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Multiplicity-dependent modification of $\chi_{c1}(3872)$ and $\psi{(2S)}$ production in $pp$ collisions at $\sqrt{s}$= 8 TeV

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Abstract

In $pp$ collisions at $\sqrt{s}$ = 8 TeV, $\chi_{c1}(3872)$ and $\psi {(2S)} $ states are produced promptly at the primary collision vertex or in decays of $b$ hadrons. We study their relative production rates as a function of particle multiplicity. The fraction of promptly produced states is found to decrease with multiplicity for both $\chi_{c1}(3872)$ and $\psi {(2S)} $. This evolution differs between the two species: the ratio of cross-sections for promptly produced particles, $\sigma_{\chi_{c1}(3872)}$/$\sigma_{\psi {(2S)} }$, is found to decrease with increasing multiplicity. By contrast, no significant dependence on multiplicity is observed for the equivalent ratio for $\chi_{c1}(3872)$ and $\psi {(2S)} $ produced in $b$-hadron decays. This behaviour is consistent with the interpretation of the $\chi_{c1}(3872)$ as a weakly bound state, such as a $ D ^0 \overline{ D } {}^{*0} $ hadronic molecule.

Figures and captions

The $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+ \pi^-$ invariant-mass spectrum. The insert shows the region of the $\chi_{c1}(3872)$ resonance.

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Simultaneous fits to the mass (left) and pseudo proper-time $t_{z}$ (right) of the $\psi {(2S)} $, in the event activity range $60 < N_{\mathrm{tracks}}^{\mathrm{VELO}} < 80$. The data is shown as black points and the total fit is shown as a blue line. The fit components consist of background (red dashed line), the prompt signal component (green dashed line), and the $b$-decay signal component (shaded green).

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The fraction $ f_{\mathrm{prompt}}$ of promptly produced $\chi_{c1}(3872)$ and $\psi {(2S)} $ hadrons, as a function of the number of tracks reconstructed in the VELO.

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The ratio of the $\chi_{c1}(3872)$ and $\psi {(2S)} $ cross sections measured in the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- $ channel as a function of the number of tracks reconstructed in the VELO.

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Created on 06 March 2021.