$\tau \to \ell +$ invisible through invisible-savvy collider variables

TL;DR

This paper introduces novel kinematic variables leveraging the $M_2$ and MAOS methods to improve the search for invisible particles in tau decays at colliders, significantly enhancing sensitivity over existing techniques.

Contribution

It develops new collider variables based on $M_2$ and MAOS to better discriminate signals involving invisible particles in tau decays, applicable to a wider range of topologies.

Findings

Achieves nearly a threefold improvement in branching ratio limits.

Demonstrates applicability to various decay topologies.

Projects sensitivity of $1.7 imes 10^{-5}$ before 2022 shutdown.

Abstract

New particles $\phi$ in the MeV-GeV range produced at colliders and escaping detection can be searched for at operating $b-$ and $\tau-$factories such as Belle II. A typical search topology involves pair-produced $\tau$s (or mesons), one of which decaying to visibles plus the $\phi$, and the other providing a tag. One crucial impediment of these searches is the limited ability to reconstruct the parents' separate boosts. This is the case in the 'typical' topology where both decay branches include escaping particles. We observe that such topology lends itself to the use of kinematic variables such as $M_2$, designed for pairwise decays to visibles plus escaping particles, and endowed with a built-in ('MAOS') way to efficiently guess the parents' separate boosts. Starting from this observation, we construct several kinematic quantities able to discriminate signal from background, and apply them to a benchmark search, $\tau \to e + \phi$, where $\phi$ can be either an axion-like particle or a hidden photon. Our considered variables can be applied to a wider range of topologies than the current reference technique, based on the event thrust, with which they are nearly uncorrelated. Application of our strategy leads to an improvement by a factor close to 3 in the branching-ratio upper limit for $\tau \to e \phi$, with respect to the currently expected limit, assuming $m_\phi \lesssim 1$ MeV. For example, we anticipate a sensitivity of $1.7 \times 10^{-5}$ with the data collected before the 2022 shutdown.