The two-photon decay of X(6900) from light-by-light scattering at the LHC

TL;DR

This paper suggests the X(6900) structure observed at LHC could be a fully-charmed tetraquark with a notable two-photon decay rate, supported by light-by-light scattering data, and discusses its possible quantum numbers.

Contribution

It proposes a connection between the X(6900) tetraquark candidate and light-by-light scattering data, estimating its two-photon decay rate and analyzing its quantum number assignments.

Findings

X(6900) may have a two-photon branching ratio around 10^{-4}

Light-by-light scattering data can accommodate the X(6900) state

Spin-parity $0^{-+}$ fits better with VMD predictions

Abstract

The LHCb Collaboration has recently discovered a structure around 6.9 GeV in the double-$J/\psi$ mass distribution, possibly a first fully-charmed tetraquark state $X(6900)$. Based on vector-meson dominance (VMD) such a state should have a significant branching ratio for decaying into two photons. We show that the recorded LHC data for the light-by-light scattering may indeed accommodate for such a state, with a $\gamma \gamma$ branching ratio of order of $10^{-4}$, which is larger even than the value inferred by the VMD. The spin-parity assignment $0^{-+}$ is in better agreement with the VMD prediction than $0^{++}$, albeit not significantly at the current precision. Further light-by-light scattering data in this region, clarifying the nature of this state, should be obtained in the Run 3 and probably in the high-luminosity phase of the LHC (Run 4 etc.).