Grands principes de symétrie à l'épreuve de l'expérience

Laboratoire de Physique Nucléaire, Université de Montréal, Case Postale 6128, Succursale « A », Montréal, Québec, Canada H3C 3J7

Résumé

Cet article est une version révisée d'un cours donné à l'Ecole Joliot-Curie de Physique Nucléaire, en 1986, et destiné à des physiciens nucléaires. Le texte a été remanié et complété pour le rendre plus conforme à un article de synthèse. Après un court préambule (chapitre 1), on rappelle quelques définitions et théorèmes dans le chapitre 2. Une courte introduction historique est donnée dans le chapitre 3. Dans le chapitre 4 on expose le formalisme du modèle standard qui servira de cadre théorique pour l'ensemble de l'article. Le chapitre 5 est consacré à la violation de la parité, due aux interactions faibles, et qui se manifeste dans divers domaines : physique nucléaire, physique des particules, mais aussi physique atomique. On passe en revue les effets dus aux courants faibles chargés et neutres, en insistant sur les aspects expérimentaux. Le chapitre 6 est consacré à la violation de l'invariance CP, qui n'a été observée que dans le système des mésons K. En postulant l'invariance CPT on est conduit à se poser la question de la validité de l'invariance sous le renversement du temps. On discute des expériences qui ont été effectuées pour rechercher une violation de cette invariance, en physique nucléaire notamment. Dans le chapitre 7 on introduit à la grande unification des interactions faibles, électromagnétiques et fortes, dont une conséquence spectaculaire est la nonconservation du nombre baryonique, avec comme conséquences expérimentales l'instabilité du nucléon et les oscillations neutron-antineutron. Le chapitre 8 s'adresse à la violation du nombre leptonique. Cette question est importante dans le cadre des théories unificatrices et elle se rattache à la nature du neutrino (particule de Dirac ou particule de Majorana massive). La recherche de la double désintégration β est devenue une activité expérimentale de très grande importance. Le chapitre 9 traite de la non-conservation des nombres leptoniques partiels. Dans la plupart des extensions du modèle standard on met en évidence plusieurs mécanismes de conversion d'un lepton en un lepton d'une autre famille, avec comme conséquences expérimentales : les désintégrations μ → e γ , μ → e γ γ et μ → e e e la conversion muon-électron dans un noyau, les oscillations de neutrinos. La possibilité d'obtenir une résonance lors de l'oscillation des neutrinos dans la matière a des conséquences importantes pour l'astrophysique. En principe, l'isospin n'était pas au menu du cours, puisque traité par un autre professeur. On a cependant ajouté, à la demande de l'éditeur, un chapitre sur les expériences recherchant une violation de la symétrie de charge dans les forces nucléaires (chapitre 10).

