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Transcript
Physics 5333
Spring 2015
(Chapter 1: getting started)




the elementary particles
the forces
the model
how can we understand it?
Elementary particle:
an entity not able to be further decomposed
with a unique set of properties
mass, m
charge, Q
spin : s =½ integer (fermion),
s = 0, 1, 2…(boson)
 flavor



spin, charge & mass(energy)
Intrinsic property  “constituents” do not exist
We don’t know how to account for the property by classical,
quantum mechanical or relativistic (field theoretic) models
what is charge?
Charge (Q) is a quantity we have defined in order to describe how
certain particles (with this charge) interact. If the particles don’t
interact in the prescribed way, they don’t have charge.
The force, F, between two charges (and the classical mathematical
model, Coulomb’s Law, kQ1Q2/r2), was derived experimentally.
Subsequent to this we developed the ideas of electric fields, E=F/Q1
electrostatic potentials, Ф, magnetic fields, B, (from moving Q or
changing E fields) and Maxwell’s equations, the most rigorous model
in physics. Still, this does not tell us what charge is.
The models above have been extended to a startling new model
(Quantum Electrodynamics) which “explains” why two charges
interact: they exchange photons (a new kind of particle with no
charge, travelling with the speed of light).
Still, we do not know what charge is.
about charge and the electron
We do know that charge is “quantized”: it comes only
in multiples of the electronic charge, e = 1.6 x 10-19
Coulombs.
Furthermore, the electron itself, although having both
mass and charge, e , has a “size” so small that we are
able only to say it is smaller than what we can detect!
This is indeed a phenomenon!
the elementary particles
(as far as we know at this time)
six quarks (u d
 six leptons (e ne

cs
m nm
t b)
t nt)
all have spin = ½  they are fermions
that’s it!
size
Like the electron, these elementary particles have
“sizes” smaller than we can detect.
Another phenomenon!
mn <
0.3 eV
Mass of proton = 0.938 GeV/c2
Particle  Antiparticle
Q  -Q
mm
an antiparticle is like a particle
‘going backwards in time’
Building composite particles –
with sizes we can detect:
Quarks (q) can be bound together to form composite
particles, like protons, neutrons and pions.
But, we only find in the laboratory composite particles
corresponding to quark-antiquark or qqq combinations.
(LHC November 2015: possibly 5-quark particle )
These composite particles of quarks are held together by
the strong force mediated by the exchange of gluons.
Like the electric charge which produces the Coulomb
force, the color “charge” is carried by the quarks.