Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Dark-sector physics in the search for the rare decays $K^+\rightarrow
  \pi^+ \bar \nu \nu$ and $K_L\rightarrow \pi^0 \bar \nu \nu$   We compute the contribution of the decays $K_L \rightarrow \pi^0 Q \bar Q$
and $K^+ \rightarrow \pi^+ Q \bar Q$, where $Q$ is a dark fermion of the dark
sector, to the measured widths for the rare decays $K^+\rightarrow \pi^+ \nu
\bar \nu$ and $K_L\rightarrow \pi^0 \nu \bar \nu$. The recent experimental
limit for $\Gamma (K^+ \rightarrow \pi^+ \nu \bar \nu)$ from NA62 sets a new
and very strict bound on the dark-sector parameters. A branching ratio for $K_L
\rightarrow \pi^0 Q \bar Q$ within the reach of the KOTO sensitivity is
possible. The Grossman-Nir bound is weakened by the asymmetric effect of the
different kinematic cuts enforced by the NA62 and KOTO experiments. This last
feature holds true for all models where the decay into invisible states takes
place through a light or massless intermediate state.

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The categories of this paper are: eess
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph
27
The categories of this paper are: comp-gas
13
The categories of this paper are: quant-ph
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The categories of this paper are: adap-org
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The categories of this paper are: chao-dyn
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The categories of this paper are: nucl-ex
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The categories of this paper are: atom-ph
33
The categories of this paper are: plasm-ph
20
The categories of this paper are: acc-phys
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The categories of this paper are: hep-th
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The categories of this paper are: funct-an
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The categories of this paper are: mtrl-th