Chiral And Reflection Symmetry Breaking In Odd-odd Nucleus ⁷⁸Br

Chirality in atomic nuclei has attracted considerable attention and intensive discussion since it was first predicted by Frauendorf and Meng. They pointed out that the rotation of triaxial nuclei attains a chiral character, which gives rise to pairs of nearly degenerate △I=1 bands with the same parity, called chiral doublet bands. So far, more than 30 experimental candidates have been reported in the A-80,100,130, and 190 mass regions. In recent years, several studies based on the relativistic mean field theory suggested that more than one pair of chiral bands could exist in a single nucleus. The experimental evidences for multiple chiral bands were reported in 133Ce,103Rh, and possibly in 107Ag.Total Routhian surfaces （TRS） calculations suggested that ⁷⁸Br has an obvi-ous triaxial shape for the rotational band involving the πg9/2（?）vgg/2 configuration, which is suitable for the construction of chiral doublet bands. In addition, for the odd-odd nucleus ⁷⁸Br with Z=35 and N=43, both proton and neutron Fermi surfaces lie near the g9/2,f5/2 and p3/2 orbits. Among the orbits mentioned above, the g9/2 and p3/2 orbits have opposite parities and △j=△l=3h, which might lead to the octupole correlation effect.High-spin states in ⁷⁸Br have been studied via the 70Zn(¹²C, 1p3n) reaction. A total of 44 new transitions and 21 new levels have been added to the level scheme. The previously known level scheme has been extended up to （20+） at 9132.9 keV. Five rotational bands are observed in the present work, three of them are newly observed. The relativistic mean field theory, the microscopic multidimensional constrained covariant density functional approach and the tri-axial particle rotor model were employed to investigate the rotational bands in ⁷⁸Br. The observed two positive-parity doublet bands and two negative-parity doublet bands are proposed to be multiple chiral doublet bands with πg9/2（?）vgg/2 and πg9/2（?）vgg/2 configurations, respectively, thereby forming a multiple chiral doublet. In addition, several new strong E1 transitions between members of two distinct sets of doublet bands have been observed, which indicates the existence of octupole correlation in ⁷⁸Br. The octupole correlations in ⁷⁸Br are support-ed by the microscopic multidimensional constrained covariant density functional approach calculations and macroscopic-microscopic method as well. This obser-vation reports the first example of chiral geometry in octupole soft nuclei and indicates that nuclear chirality can be robust against the octupole correlations.