The discovery of the fourth family at the LHC: what if?

The first evidence of new strong interactions may be a sufficiently massivefourth family observed at the LHC. The fourth family masses, of the leptons inparticular, are constrained by the electroweak precision data, and this leadsto signatures at the LHC that may imply early discovery. We study theimplications of this discovery from a bottom-up perspective, where effective4-fermion operators model the dominant effects of the new dynamics. We identifysimple approximate symmetries of these operators that may be required forrealistic masses of the third and fourth families. The large top mass forinstance is related to the structure of these operators.Comment: 15 pages, 1 figure, now matches published versio