Progress in Particle and Nuclear Physics最新文献

We review the status and prospects of heavy-ion double charge exchange (HI-DCE) reactions. Their important role for nuclear reaction, nuclear structure and double beta-decay investigations is outlined. From the experimental side the characteristically tiny cross sections for these processes and the high background generated by other more probable competing reactions is the main challenge, which has hindered HI-DCE spectroscopy until recent years. Modern magnetic spectrometers have proven to possess the right requisites to overcome past limitations, fostering the present and future development of the field. From the theory side, the description of the measured HI-DCE cross sections poses manifold challenges. Dealing with processes which involve composite nuclei, HI-DCE reactions can, in principle, proceed through several alternative paths. These, in turn, correspond to different reaction mechanisms probing competing aspects of nuclear structure, from mean field to various classes of nucleon–nucleon interactions and correlations. A powerful way to scrutinize the nuclear response to HI-DCE is to consistently link it to the information extracted from the competing direct reactions. Indeed, these complementary studies are mandatory in order to minimize the systematic errors in the data analyses and build a many-facets and parameter-free representation of the systems under study.

We review the current state of research on electromagnetic probes in the context of heavy-ion collisions. The focus is on thermal photons and dileptons which provide unique insights into the properties of the created hot and dense matter. This review is intended to provide an introductory overview of the topic as well as a discussion of recent theoretical and experimental results. In particular, we discuss the role of vector-meson spectral functions in the calculation of photon and dilepton rates and present recent results obtained from different frameworks. Furthermore, we will highlight the special role of photons and dileptons to provide information on observables such as the temperature, the lifetime, the polarization and the electrical conductivity of the produced medium as well as their use to learn about chiral symmetry restoration and phase transitions.

The basic materials of Berry’s phase and chiral anomalies are presented to appreciate the phenomena related to those notions recently discussed in the literature. As for Berry’s phase, a general survey of the subject including the anomalous Hall effect is presented using both Lagrangian and Hamiltonian formalisms. The canonical Hamiltonian formalism of the Born–Oppenheimer approximation, when applied to the anomalous Hall effect, can incorporate the gauge symmetry of Berry’s connection but unable to incorporate the completely independent gauge symmetry of the electromagnetic vector potential simultaneously. Thus the Nernst effect is not realized in the canonical formalism. Transformed to the Lagrangian formalism with a time-derivative term allowed, the Born–Oppenheimer approximation can incorporate the electromagnetic vector potential simultaneously with Berry’s connection, but the consistent canonical property is lost and thus becomes classical. The Lagrangian formalism can thus incorporate both gauge symmetries simultaneously but spoils the basic quantum symmetries, and thus results in classical anomalous Poisson brackets and the classical Nernst effect as in the conventional formalism. These properties are taken as the bases of the applications of Berry’s phase to the anomalous Hall effect in the present review.

As for chiral anomalies, we present basic materials by the path integral formulation with an emphasis on fermions on the lattice. A chiral fermion defined by ${\gamma }_5$ on the lattice does not contain the chiral anomaly for the non-vanishing lattice spacing $a\ne 0$. Each species doubler separately does not contain a well-defined chiral anomaly either, since each species doubler defined in a part of the Brillouin zone is not a local field for $a\ne 0$. The idea of a spectral flow on the lattice does not lead to an anomaly for each species doubler separately but rather to a pair production in a general sense. We also mention that a specific construction called the Ginsparg–Wilson fermion, which is free of species doublers, may practically be useful in the theoretical analysis of an Abelian massless Dirac fermion in condensed matter physics.

We discuss a limited number of representative applications of Berry’s phase and chiral anomalies in nuclear physics and related fields to illustrate the use of these two basic notions presented in this article.

