The Neutron and Its Role in Cosmology and Particle Physics
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The neutron and its role in cosmology and particle physics Dirk Dubbers* Physikalisches Institut, Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany Michael G. Schmidt † Institut für Theoretische Physik, Universität Heidelberg, Philosophenweg 16, D-69120 Heidelberg, Germany arXiv 18 May 2011, extended version of: Reviews of Modern Physics, in print. Experiments with cold and ultracold neutrons have reached a level of precision such that problems far beyond the scale of the present Standard Model of particle physics become accessible to experimental investigation. Due to the close links between particle physics and cosmology, these studies also permit a deep look into the very first instances of our universe. First addressed in this article, both in theory and experiment, is the problem of baryogenesis, the mechanism behind the evident dominance of matter over antimatter in the universe. The question how baryogenesis could have happened is open to experimental tests, and it turns out that this problem can be curbed by the very stringent limits on an electric dipole moment of the neutron, a quantity that also has deep implications for particle physics. Then we discuss the recent spectacular observation of neutron quantization in the earth’s gravitational field and of resonance transitions between such gravitational energy states. These measurements, together with new evaluations of neutron scattering data, set new constraints on deviations from Newton’s gravitational law at the picometer scale. Such deviations are predicted in modern theories with extra-dimensions that propose unification of the Planck scale with the scale of the Standard Model. These experiments start closing the remaining “axion window” on new spin- dependent forces in the submillimeter range. Another main topic is the weak-interaction parameters in various fields of physics and astrophysics that must all be derived from measured neutron decay data. Up to now, about 10 different neutron decay observables have been measured, much more than needed in the electroweak Standard Model. This allows various precise tests for new physics beyond the Standard Model, competing with or surpassing similar tests at high-energy. The review ends with a discussion of neutron and nuclear data required in the synthesis of the elements during the “first three minutes” and later on in stellar nucleosynthesis. DOI: xx,xxxx/RevModPhys.xx.xxx PACS number(s): 12.15.-y, 12.60.-i, 14.20.Dh, 98.80.-k *[email protected] † [email protected] 1 CONTENTS Sections marked with an asterisk* contain complementary material not included in the RMP article. I. Introduction 3 C. Neutron constraints on new forces and on A. Background 3 dark matter candidates 38 B. Scope of this review 4 1. Constraints from UCN gravitational levels 38 II. History Of The Early Universe 7 2. Constraints from neutron scattering 40 III. Baryogenesis, CP Violation, and the Neutron 3. Constraints on spin-dependent new forces 41 Electric Dipole Moment 9 D. More lessons from the neutron mass 42 A. Baryogenesis 9 1. Gravitational vs. inertial mass 42 1. The dominance of matter over antimatter 9 2. Neutron mass, and test of E= mc 2 42 2. Some quantities of the Standard Model 10 3. The fine structure constant 43 3. First Sakharov criterion: Violation of 4. More experiments on free falling neutrons 43 baryon number 11 VI. The Electroweak Standard Model and the 4. Second criterion: CP Violation 12 β-Decay Of Free Neutrons 44 5. Third criterion: Nonequilibrium 13 A. Neutron decay theory 44 6. Baryogenesis in the Standard Model (SM) 13 1. The example of electromagnetism 44 7. Baryogenesis beyond the Standard Model 14 2. Weak interactions of leptons and quarks 45 B. Electric dipole moments (EDMs) 16 3. Weak interactions of nucleons 46 1. EDMs and symmetry violations 16 4. The 20 and more observables accessible 2. Neutron EDM experiments 17 from neutron decay 48 3. *Upcoming neutron EDM experiments 20 5. Rare allowed neutron decays 50 4. Limits on atomic and particle EDMs 23 B. Experiments in neutron decay 51 C. EDMs in particle physics and cosmology 24 1. The neutron lifetime 51 1. EDMs in the Standard Model 24 2. Correlation coefficients 55 2. EDMs in supersymmetry 26 3. *Upcoming neutron decay experiments 59 3. Other approaches 29 4. Comparison with muon decay data 60 4. Conclusions on EDM constraints 30 C. Weak interaction results and the Standard D. The other electromagnetic properties of the Model 60 neutron 30 1. Results from neutrons and other particles 60 1. Electric charge 30 2. Results from nuclear β -decays 61 2. Magnetic monopole moment 30 3. Unitarity of the CKM matrix 62 3. Magnetic dipole moment 31 4. *Induced terms 63 4. Mean square charge radius 31 D. Symmetry tests beyond the SM 64 5. Electromagnetic polarizability 31 1. Is the electroweak interaction purely IV. Baryon Number Violation, Mirror Universes, V− A ? 