Prospects of the drift chamber beta-ray momentum analyzer with superconducting magnet to search for lepton number violating processes

Lepton number violation (LNV) would be required in leptogenesis explaining why the observable universe has more matter than untimatter. There exist two theoretical predictions about the nuclear β-decay with LNV: one is neutrinoless double β-decay (0ν2β) and the other neutrinoless quadruple β-decay (0ν4β). In order to search for these events, we have been developing a magnetic Drift Chamber Beta-ray Analyzer (DCBA) at KEK. It consists of self-triggered drift chambers for β-ray tracking, nuclear decay source plates and a superconducting solenoid magnet. The β-ray momentum is obtained by measuring the helical radius, the pitch angle and the magnetic field density, and then its kinetic energy is calculated from the momentum. The test facility DCBA-2.5 has measured the half-life of the two-neutrino double beta decay (2ν2β) of 100Mo to the ground state of 100Ru, as $T^{2\nu 2\beta }_{1/2}=[7.23^{+0.99}_{-0.77}(\mathrm{stat})\pm 0.76(\mathrm{syst})]\times 10^{18} \mathrm{yr}$. The DCBA-T3 has been proposed to search for LNV events, especially for 0ν4β of 150Nd transition to 150Gd. It is essential to detect four β-rays having the sum energy of 2.08 MeV (Q-value) from a common start point in the source plate. This article describes the present status and the future prospects of DCBA.