The Past 25 Years: Establishing the Standard Model
This chapter summarizes the most important experimental results of the past 25 years, those that established the Standard Model as the correct description of essentially all phenomena in the field of elementary-particle physics. Some experiments were designed to test predictions of the new theoretical framework; other discoveries came as surprises. All of the results depended on the construction of new accelerators and the development of new experimental techniques to exploit these accelerators.
THE WORLD OF ELEMENTARY-PARTICLE PHYSICS CIRCA 1972
In 1972, both theoretical and experimental particle physics were unknowingly on the verge of vast and exciting changes. A crude but effective model existed to explain how hadrons were held together by strong interactions, and there were rules to allow the calculation of weak interaction processes. Yet quantum electrodynamics (QED) was the only example of a precise theory that could explain a wide range of experimental results. Papers proposing the electroweak theory were starting to gain wide attention, however, and they were the first harbingers of changes to come.
Of the particles now recognized as truly fundamental that are shown in Tables 2.1 and 2.3, only the first two leptons, their accompanying neutrinos, three quarks, and the photon had been observed by 1972. Even the idea that
protons and neutrons were made of quarks was still relatively new: The experiment providing the first evidence for this occurred in 1969.
Although physicists did not realize it at the time, the field of particle physics was about to undergo a revolutionary change caused by the combination of new accelerators and the new theoretical tools called gauge theories. In 1972, the proton accelerator at Brookhaven was delivering beams of unprecedented intensity. The proton accelerator at CERN (the European Laboratory for Particle Physics) was being used to produce intense beams of neutrinos. A new high-energy proton accelerator at Fermilab was just beginning to produce new physics results, and SPEAR, an electron-positron collider, was starting to operate at the Stanford Linear Accelerator Center (SLAC).
There have been two major developments in the understanding of fundamental forces over the past 25 years. One was the establishment of the idea that electromagnetic and weak forces are unified into a single force that could be described by a theory formulated around the principle of gauge invariance: a gauge theory. The other was the discovery of a theory of the strong force, the force that holds quarks together in the proton, that was similarly a gauge theory.
The Electroweak Force
A central idea of the Standard Model is that the electromagnetic force and the weak force are different manifestations of a single unified force called electroweak. This was not evident for many years because the weak force acts over only very short distances and is completely negligible at the atomic distance scales at which the electromagnetic force acts to bind electrons to the nucleus. Electroweak theory, developed in the 1960s, gave a natural explanation for this difference. The reason for the difference is that the force carriers for the weak force, W and Z bosons, are extremely massive, whereas the force carrier for the electromagnetic force, the photon, is massless. This theory predicted many new phenomena that could be explored with the new, higher-energy accelerators whose use was beginning in the early 1970s.
Demonstrating the Unification of Weak and Electromagnetic Forces
A dramatic prediction of electroweak theory was that there would be a new kind of interaction involving quarks and leptons, called the neutral weak current. One manifestation of this new interaction was that a neutrino could strike a quark or an electron and recoil, remaining a neutrino. Up to this point, the only neutrino interactions observed were those in which a neutrino was transformed
into a charged lepton, like the electron. These two types of scattering are illustrated in Figure 4.1.
Experimentally, observing an elastic scattering of a neutrino is extremely difficult because the neutrino leaves no trace. The scattering can be observed only by seeing that a proton sitting in a neutrino beam is given a kick, without any charged lepton emerging from the event. In 1973, an experiment at CERN produced the first evidence of neutral current scattering, which was soon confirmed by experiments at Fermilab. The number of events observed was consistent with the prediction of the electroweak theory, and this discovery provided the first real evidence that theorists were on the right path. Theorists had successfully predicted the existence of new particles, but this was the first time that a fundamental particle physics interaction had first been predicted by theory and then discovered experimentally.
From that time on, a wide range of many different types of experiments studied neutral currents, and electroweak theory was able to accurately predict the results of all of them. Some experiments used neutrinos, and others were
able to measure the effect of neutral currents in scattering electrons off quarks. A precise measurement of electron scattering at SLAC saw a tiny difference of about one part in one hundred thousand in the scattering rate of left- and righthanded electrons, exactly as predicted by electroweak theory. Evidence that the electromagnetic and weak forces were unified was overwhelming; the next task was to actually discover the predicted gauge bosons of the weak interaction.
