v1.0.2 / chapter 16 of 20 / 01 aug 09 / greg goebel / public domain
* Given a disorderly pile of elementary particles, physicists began to look for ways to make sense of it all. The result was the discovery of an entirely new set of particle quantum values, such as "strangeness" and "isotopic spin", and insights into the broad structure of the Universe.
* 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. This made for a somewhat untidy table of particles. The trick was to figure out common threads in that table to allow the particles to be organized, hopefully revealing some order among the chaos.
One of the guidelines in this effort were conservation laws. Particle physics of course obeyed the classic physical laws of conservation of mass and energy (which are basically the same thing, "mass-energy", as far as modern physics is concerned); conservation of linear and angular momentum, with angular momentum modified to the concept of "spin" as used by quantum mechanics; and conservation of electric charge.
Particle physicists discovered that particle interactions also obeyed, or at
least partly obeyed conservation laws of their own. One was that the decay
of baryons like the neutron or lambda particle always had to have other
baryons in their breakdown processes; they might generate other particles,
but the products would always include baryons. The physicists defined a
value named "baryon number" and a conservation rule to go along with it. A
baryon had a baryon number of 1, while an antibaryon had a baryon number of
-1, and mesons and leptons of course had a baryon number of 0. Conservation
of baryon number meant that the particle interaction:
proton + proton --> pion+ + pion+ + pion0
-- wouldn't happen, since each proton had a baryon number of 1, while the
pions, which were mesons, had a baryon number of 0. However, the
interaction:
proton + antiproton --> pion+ + pion- + pion0
-- does work, because the proton has a baryon number of 1 and the antiproton
has a baryon number of -1. One item of significance here is that baryons
have to be created in particle-antiparticle pairs, though the antiparticle
doesn't necessarily have to be the same type as the particle.
Leptons also seemed to be conserved in particle interactions, leading to definition of a similar "lepton number" for the electron, muon, neutrino, and their antiparticles. Leptons, like baryons, have to be created in pairs, such as an electron-positron pair.
* Another interesting feature that particle physicists noticed was, as mentioned, that some of the new particles discovered -- the kaon, lambda0, xi, and sigma -- lived longer, much longer, than anyone would have expected, decaying slowly via the electromagnetic or weak forces instead of decaying quickly through the strong force.
It was further discovered that these strange particles were generally created in pairs, though there were some exceptions. In 1953, the American physicist Murray Gell-Mann (born 1929) and the Japanese physicist Kazuhiko Nishijima (1926:2009) independently proposed that the strange particles had some property that was conserved in particle interactions involving the electromagnetic and strong forces, but not the weak force. Gell-Mann called this property, logically enough, "strangeness", establishing it as quantum number, and so of course the strange particles became, officially, "strange particles". The strangeness number prohibited the decay of the particle by the strong force, meaning the particle would only decay by the weak or electromagnetic forces.
The editors of the PHYSICAL REVIEW journal didn't like the term "strange particles" and insisted that Gell-Mann refer to them with the awkward phrase "new unstable particles", which Gell-Mann commented was the only thing that they felt was "sufficiently pompous" for their taste. Not too surprisingly, physicists preferred Gell-Mann's terminology, giving him a bit of revenge.
Gell-Mann, incidentally, is not noted for his modesty but is known for his outspokenness, a lively and sometimes edged sense of humor, and a certain lack of subtlety. Much later in life, he would be interviewed by a reporter from SCIENTIFIC AMERICAN magazine and tread in an entirely cheerful fashion all over the reporter's toes, for example saying that popular science writers were all "idiots". The reporter, a popular science writer, dutifully described the encounter with minimal comment to the magazine's readers, leading to a somewhat embarrassed but not exactly contrite letter to the editors from Gell-Mann, in which he implied that he had been misunderstood.
In Gell-Mann's strangeness scheme, a strange particle that decayed into a proton was assigned a strangeness value of -1, and one that decayed into an antiproton was assigned a strangeness value of 1. If a strange particle decayed into another strange particle, it was given a strangeness value of 2 or -2, since with each step of the decay cascade one value of strangeness was discarded. The conservation of strangeness is a funny sort of conservation law, being more a "non-conservation" law: although creation of strange particles must be in pairs, with matching positive and negative strangeness values in the pair, instead of a strangeness value being preserved in a decay, a value is thrown away.
