On Tuesday, 4th of April, at the 25th DIS international workshop in Birmingham, UK, the CMS experiment extended the very first cross section measurement of the top pair (tt) production at 5.02 TeV center-of-mass (c.o.m) energy. The measurement tracks the evolution of the tt cross section as a function of c.o.m energy, crucial for the extraction of the top-quark pole mass, while it is found to constrain the gluon distribution function at the un-explored large longitudinal parton momentum fraction regime. Future measurements of the tt cross section in nuclear collisions at the same nucleon-nucleon c.o.m energy would profit from the availability of such a reference measurement in pp collisions, without the need to extrapolate from measurements at different c.o.m energies.
In November 2015 the LHC delivered proton-proton collisions at 5.02 TeV, with CMS having being fully efficient and having managed to collect 27.4pb-1 of low pileup proton-proton collision data. The fraction of tt events initiated by gluon-gluon collisions grows monotonically with center-of-mass energy, rendering this 'peculiar' data set partially complementary to the higher energy samples. This study exploited candidate events with at at least one charge lepton (muon or electron), accompanied by the presence of at least two jets (from the hadronization of the b quarks). The cross section is separately extracted with an elaborate distribution fit (l+jets channel) and a simpler, but robust, event-counting experiment approach (dilepton channel), and found in agreement with the SM expectation (CMS-PAS-TOP-16-023). The final uncertainty of the measurement is now measured 12%, the latter almost equally dominated by the statistical precision and systematic considerations.