Measurement of the relative prompt production rate of chi(c2) and chi(c1) in pp collisions at sqrt(s) = 7 TeV

TL;DR

CMS measures the prompt chi_c2/chi_c1 production ratio Rp in pp collisions at sqrt(s)=7 TeV using chi_c -> J/psi gamma with photons reconstructed from conversions, based on 4.6 fb^-1 of data. Rp is extracted with an unbinned likelihood fit to the Q distribution and MC-derived acceptance corrections under an unpolarized production assumption, providing a precise test of quarkonium production mechanisms. The results are compared to k_T-factorization and NRQCD predictions, with NRQCD showing compatibility within uncertainties and k_T-factorization capturing the trend but overestimating the normalization by about a factor of two. The analysis systematically assesses polarization and feed-down effects, delivering constraints that inform future theoretical developments in charmonium production.

Abstract

A measurement is presented of the relative prompt production rate of chi(c2) and chi(c1) with 4.6 inverse femtobarns of data collected by the CMS experiment at the LHC in pp collisions at sqrt(s) = 7 TeV. The two states are measured via their radiative decays chi(c) to J/psi + gamma, with the photon converting into a dielectron pair for J/psi rapidity abs(y(J/psi)) < 1.0 and photon transverse momentum pt(gamma) > 0.5 GeV. The measurement is given for six intervals of pt(J/psi) between 7 and 25 GeV. The results are compared to theoretical predictions.

Figures (4)

• Figure 1: Distribution of the conversion radius for the $\chi_{\mathrm{c}}\xspace$ photon candidates.
• Figure 2: The distribution of the variable $Q = m_{\mu\mu \gamma} - m_{\mu \mu} + m_{{J}\space/\space\psi\xspace}$ for $\chi_c$ candidates with $p_{\mathrm{T}}\xspace({J}\space/\space\psi\xspace)$ ranges shown in the figures. The line shows the fit to the data.
• Figure 3: Ratio of the $\chi_{\mathrm{c}2}\xspace$ to $\chi_{\mathrm{c}1}\xspace$ production cross sections (circles) and ratio of the cross sections times the branching fractions to ${J}\space/\space\psi\xspace + \gamma$ (squares) as a function of the ${J}\space/\space\psi$ transverse momentum with the hypothesis of unpolarized production. The error bars correspond to the statistical uncertainties and the green band corresponds to the systematic uncertainties. For the cross section ratios, the 5.6% uncertainty from the branching fractions is not included.
• Figure 4: Comparison of the measured $\frac{\sigma({\chi_{\mathrm{c}2}\xspace}){\cal B}(\chi_{\mathrm{c}2}\xspace) }{ \sigma({\chi_{\mathrm{c}1}\xspace}){\cal B}(\chi_{\mathrm{c}1}\xspace)}$ values with theoretical predictions from the $k_{\mathrm{T}}$-factorization baranov (top) and NRQCD nlonrqcd (bottom) calculations (solid red lines). The error bars and green bands show the experimental statistical and systematic uncertainties, respectively. The measurements in the top plot use an acceptance correction assuming zero helicity for the $\chi_{\mathrm{c}}\xspace$, as predicted by the $k_{\mathrm{T}}$-factorization model. The measurements in the bottom plot are corrected to match the kinematic range used in the NRQCD calculation and assume the $\chi_{\mathrm{c}}\xspace$ are produced unpolarized. The measurements assuming two different extreme polarization scenarios are shown by the long-dashed blue and short-dashed green lines in the plot on the bottom. The 1-standard-deviation uncertainties in the NRQCD prediction, originating from uncertainties in the color-octet matrix elements, are displayed as the dotted red lines.