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New sensitivity of current LHC measurements to vector-like quarks

by A. Buckley, J. M. Butterworth, L. Corpe, D. Huang, P. Sun

This Submission thread is now published as

Submission summary

Authors (as registered SciPost users): Andy Buckley · Jonathan Butterworth · Louie Corpe
Submission information
Preprint Link:  (pdf)
Date accepted: 2020-10-27
Date submitted: 2020-09-08 10:32
Submitted by: Corpe, Louie
Submitted to: SciPost Physics
Ontological classification
Academic field: Physics
  • High-Energy Physics - Phenomenology
Approaches: Computational, Phenomenological


Quark partners with non-chiral couplings appear in several extensions of the Standard Model. They may have non-trivial generational structure to their couplings, and may be produced either in pairs via the strong and EM interactions, or singly via the new couplings of the model. Their decays often produce heavy quarks and gauge bosons, which will contribute to a variety of already-measured "Standard Model" cross-sections at the LHC. We present a study of the sensitivity of such published LHC measurements to vector-like quarks, first comparing to limits already obtained from dedicated searches, and then broadening to some so-far unstudied parameter regions.

Author comments upon resubmission

We thank the editors and referees for the positive comments and helpful suggestions, which we have implemented, and which we believe have helped us to improve the manuscript. In particular, the comments regarding the most well-motivated VLQ multiplets and W/Z/H admixtures have led to some important improvements to the paper. We have decided to add many additional results in the appendix which cover the scenarios mentioned (namely, considering singlets, doublets and triplets, and changing to a WZH=011 or WZH=211 scenario as required, instead of an unrealistic WHZ=111 case). We have also changed the plots in the main body to represent the WZH=211 case instead of the democratic one. We have kept the plots with four VLQs in the main body as the maximal case study, but the comparison with other results shows that the overall message is unchanged, and the more realistic cases are now available in the appendix, which we hope will satisfy the reviewers.

List of changes

We have introduced some other changes to the manuscript as requested, with details provided below.

In the introduction, we have specified the charges of the VLQs, as suggested. The reviewers asked whether the signal predictions were LO or NLO. We confirm they are all LO, and mention this in the text and relevant figure captions (where we also increased the label size as requested). We are aware that NLO predictions exist and have now mentioned them, but using those is beyond the scope of this work.

For Section 2, in the second paragraph , we agree with the reviewers that the statement was confusing, and we have opted to delete the statement, which in the end was not really needed. We now note that the effect of first-generation quark couplings on EW pair-production has been remarked upon in the previous papers. We have remarked upon the W/Z -> QQ coupling as suggested. The arrow on the diagram in fig 1 e) has been fixed. Also in this section, as requested, we have clarified/reworded the statements on: phase space suppression in pair production versus single production; and the suppression T+q/B+q channels.

In Section 3, when comparing to LHC results, we have noted the integrated luminosities of the relevant analyses, and made a statement and added references for the CMS coverage.

Regarding Section 4/Fig 6, we have clarified that this relates to third-generation couplings only. We take on board the comment about the BTXY-multiplet being unrealistic, and now provide results for the various singlets, doublets and triplets which are allowed (in addition to the unrealistic BTXY case as a benchmark). As suggested, we have also commented that these bounds cannot be directly compared to searches without accounting for enhancements due to degenerate VLQs.

For section 5, we have kept in the main body the BTXY results as a useful benchmark, but noted that the more realistic multiplet results can be found in the appendix. We have also added Section 5.4 which describes the main differences between the BTXY and multiplet results. We have also replaced the WHZ=111 with a more appropriate admixture in each case. We have clarified as requested which diagrams from Fig 1 are included in the study.

In addition, we have added additional references where appropriate.

Published as SciPost Phys. 9, 069 (2020)

Reports on this Submission

Anonymous Report 2 on 2020-10-7 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:2006.07172v2, delivered 2020-10-07, doi: 10.21468/SciPost.Report.2053


1- Highlighting a novel way to constrain a class of models, beyond dedicated searches
2- coverage of a broad class of new physics models
3- use of a new source of data, in the form of standard model measurements




The authors have replied to all issues raised in the previous reports, so I think that the manuscript can be published in the current form. I particularly appreciate that the authors provided many new plots in the Appendices, as they provide very useful information for establishing bounds on this class of models.

Requested changes


  • validity: top
  • significance: high
  • originality: top
  • clarity: high
  • formatting: excellent
  • grammar: perfect

Anonymous Report 1 on 2020-9-15 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:2006.07172v2, delivered 2020-09-15, doi: 10.21468/SciPost.Report.1991


1- Broad results, complementary to standard analyses
2- The methodology used can be easily exported to other experimental analyses and have significant positive implications.


1- No significant weaknesses.


I think the improvements introduced in the second version of the article make it suitable for publication in SciPost.

Incidentally, I'm a bit confused by the choice of 0:1/2:1/2 branching fractions for the doublet case. I understand it was motivated by the request by the reviewer but this BRs only make sense if one is considering one of the two quarks in the doublet. If both quarks in the doublet are included, then we recover the 1/2:1/4:1/4 ratios when adding both quarks (as dictated by the equivalence theorem). For instance in the (X, T) case Gamma(X->Wt)=2Gamma(T->Z t)=2Gamma(T->H t) in the asymptotic limit. Probably this is what the authors do (they explicitly say that the X->Wt BR is always 1) but it is not entirely obvious from the text in the appendix. In any case, I think this is a minor issue that I don't feel needs any change.

  • validity: top
  • significance: top
  • originality: top
  • clarity: top
  • formatting: perfect
  • grammar: perfect

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