The theta meson (
θ
)^[1] is a hypothetical form of quarkonium (i.e. a flavourless meson) formed by a top quark (
t
) and top antiquark (
t
). As a P-odd and C-odd
t

t
state, it is analogous to the
ϕ
(
s

s
),
J/ψ
(
c

c
) and
ϒ
(
b

b
) mesons. Due to the top quark's short lifetime, the theta meson is expected to not be observed in nature.^[2]