4. The Strong Force Flashcards
What is strong charge?
Colour
- RGB
What is the strong force boson, and state its properties
Gluon
- Massless, spin 1
- Bicoloured (colour anti colour), can never be a colourless gluon
What is conserved by the strong interaction?
Colour
C, P, T
CP, CPT
Can gluons interact with other gluons?
Yes as they carry colour charge
- Tri-linear and quartic coupling
Which mesons can’t decay into a single gluon?
Mesons of the same particle antiparticle pair e.g. r r bar
How does the strong coupling factor evolve with relation to Q^2?
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
Why does the strong coupling evolve in the opposite way to the EM force?
Due to colour screening
- Gluons experience screening effects due to gluon-gluon interactions
- Experience gluon anti-screening
What is gluon anti-screening?
The screening due to gluon-gluon interactions
- Gluon loops increase colour charge at large distances
How does the coupling effect quarks and gluons at SMALL distances?
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
How does the coupling effect quarks and gluons at LARGE distances?
V_QCD ~ Kr
- Force is a constant and indep. of dist.
What is hadronisation?
The snapping of the colour field when lots of energy is put into separating two quarks
- Two new particles are created
What is confinement?
The observation that quarks are never observed by themselves in the universe
How does the value of the strong coupling change over distance?
Small distances: alpha_s < 1
Large distances: alpha_s > 1
What is the OZI rule?
In a strong interaction diagram, if you can split it into two pieces only by cutting gluon lines, the diagram is surpressed
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?
Large when the gluons carry a small amount of mom.
Small when the gluons carry a large amount of mom.
What is the eightfold path?
The observation that particles could be arranged in patterns that reflected underlying structures
What are the R ratioes?
Ratioes of cross section
How does R change?
It is a constant dependent on sqrt(S)
- Changes in jumps as the energy thresholds increase
How do the prediction energy ranges predict new quarks?
There is an existence of a new q qbar final state when a quark mass threshold is passed
What is the R number sensitive to?
The number of colours and the number of quark charges
How does the graph of dσ/dQ^2 look for an indivisible target, and one that has structure?
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
Why does the shape of a dσ/dQ^2 graph tail off for high Q^2?
High Q^2 means small de Broglie wavelength
- Can scatter of constituents
What is x_Bj?
The fraction, x, of the proton momentum in the deep inelastic scattering
What does x_Bj look like in deep inelastic scattering if we predict the proton is indivisible, or composed of N non-interacting particles?
Spikes at 1 for the indivisible proton
Spike at 1/N for the N non interacting constituents
What does x_Bj look like in deep inelastic scattering if we predict the proton is composed on N interacting constituents?
Narrow bell curve centered about 1/N
What does the x_Bj look like in reality and why?
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
What do we call the stable constituents in a proton?
Valence quarks
What are the 3 pieces of evidence for the evidence of colour?
- R ratio
- pi-0 decay into 2 photons
- Corner states in the eight fold path
Why do the corner states in the eightfold path show evidence for colour?
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
