Abstract
Searches for the exclusive decays of Higgs and
Z
bosons into a vector quarkonium state and a photon are performed in the
μ
+
μ
-
γ
final state with a proton–proton collision data sample corresponding to an integrated luminosity of 139 fb
-
1
collected at
s
=
13
TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into
J
/
ψ
γ
,
ψ
(
2
S
)
γ
, and
Υ
(
1
S
,
2
S
,
3
S
)
γ
are found to be
2.0
×
10
-
4
,
10.5
×
10
-
4
, and
(
2.5
,
4.2
,
3.4
)
×
10
-
4
, respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the
Z
boson decays are
1.2
×
10
-
6
,
2.4
×
10
-
6
, and
(
1.1
,
1.3
,
2.4
)
×
10
-
6
. An observed 95% CL interval of
(
-
133
,
175
)
is obtained for the
κ
c
/
κ
γ
ratio of Higgs boson coupling modifiers, and a 95% CL interval of
(
-
37
,
40
)
is obtained for
κ
b
/
κ
γ
.