* The development of improved particle accelerators and detectors led to a proliferation of discoveries of elementary particles. The initial response of excitement gradually gave way to frustration as the jumble of particles piled up, ultimately leading to the discovery of an underlying structure, defined by the particle called the "quark".
* Even as the concept of the atom became solidly established early in the 20ths century, the suspicion was growing that it wasn't as indivisible as it first seemed. The electron had already been discovered, though after some initial adjustment it didn't seem very disturbing, being no more than the "plum" in an atomic "plum pudding". Einstein's theoretical discovery of the photon was also not very disturbing, at least from the point of view of atomic theory, since photons were simply particles of a sort that an atom absorbed and released.
The experiments by Rutherford and others on radioactivity did begin to show that atoms were mutable, suggesting they had an internal structure. Rutherford then discovered the nuclear atom, followed by the proton. Chadwick complemented this discovery by the discovery of the neutron. That gave a picture of atoms, correct enough as far as it went, as composed of a nucleus of protons and neutrons, surrounded by orbiting electrons. The following table summarizes the known particles as of 1932:
mass in MeV charge spin
________________________________________________
electron 0.511 -1 1/2
proton 938.27 +1 1/2
neutron 939.57 0 1/2
photon 0 0 0
________________________________________________
* Carl Anderson's discovery of the anti-electron or positron did throw a bit of a curve at physicists, but it didn't seem unreasonable to think that particles might have corresponding reverse-polarity antiparticles, there being a certain neat symmetry to it: if we have matter, why not antimatter? It was likely that a negative proton, the "antiproton", existed to complement the well-established proton, but if so it was also a massive particle and hard to hunt down.
The antiproton would be finally discovered in 1955, by a team under Emilio Segre using the new Bevatron at UC Berkeley, coupled to an elaborate detector system. Segre and one of his team members, Owen Chamberlain (1920:2006) won the Nobel Prize in physics in 1959 for discovering the antiproton. It might seem that the logical step after discovering the antiproton would be to mix antiprotons with positrons to create antihydrogen atoms, but this is not trivial to do. It didn't happen until 1995, when a team at CERN led by a German physicist named Walter Oelert fed a beam of antiprotons through a jet of xenon gas; collisions with the xenon atoms produced positrons that in a few rare cases were pulled into orbit about the antiprotons. Only about eleven events were recorded by the detector system, but the physics community accepted that anti-atoms had finally been created.
By the way, in some cases, a proton and an antiproton might not quite collide but would come close enough to neutralize each other's charge, producing a neutron and an "antineutron" (with negative spin). The antineutron quickly encounters a neutron, resulting in their mutual annihilation. A UC Berkeley team discovered the antineutron in 1957.
* The discovery of antimatter would lead to a puzzle: why does the Universe appear to be dominated by matter, with so little antimatter? Being mirror images, why would the Universe prefer one over the other? Why insist on turning right instead of left?
There have been suggestions that the Universe really is balanced between matter and antimatter: the light emitted by distant galaxies looks the same no matter if it's emitted by matter or antimatter, so as far as that goes, the Universe might be a mosaic of clusters of matter galaxies and clusters of antimatter galaxies. However, when matter encounters antimatter, the result is gamma rays, and so the boundaries between matter and antimatter regions in space would be marked by the production of gamma rays due to the mutual annihilation of the thin extragalactic medium. The range of intensities of such a "gamma-ray background" can be calculated, and no observation has found any such thing. Besides, how could matter and antimatter have become segregated in this way in the first place?
In 1965, the brilliant Russian physicist Andrei Sakharov (1921:1989) -- one of the fathers of the Red H-bomb, later a leading Soviet human-rights activist -- conducted a study to figure out why there is an all-matter Universe. It was generally if not universally accepted by that time that the Universe had been born in the Big Bang. Why did the Big Bang only produce matter, not antimatter? Sakharov also wondered why, on the average, a cubic meter of space contains a billion photons, but only a single proton.
Sakharov published his work in 1967, suggesting that at the initial stages of the Big Bang, particle-antiparticle pair production was actually the dominant process, or in other words the Universe was not in "thermal equilibrium". The particles and antiparticles then annihilated each other. There was slightly more matter than antimatter, and so matter was the "last man standing" after this "massacre". The comparative floods of photons are the ghosts left behind from the annihilation of matter.
