MASS LIMITS FOR NEUTRAL HIGGS BOSONS IN EXTENDED HIGGS MODELS

This Section covers models which do not fit into either the Standard Model or its simplest minimal Supersymmetric extension (MSSM), leading to anomalous production rates, or nonstandard final states and branching ratios. In particular, this Section covers limits which may apply to generic two-Higgs-doublet models (2HDM), or to special regions of the MSSM parameter space where decays to invisible particles or to photon pairs are dominant (see the review on “Status of Higgs Boson Physics''). Concerning the mass limits for ${{\mathit H}^{0}}$ and ${{\mathit A}^{0}}$ listed below, see the footnotes or the comment lines for details on the nature of the models to which the limits apply.
The observed signal at about 125 GeV, see section “${{\mathit H}^{0}}$ '', can be interpreted as one of the neutral Higgs bosons of an extended Higgs sector.

Mass Limits for Light ${{\mathit A}^{0}}$

These limits are for a pseudoscalar ${{\mathit A}^{0}}$ in the mass range below $\cal O$(10) GeV.
VALUE (GeV) DOCUMENT ID TECN  COMMENT
• • We do not use the following data for averages, fits, limits, etc. • •
1
 2020 AE
ATLS ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit Z}}{{\mathit A}^{0}}$
2
 2018 AP
ATLS ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit A}^{0}}$
3
 2017 AZ
CMS ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit A}^{0}}$
4
 2016 E
BES3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
5
 2016 F
CMS ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit A}^{0}}$
6
 2015 H
BABR ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
7
 2013 C
BABR ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
8
 2013 L
BABR ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
9
 2013 R
BABR ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
10
 2012
BES3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
11
 2012 V
CMS ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
12
 2011 P
CDF ${{\mathit t}}$ $\rightarrow$ ${{\mathit b}}{{\mathit H}^{+}}$ , ${{\mathit H}^{+}}$ $\rightarrow$ ${{\mathit W}^{+}}{{\mathit A}^{0}}$
13, 14
 2011 A
KTEV ${{\mathit K}_{{L}}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
15
 2011 J
BABR ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
16
 2011 H
BABR ${{\mathit \Upsilon}{(2S,3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
17
 2010
RVUE
18, 14
 2010
BELL ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit K}^{*0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
19, 14
 2010
BELL ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit \rho}^{0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
20
 2009 P
BABR ${{\mathit \Upsilon}{(3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
21
 2009 Z
BABR ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
22
 2009 Z
BABR ${{\mathit \Upsilon}{(3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
23, 14
 2009
K391 ${{\mathit K}_{{L}}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$
24
 2008
CLEO ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
25
 2007
CLEO ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit \eta}_{{b}}}{{\mathit \gamma}}$
26
 2005
HYCP ${{\mathit \Sigma}^{+}}$ $\rightarrow$ ${{\mathit p}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
27
 1995
CLE2 ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
28
 1990 C
CBAL ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$
 1 AAD 2020AE search for the decay ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit Z}}{{\mathit A}^{0}}$ , ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$ , ${{\mathit A}^{0}}$ decaying hadronically ( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit g}}{{\mathit g}}$ or ${{\mathit s}}{{\overline{\mathit s}}}$ ), in 139 fb${}^{-1}$ of ${{\mathit p}}{{\mathit p}}$ collisions at $\mathit E_{{\mathrm {cm}}}$ = 13 TeV. Limit on the product of production cross section and the ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit Z}}{{\mathit A}^{0}}$ branching ratio in the range $17 - 340$ pb (95$\%$ CL) is given for ${\mathit m}_{{{\mathit A}^{0}}}$ = $0.5 - 4.0$ GeV, see their Table I.
 2 AABOUD 2018AP search for the decay ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ in 36.1 fb${}^{-1}$ of ${{\mathit p}}{{\mathit p}}$ collisions at $\mathit E_{{\mathrm {cm}}}$ = 13 TeV. See their Fig. 10(b) for limits on B( ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit A}^{0}}$ ) in the range ${\mathit m}_{{{\mathit A}^{0}}}$ = $1 - 2.5$, $4.5 - 8$ GeV, assuming a type-II two-doublet plus singlet model with tan $(\beta )$ = 5.
