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The NFLikelihood: an unsupervised DNNLikelihood from Normalizing Flows
by Humberto ReyesGonzalez, Riccardo Torre
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Submission summary
Authors (as registered SciPost users):  Humberto ReyesGonzález 
Submission information  

Preprint Link:  https://arxiv.org/abs/2309.09743v1 (pdf) 
Code repository:  https://github.com/NF4HEP/NFLikelihoods 
Data repository:  https://zenodo.org/record/8349144 
Date submitted:  20230920 11:49 
Submitted by:  ReyesGonzález, Humberto 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Experimental, Computational, Phenomenological 
Abstract
We propose the NFLikelihood, an unsupervised version, based on Normalizing Flows, of the DNNLikelihood proposed in Ref.[1]. We show, through realistic examples, how Autoregressive Flows, based on affine and rational quadratic spline bijectors, are able to learn complicated highdimensional Likelihoods arising in High Energy Physics (HEP) analyses. We focus on a toy LHC analysis example already considered in the literature and on two Effective Field Theory fits of flavor and electroweak observables, whose samples have been obtained throught the HEPFit code. We discuss advantages and disadvantages of the unsupervised approach with respect to the supervised one and discuss possible interplays of the two.
Current status:
Reports on this Submission
Anonymous Report 1 on 2024124 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2309.09743v1, delivered 20240124, doi: 10.21468/SciPost.Report.8449
Strengths
 towards ML implementation of LHC likelihoods: very much needed and method looks very promising
 chosen examples are highdimensional and seem realistic
Weaknesses
 choice of evaluation metrics might not be capturing all correlations.
 one paragraph contradicts the tables it describes
 presentation is not good: lots of typos and other issues
Report
Review of the scipost submission 2309.09743v1, entitled "The NFLikelihood: an unsupervised DNNLikelihood from Normalizing Flows", by Humberto ReyesGonzalez and Riccardo Torre.
The authors describe the use of normalizing flows, a type of deep generative network with an explicit likelihood, to encode and preserve highdimensional experimental likelihoods for highenergy physics analyses. They consider 3 different likelihoods (a toy, an electroweak, and a flavor example) and show how well the model learns the distribution of the likelihood, using different evaluation metrics. To learn the likelihoods, the authors use a set of sampled points in parameter space that they obtained from other sources (like the HEPfit code).
The efficient encoding of experimental likelihoods is an important problem and normalizing flows seem a promising solution to it. I therefore think that the manuscript should be published in SciPost physics. However, there are a few things, mostly regarding the presentation of the results, that need to be improved before.
main points:
 The suggested evaluation metrics seem to only look at lower dimensional projections and not the entire distribution with all its correlations. The KS test looks at onedimensional marginals. For the SWD, it is not clear to me how large the subspace is. The text says 2D directions, but a sample is only Ddimensional. Do the authors mean 2 directions?
 Instead of metrics that only evaluate subspaces, the authors could consider a classifierbased approach, as was discussed in 2305.16774, to ensure that all correlations are learned correctly.
 The authors say that they evaluated all metrics 100 times to include statistical uncertainty, yet all tables only report a central value and no errorbar. Please add the error so the numbers can be judged better.
 Tables 3, 6, and 9: What does it mean to compute the statistical tests for parameters of interest? Do they sample from the entire (joint) distribution of POI+nuissances and look at the onedimensional distribution of the POI(s) only to compute the numbers? Please explain that better.
 Please give a formula for soft clipping and explain what a hinge factor is.
 When describing Table 6, I'm not sure "generally well described" is the best way to express when 3 of the KStests are of order 0.2.
 The paragraph explaining tables 8 and 9 contradicts the tables. The KStest is 0.42 in the text and 0.31 in the table. The HPDIes are of order 10^3 in the text and 10^2 in the table. Training time is 20000s in the text and 9550s in the table. I strongly disagree with the statement "almost always above 0.4" when the KStest of the POIs is described: only 4(!) of 12 POIs are above 0.4.
 Could the authors please be more specific why the large value for HPDIe_1sigma for c'_22^LedQ is due to the algorithm?
minor points:
 in the introduction, the authors state that "The aim of this paper is (...) to give explicit physics examples of the performances of the autoregressive flows studied in (3)". I think this sentence is misleading, it suggests that this was not done before. However, autoregressive flows have been used a lot in HEP on various physics datasets (event generation, detector simulation, anomaly detection, unfolding, ...).
