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Observation of multiplicity-dependent prompt $\chi_{c1}(3872)$ and $\psi(2S)$ production in $pp$ collisions

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Abstract

The production of $\chi_{c1}(3872)$ and $\psi(2S)$ hadrons is studied as a function of charged particle multiplicity in $pp$ collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 2 fb$^{-1}$. For both states, the fraction that is produced promptly at the collision vertex is found to decrease as charged particle multiplicity increases. The ratio of $\chi_{c1}(3872)$ to $\psi(2S)$ cross-sections for promptly produced particles is also found to decrease with multiplicity, while no significant dependence on multiplicity is observed for the equivalent ratio of particles produced away from the collision vertex in $b$-hadron decays. Comparisons with model calculations and implications for the binding energy of the $\chi_{c1}(3872)$ state are discussed.

Figures and captions

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

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The $\psi {(2S)} $ (left) invariant-mass and (right) $t_z$ spectrum in the $ p_{\mathrm{T}} $ and multiplicity ranges $ p_{\mathrm{T}} > 5 \text{ Ge V /}c $ and $60 <\mathrm{N_{tracks}^{VELO}} < 80$, with the simultaneous fit superimposed.

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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. The vertical error bars (boxes) represent the uncorrelated (correlated) uncertainties, while the horizontal error bars indicate bin widths.

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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 } \pi ^+ \pi ^- $ channel as a function of the number of tracks reconstructed in the VELO. The point-to-point uncorrelated (correlated) uncertainties are shown as vertical error bars (boxes) and the bin widths are shown as horizontal error bars. See text for details on calculations from Ref. [43]

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Created on 06 March 2021.