 3 KHACHATRYAN 2017AZ search for the decay ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}{{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ , ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{{\mathit b}}{{\overline{\mathit b}}}$ , and ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ in 19.7 fb${}^{-1}$ of ${{\mathit p}}{{\mathit p}}$ collisions at $\mathit E_{{\mathrm {cm}}}$ = 8 TeV. See their Figs. 4, 5, and 6 for cross section limits in the range ${\mathit m}_{{{\mathit A}^{0}}}$ = $5 - 62.5$ GeV. See also their Figs. 7, 8, and 9 for interpretation of the data in terms of models with two Higgs doublets and a singlet.
 4 ABLIKIM 2016E search for the process ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ with ${{\mathit A}^{0}}$ decaying to ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ and give limits on B( ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range $2.8 \times 10^{-8} - 5.0 \times 10^{-6}$ (90$\%$ CL) for 0.212 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 3.0 GeV. See their Fig. 5.
 5 KHACHATRYAN 2016F search for the decay ${{\mathit H}^{0}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}{{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ in 19.7 fb${}^{-1}$ of ${{\mathit p}}$ ${{\mathit p}}$ collisions at $\mathit E_{{\mathrm {cm}}}$ = 8 TeV. See their Fig. 8 for cross section limits for ${\mathit m}_{{{\mathit A}^{0}}}$ = $4 - 8$ GeV.
 6 LEES 2015H search for the process ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit \Upsilon}{(1S)}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ with ${{\mathit A}^{0}}$ decaying to ${{\mathit c}}{{\overline{\mathit c}}}$ and give limits on B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit c}}{{\overline{\mathit c}}}$ ) in the range $7.4 - 2.4$ (90$\%$ CL) for 4.00 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 8.95 and 9.10 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 9.25 GeV. See their Fig. 6.
 7 LEES 2013C search for the process ${{\mathit \Upsilon}}$ (2S, 3S) $\rightarrow$ ${{\mathit \Upsilon}{(1S)}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ with ${{\mathit A}^{0}}$ decaying to ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ and give limits on B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range ($0.3 - 9.7){\times }10^{-6}$ (90$\%$ CL) for 0.212 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 9.20 GeV. See their Fig. 5(e) for limits on the ${{\mathit b}}$ $−{{\mathit A}^{0}}$ Yukawa coupling derived by combining this result with AUBERT 2009Z.
 8 LEES 2013L search for the process ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit \Upsilon}{(1S)}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ with ${{\mathit A}^{0}}$ decaying to ${{\mathit g}}{{\mathit g}}$ or ${{\mathit s}}{{\overline{\mathit s}}}$ and give limits on B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit g}}{{\mathit g}}$ ) between $1 \times 10^{-6}$ and $0.02$ (90$\%$ CL) for 0.5 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 9.0 GeV, and B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit s}}{{\overline{\mathit s}}}$ ) between $4 \times 10^{-6}$ and $1 \times 10^{-3}$ (90$\%$CL) for 1.5 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 9.0 GeV. See their Fig. 4.
 9 LEES 2013R search for the process ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit \Upsilon}{(1S)}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ with ${{\mathit A}^{0}}$ decaying to ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ and give limits on B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ ) in the range $0.9 - 13 \times 10^{-5}$ (90$\%$ CL) for 3.6 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 9.2 GeV. See their Fig. 4 for limits on the ${{\mathit b}}$ $−{{\mathit A}^{0}}$ Yukawa coupling derived by combining this result with AUBERT 2009P.
 10 ABLIKIM 2012 searches for the process ${{\mathit \psi}{(3686)}}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit \pi}}{{\mathit J / \psi}}$ , ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ with ${{\mathit A}^{0}}$ decaying to ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ . It gives mass dependent limits on B( ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range $4 \times 10^{-7} - 2.1 \times 10^{-5}$ (90$\%$ C.L.) for 0.212 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 3.0 GeV. See their Fig. 2.