At the time, physicists had indeed identified slight asymmetries in the behavior of particles, but they were not remotely enough to account for the gross imbalance between matter and antimatter, and nobody is yet sure why matter predominates. However, Sakharov's analysis also led to a radical assertion: the proton, seemingly stable forever, had to decay, if at such a slow rate that it would be very hard to detect. There was no experimental proof that this was so at the time, but it was a notion that physicists would take to heart later.
BACK_TO_TOP* The electron, proton, neutron, and photon -- and, except for the photon, their antiparticles -- are still regarded as "fundamental" particles, at least in the sense that for most practical purposes, few need to worry about any other details. Even as early as the late 1940s, however, physicists knew they weren't everything there was to the story:
The muon, unfortunately, turned out to be only the beginning of trouble, with new unstable particles cropping up rapidly:
To further complicate matters, physicists began to find "resonances", which seemed at first to be new, very short-lived particles, but which were eventually seen as energetic states of known particles. At first, resonances were placed in their own category, but eventually particle physicists decided there was no dividing line between a resonance and other unstable particles other than the short decay time of the resonance, and stopped making the distinction, designating the resonances with a "*" to distinguish them from their ground-state counterparts: "xi*-", "xi*0", and so on.
Resonances were also found as transitory associations of multiple particles, the first being an association of a pion+ and a proton -- discovered in 1953 and designated "delta++", since it had two positive charges. Three other deltas were discovered later: the delta+, the delta0, and the delta-. Higher-order resonances of the delta were discovered as well.
To then pile up the confusion, in the early 1960s particle physicists determined that the electron and muon had their own distinct neutrinos. The proliferation of particles became increasingly frustrating. Ernest Rutherford once said that all science was either physics or stamp collecting, and hunting particles was beginning to seem more like stamp collecting than physics. At one point, a joke made the rounds among the physics community that discoverers of new particles should no longer be given the Nobel Prize -- they should be heavily fined instead.
BACK_TO_TOP* By the early 1960s, the particle zoo had been expanded to include, counting resonances, hundreds of particles. There seemed to be two main groups:
The photon seemed to be in a class by itself. It was called an "intermediate boson", with "intermediate" here meaning that it mediated the electromagnetic force; the more obscure term "gauge boson" was sometimes used as well. The pion was also an intermediate boson, but puzzlingly it was part of the hadron family.
Nobody was particularly pleased with this untidy mess. To try to sort matters out, physicist resorted to conservation laws. Some were well-established, such as mass conservation and charge conservation. For example, in electron-positron pair production, the electron and positron have the same mass-energy as the gamma rays that produce them, and since the gamma rays didn't have an electric charge, the negative charge of the electron is nulled by the positive charge of the positron. Other conservation laws were made up by the physicists as they went along:
Isospin, in more detail, was derived from some ideas put forth by Heisenberg in the 1930s. In development, the concept postulated that if there were families of particles with similar characteristics, or in other words the particles in a family were "isotopes" of each other, then the isotopes could be assigned an isospin value according to the formula:
number_of_isotopes - 1
isotopic_spin = --------------------------
2
For example:
It was called "isospin" because it suggested an arrangement along the lines of that defined by quantum spin, though isospin has nothing really to do with angular momentum. Isospin values, it turned out, were conserved in particle reactions -- but only those controlled by the strong force.
BACK_TO_TOP* As physicists sorted through the various parameters and relationships of particles, they began to zero in on symmetries among them. One important element of the effort was work on quantum field theories, or what were known in the jargon as "gauge theories", with such work establishing a connection to a branch of mathematics known as "group theory", established in the 19th century.
A "group" is a mathematical system based on sets of values or entities that could be manipulated by operations so that any result of an operation was also in the set, or "symmetrical". There also had to be an "identity element", in which an operation between some specified element and the identity element yielded the same value of the specified element; and an "inverse element" associated with every element, in which performing an operation on any specified element and its inverse yielded the identity element. For example:
Group theorists had come up with a "catalog" of different groups. Their work had been done almost completely as an exercise in abstract mathematics, with little or no concern about applications. One subset of groups, known as "Lie groups" that had been devised by a Norwegian mathematician named Sophus Lie (pronounced "Lee" / 1842:1899), were non-Abelian groups that involved possible rotations of solids in space. Physicists would find the Lie groups very handy. As one physicist put it, it was like Neil Armstrong setting foot on the Moon and finding the footsteps of Jules Verne.
