EXA online conference 2021

A nonzero permanent electric dipole moment (EDM) of a particle implies the violation of time-reversal symmetry. Considering $CPT$ conservation, this also indicates the violation of the combined symmetry of charge conjugation and parity ($CP$), which is one of the three essential criteria to explain the baryon asymmetry of the Universe.

At the Paul Scherrer Institute, we measured the neutron EDM by applying Ramsey's method of separated oscillatory fields with stored ultracold neutrons (UCN) in vacuum at room temperature. In this experiment, a $^{199}$Hg comagnetometer and an array of vapor $^{133}$Cs magnetometers [1] were deployed to monitor, control, and correct for magnetic-field changes. In addition, dedicated measurements [2] were carried out to assess the systematic effects related to magnetic-field nonuniformity. Unprecedented understanding and assessment of systematic effects have been achieved. The statistical analysis was performed on blinded data [3] by two independent groups to avoid any bias on the data-selection criteria. The final value taken from the midpoint of the results from the two analysis groups, adding various systematic effects that have been estimated independently, gives $d_{\rm n} = \left(0.0 \pm 1.1_{\rm stat} \pm 0.2_{\rm sys} \right)\times 10^{-26}\, e\cdot{\rm cm}$. This may be interpreted as an upper limit of $\left| d_{\rm n} \right| < 1.8 \times 10^{-26}\, e\cdot{\rm cm}$ (90% C.L.), which sets the current best limit on the EDM of the neutron [4].

References:
[1] C. Abel et al. Phys. Rev. A 101, 053419 (2020).
[2] C. Abel et al. arXiv:2103.09039 [physics.ins-det] (2021).
[3] N. J. Ayres et al. Eur. Phys. J. A 57:152 (2021).
[4] C. Abel et al. Phys. Rev. Lett. 124, 081803 (2020).

The strong-interaction effects both in pionic hydrogen and deuterium have been re-determined with improved precision. The hadronic shift and width in pionic hydrogen together with the hadronic shift in pionic deuterium constitute a one-fold constraint for the two independent pion-nucleon scattering lengths. Furthermore, the hadronic width in pionic deuterium measures the transition strength of s-wave pions on an isoscalar nucleon-nucleon pair which is an independent quantity not related to the pion-nucleon scattering lengths. The experiment was performed at the high-intensity low-energy pion beam of the Paul Scherrer Institute by using the cyclotron trap and a high resolution Bragg spectrometer with spherically bent crystals. The pion-nucleon scattering lengths and other physical quantities extracted from the atom data are in good agreement with the results obtained from pion-nucleon and nucleon-nucleon scattering experiments and confirm that a consistent picture is achieved for the low-energy pion-nucleon sector with respect to the expectations of chiral perturbation theory.

The strong interaction theory in the low energy regime, is still missing fundamental experimental results in order to achieve a breakthrough in its understanding. Among these, the investigation of the low-energy kaon nucleon/nuclei processes and of the Kaonic atoms play a key-role. The talk will give an outline of the results obtained by the AMADEUS and SIDDHARTA experiments performed at the DAFNE Collider of LNF-INFN.

In the first part of the presentation the K- single and multi-nuclear absorptions on various light nuclear targets both at-rest and in-flight (for a kaon momenta up to 120 MeV/c) will be discussed with a focus on the nature of the Λ(1405), the non-resonant hyperon pion formation amplitude below the K-N threshold and the yields and cross sections of K- single and multi-nucleon interactions.

The second part of the talk will be concerned on high sensitivity X-ray spectroscopic measurements of low-lying orbits transitions in kaonic atoms, which are presently being performed by the SIDDHARTA-2 experiment, with the main aim to perform the first ever measurement of kaonic deuterium.

I shall conclude with future plans on kaonic-nuclei interaction studies at DAFNE, beyond SIDDHARTA-2 measurement.