Photon initiated single top quark production via flavor-changing neutral currents at the LHC

Single top quark production is a powerful process to search for new physics signs. In this work we propose and investigate a search for top quark flavor-changing neutral currents (FCNCs) via a photon using direct single top quark production events in proton-proton collisions at the LHC at CERN. We show that the direct single top quark final state can provide constraints on the strengths of top-quark-$\gamma$ and top-quark-gluon FCNC couplings simultaneously. Results of a search for direct single top quark production at the LHC at a center-of-mass energy of 8 TeV performed by the ATLAS Collaboration are used to set first experimental limits on the anomalous FCNC top decay branching fractions $\mathcal{B}(\mathrm{t}\to \mathrm{u}\gamma )<0.05\%$ and $\mathcal{B}(\mathrm{t}\to \mathrm{c}\gamma )<0.14\%$ via direct single top quark production. Finally, the sensitivity of the proposed channel for probing the top-quark-$\gamma$ couplings at 13 TeV is presented.

Figure 1

Representative leading order Feynman diagrams describing (a), (b) the direct single top quark production, (c)–(f) single top quark production in association with a jet, (g),(h) single top quark production in association with a photon and (i) top quark decay to a photon and up-type quark from the SM $\mathrm{t}\overline{\mathrm{t}}$ production, via FCNC interactions in proton-proton collisions.

Figure 2

The total signal cross section of $\mathrm{pp}\to \mathrm{t}$ [elastic $(\mathrm{EPA})+\mathrm{inelastic}$ (NNPDF_QED)], $\mathrm{pp}\to \mathrm{t}\gamma$, $\mathrm{pp}\to \mathrm{t}+\mathrm{jet}$ and $\mathrm{pp}\to \mathrm{t}\overline{\mathrm{t}}\to \mathrm{t}\gamma \mathrm{q}$ via $\mathrm{tu}\gamma$ (left panels) and $\mathrm{t}\gamma \mathrm{c}$ (right panesl) FCNC interactions as a function of $\mathcal{B}(\mathrm{t}\to \mathrm{q}\gamma )$ at 8 TeV (top panels) and 13 TeV (bottom panels).

Figure 3

Distributions of four important discriminating variables. (a) The top quark transverse mass. (b) The lepton transverse momentum. (c) The $\mathrm{\Delta }R$ between the lepton and the reconstructed top quark. (d) The b-jet transverse momentum for the $\mathrm{pp}\to \mathrm{t}$, $\mathrm{pp}\to \mathrm{t}+\text{jet}$, $\mathrm{pp}\to \mathrm{t}\gamma$ and $\mathrm{pp}\to \mathrm{t}\overline{\mathrm{t}}\to \mathrm{t}\gamma \mathrm{q}$.

Figure 4

Excluded region at 95% C.L. on the branching fraction plane of ($\mathcal{B}(\mathrm{t}\to \mathrm{c}\gamma )$, $\mathcal{B}(\mathrm{t}\to \mathrm{u}\gamma )$) (left panel) and ($\mathcal{B}(\mathrm{t}\to \mathrm{ug})$, $\mathcal{B}(\mathrm{t}\to \mathrm{u}\gamma )$) (right panel). The blue hatched regions show the branching fraction values excluded when considering the $\mathrm{pp}\to \mathrm{t}$ process and the red hatched regions show the branching fraction values excluded when considering all signal channels.

Figure 5

Excluded region at 95% C.L. on the coupling constants ${\kappa }_{\mathrm{u}\gamma }$ and ${\kappa }_{\mathrm{c}\gamma }$ obtained via $\mathrm{pp}\to \mathrm{t}$ channel and all signal channels at 8 TeV (the blue and brown curves). The projection results to 13 TeV are shown with red and green curves. The most stringent result obtained by the CMS Collaboration via $\mathrm{pp}\to \mathrm{t}\gamma$ at 8 TeV is shown with a dashed black curve [16].

Figure 6

Distributions of the reconstructed top quark mass (left panel) and top quark transverse momentum (right panel) for signal processes ($\mathrm{pp}\to \mathrm{t}$ via $\mathrm{tq}\gamma$ and tqg FCNC interactions), $\mathrm{W}+\mathrm{HF}$, single top and $\mathrm{t}\overline{\mathrm{t}}$ production.