Now we will introduce some of the generic terms used in particle physics and
define their essential characteristics. These are a few of the
most-often-used terminology.
nucleons: neutrons and protons;
fermions: particles with half-integral spin;
bosons: particles with integral spin;
hadrons: all particles affected by the strong nuclear force;
baryons: hadrons that are fermions such as the nucleons;
mesons: hadrons that are bosons such as the pions;
leptons: all particles not affected by the strong nuclear force,
such as the electron and the muon.
Particles that are baryons are assigned a baryon number B which takes
the value B = 1 for nucleons, B = -1 for antinucleons, and
B = 0 for all mesons and leptons.
There is an extraordinary thing about the neutron, which Heisenberg observed
shortly after its discovery in 1932: apart from the obvious fact that the
neutron has no charge, it is almost identical to the proton. In particular,
their masses are astonishingly close
(mass of proton = 938.28 MeV/c2,
mass of neutron = 939.57 MeV/c2).
Heisenberg proposed that they be regarded as two "states" of a single
particle, the nucleon. Even though the small difference in mass might
be attributed to the fact that the proton is charged, since the electrical
energy that is stored in an electric field contributes to its mass, this cannot
be true, since the mass of the proton is not the larger of the two particles.
According to Heisenberg, if the electric charge could somehow be "turned off",
the proton and neutron would be indistinguishable. That is, the strong force
experienced by protons and neutrons are identical. The strong nuclear
force seems to ignore totally the effects of electric charge and influences
all nucleons in the same way. As far as strong forces are concerned, there
is only one nucleon. Heisenberg described this mathematically by introducing
the concept of istopic spin or isospin. By a direct analogy with
electron spin,
S, the nucleon has isospin, I. The spin of the electron
has two different orientation in real, ordinary space in which the electron
can exist. But isospin I is not in ordinary space, with components
along the coordinate directions x, y, and z, but rather
in an "isospin space" with components that may be called I1,
I2, and I3. Using this concept of
isospin space, the entire mathematical apparatus of quantum angular momentum
can be used to develop this concept of isospin. Thus the nucleon carries
isospin of 1/2, and the third component I3 has eigenvalues
of +1/2 (the proton) and -1/2 (the neutron). There is no factor of
h/2π in this case, since isospin is dimensionless, by convention.
The proton is "isospin up"; the neutron is "isospin down".
Heisenberg interpreted this notation as the physics
that the strong interactions are invariant under rotation in
isospin space, just as electric forces are invariant in ordinary
configuration space. When physical laws remain the same whatever the choice
of coordinate system so that the mathematical expression of the laws remain
the same under any of transformation of translation in time or rotation about
an axis, the corresponding quantity is said to be invariant. This invariance
of strong interaction can be called an "internal" symmetry, because it has
nothing to do with ordinary space and time, but only with the relation
between different particles. A rotation through 180 degrees about axis
1 in isospin space converts protons into neutrons, and vice versa. If the
strong force is invariant under rotation in isospin space, then it follows
from Noether's theorem that isospin is conserved in all strong interactions,
just as angular momentum is conserved in processes with rotational invariance
in ordinary space.
The electric charge Q of the nucleons are related
to their isospin assignment by the simple formula
Q = e(I3 + B/2).
So for the nucleon which has unit baryon number (B = 1), the two
isospin states with the third component I3: +1/2 and -1/2,
become the positive and neutral charge states of the proton and of the
neutron, respectively.
This simple formula can also applied to other hadrons; for the pions
which have zero baryon number (B = 0), the charges are simply
the units of the electronic charge e corresponding to the three
"third" components of isospin I3 (1, 0 , -1).
In the language of
group theory,
Heisenberg asserted that the strong interactions are invariant under an
internal symmetry group SU(2), and the nucleons (the proton and neutron)
belong to the two dimensional representation (isospin 1/2).
