Skewing the Universe
It is imaginable that the universe was born skewed—that is, having unequal numbers of particles and antiparticles to begin with. Such an initial imbalance, however, would be quickly eliminated if the early universe contained any processes that could change baryon number— the number of matter particles minus the number of antimatter particles. (In extensions of the Standard Model called Grand Unified Theories, such processes would have been very common soon after the big bang.) Theorists prefer the alternative scenario, in which particles and antiparticles were equally numerous in the early universe, but the former came to dominate as the universe expanded and cooled.
Soviet physicist (and dissident) Andrei Sakharov pointed out three conditions necessary for this asymmetry to develop. First, fundamental processes that do not conserve baryon number must exist. Second, during the expansion the universe must not attain thermal equilibrium. (When in thermal equilibrium, all states of equal energy contain equal populations of particles, and because particles and antiparticles have equal mass or energy, they would be generated at the same rate.) Third, CP symmetry—essentially, the symmetry between matter and antimatter—must be violated. Otherwise any process that changes the amount of matter would be balanced by a similar effect for antimatter.
The prevailing theory holds that when the universe was born, the quantum field associated with the Higgs particle was everywhere zero. Then, somewhere in the universe, a bubble developed, inside which the Higgs field assumed its present nonzero value. Outside the bubble, particles and antiparticles had no mass; once inside, however, they interacted with the Higgs field to acquire mass. But as the bubble grew, particles and antiparticles were swept through its surface at unequal rates because of CP violation. Any imbalances between matter and antimatter thus created outside the bubble were quickly corrected by processes that change baryon number.
Such processes were extremely rare inside the bubble, however, so the imbalance was frozen in. By the time the bubble had expanded to occupy the entire universe, it contained more particles than antiparticles. Eventually the universe cooled to a point where particles and antiparticles could no longer be generated in collisions but would annihilate when they found one another. Unfortunately, when theorists calculate how much of an imbalance between matter and antimatter this mechanism can create, it comes out too small—by many orders of magnitude. This failure suggests that there must be other ways in which CP symmetry breaks down and hence that the Standard Model may be incomplete.
A fruitful place to search for more violations is most likely among the B mesons. The Standard Model predicts the various decays of the B0 and the anti- B0 to be highly asymmetric. A B0 contains a down quark bound to an antibottom quark, whereas the anti-B0 consists of an antidown quark and a bottom quark. The B mesons behave much like the kaons discussed earlier: the observed B mesons consist of mixtures of the B0 and anti-B0.
Consider the evolution of a B0 meson produced at a certain instant. Some time later an observer has a certain probability of finding the same particle and also some probability of finding its antiparticle, the anti-B0. This peculiar meson state, oscillating between a given quarkantiquark combination and its antiparticle, is a remarkable illustration of quantum mechanics at work.
The Bottom Line
To study CP violation, experimenters need to study decays of B0 into those final states that have a definite CP number. Such decays should proceed at a different rate for a particle that is initially B0 compared with one that is initially anti-B0. This difference will indicate the extent of CP violation in the system. But rather than resulting in the one-in-1,000 effect seen in K0 decays, the predicted asymmetry for B0 decays grows so large that one decay rate can become several times larger than the other.
Models other than the Standard often have additional sources of CP violation— sometimes involving extra Higgs particles—in general offering any value for imbalances in B0 decays. Thus, measuring the pattern of asymmetries will provide a clear test of the predictions.
When the bottom quark was discovered, its mass was measured at around five giga-electron volts (GeV), or about five times the mass of a proton. Consequently, theorists calculated that it would take a little more than 10 GeV of energy to produce two B mesons (because the added down or antidown quarks are very light). In the early 1980s at Cornell University, operators of an electron-positron collider—a machine that accelerates electrons and positrons into head-on crashes—tuned it so that an electron-positron pair would release an energy of 10.58 GeV on annihilating. As predicted, this burst of energy preferentially converts to B mesons, providing a very rich source of the particles. About one in four annihilations results in a B meson and its antiparticle, leaving behind no other particles at all.
At SLAC in 1983 experimenters found an unexpectedly long lifetime of about 1.5 picoseconds for the B meson. The extended life improved the chances that a B0 would turn into an anti-B0 before decaying, making CP-violating asymmetries easier to observe. Furthermore, in 1987 experimenters at the Electron Synchrotron Laboratory (DESY) in Hamburg, Germany, measured this “mixing” probability at 16 percent, making it likely that the asymmetries would be far larger than those for the K0. Still, these large asymmetries occur in relatively rare decays of the B mesons. For a true study of CP violation, a great number of B mesons would be needed.
In 1988 at a workshop in Snowmass, Colo., the major topic of interest was the Higgs particle. A group of participants also discussed CP violation, especially in B mesons. It determined that a favorable way to study the B mesons would be with an electron-positron collider tuned to 10.58 GeV in which the electron and positron beams had different energies. This rather unusual feature would facilitate the measurement of a B meson’s life span. Experimenters identify the point of birth and the point of death (that is, decay) of a B meson from traces of particles in the detector. Dividing the distance between these two points by the calculated velocity of the meson yields its life span. But an ordinary electron-positron collider at 10.58 GeV produces two B mesons that are almost at rest; the small distances they move are hard to measure.
