-Stephanie Osborn
http://www.stephanie-osborn.com
~~~
Elementary
(particles), my dear Watson
The next step to
understanding the Higgs boson is to explore the other elements of the
Standard Model of elementary particle physics, to provide the
necessary context.
The story of the
development of the standard model is pretty much the history of
twentieth century physics. The story actually begins a few years
early, in 1896, when the British physicist J. J. Thompson discovered
that the "cathode rays" which had been observed over the
previous twenty years in gas discharge tubes were actually individual
particles (corpuscles), which eventually came to be called electrons.
These particles were shown to have a negative electric charge —
meaning they moved in the direction of increasing electric voltage —
and to have a mass less than 1/1000th of a hydrogen atom.
Then in 1912,
Ernest Rutherford discovered the atomic nucleus, which when stripped
of electrons was shown to have a positive electric charge, and by
1917 realized that the lightest atomic nucleus, that of hydrogen,
appeared to be an indivisible, positively charged particle which came
to be called the proton. The problem of more massive atomic nuclei
was solved in 1932, when James Chadwick isolated the electrically
neutral neutron as the particle which contributed
to nuclear mass
without affecting nuclear electric charge. Wolfgang Pauli had
earlier postulated the neutrino to explain the slight loss of energy
associated with nuclear beta decay (the neutrino was not discovered
as an actual particle until 1956), which led to the identification
of the beta decay mechanism,
Note the bar over
the Greek letter (nu)
representing the neutrino. In the late 1920's, British physicist
Paul Dirac had developed a complete quantum theory for the electron
which introduced the possibility — indeed, the necessity, of
antiparticles, particles which possessed electrical charges of the
same magnitude but opposite polarity as their partner particles and
which reacted with them to decompose into hard radiation.
Dirac's prediction
was cemented by the discovery of the anti-electron (positron) by Carl
Anderson in 1932, the discovery of the anti-proton (very rarely
called the negatron) in 1955 by Emilio Segrè and Owen Chamberlain,
and the discovery of the anti-neutron (at which point nobody bothered
to give it its own name) in 1956 by Bruce Cork. The anti-neutron was
particularly puzzling, because the neutron is electrically neutral,
but the anti-neutron was discovered to have the opposite magnetic
polarity of the neutron. Also, since the anti-neutron decays into an
anti-proton, a positron, and a neutrino, the convention was developed
that an anti-neutrino (hence the bar in the equation above) is
associated with the electron. Together, the electron and the
neutrino are called leptons, and a quantity called "lepton
number" as assigned so that the net change in lepton number in a
beta decay is zero. Hence, the electron and the neutrino (emitted
with the positron) have lepton number +1, and the positrion and the
anti-neutrino (emitted with the electron) have lepton number -1.
It should be noted
that all four of these particles have one-half unit of spin1
(which can be measured relative to the direction of the motion of the
particle, and is aligned either in the direction of, or opposing the
direction of, motion). This means that, when collected together,
they observe the so-called Fermi-Dirac statistics which grew out of
Dirac's theory of the electron (designed to account for spin). For
practical purposes, these particles, call fermions, resist being
packed tightly together. Conversely, particle with integral values
of spin (0, 1, …) can be packed to high density. These are called
bosons, being based on Bose-Einstein statistics.2
This nice, cozy
picture of four fundamental particles, together with their
anti-particles, was shattered before it was even fully formed with
the discovery of additional particles, starting with the muon, which
behaves like an electron, in 1936, and of two classes of particles:
a class of bosons (the mesons) and a class of fermions (the baryons,
which included the neutron and the proton), beginning in 1947.3
This increasingly crowded "particle zoo" was brought to a
semblance of sanity beginning in 1964, when Murray Gell-Mann and
George Zweig postulated that the then-known mesons and baryons were
made up of three types of smaller particles called quarks. A fourth
type of quark was discovered in 1974, and an additional pair of
quarks have been confirmed in more recent experiments. A third
electron-like particle, the tauon, and neutrinos corresponding to the
muon and tauon, have also been identified. This picture of three
generations of quarks and leptons is summarized in the table below.
