v1.0.1 / chapter 15 of 20 / 01 sep 07 / greg goebel / public domain
* 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.
* 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 things, 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 antielectron 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. 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.
Of course some fine print was needed as well, such as Yukawa's strong force exchange particle, which Cecil Powell identified after the war as the pion. There was also the neutrino (and the antineutrino), though it would take a longer time to be nailed down experimentally, which was needed to keep the accounting straight on beta decay. The weak force underlying beta decay did imply a set of appropriate exchange particles, but by the nature of the weak force, these particles were had to be weakly interacting, and obviously hard to find.
There was the puzzle of the muon, a particle that nobody had predicted, at least correctly, and the muon presented a puzzle of its own. The muon seemed like little more than a massive electron -- it has a mass of 105.7 MeV, 207 times that of the electron, but otherwise has exactly the same properties as the electron -- and there was always the question of why it existed at all: who ordered it?
* Sorting through the decay modes of these particles also gave the theorists
things to think about. The first particle decay to be understood was the
beta decay of the neutron, with a half-life of about 12.8 minutes:
neutron --> proton + electron + antineutrino
This seemed straightforward enough, though explaining the underlying
mechanisms was a challenge.
The pion is now known to be the lightest particle subject to the strong force, which means the strong force can't force it to decay further, and if that were all there was to it, the pion wouldn't decay into other particles. However, the electromagnetic and weak forces can also control particle decays, and so the charged pions -- pion+ and pion-, both with a mass of 139.6 MeV -- decay into a neutrino and the muon+ or muon- respectively in about 2.6E-8 second, which is a fairly long time by the standards of unstable particles.
Cosmic ray air showers also include plenty of gamma rays. In 1948, Robert Oppenheimer and two of his graduate students suggested that the gamma rays were produced by the breakdown of a neutral pion particle, or "pion0". Trying to track down a neutral particle that decayed quickly was obviously a challenge.
In 1949, a group under R. Bjorkland at Lawrence Berkeley Laboratory took up the challenge, using the Berkeley synchrocyclotron to accelerate protons and smash them into a metal target, producing a shower of pions. Neutral pions in the shower weren't observable in themselves, but they produced gamma rays when they decayed, which then passed through two holes in the accelerator's shielding to hit tantalum targets, producing electron-positron pairs. The electrons and positrons were diverted to each side of a target by a magnetic field, and then detected by proportional counters. The extent by which the electrons and positrons were diverted by the magnetic field and the level of ionization in the proportional counters could be used as a measurement of the energy of the particles.
The energies were consistent with the breakdown of the neutral pion as had been proposed by Oppenheimer and his students, and nobody knew of a similar candidate that would give similar results. The pion0 was the first particle to be discovered by an accelerator; it was slightly lighter than the charged pions, with a mass of 135.0 MeV. We know now that the pion0 decays with a half life of about 8E-15 second, much more quickly than the charged pions, with the decay of the pion0, as mentioned, generally producing two gamma rays -- though about 1.2% of the time it produces a gamma ray and a electron-positron pair.
Pions are generated at high rates at all times by cosmic ray particles
hitting the upper atmosphere, but only the decay products reach the ground.
The electron and positron are of course stable and can survive the trip. The
muon is a relatively stable particle, taking about 2.2 microseconds to decay,
with the decay modes consisting of:
muon+ --> positron + neutrino + antineutrino
muon- --> electron + neutrino + antineutrino
The muon's release of a neutrino and antineutrino in its decay was puzzling:
why didn't they annihilate each other? This suggested that there might well
be two different kinds of neutrinos, though since nobody had actually
directly detected neutrinos at the time, there was no way to check.
* On top of this, as if to provide job security, the "in basket" of the theorists was beginning to fill up rapidly. In the postwar period physicists would find so many particles that they would be hard-pressed to keep track of them, much less figure out a decent theory to explain them.
In 1946, two researchers at Manchester University in the UK, George Rochester (1908:2001) and Clifford Butler, were examining images of tracks of particles in a cloud chamber shielded by a lead plate. They found two particles being generated seemingly from nowhere. The two particles curved symmetrically away from each other from their point of origin in the magnetic field placed across the chamber, showing they had the same mass but opposite charges; they turned out to be a pion+ and a pion-.
