Search for the $Y(2175)$ in the photoproduction cross section measurement of $γp \to φπ^{+} π^{-} p$ at GlueX

TL;DR

This work presents the first measurement of the differential photoproduction cross section for $\gamma p \to \phi(1020)\pi^+\pi^- p$ in the GlueX experiment, using $E_\gamma$ between 8 and 11.6 GeV and 334 pb$^{-1}$ of data. The analysis searches for the strangeonium-like φ(2170) in the $\phi\pi^+\pi^-$ final state but finds no evidence when using PDG parameters, instead setting upper limits on the photoproduction cross section. A significant structure at $m(\phi\pi^+\pi^-)\approx2.24$ GeV/$c^2$ with $Z\sim5\sigma$ is observed, compatible with a BESIII/BaBar-like state (often labeled Y(2240)) rather than the PDG φ(2170), and a second structure near 1.82 GeV/$c^2$ is also seen, potentially linked to $\phi_3(1850)$ or to $K^*(892)\bar{K}^*(892)$ dynamics; the results include detailed fits with alternate resonance parameters and discuss systematic uncertainties, background contributions, and the need for further partial-wave analyses. The findings contribute to distinguishing possible interpretations of light-quark exotic states and inform future photoproduction studies of these resonances.

Abstract

Based on 334 pb$^{-1}$ of photoproduction data collected with the GlueX detector at Jefferson Lab, we have measured for the first time the cross section of the exclusive reaction $γ+ p \to φ(1020) π^+π^- p$ by reconstructing the final state $K^+K^- π^+π^- p$ produced with a photon beam of energies between $8.0 {\rm GeV}$ and $11.6 {\rm GeV}$. Based on the measured differential cross section, we have performed a search for the strangeonium-like exotic candidate $Y(2175)$, recently renamed to $φ(2170)$. This state has been reported by different $e^+e^-$ annihilation experiments and it is addressed here for the first time in a photoproduction experiment. We do not find evidence for this state when using the resonance parameters quoted by the Particle Data Group and provide upper limits on the photoproduction cross section. Instead, we find a structure at a mass of $m(φπ^+π^-)= 2.24$ GeV/$c^2$ with a statistical significance of about 5 $σ$. The parameters of this structure differ from those quoted by the Particle Data Group for the $φ(2170)$ and are consistent with a previous observation in $e^+e^-$ annihilation. In addition, there is evidence for a second structure at 1.82 GeV/$c^2$.

Figures (3)

• Figure 1: Observed $\phi(1020)$ signal. (a) Distribution of invariant masses $m(K^+K^-)$ vs. $m(K^+K^-\pi^+\pi^-)$. (b) $K^+K^-$ mass spectrum with fitted signal and background function.
• Figure 2: Measured differential cross section ${\rm d}\sigma/{\rm d}m$ for the reaction $\gamma p \to \phi(1020) \pi^+\pi^- p$ (data points) and search for the $\phi(2170)\to \phi(1020) \pi^+\pi^-$ in photoproduction. The data are modeled using Breit-Wigner functions to describe the peak-like structures at 1.8 and 2.2 GeV/$c^2$ (grey dashed curves). (a) Fit A, where the Breit-Wigner parameters for the structure at 2.2 GeV/$c^2$ are fixed to the PDG average values for the $\phi(2170)$PDG2022. (b) Fit B, where the Breit-Wigner parameters for the structure at 2.2 GeV/$c^2$ are fixed to the values measured by BESIII BesIII2019.
• Figure 3: Further supplemental studies concerning the additional structure at about 1.8 GeV/$c^2$. (a) Correlated $m(K^-\pi^+)$ vs. $m(K^+\pi^-)$ invariant mass after all event selection cuts applied, showing a strong $K^{*0}\bar{K}^{*0}$ contribution in the data. (b) Projections of the $m(K\pi)$ invariant mass distributions after requiring $|m(K^\pm\pi^\mp) - 892\,{\rm MeV/}c^2| < 100$ MeV/$c^2$. (c) Illustration of the insensitivity of the $\phi$-yield extraction method to backgrounds from $K^{*0}\bar{K}^{*0}$ events: No $K^*(892)^0$ signal is visible in the combined extracted $\phi$-yield as a function of $m(K^\pm\pi^\mp)$.