The ψ and ϒ resonances were startling and largely unanticipated. By contrast, it was apparent far in advance that the Z would be spectacular in e+e− annihilation. Indeed, within the Standard Model nearly every aspect of the Z could be predicted to the extent that sin2 θW was known. Despite this, the study of the Z in e+e− annihilation was a singular achievement in particle physics.

After initial planning as early as 1976, CERN began construction of the Large Electron Positron collider in 1983. Because ultrarelativistic electrons lose energy rapidly through synchrotron radiation, whose intensity varies as E4/ρ where ρ is the radius of curvature, LEP was designed with a large circumference, 26.67 km. The first collisions occurred on August 13, 1989.

In a daring move, SLAC aimed to reach the Z before LEP by colliding electron and positron beams generated with its linear accelerator. At the Stanford Linear Collider each bunch would be lost after colliding with the opposing bunch. While the Mark II detector, which had seen service at PEP, was refurbished, four new detectors – ALEPH, DELPHI, L3, and OPAL – were built at CERN.

In the twenty years since the first edition, the promise of the Standard Model of Particle Physics has been fulfilled. The detailed behavior of the W and Z bosons did conform to expectations. The sixth quark finally arrived. The pattern of CP violation in B mesons fit convincingly the predictions based on the Kobayashi–Maskawa model. These three developments require three new chapters. The big surprise was the observation of neutrino oscillations. Neutrino masses and oscillations were not required by the Standard Model but are easily accommodated within it. An extensive fourth new chapter covers this history.

Though the neutrino story is not yet fully known, the basics of the Standard Model are all in place and so this is an appropriate time to update the Experimental Foundations of Particle Physics. We fully anticipate that the most exciting times in particle physics lie just ahead with the opening of the Large Hadron Collider at CERN. This Second Edition provides a recapitulation of some 75 years of discovery in anticipation of even more profound revelations.

Not only physics has changed, but technology, too. The bound journals we dragged to the xerox machine are now available from the internet with a few keystrokes on a laptop. Nonetheless, we have chosen to stick with our original format of text alternating with reprinted articles, believing Gutenberg will survive Gates and that there is still great value in having the physical text in your hands.

New forms of matter, 1974–1976.

In November 1974, Burton Richter at SLAC and Samuel Ting at Brookhaven were leading two very different experiments, one studying e+e− annihilation, the other the e+e− pairs produced in proton-beryllium collisions. Their simultaneous discovery of a new resonance with a mass of 3.1 GeV so profoundly altered particle physics that the period is often referred to as the “November Revolution.” Word of the discoveries spread throughout the high energy physics community on November 11 and soon much of its research was directed towards the new particles.

Ting led a group from MIT and Brookhaven measuring the rate of production of e+e− pairs in collisions of protons on a beryllium target. The experiment was able to measure quite accurately the invariant mass of the e+e− pair. This made the experiment much more sensitive than an earlier one at Brookhaven led by Leon Lederman. That experiment differed in that μ−μ+ pairs were observed rather than e+e− pairs. Both these experiments investigated the Drell–Yan process whose motivation lay in the quark–parton model.

The Drell–Yan process is the production of e+e− or μ+μ+ pairs in hadronic collisions. Within the parton model, this can be understood as the annihilation of a quark from one hadron with an antiquark from the other to form a virtual photon. The virtual photon materializes some fraction of the time as a charged-lepton pair.

Fermi's theory of weak interactions survived nearly unaltered over the years. Its basic structure was slightly modified by the addition of Gamow–Teller terms and finally by the determination of the V-A form, but its essence as a four fermion interaction remained. Fermi's original insight was based on the analogy with electromagnetism; from the start it was clear that there might be vector particles transmitting the weak force the way the photon transmits the electromagnetic force. Since the weak interaction was of short range, the vector particle would have to be heavy, and since beta decay changed nuclear charge, the particle would have to carry charge. The weak (or W) boson was the object of many searches. No evidence of the W boson was found in the mass region up to 20 GeV.

The V-A theory, which was equivalent to a theory with a very heavy W, was a satisfactory description of all weak interaction data. Nevertheless, it was clear that the theory was not complete. As described in Chapter 6, it predicted cross sections at very high energies that violated unitarity, the basic principle that says that the probability for an individual process to occur must be less than or equal to unity.

