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Hitting two BSM particles with one lepton-jet: search for a top partner decaying to a dark photon, resulting in a lepton-jet
by K. du Plessis, M. M. Flores, D. Kar, S. Sinha, H. van der Schyf
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|Authors (as registered SciPost users):||Deepak Kar · Sukanya Sinha|
|Preprint Link:||https://arxiv.org/abs/2112.08425v2 (pdf)|
|Date submitted:||2022-02-17 07:09|
|Submitted by:||Kar, Deepak|
|Submitted to:||SciPost Physics|
A maverick top partner model, decaying to a dark photon was suggested. The dark photon decays to two overlapping electrons for dark photon masses of $\approx 100$ GeV, and results in a so-called lepton-jet. The event includes a top quark as well, which results in events with two boosted objects, one heavy and the other light. We propose a search strategy exploiting the unique signal topology. We show that for a set of kinematic selections, both in hadronic and leptonic decay channel of the SM top quark, almost all background can be eliminated, leaving enough signal events up to top partner mass of about 3 TeV for the search to be viable at the LHC.
Submission & Refereeing History
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Reports on this Submission
- Cite as: Anonymous, Report on arXiv:2112.08425v2, delivered 2022-03-24, doi: 10.21468/SciPost.Report.4765
1) The authors study an interesting signal that would be interesting to search for.
2) The authors provide a thorough collider study of the signal, showing the viability of a search.
3) The analysis seems solid and reliable.
1) The model is not well laid out in the paper, and there is some confusion in the discussion of what the decay of a top partner to a dark photon may be the dominant decay.
2) Overall, I found the discussion of the collider analysis to be a somewhat confusing. The authors could strengthen the paper by addressing many of the points in the report.
This paper studies a search for novel decays of up-type vectorlike quark, i.e. top partners. In particular, the authors investigate the signal of a top partner decaying into a top quark and very light dark photon. The dark photon then decays into highly collimated leptons that form a lepton-jet. This is an interesting signal, in particular since top partners have not been discovered in more traditional modes. The authors have a complete collider study showing the viability of searching for this signal. The paper is publishable, once some concerns/questions are addressed and the discussion made clearer. The basis of a good paper is there and the material is substantive enough for SciPost, once the authors address the "Requested changes."
I would like to stress that although the "Requested changes" are quite long, the study appears reliable and trustworthy. I do not ask the authors to redo an analysis, just to clarify the following points, all of which should be addressed in the manuscript.
(1) In the third paragraph of page 3, it's stated that the top partner decay into dark photons is dominant when the dark photon is much less massive than the top partner. However, I think the requirement is that the dark sector vev is much smaller than the electroweak vev. As stated in the next paragraph, enhancement of the decay into the dark photon goes as (v_EW/v_d)^2. In principle, there could be a small dark photon mass by having a very small gauge coupling and large dark sector vev. In that case, there would be no enhancement to the top partner decay. It would be good if the authors could clarify the discussion.
(2) On the transition between pages 6 and 7, there is a statement "Here we have scaled the cross-sections by 0.1 to mimic the electron misidentification rate." Is there a relevant citation for this? I'm a bit surprised that the misidentification rate is so large, since I would expect most hadronic jets to leave deposits in the HCAL and the lepton jets to leave deposits in the ECAL only. Are these single pion backgrounds, or something similar? A clarification in the text would be useful.
(3) (a) In the first paragraph of page 4, the authors state that single production of very heavy vector like quarks dominates the QCD pair production. However, the single production is model-dependent and depends on the mixing of the vector like quark with the SM quarks. The QCD pair production is more model independent, depending on color representation, mass, and spin. Hence, if the mixing of the vectorlike quark with the SM top quark is very small, single production rates vanish. An illustration of this can be found in Fig. 3 of arXiv: 1803.06351 [hep-ph]. It would be good if the authors could clarify this in the manuscript. For example, the authors don't state what mixing angle they assume when calculating production rates.
(b) In section 2.2, the authors introduce a variable \varepsilon. This is not (and should be) defined in the paper.
(c) In section 2.2, the authors present some cross sections. In line with points (a,b) above, what parameters did the authors choose beyond the masses? How did they choose these parameters? These are very model-dependent cross sections.
(d) In general, the paper could benefit from having a section outlining the model, the quantum numbers, the interactions, the relevant parameters, and the current constraints on those parameters.
(4) On the bottom of page 8, the authors list that there are 2,380 remaining for vectorlike quark masses of 1 TeV, 35 events for 3 TeV, and 1 event for 5 TeV. Are these the number of events remaining from the original 100,000 generated for each signal mass? Or are these the number of events expected to remain after 300 inverse femtobarns of data? The number of events expected after 300 inverse femtobarns of data, or even 3 inverse attobarns, seems more useful since realistic significances could then be calculated.
(5) From tables 1 and 2, can the authors calculate the expected significance for their parameter point? This would seem like the useful information needed to determine if this signal is indeed observable at the LHC. The remaining cross section, and significances at 300 inverse femtobarns and 3 inverse attobarns would be very useful for the reader. Even a luminosity plot showing the luminosity for 2\sigma and 5\sigma observations would be helpful in understanding how observable this signal is.
(6) In section 3, I got a little lost in the discussion of jets. The word jet seems to simultaneously mean lepton and hadronic jets. However, these objects are very different. The entire discussion could be greatly improved by being more clear when the authors mean hadronic or lepton jets. This occurs throughout the discussion. A few questions along those lines:
(a) At the top of page 5, the authors mention using the mass over transverse momentum ratio to discriminate lepton-jets. However, can't you tell a lepton-jet from a hadronic jet mostly by the activity in the ECAL and the HCAL? Are the authors considering hadronic jets faking lepton-jets?
(b) In the first paragraph of Sec. 3.2, the authors say they require at least three jets. Is this one lepton jet and two hadronic jets?
(c) At the end of the second paragraph of section 3.2, the authors say they require one small radius jet and at least three large radius jets. However, I thought the signal only had one final state top quark, which would seem to be one large radius jet and not three? Or are they resolving the decay products of the top quark, which is different than what is shown in Fig. 2? I know the authors state that Fig. 2 is "not an accurate indicator of the event kinematics", but they do discuss top jets.
(d)In general, I thought the discussion of the collider analysis could be made clearer. I had similar confusion throughout the rest of the section.
(7) I was a little confused at the end of Sec. 2.2, where the authors state that there will be a combined 300 inverse femtobarn of data at the end of Run 3. Don't CMS and ATLAS already have a combined luminosity near 300 inverse femtobarns? Or do the authors means a combination of Run 1, 2, and 3 data?
(8) I may have missed it, but is Fig. 1 ever referred to in the text? If not, it should be.
(9) At the bottom of page 5, there is a statement "The decay mode of the top quark on the other side will determine the full topology." What is meant by "on the other side"?
(10) Fig. 1 shows a diagram for single production of the vector-like quark and its decay. However, I think the t-channel production with a b-quark in the initial state is the dominant production mode.
(11) I think all the captions for figures and tables could be improved. In particular in Secs. 3 and 4 I found them confusing.
(12) In general, the paper is not well-cited. There are many earlier seminal studies of dark photons that have not been cited. The original papers studying lepton-jets are not cited, in particular the theory proposals of lepton-jets are not cited. Several of the signals discussed in Sec. 2.1 need citations. There are many citations missing throughout the paper.
(13) This comment the authors can take or leave. I thought the third paragraph of page 3 was was very motivation for this search and may be better placed in the introduction.