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Register a new userImproved strong coupling determinations from hadronic decays of electroweak bosons at N$^3$LO accuracy
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We present two new extractions of the QCD coupling constant at the Z pole, $alpha_S(m_Z)$, from detailed comparisons of inclusive W and Z hadronic decays data to state-of-the-art perturbative Quantum Chromodynamics calculations at next-to-next-to-next-to-leading order (N$^{3}$LO) accuracy, incorporating the latest experimental and theoretical developments. In the W boson case, the total width computed at N$^{3}$LO is used for the first time in the extraction. For the Z boson pseudo-observables, the N$^{3}$LO results are complemented with the full two- and partial three-loop electroweak corrections recently made available, and the experimental values are updated to account for newly estimated LEP luminosity biases. A combined reanalysis of the Z boson data yields $alpha_S(m_Z) = 0.1203 pm 0.0028$, with a 2.3% uncertainty reduced by about 7% compared to the previous state-of-the-art. From the combined W boson data, a value of $alpha_S(m_Z) = 0.101 pm 0.027$ is extracted, with still large experimental uncertainties but also reduced compared to previous works. The levels of theoretical and parametric precision required in the context of QCD coupling determinations with permil uncertainties from high-statistics W and Z boson samples expected at future $e^+e^-$ colliders such as the FCC-ee, are discussed in detail.
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Isolated lepton momenta, in particular their directions are the most precisely measured quantities in pp collisions at LHC. This offers opportunities for multitude of precision measurements. It is of practical importance to verify if precision measurements with lep- tons in the final state require all theoretical effects evaluated simultaneously or if QED bremsstrahlung in the final state can be separated without unwanted precision loss. Results for final state bremsstrahlung in the decays of narrow resonances are obtained from the Feynman rules of QED in an unambiguous way and can be controlled with a very high precision. Also for resonances of non-negligible width, if calculations are appropriately performed, such separation from the remaining electroweak effects can be expected. Our paper is devoted to validation that final state QED bremsstrahlung can indeed be separated from the rest of QCD and electroweak effects, in the production and decay of Z and W bosons, and to estimation of the resulting systematic error. The quantitative discussion is based on Monte Carlo programs PHOTOS and SANC, as well as on KKMC which is used for benchmark results. We show, that for a large classes of W and Z boson observables as used at LHC, theoretical error on photonic bremsstrahlung is 0.1 or 0.2%, depending on the program options used. An overall theoretical error on QED final state radiation, i.e. taking into account missing corrections due to pair emission and interference with initial state radiation is estimated respectively at 0.2% or 0.3% again depending on the program option used.
We review models of electroweak symmetry breaking due to new strong interactions at the TeV energy scale and discuss the prospects for their experimental tests. We emphasize the direct observation of the new interactions through high-energy scattering of vector bosons. We also discuss indirect probes of the new interactions and exotic particles predicted by specific theoretical models. [Working group summary report from the Snowmass `96 summer study, to appear in the proceedings.]
We compute the unpolarized quark and gluon transverse-momentum dependent fragmentation functions (TMDFFs) at next-to-next-to-next-to-leading order (N$^3$LO) in perturbative QCD. The calculation is based on a relation between the TMDFF and the limit of the semi-inclusive deep inelastic scattering cross section where all final-state radiation becomes collinear to the detected hadron. The required cross section is obtained by analytically continuing our recent computation of the Drell-Yan and Higgs boson production cross section at N$^3$LO expanded around the limit of all final-state radiation becoming collinear to one of the initial states. Our results agree with a recent independent calculation by Luo et al.
The analysis of hadronic vector boson decays at LHC does not normally allow for interference with QCD production. These effects are studied here using the Sherpa package and can move by several GeV/c 2 the peak positions experiments would reconstruct. However, their impact depends strongly on the kinematics involved. The shifts expected in boosted W and Z bosons, which have been the subject of experimental study, are explored for the first time. The effects in the channels examined are all very small or negligible, but this may not true if lower transverse momenta are analysed, for example in the experimental trigger systems.
We compute the quark and gluon transverse momentum dependent parton distribution functions at next-to-next-to-next-to-leading order (N$^3$LO) in perturbative QCD. Our calculation is based on an expansion of the differential Higgs boson and Drell-Yan production cross sections about their collinear limit. This method allows us to employ cutting edge techniques for the computation of cross sections to extract the universal building blocks in question. The corresponding perturbative matching kernels for all channels are expressed in terms of simple harmonic polylogarithms up to weight five. As a byproduct, we confirm a previous computation of the soft function for transverse momentum factorization at N$^3$LO. Our results are the last missing ingredient to extend the $q_T$ subtraction methods to N$^3$LO and to obtain resummed $q_T$ spectra at N$^3$LL$^prime$ accuracy both for gluon as well as for quark initiated processes.
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