The Physics of the Standard Model (2002) - Part4. Quantum Chromodynamics.pdf

THE PHYSICS OF THE STANDARD MODEL
2002
PART 4 (Lectures 12-15)
QUANTUM CHROMODYNAMICS (QCD)
L. PEAK and K. VARVELL
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QUANTUM CHROMODYNAMICS (QCD)
QCD is the part of the Standard Model that describes the strong
•
interaction.
It forms the SU(3) part of U(1) SU(2) SU(3)
•× ×
QCD is a gauge theory of interacting quarks and gluons. The gluons are
•
the gauge bosons of the theory.
It introduces a new concept and quantum number (a new type of
•
charge), colour. Hence SU(3)COLOUR.
A theory of the strong interactions must shed light on some known properties
which are different to Quantum Electrodynamics (QED).
The force is short range (the Coulomb force is infinite in range).
•
The strength decreases at shorter distances (asymptotic freedom).
•
The force is confining (free quarks are not observed).
•
1
QUARKS
Particle physicists postulate the existence of six types (flavours) of quarks,
which are spin 1/2 fermions like the electron, but possessing fractional charge.
Flavour Charge (e) Mass (MeV)
d 1 5
− 3
2
u + 3 10
s 1 200
− 3
2
c + 3 1300
b 1 4300
− 3
2
t + 3 180000
The masses are indicative only - we have never seen free, unbound quarks.
In addition, there are six flavours of antiquark,
(d, u, s, c, b, t)
with the same mass and spin, but opposite charge.
2
WHY QUARKS?
The quark concept was introduced in the 1960s. Reasons:
➀ The large number of hadrons discovered to that time, many extremely
short-lived resonances. (hadrons are particles feeling the strong force).
Perhaps there were so many because they posed of
combinations of fewer, more elementary objects.
.
Baryons (half integral spin, fermions)
p(938), n(939)
∆++(1232), ∆+(1232) ∆0(1232), ∆−(1232)
Λ(1116)
Mesons (integral spin, bosons)
π+(139), π0(135), π−(139)
ρ+(770), ρ0(770), ρ−(770)
ω(783), φ(1020)
3
➁ The quark hypothesis neatly explained patterns which were known t