Tetrahedral symmetry in strongly interacting systems would establish a new class of quantum effects at subatomic scale. Excited states in ${}^{156}\mathrm{Gd}$ that could carry the information about the tetrahedral symmetry were populated in the ${}^{155}\mathrm{Gd}(n,\gamma ){}^{156}\mathrm{Gd}$ reaction and studied using the GAMS4/5 Bragg spectrometers at the Institut Laue-Langevin. We have identified the $5_1^{-}\to 3_1^{-}$ transition of 131.983(12) keV in ${}^{156}\mathrm{Gd}$ and determined its intensity to be $1.9(3)x{10}^{-6}$ per neutron capture. The lifetime $\tau ={220}_{-30}^{+180}fs$ of the $5_1^{-}$ state in ${}^{156}\mathrm{Gd}$ has been measured using the GRID technique. The resulting $B(E2)={293}_{-134}^{+61}$Weisskopf unit rate of the 131.983 keV transition provides the intrinsic quadrupole moment of the $5_1^{-}$ state in ${}^{156}\mathrm{Gd}$ to be $Q_0={7.1}_{-1.6}^{+0.7}\mathrm{b}$. This large value, comparable to the quadrupole moment of the ground state in ${}^{156}\mathrm{Gd}$, gives strong evidence against tetrahedral symmetry in the lowest odd-spin, negative-parity band of ${}^{156}\mathrm{Gd}$.

• Received 26 February 2010

DOI:https://doi.org/10.1103/PhysRevLett.104.222502