Exotic decay modes in Be-11

In a recent letter [1] a novel explanation was proposed for the so-called neutron lifetime anomaly. To explain the difference in the rate of disappearing neutrons and appearing protons, they suggested that neutrons could decay into dark sector particles ~1% of the time. In [1], they also calculated that a few selected nuclei, as Be-11, haven an open energy window for this hypothetical dark decay.

Be-11 is the only isotope for which indirect and ambiguous evidence of beta-delayed proton emission decay exists [2]. They implanted Be-11 in a foil and then, using mass spectroscopy, measured the amount of Be-10 created, thus measuring a combination of all possible decay modes leading to Be-10.

We performed an experiment at TRIUMF using the prototype Active Target Time Projection Chamber (pATTPC) and the high purity of the ISAC Be beams to directly observe the beta-p decay channel for the first time. The use of the pATTPC allowed for the unambiguous observation of the proton emission. We will present preliminary results on the measured branching ratio, where any difference with that measured in [2] would hint to the presence of a dark decay branch in Be-11.

[1] B. Fornal and B. Grinstein. Phys. Rev. Lett., 120:191801 (2018)

[2] K. Riisager, et al. Physics Letters B, 732:305 (2014)