CONTINUATION FROM THE NUMBER OF FAMILY OF MATTER

CONTINUATION FROM THE NUMBER OF FAMILY OF MATTER 

The next step outward brings the reaction products to the electron-photon calorimeter. The products traverse the superconducting coil, which creates a 15,000-gauss magnetic field at the axis of the device, and then enter the hadron calorimeter. This device, a series of iron plates separated by gas counters, also returns the magnetic flux, just as an iron core does in a conventional electromagnet. Aleph weighs 4,000 tons and cost about $60 million to build. Half a million channels of information must be read for each event, and the computer support necessary for the acquisition and later evaluation of the data is considerable.

The data gathered in the first few months of operation of the two colliders have provided the best support yet adduced for the predictions of the electroweak theory. More important, they have delineated the curve describing the Z width with great precision.

The overwhelming majority of observed electron-positron annihilations give rise to four sets of products: 88 percent produce a quark and an antiquark; the remaining 12 percent are divided equally among the production of a tau lepton and antitau lepton, muon and antimuon, and electron and positron. (The last case simply reverses the initial annihilation.)

In the decays into electrons and muons, two tracks are seen back to back, with momenta (and energies) corresponding to half of the combined beam energy. The two products are easily distinguished by their distinct behavior in the calorimeters. The decays to tau leptons are more complex because they subsist for a mere instant—during which they travel about a millimeter—before decaying into tertiary particles that alone can be observed. A tau lepton leaves either closely packed tracks or just one track; in both cases, the signature is mirrored by that of another tau lepton moving in the opposite direction (thus conserving momentum).

The quarks that account for most reactions cannot be seen in their free, or 'naked,' state, because at birth they undergo a process called hadronization. Each quark 'clothes' itself in a jet of hadrons, numbering 15 on average, two thirds of which are charged. This, the most complex of the four main decay events, usually manifests itself as back-to-back jets, each containing many tracks. The results described here are based on the analysis of about 80,000 Z decays into quarks—the combined result of the four lep teams and the one slac team.

The Z production curve is determined in an energy scan. Production probability is measured at a number of energies: at the peak energy, as well as above and below it. A precise knowledge of the beam energy is of great importance here. It was obtained at the two colliders very differently, in both cases with a good deal of ingenuity and with a precision of three parts in 10,000.

As was pointed out earlier, the total width of the Z resonance can be determined from either the height at the peak energy or the width of the resonance curve. The height has the smaller statistical error but requires knowledge not only of the rate at which events occur but also of the rate at which particles from the two beams cross. The latter rate is called the luminosity of the collider.

In the simple case of two perfectly aligned beams of identical shape and size, the luminosity equals the product of the number of electrons and the number of positrons in each crossing bunch, multiplied by the number of bunches crossing each second, divided by the cross-sectional area of the beams. In practice, luminosity is determined only by observing the rate of the one process that is known with precision: the scattering of electrons and positrons that glance off one another at very small angles without combining or otherwise changing state. To record such so-called elastic collisions, two special detectors are placed in small angular regions just off the axis of the beam pipe. One of the detectors is in front of the collision area; the other is behind it. In the case of Aleph, these detectors are electron-photon calorimeters of high granularity.

The elastically scattered electrons and positrons are identified by the characteristic pattern in which they deposit energy in the detectors and by the way they strike the two detectors back to back, producing a perfectly aligned path. The essence here is to understand precisely the way in which particles are registered, especially in those parts of the detectors that correspond to exceedingly small scattering angles. This is important because the detection rate is extremely sensitive to changes in the angle.

When the resulting data are fitted to the theoretical resonance shape, three parameters are considered: the height at the peak, the total width and the Z mass. The data, in fact, agree well with the shape of the theoretically expected distribution. The next step, then, is to determine the number of neutrino families from two independent parameters—the width and the peak height.

The combined results of the five teams produced an average estimate of 3.09 neutrino varieties, with an experimental uncertainty of 0.09. This number closely approaches an integer, as it should, and matches the number of neutrino varieties that are already known. A fourth neutrino could exist without contradicting these findings only if its mass exceeded 40 billion eV—a most unlikely possibility, given the immeasurably small masses of the three known neutrinos.

The Z result fits the cosmological evidence gathered by those who study matter on galactic and supergalactic scales. Astronomers have measured the ratio of hydrogen to helium and other light elements in the universe. Cosmologists and astrophysicists have tried to infer the processes by which these relative abundances came about.

Shortly after the big bang, the cataclysmic explosion that created the universe and began its expansion, matter was so hot that a neutron was as likely to decay into a proton-electron pair as the latter was to combine to form a neutron. Consequently, as many neutrons as protons existed. But as the universe expanded and cooled, the slightly heavier neutrons changed into protons more readily than protons changed into neutrons. The neutron-proton ratio therefore fell steadily.

When the expansion brought the temperature of the universe below one billion kelvins, protons and neutrons were for the first time able to fuse, thereby forming some of the lighter elements, mainly helium. The resulting abundances depend critically on the ratio of neutrons to protons at the time light elements were forming. This ratio, in turn, depends on the rate at which the universe expanded and cooled. At this stage, each light neutrino family—that is, any whose constituents have a mass smaller than about a million eV—contributes appreciably to the energy density and cooling rate. The measured abundances of light elements are consistent with cosmological models that assume the existence of three light neutrino families but tend to disfavor those that assume four or more.

Many questions remain unanswered. Why are there just three families of particles? What law determines the masses of their members, decreeing that they shall span 10 powers of 10? These problems lie at the center of particle physics today. They have been brought one step closer to solution by the numbering of the families of matter.