# ${{\boldsymbol \eta}_{{c}}{(1S)}}$ MASS INSPIRE search

VALUE (MeV) EVTS DOCUMENT ID TECN  COMMENT
$\bf{ 2983.9 \pm0.5}$ OUR AVERAGE  Error includes scale factor of 1.3.
$2984.6$ $\pm0.7$ $\pm2.2$ 2673
 2018
BELL ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \eta}^{\,'}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$
$2986.7$ $\pm0.5$ $\pm0.9$ 11K 1
LHCB ${{\mathit p}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit B}^{+}}{{\mathit X}}$ $\rightarrow$ ${{\mathit p}}{{\overline{\mathit p}}}{{\mathit K}^{+}}{{\mathit X}}$
$2982.8$ $\pm1.0$ $\pm0.5$ 6.4k 2
 2017 BB
LHCB ${{\mathit p}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit b}}{{\overline{\mathit b}}}{{\mathit X}}$ $\rightarrow$ 2( ${{\mathit K}^{+}}{{\mathit K}^{-}}$) ${{\mathit X}}$
$2982.2$ $\pm1.5$ $\pm0.1$ 2.0k 3
 2015 BI
LHCB ${{\mathit p}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}$(1 ${{\mathit S}}$) ${{\mathit X}}$
$2983.5$ $\pm1.4$ ${}^{+1.6}_{-3.6}$ 4
 2014
KEDR ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$
$2979.8$ $\pm0.8$ $\pm3.5$ 4.5k 5, 6
 2014 E
BABR ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit \pi}^{0}}$
$2984.1$ $\pm1.1$ $\pm2.1$ 900 5, 7, 6
 2014 E
BABR ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit \eta}}$
$2984.3$ $\pm0.6$ $\pm0.6$ 8, 9
 2012 F
BES3 ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$
$2984.49$ $\pm1.16$ $\pm0.52$ 832 5
 2012 N
BES3 ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \gamma}}$ hadrons
$2982.7$ $\pm1.8$ $\pm2.2$ 486
 2012 A
BELL ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \eta}^{\,'}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$
$2984.5$ $\pm0.8$ $\pm3.1$ 11k
 2011 M
BABR ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \pi}^{0}}$
$2985.4$ $\pm1.5$ ${}^{+0.5}_{-2.0}$ 920 9
 2011
BELL ${{\mathit B}^{\pm}}$ $\rightarrow$ ${{\mathit K}^{\pm}}$ ( ${{\mathit K}_S^0}$ ${{\mathit K}^{\pm}}{{\mathit \pi}^{\mp}}$ )
$2982.2$ $\pm0.4$ $\pm1.6$ 14k 10
 2010
BABR 10.6 ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit K}_S^0}$ ${{\mathit K}^{\pm}}{{\mathit \pi}^{\mp}}$
$2985.8$ $\pm1.5$ $\pm3.1$ 0.9k
 2008 AB
BABR ${{\mathit B}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}{(1S)}}{{\mathit K}^{(*)}}$ $\rightarrow$ ${{\mathit K}}{{\overline{\mathit K}}}{{\mathit \pi}}{{\mathit K}^{(*)}}$
$2986.1$ $\pm1.0$ $\pm2.5$ 7.5k
 2008
BELL ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}$ $\rightarrow$ hadrons
$2970$ $\pm5$ $\pm6$ 501 11
 2007
BELL ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit J / \psi}}$( ${{\mathit c}}{{\overline{\mathit c}}}$)
$2971$ $\pm3$ ${}^{+2}_{-1}$ 195
 2006
BELL ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit p}}{{\overline{\mathit p}}}{{\mathit K}^{+}}$
$2974$ $\pm7$ ${}^{+2}_{-1}$ 20
 2006
BELL ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit \Lambda}}{{\overline{\mathit \Lambda}}}{{\mathit K}^{+}}$
$2981.8$ $\pm1.3$ $\pm1.5$ 592
 2004
