${{\mathit X}^{0}}$ (Heavy Boson) Searches in ${{\mathit Z}}$ Decays

INSPIRE  
Searches for radiative transition of ${{\mathit Z}}$ to a lighter spin-0 state ${{\mathit X}^{0}}$ decaying to hadrons, a lepton pair, a photon pair, or invisible particles as shown in the comments. The limits are for the product of branching ratios.
VALUE CL% DOCUMENT ID TECN  COMMENT
• • We do not use the following data for averages, fits, limits, etc. • •
1
RAINBOLT
2019
RVUE ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$
2
SIRUNYAN
2019AZ
 CMS ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
3
BARATE
1998U
 ALEP ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \ell}}{{\overline{\mathit \ell}}}$ , ${{\mathit q}}{{\overline{\mathit q}}}$ , ${{\mathit g}}{{\mathit g}}$ , ${{\mathit \gamma}}{{\mathit \gamma}}$ , ${{\mathit \nu}}{{\overline{\mathit \nu}}}$
4
ACCIARRI
1997Q
 L3 ${{\mathit X}^{0}}$ $\rightarrow$ invisible particle(s)
5
ACTON
1993E
 OPAL ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$
6
ABREU
1992D
 DLPH ${{\mathit X}^{0}}$ $\rightarrow$ hadrons
7
ADRIANI
1992F
 L3 ${{\mathit X}^{0}}$ $\rightarrow$ hadrons
8
ACTON
1991
OPAL ${{\mathit X}^{0}}$ $\rightarrow$ anything
$<1.1 \times 10^{-4}$ 95 9
ACTON
1991B
 OPAL ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$
$<9 \times 10^{-5}$ 95 9
ACTON
1991B
 OPAL ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
$<1.1 \times 10^{-4}$ 95 9
ACTON
1991B
 OPAL ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \tau}^{+}}{{\mathit \tau}^{-}}$
$<2.8 \times 10^{-4}$ 95 10
ADEVA
1991D
 L3 ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$
$<2.3 \times 10^{-4}$ 95 10
ADEVA
1991D
 L3 ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$
$<4.7 \times 10^{-4}$ 95 11
ADEVA
1991D
 L3 ${{\mathit X}^{0}}$ $\rightarrow$ hadrons
$<8 \times 10^{-4}$ 95 12
AKRAWY
1990J
 OPAL ${{\mathit X}^{0}}$ $\rightarrow$ hadrons
1  RAINBOLT 2019 limits are from B( ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \ell}^{+}}{{\mathit \ell}^{-}}{{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$ ). See their Figs. 5 and 6 for limits in mass-coupling plane.
2  SIRUNYAN 2019AZ search for ${{\mathit p}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit Z}}$ $\rightarrow$ ${{\mathit X}^{0}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ events in ${{\mathit p}}{{\mathit p}}$ collisions at $\sqrt {s }$ = 13 TeV. See their Fig. 5 for limits on ${\mathit \sigma (}$ ${{\mathit p}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit X}^{0}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{)}\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ ).
3  BARATE 1998U obtain limits on B( ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ )B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \ell}}{{\overline{\mathit \ell}}}$ , ${{\mathit q}}{{\overline{\mathit q}}}$ , ${{\mathit g}}{{\mathit g}}$ , ${{\mathit \gamma}}{{\mathit \gamma}}$ , ${{\mathit \nu}}{{\overline{\mathit \nu}}}$ ). See their Fig.$~$17.
4  See Fig.$~$4 of ACCIARRI 1997Q for the upper limit on B( ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ ; $\mathit E_{{{\mathit \gamma}}}>\mathit E_{{\mathrm {min}}}$) as a function of $\mathit E_{{\mathrm {min}}}$.
5  ACTON 1993E give $\sigma\mathrm {( {{\mathit e}^{+}} {{\mathit e}^{-}} \rightarrow {{\mathit X}^{0}} {{\mathit \gamma}} )}\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ )$<0.4~$pb (95$\%$CL) for ${\mathit m}_{{{\mathit X}^{0}}}=60$ $\pm2.5$ GeV. If the process occurs via $\mathit s$-channel ${{\mathit \gamma}}$ exchange, the limit translates to $\Gamma\mathrm {({{\mathit X}^{0}} )}\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}$ )${}^{2}<$20 MeV for ${\mathit m}_{{{\mathit X}^{0}}}$ = $60$ $\pm1$ GeV.
