2B or not 2B, a study of bottom-quark-philic semi-visible jets

This paper considers a signature of hidden valley models -- models containing a mediator U(1) and a strongly-coupled dark sector containing dark pions -- where dark pions decay predominantly to b-quarks. First, three existing searches are considered which cover this possibility: searches for semi-visible jets; searches for dark matter production in association with b jets; and searches for missing transverse momentum plus several b-jets. The first of these was already available in RIVET, while the second two were implemented by the authors, with validation material presented in this paper.

Secondly, the paper examines the proposed signature in detail and proposes observables which could be useful to devise a full search. This consists of relatively novel techniques such as using variable-radius jets, but also loosening lepton veto requirements. Plots of various kinematic variables for signal and background events are shown.

I believe that the work in the paper is sound and interesting, and may be used to help design a search in future. However, it requires substantial editing, including the abundant typos (even in the figures, e.g. Figure 7 contains Title=... twice).

In particular:

The introduction is too abrupt and unclear: after one paragraph of motivation, the next starts discussing how signal samples are generated, when it has not been made particularly clear what the signal is or why. I suggest that the signature be spelled out in words clearly, and discuss the various other searches for similar signals at that point before details of simualations.

The phrase "which allowed to validate the Rivet routines written for the analyses, if not already been made public." should be clarified to more clearly state at that point which analyses existed and which are novel.

The phrase "Since VR jet radius adaptive compared to RC," needs an "is".

"The Yukawa coupling" kappa is not properly defined, nor is it actually used.

I also have some worries. The first is that the quality of the recast in Table 2 is very poor. The authors seem content that the yields are "mostly within 100%" but this is barely true; such a recast, with only those benchmark points as cross-checks, would not normally be considered validated. They also make the statement that "It is remarkably difficult to reproduce [MET] distributions at particle level" which needs qualification, since I am not aware that this is true. I suppose that the left plot of figure 2 is supposed to show that that analysis is not sensitive to their signal, but I am not sure what the right-hand plot is for, since a casual reader might infer that it is a reproduction of the exclusion curve for validation purposes.

The choice of loosening the lepton veto to 75 GeV is curious, since I understood that leptons along with b-jets were normally removed by isolation requirements prior to any veto, and this is normally regarded as rather efficient. The authors should at least comment on this.
Table 3 presents selection efficiencies for different jet strategies, and aare referred to as "It is evident that the use of VR jets will enhance the signal efficiency." But this is only helpful if it enhances signal compared to background.

Finally, I am surprised that the authors shy away from proposing an actual search. Their explanation for this is that they are using particle-level information and detector effects may be important. However, Rivet is supposed to compensate for this, and a public paper would at best be used as a starting point for an experimental study, so the more details that are there the better.

I prepared this report before reading the review of the first referee until this point, and I find that we broadly agree. I would not insist, however, that the authors propose a set of cuts, even if I am surprised that they did not attempt to do so.

Ask for major revision

1- The topic of semi-visible jets is very relevant and of great interest for LHC experimentalists.
2- The specific model, dark pions decaying into bottom quarks, has not been considered previously.
3- The discussion of jet clustering and jet tagging is quite useful.

1- The paper is not very pedagogical. The introduction is very short and the outline is missing. Subsequent sections are quite technical and written for an expert audience.
2- The model is not explained clearly enough.
3- The purpose of the third section is unclear.
4- The paper ends too early. There should be a recommended set of cuts and an estimate of the resulting signal-to-background ratio.

The manuscript "2B or not 2B, a study of bottom-quark-philic semi-visible jets" considers models of strongly-interacting dark sectors giving rise to dark showers that result in semi-visible jets at the LHC. In contrast to previous studies of this set-up, the present work considers dark mesons that decay dominantly into bottom quarks. Existing analyses are blind to this signature, as they explicitly reject events with several b-tagged jets. The authors then attempt to determine whether other analyses might be sensitive to this model before proposing a possible new search strategy.

The manuscript is quite short and feels very rushed. The introduction skips over the general motivation and comes straight to the point, but does not outline the remainder of the work. The second section briefly introduces the model, skipping many details and assuming that the reader is very familiar with the literature. The third section shows that it is difficult to reproduce signal yields for existing analyses, but concludes that these details don't matter because the existing analyses are expected to be blind to this model.

The main point is finally to propose a new analysis strategy that would be better suited for constraining the model under consideration. The main focus is on identifying promising jet tagging algorithms and kinematic variables that may be used for background suppression. However, no actual conclusion is reached regarding the signal-to-background ratio and whether such an analysis could actually be sensitive to the model.

Unfortunately, the paper in its current form does not meet the requirements for publication. A publication could be considered, if a substantial amount of detail is added to the various sections, as listed below.

1- An outline needs to be added to the introduction.
2- In the second section, the structure of the confined phase does not become clear. If there is only one flavour, how can there be several different dark pions? It does also not become clear which dark mesons are assumed to be stable and which ones are assumed to decay. What is the point of the very peculiar choices of the masses of the off-diagonal dark mesons? What is the reasoning behind the choice of $R_\text{inv}$? I understand that the authors are not interested in the model-building details for the purpose of this work, but they need to at least motivate the choices that they make.
3- I don't understand the statement "The dark QCD confinement scale value
was set to 6.5 TeV". Isn't that in contradiction with the masses of the dark mesons? Presumably the authors want to say something different here.
4- The proposed model comes with several free parameters, such as the mass of $\Phi$ and its coupling. Without specifying these parameters, the signal predictions stated in the following sections are meaningless. Whether the model is allowed or excluded must surely depend on the mass and coupling of the mediator?
5- In section 3 the authors need to explain more clearly where the differences between the official yields and the simulated yields for the benchmark models stem from and which steps have been taken to improve agreement.
6- For the second analysis (MET+Higgs), it would be interesting to see a histogram of $m_{bb}$ compared to data. Ideally, this should be done for different dark meson masses, to see if one might see a peak in the invariant mass distribution.
7- In section 5 the authors need to propose a combination of cuts with which they can conceivably achieve interesting signal-to-background ratios. At the very least they need to demonstrate that the resulting sensitivity should be much better than for the analyses considered in section 3.
8- The summary needs to contain a clearer conclusion and plan for future work. What are the next steps towards applying such an analysis to real data?
8- In figure 1, one of the circles is labeled "DS", the other is labeled "HV". Why?
9- The purpose of the right panel of figure 2 is unclear. Do the authors have permission to use this figure?

Ask for major revision