A search for the CP-violating strong decays η→π + π − and η ' (958)→π + π − has been performed using approximately 2.5×10 7 events of each of the decays D + →π + π + π − and D s+ →π + π + π − , recorded by the LHCb experiment. The data set corresponds to an integrated luminosity of 3.0 fb −1 of pp collision data recorded during LHC Run 1 and 0.3 fb −1 recorded in Run 2. No evidence is seen for D (s)+ →π + η (') with η (') →π + π − , and upper limits at 90% confidence level are set on the branching fractions, B(η→π + π − )<1.6×10 −5 and B(η ' →π + π − )<1.8×10 −5 . The limit for the η decay is comparable with the existing one, while that for the η ' is a factor of three smaller than the previous limit.
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