The lambda0 and sigma particles were assigned a strangeness value of -1, and the kaons were assigned a strangeness value of +1. This demonstrated the conservation of strangeness in creation of strange pairs of particles, since a lambda0 or sigma particle was always created alongside an appropriate kaon. The "doubly strange" xi- particle, with its two-level strange decay process, was assigned a strangeness value of -2.
The invention of the notion of strangeness led in 1959 to the discovery that the xi- had a neutral partner, the "xi0". The possibility of the xi0 had been floating around for a few years, but since the xi0 was neutral, tracking it down was problematic. The giveaway would be traces of charged particles that were obviously derived from a strange particle with no charge and of a given mass / energy. A team under Luis Alvarez at UC Berkeley started the hunt in earnest in 1958, and after examining thousands of bubble chamber traces, found the smoking gun: a collision of a kaon- with a proton produced a kaon0 along one trace, which then decayed into a pion+ and pion-; and more importantly produced another neutral particle that seemed to ultimately decay into a proton and pion-.
As it turned out, the mystery particle that produced this branch had the appropriate mass and other properties to be the xi0. It had a mass of 1,314.9 MeV and decayed with a half-life of 2.9E-10 seconds into two neutral particles, a lambda0 and a pion0, with the lambda0 producing the proton and pion-, and the pion0 ultimately decaying into gamma rays somewhere off the photograph of the trace. The discovery of the xi0 was one of the first major accomplishments of the bubble chamber.
* One of the oddities of the notion of strangeness was that neutral kaon0 was found to be two particles: the kaon0 and the antikaon0, which were completely identical except for having strangeness values of 1 and -1 respectively. It might be thought that the kaon0 and antikaon0 turned out to be the kaon0L and kaon0S (or the reverse), but it didn't turn out to be that simple: the kaon0L and kaon0S turned out to be quantum superpositions of the kaon0 and antikaon0 in different proportions.
* If strangeness conservation seemed like an odd idea, it was soon joined by another, the conservation of "isotopic spin" or "isospin". While the concept of quantum-mechanical spin has a somewhat dodgy relationship to the concept of spin in the macroscale, isospin has almost no relationship to either definition of spin. It might be more clearly called "isotopic index".
It had its dim beginnings in speculations by Heisenberg in the early 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
isotopic spin (or index) value according to the formula:
number_of_isotopes - 1
isotopic_spin = --------------------------
2
Nobody paid the idea too much mind at first, but it turned out be a handy way
to arrange families of particles. For example:
As this list shows, the way in which members of particle families were assigned isospin values had a resemblance to the way a particle can be assigned spin values. For example, an electron can have spin values of UP or DOWN; reasoning purely by analogy, a proton could be thought of as being an "isospin UP" nucleon and neutron could be thought of as being an "isospin DOWN" nucleon. This is why isospin was given its name, even though isospin has nothing directly to do with angular momentum. Isospin values, it turned out, were conserved in particle reactions -- but only those controlled by the strong force.
* The next consideration of the conservation laws of particle physics was
even subtler than isospin, based on the concept of "TCP (Time Conjugation
Parity) symmetry". The first component of TCP symmetry, "time symmetry",
meant that, given any particle interaction, for example beta decay:
neutron --> proton + electron + antineutrino_e
-- then if time symmetry holds, the same operation should be able to work in
reverse:
proton + electron + antineutrino_e --> neutron
The second component, "conjugation symmetry", meant that, given any particle
interaction, an equivalent interaction could be obtained using the
appropriate antiparticles. Once again, using beta decay as an example:
neutron -> proton + electron + antineutrino_e
-- then the antiparticle interaction should be possible:
antineutron -> antiproton + positron + neutrino_e
The third component, "parity symmetry", requires some education in what
physicists mean by "parity". It was pointed out earlier in this series that
bosons have "even" Schroedinger wavefunctions, meaning the wavefunction looks
the same if it is reversed, while fermions have "odd" wavefunctions, meaning
the wavefunction is inverted if it is reversed. This is the basic reason why
Bose-Einstein statistics and Fermi-Dirac statistics are so different, why
bosons can form Bose-Einstein condensates, while fermions in contrast have to
obey the Pauli exclusion principle. In any case, particles with even
wavefunctions, the bosons, are assigned a parity value of +1, while those
with odd wavefunctions, the fermions, are assigned a parity value of -1.
