Transcript 14. Elementary Particles

CHAPTER 14
Elementary Particles
14.1
14.2
14.8
14.3
14.4
14.5
14.6
14.7
Early Discoveries
The Fundamental Interactions
Accelerators
Classification of Elementary Particles
Conservation Laws and Symmetries
Quarks
The Families of Matter
Beyond the Standard Model
Steven Weinberg (1933 - )
“I have done a terrible thing: I have postulated a particle that cannot be
detected.”
- Wolfgang Pauli (after postulating the existence of the neutrino)
“If I could remember the names of all these particles, I’d be a botanist.”
- Enrico Fermi
Elementary
Particles
Particle physics tries to answer
the most fundamental questions
about nature:
What’s inside the nucleus?
What are the basic building
blocks of matter?
What are the forces that hold
matter together and break it
apart?
What new physical laws are
required to describe these
forces?
Discovery of the Neutron
By the 1920s, most atomic nuclei were known to be heavier than Z,
the atomic number. But only protons and electrons were known, so
many thought that the nucleus contained extra protons and an
equal number of extra electrons, too, to compensate.
But electrons can’t exist within the nucleus:
Nuclear size
The uncertainty principle puts a lower limit on its kinetic energy that’s
much larger that the kinetic energy observed for any electron emitted
from nuclei.
Nuclear spin
Deuterons were known. If a deuteron consisted of protons and
electrons, then it must contain 2 protons and 1 electron. A nucleus
composed of 3 fermions must have half-integral spin. But it had
been measured to be 1.
Nuclear Properties
The nuclear charge is +e times the number (Z) of protons.
Other Hydrogen atoms:
Deuterium: Heavy hydrogen. One proton and one neutron in its
nucleus.
Tritium: Heavier hydrogen! One proton and two neutrons.
The respective nuclei are called deuterons and tritons.
Atoms with the
same Z, but
different mass
number, are
called isotopes.
Number of nucleons
Number of protons
Hydrogen
The
Nuclear
Force
Because there’s no negative charge in the nucleus,
it’s clear that a new force is involved.
The nuclear force is called the strong force for the
obvious reason!
The nuclear potential energy vs. distance
The
angular
distribution
of nucleons
scattered
by other
nucleons
tells us the
nuclear
potential.
Nuclear
potential
Coulomb
repulsion
Summary of Early Discoveries
Thomson had identified
the electron in 1897, and
Einstein had defined the
photon in 1905.
The proton is the nucleus
of the hydrogen atom (let’s
give Rutherford credit for
its discovery).
In 1932 James Chadwick
identified the neutron,
actually first seen by Bothe
and Becker.
That seemed sufficient…
Protons, neutrons, electrons, and
photons—what do you give the
universe that has everything?
The Positron
In 1928 when Dirac combined
quantum mechanics with special
relativity, he introduced the
relativistic theory of the electron.
He found that, in free space, his wave
equation had negative, as well as
positive, energy solutions.
His theory can be interpreted as the
vacuum being filled with an infinite
sea of electrons with negative
energies.
Exciting an electron from the “sea,”
leaves behind a hole with negative
energy, that is, the positron, denoted
by e+.
Paul Dirac (1902-1984)
Vacuum
Electron &
positron
Positron!
E
0
Anti-Particles
Dirac’s theory yields anti-particles for all particles, which:
Have the same mass and lifetime as their associated particles.
Have the same magnitude but opposite sign for such physical
quantities as electric charge and various quantum numbers.
All particles, even
neutral ones, have
anti-particles (with
some exceptions like
the neutral pion,
whose anti-particle is
itself).
In 1932, Carl Anderson
identified the positron in cosmic
rays. It was easy: it had
positive charge and was light.
Anderson’s cloud chamber photo of
the first recorded positron track
Discovery of
the Positron
Carl Anderson (1905-1991)
Electron-Positron Interaction
The ultimate fate of positrons (anti-electrons) is annihilation with
electrons.
After a positron slows down by passing through matter, it’s attracted
by the Coulomb force to an electron, where it is annihilated through
the reaction:
e   e   2
All anti-matter eventually meets the
same fate. A lot of energy is
released in this process: all of the
matter is converted to energy.
Star Trek’s “dilithium crystals”
supposedly contain anti-matter,
which powers the Enterprise.
