Neutron radius determination of 133Cs and constraint on the weak mixing angle

Proton-¹³³Cs elastic scattering at low momentum transfer is performed using an in-ring reaction technique at the Cooler Storage Ring at the Heavy Ion Research Facility in Lanzhou. Recoil protons from the elastic collisions between the internal H₂-gas target and the circulating ¹³³Cs ions at 199.4 MeV/u are detected by a silicon-strip detector. The matter radius of ¹³³Cs is deduced by describing the measured differential cross sections using the Glauber model. Employing the adopted proton distribution radius, a point-neutron radius of 4.86(21) fm for ¹³³Cs is obtained. With the newly determined neutron radius, the weak mixing angle sin² θW is independently extracted to be 0.227(28) by fitting the coherent elastic neutrino-nucleus scattering data. This work limits the sin² θW value in a range smaller than the ones proposed by the previous independent approaches.