6  ABREU 1992D give ${{\mathit \sigma}_{{Z}}}$ $\cdot{}$ B( ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ ) $\cdot{}$ B( ${{\mathit X}^{0}}$ $\rightarrow$ hadrons) $<(3 - 10)~$pb for ${\mathit m}_{{{\mathit X}^{0}}}$ = $10 - 78$ GeV. A very similar limit is obtained for spin-1 ${{\mathit X}^{0}}$ .
7  ADRIANI 1992F search for isolated ${{\mathit \gamma}}$ in hadronic ${{\mathit Z}}$ decays. The limit ${{\mathit \sigma}_{{Z}}}$ $\cdot{}$ B( ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit X}^{0}}$ ) $\cdot{}$ B( ${{\mathit X}^{0}}$ $\rightarrow$ hadrons) $<(2 - 10)~$pb (95$\%$CL) is given for ${\mathit m}_{{{\mathit X}^{0}}}$ = $25 - 85$ GeV.
8  ACTON 1991 searches for ${{\mathit Z}}$ $\rightarrow$ ${{\mathit Z}^{*}}{{\mathit X}^{0}}$ , ${{\mathit Z}^{*}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}$ , ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ , or ${{\mathit \nu}}{{\overline{\mathit \nu}}}$ . Excludes any new scalar ${{\mathit X}^{0}}$ with ${\mathit m}_{{{\mathit X}^{0}}}<9.5$ GeV/$\mathit c$ if it has the same coupling to ${{\mathit Z}}{{\mathit Z}^{*}}$ as the MSM Higgs boson.
9  ACTON 1991B limits are for ${\mathit m}_{{{\mathit X}^{0}}}$ = $60 - 85$ GeV.
10  ADEVA 1991D limits are for ${\mathit m}_{{{\mathit X}^{0}}}$ = $30 - 89$ GeV.
11  ADEVA 1991D limits are for ${\mathit m}_{{{\mathit X}^{0}}}$ = $30 - 86$ GeV.
12  AKRAWY 1990J give $\Gamma\mathrm {( {{\mathit Z}} \rightarrow {{\mathit \gamma}} {{\mathit X}^{0}} )}\cdot{}$B( ${{\mathit X}^{0}}$ $\rightarrow$ hadrons) $<~1.9$ MeV (95$\%$CL) for ${\mathit m}_{{{\mathit X}^{0}}}$ = $32 - 80$ GeV. We divide by $\Gamma\mathrm {({{\mathit Z}} )}$ = $2.5$ GeV to get product of branching ratios. For nonresonant transitions, the limit is B( ${{\mathit Z}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit q}}{{\overline{\mathit q}}}$ ) $<~8.2$ MeV assuming three-body phase space distribution.
References:
RAINBOLT 2019
PR D99 013004 Branching fraction for $Z$ decays to four leptons and constraints on new physics
SIRUNYAN 2019AZ
PL B792 345 Search for an $L_{\mu}-L_{\tau}$ gauge boson using Z$\to4\mu$ events in proton-proton collisions at $\sqrt{s} =$ 13 TeV
BARATE 1998U
EPJ C4 571 Search for Evidence of Compositeness at LEP I
ACCIARRI 1997Q
PL B412 201 Search for New Physics in Energetic Single Photon Production in ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ Annihilation at the ${{\mathit Z}}$ Resonance
ACTON 1993E
PL B311 391 Search for Anomalous Production of High Mass Photon Pairs in ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ Collisions at LEP
ABREU 1992D
ZPHY C53 555 Study of Final State Photons in Hadronic ${{\mathit Z}^{0}}$ Decay and Limits on New Phenomena
ADRIANI 1992F
PL B292 472 Isolated Hard Photon Emission in Hadronic ${{\mathit Z}^{0}}$ Decays
ACTON 1991B
PL B273 338 A Measurement of Photon Radiation in Lepton Pair Events from ${{\mathit Z}^{0}}$ Decays
ACTON 1991
PL B268 122 Decay Mode Independent Search for a Light Higgs Boson and New Scalars
ADEVA 1991D
PL B262 155 Search for Narrow High Mass Resonances in Radiative Decays of the ${{\mathit Z}^{0}}$
AKRAWY 1990J
PL B246 285 Evidence for Final State Photons in Multihadronic Decays of the ${{\mathit Z}^{0}}$