  The measurement of a charge asymmetry in t tbar production at LHC constitutes
more than an independent confirmation of the forward-backward asymmetry found
at Tevatron. Indeed, both measurements together can be used to identify the
source of the asymmetry. This is demonstrated for the case of new Z', W' vector
bosons and colour-sextet and triplet scalars, exchanged in t, u channels
respectively, and a very heavy axigluon in the s channel.
