4. The Strong Force

Question 1

What is strong charge?

Answer 1

Colour

- RGB

Question 2

What is the strong force boson, and state its properties

Answer 2

Gluon

• Massless, spin 1
• Bicoloured (colour anti colour), can never be a colourless gluon

Question 3

What is conserved by the strong interaction?

Answer 3

Colour
C, P, T
CP, CPT

Question 4

Can gluons interact with other gluons?

Answer 4

Yes as they carry colour charge

- Tri-linear and quartic coupling

Question 5

Which mesons can’t decay into a single gluon?

Answer 5

Mesons of the same particle antiparticle pair e.g. r r bar

Question 6

How does the strong coupling factor evolve with relation to Q^2?

Answer 6

The opposite way to EM coupling
- Goes to 0 as Q^2 goes to infinity (same as distance going to 0)
- Goes to inf as Q^2 goes to 0(same as distance going to inf)
As distance gets big, force gets big and vv

Question 7

Why does the strong coupling evolve in the opposite way to the EM force?

Answer 7

Due to colour screening

• Gluons experience screening effects due to gluon-gluon interactions
• Experience gluon anti-screening

Question 8

What is gluon anti-screening?

Answer 8

The screening due to gluon-gluon interactions

- Gluon loops increase colour charge at large distances

Question 9

How does the coupling effect quarks and gluons at SMALL distances?

Answer 9

V_QCD ~ alpha_s / r
- alpha_s is ~ r at small distances
= Force ~ 0
- Quarks and gluons are quasi-free (don’t really interact with each other) - ASYMPTOTIC FREEDOM

Question 10

How does the coupling effect quarks and gluons at LARGE distances?

Answer 10

V_QCD ~ Kr

- Force is a constant and indep. of dist.

Question 11

What is hadronisation?

Answer 11

The snapping of the colour field when lots of energy is put into separating two quarks
- Two new particles are created

Question 12

What is confinement?

Answer 12

The observation that quarks are never observed by themselves in the universe

Question 13

How does the value of the strong coupling change over distance?

Answer 13

Small distances: alpha_s < 1

Large distances: alpha_s > 1

Question 14

What is the OZI rule?

Answer 14

In a strong interaction diagram, if you can split it into two pieces only by cutting gluon lines, the diagram is surpressed

Question 15

For a decay, if the how does the coupling factor of the strong force change if the gluon carries a lot or a little of the total momentum?

Answer 15

Large when the gluons carry a small amount of mom.

Small when the gluons carry a large amount of mom.

Question 16

What is the eightfold path?

Answer 16

The observation that particles could be arranged in patterns that reflected underlying structures

Question 17

What are the R ratioes?

Answer 17

Ratioes of cross section

Question 18

How does R change?

Answer 18

It is a constant dependent on sqrt(S)

- Changes in jumps as the energy thresholds increase

Question 19

How do the prediction energy ranges predict new quarks?

Answer 19

There is an existence of a new q qbar final state when a quark mass threshold is passed

Question 20

What is the R number sensitive to?

Answer 20

The number of colours and the number of quark charges

Question 21

How does the graph of dσ/dQ^2 look for an indivisible target, and one that has structure?

Answer 21

Indivisible: Negative straight line with some positive intercept for both axis
Structure: Initial negative straight line that tails off on the x axis for for larger Q^2

Question 22

Why does the shape of a dσ/dQ^2 graph tail off for high Q^2?

Answer 22

High Q^2 means small de Broglie wavelength

- Can scatter of constituents

Question 23

What is x_Bj?

Answer 23

The fraction, x, of the proton momentum in the deep inelastic scattering

Question 24

What does x_Bj look like in deep inelastic scattering if we predict the proton is indivisible, or composed of N non-interacting particles?

Answer 24

Spikes at 1 for the indivisible proton

Spike at 1/N for the N non interacting constituents

Question 25

What does x_Bj look like in deep inelastic scattering if we predict the proton is composed on N interacting constituents?

Answer 25

Narrow bell curve centered about 1/N

Question 26

What does the x_Bj look like in reality and why?

Answer 26

Exponential decay with a bump at 1/N

• Bell curve if a photon hits a stable constituent
• Exponential type decay if the photon hits a loop particle in the quark/gluon sea

Question 27

What do we call the stable constituents in a proton?

Answer 27

Valence quarks

Question 28

What are the 3 pieces of evidence for the evidence of colour?

Answer 28

1. R ratio
2. pi-0 decay into 2 photons
3. Corner states in the eight fold path

Question 29

Why do the corner states in the eightfold path show evidence for colour?

Answer 29

If we just have the eigenstates of flavour, spin and spatial, the corner states would have symmetric wavefunctions.

• They are fermions so they must have antisymmetric wavefunctions
• The colour allows for the distinguishability