The only anticipated resonant contributions to $B^+\to D^+D^K^+$ decays are charmonium states in the $D^+D^$ channel. A modelindependent analysis, using LHCb protonproton collision data taken at centreofmass energies of $\sqrt{s}=7,8,$ and $13$ TeV, corresponding to a total integrated luminosity of 9 fb$^{1}$, is carried out to test this hypothesis. The description of the data assuming that resonances only manifest in decays to the $D^+D^$ pair is shown to be incomplete. This constitutes evidence for a new contribution to the decay, potentially one or more new charmstrange resonances in the $D^K^+$ channel with masses around 2.9 GeV$/c^2$.
(a) Invariantmass distribution for $ B $ candidates with the results of the fit superimposed, where the signal component is indicated in red and background (barely visible) in blue. (b) Dalitz plot for candidates with $m( D ^+ D ^ K ^+ )$ values in the signal window. 
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Distributions of the first nine unnormalised moments, $\langle Y_k^j\rangle$, defined in Eq. 2, as a function of $m( D ^+ D ^ )$ for the selected $ B ^+ \rightarrow D ^+ D ^ K ^+ $ candidates, after efficiency correction and background subtraction have been applied. 
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Comparison between data (points with error bars) and a weighted generated sample (filled histogram) as a function of (a) $m( D ^ K ^+ )$ and (b) $m( D ^+ K ^+ )$, where the weights account for the Legendre polynomial moments of order up to and including 29. 
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Comparison between data (points with error bars) and a weighted generated sample (filled histogram) as a function of (a) $m( D ^ K ^+ )$ and (b) $m( D ^+ K ^+ )$, where the weights account for the Legendre polynomial moments of order up to and including four. The uncertainty on the weighted shape (dark band) is also shown. 
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Comparison of the test statistic evaluated for the data (black dashed line) and for the ensemble of pseudoexperiments (blue histogram) generated according to the PDF constructed using the first four moments of the $h( D ^+ D ^ )$ distribution in data. 
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Animated gif made out of all figures. 
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Created on 05 March 2021.