Measurement of the $t\bar{t}\text{+}\gamma$ production cross-section in pp collisions at $\sqrt{s}=8~\mathrm{TeV}$

A measurement of the cross section for top quark pairs produced in association with a photon in proton-proton collisions at $\sqrt{s}=8~\mathrm{TeV}$ is presented. The analysis uses data, collected by the CMS detector at the LHC, corresponding to 19.7 $\text{fb}^{-1}$ of integrated luminosity. The signal is defined as the production of a pair of top quarks in association with a photon having a transverse energy of $E_\text{T}>25~\mathrm{GeV}$ and a pseudorapidity of $\left|\eta\right|<1.44$. The measurement is performed in the fiducial phase space corresponding to the semileptonic decay chain of the top quark pair, and the cross section is measured relative to the inclusive top quark pair production cross section. The ratio of the cross section of top quark pair plus photon production to the top quark pair production cross section is measured to be $\sigma^\text{fid}_{t\bar{t}\text{+}\gamma}/\sigma_{t\bar{t}} = 5.2\pm1.1 \times 10^{-4} (\text{stat}+\text{syst})$ per semileptonic final state. The fiducial cross section for top quark pair plus photon production is found to be $\sigma^\text{fid}_{t\bar{t}\text{+}\gamma} = 127 \pm 27 (\text{stat}+\text{syst})~\mathrm{fb}$.