CLEO ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ $\rightarrow$ ${{\mathit K}_S^0}$ ${{\mathit K}^{\pm}}{{\mathit \pi}^{\mp}}$
$2984.1$ $\pm2.1$ $\pm1.0$ 190 12
 2003
E835 ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$
• • • We do not use the following data for averages, fits, limits, etc. • • •
$2982.5$ $\pm0.4$ $\pm1.4$ 12k 13
 2011 M
BABR ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit K}_S^0}$ ${{\mathit K}^{\pm}}{{\mathit \pi}^{\mp}}$
$2982.2$ $\pm0.6$ 14
 2009
CLEO ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}}$
$2982$ $\pm5$ 270 15
 2006 E
BABR ${{\mathit B}^{\pm}}$ $\rightarrow$ ${{\mathit K}^{\pm}}{{\mathit X}}$ $_{ {{\mathit c}} {{\overline{\mathit c}}} }$
$2982.5$ $\pm1.1$ $\pm0.9$ 2.5k 16
 2004 D
BABR ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}{(1S)}}$ $\rightarrow$ ${{\mathit K}}{{\overline{\mathit K}}}{{\mathit \pi}}$
$2977.5$ $\pm1.0$ $\pm1.2$ 17, 14
 2003
BES ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$
$2979.6$ $\pm2.3$ $\pm1.6$ 180 18
 2003
BELL ${{\mathit B}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}{{\mathit K}}$
$2976.3$ $\pm2.3$ $\pm1.2$ 19, 14
 2000 F
BES ${{\mathit J / \psi}}$ ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$
$2976.6$ $\pm2.9$ $\pm1.3$ 140 20, 14
 2000 F
BES ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$
$2980.4$ $\pm2.3$ $\pm0.6$ 21
 2000 B
CLE2 ${{\mathit \gamma}}$ ${{\mathit \gamma}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}$ $\rightarrow$ ${{\mathit K}^{\pm}}{{\mathit K}_S^0}$ ${{\mathit \pi}^{\mp}}$
$2975.8$ $\pm3.9$ $\pm1.2$ 20
 1999 B
BES Sup. by BAI 2000F
$2999$ $\pm8$ 25
 1998 O
DLPH ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ +hadrons
$2988.3$ ${}^{+3.3}_{-3.1}$
 1995 F
E760 ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$
$2974.4$ $\pm1.9$ 22, 14
 1991
DM2 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}{{\mathit \gamma}}$
$2969$ $\pm4$ $\pm4$ 80 14
 1990 B
MRK3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit K}^{+}}{{\mathit K}^{-}}$
$2956$ $\pm12$ $\pm12$ 14
 1990 B
MRK3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit K}_S^0}$ ${{\mathit K}_L^0}$
$2982.6$ ${}^{+2.7}_{-2.3}$ 12
 1987 B
SPEC ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$
$2980.2$ $\pm1.6$ 22, 14
 1986
MRK3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}{{\mathit \gamma}}$
$2984$ $\pm2.3$ $\pm4.0$ 14
 1986
CBAL ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}$ X, ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}$ X
$2976$ $\pm8$ 23, 14
 1984
MRK3 ${{\mathit J / \psi}}$ $\rightarrow$ 2 ${{\mathit \phi}}{{\mathit \gamma}}$
$2982$ $\pm8$ 18 24
 1980 B
MRK2 ${{\mathit e}^{+}}{{\mathit e}^{-}}$
$2980$ $\pm9$ 24
 1980 B
CBAL ${{\mathit e}^{+}}{{\mathit e}^{-}}$
1  AAIJ 2017AD report ${\mathit m}_{{{\mathit J / \psi}}}–{\mathit m}_{{{\mathit \eta}_{{c}}{(1S)}}}$ = $110.2$ $\pm0.5$ $\pm0.9$ MeV. We use the current value ${\mathit m}_{{{\mathit J / \psi}}}$ = $3096.900$ $\pm0.006$ MeV to obtain the quoted mass.