* Leveraging off Lie's work, in 1960 Murray Gell-Mann and the Israeli physicist Yuval Ne'eman (1925:2006) independently came up with a neat scheme for organizing the known hadrons that Gell-Mann named the "Eightfold Way", for the octagonal symmetry of the scheme and with a humorous nod to the "noble eightfold path", the Buddhist analogue to the Biblical ten commandments:
BEGIN QUOTE:
Now this, O monks, is the noble truth that leads to the cessation of pain, this is the noble Eightfold Way: namely, right news, right intention, right speech, right action, right living, right effort, right mindfulness, right concentration.
END QUOTE
To Gell-Mann's annoyance, pop science writers later took him very literally and wrote books linking physics to Eastern mysticism. Such a linkage was perfectly possible, as possible as forming a link between, say, football and Eastern mysticism, but Eastern mysticism has no more inherently in common with physics than it does with football. In any case, in the Eightfold Way, the known light baryons could be placed on a plot with the isospin value across the bottom and the strangeness value along the side, as follows:
Notice that if three diagonals are drawn through the elements of this table from the top down towards the right, they cut through negative, neutral, and positive particles. The known mesons could be placed on a similar plot:
These tables had a neat hexagonal symmetry. These "octoplet" plots followed a Lie group named "special unitary group 3" or "SU(3)" -- pronounced "ess-you-three", not "sue-three". Ne'eman wanted to see if the particles could fit into a group that looked like the Star of David, but he couldn't get it to work out.
There were those who wondered out loud if the Eightfold Way was an arbitrary construct, not much more than something like a different way of arranging books on the shelf, but in July 1962, at a meeting at CERN, Gell-Mann and Ne'eman learned that two new resonances had been found, "xi*-" and "xi*0". They both realized that these two new particles would permit the construction of a table of resonances in the form of a triangle with ten members -- a "decuplet", another pattern found in SU(3) -- which could be arranged as follows:
A "?" was placed at the bottom of the triangle, since there was no known "triply strange" resonance. Gell-Mann confidently proclaimed that a particle should be found with a triple strangeness value plus a mass of 1,680 MeV, and even went so far as to give it a name: "omega-". A Brookhaven team found the particle in February 1964; CERN confirmed the discovery a few weeks later. The Eightfold Way was vindicated.
BACK_TO_TOP* The Eightfold Way worked, but there was the question of why it worked. It was a little like Dmitri Mendeleyev's periodic table, which provided a neat way to keep the various atomic elements organized, and was later seen as a clue to the internal structures of those atoms. Did the hadrons have some internal structure as well? Were they made up of smaller particles themselves?
There had long been suspicions that was the case. In 1933, Otto Stern had determined the magnetic moment of the proton, finding that it was much smaller than the magnetic moment of the electron, which had been expected, but still about three times greater than had been estimated. In 1936, the neutron was found to have a magnetic moment. That was something of a shock, since it hardly seemed obvious that a neutral particle could generate a magnetic field. These discoveries suggested that there was more to the proton and neutron than had been believed, that they were made up of fractional electric charges, which nobody wanted to swallow at the time.
In 1964, Gell-Mann bit the bullet and proposed that the hadrons were made up of smaller particles that he called "quarks", picking the name almost at random while he was reading James Joyce's elaborate and linguistically arcane novel FINNEGAN'S WAKE and found:
Three quarks for Muster Mark. Sure he hasn't got much of a bark And sure as any he has it's all beside the mark.
