A Robust Iterative Unfolding Method for Signal Processing

TL;DR

This paper introduces a new robust iterative series expansion method for signal unfolding and deconvolution that guarantees convergence and optimal information recovery without ad hoc regularizations, demonstrated through physics applications.

Contribution

It provides new convergence theorems and a universally applicable, optimal unfolding method based on a series expansion for signal processing problems.

Findings

The method guarantees unbiased and optimal recovery of initial probability densities.

It applies to deconvolution and unfolding problems without needing frequency regularizations.

Successful application demonstrated in physics, specifically in particle decay analysis.

Abstract

There is a well-known series expansion (Neumann series) in functional analysis for perturbative inversion of specific operators on Banach spaces. However, operators that appear in signal processing (e.g. folding and convolution of probability density functions), in general, do not satisfy the usual convergence condition of that series expansion. This article provides some theorems on the convergence criteria of a similar series expansion for this more general case, which is not covered yet by the literature. The main result is that a series expansion provides a robust unbiased unfolding and deconvolution method. For the case of the deconvolution, such a series expansion can always be applied, and the method always recovers the maximum possible information about the initial probability density function, thus the method is optimal in this sense. A very significant advantage of the presented method is that one does not have to introduce ad hoc frequency regulations etc., as in the case of usual naive deconvolution methods. For the case of general unfolding problems, we present a computer-testable sufficient condition for the convergence of the series expansion in question. Some test examples and physics applications are also given. The most important physics example shall be (which originally motivated our survey on this topic) the case of pi^0 --> gamma+gamma particle decay: we show that one can recover the initial pi^0 momentum density function form the measured single gamma momentum density function by our series expansion.