Measurement of the ratio of prompt ${\chi }_c$ to $J/\psi$ production in $pp$ collisions at $\sqrt{s}=7$ TeV

Abstract

The prompt production of charmonium ${\chi }_c$ and $J/\psi$ states is studied in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=7$ TeV at the Large Hadron Collider. The ${\chi }_c$ and $J/\psi$ mesons are identified through their decays ${\chi }_c\to J/\psi \gamma$ and $J/\psi \to {\mu }^{+}{\mu }^{-}$ using 36 pb${}^{-1}$ of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for ${\chi }_c$ and $J/\psi$, $\sigma ({\chi }_c\to J/\psi \gamma )/\sigma (J/\psi )$, is determined as a function of the $J/\psi$ transverse momentum in the range $2<p_{\mathrm{T}}^{J/\psi }<15$ GeV/$c$. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low $p_{\mathrm{T}}^{J/\psi }$ region.

arXiv commentPublished version

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