We report on hyperfine-resolved spectroscopic measurements of the
electric-dipole forbidden 5p3/2​→8p1/2​ transition in a sample of
ultracold 87Rb atoms. The hyperfine selection rules enable the weak
magnetic-dipole (M1) contribution to the transition strength to be
distinguished from the much stronger electric-quadrupole (E2) contribution. An
upper limit on the M1 transition strength is determined that is about 50 times
smaller than an earlier experimental determination. We also calculate the
expected value of the M1 matrix element and find that it is less than the upper
limit extracted from the experiment.Comment: 7 pages, 4 figures, 3 table