The study of $\gamma\gamma\to\gamma\psi(2S)$ at Belle

TL;DR

This paper reports the first study of the two-photon process $oldsymbol{\gamma\gamma ooldsymbol{\gamma\psi(2S)}}$ at Belle, revealing two structures in the invariant mass distribution, one consistent with known states and another potentially new, with significance levels around 3 sigma.

Contribution

The study provides the first measurement of $oldsymbol{\gamma\gamma ooldsymbol{\gamma\psi(2S)}}$ and identifies two resonant structures, one matching known charmonium states and another possibly new, expanding understanding of charmoniumlike phenomena.

Findings

Two structures observed at 3922 MeV/c^2 and 4014 MeV/c^2.

First structure consistent with X(3915) or $\chi_{c2}(3930)$, with 3.1σ significance.

Second structure does not match known states, with 2.8σ significance.

Abstract

Using $980~\rm fb^{-1}$ of data on and around the $\Upsilon(nS)(n=1,2,3,4,5)$ resonances collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider, the two-photon process $\gamma\gamma\to \gamma\psi(2S)$ is studied from the threshold to $4.2~{\rm GeV}$ for the first time. Two structures are seen in the invariant mass distribution of $\gamma\psi(2S)$: one at $M_{R_1} = 3922.4\pm 6.5 \pm 2.0~{\rm MeV}/c^2$ with a width of $\Gamma_{R_1} = 22\pm 17\pm 4~{\rm MeV}$, and another at $M_{R_2} = 4014.3\pm 4.0 \pm 1.5~{\rm MeV}/c^2$ with a width of $\Gamma_{R_2} = 4\pm 11 \pm 6~{\rm MeV}$; the signals are parametrized with the incoherent sum of two Breit-Wigner functions. The first structure is consistent with the $X(3915)$ or the $\chi_{c2}(3930)$, and the local statistical significance is determined to be $3.1\sigma$ with the systematic uncertainties included. The second matches none of the known charmonium or charmoniumlike states, and its global significance is determined to be $2.8\sigma$ including the look-elsewhere effect. The production rates are $\Gamma_{\gamma\gamma}{\cal B}(R_1\to\gamma\psi(2S)) = 9.8\pm 3.6\pm 1.2~{\rm eV}$ assuming $(J^{PC}, |\lambda|) =(0^{++}, 0)$ or $2.0\pm 0.7\pm 0.2~{\rm eV}$ with $(2^{++}, 2)$ for the first structure and $\Gamma_{\gamma\gamma}{\cal B}(R_2\to\gamma\psi(2S)) = 6.2\pm 2.2\pm 0.8~{\rm eV}$ with $(0^{++}, 0)$ or $1.2\pm 0.4\pm 0.2~{\rm eV}$ with $(2^{++}, 2)$ for the second one. Here, the first errors are statistical and the second systematic, and $\lambda$ is the helicity.