Six quarks, six leptons, together with the gluons of QCD and the photon and weak bosons, are enough to describe the tangible world and more, with remarkable economy. Only the Higgs boson is missing among the ingredients of the canonical Standard Model. And yet we know we are missing much more than this. The last ten years of cosmological observations have established that the ordinary matter of quarks and leptons accounts for just 5% of the energy density of the Universe, that another 23% is “dark matter,” outside the Standard Model, and 72% of the energy density isn't due to matter at all. Moreover, we can't answer the most basic question of all: why is there something rather than nothing? Why didn't all the matter created in the Big Bang ultimately annihilate, particle against antiparticle? Andrei Sakharov explained that CP violation must be part of the answer, but we know it isn't just the CP violation of the CKM matrix, for that wouldn't account for the amount of matter that remains. On the other hand, the strong interactions might have been CP violating but aren't. Why not?

These questions are pressed upon us by facts and demand answers. Other questions arise more from aesthetics: Are the strong and electroweak forces themselves unified? What about gravity? Are there more forces still to be discovered? Why are there three generations of quarks and leptons? Even more audaciously, why are there three spatial dimensions, or perhaps, are there more than three spatial dimensions?

The fundamental achievement of physical science is the atomic model of matter. That model is simplicity itself. All matter is composed of atoms, which themselves form aggregates called molecules. An atom contains a positive nucleus very much smaller than the full atom. A nucleus with atomic mass A contains Z protons and A − Z neutrons. The neutral atom has, as well, Z electrons, each with a mass only 1/1836 that of a proton. The chemical properties of the atom are determined by Z; atoms with equal Z but differing A have the same chemistry and are known as isotopes.

This school-level description did not exist at all in 1895. Atoms were the creation of chemists and were still distrusted by many physicists. Electrons, protons, and neutrons were yet to be discovered. Atomic spectra were well studied, but presented a bewildering catalog of lines connected, at best, by empirical rules like the Balmer formula for the hydrogen atom. Cathode rays had been studied, but many regarded them as uncharged, electromagnetic waves. Chemists had determined the atomic weights of the known elements and Mendeleev had produced the periodic table, but the concept of atomic number had not yet been developed.

The discovery of X-rays byW. C. Röntgen in 1895 began the revolution that was to produce atomic physics. Röntgen found that cathode-ray tubes generate penetrating, invisible rays that can be observed with fluorescent screens or photographic film.

The elucidation of the π → μv decay sequence left particle physics in a relatively simple state. Yukawa's particle had been found and the only unanticipated particle was the muon, of which I. I. Rabi is said to have remarked “Who ordered that?” The question remains unanswered. The cosmic-ray experiments of the next few years quickly and thoroughly destroyed the simplicity that had previously prevailed. The proliferation of new particles, many with several patterns of decay, produced great confusion. The primary source of confusion was whether each new decay mode represented a new particle or was simply an alternative decay for a previously observed particle. Continued experimentation with improved accuracy and statistics eventually resolved these ambiguities, but basic uncertainties remained. What was the nature of these particles? How were they related to the more familiar particles? The examination of these questions led to the development of the concepts of associated production, strangeness, and ultimately, parity violation and SU(3).

Remarkably, another meson seems to have been discovered before the pion. Working in the French Alps in 1943, Leprince-Ringuet and L'héritier took 10, 000 triggered pictures in a 75 cm × 15 cm × 10 cm cloud chamber placed inside a magnetic field of 2500 gauss (Ref. 3.1). This permitted careful measurements of the momenta of the charged tracks. One of the pictures showed an incident positive particle of about 500 MeV/c momentum produce a secondary of about 1 MeV/c.

Hadronic scattering experiments produced extensive and rich data revealing resonances and regularities of cross sections. While the quark model provided a firm basis for classifying the particles and resonances, the scattering cross sections were less easily interpreted. The early studies of strong interactions indicated that the couplings of the particles were large. This precluded the straightforward use of perturbation theory. While alternative approaches have yielded some important results, it is still true that even processes as basic as elastic proton–proton scattering are beyond our ability to explain in detail. In contradistinction, scattering of electrons by protons and neutrons is open to direct interpretation.

