Transcript Document

Modern Physics
LECTURE II
Atomic Particles
 Atoms are made of
protons, neutrons and
electrons
 99.999999999999%
of the atom is empty space
 Electrons have locations
described by probability
functions
 Nuclei have protons and
neutrons
nucleus
mp = 1836 me
Leptons
 An electron is the most common example of a
lepton – particles which appear pointlike
 Neutrinos are also leptons
 There are 3 generations of leptons, each has
a massive particle and an associated neutrino
 Each lepton also has an anti-lepton (for
example the electron and positron)
 Heavier leptons decay into lighter leptons
plus neutrinos (but lepton number must be
conserved in these decays)
Types of Leptons
Mass
(GeV/c2)
Lepton
Electron
neutrino
Electron
Charge
0
-1
0.000511
Muon
neutrino
0
0
Muon
-1
0.106
Tau
neutrino
0
0
Tau
-1
175
0
Quarks
 Experiments have shown
that protons and neutrons
are made of smaller
particles
 We call them “quarks”, a
phrase coined by Murray
Gellman after James
Joyce’s “three quarks for
Muster Mark”
 Every quark has an antiquark
Modern
picture of atom
Types of Quarks
Flavor
Charge
Mass
(GeV/c2)
Up
2/3
0.003
Down
-1/3
0.006
Charm
2/3
1.3
Strange
-1/3
0.1
Top
2/3
175
Bottom
-1/3
4.3
 Quarks come in
three generations
 All normal matter
is made of the
lightest 2 quarks
Combining Quarks
 Particles made of quarks are
called hadrons
 3 quarks can combine to
make a baryon (examples are
protons and neutrons)
 A quark and an anti-quark can
combine to make a meson
(examples are pions and
kaons)
proton
meson
 Fractional quark electromagnetic charges add to
integers in all hadrons
Color charges
 Each quark has a color charge
and each anti-quark has an anticolor charge
 Particles made of quarks are color
neutral, either R+B+G or color +
anti-color
Quarks are
continually changing
their colors – they
are not one color
Gluon exchange
 Quarks exchange gluons within a nucleon
movie
Atomic Forces
 Electrons are bound to
nucleus by Coulomb
(electromagnetic) force
 Protons in nucleus are held
together by residual strong
nuclear force
 Neutrons can beta-decay into
protons by weak nuclear
force, emitting an electron
and an anti-neutrino
F = k q 1 q2
r2
n=p+e+n
Protons and neutrons are made up
of quarks bound together by gluons.
Like charges repel, so why does
the positive charge within a
proton not cause the proton to
explode?
The (Coulomb) repulsion is
defeated by a new force:
The STRONG force.
Fundamental Forces
 Gravity and the electromagnetic
forces both have infinite range but
gravity is 1036 times weaker at a
given distance
 The strong and weak forces are
both short range forces (<10-14 m)
 The weak force is 108 times
weaker than the strong force
within a nucleus
Force Carriers
The Uncertainty Principle
 Classical physics
 Measurement uncertainty is due to limitations of
the measurement apparatus
 There is no limit in principle to how accurate a
measurement can be made
 Quantum Mechanics
 There is a fundamental limit to the accuracy of a
measurement determined by the Heisenburg
uncertainty principle
 If a measurement of position is made with
precision Dx and a simultaneous measurement of
linear momentum is made with precision Dp, then
the product of the two uncertainties can never be
less than h/2p
DxDpx 
The Uncertainty Principle
DxDpx 
Virtual particles: created due to the UP
DE Dt 
Force Carriers
g
 Each force has a particle
which carries the force and
is unaffected by it
 Photons carry the
electromagnetic force
between charged particles
 Gluons carry the strong
force between color
charged quarks
Force Carriers
 Separating two quarks
creates more quarks as
energy from the colorforce field increases until it
is enough to form 2 new
quarks
 Weak force is carried by W
and Z particles; heavier
quarks and leptons decay
into lighter ones by
changing flavor
Forces are mediated by
particles
 Photons mediate electric and magnetic
forces. (Faraday and Ampère demonstrated that electric
and magnetic forces were different manifestations of the same
“electromagnetic” force.)
e
e

g
e
e

Forces are mediated by
particles
Gluons mediate the strong force.
q
q
q
g
q
There is also the weak
force
 It is responsible for the process by which two
protons “fuse” together in the core of the sun.
p  p  p  n  e n e
 It is “carried” by the W and Z particles.
Neutrons transform to protons via
beta decay. It is a result of the weak
force.
Gravity is the only other
force.
 It so weak as to be negligible in particle
physics experiments.
 Einstein’s “General Theory of Relativity”
superseded Newton’s Theory of Gravity
in 1915.
 An “ultimate” theory should explain how
gravitons mediate gravity…….?
Unifying Forces
 Weak and electromagnetic forces have been
unified into the “electroweak” force


They have equal strength at 10-18 m
Weak force is so much weaker at larger distances
because the W and Z particles are massive and
the photon is massless
 Attempts to unify the strong force with the
electroweak force are called “Grand Unified
Theories”
 There is no accepted GUT at present
Gravity
 Gravity may be carried by the graviton – it
has not yet been detected
 Gravity is not relevant on the sub-atomic
scale because it is so weak
 Scientists are trying to find a “Theory of
Everything” which can connect General
Relativity (the current theory of gravity) to the
other 3 forces
 There is no accepted Theory of Everything
(TOE) at present
Theory of Everything?
Standard Model
Gravity
Electroweak
Strong
Glashow, Salam, Weinberg
Weak
Electromagnetic
Ampere, Faraday, Maxwell
Electric
Magnetic
The Standard Model
 The weak and electromagnetic forces
were unified by Glashow, Weinberg &
Salam. Electroweak force
 GWS also explained how to incorporate
QCD, the model of the strong force.
 Their model defines the laws for all
known interactions except gravity.
Force Summary
Spin
 Spin is a purely quantum mechanical property which
can be measured and which must be conserved in
particle interactions
 Particles with half-integer spin are “fermions”
 Particles with integer spin are “bosons”
* Graviton
has spin 2
Quantum numbers
 Electric charge (fractional for quarks, integer
for everything else)
 Spin (half-integer or integer)
 Color charge (overall neutral in particles)
 Flavor (type of quark)
 Lepton family number (electron, muon or tau)
 Fermions obey the Pauli exclusion principle –
no 2 fermions in the same atom can have
identical quantum numbers
 Bosons do not obey the Pauli principle
Standard Model
 6 quarks (and 6 anti-quarks)
 6 leptons (and 6 anti-leptons)
 4 forces
 Force carriers (g, W+, W-, Zo, 8 gluons, graviton)
Some questions
 Do free quarks exist? Did they ever?
 Why do we observe matter and almost no antimatter if




we believe there is a symmetry between the two in the
universe?
Why can't the Standard Model predict a particle's mass?
Are quarks and leptons actually fundamental, or made
up of even more fundamental particles?
Why are there exactly three generations of quarks and
leptons?
How does gravity fit into all of this?