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

New particles ϕ in the MeV-GeV range produced at colliders and escaping detection can be searched for at operating b- and τ-factories such as Belle II. A typical search topology involves pair-produced τs (or mesons), one of which decaying to visibles plus the ϕ, 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 <math altimg="si1.svg"><msub><mrow><mi>M</mi></mrow><mrow><mn>2</mn></mrow></msub></math>, 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, <math altimg="si2.svg"><mi>τ</mi><mo stretchy="false">→</mo><mi>e</mi><mo linebreak="goodbreak" linebreakstyle="after">+</mo><mi>ϕ</mi></math>, where ϕ 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 <math altimg="si3.svg"><mi>τ</mi><mo stretchy="false">→</mo><mi>e</mi><mi>ϕ</mi></math>, with respect to the currently expected limit, assuming <math altimg="si4.svg"><msub><mrow><mi>m</mi></mrow><mrow><mi>ϕ</mi></mrow></msub><mo>≲</mo><mn>1</mn></math> MeV. For example, we anticipate a sensitivity of <math altimg="si5.svg"><mn>1.7</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo><mn>5</mn></mrow></msup></math> with the data collected before the 2022 shutdown.