3,156 research outputs found

    High-precision αs\alpha_s measurements from LHC to FCC-ee

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    This document provides a writeup of all contributions to the workshop on "High precision measurements of αs\alpha_s: From LHC to FCC-ee" held at CERN, Oct. 12--13, 2015. The workshop explored in depth the latest developments on the determination of the QCD coupling αs\alpha_s from 15 methods where high precision measurements are (or will be) available. Those include low-energy observables: (i) lattice QCD, (ii) pion decay factor, (iii) quarkonia and (iv) τ\tau decays, (v) soft parton-to-hadron fragmentation functions, as well as high-energy observables: (vi) global fits of parton distribution functions, (vii) hard parton-to-hadron fragmentation functions, (viii) jets in e±e^\pmp DIS and γ\gamma-p photoproduction, (ix) photon structure function in γ\gamma-γ\gamma, (x) event shapes and (xi) jet cross sections in e+ee^+e^- collisions, (xii) W boson and (xiii) Z boson decays, and (xiv) jets and (xv) top-quark cross sections in proton-(anti)proton collisions. The current status of the theoretical and experimental uncertainties associated to each extraction method, the improvements expected from LHC data in the coming years, and future perspectives achievable in e+ee^+e^- collisions at the Future Circular Collider (FCC-ee) with O\cal{O}(1--100 ab1^{-1}) integrated luminosities yielding 1012^{12} Z bosons and jets, and 108^{8} W bosons and τ\tau leptons, are thoroughly reviewed. The current uncertainty of the (preliminary) 2015 strong coupling world-average value, αs(mZ)\alpha_s(m_Z) = 0.1177 ±\pm 0.0013, is about 1\%. Some participants believed this may be reduced by a factor of three in the near future by including novel high-precision observables, although this opinion was not universally shared. At the FCC-ee facility, a factor of ten reduction in the αs\alpha_s uncertainty should be possible, mostly thanks to the huge Z and W data samples available.Comment: 135 pages, 56 figures. CERN-PH-TH-2015-299, CoEPP-MN-15-13. This document is dedicated to the memory of Guido Altarell

    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
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