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${}^{8}\mathrm {Be}^{*}$ $\rightarrow$ ${}^{8}\mathrm {Be}$ ${{\mathit A}^{0}}$, ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$

$<(0.2 - 1){\times }\text{ 10}$$^{-3}$

⁹

BINI

1989

CNTR

${}^{16}\mathrm {O}^{*}$ $\rightarrow$ ${}^{16}\mathrm {O}$ ${{\mathit X}^{0}}$, ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$

¹⁰

AVIGNONE

1988

CNTR

${}^{}\mathrm {Cu}^{*}$ $\rightarrow$ ${}^{}\mathrm {Cu}$ ${{\mathit A}^{0}}$ (${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$, ${{\mathit A}^{0}}$ ${{\mathit e}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit e}}$, ${{\mathit A}^{0}}$ ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit Z}}$)

$<1.5 \times 10^{-4}$

¹¹

DATAR

1988

CNTR

${}^{12}\mathrm {C}^{*}$ $\rightarrow$ ${}^{12}\mathrm {C}$ ${{\mathit A}^{0}}$, ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$

$<5 \times 10^{-3}$

¹²

DEBOER

1988

CNTR

${}^{16}\mathrm {O}^{*}$ $\rightarrow$ ${}^{16}\mathrm {O}$ ${{\mathit X}^{0}}$, ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$

$<3.4 \times 10^{-5}$

¹³

DOEHNER

1988

SPEC

${}^{2}\mathrm {H}^{*}$, ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$

$<4 \times 10^{-4}$

¹⁴

SAVAGE

1988

CNTR

Nuclear decay (isovector)

$<3 \times 10^{-3}$

¹⁴

SAVAGE

1988

CNTR

Nuclear decay (isoscalar)

$<0.106$

¹⁵

HALLIN

1986

SPEC

${}^{6}\mathrm {Li}$ isovector decay

$<10.8$

¹⁵

HALLIN

1986

SPEC

${}^{10}\mathrm {B}$ isoscalar decays

$<2.2$

¹⁵

HALLIN

1986

SPEC

${}^{14}\mathrm {N}$ isoscalar decays

$<4 \times 10^{-4}$

¹⁶

SAVAGE

1986

CNTR

${}^{14}\mathrm {N}^{*}$

¹⁷

ANANEV

1985

CNTR

${}^{}\mathrm {Li}^{*}$, deut${}^{*}{{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$

¹⁸

CAVAIGNAC

1983

CNTR

${}^{97}\mathrm {Nb}^{*}$, deut${}^{*}$ transition ${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$

¹⁹

ALEKSEEV

1982

CNTR

${}^{}\mathrm {Li}^{*}$, deut${}^{*}$ transition ${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$

²⁰

LEHMANN

1982

CNTR

${}^{}\mathrm {Cu}^{*}$ $\rightarrow$ ${}^{}\mathrm {Cu}$ ${{\mathit A}^{0}}$ (${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$)

²¹

ZEHNDER

1982

CNTR

${}^{}\mathrm {Li}^{*}$, ${}^{}\mathrm {Nb}^{*}$ decay, ${{\mathit n}}$-capt.

²²

ZEHNDER

1981

CNTR

${}^{}\mathrm {Ba}^{*}$ $\rightarrow$ ${}^{}\mathrm {Ba}$ ${{\mathit A}^{0}}$ (${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$)

²³

CALAPRICE

1979

Carbon

¹	DERBIN 2023 use a thallium garnet bolometric detector to search for the 8.4 keV solar axion line emitted from the M1 nuclear transition of ${}^{169}\mathrm {Tm}$. Their limits are equivalent to an upper bound on the KSVZ and DFSZ axion masses of 141 eV and 244 eV, respectively.

²	DERBIN 2002 looked for the axion emission in an M1 transition in ${}^{125m}{}^{}\mathrm {Te}$ decay. They looked for a possible presence of a shifted energy spectrum in gamma rays due to the undetected axion.

³	DEBOER 1997C reanalyzed the existent data on Nuclear M1 transitions and find that a 9$~$MeV boson decaying into ${{\mathit e}^{+}}{{\mathit e}^{-}}$ would explain the excess of events with large opening angles. See also DEBOER 2001 for follow-up experiments.

⁴

TSUNODA 1995 looked for axion emission when ${}^{252}\mathrm {Cf}$ undergoes a spontaneous fission, with the axion decaying into ${{\mathit e}^{+}}{{\mathit e}^{-}}$. The bound is for ${\mathit m}_{{{\mathit A}^{0}}}$=40 MeV. It improves to $2.5 \times 10^{-5}$ for ${\mathit m}_{{{\mathit A}^{0}}}$=200 MeV.

