The first fundamental particle to be discovered was, of course, the electron, in the fall of 1897. It is still considered fundamental because it appears to be pointlike, with no internal constituents or excited states. The proton and neutron were known and understood as the constituents of the atomic nucleus, by 1932. Einstein had named the photon, the first boson to be studied, the massless, spin 1 quantum of the electromagnetic field. Was that all there was to nature? That was certainly all there was to atoms... atoms are nothing more than bound systems of electrons, protons and neutrons. It was quickly realized that the proton and neutron are not pointlike, and were probably made of constituents, but nobody knew what. The antiparticle to the electron, the positron, was discovered in 1932. Another weird product of nuclear beta decay, the neutrino and the anti-neutrino, were suspected by 1930 but not actually detected until 1956. Between 1936 and 1947 particles were discovered in cosmic radiation that had masses greater than the electron and positron, but much less than the proton and neutron. These were called mesons. The first two to be found were the muon and the pion. So by the early 1950s the situation could be summarized as follows:
Particles in the upper section feel
the strong, weak and electromagnetic forces... all of the
fundamental forces that are important on the subatomic
level. Particles in the middle section feel only the weak
and electromagnetic forces. The one particle in the last
section is the boson responsible for the electromagnetic
force. So, missing were all the bosons responsible for the
strong and weak forces! And since the proton, neutron and
pion were not pointlike particles, they were presumably made of
constituents... what??????
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To make sense of what was clearly going to become an explosion of discoveries of new particles as accelerators came into use at higher and higher energies, the initial division into fermions and bosons was broadened. Particles that felt the strong force were called hadrons, while particles that did not feel the strong force were called leptons. Among the hadrons were particles that were bosons, like the pion, and they were called (as we said) mesons. The hadrons that were fermions were called baryons, with the proton and neutron being the least massive, and the proton being the only one that was stable. During the 1950s it seemed that the discovery of new particles was going to continue without limit, with any increase in accelerator beam energy generating a flood of completely new baryons and mesons! Plus a few new leptons! The really big unanswered questions were, what were the field bosons responsible for the strong and weak nuclear forces??? And, were any of the new particles fundamental, that is, pointlike? And if not, what the heck were they made of??? [Physicist Hideki Yukawa back in 1935 had been able to describe the long-range part of the strong nuclear force in terms of exchange of a massive boson, whose properties he predicted. They matched the properties of the pion, and he received the Nobel Prize in 1949 after discovery of the pion in 1947. But the pion is not a fundamental point particle like a photon. What was the fundamental boson responsible for the strong force?]
By about 1970, a general theory of
the structure of particles had emerged, and was being
well-confirmed by experiment. This description of fundamental
particles and processes is known as the Standard
Model. In the electromagnetic interaction, one boson, the
photon, couples to all charged particles. In the weak
interaction, three bosons, all very massive and two charged, the
Z0 and W±, couple to “weak charge.” In the
strong interaction, eight bosons, the gluons, couple to “color
charge.” All baryons
are made of three “valence” quarks, and there are six different
quarks, u, d, s, c, t and b. All mesons
are made of valence quark-antiquark pairs. And all of the six
different leptons
are pointlike fundamental particles not made of any known
pieces. The bosons responsible for forces all have intrinsic
spin 1, and are called gauge bosons.
There is only one other known fundamental boson, the Higgs,
which is the only fundamental boson with zero spin.
Gravity is completely left out of this picture, because it is
far from clear that gravity even has a quantum aspect.
[Our textbook was written before the discovery of the top quark
and the Higgs
boson, but the general discussion found there is accurate
and generally complete.]
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To distinguish between particles, physicists have had to invent a number of other intrinsic quantum numbers besides spin, attached to each particle. An example is intrinsic parity. Many of these quantities turn out to be conserved for some types of interactions and not conserved for others, which created a very confusing situation before the Standard Model came along to clear things up. Additive quantum numbers are intrinsic properties like charge, that add up in a simple way, and are conserved. Examples are: baryon number, lepton number, strangeness, etc. Most of these just involve counting fermions. For example, how do you distinguish a particle from its antiparticle? An antineutron has a baryon number −1 whereas the neutron has baryon number +1. Similarly, an antiproton has baryon number −1 and a charge of −1 unit.
