The Higgs Boson: Searching for the God Particle
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SPONTANEOUS SYMMETRY BREAKING is a mechanism that could spoil a prequark theory even if it has a chiral symmetry. Both of the physical systems shown here—a simple trough and a trough with a bump in the bottom—can be described as symmetrical in the sense that exchanging left and right leaves the system unatlered. For the simple trough the system remians symmetrical when a ball is put in the trough; the ball comes to rest in the center, so that exchanging left and right still has no effect. In the trough with a bump, however, the ball takes up a position on one side or the other, and the symmetry is inevitably broken. Similarly, a prequark theory that has a chiral symmetry might nonetheless give rise to composite systems that do not observe the symmetry. Showing that a chiral symmetry can definitely remain unbroken is currently the principal challenge in formulating a theory of how prequarks move.
Illustration by Jerome Kuhl
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If a consistent pre quark theory can be worked out, it will still have to pass the test of experiment. First, it is important to establish in the laboratory whether or not quarks and leptons have any internal structure at all. If they do, experiments might then begin to d iscriminate among the various models. The experiments will have to penetrate the unknown realm of d istances smaller than 10-16 centimeter and energies higher than 100 GeV. There are two basic ways to explore this region: by doing experiments with particles accelerated to very high energy and by making precise measurements of low-energy quantities that depend on the physics of events at very small distances.
Experiments of the first kind include the investigation of the weak bosons andthe search for the Higgs particles of the standard model. When such particles can be made in sufficient numbers, a careful look at their properties should reveal much about the physics of very small distances. New accelerators now being planned or built in the U.S., Europe and Japan are expected to yield detailed information about the weak bosons and will also continue the ongoing investigation of the quarks and leptons themselves.
Equally interesting are the high-precision, low-energy experiments. One of these is the search for the decay of the proton, a particle that is known to have an average lifetime of at least 1030 years. Several experiments are now monitoring large quantities of matter, incorporating substantially more than 1030 protons, in an attempt to detect the signals emitted when a proton disintegrates. None of the forces of the standard model can induce such an event, but none of the rules of the standard model absolutely for bids it. Both the grand unified theories and the prequark models, on the other hand, include mechanisms that could convert a proton into other particles that would ultimately leave behind only leptons and photons. If the decay is detected, its rate and the pattern of decay products could offer an important glimpse beyond the standard model.
There is similar interest in the hypothetical process in which a muon emits a photon and is thereby converted into an electron. Again none of the forces of the standard model can bring about an event of this kind, but again too no fundamental law forbids it. Some of the composite models allow the transition and others do not, so that a search for the process might offer a means of choosing among the models. Experiments done up to now put a limit of less than one in 10 billion on the probability that any given muon will decay in this way. Detection of such events and a determination of their rate might illuminate the mysterious distinction between the generations.
A third class of precision experiments are those that continue to refine the measurement of the magnetic moment of the electron and of the muon. Further improvements can be expected both in experimental accuracy and in the associated calculations of quantum electrodynamics. If the results continue to agree with the predictions of the standard model, the limit on the possible size of any quark and lepton substructure will become remoter. If a discrepancy between theory and experiment is detected, it will represent a strong hint that quarks and leptons are not elementary.
It may well be a decade or two before the next level in the structure of matter comes clearly into view (if, again, there is another level). What is needed is a sound theoretical picture, one that is self-consistent, that agrees with all experiments and that is simple enough to explain all the features of the standard model in terms of a few principles and a few fundamental particles and forces. The correct picture, whether it is a grand unified theory or a composite model of the quarks and leptons, may already exist in some embryonic form. On the other hand, it is also possible the correct theory will emerge only from some totally new idea. In the words of Niels Bohr, it may be that our present ideas "are not sufficiently crazy to be correct."
-Originally published: Scientific American 248(4), 53-68. (April 1983)
The Asymmetry between Matter and Antimatter
by Helen R. Quinn and Michael S. Witherell
As far as humans can see into the universe, an essential imbalance strikes the eye. Stars, planets, asteroids, rocks—everything is made of matter. Essentially no antimatter is evident.
Is this imbalance the result of an accident, a chance occurrence during the birth of the universe? Or is it an inevitable outcome of some asymmetry in the laws of nature? Theorists believe that the excess of matter comes from fundamental disparities in how matter and antimatter behave. These differences amount to violations of a symmetry called charge-parity reversal, or CP.
After years of effort, experimental and theoretical physicists have found a natural way for CP symmetry to be broken within the prevailing theory of particle physics, called the Standard Model. Curiously, the amount of CP violation the model predicts is too small to explain the matter excess in the universe.