2  From a fit of the ${{\mathit \phi}}{{\mathit \phi}}$ invariant mass with the mass and width of ${{\mathit \eta}_{{c}}{(1S)}}$ as free parameters.
3  AAIJ 2015BI reports ${\mathit m}_{{{\mathit J / \psi}}}$ $−$ ${\mathit m}_{{{\mathit \eta}_{{c}}{(1S)}}}$ = $114.7$ $\pm1.5$ $\pm0.1$ MeV from a sample of ${{\mathit \eta}_{{c}}{(1S)}}$ and ${{\mathit J / \psi}}$ produced in ${{\mathit b}}$ -hadron decays. We have used current value of ${\mathit m}_{{{\mathit J / \psi}}}$ = $3096.900$ $\pm0.006$ MeV to arrive at the quoted ${\mathit m}_{{{\mathit \eta}_{{c}}{(1S)}}}$ result.
4  Taking into account an asymmetric photon lineshape.
5  With floating width.
6  Ignoring possible interference with the non-resonant 0${}^{-}$ amplitude.
7  Using both, ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ and ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \pi}^{0}}$ decays.
8  From a simultaneous fit to six decay modes of the ${{\mathit \eta}_{{c}}}$.
9  Accounts for interference with non-resonant continuum.
10  Taking into account interference with the non-resonant $\mathit J{}^{P} = 0-$ amplitude.
11  From a fit of the ${{\mathit J / \psi}}$ recoil mass spectrum. Supersedes ABE,K 2002 and ABE 2004G.
12  Using mass of ${{\mathit \psi}{(2S)}}$ = $3686.00$ MeV.
13  Not independent from the measurements reported by LEES 2010 .
14  MITCHELL 2009 observes a significant asymmetry in the lineshapes of ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$ and ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$ transitions. If ignored, this asymmetry could lead to significant bias whenever the mass and width are measured in ${{\mathit \psi}{(2S)}}$ or ${{\mathit J / \psi}}$ radiative decays.
15  From the fit of the kaon momentum spectrum. Systematic errors not evaluated.
16  Superseded by LEES 2010 .
17  From a simultaneous fit of five decay modes of the ${{\mathit \eta}_{{c}}}$.
18  Superseded by VINOKUROVA 2011 .
19  Weighted average of the ${{\mathit \psi}{(2S)}}$ and ${{\mathit J / \psi}{(1S)}}$ samples. Using an ${{\mathit \eta}_{{c}}}$ width of $13.2$ MeV.
20  Average of several decay modes. Using an ${{\mathit \eta}_{{c}}}$ width of $13.2$ MeV.
21  Superseded by ASNER 2004 .
22  Average of several decay modes.
23  ${{\mathit \eta}_{{c}}}$ $\rightarrow$ ${{\mathit \phi}}{{\mathit \phi}}$ .