 A notational question: when referring to the "MAF", the authors mean an autoregressive architecture with affine transformations and when they say "ARQS" they mean the same architecture, but now with a splinebased transformation? Other HEP papers used "MAF" just to indicate the autoregressive nature and not what kind of transformation is used, which might be confusing to readers.
typos:
 section 2.3: refereed > referred
 section 2.3: Fig.s 3 and 4 > Figs. 3 and 4
 Table 1: column headlines of "# of bijections" and "algorithm" should be interchanged to match the body of the table.
 Table 3: Please check the caption, I think "flavor" should be example 3, not 1.
 Figure 1: There is a space missing right before the beginning of the second sentence in the caption.
 Section 4.2: Table.4 > Table 4.
 Section 4.2: Last line of page 6: Table 3 > Table 6.
 Table 4: column headlines of "# of bijections" and "hidden layers" should be interchanged to match the body of the table.
 Acknowledgements: Luce > Luca
Requested changes
 see report for content
 please correct the typos I listed and check if there are more
 in general: please make all numbers/text in figures as big as the font of the main body of the paper. Labels in Figure 1 are borderline small, labels in Figures 2 and 4 are way too small.
Author: Humberto ReyesGonzález on 20240410 [id 4405]
(in reply to Report 1 on 20240124)We thank the Referee for their careful reading and constructive suggestions. Below we reply separately to each of their comments.
**Referee**
*The authors describe the use of normalizing flows, a type of deep generative network with an explicit likelihood, to encode and preserve highdimensional experimental likelihoods for highenergy physics analyses. They consider 3 different likelihoods (a toy, an electroweak, and a flavor example) and show how well the model learns the distribution of the likelihood, using different evaluation metrics. To learn the likelihoods, the authors use a set of sampled points in parameter space that they obtained from other sources (like the HEPfit code).The efficient encoding of experimental likelihoods is an important problem and normalizing flows seem a promising solution to it. I therefore think that the manuscript should be published in SciPost physics. However, there are a few things, mostly regarding the presentation of the results, that need to be improved before. *
**Main points:**
**Referee**
*The suggested evaluation metrics seem to only look at lower dimensional projections and not the entire distribution with all its correlations. The KS test looks at onedimensional marginals. For the SWD, it is not clear to me how large the subspace is. The text says 2D directions, but a sample is only Ddimensional. Do the authors mean 2 directions?*
**Reply**
Our implementation of the slicedWasserstein distance (SWD), discussed at more length in our previous paper of Ref. [3], consists in the average of the $1D$ Wasserstein distances computed between a number (that we choose larger than the data dimensionality $D$, and equal to $2D$) of randomly generated projections, obtained by sampling directions uniformly on the surface of a unit $D$dimensional sphere. We have added explicit reference to [3] at the end of the discussion of the SWD in the manuscript. Therefore, to answer to the question of the referee, the underlying Wasserstein distance calculation is only $1D$, but the average is taken over $2D$ projections. Notice that the number of projections is not limited to the number of dimensions. Indeed, differently from the case of the meanKS metric, we do not consider just the $D$ projections along the "axes", which are just the $1D$ marginals, but we consider random $1D$ projections in the $D$dimensional space. It is known that (see Refs. [20, 21] in the paper), under specific hypotheses, the average computed by the SWD approximates the true multidimensional Wasserstein distance (which is prohibitively expensive to compute) as the number of projections increases (and asymptotically approaches infinity). Therefore, despite being intrinsically $1D$, the SWD is able to (at least partially) capture the effect of correlations, while still remaining very efficient to compute. We would also like to add that the design and evaluation of metrics for highdimensional distributions is an open problem, to which we are actively working and on which we have a forthcoming paper appearing shortly.
**Referee**
*Instead of metrics that only evaluate subspaces, the authors could consider a classifierbased approach, as was discussed in 2305.16774, to ensure that all correlations are learned correctly.*
**Reply**
We are aware of the powerful approach of using classifiers to evaluate the performance of generative models. Nevertheless, their applicability and interpretation in high dimensions is not straightforward. Again, in a forthcoming publication, we aim at comparing the performance of averagebased metrics (like those used in this paper) with the classifierbased approach. Given the tangential nature of this issue with respect to the present work, and its intrinsic relevance in the field of generative models, we prefer to defer the discussion to a dedicated publication and use here simple, efficient, and interpretable metrics.
**Referee**
*The authors say that they evaluated all metrics 100 times to include statistical uncertainty, yet all tables only report a central value and no errorbar. Please add the error so the numbers can be judged better.*
**Reply**
Done for the overall KS and SWD metrics. For each individual dimension, the distribution is approximately uniform random, as expected.
