Annual Review of Nuclear and Particle Science - Volume 63, 2013

Wolfgang K.H. Panofsky is remembered as the legendary founder and first director of SLAC, the Stanford Linear Accelerator Center. He devoted his life to teaching and research in accelerator and particle physics, to science policy, to his work as a science advisor to both the US and foreign governments, and to world peace as an expert on arms control and international security. He was admired by all who had a chance to meet and interact with him, and who simply called him Pief.

Great strides have been made in the understanding of bound states of the bottom quark, b, and its antiquark, , since the discovery of the first Ψ resonances in 1977. These bound states, known as bottomonium, have a rich spectrum whose masses and transition amplitudes shed valuable light on the strong interactions. We review some recent developments in bottomonium physics, including the discovery of the spin-singlet states and , the first D-wave states, one or more candidates for spin-triplet excitations, and above-threshold states with strong transitions to states below threshold. We also present information about transitions, production, and signatures of new physics.

This article reviews the experimental design, analysis, and results of the LSND and MiniBooNE experiments. Their primary goal was to effect sensitive searches for neutrino oscillations in the mass region with Δm²≈1 eV². The two experiments are complementary, so comparing their results can yield additional information with respect to models with sterile neutrinos. Both experiments obtained evidence for oscillations, and MiniBooNE also observed a νμ→νe excess. We also consider the results within the global context of sterile neutrino oscillation models. The final data sets require a more extended model than the simple single–sterile neutrino model imagined at the time that LSND drew to a close and MiniBooNE began. There are apparent incompatibilities between data sets in models with two sterile neutrinos that may be explained with variations within the systematic error. Overall, models with two (or three) sterile neutrinos seem to succeed in fitting the global data, and they make interesting predictions for future experiments.

We review recent developments in axion cosmology. Topics include axion cold dark matter, axions from topological defects, axion isocurvature perturbation and its non-Gaussianity, and axino/saxion cosmology in a supersymmetric axion model.

I describe the time-dependent superfluid local density approximation, which is an adiabatic extension of the density functional theory to superfluid Fermi systems and their real-time dynamics. This new theoretical framework has been used to describe several phenomena in cold atomic gases and nuclear collective motion: excitation of the Higgs modes in strongly interacting Fermi superfluids, generation of quantized vortices, crossing and reconnection of vortices, excitation of the superflow at velocities above the critical velocity, excitation of quantum shock waves and domain walls in the collisions of superfluid atomic clouds, and excitation of collective states in nuclei.

We review collective flow, its anisotropies, and its event-to-event fluctuations in relativistic heavy-ion collisions, as well as the extraction of the specific shear viscosity of quark–gluon plasma from collective flow data collected in heavy-ion collision experiments at RHIC and the LHC. We emphasize the similarities between the Big Bang of our universe and the Little Bangs created in heavy-ion collisions.

Type Ia supernovae are valuable distance indicators for cosmology and an important component in our understanding of binary stellar evolution; the elements they synthesize and eject contribute significantly to the origin of the elements. During the past year, the discovery and intensive worldwide observational follow-up of the brightest Type Ia supernova in a generation, SN 2011fe, have greatly improved our understanding of these cataclysmic stellar deaths. The supernova was discovered when it had only one one-thousandth of the brightness it would achieve at maximum light when it was observed by thousands around the world through binoculars and small telescopes. Although several long-held assumptions have been confirmed by this discovery, this supernova has forced us to question the origin of these events and to rethink and improve the way we model them through detailed simulations. We review our current understanding of Type Ia supernovae and discuss how SN 2011fe has both improved and challenged our understanding of these most spectacular explosions.

The extremely precise extraction of the proton radius obtained by Pohl et al. from the measured energy difference between the 2P and 2S states of muonic hydrogen disagrees significantly with that extracted from electronic hydrogen or elastic electron–proton scattering. This discrepancy is the proton radius puzzle. In this review, we explain the origins of the puzzle and the reasons for believing it to be very significant. We identify various possible solutions of the puzzle and discuss future research needed to resolve the puzzle.

CP-violating phenomena and rare decays of B⁰s mesons offer interesting probes to test the quark-flavor sector of the Standard Model. In view of the many new results reported, in particular from the Large Hadron Collider, this topic received a great deal of attention in 2012. We provide an overview of the most recent experimental results and new theoretical developments, and we discuss the prospects for future exploration of the B⁰s meson system.

We review the status of precision measurements of weak neutral-current interactions, mediated by the Z⁰ boson, at Q²≪M²Z. They can be used to extract values for the weak mixing angle , a fundamental parameter of the electroweak sector of the Standard Model. Apart from providing a comprehensive test of the electroweak theory at the quantum-loop level, such measurements allow indirect access to New Physics effects at and beyond the TeV scale. After providing a theoretical introduction and a brief overview of the three most precise low-Q² weak mixing angle determinations, we describe the ongoing experimental program and prospects for future, more sensitive studies. We also compare the sensitivities of planned and proposed measurements with physics beyond the Standard Model.

