Limits on heavy neutral lepton mixing parameters

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

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
$<2 \times 10^{-4}$ 95 1
FRIEDRICH
2021
Near ${\mathit m}_{\mathrm {{}^{7}\mathrm {Be}}}−$ ${\mathit m}_{\mathrm {{}^{7}\mathrm {Li}}}$ kin. thres.
$<1 \times 10^{-9}$ 90 2
CORTINA-GIL
2020
NA62 ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $150 - 400$ MeV
$<2 \times 10^{-5}$ 95 3
AAD
2019F
 ATLS ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $15 - 40$ GeV
$<1 \times 10^{-9}$ 90 4
ABE
2019B
 T2K Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-4}$ 90 5
ABLIKIM
2019AL
 BES3 ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $0.3 - 0.7$ GeV
$<1 \times 10^{-8}$ 90 6
AGUILAR-AREVA..
2018A
 PIEN ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $60 - 120$ MeV
$<3 \times 10^{-7}$ 90 7
CORTINA-GIL
2018
NA62 ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $200 - 400$ MeV
$<1 \times 10^{-6}$ 90 8
PARK
2016
BELL ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ 1.4 GeV
$<3 \times 10^{-5}$ 90 9
LIVENTSEV
2013
BELL Near ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $2 - 2.5$ GeV
$<3 \times 10^{-5}$ 95 10
ABREU
1997I
 DLPH ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ $6 - 50$ GeV
$<2 \times 10^{-5}$ 95 11
ABREU
1997I
 DLPH Near ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ $\sim{}$ 3.5 GeV
$<1 \times 10^{-5}$ 90 12
BARANOV
1993
Near ${\mathit m}_{{{\mathit \pi}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<2 \times 10^{-7}$ 90 12
BARANOV
1993
Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-7}$ 13, 14
BERNARDI
1988
CNTR Near ${\mathit m}_{{{\mathit \pi}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<2 \times 10^{-9}$ 15, 14
BERNARDI
1988
CNTR Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-7}$ 90 16
DORENBOSCH
1986
CHRM Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
$<1 \times 10^{-7}$ 90 17
COOPER-SARKAR
1985
BEBC Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit e}}}$ kin. thres.
1  Search in electron capture decay ${}^{7}\mathrm {Be}$ $\rightarrow$ ${}^{7}\mathrm {Li}$ ${{\mathit \nu}_{{x}}}$ . Kinematic threshold is $\sim{}$850 keV.
2  Search for ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ . Assumes lifetime of ${{\mathit \nu}_{{x}}}$ $>$ 50 ns.
3  Limit from prompt lepton number violating trilepton search.
4  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , 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.
5  Searches for a Majorana Heavy Neutral Lepton producing a ${{\mathit \pi}^{-}}{{\mathit e}^{+}}$ resonance in the same sign dilepton decay ${{\mathit D}}$ $\rightarrow$ ${{\mathit K}}{{\mathit \pi}^{-}}{{\mathit e}^{+}}{{\mathit e}^{+}}$ .
6  Search for ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ .
7  Search for ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ .
8  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.
9  Search for ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ .
10  Search for prompt ${{\mathit \nu}_{{x}}}$ decay signatures.
11  Search for displaced ${{\mathit \nu}_{{x}}}$ decay signatures.
12  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 ${{\mathit \pi}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{x}}}$ , ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \nu}_{{e}}}$ .
13  ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ decay through ${{\mathit U}}$ $_{ {{\mathit e}} {{\mathit x}} }$.
14  BERNARDI 1988 also considers bounds on $\vert {{\mathit U}}$ $_{ {{\mathit e}} {{\mathit x}} }{{\mathit U}}$ $_{ {{\mathit \mu}} {{\mathit x}} }\vert $.
15  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ decay through ${{\mathit U}}$ $_{ {{\mathit e}} {{\mathit x}} }$.
16  ${{\mathit D}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\mathit \ell}^{+}}{{\mathit \nu}_{{{{\mathit \ell}}}}}$ .
17  ${{\mathit D}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}_{{x}}}$ , with ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\mathit \ell}^{+}}{{\mathit \nu}_{{{{\mathit \ell}}}}}$ or ${{\mathit \nu}_{{x}}}$ $\rightarrow$ ${{\mathit e}^{-}}{{\mathit \pi}^{+}}$ .
References:
FRIEDRICH 2021
PRL 126 021803 Limits on the Existence of sub-MeV Sterile Neutrinos from the Decay of $^7$Be in Superconducting Quantum Sensors
CORTINA-GIL 2020
PL B807 135599 Search for heavy neutral lepton production in $K^+$ decays to positrons
AAD 2019F
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
LIVENTSEV 2013
PR D87 071102 Search for Heavy Neutrinos at Belle
Also
PR D95 099903 (errat.) Erratum to LIVENTSEV 2013 : Search for Heavy Neutrinos at Belle
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
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