Limit on the production of a new vector boson in $\mathrm{e^+ e^-}\rightarrow {\rm U}γ$, U$\rightarrow π^+π^-$ with the KLOE experiment

TL;DR

This study searches for a light dark photon $U$ with kinetic mixing parameter $\varepsilon^2$ in the mass range $527$–$987$ MeV using $e^+e^- \to U \gamma$, $U \to \pi^+\pi^-$ at DAΦNE with the KLOE detector. By exploiting initial-state radiation to access the hadronic decay channel and employing data-driven sidebands for the irreducible background ( complemented by a dedicated PHOKHARA-based treatment in the $\rho-\omega$ region), the analysis achieves a robust background estimate and uses the CLs method to set 90% CL limits on $\varepsilon^2$. The results show no evidence of a $U$ resonance, with the strongest constraints (up to $\varepsilon^2 \sim 1.8\times 10^{-5}$) in the 0.5–0.7 GeV region and notably tighter limits above 0.7 GeV than previous experiments. These hadronic-channel bounds complement earlier leptonic searches and tighten the allowed parameter space for light dark photons in the sub-GeV regime.

Abstract

The recent interest in a light gauge boson in the framework of an extra U(1) symmetry motivates searches in the mass range below 1 GeV. We present a search for such a particle, the dark photon, in ${\rm e^+ e^-}\rightarrow {\rm U}γ$, U$\rightarrow π^+π^-$ based on 28 million $\mathrm{e^+ e^-} \rightarrow π^+ π^-γ$ events collected at DA$Φ$NE by the KLOE experiment. The $π^+ π^-$ production by initial-state radiation compensates for a loss of sensitivity of previous KLOE ${\rm U} \rightarrow \mathrm{e^+ e^-}$, $μ^+μ^-$ searches due to the small branching ratios in the $ρ-ω$ resonance region. We found no evidence for a signal and set a limit at 90\% CL on the mixing strength between the photon and the dark photon, $\varepsilon^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV this new limit is more stringent than previous ones.

Figures (9)

• Figure 1: Example of $M_{\rm trk}$ distributions for the $M_{\uppi\uppi}=820-840$ MeV bin. Measured data are represented in black, simulated $\uppi^+ \uppi^-\upgamma$ and $\upmu^+ \upmu^-\upgamma$ in red. Simulated $\upmu^+ \upmu^-\upgamma+\uppi^+\uppi^-\upgamma$ in blue. Events at the left of the vertical line are rejected.
• Figure 2: Fractional backgrounds, normalized to the $\uppi^+\uppi^-\upgamma$ contribution, from the $\pi+\pi^-$π^+π^-$\pi0$π^0$$, $e+e^-$e^+e^-$\gamma$γ$$, and $\upmu^+ \upmu^- \upgamma$ channels after all selection criteria.
• Figure 3: Comparison of measured data (blue squares) and simulation performed with the Gounaris-Sakurai $|F_{\pi}|^2$ parametrization (red open squares) for the $M_{\pi\pi}$ invariant mass spectrum. The figure insert shows in detail the agreement achieved in the $\rho-\omega$ mixing region (779--791 MeV).
• Figure 4: Example of a Chebyshev polinomial sideband fit for the $M_{\rm U}=936$ MeV hypothesis
• Figure 5: Distribution of the differences (pulls) data-background normalized to the data statistical error (blue points) and relative Gaussian fit (red curve).