${{\mathit A}^{0}}$ (Axion) and Other Light Boson (${{\mathit X}^{0}}$ ) Searches in Hadron Decays

INSPIRE  
Limits are for branching ratios.
VALUE CL% DOCUMENT ID TECN  COMMENT
• • We do not use the following data for averages, fits, limits, etc. • •
$<7 \times 10^{-13}$ 95 1
ABRATENKO
2021
MCBN ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ )
$<1.5 \times 10^{-7}$ 90 2
CORTINA-GIL
2021
NA62 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}}{{\mathit X}^{0}}$
$<5 \times 10^{-11}$ 90 3
CORTINA-GIL
2021A
NA62 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
$<9 \times 10^{-10}$ 90 4
CORTINA-GIL
2021C
NA62 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
$<1.5 \times 10^{-8}$ 90 5
PARK
2021
BELL ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit X}^{0}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ , ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ , ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ )
$<2.4 \times 10^{-9}$ 90 6
AHN
2019
KOTO ${{\mathit K}_L^0}$ $\rightarrow$ ${{\mathit \pi}^{0}}$ ${{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}$ = 135 MeV
$<2 \times 10^{-10}$ 95 7
AAIJ
2017AQ
LHCB ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ )
$<3.7 \times 10^{-8}$ 90 8
AHN
2017
KOTO ${{\mathit K}_L^0}$ $\rightarrow$ ${{\mathit \pi}^{0}}$ ${{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}$ = 135 MeV
$<6 \times 10^{-11}$ 90 9
BATLEY
2017
NA48 ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}$ ${{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ )
10
WON
2016
BELL ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \gamma}}$ ${{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ )
$<1 \times 10^{-9}$ 95 11
AAIJ
2015AZ
LHCB ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit K}^{*0}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ )
$<1.5 \times 10^{-6}$ 90 12
ADLARSON
2013
WASA ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ), ${\mathit m}_{{{\mathit X}^{0}}}$ = 100 MeV
$<2 \times 10^{-8}$ 90 13
BABUSCI
2013B
KLOE ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ )
14
ARCHILLI
2012
KLOE ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit X}^{0}}$ , ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$
$<2 \times 10^{-15}$ 90 15
GNINENKO
2012A
BDMP ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ )
$<3 \times 10^{-14}$ 90 16
GNINENKO
2012B
BDMP ${{\mathit \eta}}$ (${{\mathit \eta}^{\,'}}$ ) $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ )
$<7 \times 10^{-10}$ 90 17
ADLER
2004
B787 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
$<7.3 \times 10^{-11}$ 90 18
ANISIMOVSKY
2004
B949 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
$<4.5 \times 10^{-11}$ 90 19
ADLER
2002C
B787 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
$<4 \times 10^{-5}$ 90 20
ADLER
2001
B787 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{0}}{{\mathit A}^{0}}$
$<4.9 \times 10^{-5}$ 90
AMMAR
2001B
CLEO ${{\mathit B}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}$ (${{\mathit K}^{\pm}}$ )${{\mathit X}^{0}}$
$<5.3 \times 10^{-5}$ 90
AMMAR
2001B
CLEO ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit K}_S^0}$ ${{\mathit X}^{0}}$
$<3.3 \times 10^{-5}$ 90 21
ALTEGOER
1998
NOMD ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}<120$ MeV
$<5.0 \times 10^{-8}$ 90 22
KITCHING
1997
B787 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ )
$<5.2 \times 10^{-10}$ 90 23
ADLER
1996
B787 ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
$<2.8 \times 10^{-4}$ 90 24
AMSLER
1996B