The two physicists knew the two particles were being produced by the breakdown of some neutral particle, which of course would leave no trace, but examination of the tracks of the two decay products suggested that their progenitor was some unknown neutral particle with a mass of 500 MeV, about half that of the proton.
They discovered more of the mysterious "vees", and then they found a single track that similarly seemed to arise from nowhere and had a funny "kink" in it along its length. The track turned out to be due to a charged pion, with the kink due to its divergence from an invisible pion0. The vees hadn't been noticed before because they were relatively rare compared to the other particles that left tracks in the cloud chamber images, but once researchers knew what to look for, they found similar events, including some that were produced by the breakdown of a charged particle with a mass of about 500 MeV.
Positive, negative, and neutral versions of the kaon were discovered, in the form of the "kaon+", the "kaon-", and the "kaon0". All were discovered in 1947 -- sort of. The particles were indeed all discovered in that year, but it would take another decade to sort them out since initially physicists thought that they had discovered distinct neutral, positively-charged, and negatively-charged particles. This matter is discussed in more detail later.
The kaon+ and kaon- both have a mass of 493.6 MeV and decay in 1.2E-8
seconds. They have several decay modes, listed below in decreasing order of
probability:
63.5%: kaon+ --> muon+ + neutrino
kaon- --> muon- + neutrino
21%: kaon+ --> pion+ + pion0
kaon- --> pion- + pion0
5.5%: kaon+ --> pion+ + pion+ + pion-
kaon- --> pion- + pion+ + pion-
4.8%: kaon+ --> positron + pion0 + neutrino
kaon- --> electron + pion0 + neutrino
3.2%: kaon+ --> muon+ + pion0 + neutrino
kaon- --> muon- + pion0 + neutrino
1.7%: kaon+ --> pion+ + pion0 + pion0
kaon- --> pion- + pion0 + pion0
The decay was strangely much longer than expected. The kaon is generated by
the strong force, and should decay by the strong force, which by a
consideration of the strong force's range meant that it should decay in about
10^-23 seconds. The actual half-life was about 10^15 times greater, which to
provide a bit of perspective is the same ratio as 300 million years is to one
second.
The neutral kaon0 has a slightly greater mass, 497.7 MeV, and oddly it proved
to have two different decay half-lives -- there was a kaon0 with a long decay
time or "kaon0L", and a kaon0 with a short decay time or "kaon0S". The
kaon0L has a decay half-life of 5.2E-8 seconds, about four times longer than
that of the charged kaons, and the decay modes are:
21.5%: kaon0L --> pion0 + pion0 + pion0
19.4%: kaon0L --> pion+ + electron + neutrino
19.4%: kaon0L --> pion- + positron + neutrino
13.5%: kaon0L --> pion+ + muon- + neutrino
13.5%: kaon0L --> pion- + muon+ + neutrino
12.4%: kaon0L --> pion+ + pion- + pion0
The kaon0s has a decay half-life is 8.9E-11 -- much faster than the kaon0l,
but still strangely much longer than the 10^-23 seconds that would have been
expected. The decay modes include:
68.6%: kaon0S --> pion+ + pion-
31.4%: kaon0S --> pion0 + pion0
The kaon is a very unusual particle, and there is a story behind it, one that
will be revisited (several times) later.
* The next particle to hit the streets was found in cloud chamber tracks in
1951. The tracks showed a mysterious "vee" pattern of known particles
produced from some neutral particle. Closer analysis revealed the existence
of two neutral particles producing the vees, one being the kaon0, while the
other was a more massive neutral particle, which was named the "lambda0".
The lambda0 turned out to have a mass of 1,115.6 MeV, making it 2,250 times
as heavy as the electron and 1.2 times as heavy as the proton. It was the
first particle to be found that was heavier than the proton or neutron,
making it the first of the "hyperons" to be discovered. The decay modes
included:
64%: lambda0 --> proton + pion-
36%: lambda0 --> neutron + pion0
The second decay mode is troublesome to track since it only yields neutral
particles. Like the kaons, the lambda0 seemed strange, breaking down in
about 10^-10 second when it should have decayed in 10^-23 second. An
"antilambda0" would be discovered in 1958.
