[17.0] Quarks & Electroweak Unification

v1.0.1 / chapter 17 of 20 / 01 sep 07 / greg goebel / public domain

* Efforts to systematize the particle zoo led to the discovery of a neat tabular pattern that become known as the "Eightfold Way". This led in turn to the discovery that baryons were actually composed of two or three lower-level particles that became known as "quarks", which had fractional charges and were impossible to isolate even in principle.

The quark model provided a basis for a quantum field theory of the strong force. Following the discovery of quarks, further work on quantum field theory led to a model of the weak force in unification with the electromagnetic force.

[17.1] YANG-MILLS THEORY

* The proliferation of particles presented a challenge to physicists. There was another side of the coin to this challenge. QED had provided an elegant model for the electromagnetic force; the challenge was to come up with comparable models of the strong and weak forces, hopefully then obtaining an overall set of models that made sense of the particle zoo. One of the tools they acquired in pursuit of this goal was the concept of "symmetry".

As a graduate student at the University of Chicago in the late 1940s, Frank Yang had thought over the nature of the electromagnetic field, specifically Einstein's concept that the speed of light was an absolute constant, not affected by the motion of the source. As Yang realized, light was "measurement invariant", indifferent to the frame of reference of the measurement or, as the physicists like to put it a little obscurely, "gauge invariant", where "gauge" here simply means "measurement". The electromagnetic field was by extension a "gauge field", and so QED was a "gauge invariant field theory".

In 1954, when Yang was at Brookhaven, he worked with his colleague Robert Mills (1927:1999) to apply the same concepts to the strong force, describing it as a gauge invariant field and deriving the rules of the operation of the force from the implied symmetries. It was a fairly primitive and not all that workable theory in terms of actually describing the strong force; it was more important in that it introduced formal concepts of symmetries to particle physics, using them to define the particles and exchange interactions for a particular force. The particle physicists would make extensive use of such "Yang-Mills gauge theories", sometimes called "Yang-Mills theories" or just "gauge theories". QED, for example, could be defined by such a theory.

Those interested in the symmetries in particle physics were somewhat startled to find out that 19th-century mathematicians had created what was known as "group theory". 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, the set of integer numbers and the operation of addition form a simple group. The identity element is 0, and the inverse element of any number is its negative value. The set of integer numbers and the operation of subtraction also establish a group, but it is a more complicated system. Addition is "commutative", meaning that the order of addition of two elements doesn't matter -- 5 + 2 equals 2 + 5 -- while subtraction is "non-commutative", meaning the order does matter -- 5 - 2 does not equal 2 - 5. In group theory, a commutative group is said to be "Abelian", which means that a non-commutative group is "non-Abelian".

Another group consists of the equilateral triangle, and operations that would result in maintaining the exact same appearance of the triangle, such as rotation by 60 or 120 degrees, or flipping the triangle around any one of its three axes.

A group along the lines of a rectangular grid being put through any possible rotation undergoes what is known as a "gauge transformation" with its rotations, meaning that the orientation of the system may change but no other properties do. It was pointed out by the brilliant mathematician Emmy Noether (1882:1935) that this gauge invariance implied the conservation of some quantity inherent in the group. For example, the gauge transformation due to linear translation -- moving everything from one place to another -- corresponds to the conservation of linear momentum, while the gauge transformation due to rotational translation corresponds to the conservation of angular momentum. Gauge invariance over time corresponds to the conservation of energy. The general principle became known as "Noether's Theorem" to physicists, to the irritation of mathematicians who reply that Noether devised more than one important theorem.

This gauge invariance leads to the definition of forces. Suppose we have a system of electrically charged particles arranged in a specific, fixed set of positions. If we change the voltage applied to the entire system, it is invariant, but if we change the voltage on only one part of the system, new forces arise to compensate. A gauge field implies a force.

Group theorists had effectively come up with a "catalog" of different symmetry 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. Such groups could be used to organize particle systems and, using Noether's theorem, describe the conservation laws followed in the interactions among those particles.

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[17.2] THE EIGHTFOLD WAY

* There were a number of attempts to find an underlying organization to the chaos of particles, and in 1960 Murray Gell-Mann and the Israeli physicist Yeval Ne'eman (born 1925) 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: "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."

