Quarkonia Production in Nonrelativistic Quantum Chromodynamics

[摘要] Color-singlet production of quarkonia has failed to correctly predict the cross sectionsof ψ's produced at the Tevatron. Gluon fragmentation has been identified the dominantsource of high energy prompt quarkonia at hadron colliders. Fragmentationapproximations break down, however, when a quarkonium's transverse momentumbecomes comparable to its mass. We review the NRQCD formalism, which is aneffective field that improves upon the color-singlet calculations. As an example ofcolor-singlet calculation, we calculate in closed form the complete O(α²_s) color-singletdifferential cross section for e⁺e^- → γ^* → ψq + X scattering. The cross sectionreduces at high energies to a heavy quark fragmentation form. We find that the energyscale at which the approximate fragmentation result becomes reliable exceeds the ψqmass by more than an order of magnitude. We also discuss the color-singlet model'spredictions for direct J/ψ angular and energy distributions at CLEO. For productionat a pp collider, we identify a large class of color-octet diagrams that mediate quarkoniaproduction at all energies and reduce to the dominant set of gluon fragmentationgraphs in the high p⊥ limit. They contribute to quarkonia differential cross sections atthe same order as color-singlet diagrams and bring theoretical predictions for γ and ψproduction at the Tevatron into agreement with experimental measurements. Usingrecent CDF data, we extract numerical values for bottomonia and charmonia color-octetmatrix elements which are consistent with NRQCD scaling rules. We calculated the polarization of ψ'due to pp → QQ[^2S+1L_J⁽⁸⁾]X → ψ^(λ)X color-octet quarkoniaproduction at order α³_s which could be used as a test of NRQCD. We find that atlow transverse momenta the ψ' is unpolarized due to the contributions proportionalto the L = S = 0 and L = s = 1 color-octet matrix elements. As p⊥ increases, the ψ' mesons become 100% polarized, as predicted by fragmentation calculations.Polarization due to lowest order color-singlet production is also considered, whichqualitatively has a similar shape to the color-octet production.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics