Observation of structure in the $J/\psi$-pair mass spectrum

TL;DR

This paper reports the observation of structures in the $J/$-pair mass spectrum from proton-proton collisions, including a narrow resonance around 6.9 GeV/c^2, indicating potential exotic four-charm-quark states.

Contribution

First observation of resonant structures in the $J/$-pair mass spectrum, including a narrow state consistent with a four-charm-quark resonance, using LHCb data.

Findings

Narrow structure at 6.9 GeV/c^2 observed

Deviation from nonresonant production above five sigma

Measured mass and width of the $X(6900)$ resonance

Abstract

Using proton-proton collision data at centre-of-mass energies of $\sqrt{s} = 7$, $8$ and $13\mathrm{\,TeV}$ recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of $9\mathrm{\,fb}^{-1}$, the invariant mass spectrum of $J/\psi$ pairs is studied. A narrow structure around $6.9\mathrm{\,GeV/}c^2$ matching the lineshape of a resonance and a broad structure just above twice the $J/\psi$ mass are observed. The deviation of the data from nonresonant $J/\psi$-pair production is above five standard deviations in the mass region between $6.2$ and $7.4\mathrm{\,GeV/}c^2$, covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow $X(6900)$ structure are measured assuming a Breit--Wigner lineshape.