⁵

MINOWA 1993 studied chain process, ${}^{139}\mathrm {Ce}$ $\rightarrow$ ${}^{139}\mathrm {La}^{*}$ by electron capture and M1 transition of ${}^{139}\mathrm {La}^{*}$ to the ground state. It does not assume decay modes of ${{\mathit A}^{0}}$. The bound applies for ${\mathit m}_{{{\mathit A}^{0}}}<166$ keV.

⁶	HICKS 1992 bound is applicable for ${\mathit \tau}_{{{\mathit X}^{0}}}$ $<4 \times 10^{-11}$ sec.

⁷	The ASANUMA 1990 limit is for the branching fraction of ${{\mathit X}^{0}}$ emission per ${}^{241}\mathrm {Am}$ ${{\mathit \alpha}}$ decay and valid for ${\mathit \tau}_{{{\mathit X}^{0}}}$ $<$ $3 \times 10^{-11}~$s.

⁸	The DEBOER 1990 limit is for the branching ratio ${}^{8}\mathrm {Be}^{*}$ ($18.15$ MeV, 1${}^{+}$) $\rightarrow$ ${}^{8}\mathrm {Be}$ ${{\mathit A}^{0}}$, ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ for the mass range ${\mathit m}_{{{\mathit A}^{0}}}$ = 4$-$15 MeV.

⁹

The BINI 1989 limit is for the branching fraction of ${}^{16}\mathrm {O}^{*}$ ($6.05$ MeV, 0${}^{+}$) $\rightarrow$ ${}^{16}\mathrm {O}$ ${{\mathit X}^{0}}$, ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ for ${\mathit m}_{{{\mathit X}}}$ = $1.5-3.1$ MeV. ${\mathit \tau}_{{{\mathit X}^{0}}}{ {}\lesssim{} }$ $10^{-11}~$s is assumed. The spin-parity of ${{\mathit X}}$ is restricted to 0${}^{+}$ or 1${}^{−}$.

¹⁰

AVIGNONE 1988 looked for the 1115 keV transition ${}^{}\mathrm {C}^{*}$ $\rightarrow$ ${}^{}\mathrm {Cu}$ ${{\mathit A}^{0}}$, either from ${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$ in-flight decay or from the secondary ${{\mathit A}^{0}}$ interactions by Compton and by Primakoff processes. Limits for axion parameters are obtained for ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ $1.1$ MeV.

¹¹

DATAR 1988 rule out light pseudoscalar particle emission through its decay ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ in the mass range $1.02-2.5$ MeV and lifetime range $10^{-13}-10^{-8}~$s. The above limit is for $\tau $ = $5 \times 10^{-13}~$s and $\mathit m$ = $1.7$ MeV; see the paper for the $\tau -\mathit m$ dependence of the limit.

¹²

The limit is for the branching fraction of ${}^{16}\mathrm {O}^{*}$ ($6.05$ MeV, 0${}^{+}$) $\rightarrow$ ${}^{16}\mathrm {O}$ ${{\mathit X}^{0}}$, ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ against internal pair conversion for ${\mathit m}_{{{\mathit X}^{0}}}$ = $1.7$ MeV and ${\mathit \tau}_{{{\mathit X}^{0}}}$ $<$ $10^{-11}~$s. Similar limits are obtained for ${\mathit m}_{{{\mathit X}^{0}}}$ = $1.3-3.2$ MeV. The spin parity of ${{\mathit X}^{0}}$ must be either 0${}^{+}$ or 1${}^{−}$. The limit at $1.7$ MeV is translated into a limit for the ${{\mathit X}^{0}}$-nucleon coupling constant: $\mathit g{}^{2}_{{{\mathit X}^{0}} }/4{{\mathit \pi}}$ $<$ $2.3 \times 10^{-9}$.

¹³	The DOEHNER 1988 limit is for ${\mathit m}_{{{\mathit A}^{0}}}$ = $1.7$ MeV, $\tau\mathrm {({{\mathit A}^{0}})}$ $<$ $10^{-10}~$s. Limits less than $10^{-4}$ are obtained for ${\mathit m}_{{{\mathit A}^{0}}}$ = $1.2-2.2$ MeV.

¹⁴

SAVAGE 1988 looked for ${{\mathit A}^{0}}$ that decays into ${{\mathit e}^{+}}{{\mathit e}^{-}}$ in the decay of the $9.17$ MeV $\mathit J{}^{P}$ = $2{}^{+}{}^{}$ state in ${}^{14}\mathrm {N}$, $17.64$ MeV state $\mathit J{}^{P}$ = $1{}^{+}{}^{}$ in ${}^{8}\mathrm {Be}$, and the $18.15$ MeV state $\mathit J{}^{P}$ = $1{}^{+}{}^{}$ in ${}^{8}\mathrm {Be}$. This experiment constrains the isovector coupling of ${{\mathit A}^{0}}$ to hadrons, if ${\mathit m}_{{{\mathit A}^{0}}}$ = ($1.1$ $\rightarrow$ $2.2$) MeV and the isoscalar coupling of ${{\mathit A}^{0}}$ to hadrons, if ${\mathit m}_{{{\mathit A}^{0}}}$ = ($1.1$ $\rightarrow$ $2.6$) MeV. Both limits are valid only if $\tau\mathrm {({{\mathit A}^{0}})}{ {}\lesssim{} }$ $1 \times 10^{-11}$ s.