This finding is a vital clue that not all is well with the Standard Model: unknown factors are very likely at play. Two new accelerators, just now being completed in California and in Japan, will soon begin to probe violations of CP, with the aim of understanding whether the Standard Model needs to be revamped or replaced. These accelerators, which will produce enormous numbers of particles called B mesons, are known as asymmetric B factories. They are the latest tool in the search for physics beyond the Standard Model.
Everything known about the elementary properties of matter is encapsulated in the Standard Model. It describes all the hundreds of observed particles and their interactions in terms of a few types of fundamental constituents: six quarks and six leptons. (The leptons are light particles, such as the electron, the neutrino and their relatives.) In addition, each quark or lepton comes with an antiparticle, which has the same mass but opposite sign for some quantum numbers such as electric charge. These ingredients are arranged in three generations of increasing mass, the first of which provides the primary constituents of matter.
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Particles of the Standard Model
The primary constituents of matter, quarks and leptons, are divided into generations. The first generation contains up and down quarks and antiquarks as well as the electron, a neutrino and their antiparticles. Ordinary matter is made almost exclusively of first-generation particles: an atom’s nucleus contains protons and neutrons, themselves made of up and down quarks. The other generations occurred in the early universe, may still exist in hot environments such as neutron stars and are routinely observed in accelerators.
In addition, the Standard Model contains several particles that transmit force as well as a mysterious and unobserved particle called the Higgs. In the Standard Model the Higgs is responsible for the masses of all particles and for violations in charge-parity symmetry. —H.R.Q. and M.S.W.
Illustration by Slim Films
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The Standard Model describes three kinds of interactions among particles: the familiar electromagnetic force as well as the strong and the weak forces. (For objects of such low mass, gravity is too weak to be of interest.) Strong interactions confine quarks, which are never seen alone, within composite objects such as protons. Weak interactions cause instability—in particular, the slow decays of all the more massi
ve quarks and leptons into objects of lower mass. All these forces are transmitted by specialized particles that also appear in the Standard Model: the photon, the gluon, and the W and Z bosons. Last, the theory requires an as yet unobserved Higgs particle, whose interactions are held responsible for the masses of the quarks and leptons as well as for much of their behavior.
Essential to the story of CP violation is a family of composite objects called mesons. A meson contains one quark and one antiquark, in an equal mixture of matter and antimatter. A set of mesons of great significance is the kaons, or K mesons, which contain a strange quark or antiquark along with up or down quarks and antiquarks. Similar in many respects are the B mesons, which contain a bottom quark or antiquark paired with up or down partners.
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COMPOSITE PARTICLES are either baryons (such as the proton and the neutron) made up of three quarks, or mesons, which contain one quark and one antiquark. The most common meson is a pion, containing up and down quarks and antiquarks. K mesons and B mesons, important to the study of charge-parity violation, contain strange and bottom quarks (or antiquarks), respectively.
Illustration by Slim Films
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Beyond the Standard
Despite its manifold successes in describing the behavior of matter, deep questions remain about the Standard Model. Physicists do not understand the mechanisms that determine the model’s 18 parameters. For the theory to describe the world as we know it, some of those parameters must have very finely tuned values, and no one knows why those values would apply. More fundamentally, we do not understand why the model describes nature at all—why, for instance, should there be exactly three generations of leptons and quarks, no more or less? Finally, aspects of the theory that involve the Higgs particle are all untested. The Large Hadron Collider, now under construction at CERN, the European laboratory for particle physics near Geneva, will, however, allow the Higgs to be observed if its properties are as predicted by the Standard Model. The Higgs is believed to lie behind most of the mysteries of the Standard Model, including the violation of CP symmetry.
A theory of physics is said to have a symmetry if its laws apply equally well even after some operation, such as reflection, transforms parts of the physical system. An important example is the operation called parity reversal, denoted by P. This operation turns an object into its mirror reflection and rotates it 180 degrees about the axis perpendicular to the mirror. In mathematical terms, parity reverses the vectors associated with the object.
A theory has P symmetry if the laws of physics are the same in the parity-reversed world as in the real world. Particles such as leptons and quarks can be classified as right- or left-handed depending on the sense of their internal rotation, or spin, around their direction of motion. If P symmetry holds, righthanded particles behave exactly the same as left-handed ones.
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Reversal of Charge and Parity
Symmetries are vital to the study of physics, and few symmetries are more intriguing than the combination of charge and parity. Charge reversal gives the opposite sign to quantum numbers such as electric charge, changing a particle to its antiparticle. Parity reversal reflects an object and also rotates it by 180 degrees (equivalent to changing the arrow on all vectors associated with the object).
The laws of classical mechanics and electromagnetism are invariant under either of these operations, as are the strong interactions of the Standard Model. The weak interactions, however, are changed by the reversal of either charge or parity.
For many years, it appeared that parity and charge flipped in succession (“charge parity”) was invariant even for weak interactions. Experiments in 1964 shattered this illusion, posing the puzzle of why nature looks different when reflected in the charge-parity mirror. —H.R.Q. and M.S.W.
