CMS-PAS-HIN-15-003

CMS-PAS-HIN-15-003
Measurement of the $\Upsilon$ (nS) polarizations in pp collisions as a function of charged particle multiplicity
CMS Collaboration
June 2015

Abstract: As a first step in a broad study of QCD medium effects on quarkonium polarization, the polarizations of the $\Upsilon$ (1S), $\Upsilon$ (2S), and $\Upsilon$ (3S) mesons are measured in proton-proton collisions at $\sqrt{s} =$ 7 TeV as a function of the charged particle multiplicity, $N_{\mathrm {ch}}$ . The measurements are performed using a dimuon data sample collected in 2011 by the CMS experiment, corresponding to an integrated luminosity of 4.9 fb $^{-1}$ . The results are extracted from the analysis of the dimuon decay angular distributions, analyzed in three different polarization frames; they are presented as a function of $N_{\mathrm {ch}}$ , for two ranges in $\Upsilon$ transverse momentum (10 $< p_{\mathrm {T} } <$ 15 and 15 $< p_{\mathrm {T}} <$ 35 GeV), integrated within the rapidity range $\|y\|<$ 1.2. The measurement of the polarization parameters $\lambda_\vartheta$ , $\lambda_\varphi$ and $\lambda_{\vartheta\varphi}$ is complemented by the determination of the frame-invariant quantity $\tilde{\lambda}$ , which provides a very useful intrinsic test of the reliability of the whole analysis chain. The polarizations do not show significant changes as a function of $N_{\mathrm{ ch} }$ , excluding strong changes of the production processes between low- and high-multiplicity pp collisions.
Links: CDS record (PDF) ; inSPIRE record ; Public twiki page ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 761 (2016) 31. The superseded preliminary plots can be found here.

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