Speaker
Description
Nuclear beta decay is a powerful tool to test the Standard Model (SM) in the electroweak sector. The wide variety of nuclei and beta transitions allows us to choose the perfect candidate for specific tests of the SM which are competitive with high energy physics experiments [1]. In particular, the beta-neutrino angular correlation coefficient $a_{\beta\nu}$ and the Fierz interference term $b$ are directly sensitive to the possible existence of scalar (resp. tensor) currents in the well-established standard vector$-$axial-vector ($V-A$) description of pure Fermi (resp. Gamow-Teller) transitions. These coefficients can be accessed either by directly measuring the energy distribution of the daughter nucleus or the emitted beta, or from the kinematic shift of secondary particles emitted after the decay. One of the best results to date on the modified $\tilde{a}_{\beta\nu}$ coefficient for a pure Fermi transition is at $0.65\%~$[2]. It was obtained using $^{32}$Ar, from the broadening of the beta-delayed proton group emitted by the isobaric analogue state of the daughter nucleus $^{32}$Cl. In the WISArD experiment, we aim at a precision at the $0.1\%$ level for the Fermi transition of $^{32}$Ar from a new approach : we measure the kinematic shift of the beta delayed protons emitted in the same or the opposite hemisphere to the beta [3].
The proof-of-principle campaign of the experiment was performed in the fall of 2018 and already yielded the 3rd best precision level on $\tilde{a}_{\beta\nu}$. The precise analysis of this first campaign allowed us to draw a full account of the dominating systematic errors, that have since been tackled in a complete upgrade of the set-up. Four major items were optimized and will be presented : the beam control and monitoring was enhanced and the detection solid angle was maximized, both to increase statistics; the energy resolution of the proton detector was lowered by one order of magnitude and the beta detection threshold and backscattering simulations were validated on dedicated measurements, both to lower the main sources of systematic errors. This upgrade is now in its final stage, on track for the second run of data taking scheduled for the end of 2021. The potential precision at reach for this new campaign will be discussed.
[1] M. Gonzàlez-Alonso et al, Prog. Part. Nucl. Phys 104, 165 (2019)
[2] E. G. Adelberger et al.,Phys. Rev. Lett. 83, 1299 (1999)
[3] V. Aurojo-Escalona et al.,Phys. Rev. C 101, 055501 (2020)