CBAR ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}<$ 65 MeV
$<3 \times 10^{-4}$ 90 24
AMSLER
1996B
CBAR ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}$= $50 - 200$ MeV
$<4 \times 10^{-5}$ 90 24
AMSLER
1996B
CBAR ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}$= $50 - 925$ MeV
$<6 \times 10^{-5}$ 90 24
AMSLER
1994B
CBAR ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}=65 - 125$ MeV
$<6 \times 10^{-5}$ 90 24
AMSLER
1994B
CBAR ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}=200 - 525$ MeV
$<7 \times 10^{-3}$ 90 25
MEIJERDREES
1994
CNTR ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}$=25 MeV
$<2 \times 10^{-3}$ 90 25
MEIJERDREES
1994
CNTR ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ , ${\mathit m}_{{{\mathit X}^{0}}}$=100 MeV
$<2 \times 10^{-7}$ 90 26
ATIYA
1993B
B787 Sup. by ADLER 2004
$<3 \times 10^{-13}$ 27
NG
1993
COSM ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$
$<1.1 \times 10^{-8}$ 90 28
ALLIEGRO
1992
SPEC ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ )
$<5 \times 10^{-4}$ 90 29
ATIYA
1992
B787 ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$
$<1 \times 10^{-12}$ 95 30
BARABASH
1992
BDMP ${{\mathit \pi}^{\pm}}$ $\rightarrow$ ${{\mathit e}^{\pm}}{{\mathit \nu}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ , ${{\mathit \gamma}}{{\mathit \gamma}}$ ), ${\mathit m}_{{{\mathit X}^{0}}}$ = 8 MeV
$<1 \times 10^{-12}$ 95 31
BARABASH
1992
BDMP ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}$ ${{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ , ${{\mathit \gamma}}{{\mathit \gamma}}$ ), ${\mathit m}_{{{\mathit X}^{0}}}$ = 10 MeV
$<1 \times 10^{-11}$ 95 32
BARABASH
1992
BDMP ${{\mathit K}_L^0}$ $\rightarrow$ ${{\mathit \pi}^{0}}$ ${{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ , ${{\mathit \gamma}}{{\mathit \gamma}}$ ), ${\mathit m}_{{{\mathit X}^{0}}}$ = 10 MeV
$<1 \times 10^{-14}$ 95 33
BARABASH
1992
BDMP ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \eta}}$ ${{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ , ${{\mathit \gamma}}{{\mathit \gamma}}$ ), ${\mathit m}_{{{\mathit X}^{0}}}$ = 10 MeV
$<4 \times 10^{-6}$ 90 34
MEIJERDREES
1992
SPEC ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ ( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ), ${\mathit m}_{{{\mathit X}^{0}}}$= 100 MeV
$<1 \times 10^{-7}$ 90 35
ATIYA
1990B
B787 Sup. by KITCHING 1997
$<1.3 \times 10^{-8}$ 90 36
KORENCHENKO
1987
SPEC ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit A}^{0}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ )
$<1 \times 10^{-9}$ 90 37
EICHLER
1986
SPEC Stopped ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit A}^{0}}$
$<2 \times 10^{-5}$ 90 38
YAMAZAKI
1984
SPEC For 160$<\mathit m<$260 MeV
$<(1.5-4){\times }\text{ 10}$$^{-6}$ 90 38
YAMAZAKI
1984
SPEC ${{\mathit K}}$ decay, ${\mathit m}_{{{\mathit X}^{0}}}{}\ll$100 MeV
39
ASANO
1982
CNTR Stopped ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
40
ASANO
1981B
CNTR Stopped ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
41
ZHITNITSKII
1979
Heavy axion
1  ABRATENKO 2021 quoted limit is for ${\mathit m}_{{{\mathit X}^{0}}}$ = 150 MeV and the lifetime $\mathit c\tau _{{{\mathit X}^{0}} }$ = 80 m. See their Fig. 4 for the limits in the range of ${\mathit m}_{{{\mathit X}^{0}}}$ = $10 - 210$ MeV.
2  CORTINA-GIL 2021 quoted limit is for ${\mathit m}_{{{\mathit X}^{0}}}$ = 370 MeV. Limits from O($10^{-5}$) and O($10^{-6}$) are obtained for ${\mathit m}_{{{\mathit X}^{0}}}$ = $10 - 370$ MeV (see their Fig. 7).
