1 Yukawa coupling
Decays weakly
Describe the 4 processes for producing top quarks
- Gluon-gluon fusion
- T channel top production
- Quark antiquark annihilation
- Single top
Why must the top quark decay weakly, and what is the most common decay process called?
Because the Strong and EM decays require top flavour conservation
- Most common process is the W emission
In the top decay process, how might the W+ decay?
- Decay into a charged lepton and corresponding neutrino
- Decay into a q qbar
What is the idea of the grand unification theory (GUT)?
To unify the electroweak with the strong interaction into a single gauge group and single theory
What is the current hypothesis for GUT
The running of the 3 gauge coupling constants might converge towards unified values at very high energies (10^17 GeV)
Why do the couplings “run” at higher energies?
Due to self interactions
Describe the group for the GUT and the problems with its predictions
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
How many generators are in an SU(n) group?
n^2 - 1
What is the approximate predicted lifetime of the proton?
Order 10^34 years
What is the hierarchy problem?
In perturbation theory, once all loop diagrams are included, the mass of the Higgs is not undercontrol, and tends to an infinitely large value
How is the hierarchy problem solved?
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
How does SUSY affect space-time?
SUSY is a symmetry of s-t
Describe the new family of particles postualted by SUSY
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
Why has SUSY not been observed?
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
