From prompt to direct $$\mathrm{J}/\psi $$ production: new insights on the $$\chi _{{c}1} $$ and $$\chi _{{c}2} $$ polarizations and feed-down contributions from a global-fit analysis of mid-rapidity LHC data

Abstract

AbstractWhile the prompt $$\mathrm{J}/\psi $$J/ψ cross section and polarization have been measured with good precision as a function of transverse momentum, $$p_\mathrm{T}$$pT, those of the directly produced $$\mathrm{J}/\psi $$J/ψ are practically unknown, given that the cross sections and polarizations of the $$\chi _{{c}1} $$χc1 and $$\chi _{{c}2} $$χc2 mesons, large indirect contributors to $$\mathrm{J}/\psi $$J/ψ production, are only known with rather poor accuracy. The lack of precise measurements of the $$\chi _{{c}J} $$χcJ polarizations induces large uncertainties in the level of their feed-down contributions to the prompt $$\mathrm{J}/\psi $$J/ψ yield, because of the polarization-dependent acceptance corrections. The experimental panorama of charmonium production can be significantly improved through a consistent and model-independent global analysis of existing measurements of $$\mathrm{J}/\psi $$J/ψ, $$\psi \mathrm{(2S)}$$ψ(2S) and $$\chi _{c}$$χc cross sections and polarizations, faithfully respecting all the correlations and uncertainties. In particular, it is seen that the $$\chi _{{c}J} $$χcJ polarizations and feed-down fractions to $$\mathrm{J}/\psi $$J/ψ production have a negligible dependence on the $$\mathrm{J}/\psi $$J/ψ$$p_\mathrm{T}$$pT, with average values $$\lambda _\vartheta ^{\chi _{{c}1}} = 0.55 \pm 0.23$$λϑχc1=0.55±0.23, $$\lambda _\vartheta ^{\chi _{{c}2}} = -0.39 \pm 0.22$$λϑχc2=-0.39±0.22, $$R^{\chi _{{c}1}} = (18.8 \pm 1.4)\%$$Rχc1=(18.8±1.4)% and $$R^{\chi _{{c}2}} = (6.5 \pm 0.5)\%$$Rχc2=(6.5±0.5)%. The analysis also shows that $$(67.2 \pm 1.9)$$(67.2±1.9)% of the prompt $$\mathrm{J}/\psi $$J/ψ yield is due to directly-produced mesons, of polarization constrained to remarkably small values, $$\lambda _\vartheta ^{\mathrm{J}/\psi } = 0.04 \pm 0.06$$λϑJ/ψ=0.04±0.06.