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

Abstract

Despite the f${}_0$(980) hadron having been discovered half a century ago, the question about its quark content has not been settled: it might be an ordinary quark-antiquark ($\mathrm{q}\bar{\mathrm{q}}$) meson, a tetraquark ($\mathrm{q}\bar{\mathrm{q}}\mathrm{q}\bar{\mathrm{q}}$) exotic state, a kaon-antikaon ($\mathrm{K}\bar{\mathrm{K}}$) molecule, or a quark-antiquark-gluon ($\mathrm{q}\bar{\mathrm{q}}\mathrm{g}$) hybrid. This paper reports strong evidence that the f${}_0$(980) state is an ordinary $\mathrm{q}\bar{\mathrm{q}}$ meson, inferred from the scaling of elliptic anisotropies ($v_2$) with the number of constituent quarks ($n_{\mathrm{q}}$), as empirically established using conventional hadrons in relativistic heavy ion collisions. The f${}_0$(980) state is reconstructed via its dominant decay channel f${}_0$(980) $\to$ ${\pi }^{+}{\pi }^{-}$, in proton-lead collisions recorded by the CMS experiment at the LHC, and its $v_2$ is measured as a function of transverse momentum ($p_{\mathrm{T}}$). It is found that the $n_q$ = 2 ($\mathrm{q}\bar{\mathrm{q}}$ state) hypothesis is favored over $n_q$ = 4 ($\mathrm{q}\bar{\mathrm{q}}\mathrm{q}\bar{\mathrm{q}}$ or $\mathrm{K}\bar{\mathrm{K}}$ states) by 7.7, 6.3, or 3.1 standard deviations in the $p_{\mathrm{T}}$ $<$ 10, 8, or 6 GeV/$c$ ranges, respectively, and over $n_{\mathrm{q}}$ = 3 ($\mathrm{q}\bar{\mathrm{q}}\mathrm{g}$ hybrid state) by 3.5 standard deviations in the $p_{\mathrm{T}}$ $<$ 8 GeV/$c$ range. This result represents the first determination of the quark content of the f${}_0$(980) state, made possible by using a novel approach, and paves the way for similar studies of other exotic hadron candidates.

Replaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-20-002 (CMS Public Pages)