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. The exercise did provide some explanation for why Gell-Mann believed that pop science writers were all idiots.

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 meson 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.

Incidentally, the omega- is a spin 3/2 baryon, has an actual mass of 1,672.4 MeV, and a decay half-life of 8E-9 second. It has three decay modes:

   68%:    omega-  -->  lambda0  +  kaon-

   24%:    omega-  -->  xi0  +  pion-

    8%:    omega-  -->  xi-  +  pion0

[17.3] DISCOVERING QUARKS

* 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. This 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 had some kind of internal structure.

The problem with that idea was that any particles that made up the hadrons would need to have fractional electric charges, with values less than one, and that seemed too much to swallow. 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.

All the known hadrons could be described as combinations of quarks or antiquarks; baryons like the proton were made up of three quarks -- which is why the baryons are all fermions -- while mesons like the pion were made up of two -- which is why the mesons are all bosons. Strange particles of course contained strange quarks, with the strangeness value matching the number of strange quarks. Leptons remained unitary particles. For example:

  BARYONS:

  neutron     U D D
  proton      U U D
  sigma-      D D S
  sigma0      U D S
  sigma+      U D S
  xi-         D S S
  xi0         U S S

  MESONS:

  kaon0       S /D
  kaon+       U /S
  pion-       D /U
  pion0       U /U  &  D /D (equal probability of one or the other)
  pion+       U /D
  kaon-       S /U
  /kaon0      D /S

The idea that there were sub-subatomic particles with fractional charges was a bit much to swallow. A CERN physicist named Georg 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. Those who were more open-minded found the quark theory elegant, in particular liking the way the strange quark neatly accounted for the strangeness conservation law. However, even if the scheme could be swallowed, there remained the question of whether it was anything more than some arbitrary accounting scheme that couldn't be backed up by the facts.

Although Gell-Mann's stature meant that he was likely to get a more polite hearing than Zweig, Gell-Mann seemed uncharacteristically cautious in his paper announcing the idea, one colleague suggesting that the gist of the paper was: "If quarks are not found, remember I never said they would be, and if they are found, remember I thought of them first." It must be noted that Einstein was as or more cautious, and with just as much good reason, when he proposed the existence of what would be called the photon, but then Einstein didn't have a reputation for assertiveness. Gell-Mann would never cease to insist in exasperation that his intent had been misread, but if so he discovered the pitfalls of making distinctions that go over the head of the audience, or prove only too convenient to critics looking for rocks to throw.

Dick Feynman jumped into the argument by considering what the implications would be if a proton or whatever was made up of subordinate particles. Feynman called these subordinate particles "partons" and was pointedly, even contemptuously, agnostic about their nature -- so much so that the quark advocates felt snubbed and became thoroughly annoyed with him, Gell-Mann describing Feynman's comments as an "insult". Feynman simply wanted to determine what kind of tests might be performed to see if a particle was in fact made up of partons.

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 (born 1929), 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 soiled 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".

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[17.4] QUANTUM CHROMODYNAMICS

* 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"; it is also sometimes referred to as the "glue force."

In this scheme, quarks were given a "color charge", with the possible charge values assigned the arbitrary names of:

   red
   green 
   blue

   antired (cyan)
   antigreen (magenta)
   antiblue (yellow)

Of course these "colors" were just names, having nothing to do with the popular concept of colors. They could have just as legitimately 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 very 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.

As mentioned, gluons carry a combination of two color charges each. There are six such combinations that can change the color of quarks:

   red / antiblue
   red / antigreen
   green / antiblue
   green / antired
   blue / antired
   blue / antigreen

   red / antired
   green / antigreen
   blue / antiblue

For example, QCD involves the same sort of baroque series of interactions that make QED so much fun to work with it, with the generation of virtual quark-antiquark pairs making the computations just as complicated. To make things worse, the gluons can perform interactions on their own, something photons can't -- a gluon can split into two gluons, and two gluons can interact to produce two different gluons.

Since quarks have (fractional) electric charges, a single particle may contain quarks with like polarities of electric charges that repel each other. The color force is strong enough to overcome this repulsion.

