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

[to restricted-access page]

## 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. Fig1.pdf [36 KiB] HiDef png [184 KiB] Thumbnail [89 KiB] *.C file 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). Fig2.pdf [68 KiB] HiDef png [445 KiB] Thumbnail [248 KiB] *.C file 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. Fig3.pdf [14 KiB] HiDef png [136 KiB] Thumbnail [126 KiB] *.C file 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. Fig4.pdf [14 KiB] HiDef png [188 KiB] Thumbnail [176 KiB] *.C file Animated gif made out of all figures. CONF-2019-005.gif Thumbnail

Created on 06 March 2021.