In 1932 this was a bold proposal, but today with evidence all around us,
and most conspicously in the "multiplet" structure of the hadrons. In the
Eightfold Way
diagrams, the horizontal rows all display exactly the
feature that caught Heisenberg's eye in the case of nucleons; they have
very similar masses but different charges. To each of these multiplets,
a particular isospin I can be assigned and to each member of the
multiplet can be assigned a particular I3. To determine
the isospin of a multiplet, just count the number of particles it contains;
since I3 ranges from -1 to +1, in integer steps, the number
of particles in the multiplet is 2I + 1. The third component of
isospin, I3, determines the electric charge, Q,
of the particle.
Q = e[I3 + (B + S)/2],
where B is the baryon number, S is the strangeness, and
e is the electric charge of the electron. The simple formula
above for determining the electric charge of nucleons can be obtained
by setting S in this formula equal to zero.
For a brief period in 1947, it was believed that all the major
problems of particle physics had been solved. After a lengthy
detour in pursuit of the muon, Yukawa's meson (the π) had finally
been found. Dirac's positron had been found, and Pauli's neutrino,
although not experimentaly detected, its existence was accepted.
The role of the muon was something of a puzzle ("Who ordered
that?" I. I. Rabbi asked); it seemed quite unnecessary in the overall
scheme of things. But on the whole, it looked in 1947 as though
the job of elementary particle physics was done. But in December
of 1947 all that changed; two British physicists, G. D. Rochester
and C. C. Butler published cloud chamber photographs in which
they observed more new particles, about a thousand times more
massive than the electron. Since in these cloud chamber photographs
of cosmic rays, these particles were associated with V-shaped
tracks, they were at first called "V particles". Their origin and
purpose were an entire mystery. Detailed analysis showed that
the particles making the V shaped tracks were
π+ and π-.
Here, then, was a new neutral particle with at least twice the mass
of the pion; it was called the kaon, K0:
K0 → π+ + π-.
In 1949 Powell published photographs showing the decay of a positive charged kaons, K+:
K+ → π+ + π+ + π-.
(The K0 was first known as the V0, and later as the θ0; the K+ was at first called τ+.) These kaons behaved in some respects like heavy pions, and so the meson family was extended to include them. Because of their anomalous behavior they became known as the strange particles. They are produced copiously rapidly (on a time scale of about 10-23 seconds), but they decay relatively slowly (typically about 10-10 seconds). In due time, many more mesons were discovered: the η, the φ, the ω, the ρ's, and so on. Meanwhile, in 1950 another neutral "V" particle was found, this time by Anderson's group at Cal Tech. The photograph was similar to Rochester's, but this time the products were a proton and negative pion. Evidently this particle was substantially heavier than the proton; we will call it the lambda, Λ:
Λ → p+ + π-.
The lambda belongs with the proton and the neutron in the baryon
family. Stuckelberg proposed a law of the conservation of baryon
number to account for the stability of the proton. By assigning
to all baryons (which in 1938 meant protons and neutrons) a "baryon
number" B = +1, and to all antibaryons B = -1, then the total
baryon number is conserved in any physical process.
(For all nonbaryons, B = 0.) Thus, neutron beta decay
(n → p+ + e- + anti-electron-neutrino)
[(+1) → (+1) + (0) + (0)]
is allowed (B = +1 before and after).
The first step in the search for an explanation of these particles was taken in 1952 by the American physicist A. Pais. He suggested that the mechanism that governs their production is entirely different from that which governs their decay. In modern language, the strange particles are produced by the strong force (the same one that holds the nucleus together), but they decay by the weak force (the same one that accounts for beta decay by the weak force and all other neutrino processes). Pais's scheme required that the strange particles could not be produced singly by the strong interaction, but only in pairs. This was confirmed in experiments at the Brookhaven accelerator in 1953, when strange particles were man-made for the first time. The strange particles always emerged in pairs in reactions such as
π- + p+ → Λ0 + K0,
where Λ0 denotes a hyperon and K0 denotes a neutral kaon. In the same year, Gell-Mann and Nishijima explained this mechanism of associated production by proposing the introduction of a new conservation law, that of strangeness, which applies only to the strong interaction. Each particle is assigned a quantum number of strangeness, S, in addition to its quantum numbers of spin, intrinsic parity and isospin. (Isospin is a complicated concept and we won't discuss it further here -- we have discussed it elsewhere --, except to say that there are exactly two kinds of isospin for the nucleons, which can be called up and down. Protons have up isospin and neutron have down isospin.) Strangeness (like charge, lepton number, and baryon number) is conserved in any strong interaction. Thus, in any strong interaction, the total strangeness of all the particles before and after reaction must be the same. Associated productions can now be explained by assigning a positive strangeness to one of the strange particle and a negative strangeness to the other strange particle, so that the total strangeness of the final state is zero, the same as that of the non-strange initial state.
