Exotic

Mesons with quantum numbers $$J^{PC}\!=\!1^{-+}$$ cannot be represented as simple quark–antiquark pairs. We explore hybrid configurations in the light meson sector comprising a quark, an antiquark and an excited gluon, studying the properties of such states in a phenomenological model inspired by the gauge/gravity correspondence. The computed mass, compared to the experimental mass of the $$1^{-+}$$ candidates $$\pi _1(1400)$$, $$\pi _1(1600)$$ and $$\pi _1(2015)$$, favours $$\pi _1(1400)$$ as the lightest hybrid state. An interesting result concerns the stability of hybrid mesons at finite temperature: they disappear from the spectral function (i.e. they melt) at a lower temperature with respect to other states, light vector and scalar mesons and scalar glueballs