* In 1952, observations of cosmic ray traces by the University of Manchester group led to the discovery of a new massive negatively-charged particle, which featured a two-stage decay process, first decaying to the lambda0 and a pion-, with the lambda0 decaying in turn to a proton and pion-. The new particle was named the "cascade-" or "xi-" particle, it was another strange particle, with a long decay half-life of 1.6E-10 seconds, and one of its decay products was the strange lambda0 particle. The xi- particle turned out to be a half-spin fermion with a mass of 1,321.3 MeV.
In 1953, Italian physicists discovered yet another strange, slow-decaying
positively-charged particle, about 30% heavier than the proton; it seemed
something like a "superproton" and was appropriately named the "sigma+",
since sigma is the Greek alphabet equivalent of an "S". It proved to
be a half-spin fermion with a mass of 1,189.4 MeV and a decay half-life of
8E-9 seconds. It had two decay modes:
51.6% sigma+ --> proton + pion0
48.4% sigma+ --> neutron + pion+
The discovery of the sigma+ led to the discovery of a negatively-charged
"sigma-". It was slightly heavier than the sigma+, with a mass of 1,197.4
MeV, and took about twice as long, 1.5E-10 seconds, to decay, always decaying
into a neutron and a pion-. Not too surprisingly, a "sigma0" was found as
well; its mass was between that of the sigma+ and sigma-, 1,192.6 MeV, and
it decayed much more quickly, with a half-life of 7.4E-20 seconds, always
decaying into a lambda0 and a gamma ray. The sigma0 was hard to find because
its direct decay products were both neutral.
* By the mid-1950s, particle accelerators were routinely able to produce energies rarely if ever achieved by cosmic rays, and from that time on all new particles would be discovered by particle accelerators. One of the first areas of investigation were antiparticles.
The antiproton was finally discovered in 1955, by a team under Emilio Segre using the new Bevatron at UC Berkeley. The beam generated by the Bevatron, being focused by magnets, generated positively charged particles to one side and negatively charged particles to the other. It was simple to tap the beam to siphon off the negative particles. A magnetic field was used to split up the beam by mass, separating the antiprotons from lighter negative pion and heavier negative kaon particles, in much the same way a mass spectrometer separates different ionized atomic isotopes. A slit could be used to pass the antiprotons and negative pions and block the negative kaons, but then the antiprotons had to be sifted out from the negative pions.
This was done with two different detector systems. The first used two scintillation detectors set 12 meters (40 feet) apart. A coincidence counting system logged events where a scintillation took place in the first detector, followed by a scintillation in the second detector a very specific time interval later. The particles were moving at known energies; antiprotons would move a bit more slowly than negative pions and bit more quickly than negative kaons, and so detection of a coincidence implied that an antiproton had passed through the detector system. This was a type of "time of flight" detector system, used for determining particle velocities.
The second detector system used two Cerenkov detectors, one based on a liquid chlorofluorocarbon that could pick the tracks of negative pions but not the slower antiprotons, and one based on a quartz crystal that could pick up the antiprotons. Segre's group and another group in Italy went on to use photoemulsion stacks to obtain the trace of the annihilation of the proton with antiprotons, which produced a "starburst" pattern. The sum of the energy of the products of the annihilation exceeded the energy of the "incoming" antiproton, demonstrating that an annihilation had really taken place. Segre and one of his team members, Owen Chamberlain (1920:2006) won the Nobel prize in physics in 1959 for discovering the antiproton.
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.
In 1957, another UC Berkeley team under Bruce Cork used a tank of liquid scintillator to search for antineutrons produced by charge exchange of protons and antiprotons produced by the Bevatron. The scintillator "lit up" when a neutron-antineutron annihilation took place, with the characteristic burst of light caught by photomultiplier tubes. Following the discovery of the antineutron, the "antilambda" or "/lambda0" was discovered in photographic emulsions at Berkeley in 1958.
* Incidentally, 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. The Italian physicist Antonino Zichichi (born 1929), fond of theatrical measures such as wearing capes and a bit of a media star in Italy, did manage to create antideuteron nuclei -- composed of a combination of a proton and antineutron at CERN in 1965, but creation of a true antiatom didn't happen for another 30 years.
