Anomalous top charged-current contact interactions in single top production at the LHC

In an effective theory approach, the full minimal set of leading contributions to anomalous charged-current top couplings comprises various new trilinear $tbW$ as well as quartic $tbf{f}^{\prime }$ interaction vertices, some of which are related to one another by equations of motion. While much effort in earlier work has gone into the extraction of the trilinear couplings from single top measurements, we argue in this article that these structures can be assessed independently by other observables, while single top production forms a unique window to the four-fermion sector. An effective theory approach is employed to infer and classify the minimal set of such couplings from dimension six operators in the minimal flavor violation scheme. In the phenomenological analysis, we present a Monte Carlo study at detector level to quantify the expected performance of the next LHC run to bound as well as distinguish the various contact couplings. Special attention is directed toward differential final state distributions including detector effects as a means to optimize the signal sensitivity as well as the discriminative power with respect to the possible coupling structures.

Figure 1

Tree level diagrams contributing to on-shell single top production in the $s$ channel (left column) and in the $t$ channel (right column): SM pieces (top row) and anomalous $tbf{f}^{\prime }$ contact insertions (bottom row).

Figure 2

Normalized $s$ channel distributions at parton level.

Figure 3

Normalized $t$ channel distributions at parton level.

Figure 4

Channel specific sensitivities in various coupling planes (top and bottom left) and effect of the $S_R$ sensitive observables $p_T(\ell )$ and $\cos {\theta }_{\ell }$ in the $t$ channel (bottom right) at parton level.

Figure 5

The detector efficiency matrix entries ${\epsilon }_{ij}$ ($tb$ selection on each partonic input process left, $tj$ selection right) and resulting binned event numbers corresponding to $\int L=100{\mathrm{fb}}^{-1}$, for various kinematic distributions.

Figure 6

The $1\sigma$ limits resolved by final state selections at detector level in various coupling planes, always setting the other couplings to their SM values (top and center rows). The bottom row shows additional features in the $S_L\text{−}{S}_R$ plane (see text).

Figure 7

Contribution from the helicity sensitive observables $p_T(\ell )$ and $\cos {\theta }_{\ell }$ to the total ${\chi }^2$ value (in percent) inside the $1\sigma$ region from the $tj$ selection in the scalar plane.

Figure 8

Detector level histograms of $\sqrt{s}$ (left) and $\cos {\theta }_{\ell }$ (right) after $tb$ selection at the reference point $S_R=0.1$ (event numbers normalized to $\int L=100{\mathrm{fb}}^{-1}$).

Figure 9

Value of the vector–scalar discriminant $P$ inside the $1\sigma$ region from combination of $tb$ and $tj$ selections in the $V_L^{\mathrm{off}}\text{−}{S}_L$ plane, with varying $b$-tagging setups. The 0-contour is highlighted by black dashed lines.

Figure 10

Impact of various parameter variations on the combined $tb$ and $tj$ selection $1\sigma$ bounds in the $V_L^{\mathrm{off}}\text{−}{S}_R$ at detector level (cf. text).