3  CORTINA-GIL 2021A quoted limit is for ${\mathit m}_{{{\mathit X}^{0}}}$ = $160 - 250$ MeV. Limits between $5 \times 10^{-11}$ and $2 \times 10^{-10}$ are obtained in the range of ${\mathit m}_{{{\mathit X}^{0}}}$ = $0 - 110$ and $154 - 260$ MeV, assuming stable or invisibly decaying ${{\mathit X}^{0}}$ . See their Fig. 4 for mass- and lifetime-dependent limits.
4  CORTINA-GIL 2021C quoted limit is for ${\mathit m}_{{{\mathit X}^{0}}}$ = $130 - 140$ MeV, and limits of $9 \times 10^{-10} - 6 \times 10^{-7}$ are obtained in the mass range of ${\mathit m}_{{{\mathit X}^{0}}}$ = $110 - 155$ MeV, assuming ${{\mathit X}^{0}}$ escapes detection. See their Fig. 6 for mass- and lifetime-dependent limits.
5  PARK 2021 look for dark photons produced by decays of ${{\mathit B}^{0}}$ through off-shell Higgs-dark Higgs mixing. See their Fig. 5 for limits in the range of ${\mathit m}_{{{\mathit X}^{0}}}$ = $0.01 - 2.62$ GeV.
6  AHN 2019 is an update of AHN 2017 from a new data set. See their Fig. 4 for the limits in the range of ${\mathit m}_{{{\mathit X}^{0}}}$ = $0 - 250$ MeV.
7  AAIJ 2017AQ limit is for ${\mathit \tau}_{{{\mathit X}^{0}}}$ = 10 ps. See their Fig. 4 for limits in the range of ${\mathit m}_{{{\mathit X}^{0}}}$ = $250 - 4700$ MeV and ${\mathit \tau}_{{{\mathit X}^{0}}}$ = $0.1 - 1000$ ps.
8  AHN 2017 limit as a function of ${\mathit m}_{{{\mathit X}^{0}}}$ from 0 to 250 MeV is provided in their Fig. 5.
9  BATLEY 2017 limit is for ${\mathit m}_{{{\mathit X}^{0}}}$ = 216 MeV and ${\mathit \tau}_{{{\mathit X}^{0}}}{}\leq{}$ 10 ps. See their Fig. 4(c) for limits in the range of ${\mathit m}_{{{\mathit X}^{0}}}$ = $211 - 354$ MeV and longer lifetimes.
10  WON 2016 look for a vector boson coupled to baryon number. Derived limits on ${{\mathit \alpha}^{\,'}}$ $<$ $10^{-3} - E-2$ for ${\mathit m}_{{{\mathit X}^{0}}}$ = $290 - 520$ MeV at 95$\%$ CL. See their Fig. 4 for mass-dependent limits.
11  AAIJ 2015AZ limit is for ${\mathit \tau}_{{{\mathit X}^{0}}}$ = 10 ps and ${\mathit m}_{{{\mathit X}^{0}}}$ = $214 - 4350$ MeV. See their Fig. 4 for mass- and lifetime-dependent limits.
12  ADLARSON 2013 limits between $2.0 \times 10^{-5}$ and $1.5 \times 10^{-6}$ are obtained for ${\mathit m}_{{{\mathit X}^{0}}}$ = $20 - 100$ MeV (see their Fig. 8). Angular momentum conservation requires that ${{\mathit X}^{0}}$ has spin ${}\geq{}$ 1.
13  BABUSCI 2013B limit is for B( ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit X}^{0}}$ )$\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ) and applies to ${\mathit m}_{{{\mathit X}^{0}}}$ = 410 MeV. It is derived by analyzing ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$ and ${{\mathit \pi}^{-}}{{\mathit \pi}^{+}}{{\mathit \pi}^{0}}$ . Limits between $1 \times 10^{-6}$ and $2 \times 10^{-8}$ are obtained for ${\mathit m}_{{{\mathit X}^{0}}}{}\leq{}$ 450 MeV (see their Fig. 6).
