Elliptic anisotropy measurement of the f0(980) hadron in proton-lead collisions and evidence for its quark-antiquark composition

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 ( $${{\rm{q}}}\overline{{{\rm{q}}}}$$ q q ¯ ) meson, a tetraquark ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{q}}}\overline{{{\rm{q}}}}$$ q q ¯ q q ¯ ) exotic state, a kaon-antikaon ( $${{\rm{K}}}\overline{{{\rm{K}}}}$$ K K ¯ ) molecule, or a quark-antiquark-gluon ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{g}}}$$ q q ¯ g ) hybrid. This paper reports strong evidence that the f$_{0}$(980) state is an ordinary $${{\rm{q}}}\overline{{{\rm{q}}}}$$ q q ¯ meson, inferred from the scaling of elliptic anisotropies (v $_{2}$) with the number of constituent quarks (n $_{q}$), 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) → π $^{+}$ π $^{−}$, 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 $_{T}$). It is found that the n $_{q}$ = 2 ( $${{\rm{q}}}\overline{{{\rm{q}}}}$$ q q ¯ state) hypothesis is favored over n $_{q}$ = 4 ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{q}}}\overline{{{\rm{q}}}}$$ q q ¯ q q ¯ or $${{\rm{K}}}\overline{{{\rm{K}}}}$$ K K ¯ states) by 7.7, 6.3, or 3.1 standard deviations in the p $_{T}$ < 10, 8, or 6 GeV/c ranges, respectively, and over n $_{q}$ = 3 ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{g}}}$$ q q ¯ g hybrid state) by 3.5 standard deviations in the p $_{T}$ < 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.