Gell-Mann gave more or less arbitrary names to his set of quarks, assigning them what he called "flavors":
quark_flavor charge ________________________ up (U) 2/3 down (D) -1/3 strange (S) -1/3 antiup (/U) -2/3 antidown (/D) 1/3 antistrange (/S) 1/3 ________________________
They were all fermions, with the quarks having a spin of 1/2 and the antiquarks a spin of -1/2. The scheme gave very tidy results, with all known hadrons could be described as combinations of quarks or antiquarks;
The idea that there were sub-subatomic particles with fractional charges was a bit much to swallow. A CERN physicist named George Zweig (born 1937) came up with the same scheme in parallel, with Zweig calling the quarks "aces", and only succeeded in convincing in his superiors that he was a crackpot. However, in 1969 experiments were performed at SLAC by American physicists Jerome Friedman (born 1930) and Henry Kendall (1926:1999), and Canadian physicist Richard E. Taylor (1929:2018), in which high-energy electrons were "fired" at protons to observe the scattering of "jets" of particles, mostly pions, produced by the impact. The jets shot off at a sharp angle, revealing the quark structure in roughly the same way that Rutherford's bombardment of gold foil with alpha particles had revealed the existence of the atomic nucleus. Quarks were indeed for real, or at least as for real as anything is at the quantum level.
Gell-Mann won the Nobel Prize in physics in that year for his quark theory, and in 1990 Friedman, Kendall, and Taylor shared the Nobel Prize for their work. Gell-Mann had the satisfaction of being vindicated, but his triumph was slightly tainted by the fact that instead of pronouncing the term "quark" to rhyme with "cork", as he wanted, the general pronunciation was to rhyme with "mark".
BACK_TO_TOP* There was a significant problem with the quark model, most significantly associated with the omega- particle. The omega- was triply strange, meaning that it included three strange quarks. The difficulty was that the strange quark was a half-spin fermion, and there was literally no way three fermions could coexist. In 1972, Gell-Mann, along with Harold Fritsch and William Bardeen, mined the SU(3) symmetry again to suggest a way out, which the group called the "color force", sometimes called the "glue force", and in doing so provided a quantum field theory that explained the strong force.
In this scheme, quarks were given a "color charge", with the possible charge values assigned the entirely arbitrary names of:
red green blue
-- along with:
antired (cyan) antigreen (magenta) antiblue (yellow)
Massless, chargeless exchange particles called "gluons" -- which were spin-1 bosons -- mediated the color force, and carried combinations of two colors as well, allowing them to transfer color charges from one quark to another.
Again, these "colors" were just arbitrary names, having nothing to do with the popular concept of colors. They could have almost as reasonably been named "moe", "larry", and "curley", along with "antimoe", "antilarry", and "anticurley", which would suggest the name of "stooges" for the quarks themselves. The use of colors as the naming convention was convenient, however, since the way the quark colors behave is logically similar to the way actual colors mix.
* The essential point of the whole exercise was to create a system to explain the binding forces between quarks in a way very similar to the system that explains the electromagnetic force. Murray Gell-Mann decided to name the scheme "quantum chromodynamics (QCD)" in a deliberate nod to quantum electrodynamics / QED.
In QED, the electromagnetic force acts between particles that have positive or negative electromagnetic charges, using photons as the exchange particle. Like charges repel, while unlike charges attract. In QCD, the particles, the quarks, can have three different types of positive or negative color charges -- red or antired, green or antigreen, and blue or antiblue. Like color charges repel, while unlike color charges attract; a color and its anticolor, such as red and antired, have a particularly strong attraction.
The combinations of colors will always add up to color-neutral or "white" -- red / green / blue, red / antired, and so on -- which is why the color force is essentially invisible to the universe outside the particle containing the quarks. QCD was based on the same sort of baroque series of interactions that make QED so much fun to work with, only worse because the color charge system has more fundamental interactions.
The concept of the color force also explained why experiments to detect a free quark never produced duplicable results. The reason is that the color force increases with range: the farther the quarks are pulled apart, the greater the force needed to pull them farther apart. As Gell-Mann put it: "You can't get them apart with a quarkscrew."
According to QCD, when quarks are very close together, they don't trade gluons and are effectively free of the color force, a condition known as "asymptotic freedom". The farther apart they are pulled, the greater the influence of gluon exchange becomes, with the force required to pull them apart climbing as the distance is increased. This may sound counterintuitive compared to the other forces, which get weaker as the distance between interacting particles increases, but it really not all that different from thinking about particles as if they were linked together by springs. The notion of asymptotic freedom was actually developed by David Gross (born 1941), David Politzer (born 1949), and Frank Wilczek (born 1951), who took the 2004 Nobel Prize in physics for their work.