Quantum Field Theory and Feynman
Diagrams
Richard Feynman presented a particularly simple graphical
technique to describe many-particle interactions in what we now
call quantum field theory.
Electromagnetism can be interpreted as the exchange of photons.
We say that the photons
are the carriers or mediators
of the electromagnetic force.
Example of a Feynman spacetime diagram. Electrons interact
through mediation of a photon.
The axes are normally omitted.
Virtual photons mediate
electromagnetism.
Quantum field theory predicts that, when two charged particles
interact, they actually exchange a series of photons called virtual
photons, which cannot be directly observed.
Two charged particles and their virtual photons:
The strong, weak, and gravitational interactions are assumed to
operate in the same manner, but with their mediating particles.
Yukawa’s Meson
The Japanese physicist Hideki
Yukawa developed a quantum field
theory that described the force
between nucleons (protons and
neutrons)—the strong force.
To do this, he had to determine the
carrier or mediator of the nuclear
strong force analogous to the photon
in the electromagnetic force, which he
called a meson (derived from the
Greek word meso meaning “middle”
due to its mass being between the
electron and proton masses).
Hideki Yukawa (1907-1981)
Yukawa’s Meson
Yukawa’s meson, called a pion (or pi-meson or p-meson), was
identified in 1947 by C. F. Powell (1903–1969) and G. P. Occhialini
(1907–1993) in cosmic rays at sites located at high-altitude
mountains, first at Pic du Midi de Bigorre in the Pyrenees, and later
at Chacaltaya in the Andes Mountains.
Charged pions have masses of 140 MeV/c2, and a neutral pion p0
with a mass of 135 MeV/c2 was later discovered.
Feynman diagram
indicating the exchange of a
pion (Yukawa’s meson)
between a neutron and a
proton.
The Mass of a Mediator Particle and the
Range of the Force
The Uncertainty Principle allows energy conservation to be violated
by a time:
t ~ / 2 E
If we set E = mc2, the mass energy of the particle (allowing for its
creation), we have:
2
t ~ / 2 mc
But the particle can travel up to the speed of light, so, if r is the range
of the force, t ~ r/c:
r / c ~ / 2 mc 2
Or:
mc2 ~ c / 2r
the strong and
weak forces
So if r = ∞, then m = 0. But if the force is short-range, m can be large.
For the strong force, r ~ 4 × 10-15 m, so mc2 ~ 150 MeV for the meson.
Accelerators
Particle accelerators generate
particles with energies >1 TeV.
Accelerators
There are several types of accelerators
used presently in particle physics
experiments: cyclotrons, linear
accelerators, and colliders.
They’re all
based on
the same
idea: as the
particles
move, apply
a voltage
that
accelerates
them to
higher a
speed.
Cyclotrons and
Synchrotrons
A charged particle in a magnetic field travels in a circle.
Accelerating it with voltage
yields a cyclotron. A problem
with cyclotrons, however, is
that, when charged particles
are accelerated, they radiate
electromagnetic energy called synchrotron radiation. This problem
is particularly severe when electrons, moving very close to the speed
of light, move in highly curved paths. If the radius of curvature is
small, electrons can radiate as much energy as they gain.
Physicists have learned to take advantage of these synchrotron
radiation losses and now build special electron accelerators (called
light sources) that produce copious amounts of photon radiation
used for both basic and applied research.
Linear
Accelerators
Linear accelerators or
linacs typically have straight
electric-field-free regions
between gaps of RF voltage
boosts. The particles gain
speed with each boost, and
the voltage boost is on for a
fixed period of time, so the
distance between gaps
becomes increasingly larger
as the particles accelerate.
Linacs are sometimes used
as pre-acceleration device
for large circular accelerators.
Colliders
Head-on collisions
are twice as
energetic as those
involving hitting an
object at rest, so
physicists began
building collidingbeam accelerators,
in which the
particles meet
head-on.
If the colliding particles have equal masses and kinetic energies,
the total momentum is zero and all the energy is available for the
reaction and the creation of new particles.
Large
Hadron
Collider
Counterpropagating
protons each have
an energy of ~7
TeV, giving a total
collision energy of
14 TeV. The LHC
can also be used
to collide heavy
ions such as lead
(Pb) with a
collision energy of
1,150 TeV.