HHH whitepaper by Abouabid, H, Arhrib, A, Arnold, H, Azevedo, D, Brigljevic, V, Chen, M, Diaz, D, Duarte, J, du Pree, T, El Falaki, J, Ferencek, D, Ferreira, PM, Fuks, B, Ganguly, S, Karkout, O, Kolosova, M, Konigsberg, J, Landsberg, G, Liu, B, Moser, B, Mühlleitner, M, Papaefstathiou, A, Pasechnik, R, Robens, T