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 (vn) is a powerful tool to probe the dynamics of relativistic heavy ion collisions. The observed approximate scaling of vn by the number of constituent quarks (NCQ) suggests the importance of partonic degree of freedom in these collisions. Large vn 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 Ks0. 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.
- Doctor of Philosophy
- Physics and Astronomy
- West Lafayette