14  ARCHILLI 2012 analyzed ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \pi}^{0}}$ decays. Derived limits on ${{\mathit \alpha}^{\,'}}$ /${{\mathit \alpha}}$ $<$ $2 \times 10^{-5}$ for ${\mathit m}_{{{\mathit X}^{0}}}$ = $50 - 420$ MeV at 90$\%$ CL. See their Fig. 8 for mass-dependent limits.
15  GNINENKO 2012A limit is for B( ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ )$\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ) and applies for ${\mathit m}_{{{\mathit X}^{0}}}$ = 90 MeV and ${\mathit \tau}_{{{\mathit X}^{0}}}$ $\simeq{}$ $1 \times 10^{-8}$ sec. Limits between $10^{-8}$ and $2 \times 10^{-15}$ are obtained for ${\mathit m}_{{{\mathit X}^{0}}}$ = $3 - 120$ MeV and ${\mathit \tau}_{{{\mathit X}^{0}}}$ = $1 \times 10^{-11} - 1$ sec. See their Fig. 3 for limits at different masses and lifetimes.
16  GNINENKO 2012B limit is for B( ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ )$\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ) and applies for ${\mathit m}_{{{\mathit X}^{0}}}$ = 100 MeV and ${\mathit \tau}_{{{\mathit X}^{0}}}$ $≅$ $6 \times 10^{-9}$ sec. Limits between $10^{-5}$ and $3 \times 10^{-14}$ are obtained for ${\mathit m}_{{{\mathit X}^{0}}}{ {}\lesssim{} }$ 550 MeV and ${\mathit \tau}_{{{\mathit X}^{0}}}$ = $10^{-10} - 10$ sec. See their Fig. 5 for limits at different mass and lifetime and for ${{\mathit \eta}^{\,'}}$ decays.
17  ADLER 2004 limit applies for a mass near 180 MeV. For other masses in the range ${\mathit m}_{{{\mathit X}^{0}}}$ = $150 - 250$ MeV the limit is less restrictive, but still improves ADLER 2002C and ATIYA 1993B.
18  ANISIMOVSKY 2004 bound is for ${\mathit m}_{{{\mathit X}^{0}}}$=0.
19  ADLER 2002C bound is for ${\mathit m}_{{{\mathit X}^{0}}}<$60 MeV. See Fig.$~$2 for limits at higher masses.
20  The quoted limit is for ${\mathit m}_{{{\mathit X}^{0}}}$ = $0 - 80$ MeV. See their Fig. 5 for the limit at higher mass. The branching fraction limit assumes pure phase space decay distributions.
21  ALTEGOER 1998 looked for ${{\mathit X}^{0}}$ from ${{\mathit \pi}^{0}}$ decay which penetrate the shielding and convert to ${{\mathit \pi}^{0}}$ in the external Coulomb field of a nucleus.
22  KITCHING 1997 limit is for B( ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ )$\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ ) and applies for ${\mathit m}_{{{\mathit X}^{0}}}\simeq{}$50 MeV, $\tau _{{{\mathit X}^{0}} }<10^{-10}~$s. Limits are provided for 0$<{\mathit m}_{{{\mathit X}^{0}}}<100$ MeV, $\tau _{{{\mathit X}^{0}} }<10^{-8}~$s.
23  ADLER 1996 looked for a peak in missing-mass distribution. This work is an update of ATIYA 1993 . The limit is for massless stable ${{\mathit X}^{0}}$ particles and extends to ${\mathit m}_{{{\mathit X}^{0}}}$=80 MeV at the same level. See paper for dependence on finite lifetime.
24  AMSLER 1994B and AMSLER 1996B looked for a peak in missing-mass distribution.
25  MEIJERDREES 1994 limit is based on inclusive photon spectrum and is independent of ${{\mathit X}^{0}}$ decay modes. It applies to $\tau\mathrm {({{\mathit X}^{0}} )}>10^{-23}~$sec.
