CMS detector based on  [@Agostinelli:2002hh; @GEANT] and are reconstructed with the same algorithms that are used for data analysis. The pileup interactions are included in simulations to match the distribution of the number of interactions per LHC bunch crossing observed in data. The average number of pileup interactions is measured to be approximately 9 and 21 in the 7 and 8data sets, respectively.

The selection efficiency in all the simulated samples is rescaled to correct for residual differences in lepton selection efficiencies in data and simulation. This correction is based on the total lepton selection efficiencies measured in inclusive samples of $\PZ$ boson events in simulation and data using a “tag-and-probe” method [@CMS:2011aa], separately for 7 and 8collisions. More details can be found in Ref. [@Chatrchyan:2013mxa].

Event selection and background modelling {#sec:EventsBackground}
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The measurements presented in this paper are based on the event selection used in the previous measurements of Higgs boson properties in this final state [@Chatrchyan:2013mxa; @Khachatryan:2014kca]. Events are selected online requiring the presence of a pair of electrons or muons, or a triplet of electrons. Triggers requiring an electron and a muon are also used. The minimum of the leading and subleading lepton are 17 and 