Production of the Charmonium States chic_1 and chic_2 in Proton Nucleus Interactions at sqrt{s} = 41.6 GeV

TL;DR

This study measures the production of χc1 and χc2 in pA collisions at sqrt(s)=41.6 GeV via χc → J/ψ γ decays, normalizing to J/ψ production to obtain Rχc and the χc1/χc2 cross-section ratio. Using a large data set from the HERA-B detector with J/ψ decays in μ+μ− and e+e− channels, the analysis employs a mass-difference ΔM technique and a mixed-event background model to extract χc yields, supported by detailed MC simulations and data-driven efficiency checks. The results show Rχc = 0.188 ± 0.013(stat) +0.024/−0.022(sys) with a polarization-uncertainty band, R12 ≈ 1.02 ± 0.17(stat) ± 0.36(sys), and σ(χc1)/σ(χc2) ≈ 0.57 ± 0.23, with no strong kinematic or A-dependence observed within the measured range; the findings align with NRQCD expectations and provide the most extensive χc measurements in pA at this energy range. The study also pioneers an explicit assessment of potential polarization biases on Rχc and R12, quantifying possible shifts up to roughly +21% and −11%, respectively.

Abstract

A measurement of the ratio R_chic = (chic -> Jpsi + gam)/ Jpsi in pC, pTi and pW interactions at 920 GeV/c (sqrt{s}=41.6 GeV) in the Feynman-x range -0.35 < x_F(Jpsi) < 0.15 is presented. Both mu+mu- and e+e- Jpsi decay channels are observed with an overall statistics of about 15000 chic events, which is by far the largest available sample in pA collisions. The result is R_chic = 0.188+-0.013(st)(+0.024)(-0.022)(sys) averaged over the different materials, when no Jpsi and chic polarisations are considered. The chic_1 to chic_2 production ratio R_12 = R_chic1/R_chic2 is measured to be 1.02+-0.40, leading to a cross section ratio sigma(chic_1)/sigma(chic_2)=0.57+-0.23. The dependence of R_chic on the Feynman-x of the Jpsi, x_F(Jpsi), and its transverse momentum, p_T(Jpsi), is studied, as well as its dependence on the atomic number, A, of the target. For the first time, an extensive study of possible biases on R_chic and R_12 due to the dependence of acceptance on the polarization states of Jpsi and chic is performed. By varying the polarisation parameter, lambda(obs), of all produced Jpsi's by two sigma around the value measured by HERA-B, and considering the maximum variation due to the possible chic_1 and chic_2 polarisations, it is shown that R_chic could change by a factor between 1.02 and 1.21 and R_12 by a factor between 0.89 and 1.16.

Figures (7)

• Figure 1: Summary of experimental results on $R_{\chi_c}$ (a), $\frac{\sigma({\chi_{c1}})}{\sigma({\chi_{c2}})}$ (b), $\sigma({\chi_{c1}})$ (c) and $\sigma({\chi_{c2}})$ (d) in pN interactions. HERA-B results: open star (old), full star (new).
• Figure 2: Summary of experimental results on $R_{\chi_c}$ (a), $\frac{\sigma({\chi_{c1}})}{\sigma({\chi_{c2}})}$ (b), $\sigma({\chi_{c1}})$ (c) and $\sigma({\chi_{c2}})$ (d) in $\pi N$ interactions.
• Figure 3: $\mu^+\mu^-$ invariant mass spectra in the muon channel for C (a), W (b), Ti (c) and full sample (d). The bin width is 15$\, \mathrm{MeV}/c^2$.
• Figure 4: $e^+e^-$ invariant mass spectra for C (a), W (b), Ti (c) and full sample (d). The bin width is 30 $\, \mathrm{MeV}/c^2$.
• Figure 5: $\Delta M$ spectra in the muon channel for C (a), W (b), Ti (c) and full sample (d). The bin width is 10$\, \mathrm{MeV}/c^2$. In the background subtracted spectra the broken curves are the fitted ${\chi_{c1}}$ and ${\chi_{c2}}$ states.