CMS-PAS-HIN-20-002

CMS-PAS-HIN-20-002
Elliptic anisotropy measurement of the f $_0$ (980) hadron in proton-lead collisions and evidence of its quark-antiquark composition by CMS
CMS Collaboration
2 September 2023

Abstract: 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 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.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to NP. The superseded preliminary plots can be found here.

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