Discovery of W and Z Bosons
The second major prediction of electroweak theory was the existence of W and Z bosons. Results from neutrino experiments could be used to predict their masses to be around 80-90 GeV ( 1 GeV = 109 electron volts), and the search for these bosons was given the highest priority. However, a new breakthrough in accelerator technology was necessary. The experiments of the 1970s, which used proton beams hitting stationary targets, could not produce a particle with a mass greater than about 10 GeV. To reach higher energies, it was necessary to use a proton-antiproton collider, at which much higher energies were accessible.
Starting in the mid-1970s, the Super Proton Synchrotron (SPS) accelerator at CERN was converted to a proton-antiproton collider capable of reaching an energy of 540 GeV in the center of mass of the collision, about 20 times the energy possible in fixed-target experiments. For the first time since the Bevatron was built in the 1950s to produce the antiproton, a new accelerator was built with the express purpose of discovering a new particle predicted by theory. Two experiments were constructed to observe the rare events in which bosons were produced and then decayed to leptons. In 1983 W and Z bosons were both discovered at CERN, with masses in the range expected from electroweak theory. Decades after the discovery that the photon had no mass, its massive siblings the gauge bosons of the weak force—were observed in the laboratory.
Precision Tests of the Electroweak Force
During the past decade, two new electron-positron colliders dramatically improved our understanding of weak interactions. An enormous collider at CERN (called the Large Electron-Positron collider [LEP]) of the conventional circular type produced millions of Z bosons. A much smaller linear electron-positron collider at SLAC (called the Stanford Linear Collider [SLC]) produced fewer Z bosons, but its innovative design allowed experiments with polarized beams, where the spins of beam particles were aligned to a common orientation (SLC was also important as a prototype for a possible future linear collider, discussed in Chapter 6). For the first time, precise measurements of the fundamental parameters of electroweak theory could be made. These measurements could then be used to probe its validity, in much the same way that precise tests of electromagnetic theory have been made for 50 years. One outcome of these
studies was that LEP and SLC experimentalists were able to put upper limits on the mass of the t quark, a sixth quark that had not been observed directly. If the electroweak theory was assumed to be correct, then the relationships between the electroweak parameters measured at LEP and SLC depended on the t quark mass. In this way, the t quark was indirectly measured to have a mass of 180 ± 15 GeV before it was even discovered. The excellent agreement between this indirect measurement of t quark mass and the direct measurement made once it was discovered, provided a stringent test of physicists' understanding of the electroweak force. The current status is that in dozens of measurements to precisions of fractions of a percent, electroweak theory and experimental measurements are in spectacular agreement everywhere.
The Strong Force
The second innovation in the description of forces in the Standard Model is the theory of the strong force, known as quantum chromodynamics (QCD). In 1972, physicists could explain hadrons, such as the proton, as composites of quarks. From detailed experiments at SLAC, it was known that if high-energy electrons were fired into a proton, they would scatter off its quarks, which acted like hard objects much smaller than the proton itself. There was no theory to explain how three quarks would bind together to make a proton or neutron or to explain why isolated quarks were never observed.
Around 1973, however, QCD was developed. It accounted for the observation that quarks are effectively confined inside the proton. The massless force carriers, analogous to the photon, were called gluons, because they provided the ''glue" that held the proton together (gluons, like quarks, could not be observed in isolation).
Experimental study of QCD as the theory of the strong force has been much more difficult than studying electromagnetic or weak forces. The traditional calculational tools developed for QED and electroweak interactions are often inadequate for QCD. Nevertheless, QCD has been verified experimentally, and there is continuing progress in developing the calculational tools necessary for precision tests of the theory.
QCD predicted that the strength of the strong force would decrease slightly with increasing energy. In the mid-1970s, this was beautifully confirmed by a series of precision experiments at Fermilab, SLAC, and CERN, which scattered electrons, muons, and neutrinos from protons.
Discovery of the Gluon
An important verification of the theory came from indirect observation of the gluon. The gluon, like quarks, manifests itself in high-energy collisions as a collimated jet of particles. The perfect environment for observing jets is in
electron-positron annihilations at high energies. After the spectacular success of SPEAR in the early 1970s (see below), electron-positron colliders with about 35 GeV energy were built at the German laboratory DESY and at SLAC. Most of the events were of the type shown in Figure 4.2(a). Two quarks are produced in the collision that form two jets of particles with equal energy and opposite directions. QCD predicted that at high-energy colliders, three-jet events from the process illustrated in Figure 4.2(b) should also be observed; the third jet comes from a gluon produced at high-energy. Figure 4.3 shows an example of such a three-jet event, collected in 1978 at DESY. This was dramatic confirmation of a prediction of QCD.
