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Near-threshold $D\bar{D}$ spectroscopy and observation of a new charmonium state

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Abstract

Using proton-proton collision data, corresponding to an integrated luminosity of 9$fb^{-1}$, collected with the LHCb detector between 2011 and 2018, a new narrow charmonium state, the $X(3842)$ resonance, is observed in the decay modes $X(3842)\rightarrow D^0\bar{D}^0$ and $X(3842)\rightarrow D^+D^-$. The mass and the natural width of this state are measured to be \begin{eqnarray*} m_{X(3842)} & = & 3842.71 \pm 0.16 \pm 0.12 MeV/c^2 , \Gamma_{X(3842)} & = & 2.79 \pm 0.51 \pm 0.35 MeV , \end{eqnarray*} where the first uncertainty is statistical and the second is systematic. The observed mass and narrow natural width suggest the interpretation of the new state as the unobserved spin\nobreakdash-3 $\psi_3\mathrm{\left(1^3D_3\right)}$ charmonium state. In addition, prompt hadroproduction of the $\psi(3770)$ and $\chi_{c2}(3930)$ states is observed for the first time, and the parameters of these states are measured to be \begin{eqnarray*} m_{\psi(3770)} & = & 3778.1 \pm 0.7 \pm 0.6 MeV/c^2 , m_{\chi_{c2}(3930)} & = & 3921.9 \pm 0.6 \pm 0.2 MeV/c^2 , \Gamma_{\chi_{c2}(3930)} & = &36.6 \pm 1.9 \pm 0.9 MeV , \end{eqnarray*} where the first uncertainty is statistical and the second is systematic.

Figures and captions

Distributions of (left) $m_{ K ^- \pi ^+ }$ versus $m_{ K ^+ \pi ^- }$ and (right) $m_{ K ^- \pi ^+ \pi ^+ }$ versus $m_{ K ^+ \pi ^- \pi ^- }$ for selected $ D \overline{ D } $ candidates.

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The mass spectra for selected $ D \overline{ D } $ combinations. The open red histogram corresponds to $ D ^0 \overline{ D } {}^0 $ pairs, while the hatched blue histogram corresponds to $ D ^+ D ^- $ pairs. Vertical black dashed lines help to identify the peaks from (left to right) $\chi _{ c 1}\mathrm{(3872)} \rightarrow D ^{*0}\overline{ D } {}^0 $, $\psi \mathrm{(3770)} \rightarrow D \overline{ D } $, $\mathrm{X(3842)} \rightarrow D \overline{ D } $ and $\chi _{ c 2}\mathrm{(3930)} \rightarrow D \overline{ D } $ decays.

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Mass spectra of (top) $ D ^0 \overline{ D } {}^0 $ and (bottom) $ D ^+ D ^- $ candidates in the narrow $3.80<m_{ D \overline{ D } }<3.88\text{ Ge V /}c^2 $ region. The result of the simultaneous fit described in the text is superimposed.

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Mass spectra of (top) $ D ^0 \overline{ D } {}^0 $ and (bottom) $ D ^+ D ^- $ candidates in the highdash-mass $3.80<m_{ D \overline{ D } }<4.20\text{ Ge V /}c^2 $ region. The result of the simultaneous fit described in the text is superimposed.

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Mass spectra of (top) $ D ^0 \overline{ D } {}^0 $ and (bottom) $ D ^+ D ^- $ candidates in the neardash-threshold $m_{ D \overline{ D } }<3.88\text{ Ge V /}c^2 $ region. The result of the simultaneous fit described in the text is superimposed.

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Animated gif made out of all figures.

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Tables and captions

Yields, mass and width of the $\mathrm{X(3842)}$ state from the fit to $ D \overline{ D } $ mass spectra in the narrow $3.80 < m_{ D \overline{ D } } < 3.88\text{ Ge V /}c^2 $ region. Uncertainties are statistical only.

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Yields, mass and width of the $\chi _{ c 2}\mathrm{(3920)}$ state from the fit to $ D \overline{ D } $ mass spectra in the highdash-mass $3.88 < m_{ D \overline{ D } }< 4.20\text{ Ge V /}c^2 $ region. Uncertainties are statistical only.

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Yields and mass of the $\psi \mathrm{(3770)}$ state from the fit to $ D \overline{ D } $ mass spectra in the neardash-threshold $m_{ D \overline{ D } } < 3.88\text{ Ge V /}c^2 $ region. Uncertainties are statistical only.

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Summary of systematic uncertainties for the measured masses ($\upsigma_m$) and width ($\upsigma_\Gamma$) of the $\mathrm{X(3842)}$, $\chi_{ c 2}\mathrm{(3930)}$ and $\psi \mathrm{(3770)}$ states. Uncertainties for the mass (width) smaller than $10\text{ ke V /}c^2 (10\text{ ke V} )$ are not shown.

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Summary of mass and width measurements for the $\chi _{ c 2}\mathrm{(3930)}$ state.

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Summary of mass measurements for the $\psi \mathrm{(3770)}$ state.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-005. The files are: Supplementary.pdf : An overview of the extra figures *.pdf : The figures in pdf format

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Created on 18 October 2019.