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.

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 in a fashion that conceptually parallels the almost-invisible way that, say, a proton is derived from quarks. What happens is that 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 that 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 such 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.

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[17.5] THE ELECTROWEAK THEORY

* While Gell-Mann and others were working on quarks and a Yang-Mills theory of the strong force, other physicists had been working towards a Yang-Mills theory of the weak force. Julian Schwinger had tinkered with such a theory in 1957, which was refined the next year, 1958, by Sidney Bludman of the University of California at Berkeley. These were incomplete theories, with considerable doubt being expressed that they were renormalizable.

Others picked up the gauntlet and managed to create a workable model 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 (born 1933) and the British Pakistani physicist Abdus Salam (1926:1996) -- managed to come up with a Yang-Mills 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. Once it cools down, the magnetic moments of the atoms line up and lock in place, giving the bar magnet a magnetic field. The symmetry has been spontaneously broken by a reduction in the thermal energy of the magnet.

In another analogy, imagine a particle trapped in a cup. The particle can be any place in the cup: the scenario is symmetrical. However, if there is a ridge in the bottom of the cup, then at low energies the particle will be trapped in 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.) At higher energies, the particle will 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.

Incidentally, Salam, although a Muslim, was Yeval Ne'eman's academic advisor at Imperial College in London. Ne'eman actually showed up in his Israeli Defense Forces uniform -- he was an intelligence officer on academic leave -- and with a letter of recommendation from Moshe Dayan. Salam felt that Islamic science owed much to Jewish scholars and gladly took on Ne'eman, though the letter from General Dayan got short shrift. Salam was a member of the minority Ahmadis Islam sect, which is generally regarded as heretical by mainstream Sunni and Shia Muslims, giving him a non-mainstream view on matters. Ne'eman was also not the stereotype of the dusty physics professor: later on, he would become a member of the Knesset, the Israeli parliament, and news shows would film him fiddling with physics equations when a debate dragged on for too long.

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, two of which, the "W+" and "W-", have an electromagnetic charge, and one which, the "Z0", is neutral. Of course, the "W" stands for "Weak" and the "Z" stands for "Zero". They are all 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.

In a weak force interaction, a particle emits a vector boson and changes into a different particle, with the vector boson then promptly decaying into other particles of its own. Exchanging a neutral Z0 vector boson does not change the electric charge of the particles -- this action is called a "neutral current" -- but since the W+ and W- carry an electric charge, their exchange does changes the electric charge of the particles -- with this action of course referred to as a "charged current".

In beta decay, a neutron breaks down into a proton, an electron, and an electron antineutrino. Since the electron antineutrino isn't affected by the electromagnetic or strong forces, the weak force necessarily controls this process. A neutron consist of two D quarks and one U quark; in beta decay, the D quark releases a W- vector boson and decays into a U quark, converting the neutron into a proton. The W- then decays into an antineutrino and an electron.

This doesn't look like an exchange process at all, until some considerations from Feynman diagrams are factored in. In Feynman diagrams an antineutrino moving forward in time is equivalent to a neutrino moving backward in time. Beta decay can be described as an exchange of a W- between a D quark and a neutrino moving backward in time, with the exchange changing the D quark into a U quark -- changing the neutron into a proton -- and the neutrino moving backwards in time into an electron moving forward in time.

Once again, the description of the operation of the weak force has many resemblances to QED, and in fact the mathematics for weak force interactions was plagued by the same sort of infinities that hobbled QED for decades. A workable scheme for renormalizing the calculations was produced by a Dutch graduate student named Gerard 't Hooft (born 1946) in 1971. According to the story, 't Hooft was chatting with his academic advisor, Martinus Veltman (born 1931), and Veltman suggested that there was a need to obtain a "renormalizable theory with massive charged vector bosons." To which t'Hooft replied: "I can do that."

Veltman was startled: "What did you say?"

"I can do that." Possibly to Veltman's surprise, his student could do that. The two men won the Nobel prize in physics in 1979 for their work.