In 1995, 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 antiatoms had finally been created. Work is now underway at CERN to trap antiatoms for analysis, using a device called a "Penning trap", which is along the lines of a linear accelerator, except that electric fields are used to decelerate particles instead of accelerate them.
* To further complicate matters, in the meantime physicists were finding "resonances", which seemed at first to be new, very short-lived particles, but which were eventually seen as excited states of known particles.
In 1951, the University of Chicago obtained a synchrocyclotron, and in the next year, 1952, Fermi and a team of researchers used it to produce pions at six different energies levels, with the pions then fired at protons in a target of liquid hydrogen. The target was bracketed by two scintillator plates in front and two in back, with those in front counting pions coming in and those in back counting pions going out. At certain pion energies, Fermi and his colleagues found that the ratio of pions going in to pions going out dropped to a low, as if there was some "resonant interaction" between pions and protons at those energies, with a pion+ linked to a proton for an extremely short period of time.
The Chicago synchrocyclotron didn't provide high enough energies to probe the
matter further. It was picked up again in 1953 by a team at Brookhaven using
the new Cosmotron, with better data obtained that showed pions of a certain
energy definitely resonating with protons. The width of the peak in the pion
energy spectrum was broad, which by the uncertainty relation:
delta_energy * delta_time >= hbar
-- meant that the time of the resonant state was very short, about 10^-23
seconds. The resonance seemed so distinctive that it was given a name, the
"delta". Since it was a temporary association between a pion+ and a proton,
that meant it was a "delta++", with two positive charges. Three other deltas
were discovered later: the delta+, the delta0, and the delta-. Higher-order
delta resonances were discovered as well.
There matters stood until 1960, when Lawrence Berkeley Lab physicists began to run large numbers of bubble-chamber traces through computers to see what might be found. What they found were emissions (as opposed to absorptions as Fermi's group had found) of particles due to resonant energy states, corresponding to a resonant state of the lambda0, called the "Y*" or "Y-star". Other resonances were then found at an accelerating rate:
So many hundreds of resonances were found that a shorthand form was defined to describe higher-order resonances, in which the name of the basic particle was given with the energy of the resonant state in MeV in parenthesis, for example "sigma(1915)".
At first, resonances were placed in their own category, but eventually particle physicists decided that there was no arbitrary dividing line between a resonance and other unstable particles other than the short decay time of the resonance, and stopped making the distinction, invoking a doctrine of "nuclear democracy". Some might have wondered if it was more something like mob rule.
* To add to the confusion, particle physicists had also confirmed that the
electron and muon had their own distinct neutrinos. In 1962, an experiment
was conducted with the proton synchrotron at Brookhaven to use the same
reaction that Cowan and Reines had used to prove the existence of the
neutrino:
proton + antineutrino --> neutron + positron
-- and see if antineutrinos emitted by the muon could perform this
transformation. A proton beam with an energy of 15 GeV was slammed into a
beryllium target, generating a stream of pions, which traveled through space
for about 20 meters (65 feet), decaying into muons and neutrinos. This beam
then hit an iron wall 13 meters (43 feet) thick, which screened out
everything but the neutrinos.
The neutrino beam was detected by a spark chamber containing a set of thick
iron plates. The spark chamber was only activated when a particle beam was
shot into it, and was surrounded by scintillation counters that disabled the
chamber when it was hit by a cosmic ray that might confound the observations.
The neutrinos had a small but finite chance of interacting with a neutron or
proton in the nucleus of an aluminum atom, a process that would result in
changing the neutron to a proton to a neutron or the reverse, with the
emission of a charged particle. As it turned out, the charged particles were
always muons:
antineutrino + proton --> neutron + muon+
neutrino + neutron --> proton + muon-
If the neutrinos had been produced by electrons or positrons, the interaction
would have produced electrons or positrons instead. The "muon neutrino
(neutrino_m)" was clearly distinct from the "electron neutrino (neutrino_e)".
The decay modes of the muon were actually as follows:
muon- --> electron + antineutrino_e + neutrino_m
muon+ --> positron + neutrino_e + antineutrino_m
* 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. The search was on for an underlying
order to the chaos.