Beyond individually resolvable gravitational wave events such as binary black hole and binary neutron star mergers, the superposition of many more weak signals coming from a multitude of sources is expected to contribute to an overall background, the so-called stochastic gravitational wave background. In this review, we give an overview of possible detection methods in the search for this background and provide a detailed review of the data-analysis techniques, focusing primarily on current Earth-based interferometric gravitational-wave detectors. In addition, various validation techniques aimed at reinforcing the claim of a detection of such a background are discussed as well. We conclude this review by listing some of the astrophysical and cosmological implications resulting from current upper limits on the stochastic background of gravitational waves.

Finite-density lattice QCD aims for the first-principle study of QCD at finite density, which describes the system consisting of many quarks. The main targets are systems such as quark–gluon plasma, nuclei, and neutron stars. Explaining macroscopic physics from the microscopic theory is a natural path in the development of physics. To understand the strong interaction completely, we have to solve finite-density QCD. Each of the systems mentioned above has open problems which cannot easily be accessed by experiment or observation, so it is important to make progress in finite-density lattice QCD.

In this article, we summarize the past development and current status of the field of finite-density lattice QCD. The difficulty in the study of theories with the sign problem is that the numerical methods which are correct in principle do not necessarily work in practice and it is hard to know when it fails. We will introduce various approaches in this article, but all of them have pitfalls, which lead to unphysical results unless we study carefully. We will explain what kinds of studies were done in the past, to what extent they succeeded, and what kinds of obstacles they encountered, and why the approaches are correct in principle can lead to wrong answers. In this way, we would like to provide lessons from the past for ambitious researchers who plan to work on the finite-density lattice QCD.

Quark–Gluon Plasma (QGP), a QCD state of matter created in ultra-relativistic heavy-ion collisions, has remarkable properties including, for example, a low shear viscosity over entropy ratio. By detecting the collection of low-momentum particles that arise from the collision, it is possible to gain quantitative insight into the created matter. However, its fast evolution and thermalization properties remain elusive. Only the usage of high momentum objects as probes of QGP can unveil its constituents at different wavelengths. In this review, we attempt to provide a comprehensive picture of what was, so far, possible to infer about QGP given our current theoretical understanding of jets, heavy-flavor, and quarkonia. We will bridge the resulting qualitative picture to the experimental observations done at both the LHC and RHIC. We will focus on the phenomenological description of experimental observations, provide a brief analytical summary of the description of hard probes, and an outlook towards the main difficulties we will need to surpass in the following years. To benchmark QGP-related effects, we will also address nuclear modifications to the initial state and hadronization effects.

Vortex states of photons, electrons, and other particles are non-plane-wave solutions of the corresponding wave equation with helicoidal wave fronts. These states possess an intrinsic orbital angular momentum with respect to the average propagation direction, which represents a new degree of freedom, previously unexplored in particle or nuclear collisions. Vortex states of photons, electrons, neutrons, and neutral atoms have been experimentally produced, albeit at low energies, and are being intensively explored. Anticipating future experimental progress, one can ask what additional insights on nuclei and particles one can gain once collisions of high-energy vortex states become possible. This review describes the present-day landscape of physics opportunities, experimental progress and suggestions relevant to vortex states in high energy collisions. The aim is to familiarize the community with this emergent cross-disciplinary topic and to provide a sufficiently complete literature coverage, highlighting some results and calculational techniques.

Many novel quantum phenomena emerge in non-equilibrium relativistic quantum matter under extreme conditions such as strong magnetic fields and rotations. The quantum kinetic theory based on Wigner functions in quantum field theory provides a powerful and effective microscopic description of these quantum phenomena. In this article we review some of recent advances in the quantum kinetic theory and its applications in describing these quantum phenomena.

We summarize the ongoing scientific program of the 12 GeV Continuous Electron Beam Accelerator Facility (CEBAF) and give an outlook into future opportunities. The program addresses important topics in nuclear, hadronic, and electroweak physics, including nuclear femtography, meson and baryon spectroscopy, quarks and gluons in nuclei, precision tests of the standard model and dark sector searches. Potential upgrades of CEBAF and their impact on scientific reach are discussed, such as higher luminosity, the addition of polarized and unpolarized positron beams, and doubling the beam energy.