In 1932 Heisenberg proposed that protons and neutrons are manifestations of the same particle, the nucleon. This idea, further developed by E. Wigner leads to the concept of isospin. Based on this assumption, the lowest states of mirror nuclei should have the same quantum properties, which is in general the experience so-far. In a recent high statistics beta decay study performed at the RIBF facility at RIKEN (Japan) we have found that the ground state spin-parity of ⁷¹Kr is different from the mirror counterpart in ⁷¹Br. This is the first case where this phenomenon occurs in nuclei that differ by only one nucleon – where a single proton is replaced by a neutron. In the talk we will show why nuclei with isospin quantum number T=1/2 are relevant and how we reached that conclusion that solves a long-standing puzzle.

• This seminar introduces the article of the month 2025/5:

Isospin symmetry breaking in the ⁷¹Kr and ⁷¹Br mirror system,
Physical Review Letters 134, 162502 (2025)

• Tea and cookies are offered 30 minutes before the seminar.