insight - Algorithms and Data Structures - # Model-Agnostic Signal Region Detection for New Physics Discovery

Core Concepts

A data-driven method for efficiently identifying signal-rich regions in high-dimensional feature spaces to enable the discovery of new physics beyond the Standard Model.

Abstract

The content presents a novel approach for detecting new physics signals in high-energy physics experiments, particularly in the context of searching for the production of two Higgs bosons decaying into four b-jets (HH→4b).

Key highlights:

- The authors address the challenge of setting up signal and control regions when there is no prior knowledge about the expected signal, as is the case for completely new types of particles.
- They propose a method that leverages the assumption that signal events are localized in the high-dimensional feature space, without relying heavily on domain knowledge.
- The approach employs the notion of a low-pass filter to extract low-frequency components of the density ratio between 4b and 3b events, allowing the identification of high-frequency features that may correspond to the signal.
- By training a classifier to distinguish between 3b events with added noise and 4b events with added noise, the authors efficiently estimate the smoothed density ratio without directly computing the convolution operation.
- The method is demonstrated on simulated HH→4b events, showing its ability to identify a data-driven signal region that is enriched with signal events compared to its size.
- The authors discuss the importance of choosing an appropriate noise scale for the convolution kernel to balance the preservation of low-frequency features and the suppression of high-frequency features.
- Future work includes extending the method to estimate the background distribution and perform hypothesis testing to determine the presence of new physics signals.

To Another Language

from source content

arxiv.org

Stats

The following sentences contain key metrics or important figures used to support the author's key logics:
The authors used 3b event samples of size n ∈ {105, 106} and the same size of 4b events.
75% and 6.25% of all the samples were used to estimate γ and eγ, respectively.
The noise scale in each dimension for generating the training dataset for the smoothed density ratio eγ was set to η ∈ {0.01, 0.1, 1} times the length of the range of the corresponding representation.

Quotes

"Remarkably, eγ can be efficiently estimated by learning a classifier without directly evaluating the convolution operation. In particular, we can estimate it by training a classifier to distinguish (Z3b + E, 0) and (Z4b + E, 1), where Z3b and Z4b are the representations of 3b and 4b events, respectively, and E ∼ K is random noise."

Key Insights Distilled From

by Soheun Yi, J... at **arxiv.org** 09-12-2024

Deeper Inquiries

To extend the proposed method for cases where signal events are distributed in a more complex manner rather than being localized, one approach could involve employing advanced machine learning techniques that can capture intricate patterns in high-dimensional feature spaces. For instance, instead of relying solely on the assumption of localized signal events, we could utilize unsupervised learning methods such as clustering algorithms (e.g., Gaussian Mixture Models or t-SNE) to identify regions of high density that may not conform to a simple localized structure.
Additionally, incorporating deep learning architectures, such as convolutional neural networks (CNNs) or recurrent neural networks (RNNs), could allow for the modeling of more complex relationships between features. These models can learn hierarchical representations of the data, potentially capturing the underlying structure of the signal distribution.
Moreover, the method could be adapted to include a multi-scale analysis, where different resolutions of the feature space are examined to identify signals at various scales. This would involve applying the low-pass filtering technique at multiple bandwidths to capture both localized and distributed signal characteristics. By integrating these approaches, the method can become more robust in identifying signals that do not conform to simple localization assumptions.

The assumption that the signal-to-background density ratio consists solely of low-frequency features presents several limitations. One significant limitation is that this assumption may overlook important high-frequency variations that could indicate the presence of signals. In scenarios where the signal exhibits sharp peaks or intricate structures in the feature space, relying exclusively on low-frequency components may lead to missed detections or false negatives.
To adapt the method for more complex signal topologies, one could implement a hybrid approach that combines both low-pass and high-pass filtering techniques. By analyzing both the low-frequency and high-frequency components of the density ratio, the method can be fine-tuned to identify signals that may be masked by background noise or that exhibit complex distributions.
Additionally, employing a multi-resolution analysis, such as wavelet transforms, could allow for the simultaneous examination of both low and high-frequency features. This would enable the detection of signals with varying scales and complexities, enhancing the method's sensitivity to diverse signal topologies. Furthermore, incorporating domain knowledge about the expected signal characteristics could guide the selection of appropriate filtering techniques and parameters, improving the overall robustness of the detection process.

Determining the optimal noise scale for the convolution kernel bandwidth is crucial for effectively distinguishing between signal and background events. A systematic approach to finding this optimal scale could involve a combination of empirical testing and theoretical modeling based on the expected characteristics of the signal.
One method to achieve this is through cross-validation techniques, where different bandwidths are tested on a validation dataset to evaluate their performance in terms of signal enrichment in the selected signal region (SR). By measuring metrics such as the proportion of signal events within the SR and the overall sensitivity of the method, one can identify the bandwidth that maximizes these metrics.
Additionally, incorporating prior knowledge about the expected signal characteristics—such as the expected width of the signal peak or the typical scale of variations in the feature space—can inform the selection of the convolution kernel bandwidth. For instance, if prior studies suggest that the signal events are concentrated within a specific range of feature values, the bandwidth can be adjusted to reflect this range, ensuring that the filtering process retains relevant signal information while suppressing background noise.
Moreover, Bayesian optimization techniques could be employed to systematically explore the bandwidth parameter space, allowing for a more efficient search for the optimal noise scale. By modeling the relationship between bandwidth and detection performance, one can iteratively refine the choice of bandwidth based on observed outcomes, leading to a more data-driven and systematic determination of the optimal convolution kernel bandwidth.

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