It should be realized that if enough force is applied to the quarks in a hadron, the color force links can be broken, but there's a big catch in that action. Once enough energy is pumped into the process to break the links, there's also enough energy to create new quarks, and breaking up the old hadron simply results into new hadrons -- never an isolated quark.
From this explanation, it might be thought that this color force is a fifth force, alongside gravity, electromagnetism, and the strong and weak forces. Not so. The strong force is mediated by the pion, which is not a fundamental particle like other force carriers such as the photon or gluons: it is a meson, made up of two quarks. The strong force carried by the pion is derived from the color force; the gluons in one particle exert an attraction on the gluons in another, with this attraction generating the pion. The strong force is just the color force manifested outside the boundary of a particle; it might be said that the color force is the "real" strong force and that the strong force as traditionally defined is just a "residual" strong force.
In sum, QCD provides a quantum field theory for the strong force in much the same way that QED provides a quantum field theory for the electromagnetic force. Incidentally, fission power is based on release of energy bound up in "residual" strong force couplings when nuclei are shattered in atomic chain reactions. If there were some way to set up a chain reaction that broke apart nucleons themselves into their constituent quarks, the amount of energy released would be vastly greater. However, asymptotic freedom means that it's not possible to break apart a single nucleon into its constituent quarks -- much less set up a chain reaction of such processes.
BACK_TO_TOP* While Gell-Mann and others were working on quarks and a quantum field theory of the strong force, other physicists had been working towards a quantum field theory of the weak force. Following the publication of Gell-Mann's quark model, in 1967 three physicists -- the American physicists Sheldon Lee Glashow (born 1932) and Steven Weinberg (1933:2021) and the British Pakistani physicist Abdus Salam (1926:1996) -- managed to come up with a quantum field theory that not only described the weak force, but showed that it could be unified with the electromagnetic force as a single "electroweak" force.
The electroweak theory showed that at high energies, found shortly after the creation of the Universe in the "cosmic fireball" called the "Big Bang", the electromagnetic and weak forces acted identically, only becoming separate in a process of "symmetry breaking" when the Universe cooled off sufficiently.
To understand the concept of symmetry breaking, consider a bar magnet. At a certain temperature, a bar magnet loses its magnetism, since the thermal vibration of the atoms randomizes the orientations of their magnetic moments. From the point of view of magnetism, the bar magnet is symmetrical -- it's equally nonmagnetic in any direction. Once it cools down, the magnetic moments of the atoms line up and lock in place, giving the bar magnet a directional magnetic field. The symmetry has been spontaneously broken by a reduction in the thermal energy of the magnet.
For another analogy, imagine a ball trapped in a cup. If there is a ridge in the bottom of the cup, then at high energies, the ball will not be constrained by the ridge; the system is symmetrical. At low energies, the ball will be trapped on one side or another: symmetry has been "broken". (The cup configuration in this case is sometimes called a "sombrero" or "Mexican hat" configuration from the shape of its cross-section.) get over the ridge and symmetry is restored. The asymmetrical position of the ball corresponds to the existence of seemingly separate electromagnetic and weak forces; the ball at higher energies, when it is not constrained, corresponds to the unified electroweak force. In the low energy configuration, the symmetry is hidden but implied, to be revealed at higher energies.
The weak force is mediated by of virtual weak-force exchange particles that are very massive, meaning that the energy required to create them is large. The time-energy uncertainty relationship ensures that the time these massive virtual particles exist is very short, which is why the weak force is short-ranged. However, at the high energies available just after the Big Bang, the energy was available to allow the production of real, not virtual, massive particles that were not constrained by the time-energy uncertainty relationship. That means that the weak force was no longer short-ranged -- in fact, it worked identically to the electromagnetic force until the Universe cooled down and broke that symmetry.
Glashow, Weinberg, and Salam shared the 1979 Nobel Prize in physics for this theory, which along with QED became known as the "standard model". In terms of group theory, it is a composite of three such groups, "SU(3) x SU(2) x U(1)", with the elements describing quantum chromodynamics, the weak force, and the electromagnetic force respectively -- this equation is pronounced as "ess-you-three cross ess-you-two cross you-one". Salam scored a fashion splash at the Nobel awards ceremony in Stockholm by disregarding the custom of wearing a tuxedo and showing up in traditional Pakistani formal wear, including boots with curled-up toes.
