Counting b-jets at the FCC-ee as a probe of top-quark flavor physics

Abstract

We explore the sensitivity of a future FCC-ee to multi-TeV new physics (NP) that can generate vector, scalar and tensor flavor-changing (FC) $eetu_k$ contact interactions ($u_k=u,c$ for $k=1,2$), probed via single top-quark production, $e^+ e^- \to t \bar{u}_k, t \bar{u}_k \gamma$ (+ c.c.) and leading (following the top-quark decay) to two- and four-jet signals, $e^+ e^- \to 2j + X, 4j + X$, where $X$ denotes any non-jet final-state particles. Our analysis exploits an approximately conserved Standard Model (SM) quantum number introduced in [1] and termed "$b$-Parity" ($b_P$), which is applicable to scattering processes of the type $e^+ e^- \to n \cdot j_b + m \cdot j_\ell + X$, where $n$ and $m$ are the number of produced $b$-jets and light-jets ($u,d,c,s,$ and/or gluons) and $b_P=(-1)^n$. The FC $eetu_k$ four-fermion interactions can generate distinct $b_P$-odd ($b_P = -1$) signals in these multi-jet events, for which the only significant SM background stems from $b$-jet misidentification. We demonstrate that the FCC-ee, operating at a $\sqrt{s} = 240$ GeV (the $ZH$ run phase) with high luminosity and excellent flavor-tagging performance, is an ideal platform to search for these $b_P$-odd signatures. Indeed, by simply counting the number of final-state $b$-jets, the FCC-ee can probe NP scales of $\Lambda \sim 10$ TeV for the new heavy states that generate the vector, scalar and tensor $eetu$ and/or $eetc$ interactions. This reach, remarkably about 40 times the assumed FCC-ee center-of-mass energy, improves upon current bounds on some of these four-fermion operators by an order of magnitude and it critically relies on high-purity b-jet tagging.