Kaonic atoms measure the antikaon-nucleus interaction at almost zero relative energy, allowing one to determine basic low-energy quantum chromodynamics (QCD) quantities, namely, the antikaon-nucleon ( $\overline{K}$ N) scattering lengths. The latter are important for extracting the sigma terms which are built on the symmetry breaking part of the Hamiltonian, thereby providing a measure of chiral and SU(3) symmetries breaking. After discussing the sigma terms and their relations to the kaonic atoms, we describe the most precise measurement in the literature of kaonic hydrogen, performed at LNF-INFN by the SIDDHARTA experiment. Kaonic deuterium is still to be measured, and two experiments are planned. The first, SIDDHARTA-2 at LNF-INFN was installed on DA $\mathsf{\Phi }$ NE in spring 2019 and will collect data in 2020. The second, E57 at J-PARC, will become operative in 2021.

Violations of the Pauli Exclusion Principle (PEP), albeit small, could be motivated by physics beyond the Standard Model, ranging from violation of Lorentz invariance to extra space dimensions. This scenario can be experimentally constrained through dedicated, state-of-the-art X-ray spectroscopy, searching for a forbidden atomic transition from the L shell to the K shell already occupied by two electrons. The VIP-2 Experiment located at the underground Gran Sasso National Laboratories of INFN (Italy) tests PEP violations by introducing new electrons via a direct current in a copper conductor, measuring the X-ray energies through a silicon drift detector. Bayesian and frequentist analyses of approximately six months of data taken with the fully operational setup is presented, setting the strongest limit to date on the PEP violation shown by the VIP collaboration. The upper bound on PEP violation are placed at 90% CL ${\beta }^2/2\le 6.8\times {10}^{-42}$ with the Bayesian approach, and ${\beta }^2/2\le 7.1\times {10}^{-42}$ with the frequentist CL_s technique.

The photofission cross section of Au was determined in the energy range 100–300 MeV by means of a quasimonochromatic photon beam. The nuclear fissility $P_f$ was calculated using the recently measured total photoabsorption cross sections. The nuclear excitation energy $E^{\mathrm{*}}$, charge and mass of compound nucleus were obtained by means of an intranuclear cascade Monte Carlo calculation. The fissility values determined for Au, Bi, and U were compared with the predictions of the cascade-evaporation model and remarkably fitted by the calculation.

The VIolation of Pauli (VIP) experiment (and its upgraded version, VIP-2) uses the Ramberg and Snow (RS) method (Phys. Lett. B 1990, 238, 438) to search for violations of the Pauli exclusion principle in the Gran Sasso underground laboratory. The RS method consists of feeding a copper conductor with a high direct current, so that the large number of newly-injected conduction electrons can interact with the copper atoms and possibly cascade electromagnetically to an already occupied atomic ground state if their wavefunction has the wrong symmetry with respect to the atomic electrons, emitting characteristic X-rays as they do so. In their original data analysis, RS considered a very simple path for each electron, which is sure to return a bound, albeit a very weak one, because it ignores the meandering random walks of the electrons as they move from the entrance to the exit of the copper sample. These complex walks bring the electrons close to many more atoms than in the RS calculation. Here, we consider the full description of these walks and show that this leads to a nontrivial and nonlinear X-ray emission rate. Finally, we obtain an improved bound, which sets much tighter constraints on the violation of the Pauli exclusion principle for electrons.

Differential cross sections for the ${}_{}{}^2{}_{}{}^{}\mathrm{H}$(γ,p)n reaction were measured at five laboratory angles from 32.5° to 130°, for photon energies between 100 and 255 MeV, in 10 MeV steps. A quasi-monochromatic photon beam, obtained by positron in-flight annihilation on a liquid hydrogen radiator, was used, and the photon spectrum was measured on-line by a pair spectrometer. The normalization uncertainty is within ±5%. Data agree, within the total errors, with recent results of a tagged-photon experiment and of a measurement of the inverse process. The measured angular distributions were fitted by a sum of Legendre polynomials, and the results compared with recent calculations. Significant departures from the calculated angular distribution asymmetry have been found.