Pier Oddone of Lawrence Berkeley National Laboratory had pointed out that if the electrons and positrons have different energies, the B0 mesons that are produced move faster. For instance, if the electron beam has an energy of 9.0 GeV and the positron beam an energy of 3.1 GeV, the B0 mesons move at half the speed of light, traveling about 250 microns (about one hundredth of an inch) before they decay. Such a distance can yield a reasonably accurate measure of the lifetime.
An accelerator facility with two separate rings delivering different energies to the electrons and positrons would fit the task. Each ring would have to deliver very intense beams of particles, obtaining a high rate of collisions. Such a machine came to be called an asymmetric B factory: asymmetric because of the different beam energies, and B factory because of the large numbers of B mesons it would produce.
Teams at several laboratories developed designs that could generate about 30 million pairs of B mesons a year. In 1993 the U.S. Department of Energy and the Japanese agency Monbusho approved two proposals for construction: one at SLAC in California and the other at KEK, the High Energy Accelerator Research Organization in Tsukuba, Japan. The SLAC project is utilizing the existing linear tunnel to accelerate the positrons and electrons. These will then be circulated in separate rings newly constructed in a 20-year-old tunnel and set to collide at a point of crossing. The accelerator construction cost $177 million. The Japanese project is also employing extant tunnels—those that previously housed the Tristan collider.
Physicists and engineers are busy setting up a large experiment that can identify the rare decays of a B meson and measure their positions to within the requisite 80 microns. This accuracy is obtained by using the silicon microstrip technology that helped to unearth the top quark [see “The Discovery of the Top Quark,” by Tony M. Liss and Paul L. Tipton; Scientific American, September 1997]. Exp
erimenters aim to identify almost every particle that emerges from the decays of the B mesons in order to isolate the rare events that shed light on charge-parity questions.
In the BABAR detector that is being built for SLAC, the silicon microstrip will be the innermost layer, forming a cylinder roughly 30 centimeters in diameter and 60 centimeters long. Outer layers will measure energy, velocity and penetration power for each particle created, allowing physicists to reconstruct the original events. More than 500 participants—including both of us—from 70 institutions in nine nations are building the detector and also sharing its cost of $85 million. (It was, in fact, to facilitate international collaborations of this kind that the World Wide Web was invented at CERN.) The BELLE collaboration that is building the Japanese experiment is also international in scope, with members from 10 countries. Both B factories are scheduled for completion later this year, with the first data arriving in early 1999.
Other kinds of violations of charge parity, less predictable than the quantum-mechanical mixing, should also occur in B decays. The Cornell collider and detector are being upgraded to search for such effects. A number of experiments on B physics are also planned at proton accelerators around the world. Both types of colliders will provide crucial, and complementary, pieces of evidence on CP violation.
The B factories could definitively tell researchers that the Standard Model concept works and then help to determine its remaining parameters. Alternatively, they could show that the model’s predictions cannot fit the data no matter what the choice of parameters. Indeed, the results could rule out entire classes of models beyond the Standard Model, thus helping theorists to zero in on a successor. And if all goes well, we may even come to understand why our world is made exclusively of matter.
-Originally published: Scientific American 279(4), 76-81 (October 1998)
SECTION 2
We Know You’re in There
The Higgs Boson
by Martinus J. G. Veltman
The truly fundamental problems of physics can always be explained in simple terms without the help of complicated equations or mathematical arguments. At least this was once told to me by Victor F. Weisskopf, an eminent physicist who often engages in such explanations, and he may very well be right. It certainly holds for a proposed but undiscovered particle called the Higgs boson and the so-called Higgs field associated with it.
The Higgs boson, which is named after Peter W. Higgs of the University of Edinburgh, is the chief missing ingredient in what is now called the standard model of elementary processes: the prevailing theory that describes the basic constituents of matter and the fundamental forces by which they interact. According to the standard model, all matter is made up of quarks and leptons, which interact with one another through four forces: gravity, electromagnetism, the weak force and the strong force. The strong force, for instance, binds quarks together to make protons and neutrons, and the residual strong force binds protons and neutrons together into nuclei. The electromagnetic force binds nuclei and electrons, which are one kind of lepton, into atoms, and the residual electromagnetic force binds atoms into molecules. The weak force is responsible for certain kinds of nuclear decay. The influence of the weak force and the strong force extends only over a short range, no larger than the radius of an atomic nucleus; gravity and electromagnetism have an unlimited range and are therefore the most familiar of the forces.
In spite of all that is known about the standard model, there are reasons to think it is incomplete. That is where the Higgs boson comes in. Specifically, it is held that the Higgs boson gives mathematical consistency to the standard model, making it applicable to energy ranges beyond the capabilities of the current generation of particle accelerators but that may soon be reached by future accelerators. Moreover, the Higgs boson is thought to generate the masses of all the fundamental particles; in a manner of speaking, particles "eat" the Higgs boson to gain weight.
