A
bstract
We introduce a new measurable quantity,
R
Δ
ϕ
, for studies of the rapidity and transverse momentum dependence of dijet azimuthal decorrelations in hadron-hadron collisions. In pQCD,
R
Δ
ϕ
...is computed as a ratio of three-jet and dijet cross sections in which the parton distribution functions cancel to a large extent. At the leading order,
R
Δ
ϕ
is proportional to α
s
, and the transverse momentum dependence of
R
Δ
ϕ
can therefore be exploited to determine α
s
. We compute the NLO pQCD theory predictions and non-perturbative corrections for
R
Δ
ϕ
at the LHC and the Tevatron and investigate the corresponding uncertainties. From this, we estimate the theory uncertainties for α
s
determinations based on
R
Δ
ϕ
at both colliders. The potential of
R
Δ
ϕ
measurements for tuning Monte Carlo event generators is also demonstrated.
A
bstract
We point out an inconsistency in perturbative QCD predictions previously used for dijet azimuthal decorrelations for azimuthal angles of Δ
ϕ
dijet
< 2π/3 between the two jets. We show how ...the inconsistency arises and how the calculations can be modified to provide more accurate results that exhibit a smaller scale dependence and give a better description of the data than the inconsistent results. We also explain how the quality of the predictions strongly depends on a perceivedly minor detail in the definition of the dijet phase space and give recommendations for future measurements.
We point out an inconsistency in perturbative QCD predictions previously used for dijet azimuthal decorrelations for azimuthal angles of Delta sub(dijet) < 2 pi /3 between the two jets. We show how ...the inconsistency arises and how the calculations can be modified to provide more accurate results that exhibit a smaller scale dependence and give a better description of the data than the inconsistent results. We also explain how the quality of the predictions strongly depends on a perceivedly minor detail in the definition of the dijet phase space and give recommendations for future measurements.
Abstract We point out an inconsistency in perturbative QCD predictions previously used for dijet azimuthal decorrelations for azimuthal angles of Delta ^sub dijet^ < 2pi/3 between the two jets. We ...show how the inconsistency arises and how the calculations can be modified to provide more accurate results that exhibit a smaller scale dependence and give a better description of the data than the inconsistent results. We also explain how the quality of the predictions strongly depends on a perceivedly minor detail in the definition of the dijet phase space and give recommendations for future measurements.
In the standard approach, predictions of perturbative Quantum Chromodynamics for ratios of cross sections are computed as the ratio of fixed-order predictions for the numerator and the denominator. ...Beyond the lowest order in the perturbative expansion, the result does, however, not correspond to a fixed-order prediction for the ratio. This article describes how exact fixed-order results for ratios of arbitrary cross sections can be obtained. The general method for computations in any order of the perturbative expansion is derived, and results for next-to-leading order and next-to-next-to-leading order calculations are given. The approach is applied to theory predictions for various multi-jet cross section ratios measured at hadron colliders. The two methods are compared with each other and with the experimental data. Recommendations are made how to obtain improved theory predictions with more realistic uncertainty estimates.
A QCD analysis of jet production in deep-inelastic scattering is presented. The studies are based on recent measurements of jet cross sections in the H1 experiment at HERA which are directly ...sensitive to the gluon density in the proton. The inclusion of H1 data on the inclusive deep-inelastic scattering cross section enables us to perform a consistent, simultaneous determination of the quark and gluon densities in the proton. The gluon density has been determined at fairly large values of 10
−2 <
x < 10
−1 at a scale of the order of the transverse jet energies
μ
f
2 = 200 GeV
2. The result is consistent with indirect determinations from HERA structure function data and extends their range of sensitivity to larger
x-values.
Recent QCD related results from the CDF and the D0 experiments are presented based on proton anti-proton collision data at sqrt(s)=1.96 TeV, taken in Run II of the Fermilab Tevatron Collider. ...Measured observables include inclusive photon and diphoton production, vector boson plus jets production, event shape variables, and inclusive multijet production. The measurement results are compared to QCD theory calculations in different approximations. A determination of the strong coupling constant from jet data is presented.
JHEP 12 (2015) 024 We point out an inconsistency in perturbative QCD predictions previously used
for dijet azimuthal decorrelations for azimuthal angles of $\Delta\phi_{\rm
dijet} < 2\pi/3$ between ...the two jets. We show how the inconsistency arises and
how the calculations can be modified to provide more accurate results that
exhibit a smaller scale dependence and give a better description of the data
than the inconsistent results. We also explain how the quality of the
predictions strongly depends on a perceivedly minor detail in the definition of
the dijet phase space and give recommendations for future measurements.
Concepts and results of determinations of the strong coupling constant in hadron collisions are discussed. A recent alpha_s result from the inclusive jet cross section in pp-bar collisions at ...sqrt(s)=1.96 TeV is presented which is based on perturbative QCD calculations beyond next-to-leading order. Emphasis is put on the consistency of the conceptual approach. Conceptual limitations in the approach of extracting alpha_s from cross section data are discussed and how these can be avoided by using observables that are defined as ratios of cross sections. For one such observable, the multijet cross section ratio R3/2, preliminary results are presented.
This document provides a comprehensive summary of the state-of-the-art, challenges, and prospects in the experimental and theoretical study of the strong coupling \(\alpha_s\). The current status of ...the seven methods presently used to determine \(\alpha_s\) based on: (i) lattice QCD, (ii) hadronic \(\tau\) decays, (iii) deep-inelastic scattering and parton distribution functions fits, (iv) electroweak boson decays, hadronic final-states in (v) e+e-, (vi) e-p, and (vii) p-p collisions, and (viii) quarkonia decays and masses, are reviewed. Novel \(\alpha_s\) determinations are discussed, as well as the averaging method used to obtain the PDG world-average value at the reference Z boson mass scale, \(\alpha_s(m^2_Z)\). Each of the extraction methods proposed provides a "wish list" of experimental and theoretical developments required in order to achieve an ideal permille precision on \(\alpha_s(m^2_Z)\) within the next 10 years.
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