LECTURE 13 QUARKS PHY492 Nuclear and Elementary Particle Physics

LECTURE 13
QUARKS
PHY492 Nuclear and Elementary Particle Physics
Elementary Particles
February 7, 2014 PHY492, Lecture 13 2 Quarks
Quarks : strongly interacting particles
fundamental constituents of matter,
but cannot be detected directly six quarks
generations
(flavors)
anti quarks
( ) ( ) ( ) u
d c
s t
b 1
2
3
u
d c
s t
b ( ) ( ) ( ) charges
+ 2/3 e
- 1/3 e
- 2/3 e
+ 1/3 e
They also interact by the weak and electromagnetic interactions, although
such effects can often be neglected compared to the strong interaction.
February 7, 2014 PHY492, Lecture 13 3 Evidence for Quarks 1
The quarks themselves have never been directly observed as single,
free particles, but these is compelling evidence for their existence.
Hadron Spectroscopy The study of the static properties of hadrons: their masses, lifetimes,
and decay modes, and their quantum numbers (spin, electric charge etc)
lead to the inference of quarks by Gell-Mann and Zweig in 1964. Example: strangeness
mass
the baryon octet
with Jπ = ½+
isospin
February 7, 2014 PHY492, Lecture 13 4 Evidence for Quarks 2
Lepton Scattering As an analogy to Rutherford scattering, high-energy lepton scattering
at large momentum transfers, revealed the existence of point-like
constituents “quarks”
Lepton Sca+ering Rutherford Sca+ering Au target
e- Nucleon
α quarks February 7, 2014 PHY492, Lecture 13 nuclei 5 Evidence for Quarks 3
Jet Production High-energy collisions can cause the quarks within hadrons, or
newly created quark – antiquark pairs, to fly apart from each other
with very high energies.
e+ + e- → q + q However, quarks have never been
observed as free particles. Quarks
exist only within hadrons (confinement).
Theoretically, this is explained by
Quantum chromodynamics (QCD).
a typical “two-jet” event
observed in the JADE chamber February 7, 2014 PHY492, Lecture 13 6 Quark masses
Quark masses are inferred indirectly from the observed masses of their
hadron bound states. the baryon octet
with Jπ = ½+
(MeV/c2) mu = md = 0.3 GeV/c2
ms = 0.5 GeV/c2
m(dss,uss)
= 0.3 + 2x 0.5 GeV/c2
= 1.3 GeV/c2
(GeV/c2) very short February 7, 2014 PHY492, Lecture 13 7 Quark decay
The decay of quarks always takes place within a hadron
( the spectator model ).
For example, in the decay,
n → p + e- + νe
the exchanged particle (w-) interacts with
only one constituent quark in the nucleons. Quark Feynman diagram
in the spectator model In the above weak interaction, total quark number
Nq = N (q) – N (q)
is conserved. Nq (n) = 3, Nq (p + e- + νe) = 3 + 0 + 0 =3.
Often, one uses baryon number defined by B = Nq/3 = [N(q) – N(q) ]/3
February 7, 2014 PHY492, Lecture 13 8 Quark Numbers
In strong and electromagnetic interactions, quarks can only be created
or destroyed by quark – antiquark pairs. Thus, each of the six quark numbers,
N q = N(q) – N (q)
( q = u,d,s,c,b,t )
is conserved.
allowed forbidden February 7, 2014 e+ + e - → c + c
e+ + e - → c + u
PHY492, Lecture 13 Nf(e+ + e-) = 0 for all f Nc(c + c) = 0 Nc(c + u) = 1 Nu(c + u) = -1 9 Quark Numbers
But for weak interactions, the quark flavor number is *NOT* conserved! February 7, 2014 PHY492, Lecture 13 10