We have done a study of the B − → K − π 0 Xð3872Þ reaction by means of a triangle mechanism via the chain of reactions: We show that this mechanism generates a triangle singularity in the π 0 Xð3872Þ invariant mass for a very narrow window of the Xð3872Þ mass, around the present measured values, and show that the peak positions and the shape of the mass distributions are sensitive to the Xð3872Þ mass, such that a measurement of the reaction can serve to improve on the present values of this

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... . In particular, we point out that the Xð3872Þ mass relative to the D 0DÃ0 threshold may be extracted from the asymmetry of the π 0 X line shape.