Annual Review of Nuclear and Particle Science - Volume 61, 2011

The study of π⁰ decay has played an important role in the development of particle physics: The π⁰→γγ decay provides key insights into the anomaly sector of quantum chromodynamics. In this review, the historical progression of π⁰ discovery, lifetime measurements, and theory are presented. A new measurement of the π⁰ radiative width via the Primakoff effect has been made at JLab. The result, Γ(π⁰→γγ)=7.82±0.14(stat.)±0.17(syst.) eV, is a factor of 2.1 more precise than the currently accepted value, and it is in agreement with the chiral anomaly prediction and with next-to-leading-order chiral perturbation theory calculations. Primakoff experiments at higher energies to measure the η and η′ radiative widths are also discussed.

Precision measurements in nuclear β decay provide sensitive means to test the structure of the standard electroweak model; they also allow the determination of fundamental properties in processes that involve the lightest quarks. The primary aim of such tests is to find deviations from the Standard Model predictions that may indicate new physics. We review here the status of precision measurements in nuclear β decay, including selected experiments in neutron decay, and discuss recent results and developments. Among these developments are the consideration of nuclear mirror transitions as a new window in which to test the unitarity condition of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, measurements of angular correlation observables to search for exotic couplings, and tests of discrete symmetries.

Big bang nucleosynthesis (BBN) theory, together with the precise WMAP cosmic baryon density, makes tight predictions for the abundances of the lightest elements. Deuterium and ⁴He measurements agree well with expectations, but ⁷Li observations lie below the BBN+WMAP prediction by a factor of three to four. This 4–5-σ mismatch constitutes the so-called cosmic lithium problem; disparate solutions are possible. First, astrophysical systematics in the observations could exist but are increasingly constrained. Second, nuclear physics experiments provide a wealth of well-measured cross-section data, but ⁷Be destruction could be enhanced by unknown or poorly measured resonances. Third, physics beyond the Standard Model could alter the ⁷Li abundance, although deuterium and ⁴He must remain unperturbed. In this review, we discuss such scenarios, highlighting decaying supersymmetric particles and time-varying fundamental constants. Present and planned experiments could reveal which (if any) of these proposed solutions is correct.

I give an overview of the effects of neutrino masses in cosmology, focusing on the role they play in the evolution of cosmological perturbations. I discuss how recent observations of the cosmic microwave background anisotropies and the large-scale matter distribution can probe neutrino masses with greater precision than current laboratory experiments. I describe several new techniques that will be used to probe cosmology in the future, as well as recent advances in the computation of the nonlinear matter-power spectrum and related observables.

Following the first full year of Large Hadron Collider (LHC) data taking, the Worldwide LHC Computing Grid (WLCG) computing environment built to support LHC data processing and analysis has been validated. In this review, I discuss the rationale for the design of a distributed system and describe how this environment was constructed and deployed through the use of grid computing technologies. I discuss the experience with large-scale testing and operation with real accelerator data, which shows that expectations have been met and sometimes exceeded. The computing system's key achievements are that (a) the WLCG infrastructure is distributed and makes use of all the dispersed resources, (b) the experiments' computing models are also distributed and can make excellent use of the infrastructure, and (c) the computing system has enabled physics output in a very short time. Finally, I present prospects for the future evolution of the WLCG infrastructure.

Semileptonic decays of B mesons play a critical role in the determination of the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements Vcb and Vub. These two quantities are fundamental parameters of the Standard Model and have to be determined experimentally. Over the past decade, the vast samples of B mesons recorded at the B factories at LEP at Cornell University, KEK at Tsukuba, and SLAC at Stanford University have allowed for detailed studies of semileptonic B decays. These decays proceed via first-order weak interactions; thus, they are expected to be free of non–Standard Model contributions and therefore are well suited for the extraction of the quark-mixing parameters. Differential decay rates are combined with theoretical calculations of hadronization effects, leading to a substantially improved knowledge of |Vcb| and |Vub|. The results are used to constrain the parameters of the CKM matrix and to test the Standard Model predictions for CP-violating effects.

