Abstract
A search for the lepton-flavour violating decay $D^0 \to e^\pm \mu^\mp$ is made with a dataset corresponding to an integrated luminosity of 3.0 fb$^{-1}$ of proton-proton collisions at centre-of-mass energies of $7$ TeV and $8$ TeV, collected by the LHCb experiment. Candidate $D^0$ mesons are selected using the decay $D^{*+} \to D^0 \pi^+$ and the $D^0 \to e^\pm \mu^\mp$ branching fraction is measured using the decay mode $D^0 \to K^-\pi^+$ as a normalisation channel. No significant excess of $D^0 \to e^\pm \mu^\mp$ candidates over the expected background is seen, and a limit is set on the branching fraction, $\mathcal{B}(D^0 \to e^\pm \mu^\mp) < 1.3 \times 10^{-8}$, at 90 % confidence level. This is an order of magnitude lower than the previous limit and it further constrains the parameter space in some leptoquark models and in supersymmetric models with R-parity violation.
Highlights
Searches for decays that are forbidden in the Standard Model (SM) probe potential contributions from new processes and particles at mass scales beyond the reach of direct searches
In Ref. [1] three extensions to the SM are considered: in a minimal supersymmetric (SUSY) SM with R-parity violation (RPV) the branching fraction B(D0 → e±μ∓) could be as large as O(10−6); in a theory with multiple Higgs doublets it would be less than about 7 × 10−10; and in the SM extended with extra fermions the branching fraction would be less than O(10−14)
In the first stage of the offline event selection, the D∗+ → D0(e±μ∓)π + and D∗+ → D0(K −π +)π + candidates that pass the trigger selection are required to have a vertex, formed from two good-quality tracks associated with particles of opposite charge, that is well separated from any primary vertex (PV), with the summed momentum vector of the two particles pointing to a PV
Summary
Searches for decays that are forbidden in the Standard Model (SM) probe potential contributions from new processes and particles at mass scales beyond the reach of direct searches. D0 → e±μ∓ is an example of a forbidden decay, in which lepton flavour is not conserved.. D0 → e±μ∓ is an example of a forbidden decay, in which lepton flavour is not conserved.1 The contributions to this process from neutrino oscillations would give a rate that is well below the reach of any currently feasible experiment. An RPV SUSY model is considered in which limits on products of couplings are obtained from the experimental upper limit on the branching fraction. A similar study of constraints on coupling constants in RPV SUSY [3], obtained from limits on the branching fraction B(D+ → π +e±μ∓), showed that. The mass spectrum in the signal region, defined as 1815–1915 MeV/c2, is not examined until all analysis choices are finalized
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