New Physics Through Flavor Tagging at FCC-ee

We deeply thank the referee for the positive feedback and insightful comments on our manuscript. We have revised the draft accordingly in the places we deemed appropriate. Below, we provide detailed replies to the comments, numbered according to their order in the report:

1) We believe that introducing a concrete example at this point in the paper would divert from the main narrative of the section. However, we would like to emphasize that our paper itself serves as a concrete example of corners at the TeV scale that have remained obscure even after LHC searches (e.g., four-fermion flavor-conserving operators involving second, third generation quarks).

2) We have added a footnote to clarify why we grouped up and down quarks together as $jj$.

3) Following the referee’s suggestion, we have swapped the paragraphs summarizing the contents of Sections 4 and 5.

4), 10) The normalization of hadronic and leptonic ratios follows the convention adopted at LEP and LEP-II, which we also applied to $R_t$. In particular, top quark pair production is excluded from the denominator normalization factor, as its experimental signature differs significantly from that of light quarks.

5) We clarify the advantage of directly fitting $N_{\rm{tot}}$ in the subsequent sentence: it ensures that any external uncertainty on $N_{\rm{tot}}$ does not propagate into additional uncertainty on the ratios.

6) In the preceding paragraph, above Eq. (3), we mention that we expect millions of events, justifying the use of the Gaussian approximation to the likelihood.

7) We agree with the referee that determining the background in a data-driven way would be ideal, and that further exploring this direction would be valuable. However, we have shown that an error of 1% is sufficient to leave our results on hadronic and leptonic ratios unchanged, and this level of precision already appears feasible through Monte Carlo modeling.

8), 9) The referee is correct in pointing out that the absence of correlation between relative errors is an assumption we make. The results we obtain for $R_b, R_c, R_s$, including their (small) correlation, depend on our set of assumptions, among which the one concerning the correlation of relative errors. We believe this assumption is consistent with the existing literature on the topic.

11) We consider the estimate provided in Ref. [30] for the improvement on $N_\nu$ with the best measurement to be reasonable, and we have decided to maintain it.

12) We confirm that this corresponds to the $\Delta \chi^2 = 2.3$ ellipsis.

13) We report the confidence intervals from LHC and HL-LHC as given in the external references cited in the text, and for consistency adopt the same convention in our results.

14) As explained in the text, the bounds from the Z-pole and those above the Z-pole are dominated by different observables, which depend on different combinations of Wilson coefficients. Consequently, it is reasonable to expect that the corresponding directions are not perfectly aligned, allowing their combined fit to resolve most of the correlations within each one individually. The fact that they are nearly orthogonal is a fortunate numerical coincidence.

15) We have added "current" to the text to clarify that we are comparing to current Bs meson bounds.

We hope that our revisions adequately address all concerns.

Best regards,

the authors.