This review summarizes exemplary secondary education and outreach programs of the particle physics community. We examine programs from the following areas: research experiences, high-energy physics data for students, informal learning for students, instructional resources, and professional development. We report findings about these programs' impact on students and teachers and provide suggestions for practices that create effective programs from those findings. We also include some methods for assessing programs.

The field of experimental particle physics has become more sophisticated over time, as fewer, larger experimental collaborations search for small signals in samples with large components of background. The search for and the observation of electroweak single top quark production by the CDF and DØ Collaborations at Fermilab's Tevatron collider are an example of an elaborate effort to measure the rate of a very rare process in the presence of large backgrounds and to learn about the properties of the top quark's weak interaction. We present the techniques used to make this groundbreaking measurement and the interpretation of the results in the context of the Standard Model.

Tracking detectors are of vital importance for most experiments in high-energy and nuclear physics. They are used to determine the charge, momentum, and energy of traversing particles and to allow quark-flavor identification through the reconstruction of secondary vertices. Gaseous and semiconductor detectors are the two main types of tracking detectors; other, more exotic ones are fiber or transition radiation tracking devices. These detectors originated with cloud and bubble chambers in the 1950s and wire chambers in the 1970s, which dominated the field until the 1980s, when silicon sensors were developed. Today, silicon strip and pixel sensors, time-projection chambers, gas electron multipliers, and micromegas define the field. More advanced detector types are described in this review, with an emphasis on application examples and future plans.

This article is devoted to the physics of pairs of electroweak gauge bosons (dibosons) produced at high-energy particle colliders. Particular emphasis is placed on published results on diboson production from the CDF and DØ experiments at the Fermilab Tevatron and the CMS and ATLAS experiments at the CERN Large Hadron Collider (LHC). The study of diboson production at colliders plays an important role in tests of electroweak theory and searches for new phenomena at the TeV-energy scale and beyond. The diboson physics program has been a highlight of Run II at the Fermilab Tevatron. The progress made on analysis methods through the study of Standard Model diboson signals has had a direct and positive impact on the sensitivity of Higgs boson and New Physics searches in the Tevatron experiments. It is anticipated that diboson physics will play an even bigger role in the TeV-scale exploration now under way at the LHC.

The discovery of dark energy by the first generation of high-redshift supernova surveys has generated enormous interest beyond cosmology and has dramatic implications for fundamental physics. Distance measurements using supernova explosions are the most direct probes of the expansion history of the universe, making them extremely useful tools with which to study the cosmic fabric and the properties of gravity at the largest scales. The past decade has seen confirmation of the original results. Type Ia supernovae are among the leading techniques to obtain high-precision measurements of the dark energy equation-of-state parameter and, in the near future, its time dependence. The success of these efforts depends on our ability to understand a large number of effects, mostly of an astrophysical nature, that influence the observed flux at Earth. The frontier now lies in understanding whether the observed phenomenon is due to vacuum energy, despite its unnatural density, or some exotic new physics. Future surveys will address the systematic effects with improved calibration procedures and will provide thousands of supernovae for detailed studies.

Each generation of high-energy physics experiments is grander in scale than the previous—more powerful, more complex, and more demanding in terms of data handling and analysis. The spectacular performance of the Tevatron and the beginning of operations at the Large Hadron Collider have placed us at the threshold of a new era in particle physics. The discovery of the Higgs boson, or another agent of electroweak symmetry breaking, and evidence of new physics may be just around the corner. The greatest challenge in these pursuits is to extract the extremely rare signals, if any, from the huge backgrounds that arise from known physics processes. The use of advanced analysis techniques is crucial in achieving this goal. In this review, I discuss the concepts of optimal analysis, some important advanced analysis methods, and a few examples. The judicious use of these advanced methods should enable new discoveries and produce results with better precision, robustness, and clarity.

