Transcript 14. Elementary Particles

CHAPTER 14
Elementary Particles
Homework due Monday December 7th
Chapter 14: 1, 3, 5, 6, 8, 10, 11, 15, 22, 25
No class this Friday, Dec. 4th!
Steven Weinberg (1933 - )
Hadrons
Hadrons are particles that
act through the strong force.
Quarks
Two classes of hadrons:
mesons and baryons.
Mesons are particles with integral spin having masses greater
than that of the muon (106 MeV/c2). They’re unstable and rare.
Baryons have masses at least as large as the proton and have
half-integral spins. Baryons include the proton and neutron, which
make up the atomic nucleus, but many other unstable baryons
exist as well. The term "baryon" is derived from the Greek βαρύς
(barys), meaning "heavy," because at the time of their naming it
was believed that baryons were characterized by having greater
mass than other particles. All baryons decay into protons.
Some
Hadrons
Energy and momentum in particle decays
1. The energy available for the decay is the difference in the rest energy
between the initial decaying particle and the particles that are produced
in the decay. We call the Q value:
Q=(mi-mf)c2
Of course Q must be positive for the decay to occur.
2. The available energy Q is shared as kinetic energy of the decay products
in such a way as to conserve linear momentum and energy.
Example
Compute the energies of the proton and the  meson that result from the
decay of a ° at rest.
Q=(m°-mp-m)c2= 1116MeV - 938MeV - 140MeV = 38MeV
So the total kinetic energy of the decay products must be:
Kp + K = 38MeV
Conservation of relativistic momentum requires pp=p
Substitute into the relativistic formula for Kinetic Energy yields
pp=p = 101MeV/c
The kinetic energies are: K=33MeV and Kp=5 MeV
Baryon Conservation
The number of nucleons (baryons) is always conserved.
So we define a new quantum number called baryon number,
which has the value B = +1 for baryons and −1 for anti-baryons,
and 0 for all other particles (mesons, leptons).
0
The conservation
of baryon number
requires the same
total baryon
number before
and after an
interaction.
0
0
1
1
Lepton Conservation
The leptons are all fundamental
particles, and there is conservation
of leptons for each of the three
kinds (families) of leptons.
The number of leptons from each family is the same both before and
after a reaction.
We let Le = +1 for the electron and the electron neutrino; Le = −1 for
their antiparticles; and Le = 0 for all other particles.
We assign the quantum numbers Lm for the muon and its neutrino and
Lt for the tau and its neutrino similarly.
Thus leptons give us three additional conservation laws.
A Veritable Zoo of Particles!
Physicists like to think that the universe is,
in the end, simple and elegant.
So maybe all these particles are in fact
composed of a smaller set of simpler
ones.
Quarks!
Murray Gell-Mann (1929- )
First proposed in 1964 by Murray Gell-Mann
and George Zweig, quarks have charges of
±1/3 and ±2/3 that of an electron. An up quark
has a charge of +2/3, and a down quark has a
charge of -1/3.
Two ups and a down make a proton. An up and
two downs make a neutron.
A proton
Baryons
and Mesons
Revisited
Mesons are made up of pairs of quarks—
a quark and an anti-quark.
Baryons are made up of three quarks.
Evidence for Quarks
In 1967, at the Stanford Linear
Accelerator Center (SLAC),
Jerome Friedman, Henry Kendall,
and Richard Taylor scattered 20GeV electrons off protons,
analogous to experiments
performed by Rutherford on the
nucleus five decades earlier, and
found back-scattered electrons
and that the proton had internal
structure (that is, quarks!).
Nevertheless, the quark idea only
caught on slowly, and it wasn’t
until 1990 that they won the Nobel
Prize.
Richard E. Taylor (1929-)
Truth, Beauty,
and Charm
A strange quark (s) was also required.
And then a fourth quark called the charmed quark (c) was proposed
to explain some additional discrepancies in the lifetimes of some of
the known particles.
A new quantum number called charm C was introduced so that the
new quark would have C = +1 while its anti-quark would have C = −1
and particles without the charmed quark have C = 0.
Charm is similar to strangeness in that it is conserved in the strong
and electromagnetic interactions, but not in the weak interactions.
This behavior was sufficient to explain the particle lifetime difficulties.
Two additional quarks, top and bottom (or truth and beauty), were
also required to construct some exotic particles (the Upsilon-meson).
Quark Properties
The spin of all quarks (and anti-quarks) is 1/2.
Quark
Description
of Particles
Baryons
normally consist
of three quarks
or anti-quarks.
-2/3e
2/3e
2/3e
-1/3e
-2/3e
2/3e
-1/3e
2/3e
-1/3e
-1/3e
2/3e
1/3e
A meson
consists of a
quark-anti-quark
pair, yielding the
required baryon
number of 0.
1/3e
-2/3e
2/3e
-1/3e
-2/3e
-1/3e
1/3e
2/3e
Gluons
The particle that mediates the very strong interaction between
quarks is called a gluon (for the “glue” that holds the quarks
together); it’s massless and has spin 1, just like the photon.
Like the photon, the gluon
has two transverse
polarization states.
There are eight
independent types of
gluon (depending on the
colors of the quarks
involved).
