The Higgs Boson: Searching for the God Particle (11 page)

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.

-Originally published: Scientific American 264(2), 70-75. (February 1991)

The Structure of Quarks and Leptons

By Haim Harari

In the past 100 years the search for the ultimate constituents of matter has penetrated four layers of structure. All matter has been shown to consist of atoms. The atom itself has been found to have a dense nucleus surrounded by a cloud of electrons. The nucleus in turn has been broken down into its component protons and neutrons. More recently it has become apparent that the proton and the neutron are also composite particles; they are made up of the smaller entities called quarks. What comes next? It is entirely possible that the progression of orbs within orbs has at last reached an end and that quarks cannot be more finely divided. The leptons, the class of particles that includes the electron, could also be elementary and indivisible. Some physicists, however, are not at all sure the innermost kernel of matter has been exposed. They have begun to wonder whether the quarks and leptons too might not have some internal composition.

The main impetus for considering still another layer of structure is the conviction (or perhaps prejudice) that there should be only a few fundamental building blocks of matter. Economy of means has long been a guiding principle of physics, and it has served well up to now. The list of the basic constituents of matter first grew implausibly long toward the end of the 19th century, when the number of chemical elements, and hence the number of species of atoms, was approaching 100. The resolution of atomic structure solved the problem, and in about 1935 the number of elementary particles stood at four: the proton, the neutron, the electron and the neutrino. This parsimonious view of the world was spoiled in the 1950's and 1960's; it turned out that the proton and the neutron are representatives of a very large family of particles, the family now called hadrons. By the mid-1960's the number of fundamental forms of matter was again roughly 100. This time it was the quark model that brought relief. In the initial formulation of the model all hadrons could be explained as combinations of just three kinds of quarks.

Now it is the quarks and leptons themselves whose proliferation is beginning to stir interest in the possibility of a simpler scheme. Whereas the original model had three quarks, there are now thought to be at least 18, as well as six leptons and a dozen other particles that act as carriers of forces. Three dozen basic units of matter are too many for the taste of some physicists, and there is no assurance that more quarks and leptons will not be discovered. Postulating a still deeper level of organization is perhaps the most straightforward way to reduce the roster. All the quarks and leptons would then be composite objects, just as atoms and hadrons are, and would owe their variety to the number of ways a few smaller constituents can be brought together. The currently observed diversity of nature would be not intrinsic but combinatorial.

It should be emphasized that as yet there is no evidence quarks and leptons have an internal structure of any kind.
In the case of the leptons, experiments have probed to within 10
^-16
centimeter and found nothing to contradict the assumption that leptons are pointlike and structureless. As for the quarks, it has not been possible to examine a quark in isolation, m uch less to discern any possible internal features. Even as a strictly theoretical conception, the subparticle idea has r un into difficulty: no one has been able to devise a consistent description of how the subparticles might move inside a quark or a lepton and how they might interact with one another. They would have, to be almost unimaginably small: if an atom were magnified to the size of the earth, its innermost constituents could be no larger than a grapefruit.
Nevertheless, models of quark and lepton substructure make a powerful appeal to the aesthetic sense and to the imagination: they suggest a way of building a complex world out of a few simple parts.

Any theory of the elementary particles of matter must also take into account the forces that act between them and the laws of nature that govern the forces. Little would be gained in simplifying the spectrum of particles if the number of forces and laws were thereby increased. As it happens, there has been a subtle interplay between the list of particles and the list of forces thro ugho ut the history of physics.

In about 1800 four forces were thought to be fundamental: gravitation, electricity, magnetism and the shortrange force between molecules that is responsible for the cohesion of matter.
A series of remarkable experimental and theoretical discoveries then led to the recognition that electricity and magnetism are actually two manifestations of the same basic force, which was soon given the name electromagnetism. The discovery of atomic structure brought a further revision. Although an atom is electrically neutral overall, its constituents are charged, and the short-range molecular force came to be understood as a complicated residual effect of electromagnetic interactions of positive nuclei and negative electrons. When two neutral atoms are far apart, there are practically no electromagnetic forces between them. When they are near each other, however, the charged constituents of one atom are able to "see" and influence the inner charges of the other, leading to various short-range attractions and repulsions.

As a result of these developments physics was left with only two basic forces. The unification of electricity and magnetism had reduced the number by one, and the molec ular interaction had been demoted from the rank of a fundamental force to that of a derivative one.
The two remaining fundamental forces, gravitation and electromagnetism, were both long-range. The exploration of nuclear structure, however, soon introduced two new short-range forces.
The strong force binds protons and neutrons together in the nucleus, and the weak force mediates certain transformations of one particle into another, as in the beta decay of a radioactive nucleus.
Thus there were again four forces.

