Nature of $X(6900)$ and its production mechanism at LHCb

We show that the Pomeron exchanges play a unique role in vector charmonium scatterings. Such a mechanism can provide a natural explanation for the nontrivial structures in the di-<math altimg="si1.svg"><mi>J</mi><mo stretchy="false">/</mo><mi>ψ</mi></math> spectrum observed by the LHCb Collaboration. The narrow structure <math altimg="si2.svg"><mi>X</mi><mo stretchy="false">(</mo><mn>6900</mn><mo stretchy="false">)</mo></math>, as a dynamically generated resonance pole, can arise from the Pomeron exchanges and coupled-channel effects between the <math altimg="si1.svg"><mi>J</mi><mo stretchy="false">/</mo><mi>ψ</mi></math>-<math altimg="si1.svg"><mi>J</mi><mo stretchy="false">/</mo><mi>ψ</mi></math>, <math altimg="si1.svg"><mi>J</mi><mo stretchy="false">/</mo><mi>ψ</mi></math>-<math altimg="si3.svg"><mi>ψ</mi><mo stretchy="false">(</mo><mn>2</mn><mi>S</mi><mo stretchy="false">)</mo></math> scatterings. A pole structure near the di-<math altimg="si1.svg"><mi>J</mi><mo stretchy="false">/</mo><mi>ψ</mi></math> threshold is also found. Meanwhile, we predict that <math altimg="si2.svg"><mi>X</mi><mo stretchy="false">(</mo><mn>6900</mn><mo stretchy="false">)</mo></math> can produce significant threshold enhancement in the <math altimg="si1.svg"><mi>J</mi><mo stretchy="false">/</mo><mi>ψ</mi></math>-<math altimg="si3.svg"><mi>ψ</mi><mo stretchy="false">(</mo><mn>2</mn><mi>S</mi><mo stretchy="false">)</mo></math> energy spectrum which can be searched for at LHCb.