In the 1950s, it was believed for good theoretical reasons that particle interactions were symmetrical with regards to the combined CPT transformations, and it didn't matter what the order of the three transformations were -- CPT, CTP, PCT, TCP, TPC, and PTC all worked the same. So far so good, but then physicists started to wonder if the transformations were symmetric if they were applied by themselves. Much to the astonishment of the physics community, the seemingly trivial P transformation was not symmetric. The roots of the matter went back to the discovery of the kaon.
As noted earlier, the kaons have a number of decay modes, with some producing three pions and others producing two. That meant that the kaon was originally thought to be two distinct particles, given the names of "tau" and "theta" respectively. The different decay modes of the theta and the tau posed a really troublesome problem. In a system of multiple particles, the parity of the system is given by multiplying the parities of the individual particles together. Kaons are bosons, meaning they have a parity of 1. Pions are fermions, meaning they have a parity of -1. A kaon breaking down into two pions produced a system with a parity of -1 * -1 = 1, which meant that parity was conserved in the decay. A kaon breaking down into three pions produced a system with a parity of -1 * -1 * -1 = -1, which meant that parity was not conserved in the decay. For this reason, physicists in general felt that the tau and the theta could not really be the same particle.
Two Chinese-American physicists, Tsung-Dao "T.D." Lee (born 1926) from Columbia University and Chen Ning "Frank" Yang (born 1922) from Princeton, set out to figure out the puzzle of the theta-tau. In 1956 they published a collaborative paper in which they challenged the idea that parity was conserved, and suggested that the theta and the tau were in fact the same particle, that parity was not necessarily conserved in the weak interaction.
Few really knew what to make of the paper, which gave Lee and Yang time to accumulate proof. A source of proof was near at hand, in the form of another Chinese-American physicist at Columbia, Chien Shiung Wu (1912:1997), known respectfully as "Madame Wu" by her colleagues. Madame Wu, a recognized expert in beta decay, knew the literature and knew that there was no experimental data that actually proved that parity was always conserved in the weak interaction. It had simply been assumed up to that time that it was.
It was, however, possible to perform an experiment to find out. It was not a trivial experiment, and Madame Wu had to call in help from the US National Bureau of Standards (NBS, now the National Institute of Standards & Technology / NIST). The NBS team used a thin film of radioactive cobalt-60 atoms deposited on a crystal and cooled to within a few degrees of absolute zero to reduce noise, immersed in a strong magnetic field to align the spins of the neutrons in the cobalt-60 nuclei. Cobalt-60's decay mode is through beta decay, so it emitted electrons, which were detected by scintillation counters. If the weak force conserved parity, there would be equal numbers of electrons in either direction of the spin axis of the neutrons -- 50% "north" and 50% "south". If the weak force did not conserve parity, the percentages would be unequal.
Wolfgang Pauli declared ahead of time that the experiment would not verify Lee and Yang's suggestion, saying: "I do not believe the Lord is a weak left-hander." The results said otherwise: the electron emission was unmistakeably asymmetrical. The theta and the tau really were the same particle, the kaon; the name of "tau" would be set aside and "recycled" for a new particle later.
Lee and Yang won the Nobel prize for physics in 1957 at age 30 and 34 respectively, making them among the youngest to achieve that distinction. Unfortunately, the two men later had such a falling out, apparently due to squabbles over who really "owned" the prize, that physicists running conferences had to arrange invitations so that the two men were never in the same room together. Much of their later work in physics was devoted to attacks on each other. It was all the more tragic because nobody is ever "half" a Nobelist: the professional distinction remains as great no matter how many share an award.
The P transformation had been proven to be asymmetric with respect to the weak force, and as the physicists put it: "Nature does distinguish between left and right." As Yang and Lee added in their paper, the failure of P symmetry also implicitly undermined C symmetry for the weak force, and observations were performed of the decay of the pion into a muon (and thence to an electron), showing that the spins of muons emitted by the pion+ and by the pion- did not agree with what theory predicted if C symmetry was obeyed.
