# Search for weak M1 transitions in $^{48}$Ca with inelastic proton scattering

## TL;DR

This study searches for weak M1 transitions in $^{48}$Ca using high-resolution proton scattering and compares results with electron scattering to understand their contribution to the total M1 strength, revealing about 25% contribution from weak transitions.

## Contribution

It provides the first detailed comparison of weak M1 transition strengths in $^{48}$Ca from proton and electron scattering experiments, confirming the significance of weak transitions and quenching factors.

## Key findings

- 29 peaks with M1 contributions identified in 7-13 MeV range
- Total B(M1) strength of 1.19(6) μ_N^2 matches electron scattering results
- Weak transitions contribute approximately 25% to the total B(M1) strength in $^{48}$Ca

## Abstract

The spinflip M1 resonance in the doubly magic nucleus $^{48}$Ca, dominated by a single transition, serves as a reference case for the quenching of spin-isospin modes in nuclei. The aim of the present work is a search for weak M1 transitions in $^{48}$Ca with a high-resolution (p,p') experiment at 295 MeV and forward angles including 0 degree and a comparison to results from a similar study using backward-angle electron scattering at low momentum transfers in order to estimate their contribution to the total B(M1) strength. M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis. The corresponding reduced B(M1) transition strengths are extracted following the approach outlined in J. Birkhan et al., Phys. Rev. C 93, 041302(R) (2016). In total, 29 peaks containing a M1 contribution are found in the excitation energy region 7 - 13 MeV. The resulting B(M1) strength distribution compares well to the electron scattering results considering different factors limiting the sensitivity in both experiments and the enhanced importance of mechanisms breaking the proportionality of nuclear cross sections and electromagnetic matrix elements for weak transitions as studied here. The total strength of 1.19(6) $\mu_N^2$ deduced assuming a non-quenched isoscalar part of the (p,p') cross sections agrees with the (e,e') result of 1.21(13) $\mu_N^2$. A binwise analysis above 10 MeV provides an upper limit of 1.62(23) $\mu_N^2$. The present results confirm that weak transitions contribute about 25% to the total B(M1) strength in $^{48}$Ca and the quenching factors of GT and spin-M1 strength are comparable in fp-shell nuclei. Thus, the role of of meson exchange currents seems to be neglible, in contrast to sd-shell nuclei.