**Referee**
*Tables 3, 6, and 9: What does it mean to compute the statistical tests for parameters of interest? Do they sample from the entire (joint) distribution of POI+nuissances and look at the onedimensional distribution of the POI(s) only to compute the numbers? Please explain that better.*
**Reply**
That is correct.
**Referee**
*Please give a formula for soft clipping and explain what a hinge factor is.*
**Reply* *
We used the soft clip bijector from tensor flowprobability. In the manuscript, we provide the reference to TensorFlow Probability SoftClip (https://www.tensorflow.org/probability/api_docs/python/tfp/bijectors/SoftClip) in the manuscript. As described in the reference, the softclip is obtained by using the softplus bijector (https://www.tensorflow.org/probability/api_docs/python/tfp/bijectors/Softplus) as a smooth approximation of $\max(x,0)$. The softlimits are obtained as: $$\begin{array}{lll} \max(x, \text{low}) & = \max(x  \text{low}, 0) + \text{low} \\ & \approx \text{softplus}(x  \text{low}) + \text{low} \\ & := \text{softlower}(x)\\ \min(x, \text{high}) & = \min(x  \text{high}, 0) + \text{high} \\ & = \max(\text{high}  x, 0) + \text{high} \\ & \approx \text{softplus}(\text{high}  x) + \text{high} \\ & := \text{softupper}(x) \end{array}$$ The soft plus, defined as $f_c(x) := c \cdot g(x / c) = c \cdot \log[1 + \exp(x / c)]$, where c is the hinge factor, maps the distribution to the domain of positive real numbers. The hinge parameter in the soft plus changes the transition at zero. In the softclip, $c<<1$ will approximate the identity mapping, but may be numerically illconditioned at the boundaries. If $c > 1.0$ translates to a smoother mapping, but creates a larger distortions. We chose $c<<1$ to avoid such distortions, since the distributions are well approximated at the tails, the error at the boundaries is generally small.
**Referee**
*When describing Table 6, I'm not sure "generally well described" is the best way to express when 3 of the KStests are of order 0.2.*
**Reply**
This has been rephrased to 'We find that most of the POIs are pretty well described, albeit small discrepancies found for $C_{\varphi l}^{1}$, $C_{\varphi l}^{3}$ and $C_{ll}$'
**Referee**
*The paragraph explaining tables 8 and 9 contradicts the tables. The KStest is 0.42 in the text and 0.31 in the table. The HPDIes are of order $10^{3}$ in the text and $10^{2}$ in the table. Training time is 20000s in the text and 9550s in the table. I strongly disagree with the statement \"almost always above 0.4\" when the KStest of the POIs is described: only 4(!) of 12 POIs are above 0.4.*
**Reply**
The discrepancies have been corrected.
**Referee**
*Could the authors please be more specific why the large value for HPDIe$_{1\sigma}$ for $c_{22}^{\prime LedQ}$ is due to the algorithm?*
**Reply**
The results have been updated. The original HPDIe$_{1\sigma}$ for $c_{22}^{\prime LedQ}$ did not correspond to the figure.
*Minor points:*
**Referee**
*in the introduction, the authors state that \"The aim of this paper is (\...) to give explicit physics examples of the performances of the autoregressive flows studied in (3)\". I think this sentence is misleading, it suggests that this was not done before. However, autoregressive flows have been used a lot in HEP on various physics datasets (event generation, detector simulation, anomaly detection, unfolding, \...).*
**Reply**
The variety of NF applications in HEP has been addressed. Our specific aim was to follow a similar testing strategy as in (3), while using the same implementations of the NFs.
**Referee**
*A notational question: when referring to the "MAF", the authors mean an autoregressive architecture with affine transformations and when they say "ARQS" they mean the same architecture, but now with a splinebased transformation? Other HEP papers used "MAF" just to indicate the autoregressive nature and not what kind of transformation is used, which might be confusing to readers.*
**Reply**
Thanks for pointing this out. The naming conventions have been clarified in the manuscript.
**Referee**
*Please correct the typos I listed and check if there are more.*
**Reply**
We have corrected the typos and checked for more.
**Referee**
*In general: please make all numbers/text in figures as big as the font of the main body of the paper. Labels in Figure 1 are borderline small, labels in Figures 2 and 4 are way too small.*
**Reply**
*We have done our best to improve readability. We admit that some of the figures are not meant to be read on paper, but rather to be zoomed in on a screen. We have saved the figures in vector format with this purpose in mind.*