If hadrons were indeed made of quarks, then physicists expected to find quarks that were not part of hadrons. Quarks that are not part of hadrons are called free quarks.
The most distinctive property of quarks is their electric charge. All particles observed in nature have an electric charge that is an integral multiple of the charge on the proton. The obvious way to find a quark would be to identify a particle that has a charge one-third or two-thirds that of the proton.
As soon as they heard about the quark hypothesis, experimenters began looking for fractionally charged particles. They studied cosmic ray photographs. They searched in the debris of accelerator collisions. But nowhere did they find fractionally charged particles.
In 1966, the Italian physicists Giacomo Morpurgo and Gaetano Gallinaro reported on an experiment they had done to search for free quarks. They placed a tiny grain of graphite in a magnetic field. Because a graphite grain acts as a small magnet, it floated in the air. When Morpurgo and Gallinaro turned on an electric field, the grain floated to a new position. The stronger the electric field, the farther the grain moved.
The two physicists then used a radioactive source to add an electron to the grain. They measured the grain's new position. A series of measurements told them whether or not the grain had a fractional charge.
Morpurgo and Gallinaro repeated their measurements over and over. They found no fractionally charged particles.
Eleven years later, Morpurgo, Gallinaro, and Mauro Marinelli, reported on a similar experiment with tiny cylinders of iron. Again, they found no evidence for free quarks.
In California, a group of physicists led by William Fairbank searched for free quarks in superconducting niobium balls. They too used magnetic levitation and electric fields to measure the electric charge on the balls. Two of their eight niobium balls seemed to have fractional charge.
By 1981, Fairbank's group had done more measurements, and found more fractional charge. Yet Morpurgo's experiment had found none. Could both experiments be right?
It is possible that free quarks can exist only in certain materials under special conditions. If that is true, there is no contradiction between the two experiments. Many physicists, however, think there may be another explanation for Fairbank's results.
Most physicists have come to believe that quarks cannot exist outside of hadrons. They are so strongly bound together that any attempt to separate them requires a tremendous amount of energy. This energy gets transformed into pairs of quarks and anti-quarks, which combine with the original quarks to create more hadrons.
We are fairly sure that we cannot break a proton apart into quarks, but why shouldn't it decay into leptons? The charged pion, which is made of a quark and an antiquark, decays into a muon and a neutrino. A charged kaon can also decay into a muon and a neutrino. But the proton, which has almost twice the mass of a kaon and nearly seven times the mass of a pion, seems to be absolutely stable. One would think it would easily disintegrate into a positron and a pi-zero, or a positron and two neutrinos.
If protons do indeed decay, such decays must be very rare. Reines, Cowan, and Goldhaber used their neutrino detector to search for decaying protons. They concluded that the half-life of a proton was at least 10**21 years. The half-life of a particle is the time it takes for half of the particles in a sample to decay.
In the 1970's, theoretical physicists Howard Georgi and Sheldon Glashow said the half-life of a proton should be about 10**30, or one million trillion trillion years. That is more than one thousand trillion years longer than we think the universe has been in existence.
Since we can't simply wait for half of the protons in the universe to decay, we must measure their lifetime another way. Radioactive decay is not predictable. We can never know exactly when a particular particle is going to decay. We can predict the average behavior of a large number of particles. We also know that some particles will decay much sooner, and others much later. If the proton lifetime is indeed 10**30 years, and we watch 10**30 protons for one year, we would expect to observe one proton decay.
The problem with observing such a rare decay is that other events, such as a cosmic ray particle, or the decay of a radioactive nucleus, could trigger the detector and give a false signal. The only way an experimenter could hope to observe a proton decay would be to put the detector deep underground, where the earth would shield it from all but the most energetic cosmic rays.
In the 1980's several groups of physicists surrounded underground vats of water with particle detectors. The largest of these experiments was in a salt mine in Ohio. Others were in a metal mine in Japan, a silver mine in Utah, a gold mine in India, an iron mine in Minnesota, and two tunnels on the border between France and Italy.
From these experiments, physicists concluded that the proton lifetime is at least 10**32 years.
