Limits on heavy neutral lepton mixing parameters

Limits on $\vert {{\boldsymbol U}}_{ {{\boldsymbol \tau}} {{\boldsymbol x}} }\vert ^2$ INSPIRE search

Quoted limits are either the best limit near the kinematic threshold of the experiment, or a characteristic value in the mass range of the experimental sensitivity
VALUE CL% DOCUMENT ID TECN  COMMENT
$<2 \times 10^{-4}$ 90 1
ORLOFF
2002
CHRM Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit \tau}}}$ kin. thres.
$<1 \times 10^{-4}$ 90 2
ORLOFF
2002
CHRM ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $200 - 250$ MeV
$<3 \times 10^{-5}$ 95 3
ABREU
1997I
 DLPH ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $6 - 50$ GeV
$<2 \times 10^{-5}$ 95 4
ABREU
1997I
 DLPH Near ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ 3.5 GeV
1  ${{\mathit D}_{{s}}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ decay via ${{\mathit U}}_{ {{\mathit \tau}} {{\mathit x}} }$.
2  ${{\mathit D}_{{s}}}$ $\rightarrow$ ${{\mathit \nu}_{{\tau}}}{{\mathit \tau}^{+}}$ , ${{\mathit \tau}^{+}}$ $\rightarrow$ ${{\mathit \nu}_{{x}}}{{\mathit X}}$ , with ${{\mathit \nu}_{{x}}}$ decay via ${{\mathit U}}_{ {{\mathit \tau}} {{\mathit x}} }$.
3  Search for prompt ${{\mathit \nu}_{{x}}}$ decay signatures.
4  Search for displaced ${{\mathit \nu}_{{x}}}$ decay signatures. Kinematical suppression of ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit \tau}}{{\mathit X}}$ at lower masses leads to rapid loosening of the $\vert {{\mathit U}}_{ {{\mathit \tau}} {{\mathit x}} }\vert $ bound compared to that for $\vert {{\mathit U}}_{ {{\mathit e}} {{\mathit x}} }\vert $ and $\vert {{\mathit U}}_{ {{\mathit \mu}} {{\mathit x}} }\vert $.
  References:
ORLOFF 2002
PL B550 8 Limits on the Mixing of ${{\mathit \tau}}$ Neutrino to Heavy Neutrinos
ABREU 1997I
ZPHY C74 57 Search for Neutral Heavy Leptons Produced in ${{\mathit Z}}$ Decays