For the scattering of an electron by a proton it is a good approximation to assume that the interaction is due to the exchange of a single virtual photon. The small corrections to this approximation may be calculated if necessary. Each coupling of the photon gives a factor of e in the scattering amplitude, so a virtual photon's two couplings typically provides a factor α = e2/4π ≈ 1/137. It is this small number that makes the approximation a good one.

The scattering of relativistic electrons (E > > me) by a known charge distribution can be calculated using the standard methods of quantum mechanics.

The detection of elementary particles is based on their interactions with matter. Swiftly moving charged particles produce ionization and it is this ionization that is the basis for most techniques of particle detection. During the 1930s cosmic rays were studied primarily with cloud chambers, in which droplets form along the trails of ions left by the cosmic rays. If the cloud chamber is in a region of magnetic field, the tracks show curvature. According to the Lorentz force law, the component of the momentum in the plane perpendicular to the magnetic field is given by p(MeV/c) = 0.300×10−3B(gauss)r (cm) or p(GeV/c) = 0.300× B(T)r (m), where r is the radius of curvature. By measuring the track of a particle in a cloud chamber it is possible to deduce the momentum of the particle.

The energy of a charged particle can be deduced by measuring the distance it travels before stopping in some medium. The charged particles other than electrons slow primarily because they lose energy through the ionization of atoms in the medium, unless they collide with a nucleus. The range a particle of a given energy will have in a medium is a function of the mass density of the material and of the density of electrons.

The striking success of the parton model in describing deep inelastic–nucleon and neutrino–nucleon scattering provided strong circumstantial evidence for the Feynman–Bjorken picture and for its complete elaboration as quantum chromodynamics (QCD). QCD describes all strong interactions as resulting from the interactions of spin-1/2 quarks and spin-1 gluons. The fundamental coupling is of the gluon to the quarks, in a fashion analogous to the coupling of a photon to electrons. In addition, the gluons couple directly to each other. SU(3) plays a central role. Just as in the Gell-Mann–Zweig model of hadrons, there are three basic constituents. The u quark, for example, comes in three versions, say, red, blue, and green. Similarly, every other kind of quark comes in these three versions or “colors.” Often it is convenient to refer to u, d, s, and c as “flavors” of quarks, to contrast with the three colors in which every flavor comes. While the SU(3) of flavor is an approximate symmetry, the SU(3) of color is an exact symmetry, thus the three colors of the u quark are exactly degenerate in mass, while the u, d, and s quarks are not degenerate.

The rules of SU(3) state that if we combine a 3 (a quark) with a 3* (an antiquark), we get 1 + 8, a singlet and an octet. In terms of mesons, this explains that combining the three quark flavors (u, d, s) with the three antiquark flavors yields an SU(3) singlet (η′) and an octet (the pseudoscalar octet of π, K, η).

The discovery of hadrons with the internal quantum number ‘strangeness’ marks the beginning of a most exciting epoch in particle physics that even now, 50 years later, has not yet reached its conclusion. As we will describe in detail, the discoveries made in this area of research have proved essential in formulating the SM of high energy physics. This is not to say, however, that it has been a particularly glorious chapter for theorists always making successful predictions. On the contrary, we have to admit that by and large experiments have driven the development, and that major discoveries came unexpectedly or even against expectations expressed by theorists. Theorists on the whole have fared better in making postdictions than predictions. Some of the relevant questions remain unanswered today and/or have led to even more profound puzzles.

These discoveries were not spread out evenly over time; some periods were clearly more fertile than others. In this chapter we will give a brief overview of important developments. We do not aim at giving a complete history of this period. Instead we try to give a flavour of the excitement accompanying these discoveries, thus illustrating the thrill and the unexpected turns that can occur in the basic sciences in general and in high energy physics in particular.

The discovery of strange particles

Tracking chambers, being based on ionization, cannot record the passage of a neutral particle. Yet when it decays into two charged particles, the tracks of the decay products can be recorded, and a so-called V pattern arises.