π- + p+ → Λ0 +
K0,
(0) + (0) → (-1) + (+1) strangeness number, S.
(-1) + (+1) → (0) + (0) electric charge, Q.
(0) + (+1) → (+1) + (0) Baryon number, B.
In the decay of the Λ0,
Λ0 → p + π-,
(-1) → (0) + (0) strangeness number, S.
(0) → (+1) + (-1) electric charge, Q.
(+1) → (+1) + (0) Baryon number, B.
The strangeness on the left is -1, and on the right, 0. Therefore, strangeness is not conserved and strangeness conservation is violated, and the Λ0-decay must go by the weak or slow process. Its decay time is 10-10 seconds, which is characteristic of weak decay processes. The decay of strange particles into non-strange particles cannot proceed by strong interactions, which must, by definition, conserve strangeness. Instead, such decays proceed by weak interaction, which need not conserve strangeness, and which allows the strange particles comparatively long lives.
Hyperons are elementary particles heavier than protons. The known hyperons fall into four classes: Λ, Σ, Ξ, and Ω, in order of increasing mass. (Λ, Σ, Ξ, and Ω are, respecitively, the Greek capital letters lambda, sigma, xi, omega.) All are unstable with extremely brief average lifetimes. The spin of all hyperons are 1/2 except for the Ω hyperon, which is 3/2. The hyperons are the strange particles which eventually decay into a proton and which, like the proton, have a spin of 1/2 and are baryons with baryon number +1. The lambda hyperon Λ0 is the least massive at 1115 MeV and has isospin zero (exists only as a neutral particle). The sigma hyperon Σ has a mass of 1190 MeV and has isospin 1 and so exists in three charged particles (Σ+, Σ0, Σ-). Finally, the xi hyperion Ξ, known as the cascade particle, has a mass of 1320 MeV, an isospin of 1/2, and a strangeness of 2. To decay into non-strange particles, it therefore needs to undergo two weak interactions, as the weak force can only change strangeness by one unit at a time. For the hyperions, the perferred distinguishing quantum number is hypercharge Y, which is the sum of baryon number B and strangeness S:
Y = B + S.
The garden that seemed so tidy in 1947 had grown into a jungle by 1960. And hadron physics could only be described as chaos. The great number of strongly interacting particles was divided into two great families: the baryons and the mesons. And the members of each family were distinguished by charge, strangeness, and mass; but beyond that there was no rhyme or reason to it at all. This situation reminded many physicists of the situation in chemistry about a century earlier, before the Periodic Table, when scores of elements had been identified, but there was no underlying order or system. The Russian chemist Dimitri Mendeleev in 1869 showed how all these elements could be arranged into a beautiful chart, called the Mendeleev periodic table. This chart, which every high school student learns in a chemistry class, suddenly made order out of the chaos. In 1960 the elementary particles were waiting for a Periodic Table.