Large (up to ∼100 kt) liquid argon (LAr) time-projection chamber detectors are presently being considered for proton decay searches and neutrino astrophysics, as well as for far detectors for the next generation of long-baseline neutrino oscillation experiments that aim to determine neutrino mass hierarchy and search for CP violation in the leptonic sector. These detectors rely on the capabilities to assemble large volumes of LAr in ultrahigh-purity conditions, possibly in an underground environment, and to achieve relatively long drifts for the ionization charge. Several proposals have been developed, each of which takes a different approach to the design of the cryogenic vessels and has different scales of modularity to reach the final mass dictated by physics. New detector concepts, with innovative designs of readout electronics and novel methods for the readout of the ionization charge and scintillation light, have been proposed.

We review the state of the art of the determination of the parton substructure of the nucleon, as expressed in terms of parton distribution functions (PDFs) and probed in high-energy lepton–hadron and hadron–hadron collisions. We also assess their implications for current precision collider phenomenology, in particular at the Large Hadron Collider (LHC). We review the theoretical foundations of PDF determination: how cross sections are expressed in terms of PDFs by use of perturbative QCD factorization and evolution, the methodology used to extract PDFs from experimental data, and how different physical processes can be used to constrain different PDFs. We summarize current knowledge of PDFs and the limitations in accuracy that this knowledge currently entails for the computation of hadron collider processes, in particular at the LHC. We discuss the current main sources of theoretical and phenomenological uncertainties and the direction of progress toward their reduction in the future.

We review the techniques used to detect photons in particle physics experiments. In particular, we describe the principles of operation, the current state of development, and progress made with vacuum photodetectors, semiconductor and hybrid sensors, and gaseous photon detectors. We also discuss applications in ongoing and planned future experiments.

For decades, the unnaturalness of the weak scale has been the dominant problem motivating new particle physics, and weak-scale supersymmetry has been the dominant proposed solution. This paradigm is now being challenged by a wealth of experimental data. In this review, we begin by recalling the theoretical motivations for weak-scale supersymmetry, including the gauge hierarchy problem, grand unification, and WIMP (weakly interacting massive particle) dark matter, and their implications for superpartner masses. These are set against the leading constraints on supersymmetry from collider searches, the Higgs boson mass, and low-energy constraints on flavor and CP violation. We then critically examine attempts to quantify naturalness in supersymmetry, stressing the many subjective choices that influence the results both quantitatively and qualitatively. Finally, we survey various proposals for natural supersymmetric models, including effective supersymmetry, focus point supersymmetry, compressed supersymmetry, and R-parity-violating supersymmetry, and summarize their key features, current status, and implications for future experiments.

We describe the discoveries of new superheavy nuclei (a) with Z=107–112 produced in cold fusion reactions between ²⁰⁸Pb and ²⁰⁹Bi and beams of A > 50 and (b) with Z=113–118 in hot fusion reactions between actinide nuclei and ⁴⁸Ca. We also discuss the facilities used in these measurements. We compare the behavior of the α-decay energies and half-lives, spontaneous fission half-lives, cross sections, and excitation functions with expectations from theoretical calculations. Finally, we outline future research directions, including studies of the detailed properties of nuclei synthesized at higher yields, searches for new elements with Z=119 and 120, and developments of new facilities.

The past decade has seen very significant progress in the measurement of the cross section for the annihilation process e⁺e⁻→hadrons. Different experimental approaches have helped achieve a more complete and fairly consistent picture. I provide a short review of the procedures used to obtain a continuous description of cross sections combining all available results from experiments. These data are necessary input to calculations of hadronic vacuum polarization effects that use dispersion integrals in an energy regime in which perturbative QCD cannot be applied. Applications include the hadronic contribution to the muon magnetic anomaly prediction in the Standard Model and the running of the electromagnetic coupling through the scale dependence of α(s), particularly its value at the Z boson mass, which is necessary for electroweak precision tests. Another application is for QCD studies that aim to accurately determine the strong coupling and its running, αS(s), and the heavy quark masses.

For more than 25 years, the Tevatron was the highest-energy accelerator in the world. It provided the first access to particle collisions beyond 1 TeV and achieved an ultimate performance that was a factor of 400 beyond the original design goals. This article reviews the many formidable challenges that were overcome, and the knowledge gained, in building, operating, and improving the Tevatron during its lifetime. These challenges included the first operation of an accelerator based on superconducting magnets; production of antiprotons in sufficient numbers to support a usable luminosity; management of beam–beam, intrabeam, and other collective effects; novel manipulations of the beam longitudinal phase space; and development and application of a wide variety of innovative technologies. These achievements established the legacy of the Tevatron as the progenitor of all subsequently constructed high-energy hadron colliders.

The proton–antiproton Tevatron Collider at Fermilab began physics operation in 1988, and by its shutdown in 2011 it had delivered more than 10 fb⁻¹ of data to the two general-purpose detectors CDF and D0. Thus far, these experiments have published more than 800 papers studying the character of the strong nuclear force, measuring the properties of hadrons containing heavy quarks, elucidating the nature of the electroweak force, discovering and measuring the properties of the top quark, seeking evidence for the Higgs boson, and searching for new phenomena beyond the standard paradigm of particle physics. We summarize the results that define the physics legacy of the Tevatron program.

Two-neutrino double-β decay is a radioactive process with the longest lifetime ever observed. It has been a subject of experimental research for more than 60 years and remains an important topic in modern nuclear and particle physics. This review examines the process in detail, covers its theoretical and experimental aspects, and describes the results obtained so far and future challenges.

We review the status of the major experiments that are searching for charged lepton flavor violation. We present and discuss recent results and prospects for future projects in the framework of theories beyond the Standard Model.