¹⁵

Limits are for $\Gamma\mathrm {({{\mathit A}^{0}}(1.8 MeV))}/\Gamma\mathrm {({{\mathit \pi}}M1)}$; i.e., for 1.8 MeV axion emission normalized to the rate for internal emission of ${{\mathit e}^{+}}{{\mathit e}^{-}}$ pairs. Valid for ${\mathit \tau}_{{{\mathit A}^{0}}}$ $<$ $2 \times 10^{-11}$s. ${}^{6}\mathrm {Li}$ isovector decay data strongly disfavor PECCEI 1986 model I, whereas the ${}^{10}\mathrm {B}$ and ${}^{14}\mathrm {N}$ isoscalar decay data strongly reject PECCEI 1986 model II and III.

¹⁶

SAVAGE 1986B looked for ${{\mathit A}^{0}}$ that decays into ${{\mathit e}^{+}}{{\mathit e}^{-}}$ in the decay of the 9.17 MeV $\mathit J{}^{P} = 2+$ state in ${}^{14}\mathrm {N}$. Limit on the branching fraction is valid if ${\mathit \tau}_{{{\mathit A}^{0}}}{ {}\lesssim{} }1. \times 10^{-11}$s for ${\mathit m}_{{{\mathit A}^{0}}}$ = (1.1$-$1.7) MeV. This experiment constrains the iso-vector coupling of ${{\mathit A}^{0}}$ to hadrons.

¹⁷	ANANEV 1985 with IBR-2 pulsed reactor exclude standard ${{\mathit A}^{0}}$ at CL = 95$\%$ masses below 470 keV (${}^{}\mathrm {Li}^{}$ decay) and below 2${}{\mathit m}_{{{\mathit e}}}$ for deuteron decay.

¹⁸	CAVAIGNAC 1983 at Bugey reactor exclude axion at any ${\mathit m}_{\mathrm {{}^{97}\mathrm {Nb}^{}decay}}$ and axion with ${\mathit m}_{{{\mathit A}^{0}}}$ between 275 and 288 keV (deuteron decay).

¹⁹	ALEKSEEV 1982 with IBR-2 pulsed reactor exclude standard ${{\mathit A}^{0}}$ at CL = 95$\%$ mass-ranges ${\mathit m}_{{{\mathit A}^{0}}}$ $<$400 keV (${}^{}\mathrm {Li}^{}$ decay) and 330 keV $<{\mathit m}_{{{\mathit A}^{0}}}$ $<$2.2 MeV. (deuteron decay).

²⁰	LEHMANN 1982 obtained ${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$ rate $<6.2 \times 10^{-5}$/s (CL = 95$\%$) excluding ${\mathit m}_{{{\mathit A}^{0}}}$ between 100 and 1000 keV.

²¹

ZEHNDER 1982 used Gosgen 2.8GW light-water reactor to check ${{\mathit A}^{0}}$ production. No 2${{\mathit \gamma}}$ peak in ${}^{}\mathrm {Li}^{*}$, ${}^{}\mathrm {Nb}^{*}$ decay (both single ${{\mathit p}}$ transition) nor in ${{\mathit n}}$ capture (combined with previous ${}^{}\mathrm {Ba}^{*}$ negative result) rules out standard ${{\mathit A}^{0}}$. Set limit ${\mathit m}_{{{\mathit A}^{0}}}$ $<$60 keV for any ${{\mathit A}^{0}}$.

²²

ZEHNDER 1981 looked for ${}^{}\mathrm {Ba}^{*}$ $\rightarrow$ ${{\mathit A}^{0}}{}^{}\mathrm {Ba}$ transition with ${{\mathit A}^{0}}$ $\rightarrow$ 2 ${{\mathit \gamma}}$. Obtained 2${{\mathit \gamma}}$ coincidence rate $<2.2 \times 10^{-5}$/s (CL = 95$\%$) excluding ${\mathit m}_{{{\mathit A}^{0}}}$ $>$160 keV (or 200 keV depending on Higgs mixing). However, see BARROSO 1981.

²³	CALAPRICE 1979 saw no axion emission from excited states of carbon. Sensitive to axion mass between 1 and 15 MeV.

References

Except where otherwise noted, content of the 2025 Review of Particle Physics is licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) license. The publication of the Review of Particle Physics is supported by US DOE, MEXT (Japan), INFN (Italy) and CERN. Individual collaborators receive support for their PDG activities from their respective institutes or funding agencies. © 2025. See LBNL disclaimers.

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