Christoph Hanhart

The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 3,200 new measurements from 903 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Most of the 118 reviews are updated, including many that are heavily revised. The Review is divided into two volumes. Volume 1 includes the Summary Tables and 96 review articles. Volume 2 consists of the Particle Listings and contains also 22 reviews that address specific aspects of the data presented in the Listings. The complete Review (both volumes) is published online on the website of the Particle Data Group ( pdg.lbl.gov ) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is available in print, as a web version optimized for use on phones, and as an Android app. The PDG API (Application Programming Interface) provides access to the data published in the Review in machine-readable format.

Radiative decays of hadronic states provide an essential source of information that can facilitate deciphering their nature and properties. However, a lot of confusion concerning radiative decays of hadronic molecules and their interpretation can be found in the literature. In this paper, we briefly review several types of such decays and pinpoint similarities and essential differences between them. In particular, we emphasise the crucial role played by the hierarchy of the scales relevant to the studied system and the resulting necessity of employing an approach that considers them appropriately. We illustrate the situation with several instructive examples.

Motivated by the discrepancies noted recently between the theoretical predictions of the electromagnetic J / ψ → π 0 γ * transition form factor and the BESIII data, we reanalyze this transition form factor using the dispersive Khuri-Treiman equations, with final-state interactions in both the direct channel and the crossed channels properly considered. This improved framework incorporates ρ – ω mixing effects. The effect of four-pion states is evaluated through a dispersively improved vector-meson-dominance model. From this information, we propose a two-parameter fit that provides an excellent description of the BESIII data over the broad energy range from 0 to 2.8 GeV. We demonstrate that the ρ π 0 decay mode of the J / ψ is dominated by strong interaction, while the ω π 0 mode is dominated by one-photon exchange. From this, we extract the relative phase between the strong and the one-virtual-photon (electromagnetic) modes in hadronic decays of J / ψ as ( 62 ± 21 ) ° . This could provide useful information in understanding the long-standing ρ π puzzle in J / ψ decays.

We study the radiative decays D s 1 ( 2460 ) → γ D s 0 * ( 2317 ) and D s 1 ( 2460 ) → γ D 0 K + / γ D + K 0 and argue them to constitute a paradigmatic case of reactions sensitive to the short-range component of the decaying meson wave function that can, in principle, be quantified in a model-independent way employing experimental data. We demonstrate how a simultaneous measurement of the above radiative decay widths, or at least a constraint on the ratio of the corresponding branching fractions, can allow one to probe the nature of the D s 0 * ( 2317 ) and D s 1 ( 2460 ) mesons.

We utilize the universality of pion-pion ( π π ) final-state interactions at small invariant masses to understand the enhanced localized C P violation in B ± → K ± π + π − , using a dispersive approach. From a fit to the integrated C P -asymmetry data, we successfully predict the Dalitz-plot kinematic distribution of the asymmetry in the low-energy π π region, including the large localized C P violation recently observed by LHCb. An essential role is played by the contributions of isospin 2. This formalism, whose parameters have a physical meaning, can be adapted straightforwardly to other systems with C P violation enhanced by final-state interactions.

In light of recent high-precision data taken by the BESIII Collaboration, we reconsider the dipion transition ψ ( 3686 ) → J / ψ π + π − . The strong pion-pion final-state interactions are taken into account model independently by using dispersion theory. We find that we can reproduce the substructure near the π + π − threshold observed experimentally without introducing an extra resonance state. While a helicity-flip amplitude plays an important role for the formation of the dip in the invariant-mass distribution, the virtual exchange of the charmoniumlike exotic Z c ( 3900 ) state improves the fit quality only slightly.