Measurement of the $\chi_\mathrm{c1}$ and $\chi_\mathrm{c2}$ polarizations in proton-proton collisions at $\sqrt{s} =$ 8 TeV

The CMS collaboration
No Journal Information, 2019

Abstract
The polarizations of promptly produced $\chi_\mathrm{c1}$ and $\chi_\mathrm{c2}$ mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at $\sqrt{s} = $ 8 TeV. The $\chi_\mathrm{c}$ states are reconstructed via their radiative decays $\chi_\mathrm{c} \to \mathrm{J}/\psi\, \gamma$, with the photons being measured through conversions to e$^+$e$^-$, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the $\chi_\mathrm{c2}$ to $\chi_\mathrm{c1}$ yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the $\mathrm{J}/\psi \to \mu^+\mu^-$ decay, in three ranges of $\mathrm{J}/\psi$ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum.

  • Table A.1, 8-12 GeV

    Values from Table A.1 in the supplementary material. The ratio is also plotted in Figure 2 (left)

    10.17182/hepdata.92245.v1/t1

    Yield ratios of chi_c2 over chi_c1 mesons as a function of phi (HX) in the J/psi pT range 8-12 GeV

  • Table A.1, 12-18 GeV

    Values from Table A.1 in the supplementary material. The ratio is also plotted in Figure 2 (left)

    10.17182/hepdata.92245.v1/t2

    Yield ratios of chi_c2 over chi_c1 mesons as a function of phi (HX) in the J/psi pT range 12-18 GeV

  • Table A.1, 18-30 GeV

    Values from Table A.1 in the supplementary material. The ratio is also plotted in Figure 2 (left)

    10.17182/hepdata.92245.v1/t3

    Yield ratios of chi_c2 over chi_c1 mesons as a function of phi (HX) in the J/psi pT range 18-30 GeV

  • Table A.2, 8-12 GeV

    Values from Table A.1 in the supplementary material. The ratio is also plotted in Figure 2 (right)

    10.17182/hepdata.92245.v1/t4

    Yield ratios of chi_c2 over chi_c1 mesons as a function of costh (HX) in the J/psi pT range 8-12 GeV

  • Table A.2, 12-18 GeV

    Values from Table A.1 in the supplementary material. The ratio is also plotted in Figure 2 (right)

    10.17182/hepdata.92245.v1/t5

    Yield ratios of chi_c2 over chi_c1 mesons as a function of costh (HX) in the J/psi pT range 12-18 GeV

  • Table A.2, 18-30 GeV

    Values from Table A.1 in the supplementary material. The ratio is also plotted in Figure 2 (right)

    10.17182/hepdata.92245.v1/t6

    Yield ratios of chi_c2 over chi_c1 mesons as a function of costh (HX) in the J/psi pT range 18-30 GeV

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