Annual Review of Nuclear and Particle Science - Volume 67, 2017

Particle physicist Martin Lewis Perl was recognized worldwide for his discovery of the τ (tau) lepton. For that achievement he received the 1982 Wolf Prize and shared the 1995 Nobel Prize in Physics. He was also a Fellow of the American Physical Society and a member of the National Academy of Sciences (elected 1981).

Martin's distinctive approach to scientific investigation had its origins in his upbringing and in the influence of I. I. Rabi, his graduate advisor at Columbia University.

After coming to Stanford University in 1963, Martin sought to understand why there should be two and only two families of leptons: the electron and its associated neutrino; and the muon and the muon neutrino. His discovery of the τ provided evidence for a third family of fundamental leptons. The bottom quark was discovered shortly afterward at the Fermi National Accelerator Laboratory, providing evidence for a third family of quarks. Direct evidence for the τ neutrino came later, thereby completing the third lepton generation, while the discovery of the top quark in 1995 completed the third generation of quarks. These achievements established leptons and quarks as fundamental constituents of matter and, along with the fundamental forces, provided the experimental basis of the “Standard Model,” our picture of how all matter is made up and how its components interact. Why there are three and only three families of leptons and quarks remains an unsolved mystery to this day.

To our present knowledge, all of the physics at the LHC can be described in the framework of the Standard Model of particle physics. Indeed, the newly discovered Higgs boson with a mass close to 125 GeV seems to confirm the predictions of the theory. Thus, in addition to looking for direct manifestations of the physics beyond the Standard Model, the LHC aims to perform ever more stringent tests of the Standard Model, in particular in the sectors of electroweak symmetry breaking and gauge interactions at high energies. In this article, I review recent experimental results in the field of electroweak interactions at the LHC.

High-energy neutrino astrophysics has come of age with IceCube's discovery of neutrinos in the TeV to PeV energy range, attributable to extragalactic sources at cosmological distances. At such energies, astrophysical neutrinos must originate in cosmic-ray interactions, providing information about the sources of high-energy cosmic rays, as well as leading to the coproduction of high-energy γ-rays. The intimate link with these two independently observed types of radiation provides important tools for the quest to identify and understand the nature of the astrophysical sources of the neutrinos. These neutrinos can set important constraints on the cosmic-ray acceleration process, and because they travel essentially unimpeded, they can probe our Universe out to the farthest cosmological distances.

The primary experimental goal of studies of hadronic parity nonconservation (PNC) has long been the isolation of the isovector weak nucleon–nucleon interaction, expected to be dominated by long-range pion exchange and enhanced by the neutral current. In meson-exchange descriptions, this interaction, together with an isoscalar interaction generated by and exchange, dominates most observables. Both amplitudes have been used to compare and check the consistency of experiments, yet no evidence for isovector hadronic PNC has been found. We argue that the emphasis on isovector hadronic PNC was misplaced. The large- expansion provides an alternative and theoretically better-motivated simplification of effective field theories (EFTs) of hadronic PNC, separating the five low-energy constants (LECs) into two of leading order (LO) and three of next-to-next-to-leading order (NLO). We show that this large- LEC hierarchy accurately describes all existing data on hadronic PNC and discuss opportunities to further test the predicted large- LEC hierarchy. This formalism—combined with future experiments—could lead to rapid progress in the next 5 years. We discuss the impact of anticipated new results and describe future experiments that can yield more precise values of the LO LECs and help to isolate the NLO 10% corrections.

The Cabibbo–Kobayashi–Maskawa (CKM) matrix is a key element in describing flavor dynamics in the Standard Model. With only four parameters, this matrix is able to describe a large range of phenomena in the quark sector, such as violation and rare decays. It can thus be constrained by many different processes, which have to be measured experimentally with high accuracy and computed with good theoretical control. Recently, with the advent of the factories and the LHCb experiment taking data, the precision has significantly improved. We review the most relevant experimental constraints and theoretical inputs and present fits to the CKM matrix for the Standard Model and for some topical model-independent studies of New Physics.