The biggest drawback of the Higgs boson is that so far no evidence of its existence has been found. Instead a fair amount of indirect evidence already suggests that the elusive particle does not exist. Indeed, modern theoretical physics is constantly filling the vacuum with so many contraptions such as the Higgs boson that it is amazing a person can even see the stars on a clear night! Although future accelerators may well find direct evidence of the Higgs boson and show that the motivations for postulating its existence are correct, I believe things will not be so simple. I must point out that this does not mean the entire standard model is wrong. Rather, the standard model is probably only an approximation–albeit a good one–of reality.
Even though the only legitimate reason for introducing the Higgs boson is to make the standard model mathematically consistent, much attention has been given to the conceptually easier proposal that the particle generates the masses of all the fundamental particles. I shall therefore begin with that topic.
Central to an understanding of how the Higgs boson would generate mass is the concept of a field. A field is simply a quantity, such as temperature, defined at every point throughout some region of space and time, such as the surface of a frying pan. In physics the term "field" is usually reserved for such entities as the gravitational field and the electromagnetic field. Fields generally make themselves felt by means of the exchange of a mediating particle; the particle that mediates the electromagnetic field, for example, is the photon, or quantum of light. The mediating particles of the gravitational field, the weak field and the strong field are respectively the graviton (which has not yet been detected), three weak vector bosons, called the W+, W- and Z0 particles, and eight gluons. In a somewhat analogous way the Higgs boson is the mediating particle of the proposed Higgs field.
It is now assumed that there is a constant Higgs field throughout all space, that is, the vacuum of outer space is not empty but contains this constant field. The Higgs field is thought to generate mass by coupling to particles. Depending on the coupling strength, a particle in space has a certain potential energy. By Einstein's famous equation, E = mc2 (energy equals mass multiplied by the square of the speed of light), the coupling energy is equivalent to a mass. The stronger the coupling, the greater the mass.
The way particles are thought to acquire mass in their interactions with the Higgs field is somewhat analogous to the way pieces of blotting paper absorb ink. In such an analogy the pieces of paper represent individual particles and the ink represents energy, or mass. Just as pieces of paper of differing size and thickness soak up varying amounts of ink, different particles "soak up" varying amounts of energy, or mass. The observed mass of a particle depends on the particle's "energyabsorbing" ability and on the strength of the Higgs field in space.
What are the characteristics of the proposed Higgs field? In order to endow particles with mass, the Higgs field, if it exists, would have to assume a uniform, nonzero value even in the vacuum. Moreover, the Higgsfield would be a scalar field, which is one of two kinds of field important in describing the interactions of particles. A scalar field is a field in which each point has associated with it a single magnitude, or number. The other important field is a vector field: a field where at each point a vector, or arrow, is drawn. A vector has both a magnitude, which is represented by the length of the arrow, and a direction. The electromagnetic, weak and strong fields are all vector fields. (The gravitational field is a special entity called a tensor field.)
The proposed Higgs field must be a scalar field, because if it were a vector field, the mass of a particle would in general depend on the particle's alignment with the field. Stated in a somewhat oversimplified way, the mass of a person would change if he or she turned around while standing in the same place. In other words, the Higgs field is "spinless."
Because the Higgs field is spinless, the Higgs boson must also be spinless. Spin, as applied to elementary particles, is a quantum-mechanical property roughly equivalent to the classical spin of a rotating ball. Elementary particles can take on only integer (0, 1, 2
and so on) and half-integer (1/2, 3/2 and so on) values of spin. Particles that have integral spin are called bosons and particles that have half-integral spin are called fermions. Bosons and fermions have sharply differing properties, but I shall not delve into that topic here.
The Higgs boson is called a scalar boson because it has a spin of 0. Most other bosons associated with fields are thought to be vector bosons: particles that have a spin of l. The photon, gluon and W+, W- and Z0 particles, for instance, are spin-1 bosons.
Since vector bosons are typically associated with the fundamental forces of nature and the Higgs boson is a scalar boson, the force by which particles couple to the Higgs field must be a new force. It is introduced explicitly and solely as a mechanism to improve the mathematical consistency of the standard model. The Higgs force behaves mathematically in a similar manner to the recently publicized "fifth force" reported by Ephraim Fischbach of Purdue University. Theproposed Higgs force is, however, weaker and has a much shorter range than the "fifth force."
The Higgs force is not a universal force, because it couples differently to different particles. Specifically, if a particle is observed to have mass, the strength of the coupling to the Higgs field is assumed to be whatever quantity is necessary to generate precisely that mass. Presumably the Higgs field does not couple to the photon, since experiment shows the photon is massless. But apparently it couples to the W+, W- and Z0 particles, because they do have mass. It should perhaps be noted that particles could have a mass of their own, in addition to what they are thought to acquire from the Higgs field. Curiously, however, in the standard model not a single particle could have a mass of its own without destroying the mathematical completeness of the theory.
The Higgs Boson: Searching for the God Particle Page 14
