Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A search for the decays $B^+\to e^+ \nu_e$ and $B^+\to \mu^+ \nu_\mu$
  using hadronic-tag reconstruction   We report on a search for the rare decay modes $B^+\to e^+ \nu_e$ and $B^+\to
\mu^+ \nu_\mu$ with data collected from the BaBar detector at the PEP-II
$e^+e^-$ storage ring. This search utilizes a new technique in which we fully
reconstruct the accompanying $B^-$ in $\Upsilon(4S)\righarrow B^+ B^-$ events,
and look for a mono-energetic lepton in the $B^+$ rest frame. No signal
candidates are observed in either of the channels, consistent with the expected
background, in a data sample of approximately 229 million $B\bar{B}$ pairs. The
branching-fraction upper limits are set at $\mathcal{B}(B^+\to e^+\nu_{e})<7.9
\times 10^{-6}$ and $\mathcal{B}(B^+\to\mu^+\nu_{\mu})<6.2 \times 10^{-6}$ at
the 90% confidence level.

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The categories of this paper are: hep-th
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The categories of this paper are: patt-sol
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The categories of this paper are: quant-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: mtrl-th
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The categories of this paper are: physics
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The categories of this paper are: econ
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The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
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The categories of this paper are: solv-int
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The categories of this paper are: gr-qc
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The categories of this paper are: chem-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: nlin
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The categories of this paper are: cmp-lg