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Register a new userRare charm decays: an experimental review
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Authors
Francesco Dettori
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Indirect searches, and in particular rare decays, have proven to be a fruitful field to search for New Physics beyond the Standard Model. While the down-quark sector (B and K) have been studied in detail, less attention was devoted to charm decays due to the smaller expected values and higher theoretical uncertainties of their observables. Recently a renewed interest is growing in rare charm searches. In this article we review the current experimental status of searches for rare decays in charmed hadrons. While the Standard Model rates are yet to be reached, current experimental limits are already putting constraints on New Physics models.
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We review current experimental results on charm mixing and CP violation. We survey experimental techniques, including time-dependent, time-independent, and quantum-correlated measurements. We review techniques that use a slow pion tag from D*+ --> pi+ D0 + c.c. decays and those that do not, and cover two-body and multi-body D0 decay modes. We provide a summary of D-mixing results to date and comment on future experimental prospects at the LHC and other new or planned facilities.
The combination of simple dynamics, small number of available decay channels, and extremely well controlled radiative and loop corrections, make charged pion decays a sensitive means for testing the underlying symmetries and the universality of weak fermion couplings, as well as for improving our understanding of pion structure and chiral dynamics. This paper reviews the current state of experimental study of the allowed rare decays of charged pions: (a) leptonic, $pi^+ to e^+ u_e$, or $pi_{e2}$, (b) radiative, $pi^+ to e^+ u_egamma$, or $pi_{e2gamma}$, and $pi^+ to e^+ u_e e^+e^-$, or $pi_{e2ee}$, and (c) semileptonic, $pi^+to pi^0 e^+ u$, or $pi_{e3}$. Taken together, the combined data set presents an internally consistent picture that also agrees well with standard model predictions. The internal consistency is illustrated well by the $pi_{e2}$ branching ratio of $(R_{e/mu}^pi)^{rm PIBETA} = (1.2366 pm 0.0064) times 10^{-4}$ extracted in this work from the PIBETA measurement of the $pi_{e3}$ decay and the current best value for the CKM matrix element $V_{ud}$. However, even after the great progress of the recent decades, experimental precision is lagging far behind that of the theoretical description for all above processes. We review the implications of the present state of knowledge and prospects for further improvement in the near term.
We review recent experimental progress in the domain of rare radiative, semileptonic and leptonic B decays. The statistical precision attained for these decays has reached a level where they start to impose meaningful constraints on the Cabibbo-Kobayashi-Maskawa matrix, which are complementary to those obtained from hadronic decays. While the current data indicate no deviations from Standard Model predictions, there is still some room for new physics in these decays.
Leptonic and semileptonic decays in the charm sector have been well studied in recent years. With the largest data sample near $Dbar D$ threshold, precision measurements of leptonic and semileptonic decays of charm meson and baryon are perfromed at BESIII. Test for letpon flavor universality is also performed. Sensitivity for rare leptonic and semileptonic charm decays is significantly improved taking advantage of the huge statistics in LHCb and the $B$ factories.
This review summarizes the current experimental results on rare B0_(s) -> mu+mu- decays of the Tevatron experiments CDF and D0, and the LHC experiments ATLAS, CMS and LHCb. The experimental branching fraction upper limits for the B0_s -> mu+mu- are already quite close to the branching fraction predicted by the Standard Model, and the first observation of the B0_s -> mu+mu- decay is expected soon. The rare decays B0_(s) -> mu+mu- are highly suppressed in the Standard Model, and therefore accurate measurements of these branching fractions provide complementary constraints to the free parameters of various extensions of the Standard Model.
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