Progress in Particle and Nuclear Physics最新文献

We discuss our present knowledge of ${\alpha }_s$, the fundamental running coupling or effective charge of Quantum Chromodynamics (QCD). A precise understanding of the running of ${\alpha }_s\left(Q^2\right)$ at high momentum transfer, $Q$, is necessary for any perturbative QCD calculation. Equally important, the behavior of ${\alpha }_s$ at low $Q^2$ in the nonperturbative QCD domain is critical for understanding strong interaction phenomena, including the emergence of mass and quark confinement. The behavior of ${\alpha }_s\left(Q^2\right)$ at all momentum transfers also provides a connection between perturbative and nonperturbative QCD phenomena, such as hadron spectroscopy and dynamics. We first sketch the origin of the QCD coupling, the reason why its magnitude depends on the scale at which hadronic phenomena are probed, and the resulting consequences for QCD phenomenology. We then summarize latest measurements in both the perturbative and nonperturbative domains. New theory developments include the derivation of the universal nonperturbative behavior of ${\alpha }_s\left(Q^2\right)$ from both the Dyson–Schwinger equations and light-front holography. We also describe theory advances for the calculation of gluon and quark Schwinger functions in the nonperturbative domain and the relation of these quantities to ${\alpha }_s$. We conclude by highlighting how the nonperturbative knowledge of ${\alpha }_s$ is now providing a parameter-free determination of hadron spectroscopy and structure, a central and long-sought goal of QCD studies.

The nuclear equation of state (EOS) is at the center of numerous theoretical and experimental efforts in nuclear physics. With advances in microscopic theories for nuclear interactions, the availability of experiments probing nuclear matter under conditions not reached before, endeavors to develop sophisticated and reliable transport simulations to interpret these experiments, and the advent of multi-messenger astronomy, the next decade will bring new opportunities for determining the nuclear matter EOS, elucidating its dependence on density, temperature, and isospin asymmetry. Among controlled terrestrial experiments, collisions of heavy nuclei at intermediate beam energies (from a few tens of MeV/nucleon to about 25 GeV/nucleon in the fixed-target frame) probe the widest ranges of baryon density and temperature, enabling studies of nuclear matter from a few tenths to about 5 times the nuclear saturation density and for temperatures from a few to well above a hundred MeV, respectively. Collisions of neutron-rich isotopes further bring the opportunity to probe effects due to the isospin asymmetry. However, capitalizing on the enormous scientific effort aimed at uncovering the dense nuclear matter EOS, both at RHIC and at FRIB as well as at other international facilities, depends on the continued development of state-of-the-art hadronic transport simulations. This white paper highlights the essential role that heavy-ion collision experiments and hadronic transport simulations play in understanding strong interactions in dense nuclear matter, with an emphasis on how these efforts can be used together with microscopic approaches and neutron star studies to uncover the nuclear EOS.

We survey the impact of nuclear three-body forces on structure properties of nuclei within the shell model. It has long been acknowledged, since the seminal works of Zuker and coworkers, that three-body forces play a fundamental role in making the monopole component of shell-model Hamiltonians, derived from realistic nucleon–nucleon potentials, able to reproduce the observed evolution of the shell structure. In the vast majority of calculations, however, their effects have been taken into account by shell-model practitioners by introducing ad hoc modifications of the monopole matrix elements. During last twenty years, a new theoretical approach, framed within the chiral perturbation theory, has progressed in developing nuclear potentials, where two- and many-body components are naturally and consistently built in. This new class of nuclear forces allows to carry out nuclear structure studies that are improving our ability to understand nuclear phenomena in a microscopic approach. We provide in this work an update on the status of the nuclear shell model based on realistic Hamiltonians that are derived from two- and three-nucleon chiral potentials, focusing on the role of the three-body component to provide the observed shell evolution and closure properties, as well as the location of driplines. To this end, we present the results of shell-model calculations and their comparison with recent experimental measurements, which enlighten the relevance of the inclusion of three-nucleon forces to master our knowledge of the physics of atomic nuclei.

One of the many physical questions that have emerged from studies of heavy-ion collisions at RHIC and the LHC concerns the validity of hydrodynamic modelling at the very early stages, when the Quark–Gluon Plasma system produced is still far from isotropy. In this article we review the idea of far-from-equilibrium hydrodynamic attractors as a way to understand how the complexity of initial states of nuclear matter is reduced so that a hydrodynamic description can be effective.

One of characteristic phenomena for nuclei beyond the proton dripline is the simultaneous emission of two protons (2p). The current status of our knowledge of this most recently observed and the least known decay mode is presented. First, different approaches to theoretical description of this process, ranging from effective approximations to advanced three-body models are overviewed. Then, after a brief survey of main experimental methods to produce 2p-emitting nuclei and techniques to study their decays, experimental findings in this research field are presented and discussed. This review covers decays of short-lived resonances and excited states of unbound nuclei as well as longer-lived, ground-state radioactive decays. In addition, more exotic decays like three- and four-proton emission are addressed. Finally, related few-body topics, like two-neutron and four-neutron radioactivity, and the problem of the tetraneutron are shortly discussed.

Phase transitions in a non-perturbative regime can be studied by ab initio Lattice Field Theory methods. The status and future research directions for LFT investigations of Quantum Chromo-Dynamics under extreme conditions are reviewed, including properties of hadrons and of the hypothesized QCD axion as inferred from QCD topology in different phases. We discuss phase transitions in strong interactions in an extended parameter space, and the possibility of model building for Dark Matter and Electro-Weak Symmetry Breaking. Methodological challenges are addressed as well, including new developments in Artificial Intelligence geared towards the identification of different phases and transitions.

Extensive experimental and theoretical explorations over the last decades showed that the nucleon (proton/neutron) is not just a simple system of 3 quarks bound by gluons, but a complex system of valence and sea quarks as well as gluons (summarized as partons) which are all interacting with each other and moving relative to each other, following the rules of quantum chromo dynamics (QCD). To understand how the properties of these colored building blocks are related to the basic properties of the nucleon like its mass, its spin or its charge, a full understanding of the relevant effective degrees of freedom and of the effective interactions at large distances is required. In the classical picture of parton dynamics in high energy interactions the description is often simplified into two cases. On the one side the classical form factors, providing a 2D picture of the transverse position distribution and on the other side, the one-dimensional picture of a fast moving nucleon as a collection of co-linearly moving quarks and gluons, described in terms of the longitudinal momentum fraction in parton distribution functions. However, recent experimental and theoretical advances during the last two decades showed, that such a simple picture is not adequate for a full description, especially if transverse spin dependent observables are involved. It turned out, that the intrinsic transverse motion of partons and also the correlation between momentum and position information have to be considered, requiring a full 3-dimensional understanding of the nucleon structure. This review will give an overview on the main experimental data for 3D nucleon structure studies, available from lepton and hadron scattering and its interpretation in terms of generalized parton distributions (GPDs) and transverse momentum dependent parton distributions (TMDs). Recent global fits of both types of distribution functions based on experimental data and their physics content will be presented and discussed on the way to a full 3D imaging of the nucleon. Furthermore, an overview of current and future trends and new perspectives in the field will be provided.