${{\mathit A}^{0}}$ (Axion) Limits from Its Electron Coupling

Limits are for $\tau\mathrm {( {{\mathit A}^{0}} \rightarrow {{\mathit e}^{+}} {{\mathit e}^{-}} )}$.
VALUE (s) CL% DOCUMENT ID TECN  COMMENT
• • We do not use the following data for averages, fits, limits, etc. • •
1
 2021
NA64 ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ invisibles)
2
 2021 B
NA64 ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
$\text{none } 4 \times 10^{-16} - 4.5 \times 10^{-12}$ 90 3
 1991
BDMP ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
4
 1990
BDMP ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
5
 1988
CALO ${{\mathit A}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ or 2 ${{\mathit \gamma}}$
6
 1988
MD1 ${{\mathit e}}$ ${{\mathit e}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}{{\mathit A}^{0}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
$\text{none } 1 \times 10^{-14} - 1 \times 10^{-10}$ 90 7
 1987
BDMP ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
$\text{none } 1 \times 10^{-14} - 1 \times 10^{-11}$ 90 8
 1986
BDMP ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
$\text{none } 6 \times 10^{-14} - 9 \times 10^{-11}$ 95 9
 1986
BDMP ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
$\text{none } 3 \times 10^{-13} - 1 \times 10^{-7}$ 90 10
 1986
BDMP ${{\mathit e}}$ ${{\mathit N}}$ $\rightarrow$ ${{\mathit e}}{{\mathit A}^{0}}{{\mathit N}}$ ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}}{{\mathit e}}$ )
 1 ANDREEV 2021 look for invisible decays of axions coupled to electrons, and set limits on $\mathit g_{ {{\mathit A}} {{\mathit e}} {{\mathit e}} }$ $<$ $4.6 \times 10^{-6} - 3.1 \times 10^{-3}$ for ${\mathit m}_{{{\mathit A}^{0}}}$ = $10^{-3} - 1$ GeV. This limits the axion contribution to the electron $\mathit g−$2 to an order of magnitude less than the current experimental uncertainty. See their Figs. 3 and 4 for mass-dependent limits.
 2 ANDREEV 2021B set limits on $\mathit g_{ {{\mathit A}} {{\mathit e}} {{\mathit e}} }$ in the range of $6.3 \times 10^{-6} - 1.6 \times 10^{-3}$ for ${\mathit m}_{{{\mathit A}^{0}}}$ = $2 - 17$ MeV at 90$\%$ CL. This excludes $6.6 \times 10^{-5}$ $<$ $\mathit g_{ {{\mathit A}} {{\mathit e}} {{\mathit e}} }$ $<$ $1 \times 10^{-4}$ at ${\mathit m}_{{{\mathit A}^{0}}}$ = 16.7 MeV corresponding to the ATOMKI anomaly. See their Fig. 2 for mass-dependent limits.
 3 The listed BROSS 1991 limit is for ${\mathit m}_{{{\mathit A}^{0}}}$ = $1.14~$MeV. B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ) =$~$1 assumed. Excluded domain in the ${\mathit \tau}_{{{\mathit A}^{0}}}-{\mathit m}_{{{\mathit A}^{0}}}$ plane extends up to ${\mathit m}_{{{\mathit A}^{0}}}$ $\approx{}~$7 MeV (see Fig.$~$5). Combining with electron $\mathit g~-~2$ constraint, axions coupling only to ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ruled out for ${\mathit m}_{{{\mathit A}^{0}}}$ $<~4.8$ MeV (90$\%$ CL).
 4 GUO 1990 use the same apparatus as BROWN 1986 and improve the previous limit in the shorter lifetime region. Combined with $\mathit g~-~$2 constraint, axions coupling only to ${{\mathit e}^{+}}{{\mathit e}^{-}}$ are ruled out for ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ $2.7$ MeV (90$\%$ CL).
 5 BJORKEN 1988 reports limits on axion parameters (${{\mathit f}_{{A}}}$ , ${{\mathit m}_{{A}}}$ , ${{\mathit \tau}_{{A}}}$ ) for ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 200 MeV from electron beam-dump experiment with production via Primakoff photoproduction, bremsstrahlung from electrons, and resonant annihilation of positrons on atomic electrons.
 6 BLINOV 1988 assume zero spin, $\mathit m$ = $1.8$ MeV and lifetime $<$ $5 \times 10^{-12}~$s and find $\Gamma\mathrm {( {{\mathit A}^{0}} \rightarrow {{\mathit \gamma}} {{\mathit \gamma}} )}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ ) $<$ 2 eV (CL=90$\%$).
 7 Assumes ${{\mathit A}^{0}}{{\mathit \gamma}}{{\mathit \gamma}}$ coupling is small and hence Primakoff production is small. Their figure 2 shows limits on axions for ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 15 MeV.
 8 Uses electrons in hadronic showers from an incident 800 GeV proton beam. Limits for ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 15 MeV are shown in their figure 3.
 9 ${\mathit m}_{{{\mathit A}^{0}}}$ = 1.8 MeV assumed. The excluded domain in the ${\mathit \tau}_{{{\mathit A}^{0}}}−{\mathit m}_{{{\mathit A}^{0}}}$ plane extends up to ${\mathit m}_{{{\mathit A}^{0}}}$ $\approx{}$ 14 MeV, see their figure 4.
 10 The limits are obtained from their figure 3. Also given is the limit on the ${{\mathit A}^{0}}{{\mathit \gamma}}{{\mathit \gamma}}$ $−$ ${{\mathit A}^{0}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ coupling plane by assuming Primakoff production.
References:
 ANDREEV 2021
PRL 126 211802 Constraints on New Physics in Electron $g-2$ from a Search for Invisible Decays of a Scalar, Pseudoscalar, Vector, and Axial Vector
 ANDREEV 2021B
PR D104 L111102
 BROSS 1991
PRL 67 2942 A Search for Short Lived Particles Produced in an Electron Beam Dump
 GUO 1990
PR D41 2924 Improved Limit on Axion Production in 800 GeV Hadronic Showers
 BJORKEN 1988
PR D38 3375 Search for Neutral Metastable Penetrating Particles Produced in the SLAC Beam Dump
 BLINOV 1988
SJNP 47 563 Limit on the Two Photon Width of the Particles with the Mass of 1.8 ${\mathrm {MeV}}/\mathit c{}^{2}$
 RIORDAN 1987
PRL 59 755 Search for Short Lived Axions in an Electron Beam Dump Experiment
 BROWN 1986
PRL 57 2101 A New Limit on Axion Production in 800 GeV Hadronic Showers
 DAVIER 1986
PL B180 295 Search for Axionlike Particles in Electron Bremsstrahlung
 KONAKA 1986
PRL 57 659 Search for Neutral Particles in Electron Beam Dump Experiment