Strength of the Strong Interaction
A fundamental property of all forces is their strength. The strength of the strong force has been very difficult to measure, however, in part because quarks and gluons are confined. Nevertheless, a series of measurements of the strength of the strong interaction (g3) was made, starting in 1978, using very different techniques. It is a severe test of the theory that all of these measurements agree and determine this fundamental constant of nature rather well. Also, it has been shown that accurate measurements of the strength of the forces can be used to test ideas of "grand unification," the possibility that all of the forces derive from a single one. As calculational tools mature, these measurements are continuing.
The Spectrum of Particles
High-energy physicists use the masses of hadrons to probe the strong interaction that binds quarks to form particles in the same way that atomic physicists early in this century used the spectra of atoms to study the electromagnetic interaction that binds electrons in the atom. As a result of a concerted experimental effort over the past 25 years, approximately 50 new quark bound states have been discovered at a variety of high-energy machines around the world. QCD has the potential to calculate the particle spectrum in terms of the quark constituents and fundamental equations describing the behavior of the strong interaction. This is a difficult and challenging task. One continuing mystery is that QCD predicts an even richer spectrum of states than has so far been observed. Quantitatively explaining the wealth of experimental data from QCD will continue to be a challenge for theorists of the next decade.
Progress over the past 25 years in understanding the constituent particles has been just as dramatic as for the fundamental forces. In 1972, only the first two generations of leptons had been observed: the electron, muon, electron neutrino, and muon neutrino. Three types of quarks were known: up, down, and strange. Experiments since that time have discovered the tau lepton and its neutrino, as well as three more quarks: charm, bottom, and top. In addition, experimental studies of Z-boson decay have determined that there are exactly three neutrinos, and therefore the familiar pattern of generations of quarks and leptons shown in Table 2.1 will not continue any further.
Discovery of the Charm Quark
In 1972, knowledge about quarks was still rudimentary. The breathtaking discovery of the charm quark in 1974 was hailed as the beginning of "the new physics." The experimental evidence was dramatic, and in an astounding quirk of fate, the charm quark was discovered simultaneously at two different laboratories with very different experiments. The Mark I collaboration at SLAC observed the charm quark using the electron-positron collider SPEAR. At the same time, a group at Brookhaven using protons on a fixed target in a completely different type of experiment also detected the new quark. The discovery established that the structure of repeating generations seen in the leptons also applied to quarks.
Discovery of the Tau Lepton
When the muon was discovered in 1936, physicists wondered why nature created this heavier copy of the electron. It was the first indication of the repeat-
ing generations of quarks and leptons. In 1976, still flush from the discovery of the charm quark, physicists on the Mark I detector operating at SPEAR discovered a third-generation lepton, the tau. Figure 4.4 shows the Mark I detector at SLAC in which this and many other discoveries were made. After years of subsequent research, the properties of the tau lepton have been measured to be precisely as predicted for a heavier repetition of the electron, leading to the conclusion that the three generations of charged leptons are distinguished only by the large differences in mass.
Discovery of the Bottom Quark
The charm quark completed the second quark generation. As soon as evidence for a third generation of leptons was found with the discovery of the tau lepton, physicists intensified their search for more quarks. However, even if such a third generation of quarks did exist, there was no guidance from theory as to what the mass of third-generation quarks might be.
The bottom quark was discovered in 1977 in an experiment using the stillnew proton accelerator at Fermilab. Figure 4.5 shows the experimental signal of the discovery. Thus, two new quarks and one new lepton were discovered at American accelerator laboratories in a period of less than 3 years, after a period of 25 years without any new particles of this type.
Starting in 1981, the energy of the electron-positron collider CESR (the Cornell Electron-positron Storage Ring) at Cornell University was tuned to a value that maximized the number of B mesons, particles containing a b quark, that could be observed. The steadily increasing collision rate of this collider, along with a similar one called DORIS at the German laboratory DESY, made possible ever more precise measurements of the decays of the bottom quark. This program has continued until the present, and CESR now has the highest rate of collisions of any collider in the world. Measurement of properties of the bottom quark continues to be a very active part of the current high-energy experimental program.
Experiments showed first that the B meson had a lifetime longer than expected and then that there was significant mixing amplitude for B, anti-B oscillations. These unexpected findings were crucial for the expected viability of performing CP measurements in the B meson system and stimulated the design of B factories.
