Measurement of f0(980) Elliptic Flow in Proton-Lead Collisions at the Large Hadron Collider and Its Possible Quark Content

Exotic hadrons are hadrons with constituent quark content different from the normal meson and baryon. The f₀(980) hadron is one of the exotic hadron candidates, which was first discovered in $\pi\pi$ scattering experiments in the 1970's. It could be a ordinary quarnk-antiquark meson, a tetraquark exotic state, a kaon-antikaon molecule, or a quark-antiquark-gluon hybrid state.

Anisotropic flow (v_n) is a powerful tool to probe the dynamics of relativistic heavy ion collisions. The observed approximate scaling of v_n by the number of constituent quarks (NCQ) suggests the importance of partonic degree of freedom in these collisions. Large v_n and NCQ-scaling have also been observed in high-multiplicity proton-lead (pPb) collisions at the LHC, suggesting similar dynamics in pPb collisions. The empirically established NCQ-scaling can be exploited to probe the constituent quark content of the f₀(980) hadron.

We measure the elliptic flow (v₂) of f₀(980) in high-multiplicity pPb collisions at 8 TeV with the Compact Muon Solenoid (CMS) detector at the LHC, through its main pion-pion ($\pi^+\pi^-$) decay channel. The invariant mass spectrum is obtained from opposite sign pion pairs, and the combinatorial background is estimated by same-sign pion pairs and subtracted. The f₀(980) signal and the remaining residual background are modeled by relativistic Breit-Wigner and polynomial functions, respectively. The yield of f₀(980) is extracted as a function of azimuthal angle relative to the event plane, and the f₀(980) v₂ is measured as function of $p_T$, corrected for the event-plane resolution. The non-flow component in f₀(980) v₂ has been estimated by that of K_s⁰. By comparing the measured v₂ of the f₀(980) to those of other hadrons, we infer the number of constituent quarks for the f₀(980) assuming NCQ scaling. The f₀(980) is found to be a 2-quark state in this work. We discuss the systematic uncertainties involved in our study, implications of our result, and possible future works in this endeavor.