64 and Neutron Oscillations 32 2. Was the universe left-handed from the A. Neutron-antineutron oscillations 32 beginning? 67 B. Neutron-mirrorneutron oscillations 32 3. Time reversal invariance 68 V. Extra Dimensions, New Forces, and the Free VII. The Role of the Neutron in Element Formation 69 Fall of the Neutron 33 A. Primordial nucleosynthesis 69 A. Large extra dimensions 33 1. The neutron density in the first three 1. The hierarchy problem 33 minutes 69 2. Bringing down the Planck mass 34 2. Nuclear reactions in the expanding universe 71 3. Competing models 35 3. Neutron lifetime, particle families, and B. Gravitational quantum levels 35 baryon density of the universe 71 1. Neutron states in the gravitational potential 35 4. Light-element abundances 72 2. Ultracold neutron (UCN) transmission B. Stellar nucleosynthesis 73 experiments 36 1. Stellar nucleosynthesis and the s-process 73 3. Observation of UCN gravitational wave 2. Explosive nucleosynthesis and the r- functions and resonance transitions 37 process 74 VIII. Conclusions 75 2 I. INTRODUCTION By pure coincidence, the size of the neutron’s mass is such that, at thermal velocities, their de-Broglie In the morning, a weight watcher sees the mass of wavelengths are in the Ångstrom range and thus match typically 40 kilograms of protons, 32 kilograms of typical atomic distances as well. Hence, neutrons can neutrons, and 22 grams of electrons. Neutrons are as give both structural information (by elastic diffraction) ubiquitous on earth as are protons and electrons. In the and dynamical information (by inelastic spectroscopy). laboratory, we can produce free electrons simply by Neutrons are the only penetrating particles with this heating a wire and free protons by igniting a hydrogen feature, which enables them to tell us both “where the gas discharge, but things are not so easy for neutrons. atoms are”, and, at the same time, “what the atoms Neutrons are tightly bound in nuclei and free neutrons do”. Dynamical information is provided separately for are unstable with a lifetime of about 15 minutes, so any length scale as chosen by the experimenter via there are very few free neutrons in our environment. momentum transfer (selected, for example, by the To set neutrons free we need a nuclear reaction, a MeV scattering angle). In the specialist’s language: rather than an eV process, and to obtain neutron fluxes Neutrons provide us with the full atomic space-time competitive for modern scientific experimentation we correlation functions of the material under study, both need big nuclear installations. for self-correlations of the atoms (from incoherent As its name says, the outstanding property of the neutron scattering) and for their pair-correlations (from neutron is its electric neutrality. In fact, the neutron is coherent neutron scattering). the only long-lived massive particle with zero charge By another unique coincidence (also linked to the (if we neglect the small neutrino mass). This makes neutron mass via the nuclear magneton), the size of the neutrons difficult to handle, as one cannot easily neutron’s magnetic moment is such that amplitudes of accelerate, guide, or focus them with electric fields. magnetic electron scattering and nuclear scattering are Furthermore, one can detect slow neutrons only in a of similar magnitude. This makes neutrons useful also destructive way, via nuclear reactions that produce for structural and dynamical studies of the magnetic secondary ionizing reaction products. Once detected, properties of matter. On the instrumentation side, the neutrons are gone, in contrast to charged particles, magnetic interaction enables powerful techniques for which leave an ionization track that we can detect in a spin-polarization, magnetic guidance, or confinement destructive-free manner. Another disadvantage of of neutrons. Furthermore, slow neutrons generally neutrons is that fluxes available are much lower than distinguish easily between elements of similar atomic photon, electron, or proton fluxes. number, and often between isotopes of the same So why take the pain and work with neutrons when element, the latter feature making powerful contrast they are so difficult to produce and to handle? The variation techniques applicable. All these are answer is, although the neutron’s neutrality is a properties that neutrons have and x-rays do not. burden, it is also its greatest asset (we find a similar Neutron-particle physicists should be well acquainted situation for the neutrino: its neutrality to all charges with these particularities of neutron-material science if except the weak charge is both its greatest asset and, they want to exploit fully the potential of the field. for the experimenter, a heavy burden). When the usually dominant long-range Coulomb interaction is not present, other more subtle effects come to the A. Background forefront. Being hadrons (i.e., particles composed of quarks), neutrons respond to all known interactions: Neutron-particle physics is part of the growing field the gravitational, the weak, the electromagnetic, and of low-energy particle physics.