Model-Independent Searches for New Physics in Multi-Body Invariant Masses

Round 1

Reviewer 1 Report

This paper illustrates how an analysis of three and four-body invariant masses can be an important tool in searching for the production of new particles at the Large Hadron Collider. This is important in order to maximise the discovery potential of the collider.

The paper is clearly and concisely written, with the correct level of detail to serve its purpose. It is clearly a summary of a very substantial body of work. The general techniques are illustrated with a number of examples of new particles predicted by interesting beyond-the-standard-model (BSM) theories.

I am not an expert in Monte Carlo simulations or particle data analysis techniques so I cannot judge the paper in that level of detail.  However, the paper appears to be sound and of very good quality. The case it makes for an increased focus on three and four-body invariant mass analyses is very well made and should influence both theorists working on BSM physics and collider experimentalists searching for new physics at the LHC. 

I am happy to recommend the paper for publication in Universe.

 

Author Response

Thank you  for your recommendation.

Reviewer 2 Report

The manuscript proposes to search for Beyond-the-Standard-Model (BSM) particles
through the analysis of three and four-body invariant masses, as for example
2 jet + lepton final states. BSM particles would initiate a cascade decay or would itself be contained in such a decay chain.  
The authors argue, that in this case also signals of broad composite states  
with $\Gamma/M \sim 0.5$ can be detected over a smooth SM background.

The paper is well written and interesting.
Before I can recommend its publication, I would like the authors to address the
following:

1. The proposed method relies on the fact that the SM background is a smoothly falling function of the relevant kinematical variable (multibody invariant mass).
It is not entirely clear to me which effects are accounted for in the SM
simulation using PYTHIA. I get the impression that the modeling is based
on standard parton model + initial/final state showering.
For multibody final states, however, one may expect a contribution from
double parton scattering (DPS) processes, where two hard scatterings happen in
one event. As in practice one puts cuts on transverse momenta, rapidities etc.
it is not clear to me that these DPS events cannot sometimes produce effects
similar to the multibody cascade decays the authors discuss.
Have the authors included DPS in Pythia? At least a comment would be in order.

2. I have a minor remark on the caption in Fig.5. Particle $E$ clearly is a fermion, it should not be called a "composite Higgs-like particle".
 

 

Author Response

Please find below our answers (blue):

1. The proposed method relies on the fact that the SM background is a smoothly falling function of the relevant kinematical variable (multibody invariant mass). It is not entirely clear to me which effects are accounted for in the SM simulation using PYTHIA. I get the impression that the modeling is based on standard parton model + initial/final state showering. For multibody final states, however, one may expect a contribution from double parton scattering (DPS) processes, where two hard scatterings happen in one event. As in practice one puts cuts on transverse momenta, rapidities etc. it is not clear to me that these DPS events cannot sometimes produce effects similar to the multibody cascade decays the authors discuss. Have the authors included DPS in Pythia? At least a comment would be in order.

The current simulations do not include DPS. We expect such effects (including pileup)  are not significant contributors to high-mass states. We include the following statement:

“Effects from double parton scattering are not included in the simulation”.(added line 109)

2. I have a minor remark on the caption in Fig.5. Particle $E$ clearly is a fermion, it should not be called a "composite Higgs-like particle".

The referee is absolutely correct. We mean “composite lepton-like particle”.

 

Reviewer 3 Report

In this manuscript, the authors study signatures of beyond standard model at the LHC. They focus on  channels characterized by three-body and four-body invariant masses. They claim that these type of analysis are promising when the two-body invariant masses cannot provide sufficient experimental signatures.

I have read carefully the work and I think the analysis is interesting since it could be applied to a large phenomenological set of frameworks. For this reason, I think it would be interesting if the authors can introduce several comments about the following subjects:

1.- Although the title is very generic, they only study three-body and four-body invariant masses. I think it would improve the general approach if the authors can comment about the possibility of invariant masses associated to five or more particles.

2.- In the same line, the authors only analyse invariant masses with jets and leptons. It would be interesting to discuss, at least briefly, the possibility of pure leptonic or pure hadronic invariant masses.

3.- On the other hand, the analyses do not take into account systematic effects in the computations of the different prospects. Although I understand that these calculations suffer from important approximations, I think it is fundamental to discuss and estimate the systematic uncertainties as far as possible.

In conclusion, I will postpone the publication of the manuscript after the questions listed above were properly answered.

 

 

Author Response

Please find below our answers (blue):

1.- Although the title is very generic, they only study three-body and four-body invariant masses. I think it would improve the general approach if the authors can comment about the possibility of invariant masses associated to five or more particles.

Thanks, you are completely correct. The method can be used for combinations with more objects/particles. This, however, requires some consideration of BSM models. To address this comment (as well as to address your following comment), we propose to include the statement given below of your second question:

2.- In the same line, the authors only analyse invariant masses with jets and leptons. It would be interesting to discuss, at least briefly, the possibility of pure leptonic or pure hadronic invariant masses.

We believe 3-jet invariant masses have been already studied (but less certain about 3-lepton masses). But, of course, this is another option that requires attention.

To address the comment 1) and 2), we propose to improve the text by adding the following text:(added line 350)

The approach described in this paper can equally be applied to the invariant masses that include more than four objects, or invariant masses that are composed from only jets or leptons. However, such studies are beyond the scope of the current paper. 

 

3.- On the other hand, the analyses do not take into account systematic effects in the computations of the different prospects. Although I understand that these calculations suffer from important approximations, I think it is fundamental to discuss and estimate the systematic uncertainties as far as possible.

Experimental systematic effects should be done by experiments that have full access to energy scale, energy resolutions of jets (these studies are fully dominated by jets). Such variations are included as  “nuisance parameters“ during limit calculations (see and example https://arxiv.org/abs/2002.11325), which cannot be done in such phenomenological papers. However, it is also should be mentioned that statistical limits are intrinsic statistical quantities that are fully dominated by statistics. 

We suggest the following statement: (added: page 11 footnote)

For 2-jet plus lepton (or two lepton) invariant masses, systematic uncertainties for exclusion limits are expected to be dominated by jet energy scale and jet energy resolution. Typically, such effects are not significant (within the 1-sigma band on the limits (reference to   https://arxiv.org/abs/2002.11325). Systematic uncertainties are included as nuisance parameters“ during  limit calculations by experiments.