525 research outputs found

    Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

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    The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, 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 χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ 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

    Measurement of tt̄ normalised multi-differential cross sections in pp collisions at √s = 13 TeV, and simultaneous determination of the strong coupling strength, top quark pole mass, and parton distribution functions

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    Normalised multi-differential cross sections for top quark pair (tt̄) production are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV using events containing two oppositely charged leptons. The analysed data were recorded with the CMS detector in 2016 and correspond to an integrated luminosity of 35.9fb⁻¹. The double-differential tt̄ cross section is measured as a function of the kinematic properties of the top quark and of the tt̄ system at parton level in the full phase space. A triple-differential measurement is performed as a function of the invariant mass and rapidity of the tt̄ system and the multiplicity of additional jets at particle level. The data are compared to predictions of Monte Carlo event generators that complement next-to-leading-order (NLO) quantum chromodynamics (QCD) calculations with parton showers. Together with a fixed-order NLO QCD calculation, the triple-differential measurement is used to extract values of the strong coupling strength αS and the top quark pole mass (m_(pole_t) using several sets of parton distribution functions (PDFs). The measurement of m_(pole)_t exploits the sensitivity of the tt̄ invariant mass distribution to m_(pole)_t near the production threshold. Furthermore, a simultaneous fit of the PDFs, α_S, and m_(pole)_t is performed at NLO, demonstrating that the new data have significant impact on the gluon PDF, and at the same time allow an accurate determination of α_S and m_(pole)_t. The values α_S(m_Z) = 0.1135+0.0021−0.0017 and m_(pole)_t = 170.5±0.8GeV are extracted, which account for experimental and theoretical uncertainties, the latter being estimated from NLO scale variations. Possible effects from Coulomb and soft-gluon resummation near the tt̄ production threshold are neglected in these parameter extractions. A rough estimate of these effects indicates an expected correction of m_(pole)_t of the order of +1 GeV, which can be regarded as additional theoretical uncertainty in the current m_(pole)_t extraction
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