Abstract
We present a search for seven lepton-flavor-violating neutral charm decays of the type $D^{0}\rightarrow X^{0} e^{\pm} \mu^{\mp}$, where $X^{0}$ represents a $\pi^{0}$, $K^{0}_{\rm S}$, $\bar{K^{*0}}$, $\rho^{0}$, $\phi$, $\omega$, or $\eta$ meson. The analysis is based on $468$ fb$^{-1}$ of $e^+e^-$ annihilation data collected at or close to the $\Upsilon(4S)$ resonance with the BaBar detector at the SLAC National Accelerator Laboratory. No significant signals are observed, and we establish 90\% confidence level upper limits on the branching fractions in the range $(5.0 - 22.5)\times 10^{-7}$. The limits are between one and two orders of magnitude more stringent than previous measurements.
Highlights
Lepton-flavor-conserving charm decays such as D → Xeþe− or D → Xμþμ−, where X is a meson, can occur in the standard model (SM) through short-distance [1,2] and long-distance [2] processes
We present a search for seven D0 → X0eÆμ∓ LFV decays, where X0 represents a π0, K0S, K Ã0, ρ0, φ, ω, or η meson, with data recorded with the BABAR detector at the PEP-II asymmetric-energy eþe− collider operated at the SLAC National Accelerator Laboratory
A multivariate selection based on a boosted decision tree (BDT) discriminant is applied to the signal modes [39]
Summary
Lepton-flavor-conserving charm decays such as D → Xeþe− or D → Xμþμ−, where X is a meson, can occur in the standard model (SM) through short-distance [1,2] and long-distance [2] processes. Some models that consider LFV and lepton-number-violating four-body charm decays, with two leptons and two hadrons in the final state, predict branching fractions up to Oð10−5Þ, approaching those accessible with current data [6,7,8,13]. The most stringent existing upper limits on the branching fractions for the LFV four-body decays of the type D0 → h0−hþeÆμ∓ are in the range ð11.0–19.0Þ × 10−7 at the 90% confidence level [23]. Decays (D0 → K0Sð→ πþπ−ÞeÆμ∓, D0 → ρ0ð→ πþπ−ÞeÆμ∓, D0 → K Ã0ð→ K−πþÞeÆμ∓, and D0 → φð→ KþK−ÞeÆμ∓), the current D0 → X0eÆμ∓ branching fraction upper limits, which are in the range ð3.4–8.3Þ × 10−5 [20,21,24], are approximately 20 times less stringent than the D0 → h0−hþeÆμ∓ limits reported in Ref. D0 mesons are produced via other processes, the use of this decay chain increases the purity of the D0 samples at the cost of a smaller number of reconstructed D0 mesons
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