26  ATIYA 1993B looked for a peak in missing mass distribution. The bound applies for stable ${{\mathit X}^{0}}$ of ${\mathit m}_{{{\mathit X}^{0}}}=150 - 250$ MeV, and the limit becomes stronger ($10^{-8}$) for ${\mathit m}_{{{\mathit X}^{0}}}=180 - 240$ MeV.
27  NG 1993 studied the production of ${{\mathit X}^{0}}$ via ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ in the early universe at $\mathit T\simeq{}$1 MeV. The bound on extra neutrinos from nucleosynthesis $\Delta {{\mathit N}_{{\nu}}}$ $<0.3$ (WALKER 1991 ) is employed. It applies to ${\mathit m}_{{{\mathit X}^{0}}}{}\ll$1 MeV in order to be relativistic down to nucleosynthesis temperature. See paper for heavier ${{\mathit X}^{0}}$ .
28  ALLIEGRO 1992 limit applies for ${\mathit m}_{{{\mathit X}^{0}}}=150 - 340$ MeV and is the branching ratio times the decay probability. Limit is $<1.5 \times 10^{-8}$ at 99$\%$CL.
29  ATIYA 1992 looked for a peak in missing mass distribution. The limit applies to ${\mathit m}_{{{\mathit X}^{0}}}=0 - 130$ MeV in the narrow resonance limit. See paper for the dependence on lifetime. Covariance requires ${{\mathit X}^{0}}$ to be a vector particle.
30  BARABASH 1992 is a beam dump experiment that searched for a light Higgs. Limits between $1 \times 10^{-12}$ and $1 \times 10^{-7}$ are obtained for 3 $<$ ${\mathit m}_{{{\mathit X}^{0}}}$ $<$ 40 MeV.
31  Limits between $1 \times 10^{-12}$ and $1$ are obtained for 4 $<$ ${\mathit m}_{{{\mathit X}^{0}}}$ $<$ 69 MeV.
32  Limits between $1 \times 10^{-11}$ and $5 \times 10^{-3}$ are obtained for 4 $<$ ${\mathit m}_{{{\mathit X}^{0}}}$ $<$ 63 MeV.
33  Limits between $1 \times 10^{-14}$ and $1$ are obtained for 3 $<$ ${\mathit m}_{{{\mathit X}^{0}}}$ $<$ 82 MeV.
34  MEIJERDREES 1992 limit applies for ${\mathit \tau}_{{{\mathit X}^{0}}}$ = $10^{-23} - 10^{-11}~$sec. Limits between $2 \times 10^{-4}$ and $4 \times 10^{-6}$ are obtained for ${\mathit m}_{{{\mathit X}^{0}}}$ = $25 - 120$ MeV. Angular momentum conservation requires that ${{\mathit X}^{0}}$ has spin ${}\geq{}$1.
35  ATIYA 1990B limit is for B( ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ )$\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ ) and applies for ${\mathit m}_{{{\mathit X}^{0}}}$ = 50 MeV, ${\mathit \tau}_{{{\mathit X}^{0}}}$ $<$ $10^{-10}~$s. Limits are also provided for 0 $<$ ${\mathit m}_{{{\mathit X}^{0}}}$ $<$ 100 MeV, ${\mathit \tau}_{{{\mathit X}^{0}}}$ $<$ $10^{-8}~$s.
36  KORENCHENKO 1987 limit assumes ${\mathit m}_{{{\mathit A}^{0}}}$ = $1.7$ MeV, ${\mathit \tau}_{{{\mathit A}^{0}}}{ {}\lesssim{} }$ $10^{-12}$ s, and B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ) = 1.
37  EICHLER 1986 looked for ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}{{\mathit A}^{0}}$ followed by ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ . Limits on the branching fraction depend on the mass and and lifetime of ${{\mathit A}^{0}}$ . The quoted limits are valid when $\tau\mathrm {({{\mathit A}^{0}} )}{ {}\gtrsim{} }3. \times 10^{-10}$s if the decays are kinematically allowed.