One of the major predictions of the standard model was to provide possible details of the weak force. The weak force affects all leptons and hadrons, but it is much weaker than the electromagnetic or strong forces. The weak force was assumed to operate by exchange particles, but nobody had much clue of the details. According to the electroweak theory, the weak force is mediated by three exchange particles:
All three are bosons and all have mass, making them unique among the fundamental force carriers. They do not carry a color charge, but do carry a "weak hypercharge" that is analogous to the electric charge or color charge. The W+ and W- are antiparticles of each other, while the Z0 is its own antiparticle. The W+, W-, and Z0 were collectively known as the "intermediate vector bosons" or just "vector bosons".
The weak force can really only come into play when the dominating electromagnetic or color forces aren't involved. For example, since neutrinos don't have an electric charge or color charge, interactions involving neutrinos can only involve the weak force or, as a much lower probability, the gravitational force.
BACK_TO_TOP* The fact that the vector bosons that mediate the weak force have mass was very puzzling; photons and gluons don't. The fact that some particles had mass and others didn't also led to the more general question of why mass existed at all. Was it just "the way things were", or was there some deeper scheme to it?
In 1964, the British physicist Peter Higgs (born 1929) postulated that there was another field permeating the Universe that was generally undetectable, which of course became known as the "Higgs field", that was mediated by a massive but also generally undetectable particle, which similarly became known as the "Higgs boson" or "higgson". In contrast to the photon and the gluons, which are spin-1 bosons, the Higgs boson is a spin-0 boson, a fact that gives it distinctive properties.
The Higgs boson is supposed to be highly interactive with almost everything, and the interactions of Higgs boson (or, equivalently, the "Higgs field") with a particle would impede the particle's motion, effectively giving the particle mass. The Higgs field has been compared to a sea of molasses, impeding the acceleration of particles -- with different particles impeded in different ways.
The same theory was developed independently at roughly the same time by two Belgian theorists, Robert Brout (1928:2011) and Francois Englert (born 1932). The whole notion of the Higgs boson / Higgs field had some quirky aspects, one being that even in theory it would be very hard to detect. Even Higgs was unsure that there was much to it, telling one of his students that he, Higgs, had discovered something "totally useless". The Higgs boson is generally accepted today, though some physicists were suspicious of the idea; Sheldon Glashow referred to the Higgs boson as a "commode down which all theoretical inconsistencies have been flushed."
The Fermilab Tevatron, upgraded with a new proton-antiproton injector system in 2001, looked for the Higgs, but it wasn't found there before the Tevatron was shut down in 2011. Observations at the LHC in 2012 tentatively identified the Higgs, with follow-on analysis reinforcing the conclusion instead of undermining it. However, CERN researchers were still reluctant to say they had nailed the Higgs. The LHC was shut down in early 2013 for two years of maintenance, in particular to fix bugs that had prevented the collider from operating at full power, with upgrades also being added to improve its capability. When it comes back online in early 2015, it will then focus on confirming the Higgs.
* Along with the Higgs, additional quarks have been found. The U, D, & S quarks could account for all known hadrons, but in 1970 Sheldon Glashow, Greek physicist John Iliopoulis (born 1940), and Italian physicist Luciano Maiani (born 1941) predicted the existence of a fourth quark, which was arbitrarily named the "charm (C)" quark. There was no strong experimental motivation for this concept; the rationale was simply that the U and D quarks seemed to be very similar, while the S quark was different from the other two. Possibly the S quark had a C quark counterpart?
The notion was tenuous, and the physics community had a hard time taking the matter very seriously -- until September 1974, when a research team under experimental physicist Sam Ting (born 1936) at Brookhaven discovered a resonance that, by all its observed properties, seemed to contain a C quark. The Brookhaven group named it "J", with most believing that it was because the Chinese character for the name "Ting" looks like a "J". In November, a SLAC team under Burton Richter found the same particle, calling it "psi". The two teams ended up compromising and calling it "J/psi" -- pronounced "jay-sigh", with the alternate pronunciation of "gipsy" not widely catching on. As it turned out, it was a spin-1 meson, composed of a C quark and /C antiquark, with a mass of 3.1 GeV and a half-life of 10^-20 second. Ting and Richter won the Nobel Prize for physics in 1976 for finding J/psi.
