Abstract
The polarizations of the Y(1S), Y(2S), and Y(3S) mesons are measured as a function of the charged particle multiplicity in proton-proton collisions at sqrt(s) = 7 TeV. The measurements are performed with a dimuon data sample collected in 2011 by the CMS experiment, corresponding to an integrated luminosity of 4.9 inverse femtobarns. The results are extracted from the dimuon decay angular distributions, in two ranges of Y(nS) transverse momentum (10-15 and 15-35 GeV), and in the rapidity interval abs(y) < 1.2. The results do not show significant changes from low- to high-multiplicity pp collisions, although large uncertainties preclude definite statements in the Y(2S) and Y(3S) cases.
Highlights
Studies of heavy-quarkonium production contribute to an improved understanding of hadron formation within the context of quantum chromodynamics (QCD) [1]
In the HX frame, the λ parameters are measured with negligible correlations, as illustrated by Fig. 4, which shows the two-dimensional marginals of the posterior probability density (PPD) in the λφ vs. λθ and λθφ vs. λφ planes, for a representative analysis bin
The λvalues have been measured in the Collins–Soper frame (CS) [22], whose z axis is the average of the two beam directions in the Υ rest frame, and in the perpendicular helicity frame (PX) [23], orthogonal to the CS frame
Summary
Studies of heavy-quarkonium production contribute to an improved understanding of hadron formation within the context of quantum chromodynamics (QCD) [1]. The short-distance QQ production cross sections are functions of the QQ momentum and are calculated in perturbative QCD [3,4,5,6], while the probabilities that QQ pairs of different quantum properties form the observed quarkonium state are parametrized by momentum-independent long-distance matrix elements (LDMEs). Since they are expected to scale with powers of the heavy-quark velocity squared, v2, in the nonrelativistic limit (v2 1) most LDMEs are negligible and S-wave vector quarkonia should be dominantly formed from QQ pairs produced as coloursinglet, S [1] 1. As in the previous CMS quarkonium polarization measurements [13,17], the analysis is exclusively based on measured data: the 3-momentum vectors of the two muons (containing the spin alignment information of the decaying Υ(nS) mesons) and the muon detection efficiencies
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