38  YAMAZAKI 1984 looked for a discrete line in ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}$ X. Sensitive to wide mass range (5$-$300 MeV), independent of whether X decays promptly or not.
39  ASANO 1982 at KEK set limits for B( ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ ) for ${\mathit m}_{{{\mathit X}^{0}}}$ $<$100 MeV as BR $<4. \times 10^{-8}$ for $\tau\mathrm {( {{\mathit X}^{0}} \rightarrow {{\mathit n}} )}$ $>1. \times 10^{-9}$ s, BR $<1.4 \times 10^{-6}$ for $\tau $ $<1. \times 10^{-9}$s.
40  ASANO 1981B is KEK experiment. Set B( ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ ) $<3.8 \times 10^{-8}$ at CL = 90$\%$.
41  ZHITNITSKII 1979 argue that a heavy axion predicted by YANG 1978 (3 $<\mathit m$ $<$40 MeV) contradicts experimental muon anomalous magnetic moments.
References:
ABRATENKO 2021
PRL 127 151803
CORTINA-GIL 2021C
JHEP 2102 201 Search for $\pi^0$ decays to invisible particles
CORTINA-GIL 2021A
JHEP 2103 058 Search for a feebly interacting particle $X$ in the decay $K^{+}\rightarrow\pi^{+}X$
CORTINA-GIL 2021
PL B816 136259 Search for $K^+$ decays to a muon and invisible particles
PARK 2021
JHEP 2104 191 Search for the dark photon in $B^0 \to A^{\prime} A^{\prime}$, $A^{\prime} \to e^+ e^-$, $\mu^+ \mu^-$, and $\pi^+ \pi^-$ decays at Belle
AHN 2019
PRL 122 021802 Search for the $K_L \!\to\! \pi^0 \nu \overline{\nu}$ and $K_L \!\to\! \pi^0 X^0$ decays at the J-PARC KOTO experiment
AAIJ 2017AQ
PR D95 071101 Search for Long-Lived Scalar Particles in ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit \chi}}$ (${{\mathit \mu}^{+}}$ ${{\mathit \mu}^{-}}$ )} Decays
AHN 2017
PTEP 2017 021C01 A New Search for the ${{\mathit K}_L^0}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$ and ${{\mathit K}_L^0}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit X}^{0}}$ Decays
BATLEY 2017
PL B769 67 Searches for Lepton Number Violation and Resonances in ${{\mathit K}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit \mu}}{{\mathit \mu}}$ Decays
WON 2016
PR D94 092006 Search for a Dark Vector Gauge Boson Decaying to ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ using ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \gamma}}$ Decays
AAIJ 2015AZ
PRL 115 161802 Search for Hidden-Sector Bosons in ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit K}^{*0}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ Decays
ADLARSON 2013
PL B726 187 Search for a Dark Photon in the ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \gamma}}$ Decay
BABUSCI 2013B
PL B720 111 Limit on the Production of a Light Vector Gauge Boson in ${{\mathit \phi}}$ Meson Decays with the KLOE Detector
ARCHILLI 2012
PL B706 251 Search for a Vector Gauge Boson in ${{\mathit \phi}}$ Meson Decays with the KLOE Detector
GNINENKO 2012B
PL B713 244 Constraints on sub-GeV Hidden Sector Gauge Bosons from a Search for Heavy Neutrino Decays
GNINENKO 2012A
PR D85 055027 Stringent Limits on the ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}}$ , ${{\mathit X}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ Decay from Neutrino Experiments and Constraints on New Light Gauge Bosons
ADLER 2004
PR D70 037102 Further Search for the Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$ in the Momentum Region P $<$ 195 MeV/$\mathit c$
ANISIMOVSKY 2004
PRL 93 031801 Improved Measurement on the ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \nu}}{{\overline{\mathit \nu}}}$ Branching Ratio
Also