The Mendeleev of elementary particle physics was Murray Gell-Mann, a physicist at Cal Tech, who introduced the so-called Eightfold Way in 1961 (Essentially the same scheme was proposed independently by Yuval Ne'eman, an Israeli intelligence officer turned physicist. Together, acting as editors, Gell-Mann and Ne'eman produced a book in 1964, The Eightful Way, in which their own original papers and other key contributions to the new understanding of the particle zoo were reprinted.) They noticed a pattern in the already classified hadron. They based their research on a mathematical symmetry which incorporated the known conservation of various hadron charges. (There underlying mathematical formalism was a branch of modern mathematics called group theory) But the mathematical symmetry, which implied the pattern, Gell-Mann called the Eightfold Way, went far beyond the conservation law it incorporated. (Because some of the patterns found initially involved particles in groups of eight, Gell-Mann whimsically called his mathematical theory the Eightfold Way, which is also the name of the Buddhist doctrine of the eightfold path to Nirvanna. Incidently, the eight are: right belief, right resolve, right speech, right conduct, right living, right effort, right contemplation, and right ecstasy.)
According to the Eightfold Way, every hadron must be a member of a specific family of hadrons. These families consisted of definite number of members; the smallest had 1, 8, 10, and 27 members. Families containing only a single member (hardly a family!) are called singlets, other families with 8 members are called octets, with 10 members decuplets. All hadron members of a specific family have the same spin, but their electric charge, isotopic charge, and strangeness charge differred. The Eightfold Way arranged the baryons and mesons into a geometrical pattern, according to their charge and strangeness.
The eight lightest baryons fit into a hexagonal array, with two particles
at the center. This group is known as the baryon octet. Notice that
particles of like charge Q lie along the downward-sloping diagonal lines:
Q = +1 (in units of the proton charge) for the proton and
Σ+;
Q = 0 for the neutron, lambda, the Σ0,
and the Ξ0;
Q = -1 for Σ- and Ξ-.
Horizontal lines associate particles of the same strangeness:
S = 0 for proton and neutron;
S = -1 for the middle line with Σ-, Σ0,
&lamda;, and Σ+;
S = -2 for the two Ξ particles, Ξ- and Ξ0.
The eight lightest mesons fill a similar hexagonal pattern, forming the (pseudo-scalar) meson octet. Hexagon patterns were not the only figures allowed by the Eightfold Way; there was also, for example, a triangular array incorporating 10 heavier baryons, called the baryon deuplet. The fitting hadrons into families worked beautifully, just like the periodic table of chemical elements. Many properties of a given family of particles, such as their different masses, could be related using mathematical symmetry. Exploring these and many other consequences of the Eightfold Way preoccupied physicists during the mid-1960s.
Some critics of the success of the eightfold-way symmetry thought that its success was accidental. After all, they argued, it arranged hadrons that were already known from experiment, and it was just set up to fit the facts and no more. But when Gell-Mann fitted particles into the decuplet, he found that nine of the particles were already known experimentally, but at that time the tenth particle, one at the very bottom with a charge of -1 and a strangeness of -3, was missing. No particle with these properties had every been detected in the laboratory. Gell-Mann boldly predicted that such a particle would be found, and told the experimentalist exactly how to produce it. In addition, he calculated its mass and its lifetime. And sure enough in 1964 the famous omega-minus particle (Ω-) was discovered. In November 1963, a large group of physicists at Brookhaven Laboratory in New York devoted its resources to the search for the Ω- particle. They took over fifty thousands bubble chamber photographs and on one of them there appeared the track of the Ω- particle. That December the elated experimentalists at the laboratory sent out a greeting card which had the bubble-chamber photograph of the track of Ω- particle. When the Ω- particle was discovered with the predicted mass value, all but the most hardened critics were won over to the Eightfold Way.
Since the discovery of the Ω- particle, no one seriously doubted that the Eightfold Way is correct. Over the next 10 years, every new hadron found a place in one of the Eightfold Way supermultiplets, patterns beyond octet, decuplet. In addition to the baryon octet, decuplet, and so on, there exist of course an antibaryon octet, decuplet, etc., with opposite charge and opposite strangeness. But in the case of the mesons, the antiparticles exist in the same supermultiplet as the corresponding particles, in diametrically opposite direction.