 11 CHATRCHYAN 2012V search for ${{\mathit A}^{0}}$ production in the decay ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ with 1.3 fb${}^{-1}$ of ${{\mathit p}}{{\mathit p}}$ collisions at $\mathit E_{{\mathrm {cm}}}$ = 7 TeV. A limit on $\sigma ({{\mathit A}^{0}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range ($1.5 - 7.5$) pb is given for ${\mathit m}_{{{\mathit A}^{0}}}$ = ($5.5 - 8.7$) and ($11.5 - 14$) GeV at 95$\%$ CL.
 12 AALTONEN 2011P search in 2.7 fb${}^{-1}$ of ${{\mathit p}}{{\overline{\mathit p}}}$ collisions at $\mathit E_{{\mathrm {cm}}}$ = 1.96 TeV for the decay chain ${{\mathit t}}$ $\rightarrow$ ${{\mathit b}}{{\mathit H}^{+}}$ , ${{\mathit H}^{+}}$ $\rightarrow$ ${{\mathit W}^{+}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ with ${\mathit m}_{{{\mathit A}^{0}}}$ between 4 and 9 GeV. See their Fig.$~$4 for limits on B( ${{\mathit t}}$ $\rightarrow$ ${{\mathit b}}{{\mathit H}^{+}}$ ) for 90 $<$ ${\mathit m}_{{{\mathit H}^{+}}}<$ 160 GeV.
 13 ABOUZAID 2011A search for the decay chain ${{\mathit K}_{{L}}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ and give a limit B( ${{\mathit K}_{{L}}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit A}^{0}}$ ) $\cdot{}$ B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) $<$ $1.0 \times 10^{-10}$ at 90$\%$ CL for ${\mathit m}_{{{\mathit A}^{0}}}$ = 214.3 MeV.
 14 The search was motivated by PARK 2005 .
 15 DEL-AMO-SANCHEZ 2011J search for the process ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit \Upsilon}{(1S)}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ with ${{\mathit A}^{0}}$ decaying to invisible final states. They give limits on B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ invisible) in the range ($1.9 - 4.5){\times }10^{-6}$ (90$\%$ CL) for 0 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 8.0 GeV, and ($2.7 - 37){\times }10^{-6}$ for 8.0 ${}\leq{}{\mathit m}_{{{\mathit A}^{0}}}{}\leq{}$ 9.2 GeV.
 16 LEES 2011H search for the process ${{\mathit \Upsilon}{(2S,3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ with ${{\mathit A}^{0}}$ decaying hadronically and give limits on B( ${{\mathit \Upsilon}{(2S,3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ hadrons) in the range $1 \times 10^{-6} - 8 \times 10^{-5}$ (90$\%$ CL) for 0.3 $<$ ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 7 GeV. The decay rates for ${{\mathit \Upsilon}{(2S)}}$ and ${{\mathit \Upsilon}{(3S)}}$ are assumed to be equal up to the phase space factor. See their Fig. 5.
 17 ANDREAS 2010 analyze constraints from rare decays and other processes on a light ${{\mathit A}^{0}}$ with ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 2${\mathit m}_{{{\mathit \mu}}}$ and give limits on its coupling to fermions at the level of $10^{-4}$ times the Standard Model value.
 18 HYUN 2010 search for the decay chain ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit K}^{*0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ and give a limit on B( ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit K}^{*0}}{{\mathit A}^{0}}$ ) $\cdot{}$ B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range ($2.26 - 5.53){\times }10^{-8}$ at 90$\%$CL for ${\mathit m}_{{{\mathit A}^{0}}}$ = $212 - 300$ MeV. The limit for ${\mathit m}_{{{\mathit A}^{0}}}$ = 214.3 MeV is $2.26 \times 10^{-8}$.
 19 HYUN 2010 search for the decay chain ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit \rho}^{0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ and give a limit on B( ${{\mathit B}^{0}}$ $\rightarrow$ ${{\mathit \rho}^{0}}{{\mathit A}^{0}}$ ) $\cdot{}$ B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range ($1.73 - 4.51){\times }10^{-8}$ at 90$\%$CL for ${\mathit m}_{{{\mathit A}^{0}}}$ = $212 - 300$ MeV. The limit for ${\mathit m}_{{{\mathit A}^{0}}}$ = 214.3 MeV is $1.73 \times 10^{-8}$.