PR D77 052003 Measurement of the ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \nu}}{{\mathit \nu}_{{bar}}}$ Branching Ratio
ADLER 2002C
PL B537 211 Search for the Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$ in the Momentum Region P$_{{{\mathit \pi}} }$ $<$ 195-MeV/$\mathit c$
ADLER 2001
PR D63 032004 Search for the Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{0}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$
AMMAR 2001B
PRL 87 271801 Search for the Familon via ${{\mathit B}^{\pm}}$ $\rightarrow$ ${{\mathit \pi}^{\pm}}{{\mathit X}^{0}}$ , ${{\mathit B}^{\pm}}$ $\rightarrow$ ${{\mathit K}^{\pm}}{{\mathit X}^{0}}$ , and ${{\mathit B}^{0}}$ ${{\mathit X}^{0}}$ Decays
ALTEGOER 1998
PL B428 197 Search for a New Gauge Boson in ${{\mathit \pi}^{0}}$ Decays
KITCHING 1997
PRL 79 4079 Observation of the Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \gamma}}{{\mathit \gamma}}$
ADLER 1996
PRL 76 1421 Search for Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$
AMSLER 1996B
ZPHY C70 219 Search for a New Light Gauge Boson in ${{\mathit \pi}^{0}}$ , ${{\mathit \eta}}$ and ${{\mathit \eta}^{\,'}}$ Decays
AMSLER 1994B
PL B333 271 Search for a New Light Gauge Boson in Decays of ${{\mathit \pi}^{0}}$ and ${{\mathit \eta}}$
MEIJERDREES 1994
PR D49 4937 Search for the Decay ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}}$
ATIYA 1993B
PR D48 1 Search for the Decays ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$ and ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$ for 150 $<$ $\mathit M_{{{\mathit X}^{0}} }$ $<$ 250 ${\mathrm {MeV}}/\mathit c{}^{2}$
NG 1993
PR D48 2941 Cosmological Bound on the Decay ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}}$
ALLIEGRO 1992
PRL 68 278 Study of the Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
ATIYA 1992
PRL 69 733 Search for the Decay ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}}$
BARABASH 1992
PL B295 154 Restraints on Light Higgs Boson Mass from ${{\mathit \pi}}$ -, ${{\mathit K}}$ -, and ${{\mathit \eta}^{\,'}{(958)}}$ -Meson Decays in Proton Beam-Dump Experiment
Also
SJNP 55 1810 Restraints on Light Higgs Boson Mass from ${{\mathit \pi}}$ -, ${{\mathit K}}$ -, and ${{\mathit \eta}^{\,'}{(958)}}$ -Meson Decays in Proton Beam-Dump Experiment
MEIJERDREES 1992
PRL 68 3845 Search for Weakly Interacting Neutral Bosons Produced in ${{\mathit \pi}^{-}}$ ${{\mathit p}}$ Interactions at Rest and Decaying into ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ Pairs
ATIYA 1990B
PRL 65 1188 Search for the Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \gamma}}{{\mathit \gamma}}$
KORENCHENKO 1987
SJNP 46 192 On Decays ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit \nu}}$ ( ${{\mathit e}^{+}}{{\mathit e}^{-}}$), ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{-}}{{\mathit e}^{+}}{{\mathit e}^{+}}{{\mathit \nu}}$
EICHLER 1986
PL B175 101 Limits for Short Lived Neutral Particle Emitted in ${{\mathit \mu}^{+}}$ or ${{\mathit \pi}^{+}}$ Decay
YAMAZAKI 1984
PRL 52 1089 Search for a Neutral Boson in a Two Body Decay of ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit X}^{0}}$
ASANO 1982
PL 113B 195 A New Experimental Limit for the Decay ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \gamma}}{{\mathit \gamma}}$
ASANO 1981B
PL 107B 159 Search for a Rare Decay Mode ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \nu}}{{\overline{\mathit \nu}}}$ and Axion
ZHITNITSKII 1979
SJNP 29 517 Does Yang's Heavy Axion Exist?