Study of $e^{+}e^{-}\to h^{+}h^{-}J/ψ~(h=π,~K,~p)$ via initial-state radiation at Belle~II

TL;DR

This work analyzes e+e- -> h+h- J/ψ (h = π, K, p) via initial-state radiation using 427.9 fb^-1 of Belle II data to map the cross sections from the production threshold up to about 7 GeV. The study employs data-driven efficiency corrections through Dalitz-plot reweighting, performs a Dalitz analysis to search for intermediate states, and uses a control channel to mitigate data/MC discrepancies; it also reports the first Belle II measurement of e+e- -> p p̄ J/ψ and sets upper limits. A clear Zc(3900)± signal is observed in π± J/ψ with 5.3–5.6σ significance, while Y(4230/4320) is indicated around 4.26 GeV in π+π- J/ψ and no significant structures are found in KK J/ψ. The cross sections for π+π- J/ψ and K+K- J/ψ are consistent with prior results, and the pp̄ J/ψ results provide important new constraints for exotic-state production mechanisms; a larger Belle II dataset will enhance sensitivity to intermediate states and dynamics in these channels.

Abstract

Using a data sample of 427.9 fb$^{-1}$ collected by the Belle~II detector at or near the $Υ(4S)$ and $Υ(10753)$ resonances, the cross sections for $e^+e^-\to h^+h^-J/ψ$ $(h=π/K/p)$ at center-of-mass energies ranging from 3.8 GeV or the production threshold to 5.5/6.0/7.0 GeV have been measured via initial-state radiation. The cross sections for the processes $e^+e^-\to π^+π^-J/ψ$ and $e^+e^-\to K^+K^-J/ψ$ are consistent with previously published results. The cross sections for these channels obtained by combining with previous Belle results are also given. The process $e^+e^-\to p\bar p J/ψ$ is investigated for the first time. The yields are small and no significant structure is observed in the cross section versus energy. Searches for vector charmonium-like states in the $h^+h^-J/ψ$ systems, and for associated intermediate states in the $h^{\pm} J/ψ$ systems, are also presented.

Figures (5)

• Figure 1: Invariant mass distributions of (a) $\pi^+\pi^-J/\psi$, (b) $K^+K^-J/\psi$, and (c) $p\bar{p}J/\psi$. The black dots with error bars represent the selected data and the green shaded histograms are from the normalized $J/\psi$ sidebands. The inset in (a) shows the distribution extended to lower masses on a logarithmic vertical scale, and the large peak near 3.686 GeV is from $\gamma_{\rm ISR}\psi(2S)$.
• Figure 2: The Dalitz plots of (a) $e^+e^-\to\pi^+\pi^-J/\psi$ in the $Y(4230/4320)$ region and (b) $e^+e^-\to K^+K^-J/\psi$ with $M(K^+K^-J/\psi)<6.0~{\rm GeV}/c^2$. The insets show the background events from the normalized $J/\psi$ mass sidebands.
• Figure 3: The distribution of the $M(\pi^+\pi^-)$ in the $Y(4230/4320)$ region.
• Figure 4: (a) An unbinned extended maximum likelihood to the distribution of the $M_{\rm max}(\pi J/\psi)$ in the $Y(4230/4320)$ region. (b) The distribution of the $M_{\rm max}(KJ/\psi)$ with $M(K^+K^-J/\psi)<6.0~{\rm GeV}/c^2$.
• Figure 5: The measured Born cross sections of (a) $e^+e^-\to\pi^+\pi^-J/\psi$ for $\sqrt{s}$ between 3.8 and 5.5 GeV, (b) $e^+e^-\to K^+K^-J/\psi$ for $\sqrt{s}$ between 4.0 and 6.0 GeV, and (c) $e^+e^-\to p\bar{p}J/\psi$ for $\sqrt{s}$ between 5.0 and 7.0 GeV. The error bars include both statistical and systematic uncertainties. The blue open circles with error bars in panels (a) and (b) represent the Belle results Liu:2013Shen:2014, while the red full squares with error bars indicate the combined results. The blue inverted triangles (c) are 90% C.L. upper limits for the measured cross sections of $e^+e^-\to p\bar{p}J/\psi$.