The development of the quark model and the accompanying theory of quark interactions was the next occasion for revising the list of forces. The quarks in a proton or a neutron are thought to be held together by a new long-range fundamental force called the color force, which acts on the quarks because they bear a new kind of charge called color.
(Neither the force nor the charge has any relation to ordinary colors.) Just as an atom is made up of electrically charged constituents but is itself neutral, so a proton or a neutron is made up of colored quarks but is itself colorless.
When two colorless protons are far apart, there are essentially no color forces between them, but when they are near, the colored quarks in one proton
"see" the color charges in the other proton.
The short-range attractions and repulsions that result have been identified with the effects of the strong force. In other words, just as the short-range molecular force became a residue of the long-range electromagnetic force, so the short-range strong force has become a residue of the long-range color force.

One more chapter can be added to this abbreviated history of the forces of nature. A deep and beautiful connection has been found between electromagnetism and the weak force, bringing them almost to the point of full unification.
They are clearly related, but the connection is not quite as close as it is in the case of electricity and magnetism, and so they must still be counted as separate forces. Therefore the current list of fundamental forces still has four entries:
the long-range gravitational, electromagnetic and color forces and the short-range weak force. Within the limits of present knowledge all natural phenomena can be understood through these forces and their residual effects.

The evolution of ideas about particles and that of ideas about forces are clearly interdependent. As new basic particles are found, old ones turn out to be composite objects. As new forces are discovered, old ones are unified or reduced to residual status. The lists of particles and forces are revised from time to time as matter is explored at smaller scale and as theoretical understanding progresses.
Any change in one list inevitably leads to a modification of the other. The recent speculations about quark and lepton structure are no exception; they too call for changes in the complement of forces. Whether the changes represent a simplification remains to be seen.

Of the four established fundamental forces, gravitation must be put in a category apart. It is too feeble even to be detected in the interactions of individual particles, and it is not understood in terms of microscopic events. For the other three forces successful theories have been developed and are now widely accepted. The three theories are distinct, but they are consistent with one another; taken together they constitute a comprehensive model of elementary particles and their interactions, which I shall refer to as the standard model.

In the standard model the indivisible constituents of matter are the quarks and the leptons. It is convenient to discuss the leptons first. There are six of them: the electron and its companion the electron-type neutrino, the muon and the muon-type neutrino and the tau and the tau-type neutrino. The electron, the muon and the tau have an electric charge of -1; the three neutrinos are electrically neu tral.

There are also six basic kinds of quark, which have been given the names up, down, charmed, strange, top and bottom, or
u, d, c, s, t
and
b
. (The top quark has not yet been detected experimentally, and neither has the tau-type neutrino, but few theorists doubt their existence.) The
u, c
and
t
quarks have an electric charge of + 2/3, the
d, s
and
b
quarks a charge of -1/3. In addition each quark type has three possible colors, which I shall designate red, yellow and blue. Thus if each colored quark is counted as a separate particle, there are 18 quark varieties altogether. Note that each quark carries both color and electric charge, but none of the leptons are colored.

For each particle in this scheme there is an antiparticle with the same mass but with opposite values of electric charge and color. The antiparticle of the electron is the positron, which has a charge of + 1. The antiparticle of a red
u
quark, with a charge of +2/3, is an antired
u
¹
antiquark, with a charge of -2/3.

The color property of the quarks is analogous in many ways to electric charge, but because there are three possible colors it is appreciably more complicated.
Electrically charged particles can be brought together to form an electrically neutral system in only one way:
by combining equal quantities of positive and negative charge.
A colorless composite particle can be formed out of colored quarks in much the same way, namely by combining a colored quark and an anticolored antiquark.
In the case of color, however, there is a second way to form a neutral state:
any composite system with equal quantities of all three colors or of all three anticolors is also colorless.
For this reason a proton consisting of one red quark, one yellow quark and one blue quark has no net color.

One further property of the quarks and leptons should be mentioned:
each particle has a spin, or intrinsic angular momentum, equal to one-half the basic quantum-mechanical unit of angular momentum.
When a particle with a spin of 1/2 moves along a straight line, its intrinsic rotation can be either clockwise or counterclockwise when the particle is viewed along the direction of motion.
If the spin is clockwise, the particle is said to be right-handed, because when the fingers of the right hand curl in the same direction as the spin, the thumb indicates the direction of motion.
For a particle with the opposite sense of spin a left-hand rule describes the motion, and so the particle is said to be left-handed.