Towards a D correlation measurement with MORA

by Marah Jbayli (1^st year PhD student)

Supervisor: Pierre Delahaye

The observed matter–antimatter asymmetry in the Universe remains one of the major open questions in modern physics. CP violation, one of the three Sakharov conditions [1] required for baryogenesis, may play a key role in understanding this asymmetry. The Matter’s Origin from RadioActivity (MORA) experiment [2], currently located at the IGISOL facility of the University of Jyväskylä, searches for CP violation through the measurement of the D-correlation parameter in nuclear beta decay [3]. This parameter is sensitive to time-reversal symmetry violation and, via the CPT theorem, to CP violation. The D correlation is a triple correlation between the nuclear spin and the momenta of the decay products (the electron/positron and the antineutrino/neutrino). Experimentally, it is accessed by measuring an angular asymmetry between the recoil ions, used in place of the neutrino/antineutrino, and the charged leptons in the plane perpendicular to the nuclear spin. MORA uses a Paul trap to confine the ions. Around the trap, four phoswich detectors detect the beta particles, while four MCP-based detectors detect the recoil ions. Two silicon detectors located along the trap axis are used to measure the polarization of the trapped ions.
The first part of this talk will present the current status of MORA towards a D-correlation measurement, focusing on three key aspects: reducing beam contamination, measuring the polarization of the trapped ions, and reaching 104 trapped ions per bunch.The second part will focus on the development of a reliable calibration procedure for the phoswich detectors, which is required for the subsequent data analysis. The phoswich detector is made of a fast thin scintillator and a slow thick one, enabling the discrimination between emitted betas and gammas. This adds complexity to the calibration process as the signals are correlated and each layer has a different gain and resolution.
In particular, a reliable calibration is essential for applying a common energy threshold to all detectors and thereby avoiding the generation of a fake asymmetry. It is also necessary for testing theoretical predictions of final-state interaction effects, which depend on the energy of the emitted beta particle. Such effects will become particularly relevant for MORA at DESIR, where a sensitivity to D at the 10^-5 level is targeted.

References
[1] Sakharov, A. D., Violation of CP invariance, C asymmetry, and baryon asymetry of the universe. JETP Letters, 5, 24 (1967).
[2] Goyal, N., Singh, A., Daumas-Tschopp, S. et al., Performance of the MORA apparatus for testing time-reversal invariance in nuclear beta decay. Eur. Phys. J. A 61, 221 (2025). https://doi.org/10.1140/epja/s10050-025-01694-3
[3] Jackson, J. D., Treiman., S. B. and Wyld, H. W. Possible Tests of Time Reversal Invariance in Beta Decay. Phys. Rev., 106(3), 517 (1957).