The discovery of the J/psi did much to eliminate remaining resistance to the quark model. Now the scheme gave a neat symmetrical table of four leptons and four quarks, which can be divided into two "generations", as follows:
leptons quarks
______________________________ _________________
1st generation electron neutrino_e up down
2nd generation muon neutrino_m strange charm
______________________________ _________________
Of course, there are also antiparticles for all these particles. The leptons are further subdivided by the fact that the electron and muon have electric charges, while their corresponding neutrinos are, as their name more than suggests, electrically neutral. Similarly, the U and S quarks as a set have different properties from the D and C quarks as a set. A separate table is needed to cover the exchange particles that mediate forces, such as the photon, gluons, and vector bosons.
* Probably to no great surprise to those who had been struggling with particle physics for a long time, even at the time of the discovery of "charmed" hadrons there were signs that things were a bit more complicated.
In 1973, the Japanese theoretical physicists Kobayashi Makoto (born 1944) and Masukawa Toshihide (1940:2021) suggested that if there were a third generation of particles, the weak interaction would show a preference for matter over antimatter -- or in other words, explain matter-antimatter asymmetry. The paper was very influential, and the two men would share the Nobel Prize in physics in 2008 for it.
Two years later, in 1975, American physicist Martin Perl (1927:2014) and his colleagues discovered a third charged lepton, which became known as the "tau". It was a very heavy particle, with about twice the mass of the proton, and broke down in about 0.3 picoseconds. There was likely a "tau neutrino" associate with it, forming a "third generation" of leptons, but it wasn't discovered until 2000, when Fermilab researchers managed to pin it down. Perl shared the Nobel Prize with Reines in 1995 for the discovery of the tau.
A third generation of leptons suggested the existence of a third generation of quarks, which had been also hinted at in the standard model, and in 1977 the American physicist Leon Lederman (1922:2018) and his colleagues discovered the "upsilon" meson, which contained a fifth type of quark, which was arbitrarily named the "bottom (B)" quark and was in the same group with the D and C quarks. The upsilon meson, incidentally, is a neutral spin-0 boson with a mass of 9.46 GeV, a half-life of 10^-20 second, and consisting of a B and /B quark.
The B quark of course suggested a partner in the U quark and S quark group, which was unsurprisingly named the "top (T)" quark. After some effort, it was finally discovered at Fermilab in 1995. It wasn't surprising that it was so hard to find the T quark, since it is a highly energetic particle, with 174 times the mass of the proton.
Whatever the incidental details, the end result was a table like this:
leptons quarks
________________________ __________________
1st generation electron neutrino_e up down
2nd generation muon neutrino_m strange charm
3rd generation tau neutrino_t top bottom
________________________ __________________
The six flavors of quarks can be described as follows, with a rough index of relative mass given in the right-hand column for comparison:
quark charge spin mass relative mass ___________________________________________________________ up +2/3 1/2 1.5 to 4 MeV 1 down -1/3 1/2 4 to 8 MeV ~2 strange -1/3 1/2 80 to 130 MeV ~100 charm +2/3 1/2 1.15 to 1.35 GeV ~1,200 bottom -1/3 1/2 4.1 to 4.4 GeV ~4,300 top +2/3 1/2 172.7 +/- 2.9 Gev ~170,000 ___________________________________________________________
There have been hints of particles made up of five quarks -- "pentaquarks" -- and that gluons can form transient assemblies among themselves -- "glueballs". Nobody's actually verified them, and for the time being they're not being taken very seriously.
BACK_TO_TOP* The discovery that there were three different flavors of neutrino led to the discovery that these three neutrinos could transform into each other. That story actually began in the 1960s, when a detector to pick up neutrinos from the core of the Sun was built by Raymond Davis JR (1914:2006) and his group from Brookhaven. The detector consisted of a tank containing 615 tonnes of perchloroethylene (C2Cl4), a dry cleaning fluid, stuck deep in the Homestead Mine in Lead, South Dakota, to reduce confounding radiation from other sources. Interaction of a neutrino with a chlorine atom in the perchloroethylene molecule would produce an atom of a radioactive isotope of argon as follows:
neutrino + Cl<37/17> --> electron + Ar<37/18>
The radioactive argon could be detected by a flushing system. The detection rate was estimated to be about one argon atom per day. Although it might be hard to believe that a detector could be built that could reliably pick one radioactive argon atom out of a huge vat of cleaning fluid every day or so, Davis liked to say: "It's just plumbing." -- though anybody else would have to reply that it didn't exactly sound like bathroom plumbing. Davis's work won him the 2002 Nobel Prize in physics, which he shared with Japanese neutrino researcher Koshiba Masatoshi (1926:2020) of the University of Tokyo.
