  A search for narrow resonances in dielectron and dimuon invariant mass
spectra has been performed using data obtained from proton-proton collisions at
sqrt(s) = 13 TeV collected with the CMS detector. The integrated luminosity for
the dielectron sample is 2.7 inverse femtobarns and for the dimuon sample 2.9
inverse femtobarns. The sensitivity of the search is increased by combining
these data with a previously analysed set of data obtained at sqrt(s) = 8 TeV
and corresponding to a luminosity of 20 inverse femtobarns. No evidence for
non-standard-model physics is found, either in the 13 TeV data set alone, or in
the combined data set. Upper limits on the product of production cross section
and branching fraction have also been calculated in a model-independent manner
to enable interpretation in models predicting a narrow dielectron or dimuon
resonance structure. Limits are set on the masses of hypothetical particles
that could appear in new-physics scenarios. For the Z'[SSM] particle, which
arises in the sequential standard model, and for the superstring inspired
Z'[psi] particle, 95% confidence level lower mass limits for the combined data
sets and combined channels are found to be 3.37 and 2.82 TeV, respectively. The
corresponding limits for Kaluza-Klein gravitons arising in the Randall-Sundrum
model of extra dimensions with coupling parameters 0.01 and 0.10 are 1.46 and
3.11 TeV, respectively. These results significantly exceed the limits based on
the 8 TeV LHC data.
