Evidence for $t\overline{t}\gamma$ production and measurement of ${\sigma }_{t\overline{t}\gamma }/{\sigma }_{t\overline{t}}$

Using data corresponding to $6.0{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the CDF II detector, we present a cross section measurement of top-quark pair production with an additional radiated photon in the central region with 10 GeV or more of transverse energy $t\overline{t}\gamma$. The events are selected by looking for a lepton ($\ell$ or $\mu$), a photon ($\gamma$), significant transverse momentum imbalance (${\mathrm{E̸}}_T$), large total transverse energy, and three or more jets, with at least one identified as containing a $b$ quark ($b$). Using an event selection optimized for the $t\overline{t}\gamma$ candidate sample, we also measure the cross section of $t\overline{t}$ (${\sigma }_{t\overline{t}}$). We measure the $t\overline{t}\gamma$ cross section (${\sigma }_{t\overline{t}\gamma }$) to be $0.18\pm 0.08\mathrm{pb}$, and the ratio of ${\sigma }_{t\overline{t}\gamma }$ to ${\sigma }_{t\overline{t}}$ to be $0.024\pm 0.009$. We observe a probability of 0.0015 (3.0 standard deviations) of the background (non-$t\overline{t}\gamma$ events alone producing 30 events or more.

Figure 1

The distributions for events in the $t\overline{t}$ sample (points) of (a) the $E_T$ of the lepton; (b) the missing transverse energy ${\mathrm{E̸}}_T$; (c) the $E_T$ of the highest $E_T$ jet; and (d) the total transverse energy $H_T$. The histograms show the expected SM contributions from top production ($t\overline{t}$), and miscellaneous backgrounds (Misc), which include diboson production, single-top, $W+\mathrm{HF}$, and $Z+\mathrm{HF}$ production as well as jets misidentified as leptons (QCD), and misidentified $b$ tags.

Figure 2

The distributions for events in the $t\overline{t}\gamma$ sample (points) in (a) the $E_T$ of the lepton; (b) the missing transverse energy, ${\mathrm{E̸}}_T$; (c) the $E_T$ of the $b$ jet; and (d) the $E_T$ of the photon. The histograms show the expected SM contributions from radiative top production ($t\overline{t}\gamma$), $W\gamma$ production with heavy flavor (HF), and miscellaneous backgrounds (Misc), which include $WW$ and $WZ$ production as well as jets, $\tau$ leptons, electrons, and jets misidentified as photons, jets misidentified as leptons (QCD), and misidentified $b$ tags.

Figure 3

The distributions for events in the $t\overline{t}\gamma$ sample (points) of (a) the total transverse energy $H_T$, the sum of the transverse energies of the lepton, photon, jets and ${\mathrm{E̸}}_T$, for the $t\overline{t}\gamma$ events; (b) the total number of jets. The histograms show the expected SM contributions from radiative top-quark pair production ($t\overline{t}\gamma$), $W\gamma$ production with heavy flavor ($W\gamma +\mathrm{HF}$), and miscellaneous backgrounds (Misc), which include SM $WW$ and $WZ$ production as well as jets, $\tau$ leptons, electrons, and jets misidentified as photons, jets misidentified as leptons (QCD), and misidentified $b$ tags.