Measurement of the semileptonic $ \mathrm{t}\overline{\mathrm{t}} $ + γ production cross section in pp collisions at $ \sqrt{s}=8 $ TeV

A measurement of the cross section for top quark-antiquark ( $\mathrm{t}\overline{\mathrm{t}}$ ) pairs produced in association with a photon in proton-proton collisions at $\sqrt{s}=8$ TeV is presented. The analysis uses data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.7 fb$^{1}$. The signal is defined as the production of a $\mathrm{t}\overline{\mathrm{t}}$ pair in association with a photon having a transverse energy larger than 25 GeV and an absolute pseudorapidity smaller than 1.44. The measurement is performed in the fiducial phase space corresponding to the semileptonic decay chain of the $\mathrm{t}\overline{\mathrm{t}}$ pair, and the cross section is measured relative to the inclusive $\mathrm{t}\overline{\mathrm{t}}$ pair production cross section. The fiducial cross section for associated $\mathrm{t}\overline{\mathrm{t}}$ pair and photon production is found to be 127 ±27 (stat+syst) fb per semileptonic final state. The measured value is in agreement with the theoretical prediction.