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ASTIER 2002 search for anomalous pion decay into a 33.9 MeV neutral particle. No evidence was found and the sensitivity to the branching ratio B( ${{\mathit \pi}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit X}}$ )$\cdot{}$B( ${{\mathit X}}$ $\rightarrow$ ${{\mathit \nu}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ ) is as low as $3.7 \times 10^{-15}$, depending on the ${{\mathit X}}$ lifetime.

²	DAUM 2000 search for anomalous pion decay into a 33.9$~$MeV neutral particle that might be responsible for the time-distribution anomaly observed by the KARMEN Collaboration.

FORMAGGIO 2000 search for anomalous pion decay into a 33.9$~$MeV neutral particle ${{\mathit Q}^{0}}$ that might be responsible for the time-distribution anomaly observed by the KARMEN Collaboration. In the E815 (NuTeV) experiment at Fermilab no evidence was found, with sensitivity for the pion branching ratio B( ${{\mathit \pi}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit Q}^{0}}$ )$\cdot{}$B( ${{\mathit Q}^{0}}$ $\rightarrow$ visible) as low as $10^{-13}$.

⁴

ASSAMAGAN 1998 obtain a limit on heavy neutrino admixture from ${{\mathit \pi}^{+}}$ decay essentially at rest, by measuring with good resolution the momentum distribution of the muons. However, the search uses an ad$~$hoc shape correction. The authors report upper limit for $\vert \mathit U_{{{\mathit \mu}}\mathit x}\vert ^2$ of $0.22$ for ${\mathit m}_{{{\mathit \nu}}}$ = $0.53$ MeV, $0.029$ for ${\mathit m}_{{{\mathit \nu}}}$ = $0.75$ MeV, and $0.016$ for ${\mathit m}_{{{\mathit \nu}}}$ = $1.0$ MeV at 90$\%$CL.

⁵	BRYMAN 1996 search for massive unconventional neutrinos of mass ${\mathit m}_{{{\mathit \nu}_{{x}}}}$ in ${{\mathit \pi}^{+}}$ decay.

⁶

ARMBRUSTER 1995 study the reactions ${}^{12}\mathrm {C}({{\mathit \nu}_{{e}}},{{\mathit e}^{-}}$) ${}^{12}\mathrm {N}$ and ${}^{12}\mathrm {C}({{\mathit \nu}},{{\mathit \nu}^{\,'}}$) ${}^{12}\mathrm {C}^{*}$ induced by neutrinos from ${{\mathit \pi}^{+}}$ and ${{\mathit \mu}^{+}}$ decay at the ISIS neutron spallation source at the Rutherford-Appleton laboratory. An anomaly in the time distribution can be interpreted as the decay ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{x}}}$ , where ${{\mathit \nu}_{{x}}}$ is a neutral weakly interacting particle with mass $\approx{}33.9$ MeV and spin 1/2. The lower limit to the branching ratio is a function of the lifetime of the new massive neutral particle, and reaches a minimum of a few ${\times }10^{-16}$ for ${{\mathit \tau}_{{x}}}\sim{}5~$s.

⁷	From experiments of ${{\mathit \pi}^{+}}$ and ${{\mathit \pi}^{-}}$ decay in flight at PSI, to check the claim of the KARMEN Collaboration quoted above (ARMBRUSTER 1995 ).

⁸	${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}$ peak search experiment.

⁹	${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}$ peak search experiment.

References:

ASTIER

2002

PL B527 23

New Results on a Search for a 33.9-Mev/$\mathit c{}^{2}$ Neutral Particle from ${{\mathit \pi}^{+}}$ Decay in the NOMAD Experiment

DAUM

2000

PRL 85 1815

The KARMEN Time Anomaly: Search for a Neutral Particle of Mass 33.9 MeV in Pion Decay

FORMAGGIO

2000

PRL 84 4043

Search for a 33.9 ${\mathrm {MeV}}/\mathit c{}^{2}$ Neutral Particle in Pion Decay

ASSAMAGAN

1998

PL B434 158

Search for a Heavy Neutrino State in the Decay ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{\mu}}}$

BRYMAN

1996

PR D53 558

Search for Massive Neutrinos in ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}}$ Decay

ARMBRUSTER

1995

PL B348 19

Anomaly in the Time Distribution of Neutrinos from a Pulsed Beam Stop Source

BILGER

1995

PL B363 41

Search for the Hypothetical ${{\mathit \pi}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit X}}$ Decay

DAUM

1995B

PL B361 179

Search for a Neutral Particle of Mass 33.9 MeV in Pion Decay

DAUM

1987

PR D36 2624

Search for Admixture of Massive Neutrinos in the Decay ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}}$

MINEHART

1984

PRL 52 804

Search for Admixtures of Massive Neutrinos in the Decay ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\overline{\mathit \nu}}}$

HAYANO

1982

PRL 49 1305

Heavy Neutrino Search using ${{\mathit K}_{{\mu2}}^{+}}$ Decay

ABELA

1981

PL 105B 263

Search for an Admixture of Heavy Neutrino in Pion Decay

Except where otherwise noted, content of the 2023 Review of Particle Physics is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) license. The publication of the Review of Particle Physics is supported by US DOE, MEXT (Japan), INFN (Italy), JPS and CERN. Individual collaborators receive support for their PDG activities from their respective institutes or funding agencies. © 2023. See LBNL disclaimers.

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