Test du Modèle Standard à basse énergie

Precise measurements of Fermi superallowed $0^{+} \rightarrow 0^{+} \beta$-decays provide a powerful tool to study the weak interaction properties in the framework of the Standard Model (SM). Collectively, the comparative half-lives (ƒt) of these transitions allow a sensitive probe of the CVC hypothesis and contribute to the most demanding test of the unitarity of the quarks-mixing CKM matrix top-row, by providing, so far, the most accurate determination of its dominant element (Vud). Until recently, an apparent departure from unity enhanced a doubt on the validity of the minimal SM and thus stimulated a considerable effort in order to extend the study to other Fermi emitters available. The$^{62}$Ga and $^{38}$Ca are among key nuclei to achieve these precision tests and verify the reliability of the corrections applied to the experimental ƒt-values. The $^ {62}$Ga $\beta$-decay was investigated at the IGISOL separator in Jyväskylä, with an experimental setup composed of 3 EUROBALL Clovers for $\gamma$-ray detection. Very weak intensity (<1‰) $\gamma$-rays were identified, via $\beta-\gamma$ and $\beta -\gamma-\gamma$ correlations, and allowed a partial decay scheme reconstruction in $^{62}$Zn. The newly established analog branching-ratio (B.R.A = 99.893(24) %) was used to compute the universal Ft-value ($^{62}$Ga). The latter turned out to be in good agreement with the 12 well-known cases. Compatibility between the upper limit set here on the term (d1IM) and the theoretical prediction suggests that the isospin-symmetry-breaking correction is indeed large for the heavy (A = 62) $\beta$-emitters. The study of the $^{38}$Ca decay was performed at the CERN- ISOLDE facility. Injection of fluorine into the ion source, in order to chemically select the isotopes of interest, assisted by the REXTRAP Penning trap facility and a TOF analysis, enabled us to eliminate efficiently the troublesome 38mK. For the first time, the $^{38}$Ca half-life is measured with a highly purified radioactive sample. The preliminary result obtained, T$_{1/2}$($^{38}$Ca) = 445.8(10) ms, improves the precision on the half-life as determined from previous measurements by a factor close to 10.