We investigate a charge asymmetry in $t bar t gamma$ production at the LHC that provides complementary information to the measured asymmetries in $t bar t$ production. We estimate the experimental uncertainty in its measurement at the LHC with 8 and
14 TeV. For new physics models that simultaneously reproduce the asymmetry excess in $t bar t$ at the Tevatron and the SM-like asymmetry at the LHC, the measurement in $t bar t gamma$ at the LHC could exhibit significant deviations with respect to the SM prediction.
We have implemented a code for Z + n jets production in ALPGEN, with Z decays into several final states, including l+ l- and t tbar. The MLM prescription is used for matching the matrix element with the parton shower, including in this way the leadin
g soft and collinear corrections. In order to demonstrate its capabilities, we perform a combined analysis of Z -> t tbar and Z -> t tbar j production for a heavy leptophobic gauge boson. It is found that the effect of the extra jet cannot only be accounted for by a K factor multiplying the leading-order cross section. In fact, the combined analysis for Z -> t tbar and Z -> t tbar j presented improves the statistical significance of the signal by 25% (8.55 sigma versus 6.77 sigma for a Z mass of 1 TeV), compared with the results of an inclusive analysis carried out on the same sample of t tbar + t tbar j events.
