Department of Experimental Particle Physics and Applications

SIDDHARTA-2

Strong interaction, one of the four fundamental interactions in the Universe, is described by quantum chromodynamics (QCD). Many aspects of QCD, especially at low energies, remain unclear due to the lack of experimental data. The SIDDHARTA-2 experiment has as its objective to make unique measurements at a global level on strong interaction, using particles called kaons, which consist of a quark and an antiquark (one of the two with “strangeness”), produced by the DAΦNE accelerator of the National Laboratory of Frascati. SIDDHARTA-2, successor of SIDDHARTA which made the best measurement worldwide of kaonic hydrogen, studies the formation and decay of exotic atoms, in particular of kaonic deuterium. Kaonic atoms are atomic systems in which a negatively charged kaon (K^-) replaces an electron in the atomic orbit. The K^- is captured in a highly excited atomic level and starts an electromagnetic cascade process towards the more internal atomic levels. The cascade ends with the nuclear absorption of the K^-. The strong interaction between the kaon on the innermost atomic level and the nucleus adds up to the electromagnetic one, producing a detectable shift and broadening of the energy level that can be extracted with dedicated X-ray spectroscopy measurement. The kaonic atom formation one can see HERE.

Comparison of hydrogen atom (left) with the kaonic hydrogen atom (right).

The first-ever kaonic deuterium measurements have been performed by SIDDHARTA-2 in 2023 - 2024 and will allow to extract, using also the SIDDHARTA kaonic hydrogen measurement result, the isospin-dependent antikaon-nucleon scattering lengths, which are quantities of fundamental importance for understanding the low energy QCD in the strangeness sector (as for example aspects of the interplay between spontaneous and explicit chiral symmetry breaking) having also important consequences for astrophysics (equation of state of neutron stars). SIDDHARTA-2 will perform precision X-ray spectroscopy of the kaonic deuterium transitions to the 1s level, which allows to measure the strong interaction induced observables, shift and width of the ground states (1s). SIDDHARTA-2 group performed also the first high precision measurements of X-ray transitions in kaonic helium atoms and intermediate mass kaonic atoms like C, O, N and Al.