Weak-interaction studies with 32Ar decay – WISArD

To detect proton and beta simultaneously, the experimental device is installed in a strong magnetic field of 4 Tesla. This field guides the beta particles towards a specific detection separated from the proton detection, so the two detection systems can be optimized separately.

A test experiment was successfully conducted in November 2018 which validated the approach and predicted with certainty the achievable precision in a dedicated future experiment. A paper on this test experiment was recently accepted in PRC. Optimization measures and tests are currently underway to develop a device for a future data-taking campaign.

We are currently developing the experimental device for a future measurement campaign which can start after the resumption of operation of CERN accelerators in 2021. Thus all the detectors are optimized via laboratory tests, simulation models are developed and a new control system has been built. Everything will be ready for a restart of measurements in spring 2021.

V. Araujo-Escalona et al., accepted in PRC

The weak interaction mediating nuclear beta decay is described in the standard model of particle physics as consisting of two different currents, the vector current responsible for Fermi decays and the axial-vector current responsible for Gamow-Teller decays. All experimental findings can be described with these two interactions. However, from a more global theoretical picture, also scalar, tensor and pseudo-scalar (at relativistic energies only) currents are allowed. In such a scenario, scalar currents would appear with the vector currents and tensor currents with axial-vector currents.
Limits on these ‘exotic’ currents can either be obtained from high-energy physics experiments trying to directly produce the bosons responsible for such new interactions or from precision experiments at low energy, e.g. in nuclear beta decay, searching for small deviations from the standard model predictions. In the present proposal we follow this latter path.
Presently the most stringent limits on the presence of scalar currents come from the average corrected Ft-value of the super-allowed Fermi transitions and measurements of the ß-? correlation in the decay of 38mK (precision 0.5%) and 32Ar (0.65%). For tensor current searches the present best precision is 1 % from a measurement of the energy difference between the alpha particles from the breakup of the 8Be daughter nucleus of 8Li beta decay.
The previous experiment performed with 32Ar is close to the one we will present here. 32Ar decays, beneath other channels, by a super-allowed Fermi decay to a state in 32Cl, its isobaric analogue state, which is unbound to proton emission. Due to the recoil the daughter nucleus gets from the emission of the positron and the neutrino, the proton is emitted from a moving source and is subject to Doppler Effect. As the angular distribution of the positron and the neutrino is different between the dominant vector part and a possible scalar current, the measurement of the Doppler broadening of the proton energy peak from the decay of this state and the comparison to model predictions with or without exotic contributions allowed the determination of the one of the most stringent limit on scalar currents.
In order to achieve this precise result, the experimental set-up was installed in a strong magnetic field to guide the positrons away from the proton detectors. However, the positrons themselves were not detected. In the present proposal we suggest to perform a first-time measurement of the Doppler shift, measuring positron-proton coincidences in b-delayed proton decay, instead of the Doppler broadening, keeping in mind that a shift is easier to measure with high precision than a broadening.
The precision aimed at for the correlation coefficient, to be extracted from the Doppler shift is 0.1 % (factor 5 improvement). The higher precision with respect to the previous experiment is possible because a measurement of the Doppler shift is less encumbered by systematic errors. Another important factor is that we propose a long-term installation of WISArD at ISOLDE which will allow us to improve successively the set-up. The beauty of the present project is also that 32Ar can be replaced with e.g. 20Mg allowing similar measurement to be performed. Gamow-Teller fed states could allow for the search for tensor currents with the same technique.
It was recently shown that for precisions at the few per mill level, beta-neutrino correlation measurements remain competitive with LHC searches for exotic weak currents. Our result would thus bring the sensitivity to right-handed scalar weak currents to the same level as projected bounds of the 14TeV run of the LHC.
The installation of this new experiment at ISOLDE was recently approved by the ISOLDE Collaboration Committee, the political body of ISOLDE. A letter of intent submitted to the ISOLDE and n-TOF Program Advisory Committee requesting a first test beam time was well received and approved as well.