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
Pietro Faccioli (LIP, Lisbon, Portugal); Carlos Lourenço (CERN, Geneva, Switzerland); Thomas Madlener (DESY, Hamburg, Germany)
While the prompt $$\mathrm{J}/\psi $$ cross section and polarization have been measured with good precision as a function of transverse momentum, $$p_\mathrm{T}$$ , those of the directly produced $$\mathrm{J}/\psi $$ are practically unknown, given that the cross sections and polarizations of the $$\chi _{{c}1} $$ and $$\chi _{{c}2} $$ mesons, large indirect contributors to $$\mathrm{J}/\psi $$ production, are only known with rather poor accuracy. The lack of precise measurements of the $$\chi _{{c}J} $$ polarizations induces large uncertainties in the level of their feed-down contributions to the prompt $$\mathrm{J}/\psi $$ 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 $$\mathrm{J}/\psi $$ , $$\psi \mathrm{(2S)}$$ and $$\chi _{c}$$ cross sections and polarizations, faithfully respecting all the correlations and uncertainties. In particular, it is seen that the $$\chi _{{c}J} $$ polarizations and feed-down fractions to $$\mathrm{J}/\psi $$ production have a negligible dependence on the $$\mathrm{J}/\psi $$ $$p_\mathrm{T}$$ , with average values $$\lambda _\vartheta ^{\chi _{{c}1}} = 0.55 \pm 0.23$$ , $$\lambda _\vartheta ^{\chi _{{c}2}} = -0.39 \pm 0.22$$ , $$R^{\chi _{{c}1}} = (18.8 \pm 1.4)\%$$ and $$R^{\chi _{{c}2}} = (6.5 \pm 0.5)\%$$ . The analysis also shows that $$(67.2 \pm 1.9)$$ % of the prompt $$\mathrm{J}/\psi $$ yield is due to directly-produced mesons, of polarization constrained to remarkably small values, $$\lambda _\vartheta ^{\mathrm{J}/\psi } = 0.04 \pm 0.06$$ .
