Abstract
Rare charm decays offer the unique possibility to explore flavor-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 [Formula: see text] 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-flavor conserving and lepton-flavor violating Wilson coefficients.
Highlights
Despite being consistent with an enormous amount of experimental results, there are undoubtedly phenomena that the Standard Model (SM) fails to explain and a more fundamental theory has to exist
Since C10 corresponds to an axial vector coupling for the leptonic part, effects on the V − A structure of the SM are shut off at the charm scale and charm physics within the SM is dominated by quantum chromodynamics (QCD) and quantum electrodynamics (QED) effects
New Physics (NP) searches in rare decays have mainly focused on K and B systems, and less attention has been devoted to rare charm decays
Summary
Despite being consistent with an enormous amount of experimental results, there are undoubtedly phenomena that the Standard Model (SM) fails to explain and a more fundamental theory has to exist. While in the past most of the experimental effort has focused on studying rare processes in the kaon and beauty sectors, investigations of rare charm decays, which are sensitive to |∆c| = |∆u| = 1 transitions, have only started. The SM symmetries in the charm system offer the possibility to define null-test observables with small or negligible theory uncertainties in resonance-dominated semi-leptonic and radiative decays. The possibility to investigate angular distributions, CP asymmetries and tests for lepton universality experimentally in these decays with typical branching fractions of 10−5 − 10−7 has only opened recently, and precision measurements are expected to be possible in the near future at the current flavour experiments LHCb, Belle II and BES III. We present a summary of experimental searches for rare and forbidden decays, and how experimental limits can be translated in model-independent bounds on Wilson coefficients in Sect.
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