Kaonic atoms (hydrogen & deuterium)
A kaonic atom is formed when a negatively charged kaon is slowed down and captured by an atom. The kaon replaces an electron and leaves the atom in a highly excited state. A complex cascade process begins, during which further electrons, and eventually X-rays, can be emitted, until – for hydrogen and deuterium atoms – the kaon reaches the ground state, from which it will be absorbed by the nucleons. Since the kaon is a hadron, it is subject to the strong interaction with the nucleons, which influences the kaonic atom ground state. The state’s energy is shifted with respect to the electromagnetic value, and broadened. By precisely measuring the ground state shift and width in both kaonic hydrogen and deuterium via X-ray spectrocscopy, one can obtain new information about the antikaon-nucleon interaction at low energies, which is not easily accessible via scattering experiments.
The Apparatus
SIDDHARTA-2 is located at the interaction point of the electron-positron collider DAΦNE, a Φ machine, producing monochromatic charged kaons at very low momenta. The measurement of the ground state transition in kaonic deuterium is experimentally very challenging and the signal-to-background ratio has to increased by one order of magnitude compared to the previous kaonic hydrogen measurement of SIDDHARTA. To achieve this, the SIDDHARTA-2 apparatus employs specific updates:
- 48 newly developed, large area, monolithic Silicon Drift Detector (SDD) arrays for X-ray detection
- A new, lightweight cryogenic target cell for the formation of the kaonic atoms,
- And a dedicated, two-stage veto system for the active suppression of background.
The target cell is made of thin Kapton walls with aluminium structures to ensure optimum tranismission of X-rays to the SDDs.
The Silicon Drift Detectors were developed by a collaboration of Fondazione Bruno Kessler, Politechnico di Milano, INFN-LNF (Italy) and Stefan Meyer Institute (Austria). They operate stably at temperatures of 170 K, with improved energy and time resolutions and enable a close packing around the target cell, thus maximising the solid angle. Additionally, a new front-end analog processing electronic chip, called SFERA (SDDs Front-End ReadoutASIC), was developed by Politechnico di Milano.
To actively reduce the synchronous background, an external and internal veto system, the Veto-1 and Veto-2, were developed. The Veto-1 system surrounds the SIDDHARTA-2 vacuum chamber and uses timing information to suppress signals from kaon stops in the solid setup materials. It consists of plastic scintillators with PhotoMultiplier (PM) read-out. The internal system, the Veto-2, is placed surrounding the SDDs. Its purpose is the suppression of signals produced by Minimum Ionizing Particles (MIPs) from the final kaon absoprtion grazing the SDDs with only little charge deposit. It consists of plastic scintillator units with SiPM read-out.
Moreover, a dedicated luminosity monitor is employed to precisely study the beam and background qualities in real-time. A system of scintillators, one placed below the beam pipe, one placed in front of the entrance window of the vacuum chamber, acts as kaon trigger, taking advantage of the fact that the charged kaon pairs from the Φ decay are emitted back-to-back.
The apparatus was installed on-site in 2019 and has already undergone a successful preparatory run with kaonic helium in 2021.