As for Pauli, although he liked to proclaim that he didn't make mistakes -- "I, never!" -- he had to eat crow. It should be noted that despite Pauli's assertions of infallibility, his inclination to criticism also applied to himself, and he was proven wrong he accepted it. He commented to a friend that people "now have the right to laugh at me." Still, Pauli was always at heart a critic, and remained famous, even accepted for it, up to his death in 1958. A joke then made the rounds that after Pauli passed through the pearly gates, he asks the Lord what the rationale was behind the "fine structure constant" -- a mysterious value of 137 that shows up persistently in computations about electromagnetism, resulting in something of a dubious cottage industry of research papers trying to establish, so far without much success, its significance. The Lord handed Pauli a paper describing the matter, saying: "It's all here." Pauli paged through it and indignantly declared: "This is completely WRONG!"
Pauli was also not the only one to be surprised, in fact the discovery was something of a shock to the physics community. As I.I. Rabi put it: "In a certain sense, a rather complete theoretical structure has been shattered at the base, and we are not sure how the pieces will be put together."
* As it turned out, the C asymmetry seemed to be something of an "inverse" to the P asymmetry, with the two asymmetries cancelling each other out and maintaining the combined CP symmetry. Physicists settled down considerably when they realized that CP symmetry still seemed valid.
It would take a few more years to find out that CP symmetry wasn't quite as solid as assumed. In 1964 two Brookhaven physicists, James W. Cronin (born 1931) and Val L. Fitch (born 1923), conducted a study of beams of neutral kaons that gave some interesting results. As noted, there are two types of kaon0: a "kaon0S" with a short decay time that decays into two pions, and a "kaon0L" with a long decay time that decays into three pions. If a beam of neutral kaons is produced, all the kaon0S particles will decay in a short period of time, leaving a relatively stable beam of kaon0L particles. What Cronin and Fitch discovered was that about one kaon0L in 500 would decay into two pions instead of three pions. This was a clear violation of CP symmetry. The two physicists shared the Nobel prize for physics in 1980 for this discovery.
The combined TCP symmetry has not been overthrown, and nobody expects it to be. Since CP symmetry is broken in certain rare cases, the validity of TCP symmetry means that T symmetry is also broken in certain rare cases, but in exactly an opposite fashion to CP symmetry, allowing the two "errors" to cancel out. Incidentally, the breaking of T symmetry means that at the level of elementary particle interactions, nature does at least subtly distinguish between time going forward and time going into reverse: in other words, there is a quantum-physical "arrow of time". This is a bit embarrassing for concepts like QED, which take time reversal as a given, but the consensus on CP and T operations is that they can be regarded as symmetrical, except under some unusual "fine print" conditions.
The following table gives the conservation laws of particle physics relative
to the strong force, the electromagnetic (EM) force, and the weak force, as
now known:
_________________________________________________
STRONG EM WEAK
_________________________________________________
linear momentum YES YES YES
mass-energy YES YES YES
angular momentum YES YES YES
electric charge YES YES YES
baryon number YES YES YES
lepton number YES YES n
strangeness YES YES n
isospin YES n n
time reversal (T) YES YES yes
charge conjugation (C) YES YES n
parity (P) YES YES n
CP YES YES yes
TCP YES YES YES
_________________________________________________
The "n" of course stands for "no"; the lowercase
"yes" for the T / CP transformation means "only
violated under very unusual circumstances."
_________________________________________________
BACK_TO_TOP
* The discussion of particle symmetries in the previous section seems abstract, to put it mildly, but there is an apparent asymmetry in the cosmos that is hard to ignore and which is still being energetically debated: the Universe seems to be all matter and little or no antimatter.
It hardly seems obvious why nature would choose one over the other. 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, as mentioned earlier 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 contained 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.
The real question in the scenario was why there was this slight imbalance. Sakharov suspected that the CP asymmetry recently discovered at Brookhaven by Cronin and Fitch was behind it, though later many physicists would question that assumption, seeing the asymmetry as too slight, about ten billion times too small to do the job. It seemed likely to them that there was a more significant CP asymmetry that was only seen in the high energies that prevailed just after the Big Bang, energies far beyond those that physicists could hope to duplicate with their particle accelerators. However, other physicists would concoct theoretical models in which the Cronin-Fitch CP asymmetry would be adequate to ensure a matter Universe.
Sakharov's analysis also led to a particularly radical assertion: the conservation of baryon number could be broken -- or in other words 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.