Spectroscopic signatures and origin of a hidden order in Ba$_2$MgReO$_6$
J. Soh, Maximilian E. Merkel, L. Pourovskii, I. vZivkovi'c, Oleg Malanyuk, Jana P'asztorov'a, S. Francoual, Daigorou Hirai, A. Urru, Davor Tolj, Dario Fiore-Mosca, O. Yazyev, N. Spaldin, C. Ederer, H. M. Rønnow