On Monday, 18th of September, at the 10th International Workshop on Top Quark Physics, TOP 2017, in Braga, Portugal, the CMS experiment reported the first observation of the top pair (tt) production using proton-nucleus collisions at 8.16 TeV center-of-mass (c.o.m) energy. The excess of events with respect to the background-only hypothesis corresponds to a significance of more than 5σ (the conventional “golden standard” for observation) even under the most conservative assumptions. The measured top-pair cross section is consistent with the expectations from the scaled proton-proton data as well as pQCD predictions at next-to-next-to-leading order with next-to-next-to-leading-log accuracy.
Until recently, studies of top quark production in nuclear collisions remained out of reach, due to the small integrated luminosities of the first heavy ion runs at the LHC. This situation changed with the proton-lead run at 8.16 TeV, performed in November 2016, when a data set corresponding to an integrated luminosity of 174 nb-1 was recorded, allowing the CMS Collaboration to perform the first-ever study of top quark production in nuclear collisions. The amount of signal in the selected sample is inferred by a fit to the invariant mass of the two untagged jets, interpreted as W boson decay products (W→qq’). The outcome of the fit is used to model the signal and background invariant mass distributions of the top quark candidate in the hadronic decay channel (t→Wb→qq’b). This result paves the way towards future studies of top quark production in the hot and dense matter created in nucleus-nucleus collisions.