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

Information

LHCb-PAPER-2020-023
CERN-EP-2020-161
arXiv:2009.06619 [PDF]

(Submitted on 14 Sep 2020)
Phys. Rev. Lett. 126 (2021) 092001
Inspire 1817021

Tools

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. This behavior is consistent with a calculation that models the $\chi_{c1}(3872)$ structure as a compact tetraquark. 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.	Fig1_v2.pdf [37 KiB] HiDef png [152 KiB] Thumbnail [80 KiB] *.C file
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.	Fig2.pdf [62 KiB] HiDef png [387 KiB] Thumbnail [214 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. The vertical error bars (boxes) represent the uncorrelated (correlated) uncertainties, while the horizontal error bars indicate bin widths.	Fig3.pdf [15 KiB] HiDef png [154 KiB] Thumbnail [157 KiB] *.C file
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]	Fig4_u[..].pdf [21 KiB] HiDef png [460 KiB] Thumbnail [220 KiB] *.C file
Animated gif made out of all figures.	PAPER-2020-023.gif Thumbnail