| Charge | Type/Anti | 1st Generation |
2nd
Generation
|
3rd Generation |
+ 1
|
Anti-lepton
|
Positron,
e+
|
Antimuon,
+
|
Anti-tauon,
+
|
+
2/3
|
Quark
|
Up
quark, u
|
Charm
quark, c
|
Top
quark, t
|
+
1/3
|
Anti-quark
|
Anti-down
quark,
|
Anti-strange
quark,
|
Anti-bottom
quark,
|
0
|
Lepton
|
Electron
neutrino,
|
Muon
neutrino,
|
Tauon
Neutrino,
|
0
|
Anti-lepton
|
Electron
antineutrino,
|
Muon
antineutrino
|
Tauon
Antineutrino,
|
-
1/3
|
Quark
|
Down
quark, d
|
Strange
quark, s
|
Bottom
quark, b
|
-
2/3
|
Anti-quark
|
Anti-up
quark,
|
Anti-charm
quark,
|
Anti-top
quark,
|
- 1
|
Lepton
|
Electron,
e-
|
Muon,
-
|
Tauon,
-
|
In this picture,
the mesons are found to be made of pairs of quarks and anti-quarks;
while the baryons are composed of three quarks. These compositions
obey mathematical rules which look very much like the much more
comprehensible concept of spin, which results in beautifully
symmetric structures. In the graphics on the left below4,
chart (a) shows the composition of scalar mesons (quark and
anti-quark with opposing spins that cancel to zero net spin) composed
of the up, down, strange, and charm quark, while chart (b) shows the
corresponding vector (aligned spins adding to one unit of spin)
mesons. The central octets represent the particles known at the time
the quark theory was formed. The graphic on the right5
shows the basic spin one-half baryons (two quarks with aligned spin,
one with opposing spin) on the bottom octet, and then the structure
as one, two, or three bottom baryons replace the lighter quarks (a
similar structure has been defined with charm quarks, and additional
combinations with both charm and bottom mesons have also been
discovered).
The quarks have
been found to be very tightly bound together; although there have
been a few tantalizing hints, there is no firm evidence that quarks
exist as free particles outside of the elementary particles which are
composed of them. Similarly, there have been hints, but no
confirmation, of more complicated particles, such as meson-like
particles composed of two quarks and two antiquarks, or hybrid
meson-baryons with four quarks and one anti-quark. The significance
and/or rarity of such events remains a subject of study.
The most notable
difference between the three generations is the mass of the
particles. This is illustrated graphically below, where the mass of
the three generations of particles, excluding the neutrinos (which
are now known to have mass but are very light), is given in the
conventional units of high energy physics, MeV/c2.6
As can be seen, the mass of particles increases more or less
exponentially between generations. This is far from fully
understood.
1
"Spin" is
a quantum property of elementary particles which behaves
mathematically in many ways like spin in ordinary life, from an ice
skater twirling to the Earth's rotation about its axis. However, as
a quantum property, it only occurs in discrete values: 0, 1/2, 1,
3/2, 2, etc.
However, while spin at the quantum level behaves like spin in real
life, we cannot directly observe elementary particles to see if they
are really spinning, or if something else is happening that we can't
observe. When particles with spin combine to form composite
particles, the spins can be aligned with or against each other; the
particles can also have quantized (values of 0, 1, 2, etc.) orbital
angular momentum from their orbits around each other. This is not
the most confusing thing about elementary particles.
2
Satyendra Nath Bose
was an Indian physics professor who developed his theory of boson
behavior in 1924 and sent it to Einstein, who was so impressed he
translated it into German himself and submitted it for publication.
3
For the sake of
simplifying the discussion, I will not discuss the confusion of
meson and baryon physics in the 1950's, as almost a hundred
different particles were discovered and quantified, and the muon was
not yet fully distinguished from the true mesons.
4
http://fafnir.phyast.pitt.edu/particles/conuni6.html
(accessed 16OCT2011)
5
http://cerncourier.com/cws/article/cern/40104
(accessed 16OCT2011)
6
One MeV is the
kinetic energy of a charged particle which has accelerated through a
volatage of one million volts, about the scale of most lightning
generators found in high school physics labs. The effective
accelerating voltage of a facility such as the Large Hadron Collider
at CERN is about 10 million MeV. The division by c2
is reflective of Einstein's equivalence of energy and mass.
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