With the discovery of the bottom quark, a new question jumped to the top of the list for further experiments to answer: What is the mass of the quark that is the partner of the bottom quark in the third generation?
Discovery of the Top Quark
The top quark was by far the most elusive of all. The top quark was a necessary component of the Standard Model of electroweak interactions, but there was no consistent theoretical guidance as to what its mass should be. By 1988, the search had extended to the lofty mass of 41 GeV, almost 10 times the mass of the bottom quark, with no success. With the onset of operations at LEP and SLC in 1989, physicists could begin to extract limits on the top quark mass from very precise measurements of the properties of the Z boson. By 1992, it was indicated that the top quark mass must be between 100 and 200 GeV if the
Standard Model was correct. This was a startlingly high mass, given the masses of other quarks.
The only way to observe a top quark with such a high mass was at the collider with the highest-energy, the Tevatron antiproton-proton collider at Fermilab. With each run, the rate of collisions at the Tevatron was increased, allowing an extension of the search for the top quark. Finally, in 1994, the Collider Detector at Fermilab (CDF) experiment announced the first evidence of top quark production. The existence of the top quark was firmly established in 1995 with simultaneous announcements by both the CDF and the DO experiments of results that demonstrated a mass of around 175 GeV for the top quark. Figure 4.6 (in color well following p. 112) shows a top quark event from the CDF experiment. After an 18-year search, the last quark (at least within the Standard Model) was found.
Counting the Number of Generations
The pattern of generations, each with a charged lepton, a neutrino, and two quarks, has been repeated three times. Could there be more? An example of the important physics coming from the Z factories, accelerator facilities that produce large numbers of Z decays, was the counting of lepton generations. The Z decays very quickly into any lepton or quark and its antiparticle (except for the top quark, which is too heavy). By very precisely measuring the rate at which the Z decays, experiments were able to determine the number of neutrino types and found that there are exactly three. (In principle, there could be more generations, but the associated neutrinos would have a mass of more than 45 GeV.) This means that the pattern of repeating lepton generations, each with a heavier lepton than the one before, stops at three, and since in the Standard Model it is necessary to have the same number of leptons and quarks, all the leptons and quarks that nature has to offer may have been found. Such a conclusion would have been impossible before the framework of the Standard Model had been developed. Figure 4.7 shows some of the data that led to measurement of the number of neutrino types.
In 1964, physicists made an unexpected and astounding discovery: They found a difference in the ways that matter and antimatter behave. They observed about 45 K meson decay events that would have been forbidden if particles and antiparticles behaved symmetrically. Until this astonishing experiment, it had always been assumed that the laws of nature operate identically on matter and antimatter.
This particle-antiparticle asymmetry known as CP violation is one of the most profound mysteries of particle physics. There are only two manifestations
of this violation known to date. One is the CP violation discovered in the decay of K mesons in 1964, which has been under continual investigation since. The other is the fact that there is matter left over from the "big bang," which is in the form of stars and planets. If there were a perfect symmetry between matter and antimatter, equal amounts of each would have been produced in the very early universe and would have largely been annihilated, leaving little raw material on which to build structure in the universe.
Description of the transitions of quarks from one generation to another provided a framework for understanding CP violation. In this model, CP violation could be described by the same parameters that describe quark transitions from
one generation to another, as discussed in Chapter 3. Ever more demanding experiments on the K meson system have explored the asymmetry in greater detail throughout the past 25 years and continue through the present. These experiments are all consistent with the source of CP violation being in the flavor changing quark transitions, although some other explanations still survive. Furthermore, the search for an understanding of particle-antiparticle asymmetry has motivated many detailed studies of the bottom quark in an experimental program that started in the early 1980s and continues to be very active today.
Measuring the Mass of Neutrinos
It has long been realized that neutrinos are peculiar. All of the other leptons and all of the quarks are massive. Neutrinos, on the other hand, have masses that are either zero or so small as to be, thus far, experimentally indistinguishable from zero. However, there are no known reasons for the neutrino mass to be exactly zero, so for the past 25 years, there has been a very dedicated experimental effort to look for a mass for any of the neutrinos. This effort continues today.
Direct measurements of neutrino masses are very difficult, and many different techniques have been employed. The best limits are on the mass of the electron neutrino, coming from studies of the spectrum of electrons in tritium beta decay. In a stroke of good fortune, the spectacular explosion of the supernova SN1987a also helped to limit the electron neutrino mass. The supernova released a burst of neutrinos that were detected in large experiments deep underground that had been designed to search for proton decay. From the distribution of arrival times of neutrinos from SN1987a, a limit on their mass could be deduced. Today, the electron neutrino mass is known to be less than 0.003% of the electron mass itself.