Illustration by Slim Films
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The laws of electrodynamics and the strong interactions are the same in a parity-reflected universe. But in a famous experiment in 1957 Chien-Shiung Wu of Columbia University and her collaborators found that the weak interactions are very different for particles of different handedness. Peculiarly, only left-handed particles can decay by means of the weak interaction, not right-handed ones. Moreover, so far as we know there are no left-handed neutrinos: these particles are always right-handed. Because neutrinos have only weak interactions with the rest of the universe, this asymmetry is attributed to the weak force. So the weak force violates P.
Another basic symmetry of nature is charge conjugation, or C. This operation changes the quantum numbers of every particle into those of its antiparticle. Charge symmetry is also violated in weak interactions: antineutrinos are not left-handed, only right-handed.
Theorists combine C and P to get the operation CP, which turns all particles into their antiparticles and also reverses the direction of all vectors. When subjected to CP, the left-handed neutrino becomes a right-handed antineutrino. Not only does the right-handed antineutrino exist, but its interactions with other particles are the same as they are for lefthanded neutrinos. So although charge and parity symmetry are individually broken by neutrinos, in combination their dictates would seem to be obeyed.
Much to the surprise of physicists, the story of CP turned out to be far from simple. A mathematical theorem proved in 1917 by German mathematician Emmy Noether states that every symmetry implies the existence of a related quantity that is conserved, or immutable. For instance, the fact that spacetime is the same in all directions—that is, has rotational symmetry—leads to the conservation of angular momentum. Noether’s theorem implies that if charge parity were an exact symmetry of nature, then a quantity called CP number would be conserved.
CP Violated
A particle and its antiparticle moving in opposite directions with equal energies form a pair with charge-parity symmetry: the CP operation does not change the system (taken as a whole), except that its mathematical representation acquires an overall factor. This factor is the CP number.
Either C or P, if acting twice on a system, returns it to the original state. This property is expressed as C2 = P2 = 1 (where 1, the identity operation, imparts no change at all). As a result, the CP number can be only +1 or –1. If nature has perfect charge-parity symmetry, Noether’s theorem rules that no physical state with CP number –1 can transform into a state with CP number +1.
Consider the electrically neutral kaons. The K0 consists of a down quark and an antistrange quark, whereas the anti-K0 consists of an antidown quark and a strange quark. Because CP transposes quarks and antiquarks, it would turn each kaon into the other instead of leaving it unchanged. Hence, neither of these kaons has a definite CP number. Theorists can, however, construct a pair of kaons with definite CP numbers by superposing the wave functions for K0 and anti-K0. By the rules of quantum mechanics, these mixtures correspond to real particles and have definite mass and lifetime.
The conservation of CP number would explain an odd detail: the two “combination” kaons, though apparently similar, differ in their life spans by a factor of about 500. The kaon with CP number +1 can change to two pions, a state that has the same CP number. This decay proceeds rapidly, because the kaon is massive enough to yield two pions readily. But the kaon with CP number –1 can decay only to another state with CP number –1: three pions. This latter breakdown takes time, because the kaon has barely enough mass to generate three pions. So when physicists found a long-lived kaon in addition to a short-lived one, they acquired strong evidence that the combination kaons obeyed CP symmetry.
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NEUTRAL KAONS, or K mesons, are observed to have two very different life spans. One type of kaon decays quickly into two pions, whereas the other decays slowly into three pions. The different behavior comes from the two kaons having opposite chargeparity symmetry. On rare occasions, however, the second type of kaon also decays to two pions, proving that charge parity can be violated.
Illustration by Slim Films
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This tidy picture was shattered in 1964, when in a groundbreaking experiment at Brookhaven National Laboratory on Long Island, James Christenson, James Cronin, Val Fitch and René Turlay observed that about one out of every 500 of the long-lived kaons (those with CP number –1) decays into two pions. If CP were an exact symmetry of nature, it would forbid such a decay. Few experiments in particle physics have produced a result as surprising as this one. Theorists found it hard to see why CP symmetry should be broken at all and even harder to understand why any imperfection should be so small.
In 1972 Makoto Kobayashi and Toshihide Maskawa of Nagoya University showed that charge parity could be violated within the Standard Model if three or more generations of quarks exist. As it happened, only two generations of quarks—the first, containing the up and down, and the second, with the strange and charm—were known at the time. So this explanation began to gain currency only when Martin L. Perl and others at the Stanford Linear Accelerator Center (SLAC) spied τ (tau) leptons, the first particles of the third generation, in 1975. Two years later experimenters at Fermi National Accelerator Laboratory in Batavia, Ill., found the bottom quark. But only recently, with the top quark being nailed down, also at Fermilab, has the third generation been completed.