The weak force provides an accounting of the generations of particles. Leptons and quarks can only change into different leptons and quarks through the weak force. Since second generation leptons and quarks are heavier than first generation, they tend to emit vector bosons and decay into first generation leptons and quarks. Since there are no lighter leptons and quarks, first generation particles cannot decay further: they are at "the bottom of the hill" and they can't go any lower energetically.

The standard model told the experimental physicists where to look for the vector bosons, The W+ and W- were finally discovered in 1983 by Carlo Rubbia and his colleagues at CERN. Rubbia and Simon Van der Meer shared the 1984 Nobel prize in physics for this discovery. The Z0 particle was discovered in parallel at SLAC and CERN in 1988.

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[17.6] THE HIGGS BOSON

* The fact that the vector bosons that mediate the weak force have mass was very puzzling; photons and gluons don't. Early work on the electroweak theory by Schwinger and others unsurprisingly assumed that the vector bosons were massless, but nobody could get things to work that way. The fact that some particles had mass and others didn't also led the more general question of why mass existed at all. Was it just "the way things were", or was their some underlying 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 and Francois Englert. The whole notion of the Higgs field and Higgs boson had some bizarre aspects, for example that the Higgs field was scalar only and uniform everywhere -- which was why it was all but impossible to detect, sort of like the "little man who wasn't there". Even Higgs was unsure that there was much to it, telling one of his students: "This summer I have discovered something that is totally useless."

However, Weinberg and Salam figured out that it was needed to get electroweak theory to work, and incorporated it into the standard model as a way of explaining why the W and Z exchange particles had mass, but the photon didn't. The Higgs boson is generally accepted today, though some physicists remain suspicious, believing the idea is basically "dippy". Sheldon Glashow has referred to the Higgs boson as a "commode down which all theoretical inconsistencies have been flushed."

Attempts to find the Higgs boson with the Fermilab Tevatron have had ambiguous results. One of the main initial objectives of the CERN LHC after it comes online in 2007 will be to hunt for the Higgs boson. The Fermilab Tevatron, upgraded with a new proton-antiproton injector system in 2001, has also been looking for the Higgs, though getting everything to work right has been troublesome; however, Fermilab physicists believe they have a shot at finding the Higgs before the Tevatron is shut down in 2009.

LHC physicists are certain their system will be able to detect the Higgs boson; if the LHC doesn't find the Higgs boson, that will be conclusive evidence that it doesn't exist, at least not in the various forms that have been described so far.

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[17.7] NEW GENERATIONS OF PARTICLES

* 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".

This led to some horse trading about who got priority: in the end, the two groups compromised and published their papers in the same 2 December 1974 issue of PHYSICAL REVIEW LETTERS, with the particle being named "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
                   ______________________________  _________________

* However, 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 Makoto Kobayashi and Toshihide Masukawa 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. Two years later, in 1975, American physicist Martin Perl (born 1927) and his colleagues discovered a third charged lepton, which became known as the "tau", recycling the original name of the kaon0S. 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 (born 1922) and his colleagues discovered the "upsilon" meson, which contained a fifth type of quark, which was arbitarily 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. There was a suggestion that physicists would have to "fall on their fountain pens" if it wasn't found, but 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. Incidentally, some physicists called the bottom quark the "beauty" quark and the top quark the "truth" quark, but those usages never caught on.

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
                   ________________________  __________________

   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
   ___________________________________________________________

Since that time, a large number of experiments have also provided evidence of the pentaquark, but there are inconsistencies between the different experimental results, and other experiments don't reveal it at all. From a theoretical point of view the pentaquark seems very dodgy, since a five-quark assemblage should immediately break down into a three-quark baryon and a two-quark meson. The fact that the pentaquark seems so bizarre makes it interesting: if it does in fact exist, it would open the door to previously unknown physics. Researchers are being cautious about the matter.

There is also a possibility that gluons could join together -- no quarks involved, except for the virtual quark-antiquark pairs inherent in QCD -- to form a "glueball". Candidate events for glueballs were observed as far back as 1982, though these events could also be states of quark-antiquark pairs. If the existence of the glueball is confirmed, that would provide direct evidence for the existence of gluons: although particle accelerators have been able to confirm the existence of quarks themselves through scattering experiments, such tricks don't work for gluons, and they remain somewhat more of a theoretical fiction than quarks themselves.

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