24  Mass adjusted by us to correspond to ${{\mathit J / \psi}{(1S)}}$ mass = 3097 MeV.

${{\mathit \eta}_{{c}}{(1S)}}$ mass (MeV)
References:
 XU 2018
PR D98 072001 Measurement of eta_c(1S), eta_c(2S) and non-resonant eta' pi+ pi- production via two-photon collisions
 AAIJ 2017BB
EPJ C77 609 Study of Charmonium Production in hadron Decays and First Evidence for the Decay ${{\mathit B}_{{s}}^{0}}$ $\rightarrow$ ${{\mathit \phi}}{{\mathit \phi}}{{\mathit \phi}}$
PL B769 305 Observation of ${{\mathit \eta}_{{c}}{(2S)}}$ $\rightarrow$ ${{\mathit p}}{{\overline{\mathit p}}}$ and Search for ${{\mathit X}{(3872)}}$ $\rightarrow$ ${{\mathit p}}{{\overline{\mathit p}}}$ Decays
 AAIJ 2015BI
EPJ C75 311 Measurement of the ${{\mathit \eta}_{{c}}{(1S)}}$ Production Cross-Section in Proton-Proton Collisions Via the Decay ${{\mathit \eta}_{{c}}{(1S)}}$ $\rightarrow$ ${{\mathit p}}{{\overline{\mathit p}}}$
 ANASHIN 2014
PL B738 391 Measurement of ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$ Decay Rate and ${{\mathit \eta}_{{c}}}$ Parameters at KEDR
 LEES 2014E
PR D89 112004 Dalitz Plot Analysis of ${{\mathit \eta}_{{c}}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit \eta}}$ and ${{\mathit \eta}_{{c}}}$ $\rightarrow$ ${{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit \pi}^{0}}$ in Two-Photon Interactions
 ABLIKIM 2012N
PR D86 092009 Study of ${{\mathit \psi}{(3686)}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit h}_{{c}}}$ , ${{\mathit h}_{{c}}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$ via ${{\mathit \eta}_{{c}}}$ Exclusive Decays
 ABLIKIM 2012F
PRL 108 222002 Measurements of the Mass and Width of the ${{\mathit \eta}_{{c}}}$ Using the Decay ${{\mathit \psi}{(3686)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}_{{c}}}$
 ZHANG 2012A
PR D86 052002 First Study of ${{\mathit \eta}_{{c}}{(1S)}}$, ${{\mathit \eta}{(1760)}}$ and ${{\mathit X}{(1835)}}$ Production via ${{\mathit \eta}^{\,'}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}$ Final States in Two-Photon Collisions
 DEL-AMO-SANCHEZ 2011M
PR D84 012004 Observation of ${{\mathit \eta}_{{c}}{(1S)}}$ and ${{\mathit \eta}_{{c}}{(2S)}}$ decays to ${{\mathit K}^{+}}{{\mathit K}^{-}}{{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \pi}^{0}}$ in Two-Photon Interactions
 VINOKUROVA 2011
PL B706 139 Study of ${{\mathit B}^{\pm}}$ $\rightarrow$ ${{\mathit K}^{\pm}}$ ($\mathit KSK{{\mathit \pi}}$) Decay and Determination of ${{\mathit \eta}_{{c}}}$ and ${{\mathit \eta}_{{c}}{(2S)}}$ Parameters
 LEES 2010
PR D81 052010 Measurement of the ${{\mathit \gamma}}$ ${{\mathit \gamma}^{*}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}$ Transition Form Factor
 MITCHELL 2009
PRL 102 011801 ${{\mathit J / \psi}}$ and ${{\mathit \psi}{(2S)}}$ Radiative Decays to ${{\mathit \eta}_{{c}}}$
 AUBERT 2008AB
PR D78 012006 Study of ${{\mathit B}}$-Meson Decays to ${{\mathit \eta}_{{c}}}{{\mathit K}^{(*)}}$ , ${{\mathit \eta}_{{c}}{(2S)}}{{\mathit K}^{(*)}}$ , and ${{\mathit \eta}_{{c}}}{{\mathit \gamma}}{{\mathit K}^{(*)}}$
 UEHARA 2008
EPJ C53 1 Study of Charmonia in Four-Meson Final States Produced in Two-Photon Collisions