 20 AUBERT 2009P search for the process ${{\mathit \Upsilon}{(3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ with ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ for 4.03 $<$ ${\mathit m}_{{{\mathit A}^{0}}}<$ 9.52 and 9.61 $<$ ${\mathit m}_{{{\mathit A}^{0}}}<$ 10.10 GeV, and give limits on B( ${{\mathit \Upsilon}{(3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ ) in the range ($1.5 - 16){\times }10^{-5}$ (90$\%$ CL).
 21 AUBERT 2009Z search for the process ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ with ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ for 0.212 $<$ ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 9.3 GeV and give limits on B( ${{\mathit \Upsilon}{(2S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range ($0.3 - 8){\times }10^{-6}$ (90$\%$ CL).
 22 AUBERT 2009Z search for the process ${{\mathit \Upsilon}{(3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ with ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ for 0.212 $<$ ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 9.3 GeV and give limits on B( ${{\mathit \Upsilon}{(3S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) in the range ($0.3 - 5){\times }10^{-6}$ (90$\%$ CL).
 23 TUNG 2009 search for the decay chain ${{\mathit K}_{{L}}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit A}^{0}}$ , ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ and give a limit on B( ${{\mathit K}_{{L}}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit A}^{0}}$ ) $\cdot{}$ B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ ) in the range ($2.4 - 10.7){\times }10^{-7}$ at 90$\%$CL for ${\mathit m}_{{{\mathit A}^{0}}}$ = $194.3 - 219.3$ MeV. The limit for ${\mathit m}_{{{\mathit A}^{0}}}$ = 214.3 MeV is $2.4 \times 10^{-7}$.
 24 LOVE 2008 search for the process ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ with ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ (for ${\mathit m}_{{{\mathit A}^{0}}}$ $<$ 2${\mathit m}_{{{\mathit \tau}}}$) and ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ . Limits on B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ ) $\cdot{}$ B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$ ) in the range $10^{-6} - 10^{-4}$ (90$\%$ CL) are given.
 25 BESSON 2007 give a limit B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit \eta}_{{b}}}{{\mathit \gamma}}$ ) $\cdot{}$ B( ${{\mathit \eta}_{{b}}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$ ) $<$ 0.27$\%$ (95$\%$ CL), which constrains a possible ${{\mathit A}^{0}}$ exchange contribution to the ${{\mathit \eta}_{{b}}}$ decay.
 26 PARK 2005 found three candidate events for ${{\mathit \Sigma}^{+}}$ $\rightarrow$ ${{\mathit p}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ in the HyperCP experiment. Due to a narrow spread in dimuon mass, they hypothesize the events as a possible signal of a new boson. It can be interpreted as a neutral particle with ${\mathit m}_{{{\mathit A}^{0}}}$ = $214.3$ $\pm0.5~$MeV and the branching fraction B( ${{\mathit \Sigma}^{+}}$ $\rightarrow$ ${{\mathit p}}{{\mathit A}^{0}}$ )$\cdot{}$B( ${{\mathit A}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ) = ($3.1$ ${}^{+2.4}_{-1.9}$ $\pm1.5){\times }10^{-8}$.
 27 BALEST 1995 give limits B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ ) < $1.5 \times 10^{-5}$ at 90$\%$ CL for ${\mathit m}_{{{\mathit A}^{0}}}<$ 5 GeV. The limit becomes $<10^{-4}$ for ${\mathit m}_{{{\mathit A}^{0}}}$ $<7.7$ GeV.
 28 ANTREASYAN 1990C give limits B( ${{\mathit \Upsilon}{(1S)}}$ $\rightarrow$ ${{\mathit A}^{0}}{{\mathit \gamma}}$ ) < $5.6 \times 10^{-5}$ at 90$\%$ CL for ${\mathit m}_{{{\mathit A}^{0}}}<$ 7.2 GeV. ${{\mathit A}^{0}}$ is assumed not to decay in the detector.
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