Nuclear dynamics at short distances is one of the most fascinating topics of strong interaction physics. The physics of it is closely related to the understanding of the role of the QCD in generating nuclear forces at short distances, as well as of the dynamics of the superdense cold nuclear matter relevant to the interior of neutron stars. The emergence of high-energy electron and proton beams has led to significant recent progress in high-energy nuclear scattering experiments investigating the short-range structure of nuclei. These experiments, in turn, have stimulated new theoretical studies resulting in the observation of several new phenomena specific to the short-range structure of nuclei. We review recent theoretical and experimental progress in studies of short-range correlations in nuclei and discuss their importance for advancing our understanding of the dynamics of nuclear interactions at short distances.

In the last decade, cryogenic bolometers have provided increasingly improved resolution and sensitivity in particle and radiation detectors. Thermal particle detectors have proven their outstanding capabilities in different fields of fundamental physics, especially in rare event detection. Cryogenic incoherent detector arrays designed to detect millimeter-wave photons have helped enable precision measurements of anisotropies in the cosmic microwave background (CMB), providing a unique probe of early universe physics and helping to constrain parameters of particle physics such as the sum of the neutrino masses. We review the latest achievements of cryogenic particle detectors for direct detection searches for dark matter and double- decay, as well as for CMB measurements, and we discuss expected improvements aiming to increase the sensitivities of these experiments. An important challenge is the large-scale implementation of arrays of detectors such as transition edge sensors, especially in CMB polarization experiments. We describe the challenges of scaling up to these larger arrays, including fabrication throughput and development of new multiplexing electronics.

Reactor neutrinos have been an important tool for both discovery and precision measurement in the history of neutrino studies. Since the first generation of reactor neutrino experiments in the 1950s, the detector technology has advanced greatly. New ideas, new knowledge, and modern software have also enhanced the power of the experiments. The current reactor neutrino experiments, Daya Bay, Double Chooz, and RENO, have led neutrino physics into the precision era. In this article, we review these developments and advances, address the key issues in designing a state-of-the-art reactor neutrino experiment, and explain how the challenging requirements of determining the neutrino mass hierarchy with the next-generation experiment JUNO could be realized in the near future.

At modern laser facilities, energy densities ranging from 1 Mbar to many hundreds of gigabars can regularly be achieved. These high-energy states of matter last for mere moments, measured in nanoseconds to tens of picoseconds, but during those times numerous high-precision instruments can be employed, revealing remarkable compressed matter physics, radiation–hydrodynamics physics, laser–matter interaction physics, and nuclear physics processes. We review the current progress of high-energy-density physics at the National Ignition Facility and describe the underlying physical principles.

The China Jinping Underground Laboratory, inaugurated in 2010, is an underground research facility with the deepest rock overburden and largest space by volume in the world. The first-generation science programs include dark matter searches conducted by the CDEX and PandaX experiments. These activities are complemented by measurements of ambient radioactivity and the installation of low-background counting systems. Phase II of the facility is being constructed, and its potential research projects are being formulated. In this review, we discuss the history, key features, results, and status of this facility and its experimental programs, as well as their future evolution and plans.

The existence of neutron star mergers has been supported since the discovery of the binary pulsar and the observation of its orbital energy loss, consistent with General Relativity. They are considered nucleosynthesis sites of the rapid neutron-capture process (r-process), which is responsible for creating approximately half of all heavy elements beyond Fe and is the only source of elements beyond Pb and Bi. Detailed nucleosynthesis calculations based on the decompression of neutron star matter are consistent with solar r-process abundances of heavy nuclei. Neutron star mergers have also been identified with short-duration -ray bursts via their IR afterglow. The high neutron densities in ejected matter permit a violent r-process, leading to fission cycling of the heaviest nuclei in regions far from (nuclear) stability. Uncertainties in several nuclear properties affect the abundance distributions. The modeling of astrophysical events also depends on the hydrodynamic treatment, the occurrence of a neutrino wind after the merger and before the possible emergence of a black hole, and the properties of black hole accretion disks. We discuss the effect of nuclear and modeling uncertainties and conclude that binary compact mergers are probably a (or the) dominant site of the production of r-process nuclei in our Galaxy.