Particle

Question 1

What are the compositions of pi and K mesons?

Answer 1

Pi mesons are a combination of u and d.

K mesons are a combination of u and s.

Question 2

What are D and B mesons made from?

Answer 2

D0 is c and anti u.

B0 is b and anti d.

Question 3

What is meant by speed saturation? What is bunch stability?

Answer 3

LINACS are more effective when particle speed is uniform. This is the case for electrons above 5 MeV.

Fast, early particles are accelerated less and late particles are accelerated more. Hence the bunch is compressed.

Question 4

What is meant by the cross section of an interaction?

Answer 4

The physical area corresponding to that process. The scattering probability is P = δσ/Area(beam).

Question 5

What are the Rutherford scattering assumptions?

Answer 5

Nuclear recoil is neglected (breaks down for light nuclei).
Electrons are ignored (really they screen nuclear charge and take energy by being accelerated).
Used classical mechanics.

Only considered EM force, not strong force.
Nuclear target assumed to be at rest.

Question 5

What is the available energy in a linear accelerator?

Answer 5

E = qV and only the electric field can increase velocity as v x B is perpendicular to motion.

Question 6

What are the principles of the van der graff and Cockcroft-Walton accelerators?

Answer 6

Van der graff - charge carried by static on a rotating belt (low current, high voltage)
Cockcroft-Walton - similar, but charged via a system of capacitors and diodes which let current flow up only. All capacitors charge to |Vac| and Vdc = Nstage X |Vac| where each stage consists of 2 capacitors and 2 diodes.

Question 7

What is a folded tandem?

Answer 7

Essentially a folded Cockcroft-Walton; the accelerating electric field is used twice. H- in one direction, electrons stripped at anode and protons sent the other direction.

Question 8

What are advantages of a cyclotron compared to a LINAC?

Answer 8

Particles pass over the same accelerating stage many times so no need for many expensive cavities.
Static magnets, only the RF field changes between plates changes. This eases load in operation and gives high duty cycle.

Question 9

What is a disadvantage of synchrotrons?

Answer 9

They require the magnetic field strength to be ramped up over a period of a few hours, so electromagnets are required. The LHC has a cycle time of 24 hours.

Question 10

What is the power lost as bremsstrahlung radiation in synchrotrons?

Answer 10

U is proportional to gamma^4 / r.

Question 11

What is the method of finding new particles?

Answer 11

Define criteria for identifying the signal.
Estimate the expected background in the signal region.
Predict the rate in a matching signal free verification region.

Question 12

What is the criteria for a discovery?

Answer 12

P-value: probability of observing the data seen or a more extreme set (2.9 x 10^-7).

5 sigma: a deviation 5 times the expected fluctuation is considered statistically significant and has a probability given by the p-value.

Question 13

What is the composition of kaons, D, J/phi and B mesons?

Answer 13

K - involves a strange quark.
D - involves a charm quark.
J/Phi - c cbar.
B - involves a bottom quark.

Question 14

What are the assumptions of the Bethe-Block formula?

Answer 14

Only describes dE/dx loss.
Assume charged particle passes a free, stationary electron.
Ignore energy transfer to the nuclei.
Energy loss to individual electrons assumed negligible.

Question 15

What are the approximations on b min and max for the Bethe-Bloch formula?

Answer 15

B min is the de Broglie wavelength of the electron as seen by the incoming charged particle, h/gamma mv. We cannot use classical mechanics if the distance scale is less than the electron wavelength.

B max uses the interaction time, b/v. The particle sees a time dilated electron orbit period of gamma/. Equate the two.

Set gamma v = c.

Question 16

What is a delta ray?

Answer 16

A rare electron with a very high kinetic energy.

Question 17

What are the bremsstrahlung and pair production probabilities proportional to?

Answer 17

Z^2p/m^2
Hence nuclei dominate this interaction and high Z elements cause more radiation.
Also the 1/m^2 factor means electrons bremsstrahlung is always from electrons.

Question 18

What is the critical energy for electrons.

Answer 18

Above the critical energy, bremsstrahlung losses exceed ionisation losses. Ec is 0.4 GeV for electrons.
An EM shower can only occur above Ec.

Question 19

What makes hydronic showers less predicable?

Answer 19

The short range of the strong force means that the cross section for hadron is interactions ~ size of the nucleus. Energy is scattered by nuclear interactions and mostly into new particles.

Question 20

What is the thinking behind sampling calorimeters?

Answer 20

Alternating dense and sensitive material to detect the particle. This gives more information about the shower progression, but is less precise as the fraction of the ionisation detected is reduced.

Question 21

What are the primary design goals of the LHC?

Answer 21

Be able to identify electrons, muons and photons and measure momentum at 50 GeV/c with 1% precision and a few percent for jets.

Question 22

Why is a trigger needed?

Answer 22

Detector readout for one crossing takes ~1MB. A 40 MHz collision rate would mean 40 TB/s (not possible) so a trigger selects events of interest.

Question 23

Summarise the LHC detector layout?

Answer 23

Tracking; 1m radius and mostly Si.
EM calorimeter; 30 - 40 cm, 1 - 2 % energy resolution @ 50 GeV/c.
Hadron calorimeter; 1 - 2 m, 10 times worse energy resolution.
Muon system; diffuse magnetic field a length of a few meters, low particle density makes wire chambers the best choice, make very good momentum measurements.