Elliptic anisotropy measurement of the $\rm{f}_0(980)$ hadron in proton-lead collisions and evidence of its quark-antiquark composition by CMS

Despite its discovery half a century ago, the question about the quark content of the $\rm{f}_0(980)$ hadron has not been settled, whether its being an ordinary quark-antiquark meson, a tetraquark exotic state, a kaon-antikaon molecule, or a quark-antiquark-gluon hybrid. In this note, evidence that the $\rm{f}_0(980)$ hadron is an ordinary quark-antiquark meson is reported, by employing the number-of-constituent-quark (NCQ) scaling of elliptic flow anisotropies ($v_2$), empirically established by conventional hadrons up to transverse momentum ($p_{\rm {T}}$) of about 10~GeV/$c$ in relativistic heavy ion collisions. The $\rm{f}_0(980)$ is reconstructed via the invariant mass of its main decay channel ${\rm{f}_0(980)} \to \pi ^ + \pi ^ -$ in proton-lead collisions recorded by the CMS experiment at the LHC. The $\rm{f}_0(980)$ yield is measured relative to the second-order symmetry plane as reconstructed from the energy deposited in the forward/backward region of the CMS detector, and its $v_2$ parameter is extracted as a function of $p_{\rm {T}}$. It is found that the $\rm{f}_0(980)$ explanation as an ordinary quark-antiquark state is preferred over a tetraquark or ${\rm{K}\overline{K}}$ molecule hypothesis at 7.7, 6.3, or 3.1 standard deviations in the $p_{\rm {T}}<10$, 8, or $6 ~\rm{GeV/c}$ ranges, respectively. The quark-antiquark hypothesis is also preferred by 3.5 standard deviations over the $p_{\rm {T}} <8 ~\rm{GeV/c}$ range for $n_{\rm{q}}=3$, characteristic of a quark-antiquark-gluon hybrid state. The first determination of the $\rm{f}_0(980)$ state quark content with high confidence using this novel approach advances the study of quantum chromodynamics, the fundamental theory governing the physics of hadrons.