Abstract

This review article has its origin in a series of lectures (P, C, T, baryon number, lepton numbers) given at the « Joliot-Curie » School of Nuclear Physics (Maubuisson, France), in September 1986. The manuscript has been modified, a chapter on charge symmetry has been added and the bibliography has been updated. The lectures were intended for a nuclear physics audience, something which is reflected in the way this article has been written. The contribution of nuclear physics to the understanding of the fundamental interactions is stressed, as well as the relationship of nuclear to particle physics. The definitions of the various symmetry operations in question are recalled briefly in chapter 2. A short historical account is given in chapter 3. Since the presentation of the various parts of the article requires a theoretical framework, the « Standard Model » is briefly described in chapter 4. Its limitations are discussed together with the various avenues which have been explored to generalise it and to cure its diseases. The relations between quark families and the problem of neutrino masses are discussed. Chapter 5 contains a detailed presentation of the various manifestations of parity violation : existence of pseudoscalar observables, violation of selection rules. Parity violation is maximal in the charged current of weak interactions. In the pure leptonic sector it manifests itself in the asymmetry of electrons (positrons) in muon decay with respect to the muon spin. In the semileptonic sector (lepton-quark coupling) it is responsible for the asymmetry in the β-decay of oriented nuclei, the β-circularly polarized γ correlation and the longitudinal polarization of electrons (positrons). Together with the determination of the helicity of the neutrino these observations lead to the V - A theory of the weak interactions. The Standard Model also predicts a weak neutral current which affects the lepton and quark sectors. But for processes involving neutrinos the weak neutral current interferes with the electromagnetic current in a way which is predicted by the Standard Model and introduces a very important parameter, the Weinberg angle θ w. Purely leptonic processes include Møller and Bhabha scatterings, the reactions e+ e- → μ+ μ - and e+ e- → τ+ τ -, where spatial asymmetries and polarizations can be observed, especially at high energies. In the semileptonic sector (lepton-quark coupling) the weak neutral current interferes with the electromagnetic current to produce parity-violating effects. Atomic physics is affected and very tiny effects have been observed in heavy atoms, such as a rotation of the plane of polarization when light passes through an atomic vapor. The scattering of longitudinally polarized electrons from nuclei gives a forward-backward asymmetry which has been observed, in beautiful agreement with Standard Model predictions. According to the Standard Model there is a weak interaction between quarks which gives rise to a violation of parity in nuclear forces and nuclear physics. It takes very careful and difficult experiments to extract the very tiny signal of the weak interactions in the huge background of the strong interaction. A well known selection rule in α decay is violated and the forbidden α transition from 16O (2- , 8.87 MeV) to the ground state of 12C has been observed. Electromagnetic selection rules are modified and the interference between, for instance, M1 and E1 monopoles, gives a circular polarization to photons emitted from non-oriented nuclei. After neutron-proton capture the emitted photons are circularly polarized. With oriented nuclear states (as obtained in polarized neutron capture) one observes an asymmetry in the emission of γ-rays. Parity violation has also been studied in nucleon-nucleon scattering and few-body reactions. The theory is a difficult one since there is a long way to go from the Standard Model to complicated systems such as nucleons and a fortiori nuclei. A qualitative agreement emerges from the comparison between experiment and theory. The origin of parity violation is not explained by the Standard Model. It is not known whether parity violation is a fundamental property of nature or if it is a low-energy phenomenon. It is possible to construct gauge theories which are basically symmetric but where this symmetry is spontaneously broken at some energy scale. In such theories there are additional weak bosons of right-handed character. These theories make interesting predictions, even at low energy (deviations from V — A) but they will be tested efficiently with the high energy electron-positron colliders. Parity violation has useful applications outside the field of particle and nuclear physics : muon spin rotation is widely used by solid state physicists and chemists. Chapter 6 discusses briefly CP violation in the neutral-kaon system and dwells on the experiments aimed at the verification of time-reversal invariance in various parts of physics. Time reversal invariance has several consequences for nuclear reactions. The theorem of detailed balance relates the cross-sections of inverse reactions. The polarization-asymmetry theorem relates the polarization observed in a reaction (induced by unpolarized particles) and the asymmetry observed in the inverse reaction (induced by polarized particles). No convincing evidence for time-reversal noninvariance has been obtained. In neutron and nuclear beta decay some correlations involving one spin and two momenta are odd under time reversal. Upper limits have been obtained for the phase between the V and A coupling constants in weak interactions. Electromagnetic transitions in nuclei can be used to test time reversal in various experimental conditions (oriented nuclei, measurement of linear polarization of γ-rays). One gets upper limits for the phase between electromagnetic nuclear matrix elements. All experiments are in agreement with no violation of time-reversal invariance. The most interesting observable seems to be the electric dipole moment of the neutron, which vanishes under time-reversal invariance (assuming parity violation). The magnitude of the theoretical predictions varies considerably, therefore the electric dipole moment of the neutron constitutes a very valuable test of time-reversal invariance. The conservation of parity and time reversal in the strong interaction raises a delicate problem in Quantum Chromodynamics. In order to get rid of parity and time reversal violating terms in the QCD Lagrangian one invokes a new symmetry which introduces a light pseudoscalar particle, the axion. This particle has been searched for but not found. Recent findings in heavy-ion collisions (the famous e+ -e- pairs) have probably nothing to do with axions. Chapter 7 deals with baryon number nonconservation. Grand unification theories have been introduced to cure several deficiencies of the Standard Model. One of the most dramatic consequences of these theories is the violation of baryon number conservation, resulting in the instability of the nucleon and other effects like neutron-antineutron oscillations. The economical model based on the unification group SU(5) fails in the prediction of the proton lifetime. Alternative unification groups have been proposed. Proton decay and neutron-antineutron searches are fundamental experiments which are pushed very strongly with a variety of experimental techniques. Lepton number nonconservation is the subject of chapter 8. The search for nuclear neutrinoless double beta decay is another activity which has become important in the attempt to elucidate the nature of the neutrino. Neutrinoless double beta decay can only occur with Majorana neutrinos if these neutrinos are massive and/or weak currents are not exactly V — A. In the context of the gauge theories the observation of this process would be a proof of massive Majorana neutrinos. Various isotopes can be used to search for double beta decay (with or without neutrinos) and detectors can be large-volume GeLi crystals, Time Projection Chambers, etc... Coincidences between beta particles and γ-rays are useful for shedding information on the mechanism responsible for lepton-number violation. A new particle, the majoron, has been invented. It is a Goldstone boson associated with the breaking of lepton-number symmetry. No convincing observation of neutrinoless double beta decay has been reported so far. Whereas quark families communicate via the Kobayashi-Maskawa matrix, leptons seem to obey separate conservation laws of lepton number (or lepton-flavor). This problem, which has received much attention recently, is presented in chapter 9. Searches for μ → eγ, μ → e γ γ, μ, → eee and muon-to-electron conversion in a nucleus have reached very low upper limits (down to the 10-11-10-12 range), although gauge theories provide several natural flavor-violating mechanisms. Neutrino oscillations in a vacuum open another window on the problem of neutrino masses and experiments are pursued actively at high-flux nuclear reactors and high-energy accelerators. There are some indications for neutrino oscillations but experimental results are still in conflict. The old Wolfenstein theory of neutrino oscillations in matter has been exploited by Mekheyev and Smirnov who showed that under suitable conditions the mixing angle between neutrinos of different flavors goes through a resonance. This gives rise to the MSW effect which has an important bearing on the solar neutrino problem. There is a good hope that nuclear forces will be eventually explained by Quantum Chromodynamics, in terms of quarks and gluons. Therefore it is important to establish the symmetry properties of these forces and in particular to study possible violations of isospin symmetry to a high level of accuracy. The subject of isospin is dealt with in a companion article by Galès and Van Giai, Ann. Phys. Fr. 12 (1987). Chapter 10 discusses a few tests of charge symmetry of nuclear forces.