Ultrahigh-resolution γ-ray spectroscopy of 156Gd: a test of tetrahedral symmetry.

Tetrahedral symmetry in strongly interacting systems would establish a new class of quantum effects at subatomic scale. Excited states in {sup 156}Gd that could carry the information about the tetrahedral symmetry were populated in the {sup 155}Gd(n,{gamma}){sup 156}Gd reaction and studied using the GAMS4/5 Bragg spectrometers at the Institut Laue-Langevin. We have identified the 5{sub 1}{sup -{yields}}3{sub 1}{sup -} transition of 131.983(12) keV in {sup 156}Gd and determined its intensity to be 1.9(3)x10{sup -6} per neutron capture. The lifetime {tau}=220{sub -30}{sup +180}fs of the 5{sub 1}{sup -} state in {sup 156}Gd has been measured using the GRID technique. The resulting B(E2)=293{sub -134}{sup +61}Weisskopf unit rate of the 131.983 keV transition provides the intrinsic quadrupole moment of the 5{sub 1}{sup -} state in {sup 156}Gd to be Q{sub 0}=7.1{sub -1.6}{sup +0.7} b. This large value, comparable to the quadrupole moment of the ground state in {sup 156}Gd, gives strong evidence against tetrahedral symmetry in the lowest odd-spin, negative-parity band of {sup 156}Gd.