By the mid-1960s, the Eightfold Way and the mathematical symmetry, which it applied, brought order to the realm of the hadrons. The infinite set of hadrons, the baryons and mesons, could be classified and the pattern of their properties clearly seen. But classification is the first stage of development in any science and the next phase of scientific method was to seek an explanation of the classification. The main problem of this phase was: Why did the Eightfold Way work? This problem was especially puzzling in the 1960s because of the view that physicists held at that time about the structure of hadrons, that is, that they didn't seem to have any structure. If a hadron was smashed apart all that emerged was more hadrons created from the energy supplied to the smashing. The best explanation for this observation back in the 1960s was the bootstrap hypothesis, which asserted that all the hadrons were composed out of one another; that is, they had lifted themselves into existence by "pulling on their own bootstraps".
Many physicists in 1960s were attracted to the bootstrap hypothesis, which they applied to the infinite set of hadrons, not just three, because it seemed to account for the fact that no new fundamental particle had been seen in hadron collisions, just more of the same old hadrons. The difficulty with the bootstrap hypothesis is that it gave no explanation for Eightfold Way, that is, the observed symmetry properties of the hadrons. The bootstrap hypothesis did not seem to answer the riddle of hadrons. The answer to the riddle came from the imagination of the theoretical physicists. Murray Gell-Mann, and independently George Zweig, who noticed that all the hadron families could nicely be accounted for if one imagined that the hadrons were built up out of more fundamental particles, which Gell-Mann called "quarks". Using simple rules for combining quarks, all the infinite set of hadrons and the observed families were explained. The way to think of a hadron is as a little bag filled with a few elementary, pointlike quarks moving around inside. The observed new laws of charge conservation were just the consequence of the fact that different quarks were numerically conserved in hadron reactions; they were like atoms in a chemical reaction. The answer to the riddle of the hadrons was that the hadrons are quark "molecules". In 1969 Gell-Mann won a Nobel Prize for unraveling the symmetries of the hadrons.
Hadrons are made out of quarks. And that is the answer to the riddle of the hadrons. But what are quarks? Quarks are point quantum particles similar to the electron with the same spin of 1/2. But they have a fractional electric charge compared to the electron's one unit of charge. Also unlike the electron, no one has ever detected a quark. Quarks came into modern physics, not as spectacular experimental discovery with a shout of "Eureka" ("I found it"), but as a mathematical creation of the theoretical physicists. One day in 1963, Murray Gell-Mann was visiting Columbia University to give a lecture. Stimulated by the questions and the suggestions from an eminent member of the Columbia faculty, the theoretical physicist Robert Serber, Gell-Mann got the idea of a substructure for the hadrons. Later Gell-Mann got the name for the particles in this substructure after a line in James Joyce's novel Finnegan's Wake: "Three quarks for Muster Mark". Joyce probably intended the word to rhyme with the word "Mark" and the three quarks referred to three quarts of drink. Gell-Mann went ahead and named the particles in the substructure of hadrons "quarks". The word "quark" is a German word for a curd of cheese, but this did not enter into Gell-Mann naming of the particles "quarks". George Zweig, who was visiting CERN, the large international European nuclear laboratory near Geneva, came to the same idea, but he called particles in the substructure "aces". Zweig wrote an article about his idea and he wanted to publish his article in an American physics journal, but the European leadership of CERN, intent on fostering their independence from American physics, had a policy that research done at CERN had to appear in European journals. Zweig's article was never published and term "aces" never caught on.
