Transcript More on the Standard Model

More on the Standard Model
•
•
•
•
•
•
Particles from quarks
Particle interactions
Particle decays
More conservation laws
Quark confinement
Spin
The fundamental particles
u
c
t
+2/3
d
s
b
-1/3
e
e
µ
µ
 0

six quarks
six leptons
-1
These particles interact through the
exchange of a force carrier.
Particles from Quarks
The quarks combine in two ways to form
the particles we see in the lab:
Quark-antiquark pairs from mesons:
u and anti-d = +
u and anti-s = K+
and so on, making hundreds of particles.
Quiz: How would I make a neutral K
particle?
Particles from Quarks
Three quarks combine to form a baryon such as
the proton and neutron
uud = proton
udd = neutron
uds = lambda ()
and so on, making hundreds of baryons
Quiz: What is the charge of the lambda?
Force carriers
Particles interact through the
exchange of force carriers.
There are four force carriers:
Photon () for EM interaction
Gluon (g) for the strong interaction
W+, W- and Z0 for the weak
interaction
Electromagnetic interaction example
The photon is the force carrier, and it couples
to electric charge.
Weak Interaction examples
The Ws and Zs are the
carriers of the weak
force, and they couple
to all known particles
Quarks can change from one
type to another (flavor changing),
but ONLY through the exchange
of a W.
Strong interaction example
The leptons do NOT
see the strong
interaction.
Only the quarks
interact strongly.
Heavier particles decay into ligher particles
But subject to the rules:
conservation of energy, momentum and
angular momentum
conservation of electric charge
conservation of the number of baryons
and
leptons
So for example
n p e e
  µ µ
µ  e e µ
Particles decay as far as possible
Particles keep on decaying until there is some
conservation rule that forbids them from
decaying further.
The electron is the lightest charged particle,
so it is stable.
But wait…why doesn’t a proton decay into a
positron and say a 0 ?
Baryon number/lepton number
In particle physics we have two additional
conservation laws that all decays ever observed
obey.
The total number of baryons is constant.
The total number of leptons is constant.
Anti-particles carry opposite baryon/lepton
number.
_
n p e e
(bar = antiparticle)
But what about the matter-antimatter
asymmetry of the universe?
If the number of baryons is a constant in the universe,and
we make matter and antimatter in pairs…
How did we arrive at a matter-dominated universe?
We don’t know the answer completely, but we do know it
is related to the violation of the fundamental symmetries
of parity (mirror reflection) and charge conjugation
(matterantimatter)
Why no free quarks?
There are no free quarks—why?
People have looked for decades for free quarks, but
none have been observed.
We can pull an electron off an atom, so why can’t
we pull a quark out of a proton?
The answer has to do with the nature of the force
Holding them together, the so-called color force.
Why no free quarks?
The potential energy two electric charges:
U=-kqQ/r
The potential energy between two color
charges is Coulomb-like but has an extra
term:
U=-kqQ/r+Kr
The PE increases with distance!
Quark- quark potential energy
The quark potential looks
like the Coulomb potential
close up, but when the
separation is large, the
linear term dominates
In these units 1=the
proton radius.
Quark -quark potential energy
As r gets large, the PE
increases without bound
and the quark cannot
escape!!
We call this
CONFINEMENT!
Electric field between electric charges
Electric Dipole
Color field between color charges
Color Dipole
When the distance beteen the quarks gets large, a
great deal of PE is stored in the color field.
Confinement!
When two quarks separate, at some point there is
enough energy in the field to create a quarkantiquark pair. Then the color field lines snap, and
two mesons are created!
Jet Production
The color charge holding the quarks together
is so strong, you can never pull a free quark
out of a proton.
The harder you try to pull a quark out of a
proton, the more mesons you get!
The stream of mesons that marks the path of
a quark we call a jet.
We can see quarks
The jet of particles
produced by the
quark allows us to
get some measure
of its energy and
momentum.
Spin
All the fundamental particles have one other
property-- intrinsic angular momentum. They all
have ½ unit of angular momentum, where the unit is
h/2.
Particles with ½ integral spin are called fermions.
Particles with integral spin are called bosons.
Combining two spin ½ fermions gives a boson
Adding spins
A state of two electrons will have spin either 1
or 0, both of which are bosons.
Spins add, total spin=1
Spins cancel, spin=0
Fermions and the exclusion principle
Only one fermion can be an any state, which explains
most of chemistry…the electrons fill up the energy
levels with only one per state. This is the Pauli
exclusion principle.
But can’t I put two electrons per state? Yes, but their
spins are in different directions, so they are not really
in the same state.
Supersymmetry
Supersymmetry is an
unproven theory that
postulates a boson for every
fermion and a fermion for
every boson!
It solves some problems
deep in the mathematics of
the Standard Model.
But…ugh…too many fundamental particles. And
where are they anyway? (Must be heavy or we would
have seen them by now.)
Supersymmetry and dark matter
Supersymmetric particles are
prime candidates to be the
dark matter of galactic halos.
But there is as yet no
evidence for supersymmetric
particles, although we are
looking very hard!
Supersymmetry?
Supersymmetry is an offer nature can’t refuse.
D.V. Nanopolus, theorist
There ain’t no Supersymmetry.
Leo Bellantoni, Fermilab physicist
Experiment is the sole judge of scientific truth.
Richard Feynman
Supersymmetric names
The supersymmetric partner of the top quark is called
stop.
The supersymmetric partners of any quark is a squark.
The supersymmetric partner of the photon is the
photino.
The supersymmetric partner of the W is the …
(I am not making this up..)