In the Kamioka Mine, 200 kilometers north of Tokyo, physicists from Japan and the United States have built the world's largest underground neutrino observatory. Super-Kamiokande is a tank of ultra-pure water 40 meters in diameter and 40 meters tall, surrounded by thousands of phototubes. In addition to neutrinos, Super-Kamiokande is able to observe proton decays. In April of 1996, Super-Kamiokande began taking data. If it does not observe any proton decays, the lower limit on the proton's lifetime will continue to rise.
Why are protons stable? This is one of the mysteries that today's physicists are trying to solve.
The quark model reduced the number of elementary particles from more than 80 to 14. There were the up, down, and strange quarks, and their anti-quarks. There were the electron, the muon, the electron neutrino, the muon neutrino, and their anti-particles.
In 1974, a group of researchers at Brookhaven, led by Samuel Ting, and a group at SLAC led by Burton Richter announced that they had observed a hadron containing a fourth flavor of quark, called charm. The new hadron, named the J/psi, has a mass about three times that of the proton. As particle accelerators produced beams of higher energy, physicists found other hadrons containing the charmed quark.
In 1976, the American physicist Martin Perl and his collaborators discovered a fifth lepton, called tau. Like the electron and the muon, the tau lepton has an electric charge of -1. The tau is nearly four thousand times as massive as the electron.
In 1977, Leon Lederman and his collaborators at Fermilab in the United States discovered hadrons made of a fifth kind of quark, the bottom quark. The bottom quark has an electric charge of -1/3.
In 1983, a group of physicists led by Carlo Rubbia at CERN, the European Center for Nuclear Research, detected a new class of elementary particles, called intermediate vector bosons. There are three intermediate vector bosons, the W+, the W-, and the Z0. These particles are the intermediaries in the weak interaction, transforming quarks from one flavor to another, and enabling a muon to decay into an electron and two neutrinos. The Z and W are very massive, 80 to 90 times the mass of a proton.
Just last year, using the Tevatron accelerator, physicists at Fermilab observed a sixth type of quark, called top.
Physicists add one more elementary particle to the list--the photon. A photon is a quantum of light. A photon is also a gauge boson, like the Z and W. It carries the electromagnetic force from one charged particle to another.
According to the standard model, which is our best guess at the subatomic world, there are now 28 elementary particles. We have the up, down, strange, charm, top, and bottom quarks and their anti-quarks. We have the electron, muon, and tau lepton, and their anti-particles. We have the electron neutrino, the muon neutrino, the tau neutrino, and their anti-particles. And we have the four gauge bosons: the Z, the positive and negative W's, and the photon.
Physicists, who always look for simplicity, are not entirely pleased with this new particle zoo. Many of them are already looking for simpler ways to describe subatomic particles.
Underneath the city of Hamburg, Germany, is a tunnel over six kilometers in circumference. Inside this tunnel, electrons meet protons in a head-on collision. The energy of these collisions is enough for the electrons to "see" particles 10^-18 meters in diameter, 1000 times smaller than the proton.
The accelerator is called the Hadron-Electron Ring Accelerator, or HERA. Two collaborations of physicists at HERA are trying to find out if quarks are indeed pointlike particles. The ZEUS collaboration has approximately 430 physicists from twelve countries. The H1 collaboration includes about 400 physicists from twelve countries.
In their data from the past few years, these physicists have found more deep inelastic scattering events than they expected. This may mean that quarks are indeed composed of more fundamental particles. The data collected in 1997 will show whether the result is indeed something new, or if it is a statistical fluctuation.
Ironically, the electron, the first elementary particle to be discovered, is still considered to be elementary. Although we use electrons every time we turn on a light switch, we really don't know that much about them.
In the future, physicists hope to answer questions about elementary particles. Are quarks and leptons elementary particles? Why are there three generations of particles in the standard model? How do particles get their mass? Why do we observe many particles and very few antiparticles?
Physicists hope to find a simple theory that will explain everything in the universe. As experimental results continue to come in, physicists around the world are drawing closer to that goal.