However, if the neutrino masses are different from zero even by such a small amount, then they can undergo transitions from one generation to another just as quarks do. For example, if a neutrino is produced as a muon neutrino, it could change into an electron neutrino with time. This process is called "neutrino oscillations," and its rate depends, among other things, on the masses of the neutrinos involved. Oscillations give a method of seeing the effect of nonzero neutrino mass at levels well below what could be measured directly.
The strongest evidence that neutrinos may oscillate, and therefore have nonzero mass, has come from solar neutrinos. In a truly groundbreaking experiment located deep in the Homestake mine in South Dakota, physicists have collected the first evidence of neutrinos originating from the Sun. However, the number of neutrino interactions they see is well below the calculated rate of solar neutrinos that should reach Earth. This discrepancy could occur if electron antineutrinos, which form the bulk of solar neutrinos, oscillate to some other neutrino
type. New experiments that could search for these neutrinos in other ways began to obtain data in the late 1980s. By now, five experiments using three different techniques have observed a similar deficit in the rate of solar neutrinos reaching Earth. These results are not yet claimed as final evidence of neutrino mass, however. New experiments are about to start that can measure the rate of all neutrino types together. They could provide the last bit of information necessary to nail down the discovery that the solar neutrino deficit comes from neutrino oscillations; this in turn would imply that neutrinos have a mass.
Searching for Proton Decay
During this period, several ingenious experiments were built to look for the very rare decays of protons in matter, which is predicted if all forces are unified. For example, one experiment consisted of an instrumented tank of 8,000 m¬ of purified water in a salt mine near Cleveland. After years of searching, no proton decays were seen, thus eliminating the simplest grand unified theories from contention. In this case, successful experiments shaped particle physics by discovering that something did not happen.
Other Physics Beyond the Standard Model
One way of searching for physics at mass scales above the reach of accelerators is to search for rare decays. These decays would be caused by very heavy particles that interact in ways forbidden in the Standard Model. To observe rare decays, very large data samples are needed and such searches often push the limits of available accelerator and detector technology. Such programs have been under way at Brookhaven National Laboratory, Fermi National Accelerator Laboratory (FNAL), the Japanese High-Energy Accelerator Research Organization (KEK), and CERN in searches for very rare K meson decays; at CESR, for rare B meson and tau lepton decays; and at Los Alamos, for rare muon decays. No evidence has been found for decays that violate the predictions of the Standard Model, even though rare K decays have been searched for with a sensitivity of better than one part in 1011. Rare decays of the muon have been searched for with a sensitivity of one part in 1012. Again, such studies are important in limiting the scope of new physics.
Over the past 25 years, particle physics has undergone a period of spectacular development. This period began with a wealth of interesting phenomena and a patchwork quilt of theoretical ideas, each of which explained some part of the data. All of the available experimental data that have been collected are now well described by a theory called the Standard Model, which has been verified
experimentally to great precision in an extraordinarily diverse set of measurements. It has proven to be frustratingly accurate. Every experimental result so far either has agreed with the Standard Model prediction or has turned out to be wrong! Some experiments made startling new discoveries, which helped to develop the model, and others made measurements of unprecedented precision in order to test it.
One measure of the breakthroughs recognized during this 25-year period is the Nobel Prizes that have been awarded for experimental or theoretical work in this field. These are listed here:
Burton Richter and Samuel Ting for the discovery of the charm quark;
Martin Perl for the discovery of the tau lepton;
Carlo Rubbia and Simon Van der Meer for the discovery of W and Z bosons;
James Cronin and Val Fitch for the discovery of particle-antiparticle asymmetry;
Sheldon Glashow, Abdus Salam, and Steven Weinberg for development of the unified electroweak theory;
Jerome Friedman, Henry Kendall, and Richard Taylor for the first observation of quarks inside the proton;
Frederick Reines for the first observation of the electron neutrino;
Leon Lederman, Melvin Schwartz, and Jack Steinberger for the experiment establishing that the muon neutrino and the electron neutrino are separate particles; and
Georges Charpak for the development of particle detectors.
Since 1972, high-energy physics has advanced to the stage at which almost the entire Standard Model has been established. Three lepton generations, all six quarks, and the gauge bosons for strong, electromagnetic, and weak interactions have all been observed. All of the fundamental particles have been seen, except the Higgs boson or whatever takes its place.