The basic idea of quarks is that all the hadrons can be built out of three quarks, called the "up" quark, the "down" quark, and the "strange" quark, and their three anti-quark partners (that is, the antimatter versions of the three quarks with opposite electric charge). "Up", "down", and "strange" are called the flavors of the quarks -- a curious use of the word "flavor". At one point the physicists whimsically referred to the three quarks as "chocolate", "vanilla", and "strawberry" instead of "up", "down", and "strange" and hence they were called "flavors". The ice cream terminology never caught on, but the use of "flavor" as a generic label distinguishing the three quarks did. The physicists find it convenient to give letter names to particles, English or Greek, and so the three quarks are denoted by u for up quark, d for down quark, and s for strange quark. These three types (or "flavors") of quarks form a triangular "Eightfold-Way" pattern. The u (for "up") quark carries a charge of 2/3 and a strangeness of zero (S = 0), the d (for "down") quark carries a charge of -1/3 and a strangeness of S = 0, and s (originally for "sideways", now commonly called "strange") quark has a charge Q = -1/3 and S = -1. To each quark there corresponds an antiquark with the opposite charge and strangeness. The quark model says that
| qqq | Q | S | Baryon |
|---|---|---|---|
| uuu | 2 | 0 | Δ++ |
| uud | 1 | 0 | Δ+ |
| udd | 0 | 0 | Δ0 |
| ddd | -1 | 0 | Δ- |
| uus | 1 | -1 | Σ*+ |
| uds | 0 | -1 | Σ*0 |
| dds | -1 | -1 | Σ*- |
| uss | 0 | -2 | Ξ*0 |
| dss | -1 | -2 | Ξ*- |
| sss | -1 | -3 | Ω- |
A similar ennumeration of the quark-antiquark combinations yields a meson table.
| q‾q | Q | S | Meson |
|---|---|---|---|
| u‾u | 0 | 0 | π0 |
| u‾d | 1 | 0 | π+ |
| d‾u | -1 | 0 | π- |
| d‾d | 0 | 0 | η |
| u‾s | 1 | 1 | K+ |
| d‾s | 0 | 1 | K0 |
| s‾u | -1 | -1 | K- |
| s‾d | 0 | -1 | K0 |
| s‾s | 0 | 0 | ?? |
Indeed, the Eightfold Way supermultiplets emerge in a natural way from the quark model. The Δ+ and the proton are both composed of two u's and a d. Just as the hydrogen atom (electron plus proton) has many different energy levels, so a given collection of quarks can bind together in many different ways. But whereas the various levels in the electron/proton system are relatively close together (the spacings are typically several electron volts, in an atom whose rest energy in nearly 109 electron volts), so that we naturally think of them all as "hydrogen", energy spacings for different states of a bound quark system are very large, and normally regarded as distinct particles. Thus we can, in principle, construct an infinite number of hadrons out of only three quarks. But note that some things are absolutely excluded in the quark model: for example, a baryon with S = 0 and Q = -2; no combination of the three quarks can produce these numbers. Nor can there be a meson with a charge of +2 (like the Δ++ baryon) or a strangeness of -3 (like the Ω-). For a long time there were major experimental searches for these so-called "exotic" particles; their discovery would be devastating for the quark model, but none has ever been found.
But the quark model does suffer from one profound embarrassment: in spite of the most diligent search over a period of 20 years, no one has ever seen an individual quark. Now, if a single proton is really made out of three quarks, then you'd think that if you hit hard enough, the quark ought to pop out. Nor, would they be hard to be detected, carrying as they do the conspicuous fractional charge; an ordinary Millikan oil drop experiment would clinch the identification. Moreover, at least one of the quarks should be absolutely stable; what could it decay into, since there is no lighter particle with fractional charge? So quarks ought to be easy to produce, easy to identify, and easy to store, and yet, no one has ever found one.
The failure of experiments to produced isolated quarks has occasioned wide-spread skepticism about the quark model in the late 1960s and 1970s. Thus those who have clung to the quark model tried to conceal their disappointment by introducting the concept of quark-confinement: perhaps, for reasons not yet understood, quarks are absolutely confined within baryons and mesons, so that no matter how hard we try, one cannot get them out. Of course, this does not explain why quarks can not be isolated. But it poses sharply what has become a crucial theoretical problem: to discover the mechanism responsible for quark confinement.