Most new particles predicted by theories beyond the Standard Model of particle physics decay to jets and electroweak gauge bosons, W and Z. The search and study of these particles therefore require a solid understanding of the associated production of W/Z and jets. This review provides an introduction to the theoretical and experimental aspects of these processes in the context of the Standard Model. First, we introduce the challenges presented by the calculation of the production properties of W/Z plus jets in hadronic collisions and review the tools developed to quantitatively characterize such final states. Then, we summarize the current experimental results at the Tevatron and LHC colliders and discuss the comparison between the available data and the theoretical predictions.

We review the current status and future prospects of rare kaon and pion decay research programs. Our emphasis is on experimental probes of New Physics beyond the Standard Model via the theoretically pristine decays and precision tests of electron-muon universality. These studies test the Standard Model at the level of its quantum-loop predictions and have the potential to uncover new interactions beyond the O(1,000 TeV) scale.

The study of neutrino oscillations has necessitated a new generation of neutrino experiments that are exploring neutrino-nuclear scattering processes. We focus in particular on charged-current quasi-elastic scattering, a particularly important channel that has been extensively investigated both in the bubble-chamber era and by current experiments. Recent results have led to theoretical reexamination of this process. We review the standard picture of quasi-elastic scattering as developed in electron scattering, review and discuss experimental results, and discuss additional nuclear effects such as exchange currents and short-range correlations that may play a significant role in neutrino-nucleus scattering.

The Tevatron experiments are currently the world's most sensitive experiments for searching for the Higgs boson. They are sensitive enough to find evidence of the Standard Model Higgs boson, if it has a large mass, and are nearly sensitive enough to find evidence at lower mass or to exclude the Standard Model Higgs boson at all masses, if it does not exist. This review describes the recent experimental progress achieved at the Tevatron at a time when the discovery of the Higgs boson is considered imminent.

The Bates Large Acceptance Spectrometer Toroid (BLAST) experiment was operated at the MIT-Bates Linear Accelerator Center from 2003 until 2005. The experiment was designed to exploit the power of a polarized electron beam incident on polarized targets of hydrogen and deuterium to measure, in a systematic manner, the neutron, proton, and deuteron form factors as well as other aspects of the electromagnetic interaction on few-nucleon systems. We briefly describe the experiment, and present and discuss the numerous results obtained.

The Large Hadron Collider (LHC) is the most complex instrument ever built for particle physics research. It will, for the first time, provide access to the TeV-energy scale. Numerous technological innovations are necessary to achieve this goal. For example, two counterrotating proton beams are guided and focused by superconducting magnets whose novel two-in-one structure saves cost and allowed the machine to be installed in an existing tunnel. The very high (>8-T) field in the dipoles can be achieved only by cooling them below the transition temperature of liquid helium to the superfluid state. More than 80 tons of superfluid helium are needed to cool the whole machine. So far, the LHC has behaved reliably and predictably. Single-bunch currents 30% above the design value have been achieved, and the luminosity has increased by five orders of magnitude. In this review, I briefly describe the design principles of the major systems and discuss some initial results.

At energies greater than 10¹⁵ eV, cosmic-ray particles can be measured only indirectly by detecting the extensive showers of secondary particles they create in the Earth's atmosphere. A detailed simulation of these particle showers is needed to reconstruct the properties of the primary particles. Key to understanding extensive air showers is the modeling of hadronic multiparticle production at energies from the particle-production threshold up to 10²⁰ eV—far beyond the reach of man-made accelerators. In this article, we introduce the relation between extensive air showers and hadronic interactions at high energy. We review air shower predictions of commonly used interaction models and discuss their uncertainties. Finally, we illustrate the importance of accelerator measurements for air shower simulation and, complementarily, the information that can be obtained from air showers on particle production.

With the next-generation experiments, flavor physics is fully entering the era of precision measurements. Its focus is shifting from testing the Standard Model to finding and characterizing New Physics contributions. We review the opportunities offered by future flavor experiments and discuss the expected sensitivities of the most important measurements. We also present some examples of measurable deviations from the Standard Model in the flavor sector generated in a selection of New Physics models, which demonstrate the potentially major contribution of precision flavor physics to the effort of going beyond the Standard Model.