 ABE 2007
PRL 98 082001 Observation of a Charmoniumlike State Produced in Association with ${{\mathit J / \psi}}$ in ${{\mathit e}^{+}}{{\mathit e}^{-}}$ Annihilation at $\sqrt {s }$ $\sim{}$ 10.6 GeV
 AUBERT 2006E
PRL 96 052002 Measurements of the Absolute Branching Fractions of ${{\mathit B}^{\pm}}$ $\rightarrow$ ${{\mathit K}^{\pm}}{{\mathit X}}$ $_{ {{\mathit c}} {{\overline{\mathit c}}} }$
 WU 2006
PRL 97 162003 Study of ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit p}}{{\overline{\mathit p}}}$ , ${{\mathit \Lambda}}{{\overline{\mathit \Lambda}}}$ and Observation of ${{\mathit \eta}_{{c}}}$ $\rightarrow$ ${{\mathit \Lambda}}{{\overline{\mathit \Lambda}}}$ at Belle
 ASNER 2004
PRL 92 142001 Observation of ${{\mathit \eta}_{{c}}^{\,'}}$ Production in ${{\mathit \gamma}}{{\mathit \gamma}}$ Fusion at CLEO
 AUBERT 2004D
PRL 92 142002 Measurements of the Mass and Width of the ${{\mathit \eta}_{{c}}}$ Meson and of an ${{\mathit \eta}_{{c}}{(2S)}}$ Candidate
 AMBROGIANI 2003
PL B566 45 Measurement of the Resonance Parameters of the Charmonium Ground State, ${{\mathit \eta}_{{c}}}(1_{1}S_{0}$)
 BAI 2003
PL B555 174 Measurement of the Mass and Full Width of the ${{\mathit \eta}_{{c}}}$ Meson
 FANG 2003
PRL 90 071801 Measurement of Branching Fractions for ${{\mathit B}}$ $\rightarrow$ ${{\mathit \eta}_{{c}}}{{\mathit K}}{}^{(*)}$ Decays
 BAI 2000F
PR D62 072001 A Measurement of the Mass and Full Width of the ${{\mathit \eta}_{{c}}}$ Meson
 BRANDENBURG 2000B
PRL 85 3095 Measurements of the Mass, Total Width and Two Photon Partial Width of the ${{\mathit \eta}_{{c}}}$ Meson
 BAI 1999B
PR D60 072001 Study of the Hadronic Decays of ${{\mathit \chi}_{{c}}}$ States
 ABREU 1998O
PL B441 479 A Search for ${{\mathit \eta}_{{c}}^{\,'}}$ Production in Photon-Photon Fusion at LEP
 ARMSTRONG 1995F
PR D52 4839 Study of the ${{\mathit \eta}_{{c}}}(1{}^{1}{{\mathit S}_{{0}}}$) State of Charmonium Formed in ${{\overline{\mathit p}}}{{\mathit p}}$ Annihilations and a Search for the ${{\mathit \eta}_{{c}}}(2{}^{1}{{\mathit S}_{{0}}}$)
 BISELLO 1991
NP B350 1 Study of the ${{\mathit \eta}_{{c}}}$ Decays
 BAI 1990B
PRL 65 1309 Observation of a Pseudoscalar State in ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \phi}}{{\mathit \phi}}$ near ${{\mathit \phi}}{{\mathit \phi}}$ Threshold
 BAGLIN 1987B
PL B187 191 Direct Observation and Partial Width Measurement of ${{\mathit \gamma}}{{\mathit \gamma}}$ Decay of Charmonium States
 BALTRUSAITIS 1986
PR D33 629 Hadronic Decay of the ${{\mathit \eta}_{{c}}{(2980)}}$
 GAISER 1986
PR D34 711 Charmonium Spectroscopy from Inclusive ${{\mathit \psi}^{\,'}}$ and ${{\mathit J / \psi}}$ Radiative Decays
 BALTRUSAITIS 1984
PRL 52 2126 Observation of the Decay ${{\mathit \eta}_{{c}}}$ $\rightarrow$ ${{\mathit \phi}}{{\mathit \phi}}$ and Determination of the ${{\mathit \eta}_{{c}}}$ Spin and Parity
 HIMEL 1980B
PRL 45 1146 Observation of the ${{\mathit \eta}_{{c}}{(2980)}}$ Produced in the Radiative Decay of the ${{\mathit \psi}^{\,'}{(3684)}}$
 PARTRIDGE 1980B
PRL 45 1150 Observation of an ${{\mathit \eta}_{{c}}}$ Candidate State with Mass $2978$ $\pm9$ MeV