Even if all quarks are stuck inside hadrons, this does not mean that they are inaccesible to experimental study. One can probe the inside of a proton in much the same way as Rutherford probed the inside of the atom, by firing something into it. Such experiments were carried out in the late 1960s at the Stanford Linear Accelerator Center (SLAC) using high-energy electrons. They were repeated in the early 1970s at CERN using neutrinos, and later using protons. The result of these so-called "deep inelastic scattering" experiments were striking reminiscent of Rutherford's experiment. Most of these incident particles passed right through, but a small number bounced back sharply. This means that the charge of the proton is concentrated in small lumps, just as Rutherford's results indicated that the positive charge of atom is concentrated on the nucleus. But, in the case of the proton, the evidence suggests three lumps, not one. This is strong support for the quark model, obviously, but still not conclusively.
Finally, there was a theoretical objection to the quark model: it appeared to violate the Pauli exclusion principle. In Pauli's original formulation, the exclusion principle stated that no two electrons can occupy the same quantum state. But it was later realized that the same rule applied to all particles at half-integer spin (the proof of this is one of the most important achievements of the quantum field theory). In particular, the exclusion principle should apply to quarks which must carry spin 1/2. Now the Δ++ particle, for instance, is supposed to consist of three identical u quarks in the same state; it (and also the Δ- and the Ω-) appear to be inconsistent with the Pauli principle. In 1964, O. W. Greenberg proposed a way out of this dilemma: he suggested that quarks not only come in three flavors (u, d, and s) but each of these also come in three colors ("red", "green", and "blue"). To make a baryon, we simply take one quark of each color, then the the three u's of Δ++ are no longer identical (one's red, one's green, and one's blue). Since the exclusion principle only applies to identical particles, and problem disappears.
The color hypothesis sounds like a mental slight of hand, and many people initially considered it the last gasp of the quark model. As it turn out, the introduction of color was one of the most fruitful ideas of our time. It is hardly necessary to say that the term "color" here has nothing to do with the usual meaning of that word. Redness, greeness, and blueness are simply labels used to denote three new properties that, in addition to charge and strangeness, the quarks possess. A red quark possess one unit of redness, zero blueness, and zero greeness; its antiparticle carries minus one unit of redness, and so on. We could just as well had X-ness, Y-ness, and Z-ness, for instance. But the color terminology has one especially nice feature; it suggests a delightful simple characterization of the particlar quark combination that are found in nature.
All naturally occurring particles are colorless.
By "colorless" I mean that either the total amount of each color is zero, or all three colors are present in equal amounts. (The latter case mimics the optical fact that light beams of three primary colors combine to make white.) This clever rule "explains" why we can't make a particle out of two quarks, or four quarks, and for that matter why individual quarks do not occur in nature. The only colorless that can be made mesons (quark + antiquark), baryons (quark + quark + quark), and antibaryons (antiquark + antiquark + antiquark).
The decade from 1964 to 1974 was a barren time for elementary particle physics. The quark model, which had seemed so promising at the beginning, was in a state of limbo. It had striking success of neatly explaining the EightFold Way and correctly predicting the lumpy structure of the proton. But it had two defects: the experimental absence of free electrons, and the inconsistency with Pauli exclusion principle. What rescued the quark model was, not the discovery of free quarks, nor confirmation of the color hypothesis, but the completely unexpected discovery of the psi (ψ) meson. The ψ was first observed in the summer of 1974 at the Brookhaven National Laboratory on Long Island by a group lead by Samuel C. C. Ting of the Massachusetts Institute of Technology. But Ting wanted to check his results before announcing them publicly, and the discovery remained secret until November 10-11, when the new particle was independently dicovered by Burton Richter's group at SLAC. The two teams published simultaneously. The discovery was so dramatic that the announcement of the experimental results, in November 1974. is referred to by physicists today as the "November Revolution". Ting named the particle J, and Richter called it ψ. The J/ψ was an electrically neutral, extremely heavy meson - more than three times the mass of the proton. But what made this particle so unusual was its extraordinarly long lifetime of 10-20 seconds before disintegrating. In the months that followed, the true nature of the ψ meson was the subject of lively debate, but the quark model provided the winning explanation. The ψ represented a bound state of a new (fourth) quark, the c (for charm) and its antiquark. Actually, the idea of a fourth flavor, and even its whimisical name, had been introduced years early before by Bjorken and Glashow. So when the ψ was discovered, the quark model was ready and waiting with an explanation. And the story does not end there. Some theorists had argued that there "ought" to be two more quarks. These were called top and bottom quarks. Two years later in 1976, an new heavy meson (called the upsilon) was discovered and it was quickly recognized in 1977 as the carrier of the fifth quark, b (for beauty or bottom). Immediately, the search began for mesons and baryons exhibiting this quark. They were found in 1981 and 1983. The search for the top quark, t (sometimes called truth) came to an end in 1994, when it was found in the debris from high-energy experiments at Fermilab, near Chicago.
