Deciphering top flavor violation at the LHC with B factories

TL;DR

This work establishes a model-independent EFT framework to relate top FCNC decays $t\to cZ$ and $t\to c\gamma$ to low-energy flavor observables. By performing weak-scale matching and translating $B$-physics data into bounds on seven dimension-6 operators, the authors map how various NP scenarios could manifest at the LHC. The analysis finds that most operators, especially those involving left-handed light-quark fields, are tightly constrained, but a window remains for operators with right-handed charm or up quarks. If the LHC observes FCNC top decays, the observed pattern will help identify the underlying NP, with a typical implication that the new physics scale is $\Lambda$ of a few TeV and thus within reach of direct discovery at the LHC.

Abstract

The LHC will have unprecedented sensitivity to flavor-changing neutral current (FCNC) top quark decays, whose observation would be a clear sign of physics beyond the standard model. Although many details of top flavor violation are model dependent, the standard model gauge symmetries relate top FCNCs to other processes, which are strongly constrained by existing data. We study these constraints in a model independent way, using a low energy effective theory from which the new physics is integrated out. We consider the most important operators which contribute to top FCNCs and analyze the current constraints on them. We find that the data rule out top FCNCs at a level observable at the LHC due to most of the operators comprising left-handed first or second generation quark fields, while there remains a substantial window for top decays mediated by operators with right-handed charm or up quarks. If FCNC top decays are observed at the LHC, such an analysis may help decipher the underlying physics.

Figures (5)

• Figure 1: A one-loop contribution from $O_{RL}^w$ (denoted by $\otimes$) to $O_{7\gamma}$.
• Figure 2: Constraints from $B\to X_s \gamma$ and $B\to X_s \ell^+\ell^-$ in the $C_{LL}^u$ -- $C_{LL}^h$ plane. The red, green, and blue regions denote 68%, 95%, and 99% CL, respectively. The region between the dashed lines is beyond the LHC sensitivity.
• Figure 3: Constraints on $O_{LR}^w$ in the ${\rm Re}(C_{LR}^w)$ -- $|V_{cb}|$ plane from semileptonic $B\to X_c \ell\bar{\nu}$ (solid curves) and $B\to D^{(*)}\ell\bar{\nu}$ decays (dashed curves) and their combination (shaded areas). For each constraint the 68%, 95% and 99% CL regions are shown.
• Figure 4: Constraints on $O_{LR}^{\prime w}$ in the ${\rm Re}(C_{LR}^{\prime w})$ -- $|V_{ub}|$ plane from $B\to X_u \ell\bar{\nu}$ (solid curves) and $B\to \pi \ell\bar{\nu}$ (dashed curves) and their combination (shaded areas). For each constraint the 68%, 95% and 99% CL regions are shown.
• Figure 5: Constraints from $B\to \rho \gamma$ and $B \to \mu^+\mu^-$ in the $C_{LL}^{\prime u}$ -- $C_{LL}^{\prime h}$ plane. The red, green, and blue regions denote 68%, 95%, and 99% CL, respectively. The region between the dashed lines is beyond the LHC sensitivity.