Measurement of the inclusive top-quark pair + photon production cross section in the muon + jets channel in pp collisions at 8 TeV

The production cross section of top-quark pairs associated with a photon is measured in pp-collisions at a centre-of-mass energy of at $8\,$TeV, with the CMS detector at LHC, using data corresponding to an integrated luminosity of ${\cal L}_{{\rm int}} = 19.7\,{\rm fb}^{-1}$. The measurement is performed in the muon + jets decay channel. The signal region is defined by the final state of the process ${{\rm pp \rightarrow W^+W^-b \bar b} \gamma}$, with a transverse energy of the photon of $E_T(\gamma)>20\,$GeV and a distance between the photon and the b quark in $\eta-\phi$ space, $\Delta R(\gamma, {\rm b/\bar b}) > 0.1$. The normalized cross-section is measured to be $R = \sigma_{\bar t t + \gamma} / \sigma_{\bar t t} = (1.07 \;\pm 0.07\,{\rm(stat.)}\;\pm 0.27\,{\rm(syst.)})\times 10^{-2}$. Using a CMS cross section measurement of inclusive $\bar t t$ production at $8\,$TeV, $\sigma_{t \bar t}^{\rm CMS} = 227 \pm 15\,$pb, the top-quark pair + photon production cross section is $\sigma_{\bar t t + \gamma}^{\rm CMS} =2.4\;\pm0.2\,{\rm(stat.)}\;\pm0.6\,{\rm(syst.)}\,$pb. The result agrees with the standard model expectation of $\sigma_{\bar t t + \gamma}^{SM} = 1.8\pm 0.5\,$pb.