| Name | Symbol | Approximate Mass
in Units of Electron Mass |
Electric Charge
in Units of Proton Charge |
|---|---|---|---|
| UP | u | 2 | 2/3 |
| DOWN | d | 6 | -1/3 |
| STRANGE | s | 200 | -1/3 |
| CHARM | c | 3000 | 2/3 |
| BOTTOM | b | 9000 | -1/3 |
| TOP | t | ? | 2/3 |
For a long time physicists thought that the electron, the muon and their associated two neutrinos were the only leptons. But in 1977, they were surprised by the discovery of the tau particle (τ). The discovery of the tau came slowly. As early as 1975, physicists at an electron-positron storage ring of colliding, counterrotating electrons and positrons near Stanford were seeing peculiar effects. The leader of the experimental group, Martin Perl (Nobel Prize shared with F. Reines in 1995), with cautious persistence, suggested that these effects could be due to a new lepton, although other explanations could not be ruled out. In 1977-1978, with confirming evidence coming from a similar experimental facility in Hamburg, Germany, it was clear that there was a new lepton with a hugh mass, 3500 times the mass of the electron. This new lepton that has a mass of 1.8 GeV (1 GeV = 1000 MeV) was called the tau particle. Like the electron and the muon, the tau presummably has a chargeless neutrino associated with it, although there was little evidence for this. The tau, because it is so massive, can decay into a lighter muon particle and a pair neutrinos. The only difference between the electron, the muon, and the tau seems be their different masses. If the muon is a fat electron, the tau is a fat muon.
| Name | Symbol | Mass
in Units of Electron Mass |
Electric Charge
in Units of Electron Charge |
|---|---|---|---|
| Electron | e- | 1 | -1 |
| Electron-Neutrino | νe | less than 0.00012 | 0 |
| Muon | μ- | 207 | -1 |
| Muon-Neutrino | νμ | less than 11 | 0 |
| Tau | τ- | 3491 | -1 |
| Tau-Neutrino | ντ | less than 500 | 0 |
One can almost hear again Rabi's question, "Who ordered that?". Rabi's question expresses the puzzle of the leptons. Who needs any of the leptons beyond the electron and its neutrino? We need only electrons to build atoms. Yet the muon and the tau are just as fundamental as the electron. All the leptons have the remarkable feature that they have never revealed an interor structure. They appear to be pure point particles even at the highest energies. This suggests that they are truly elementary particles and not composite. They are at the rock-bottom of the levels of matter; a feature that compounds the puzzle of the leptons.
Physicists do not know the answer to Rabi's question. The theoretical physicists can fit these leptons into their theories and make predictions about their interactions, but they do not why they exist, why they have the masses they do have, or any further explanation. The discovery of the tau seems to be the end of lepton story for now. But many physicists would probably wager that heavier leptons are waiting to be discovered once the machines with sufficient energy capable of creating them are built.
Both the quarks and leptons seem to be the "rock-bottom" of matter. Leptons exist as real particle, while the quarks are trapped inside the hadrons. This raises another problem: how do the quarks and leptons interact with each other? The physicists answers this question by introducing another set of quanta, the gluons.