Limits on heavy neutral lepton mixing parameters

Limits on $\vert {{\mathit U}}$ $_{ {{\mathit \tau}} {{\mathit x}} }\vert ^2$

INSPIRE  
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
$<3 \times 10^{-4}$ 90 1
ACCIARRI
2021
ARNT Near ${\mathit m}_{{{\mathit \nu}_{{x}}}}{ {}\lesssim{} }$ 970 MeV
$<3 \times 10^{-6}$ 90 2
BOIARSKA
2021
RVUE Near ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $0.8 - 1.6$ GeV
$<2 \times 10^{-4}$ 90 3
ORLOFF
2002
CHRM Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit \tau}}}$ kin. thres.
$<1 \times 10^{-4}$ 90 4
ORLOFF
2002
CHRM ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $200 - 250$ MeV
$<3 \times 10^{-5}$ 95 5
ABREU
1997I
 DLPH ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $6 - 50$ GeV
$<2 \times 10^{-5}$ 95 6
ABREU
1997I
 DLPH Near ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ 3.5 GeV
1  Search for ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{{\mathit \nu}}$ .
2  Reanalysis of CHARM results (cf. ORLOFF 2002 ) to include searches for ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit \nu}}{{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$ decays, and including the production of HNLs from ${{\mathit \tau}}$ decays.
3  ${{\mathit D}_{{s}}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ decay via ${{\mathit U}}$ $_{ {{\mathit \tau}} {{\mathit x}} }$.
4  ${{\mathit D}_{{s}}}$ $\rightarrow$ ${{\mathit \nu}_{{\tau}}}{{\mathit \tau}^{+}}$ , ${{\mathit \tau}^{+}}$ $\rightarrow$ ${{\mathit \nu}_{{x}}}{{\mathit X}}$ , with ${{\mathit \nu}_{{x}}}$ decay via ${{\mathit U}}$ $_{ {{\mathit \tau}} {{\mathit x}} }$.
5  Search for prompt ${{\mathit \nu}_{{x}}}$ decay signatures.
6  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:
ACCIARRI 2021
PRL 127 121801
BOIARSKA 2021
PR D104 095019
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
Also
ZPHY C75 580 (errat.) Erratum: ABREU 1997I Search for Neutral Heavy Leptons Produced in ${{\mathit Z}}$ Decays