However, although the detector worked, the detection rate was much lower than expected, a matter that physicists found very interesting. Theoretical analysis by astrophysicist John Bahcall (1934:2005), who was working closely with Davis, indicated that the detector should have acquired one radioactive argon atom, created by a collision with a neutrino, per day -- but in reality, Davis got one atom every 2.5 days.
Although there was considerable skepticism at the outset that Davis's experiment could actually deliver useful results, the experiment was checked and rechecked, and if there was a flaw there nobody could figure out what it was. Astrophysicists refined their models of the workings of the Sun and everything seemed to fit the data, except for the neutrino deficit.
The discovery that there were three different flavors of neutrino hinted at a possible solution. Davis's detector could only pick up electron neutrinos; what if electron neutrinos somehow changed their flavor to muon or tau neutrinos, "oscillating" as they traveled the distance from the Sun's core to the Earth?
New deep-underground particle detectors had been built following Davis's experiment. Observations performed in the late 1990s showed that muon neutrinos produced by cosmic rays tended to disappear at a rate proportional to the distance of the cosmic-ray collision to the detector, which hinted at neutrino oscillations. Experiments with neutrino beams shot over long distances through the Earth to neutrino detector systems seemed to show that neutrinos oscillated, but what was really needed was a tool to probe the neutrino emissions of the Sun more directly. That tool was built, in the form of the "Sudbury Neutrino Observatory (SNO)", buried two kilometers under the ground in a nickel mine in Sudbury, Ontario, Canada.
While the Homestake detector could only pick up neutrinos through a single type of interaction, SNO could detect neutrinos through two different classes of interactions. One of these interactions was only with electron neutrinos, while the other two were with all flavors of neutrinos. Analysis showed that if neutrinos weren't changing flavor, the count of interactions with electron neutrinos would be basically the same as the count of interactions with all three flavors of neutrinos. If neutrinos did change flavor, the count of interactions with electron neutrinos would be an obviously lower proportion.
At the core of the SNO detector was a 12-meter-wide tank made of acrylic plastic that contained the 1,000 tonnes of heavy water. The tank was in turn encapsulated inside a geodesic spheroid 18 meters in diameters that was studded with more than 9,500 sensitive photomultiplier tubes, each capable of picking up a single photon. The entire assembly was then submerged into a cavity dug into the bottom of the Sudbury mine and filled with ultrapure ordinary water, The three interactions captured by SNO included:
Interactions were expected to occur about ten times a day. There was no way to assign any single detection event to any one of these three possibilities, but there were indirect clues that could be used to statistically sort out the events.
Muons from cosmic rays could also produce Cerenkov light, with about three such events an hour at the underground depth of SNO, but they produced events in both the normal water outside the detector core and in the heavy water inside the core. A worse problem was traces of radioactive materials in the immediate environment; all was done to minimize this problem, and the experiment was "calibrated" to determine the level of background radiation.
The first test run began in late 1999 and went into May 2001, with a half billion events detected, with just under 3,000 validated after screening. The detected electron neutrino flux was about a third of the total rate, which strongly suggested that neutrinos were oscillating in flavor as they passed from the Sun.
The fact that the neutrino oscillated clearly indicated that it has mass. If it were massless, it would travel at the speed of light; if it traveled at the speed of light, time would not exist for it; if time did not exist for it, it would not be able to change in any way and so oscillations would be ruled out. Neutrinos had to have at least a very small mass, and they didn't travel at the speed of light.
The subject of deep-underground particle detectors such as SNO -- as well as underwater and deep-ice particle detectors -- is an interesting one, with a substantial number of detector systems now available, and more coming online all the time. However, the number and diversity of these detector installations, as well the ongoing change in the field, makes useful discussion of them beyond the scope of this survey.
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