¹ BACK 2003A searched for heavy neutrinos emitted from ${}^{8}\mathrm {B}$ decay in the Sun using the decay ${{\mathit \nu}_{{h}}}$ $\rightarrow$ ${{\mathit \nu}_{{e}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ in the Counting Test Facility (the prototype of the Borexino detector) and obtained limits on heavy neutrino admixture for the ${{\mathit \nu}_{{h}}}$ mass range $1.1 - 12$ MeV.

² ABREU 1997I long-lived ${{\mathit \nu}_{{x}}}$ analysis. Short-lived analysis extends limit to lower masses with decreasing sensitivity except at $3.5$ GeV, where the limit is the same as at 6 GeV.

³ HAGNER 1995 obtain limits on heavy neutrino admixture from the decay ${{\mathit \nu}_{{h}}}$ $\rightarrow$ ${{\mathit \nu}_{{e}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ at a nuclear reactor for the ${{\mathit \nu}_{{h}}}$ mass range $2 - 9$ MeV.

⁴ BARANOV 1993 is a search for neutrino decays into ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \nu}_{{e}}}$ using a beam dump experiment at the 70 GeV Serpukhov proton synchrotron. The limits are not as good as those achieved earlier by BERGSMA 1983 and BERNARDI 1986 , BERNARDI 1988 .

⁵ BURCHAT 1990 includes the analyses reported in JUNG 1990 , ABRAMS 1989C, and WENDT 1987 .

⁶ OBERAUER 1987 bounds from search for ${{\mathit \nu}}$ $\rightarrow$ ${{\mathit \nu}^{\,'}}{{\mathit e}}{{\mathit e}}$ decay mode using reactor (anti)neutrinos.

⁷ COOPER-SARKAR 1985 also give limits based on model-dependent assumptions for ${{\mathit \nu}_{{\tau}}}$ flux. We do not list these. Note that for this bound to be nontrivial, $\mathit x$ is not equal to 3, i.e. ${{\mathit \nu}_{{x}}}$ cannot be the dominant mass eigenstate in ${{\mathit \nu}_{{\tau}}}$ since ${\mathit m}_{{{\mathit \nu}_{{3}}}}$ $<$70 MeV (ALBRECHT 1985I). Also, of course, $\mathit x$ is not equal to 1 or 2, so a fourth generation would be required for this bound to be nontrivial.

⁸ BERGSMA 1983B also quote limits on $\vert \mathit U_{{{\mathit e}}3}\vert {}^{2}$ where the index 3 refers to the mass eigenstate dominantly coupled to the ${{\mathit \tau}}$. Those limits were based on assumptions about the ${{\mathit D}_{{s}}}$ mass and ${{\mathit D}_{{s}}}$ $\rightarrow$ ${{\mathit \tau}}{{\mathit \nu}_{{\tau}}}$ branching ratio which are no longer valid. See COOPER-SARKAR 1985 .

References:

BACK

2003A

JETPL 78 261

New Experimental Limits on Heavy Neutrino Mixing in ${}^{8}\mathrm {B}$ Decay Obtained with the Borexino Counting Test Facility

ACHARD

2001

PL B517 67

Search for Heavy Isosinglet Neutrino in ${{\mathit e}^{+}}{{\mathit e}^{-}}$ Annihilation at LEP

ACCIARRI

1999K

PL B461 397

Search for Heavy Isosinglet Neutrinos in ${{\mathit e}^{+}}{{\mathit e}^{-}}$ Annihilation at 130 $<$ $\sqrt {s }$ $<$ 189 GeV

ABREU

1997I

ZPHY C74 57

Search for Neutral Heavy Leptons Produced in ${{\mathit Z}}$ Decays

HAGNER

1995

PR D52 1343

Experimental Search for the Neutrino Decay ${{\mathit \nu}}$ $\rightarrow$ ${{\mathit \nu}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ and Limits on Neutrino Mixing

BARANOV

1993

PL B302 336

Search for Heavy Neutrinos at the IHEP-JINR Neutrino Detector

ADEVA

1990S

PL B251 321

A Search for Heavy Charged and Neutral Leptons from ${{\mathit Z}^{0}}$ Decays

BURCHAT

1990

PR D41 3542

A Search for Decays of the ${{\mathit Z}^{0}}$ to Unstable Neutral Leptons with Mass between 2.5 and 22 GeV