Particle and Nuclear Physics

  • Oct 05, 2019

SIDDHARTA-2 experiment
A kaonic atom is formed when a kaon stops into a target and replaces the electron of an atom. The kaonic atom experiences then a series of de-excitation processes, accompanied by the emission of radiation which is in the X-rays energy domain for transitions to the lowest lying levels. When the kaon reaches the ground state the kaon and the nucleus interact by the strong interaction. The main effect of the presence of the strong interaction on the top of the electromagnetic one is a shift of the energy level with respect to the electromagnetically calculated value and a broadening of the level. The measurement of the X-rays transitions towards the lowest lying states allows to obtain the shift and the width of the levels. The experimental quantities, shifts and widths, measured for the lightest kaonic atoms, namely kaonic hydrogen and kaonic deuterium, are then used to determine the isospin-dependent antikaon-nucleon scattering lengths, fundamental quantities for a better understanding of the QCD with strangeness in non-perturbative regime.

The SIDDHARTA-2 experiment aims to perform the first measurement of the kaonic deuterium exotic atom, by measuring the X-rays transitions to the fundamental 1s level. The setup consists of advanced spectroscopic Silicon Drift Detectors (SDD), two veto-systems and a trigger to enhance the signal/background ratio.

The SIDDHARTA-2 setup is installed on an interaction region of DAΦNE; the kaons are stopped inside a high-density gaseous cryogenic deuterium target by using a dedicated degrader system; the kaonic atoms are formed in a highly excited orbit. The X rays are measured by the SDD detectors in coincidence with a trigger signal given by a scintillators system placed below and above the beam pipe at the interaction point, measuring the back-to-back charged kaons coming from the φ-decay. Two veto systems, inside and outside the vacuum chamber further reduce the background generated by the beam losses (electromagnetic background) and by the kaons themselves (hadronic background). The goal is to extract the 1s level shift and width for the kaonic deuterium induced by the strong interaction with respect to the QED calculated values. These quantities, together with the kaonic hydrogen ones, measured by the previous SIDDHARTA experiment on DAΦNE, allow to extract the antikaon-nucleon isospin dependent scattering lengths. A research program at DAΦNE focused on kaon-nucleon/nuclei interaction experimental studies at low-energies including the SIDDHARTA-2 experiment and R&D for future experiments, is planned. Test measurements of other kaonic atoms, with light and heavy nuclei, using various detector systems, such as dedicated Germanium, HAPG crystal systems (VOXES system) and TES detectors, will be performed.

The PADME experiment
One of the biggest mysteries in physics today is that the matter seen in the universe accounts only for about 5% of the observed gravity. This has triggered the idea that enormous amounts of invisible matter (dark matter) should be present. Among the different theoretical models that try to define what dark matter could be, there are those that postulate the existence of a “Hidden Sector” populated by new particles that do not couple with those of the Standard Model (SM). The only connection within these two worlds could be realized by a low-mass spin-1 particle, named A', that would possess a gauge coupling of electroweak strength to dark matter, and a much smaller coupling to the SM hypercharge. This Dark Photon (DP) could be the portal connecting ordinary and dark world.

The Positron Annihilation into Dark Matter Experiment (PADME) aims to search for the production of a Dark Photon (DP) in the process e+ e- → A'γ, by using the 550 MeV positron beam provided by the DAΦNE linac and interacting with a Carbon target. To measure such a reaction, the PADME apparatus has been constructed composed by the following components:

  • a diamond active target, able to measure the position and the intensity of the beam;
  • a beam monitor device, made of ultra-thin silicon pixel detectors to study beam characteristics in terms of intensity and divergence;
  • a charged particle spectrometer, made of scintillator slabs, measuring momenta in the range 50-400 MeV/c;
  • a dipole magnet, to deflect the primary positron beam;
  • a vacuum chamber, to minimize the unwanted interactions of primary and secondary particles;
  • a finely segmented, high resolution electromagnetic calorimeter, to measure 4-momenta and/or veto final state photons.
The primary positron beam crosses the diamond target and if it does not interact, it is bent by the dipole in between the end of the spectrometer and the calorimeter, impinging on a silicon pixel detector that characterizes it. If any kind of interaction causes the positron to lose more than 50 MeV of energy, the magnet bends it into the spectrometer acceptance, providing a veto signals against the Bremsstrahlung background. If eventually a DP is produced after positron annihilation, the photon accompanying it is detected by the electromagnetic calorimeter. A single kinematic variable characterizes the process, the missing mass which is computed event-by event and its distribution should peak at M2A’ for A' production, at zero for the concurrent e+ e- → γγ process and should be smooth for the remaining background.