We perform a systematic study of the $\alpha$-particle excitation from its ground state $0_1^{+}$ to the $0_2^{+}$ resonance. The so-called monopole transition form factor is investigated via an electron scattering experiment in a broad $Q^2$ range (from 0.5 to $5.0{\mathrm{fm}}^{-2}$). The precision of the new data dramatically supersedes that of older sets of data, each covering only a portion of the $Q^2$ range. The new data allow the determination of two coefficients in a low-momentum expansion, leading to a new puzzle. By confronting experiment to state-of-the-art theoretical calculations, we observe that modern nuclear forces, including those derived within chiral effective field theory that are well tested on a variety of observables, fail to reproduce the excitation of the $\alpha$ particle.

A large area silicon drift detectors (SDDs) system and its readout electronics have been developed by the SIDDHARTA-2 Collaboration, aiming to perform high-precision light kaonic atoms x-ray spectroscopy for the investigation of the K ˉ N strong interaction in the low-energy QCD regime. To perform these measurements, a linear energy response and a good energy resolution are mandatory requirements for the system, to be preserved along the whole DAQ (analog and digital) chain; such task is made even harder in the experimental environment of particles colliders, where the high background due to ionizing particles and radiation is present. The energy response of the SDDs system has been characterized with the beam-originating background generated during the commissioning phase of the DAΦNE electron–positron collider (INFN-LNF) in early 2020. The data analysis has been optimized to describe the system’s response and the background. The calibration procedure demonstrates that, despite the high and variable background of the collider, the energy response of the system is linear at the level of few eV ( ( Δ E / E ) < 10 − 3 ), with an energy resolution of 157.8 ± 0.3 − 0.2 + 0.2 eV for the Fe K α line.

When a positron annihilates, two gamma photons are created with orthogonal polarizations. It is possible to use coincidence measurements where both photons undergo Compton scattering to estimate their initial relative polarization orientation. This information is of great interest in gamma imaging systems, such as Positron Emission Tomography, where it may be used as an additional tool to distinguish true coincidence events from scatter and random background. The successful utilization of this principle critically depends on the detector’s angular and energy resolution, which determine its polarimetric performance. In this study, we use Monte Carlo simulations based on the Geant4 toolkit to model two multi-pixel detector configurations identified as prospective for the measurement of gamma-ray polarization in PET. One is based on 2 mm × 2 mm × 20 mm LYSO scintillators and the other is based on 3 mm × 3 mm × 20 mm GAGG scintillators. Each configuration has a pair of modules, each consisting of 64 crystals set up in a single 8 × 8 matrix, where both the recoil electron and the Compton-scattered photon are absorbed. We simulate positron annihilation by generating two back-to-back gamma photons of 511 keV with orthogonal polarizations. The Compton scattering is successfully identified and the modulation of the azimuthal angle difference is clearly observed. The configuration based on GAGG crystals demonstrates slightly better polarimetric performance than the one based on LYSO crystals, reflected in the more pronounced azimuthal modulation.

The current work presents the optimization of large area silicon drift detectors developed by the SIDDHARTA-2 collaboration for high precision x-ray measurements of light exotic atom transitions. Two different radiation sources were employed in the study: an x-ray tube, for investigating the energy resolution and the charge collection efficiency of the device in the range 4000 eV–13 000 eV, and a β ^{− 90} Sr radioactive source for measuring the timing response, thus qualifying the charge drift parameters inside the semiconductor. The study reports the spectroscopic response optimization, together with the tuning of the electron dynamics for the given Silicon technology, by adjusting the applied electric field and the working temperature, which allow a good control of the device’s performances for high precision, timed x-ray spectroscopy applications.

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.