Computed image of quarks
and gluons in a nucleon
Quark-Antiquark
Creation
No one’s ever measured a
free quark.
Physicists now believe that
free quarks cannot be
observed; they can only exist
within hadrons. This is called
confinement.
This occurs because the force
between the quarks increases
rapidly with distance, and the
energy supplied to separate
them creates new quark-antiquark pairs.
J/y
Fundamental and Composite Particles
We call certain particles
fundamental; this means that
they aren’t composed of other,
smaller particles. We believe
leptons, quarks, and gauge
bosons are fundamental
particles.
Other particles are composites,
made from the fundamental
particles.
Some of these fundamental
particles (W, Z, m, t) have short
lifetimes and decay, but this is
okay.
The Families
of Matter
The three generations (or
families) of matter. Note that
both quarks and leptons exist in
three distinct sets. One of each
charge type of quark and lepton
make up a generation.
All visible matter in the
universe is made from
the first generation.
Second- and third-generation particles are unstable and decay into firstgeneration particles. Second-generation particles occur in astrophysical
objects and cosmic rays. Third-generation particles were probably
important in the early universe. All are produced in accelerators.
The Four Fundamental Interactions
The Four Fundamental Interactions
Photons and gravitons are massless. W and Z bosons are heavy.
Gluons are also massless and appear to violate our calculation of
the inverse relation between the range and mass of such particles,
but, because quarks are confined within hadrons, this effectively
limits the range of the strong interaction to 10−15 meters, roughly the
size of an atomic nucleus.
But why are there four fundamental interactions?
The Standard Model
Over the latter half of the 20th
century, numerous physicists
combined efforts to model the
electromagnetic, weak, and strong
interactions, which has resulted in
The Standard Model.
It is currently widely accepted.
It is a relatively simple, comprehensive theory that explains hundreds
of particles and complex interactions with six quarks, six leptons, and
four force-mediating particles.
It’s based on three independent interactions, symmetries and
coupling constants.
The Higgs Boson
What about all the particle masses?
The Standard Model of particle physics proposes that there’s a field
called the Higgs field that permeates all of space.
By interacting with this field, particles acquire mass. Particles that
interact strongly with the Higgs field have heavy mass; particles that
interact weakly have small mass.
The Higgs field requires another boson.
It’s called the Higgs particle (or
Higgs boson) after Peter Higgs.
.
The Higgs boson was recently detected!
That little bump?
That's where
CERN has seen
a significant
number of
unusual events
at about 125
GeV, which
means that
something new
is going on.
Are we sure it’s
the Higgs? All
evidence to date
suggests yes.
Higgs!
In the mid-19th century, Maxwell unified
electricity and magnetism into a single force
with his now famous equations.
Ampère’s
Law
In free space:
E  0
B  0
B
  E  0
t
1 E
 B  02
c t
Maxwell’s term (displacement current)
where E is the electric field, B is the
magnetic field, and c is the velocity of light.
James Clerk Maxwell
(1831-1879)
Unifying the Electromagnetic and Weak
Forces: the Electroweak Theory
Sheldon
Glashow
(1932- )
Steven
Weinberg
(1933 - )
In the 1960s, Sheldon
Glashow, Steven
Weinberg, and Abdus
Salam unified the
electromagnetic and weak
interactions into what they
called the electroweak
theory.
Grand Unifying Theories
Several grand unified theories now combine the weak,
electromagnetic, and strong interactions into one interaction
and predict that, at extremely high energies (>1014 GeV), the
electromagnetic, weak, and strong forces fuse into a single unified
field.
Currently, they can explain why:
Neutrinos have a small, nonzero mass.
The proton and electron charges have the same magnitude.
Massive magnetic monopoles may exist. Just one anywhere in
the universe would explain charge quantization.
But current experimental measurements have shown the proton
lifetime to be greater than 1035 years. Current theory has it at 1029 to
1031 years.
Including Gravity: String Theory
For the last two decades there has been a tremendous amount of
effort by theorists in string theory, which has had several variations.
The addition of super-symmetry resulted in the name theory of
super-strings.
In super-string theory, elementary particles don’t exist as points, but
rather as tiny, wiggling loops that are only 10−35 m in length.
Presently super-string theory is a promising approach to unify the
four fundamental forces, including gravity.
However, because experiments that can confirm or falsify string
theory require orders of magnitude more energetic particles than
can currently be produced, these theories are very controversial.
Super-Symmetry
Super-symmetry is a
necessary ingredient
in many of the theories
trying to unify all four
forces of nature.
The symmetry relates fermions
and bosons. All fermions have a
super-partner: a boson of equal mass, and vice versa.
These particles include sleptons, squarks, axions, winos, photinos,
zinos, gluinos, and preons.
The super-partner spins differ by ħ/2.
Presently, none of the known leptons, quarks, or gauge bosons can
be identified with a super-partner of any other particle type.
M-Theory
Recently theorists
have proposed
a successor to
super-string theory
called M-theory.
M-theory has 11 dimensions (ten spatial and one temporal) and
predicts that strings coexist with membranes, called “branes” for short.
Only through experiments (which no one currently knows how to do,
since they require > 1,000,000 times more energy than current
accelerators can produce) will scientists be able to wade through the
vast number of unifying theories.