Theoretical and experimental status of rare charm decays

TL;DR

This paper reviews the current theoretical and experimental understanding of rare charm decays, highlighting their potential for discovering New Physics through null-test observables and summarizing recent experimental limits.

Contribution

It provides a comprehensive overview of the status of rare charm decays, emphasizing new strategies for detecting New Physics despite theoretical challenges.

Findings

Updated experimental limits on Wilson coefficients

Identification of null-test observables for New Physics

Assessment of future experimental opportunities

Abstract

Rare charm decays offer the unique possibility to explore flavour-changing neutral-currents in the up-sector within the Standard Model and beyond. Due to the lack of effective methods to reliably describe its low energy dynamics, rare charm decays have been considered as less promising for long. However, this lack does not exclude the possibility to perform promising searches for New Physics per se, but a different philosophy of work is required. Exact or approximate symmetries of the Standard Model allow to construct clean null-test observables, yielding an excellent road to the discovery of New Physics, complementing the existing studies in the down-sector. In this review, we summarize the theoretical and experimental status of rare charm $|\Delta c|=|\Delta u|=1$ transitions, as well as opportunities for current and future experiments such as LHCb, Belle II, BES III, the FCC-ee and proposed tau-charm factories. We also use the most recent experimental results to report updated limits on lepton-flavour conserving and lepton-flavour violating Wilson coefficients.