# Observation of charmonium pairs produced exclusively in $pp$ collisions

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## Abstract

A search is performed for the central exclusive production of pairs of charmonia produced in proton-proton collisions. Using data corresponding to an integrated luminosity of $3{\rm\ fb}^{-1}$ collected at centre-of-mass energies of 7 and 8 TeV, $J/\psi J/\psi$ and $J/\psi\psi(2S)$ pairs are observed, which have been produced in the absence of any other activity inside the LHCb acceptance that is sensitive to charged particles in the pseudorapidity ranges $(-3.5,-1.5)$ and $(1.5,5.0)$. Searches are also performed for pairs of P-wave charmonia and limits are set on their production. The cross-sections for these processes, where the dimeson system has a rapidity between 2.0 and 4.5, are measured to be $$\begin{array}{rl} \sigma^{J/\psi J/\psi} &= 58\pm10{(\rm stat)} \pm 6{(\rm syst)} {\rm\ pb} , \\ \sigma^{J/\psi\psi(2S)} &= 63 ^{+27}_{-18}{(\rm stat)}\pm 10{(\rm syst)} {\rm\ pb} , \\ \sigma^{\psi(2S)\psi(2S)} &< 237 {\rm\ pb}, \\ \sigma^{\chi_{c0}\chi_{c0}} &< 69 {\rm\ nb}, \\ \sigma^{\chi_{c1}\chi_{c1}} &< 45 {\rm\ pb}, \\ \sigma^{\chi_{c2}\chi_{c2}} &< 141 {\rm\ pb}, \\ \end{array}$$ where the upper limits are set at the 90 confidence level. The measured $J/\psi J/\psi$ and $J/\psi\psi(2S)$ cross-sections are consistent with theoretical expectations.

## Figures and captions

 Representative Feynman diagrams for pairs of charmonia produced through double pomeron exchange. In the left, one $t$-channel gluon is much softer than the other while in the right, they are similar. Fig1a.pdf [33 KiB] HiDef png [52 KiB] Thumbnail [28 KiB] *.C file Fig1b.pdf [33 KiB] HiDef png [53 KiB] Thumbnail [31 KiB] *.C file Left: Invariant masses of pairs of oppositely charged muons in events with exactly four tracks. Of the two possible ways of combining the muons per event, the one with the higher value for the lower-mass pair is plotted. Right: Invariant mass of the second pair of tracks where the first pair has a mass consistent with the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ or $\psi {(2S)}$ meson. When both masses are consistent with a charmonium, only the candidate with the higher mass is displayed. The curve shows an exponential fit in the region below 2500 $\mathrm{ Me V}$ . Fig2a.pdf [13 KiB] HiDef png [114 KiB] Thumbnail [72 KiB] *.C file Fig2b.pdf [15 KiB] HiDef png [112 KiB] Thumbnail [111 KiB] *.C file Invariant mass of the four-muon system in (left) ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and (right) ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \psi {(2S)}$ events. Fig3a.pdf [13 KiB] HiDef png [65 KiB] Thumbnail [36 KiB] *.C file Fig3b.pdf [13 KiB] HiDef png [65 KiB] Thumbnail [31 KiB] *.C file Number of tracks passing the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ exclusive selection after having removed the requirement that there be no additional charged tracks or photons. The shaded histogram is the expected feed-down from exclusive ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \psi {(2S)}$ events. Fig4.pdf [14 KiB] HiDef png [96 KiB] Thumbnail [108 KiB] *.C file Transverse momentum squared distribution of candidates for exclusively produced (left) ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and (right) dimuons whose invariant mass is between 6 and 9 $\mathrm{ Ge V}$ . The curves are fits to the data as described in the text. Fig5a.pdf [14 KiB] HiDef png [120 KiB] Thumbnail [110 KiB] *.C file Fig5b.pdf [16 KiB] HiDef png [123 KiB] Thumbnail [126 KiB] *.C file Animated gif made out of all figures. PAPER-2014-027.gif Thumbnail

## Tables and captions

 Summary of numbers entering the cross-section calculation. Table_1.pdf [59 KiB] HiDef png [63 KiB] Thumbnail [27 KiB] tex code Percentage uncertainties on the quantities entering the denominator of Eq. (2). Table_2.pdf [54 KiB] HiDef png [41 KiB] Thumbnail [18 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [60 KiB] Fig6.pdf [14 KiB] HiDef png [124 KiB] Thumbnail [115 KiB] *C file

Created on 20 February 2021.