A novel approach to meson or lepton decays to escaping particles – InvISYble

Meson or lepton decays to escaping particles provide a key method to probe the existence of light new particles, or to measure very rare or forbidden processes. Such processes are searched for primarily through pair-produced mesons or leptons, one of which yields the decay of interest, and the other is often used as a "tag", exploiting entanglement to enhance signal reconstruction. Typical tags are fully reconstructed decays, whereas channels with partially undetected final states pose a challenge. This represents a considerable limitation, because such decays are often clean and abundant, as is the case of semi-leptonic tags.

Aim of the proposal is to substantially improve reconstruction techniques for meson or lepton decays to escaping particles, via a systematic approach to the underlying decay topology. Importantly, this topology — pairwise decays to sets of visible plus escaping particles - lends itself to the use of a collection of kinematic variables conceived precisely to marginalize over the unknown kinematic degrees of freedom.

Collectively denoting these variables as M2, we identify five of their properties that are key for our stated aim. (1): M2 and siblings come with a built-in method to best-guess the total invisible momenta for, separately, the two decay branches; (2): suitable twists of method (1) can help estimate the number of invisibles in each branch, which provides a further handle toward signal/background separation; (3) & (4): M2 may be augmented with constraints that leverage the known decaying-parent mass and its flight direction — accessible thanks to the exquisite vertexing capabilities nowadays available; (5): all M2 constraints may be implemented with their finite uncertainty, namely as inequalities, which are algorithmically more flexible than strict equalities. This way many pieces of uncertain information may collectively build up strong constraints.

The five features above constitute the backbone of our methodology, and allow to identify three specific objectives towards our stated aim. O1: A complete study of certain reference decays, either with new final-state particles, or highly sensitive to new short-distance dynamics; O2: The development of a structured strategy for tag decays with invisibles — e.g., semi-leptonic ones; O3: The exploration of M2 properties discerning invisibles of different spin.

The project, if successful, will lead to several results of high and broad impact, including among the others: (a) A complete analysis of tau -> l phi, with l = e or mu, and phi a new boson, where we expect to sizeably improve over the state-of-the-art method — e.g. by an estimated factor-of-3 for small m_phi; (b) A new approach to measuring B -> K(*) nu nu and likewise a complete analysis. This task may deliver the very first measurement (as opposed to "just" an improved limit) for this decay. Such result would be a breakthrough and would, alone, justify the whole project. Besides, it will provide key pieces of information for theory; (c) A novel library for the calculation of M2 variables, allowing for the first time to include all kinematics and positional constraints available, with their uncertainties, and regardless of their number. Such general design will warrant wide applicability.

The project's objectives have theoretical and experimental facets intertwined. Our "hybrid" team of theorists and experimentalists has a tried-and-true collaborative record, proven among the other ways by recent work, foundational for the proposal. The project's success rests on appropriate HRs: a PhD student in exp-th co-supervision, and a 3-year postdoc with broad expertise on particle-physics phenomenology and also acquainted with collider-data simulation and processing. The project's results would not be achievable without dedicated funding, as they require an exp-th interplay between the method's practitioners, and because the implementation demands dedicated HRs.