Abstract:
A search is presented for flavour-changing neutral-current interactions involving the top quark, the Higgs boson and an up-type quark ($$q=u,c$$
q
=
u
,
c
) with the ATLAS detector at the Large Hadron Collider. The analysis considers leptonic decays of the top quark along with Higgs boson decays into two $$W$$
W
bosons, two $$Z$$
Z
bosons or a $$\tau ^{+}\tau ^{-}$$
τ
+
τ
-
pair. It focuses on final states containing either two leptons (electrons or muons) of the same charge or three leptons. The considered processes are $$t\bar{t}$$
t
t
¯
and Ht production. For the $$t\bar{t}$$
t
t
¯
production, one top quark decays via $$t\rightarrow Hq$$
t
→
H
q
. The proton–proton collision data set analysed amounts to $$({140}\,{\hbox {fb}^{-1}})$$
(
140
fb
-
1
)
at $$(\sqrt{s}={13}\,\hbox {TeV})$$
(
s
=
13
TeV
)
. No significant excess beyond Standard Model expectations is observed and upper limits are set on the $$t\rightarrow Hq$$
t
→
H
q
branching ratios at 95 % confidence level, amounting to observed (expected) limits of $$\mathcal {B}(t\rightarrow Hu)<2.8\,(3.0) \times 10^{-4}$$
B
(
t
→
H
u
)
<
2.8
(
3.0
)
×
10
-
4
and $$\mathcal {B}(t\rightarrow Hc)<3.3\,(3.8) \times 10^{-4}$$
B
(
t
→
H
c
)
<
3.3
(
3.8
)
×
10
-
4
. Combining this search with other searches for $$tHq$$
tHq
flavour-changing neutral-current interactions previously conducted by ATLAS, considering $$H\rightarrow b\bar{b}$$
H
→
b
b
¯
and $$H\rightarrow \gamma \gamma $$
H
→
γ
γ
decays, as well as $$H\rightarrow \tau ^{+}\tau ^{-}$$
H
→
τ
+
τ
-
decays with one or two hadronically decaying $$\tau $$
τ
-leptons, yields observed (expected) upper limits on the branching ratios of $$\mathcal {B}(t\rightarrow Hu)<2.6\,(1.8) \times 10^{-4}$$
B
(
t
→
H
u
)
<
2.6
(
1.8
)
×
10
-
4
and $$\mathcal {B}(t\rightarrow Hc)<3.4\,(2.3) \times 10^{-4}$$
B
(
t
→
H
c
)
<
3.4
(
2.3
)
×
10
-
4
.
