# Limits on $\vert {{\boldsymbol U}}_{ {{\boldsymbol e}} {{\boldsymbol x}} }\vert ^2$ 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
$<2 \times 10^{-5}$ 95 1
 2019 F
ATLS ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $15 - 40$ GeV
$<1 \times 10^{-9}$ 90 2
 2019 B
T2K Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-4}$ 90 3
 2019 AL
BES3 ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $0.3 - 0.7$ GeV
$<1 \times 10^{-8}$ 90 4
 2018 A
PIEN ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $60 - 120$ MeV
$<3 \times 10^{-7}$ 90 5
 2018
NA62 ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $200 - 400$ MeV
$<3 \times 10^{-5}$ 95 6
 1997 I
DLPH ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $6 - 50$ GeV
$<2 \times 10^{-5}$ 95 7
 1997 I
DLPH Near ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ 3.5 GeV
$<1 \times 10^{-5}$ 90 8
 1993
Near ${\mathit m}_{{{\mathit \pi}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<2 \times 10^{-7}$ 90 8
 1993
Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-7}$ 9, 10
 1988
CNTR Near ${\mathit m}_{{{\mathit \pi}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<2 \times 10^{-9}$ 11, 10
 1988
CNTR Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-7}$ 90 12
 1986
CHRM Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-7}$ 90 13
 1985
BEBC Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
• • • We do not use the following data for averages, fits, limits, etc. • • •
14
 2016
BELL ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $0.2 - 1.4$ GeV
1  Limit from prompt lepton number violating trilepton search.
2  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ decay through ${{\mathit U}}_{ {{\mathit e}} {{\mathit x}} }$. ABE 2019B also considers bounds on $\vert {{\mathit U}}_{ {{\mathit \ell}} {{\mathit x}} }{{\mathit U}}_{ {{\mathit \ell}^{\,'}} {{\mathit x}} }\vert$ for combinations of lepton flavors in the ${{\mathit \nu}_{{x}}}$ decay final state.
3  Searches for a Majorana Heavy Neutral Lepton producing a ${{\mathit \pi}^{-}}{{\mathit e}^{+}}$ resonance in the same sign dilepton decay ${{\mathit D}}$ $\rightarrow$ ${{\mathit K}}{{\mathit \pi}^{-}}{{\mathit e}^{+}}{{\mathit e}^{+}}$ .
4  Search for ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ .
5  Search for ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ .
6  Search for prompt ${{\mathit \nu}_{{x}}}$ decay signatures.
7  Search for displaced ${{\mathit \nu}_{{x}}}$ decay signatures.
8  Searches for ${{\mathit K}}$ or ${{\mathit \pi}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \nu}_{{e}}}$ using a beam dump experiment at the 70 GeV Serpukhov proton synchrotron. BARANOV 1993 also considers limits for $\vert {{\mathit U}}_{ {{\mathit e}} {{\mathit x}} }{{\mathit U}}_{ {{\mathit \mu}} {{\mathit x}} }\vert$ from ${{\mathit K}}$ or ${{\mathit \pi}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{x}}}$ , ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \nu}_{{e}}}$ .
9  ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ decay through ${{\mathit U}}_{ {{\mathit e}} {{\mathit x}} }$.
10  BERNARDI 1988 also considers bounds on $\vert {{\mathit U}}_{ {{\mathit e}} {{\mathit x}} }{{\mathit U}}_{ {{\mathit \mu}} {{\mathit x}} }\vert$.
11  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ decay through ${{\mathit U}}_{ {{\mathit e}} {{\mathit x}} }$.
12  ${{\mathit D}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\mathit \ell}^{+}}{{\mathit \nu}_{{{{\mathit \ell}}}}}$ .
13  ${{\mathit D}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\mathit \ell}^{+}}{{\mathit \nu}_{{{{\mathit \ell}}}}}$ or ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\mathit \pi}^{+}}$ .
14  PARK 2016 quotes an approximate limit B( ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ ) $<$ $3 \times 10^{-6}$ in the mass range ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $0.2 - 1.4$ GeV.
References:
JHEP 1910 265 Search for heavy neutral leptons in decays of $W$ bosons produced in 13 TeV $pp$ collisions using prompt and displaced signatures with the ATLAS detector
 ABE 2019B
PR D100 052006 Search for heavy neutrinos with the T2K near detector ND280
 ABLIKIM 2019AL
PR D99 112002 Search for heavy Majorana neutrino in lepton number violating decays of $D\to K \pi e^+ e^+$
 AGUILAR-AREVALO 2018A
PR D97 072012 Improved search for heavy neutrinos in the decay $\pi\rightarrow e\nu$
 CORTINA-GIL 2018
PL B778 137 Search for heavy neutral lepton production in $K^+$ decays
 PARK 2016
PR D94 012003 Search for a Massive Invisible Particle ${{\mathit X}^{0}}$ in ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit X}^{0}}$ and ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit X}^{0}}$ Decays
 ABREU 1997I
ZPHY C74 57 Search for Neutral Heavy Leptons Produced in ${{\mathit Z}}$ Decays
 BARANOV 1993
PL B302 336 Search for Heavy Neutrinos at the IHEP-JINR Neutrino Detector
 BERNARDI 1988
PL B203 332 Further Limit on Heavy Neutrino Coupling
 DORENBOSCH 1986
PL 166B 473 A Search for Decays of Heavy Neutrinos in the Mass Range of 0.5 $−$ 2.8 GeV
 COOPER-SARKAR 1985
PL 160B 207 Search for Heavy Neutrino Decays in the BEBC Beam Dump Experiment