Examples of some additive quantum
numbers. The table shows (left to right) charge, lepton number,
muon number, tauon number, baryon number, strangeness, charm,
beauty and truth. The strangeness is just minus the number of
strange quarks in the particle, and similarly the charm and
truth are just the number of c and t quarks in the
particle! Beauty is minus the number of b quarks in the
particle. Notice quarks have fractional charge and
fractional baryon number.
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Remember the angular momentum quantum numbers like ℓ and mℓ or s and ms? In the early days of nuclear physics, Heisenberg and Wigner introduced the concept of isospin, as a convenient way to distinguish between proton and neutron when the nuclear force acts. Let's call the corresponding quantum numbers T and T3. For the proton and neutron, T = 1/2 and the proton has T3 = +1/2 while the neutron has T3 = −1/2. [The text uses capital I for isospin, but in html this looks like 1 or ell, so we use T instead, as all nuclear physicists do.] We can assign T and T3 for any of the hadrons--- particles participating in the strong interaction--- and for these processes it turns out that isospin is conserved! Isospin is related to many other additive quantum numbers. For example Q/e = T3 + B/2, where B is the baryon number, which is also called A (as in the tables above). Warning: the textbook very confusingly and weirdly refers to T3 as mI! |
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Hadrons form isospin multiplets, where all members of the multiplet have almost the same mass. For example, the proton and neutron both have a mass of about 940 MeV. A meson example is the pion, which comes in three charge states, π+, π0, and π−, all with a mass of about 140 MeV, and having T3 = ±1, 0. Because hadrons are made of quarks (baryons have 3 valence quarks and mesons have a valence quark and antiquark) it is important to assign isospin to each quark, even if we just assign zero, so that we can work upward to the isospin of some particular baryon or meson, from scratch. Here is a table, but it requires some explanation:
In the table B is the baryon number, but what is Y? Y is another important quantity called the hypercharge. It satisfies the equation Q/e = T3 + Y/2, which is a generalization of the equation we first stated. Specifically, Y = B + S - (C - B' + T')/3, where S is strangeness, C is charm, B' is beauty and T' is truth! B' and T' are the quantum numbers unique to the b and t quarks, just as S and C are unique to the s and c quarks. Notice all the quarks have zero isospin except u and d. [The masses given in this table for u and d quarks are "effective masses," the actual masses are 2 MeV for the up, and 4.8 MeV for the d quark.]
Just as the electromagnetic interaction couples to charge, and the weak interaction couples to weak charge, the strong interaction couples to color. There are three colors, red, green and blue, and three anti-colors, anti-red, anti-green and anti-blue. Just as all atoms naturally are electrically neutral, all hadrons are colorless. Thus a proton consists of three valence quarks, and each quark can have any of the three colors, as long as all three quarks have colors adding to white, where white = red + green + blue. Note that the proton also consists of a practically infinite number of gluons and virtual quark-antiquark pairs. The gluons carry a color and an anticolor, for example green + antiblue. In the same way, the mesons consist of a valence quark and an antiquark, whose colors combine to white... for example red, and anti-red. There are 8 different color-anticolor combinations for a gluon. Sounds complicated? It is! For example since the gluons have color, they can couple to one another, unlike photons, which are not charged and do not interact electromagnetically. Gluons cannot exist outside the baryons and mesons that they hold together. In general it is physically impossible to remove a quark or antiquark from inside a baryon or meson, because “naked” color is forbidden in nature.
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A summary: the hadrons are the particles that feel all the interactions, strong, weak and electromagnetic (if charged). All hadrons are made of quarks, held together by gluon exchange. There are two kinds of hadrons. The baryons have three valence quarks, and the mesons have a valence quark and antiquark. The hadrons are complicated bound states, and have an infinite binding energy. The leptons, by contrast are fundamental pointlike elementary particles. They are not made of anything and do not feel the strong interaction, only the electromagnetic (if charged) and weak interactions. There are six different leptons, and six different quarks. There are twelve fundamental force bosons, the eight gluons, the three weak bosons, and the photon.
Let's make sure we understand antiparticles. Every fundamental particle in nature has a corresponding antiparticle which has opposite additive quantum numbers compared to the particle. When a particle collides with its antiparticle, the typical result is two photons, but for higher and higher energy collisions, other boson pairs can result, even other fermion-antifermion pairs. What if a particle is its own antiparticle? This happens when a particle has no additive quantum numbers at all, like the photon. Then two such particles can annihilate one another at any time, which is not unusual for bosons, which are never conserved in number. Whatever annihilation takes place must conserve total energy and total momentum.
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