# Measurement of the relative rate of prompt $\chi_{c0}$, $\chi_{c1}$ and $\chi_{c2}$ production at $\sqrt{s}=7$TeV

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## Abstract

Prompt production of charmonium $\chi_{c0}$, $\chi_{c1}$ and $\chi_{c2}$ mesons is studied using proton-proton collisions at the LHC at a centre-of-mass energy of $\sqrt{s}=7$TeV. The $\chi_{c}$ mesons are identified through their decay to $J/\psi\gamma$, with $J/\psi\to\mu^+\mu^-$ using photons that converted in the detector. A data sample, corresponding to an integrated luminosity of $1.0\mathrm{fb}^{-1}$ collected by the LHCb detector, is used to measure the relative prompt production rate of $\chi_{c1}$ and $\chi_{c2}$ in the rapidity range $2.0<y<4.5$ as a function of the $J/\psi$ transverse momentum from 3 to 20 GeV$/c$. First evidence for $\chi_{c0}$ meson production at a high-energy hadron collider is also presented.

 Distribution of the mass difference $\Delta M\equiv M(\mu ^+\mu ^- \gamma )-M(\mu ^+\mu ^- )$ for $\chi _{c}$ candidates with $3< p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} } <20 {\mathrm{ Ge V /}c}$. dMall2011.pdf [22 KiB] HiDef png [157 KiB] Thumbnail [115 KiB] *.C file (a) Efficiency of converted photon reconstruction and selection relative to the calorimetric photon efficiency for data (red circles) and simulated events (blue triangles) as a function of $p_{\rm T}^{\gamma }$ . (b) Ratio of photon efficiencies $\varepsilon^{\gamma }_{\chi _{ c 1} }/\varepsilon^{\gamma }_{\chi _{ c 2} }$ as a function of $p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ from simulation (blue triangles) and after correcting the simulation for the converted photon efficiency measured in data (red circles) taken from plot (a). effic_[..].pdf [15 KiB] HiDef png [112 KiB] Thumbnail [95 KiB] *.C file ratio_[..].pdf [14 KiB] HiDef png [104 KiB] Thumbnail [87 KiB] *.C file Distribution of $\Delta M=M(\mu ^+\mu ^- \gamma )-M(\mu ^+\mu ^- )$ for $p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ in the range (a) 4--5 ${\mathrm{ Ge V /}c}$ and (b) 11--13 ${\mathrm{ Ge V /}c}$ . The results of the fit are also shown, with the total fitted function (blue solid curve), the $\chi _{ c 1}$ signal (green dashed curve), the $\chi _{ c 2}$ signal (red dot-dashed curve) and the $\chi _{ c 0}$ signal (purple long-dashed curve). Chic20[..].pdf [37 KiB] HiDef png [240 KiB] Thumbnail [169 KiB] *.C file Chic20[..].pdf [34 KiB] HiDef png [239 KiB] Thumbnail [175 KiB] *.C file Distribution of $\Delta M=M(\mu ^+\mu ^- \gamma )-M(\mu ^+\mu ^- )$ (blue histogram) for $4< p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} } <20 {\mathrm{ Ge V /}c}$. (a) The background estimated using fake photons (green) is superimposed on the $\Delta M$ distribution, together with the function used to parametrize it (black solid line). (b) The same $\Delta M$ distribution after background subtraction (using the shape shown in (a) and its fitted normalization): total fitted function (blue solid curve), $\chi _{ c 1}$ signal (green dashed curve), $\chi _{ c 2}$ signal (red dot-dashed curve) and $\chi _{ c 0}$ signal (purple long-dashed curve). Chic20[..].pdf [29 KiB] HiDef png [272 KiB] Thumbnail [196 KiB] *.C file Chic0F[..].pdf [30 KiB] HiDef png [245 KiB] Thumbnail [187 KiB] *.C file (left) Ratio of $\chi _{ c 2}$ to $\chi _{ c 1}$ cross-sections at $\sqrt{s}=7$ $\mathrm{ Te V}$ for $2.0 ## Tables and captions  Systematic uncertainties on the ratio of$\chi _{ c 2}$and$\chi _{ c 1}$yields for each$p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$bin (in percent). The total systematic uncertainty is defined as the quadratic sum of all the systematic uncertainties. Table_1.pdf [47 KiB] HiDef png [63 KiB] Thumbnail [24 KiB] tex code Correction factors to be applied to the final$\sigma(\chi _{ c 2} )/\sigma(\chi _{ c 1} )$results for each$p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$bin for different combinations of$\chi _{ c 1}$and$\chi _{ c 2}$polarization states$|J,m_{\chi_{cJ}}>$with$|m_{\chi_{cJ}}|=0,...,J$("unpol" means the$\chi _{c}$is unpolarized). The polarization axis is defined as the direction of the$\chi _{c}$in the laboratory frame. Table_2.pdf [58 KiB] HiDef png [113 KiB] Thumbnail [33 KiB] tex code Measurements of the ratio of$\chi _{ c 2}$to$\chi _{ c 1}$production cross-sections for the given$p_{\rm T}^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$range assuming unpolarized$\chi _{c}\$ production. The first uncertainty is statistical, the second is systematic, the third is from the branching fractions used and the last gives the maximum correction due to the unknown polarization. Table_3.pdf [53 KiB] HiDef png [143 KiB] Thumbnail [67 KiB] tex code

Created on 05 March 2021.