Measurement Of Αs Research Articles

Jet production and αS measurements at CMS

Recent CMS results related to jet production measurements as well as measurements of the strong coupling constant are discussed. In particular, results are described related to jet production cross sections, which pose a central test to perturbative QCD predictions, as well as hadronic event shape measurements, and measurements of αS with data taken at center-of-mass energy of 7 TeV and 8 TeV collected with the CMS detector.

Measurement of the strong coupling [formula omitted] in [formula omitted]-annihilation using the three-jet rate

We present a measurement of the strong coupling αS using data collected with the JADE detector at centre-of-mass energies between 14 and 44 GeV. The three-jet rate as a function of the transition parameter ycut is determined using the Durham jet algorithm and the distribution is compared to QCD predictions. Recent theoretical calculations predict the three-jet rate at next-to-next-to-leading order. For the first time a measurement of αS is presented using QCD predictions at next-to-next-to-leading order matched to predictions at next-to-leading logarithmic approximation, with subleading terms being included as well. We obtain αS(MZ0)=0.1199±0.0010(stat.)±0.0021(exp.)±0.0054(had.)±0.0007(theo.), being consistent with the world average value of αS.

Flow past two rotating cylinders

Flow past two uniformly rotating cylinders with the same rotation rates in a side-by-side configuration is studied experimentally. The experiments are carried out at Reynolds numbers, Re, of 100, 200, 300, 400, and 500 and nondimensional rotation rates, α, varying from 0 to 5. The spacing ratios, T/D, are 1.8, 2.5, 4.0, and 7.5. Two possibilities of rotations are considered with the cylinder surfaces in between the two cylinders moving upstream in one case (inward rotation case) and downstream in the other (outward rotation case). The diagnostics is done by flow visualization using hydrogen bubble technique and quantitative measurements using particle image velocimetry (PIV). We present, using extensive flow visualization, the global view of the wake structure at Re of 200 for various rotation rates, and two senses. Vortex shedding suppression is studied through flow visualization and/or PIV at various Re’s, T/D’s, and two senses. Vortex shedding is found to be suppressed in the inward rotation cases at all Re and T/D’s. The value of α corresponding to vortex shedding suppression, αs, in the inward rotation case is ∼2.0 for Re of 200–500 at all T/D’s. The value of αs for Re of 100 in the case of inward rotation shows an increasing trend with T/D from T/D=1.8 to 4.0 with αs changing from 1.2 to 1.7; further increase of T/D does not change αs. For outward rotation cases, vortex shedding suppression is clearly observed for Re of 100 and for all values of T/D; however, for higher Re, vortex shedding suppression is observed for T/D of 4.0 and 7.5 only. The measurements of αs in this case showed a decreasing trend with increasing T/D. Symmetry breaking is reported for inward rotation case near α=1.35 for T/D=2.5 at Re of 200 where the wake pattern changes from in-phase to antiphase mode.

Determination of the strong coupling constant using available experimental data

We determine the strong coupling constant αs up to 4-loop in perturbative QCD. Testing QCD requires the measurement of αs over ranges of energy scales. In this analysis, the value of αs is determined from the unpolarized structure functions data points by minimizing the χ2 function between the theory result and experimental data. Using perturbative QCD calculations from threshold corrections, we obtain αs(M2Z) = 0.1139±0.0020 at N3LO which is in good agreement with the very recently results from the inclusive jet cross section in pp̄ collisions at √s = 1.96 TeV.

One-loop electro-weak corrections to three-jet observables of b-quarks e + e − annihilations

We compute the full one-loop EW contributions of \({\mathcal{O}}(\alpha_{\mathrm{S}}\alpha_{\mathrm{EM}}^{3})\) entering the electron–positron into two b-quarks and one gluon cross section at the Z peak and LC energies. We include both factorisable and non-factorisable virtual corrections, photon bremsstrahlung but not the real emission of W± and Z bosons. Their importance for the measurement of αS from jet rates and shape variables is explained qualitatively and illustrated quantitatively. Their impact on the forward–backward asymmetry is also analysed.

Precision measurements of αs at hera

Recent determinations of αs (MZ) from the H1 and ZEUS Collaborations using inclusive-jet cross-section measurements in neutral current deep inelastic scattering at high Q2 are presented. A combined value of αs (MZ) = 0.1198±0.0019 (exp.)±0.0026 (th.) was obtained from these measurements. The determinations of αs at various scales clearly show the running of the coupling from HERA jet data alone and in agreement with the prediction of QCD.

Measurement of αs with radiative hadronic events

Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass of the Z boson with the OPAL detector at LEP. The strong coupling αs is extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies ranging from 24 GeV to 78 GeV, and the energy dependence of αs is studied. Our results are consistent with the running of αs as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different event shape observables and energies gives αs(MZ)=0.1182±0.0015(stat.)±0.0101(syst.).

Final QCD results from LEP

Recent results on QCD studies in e+e− annihilations at LEP are presented. Data recorded by the LEP experiments at centre-of-mass energies between 91.2 to 206 GeV are included. The main topic is the measurement of αs from event shape variables and associated aspects like the energy evolution or non-perturbative power-law corrections. These 'standard' measurements are complemented by new determinations using the 4-jet rate with an excellent precision. A summary of the results on QCD colour factors from angular correlations in the 4-jet system completes this report.

QCD studies at LEP

Studies of hadronic final states of e+e- annihilations at LEP are reviewed. The topics included cover hadronic event shapes, measurements of αs, determinations of QCD colour factors and tests of the non Abelian gauge structure of QCD, differences between quark and gluon jets, QCD with heavy quarks and selected results of two-photon scattering processes.

αs at Zinnowitz 2004

A review of measurements of αs is given, representing the status of April 2004. The results prove the energy dependence of αs and are in excellent agreement with the expectations of Quantum Chromodynamics, QCD. Evolving all results to the rest energy of the Z0 boson, the world average of αs(MZ0) is determined from mea- surements which are based on QCD calculations in complete NNLO perturbation theory, giving αs(MZ0)=0.1182±0.0027.

On a possible measurement of αS from B– [formula omitted] correlations in Z0 decay

Motivated by recent preliminary results from the SLD Collaboration on the measurement of angle-dependent B– B energy correlations in Z 0→b b ̄ events, we propose a class of observables that can be computed as a power expansion in the strong coupling constant α S, of order α S at the Born level and that can be used for a precision measurement of α S( M Z ). We compute their next-to-leading order O(α S 2) corrections in the strong coupling constant, including exactly quark-mass effects. We show that, in the theoretical evaluation of these quantities, large logarithms of the ratio of the mass of the final quark over the centre-of-mass energy cancel out. Thus, these variables have a well-behaved perturbative expansion in α S( M Z ). We study the theoretical uncertainties due to the renormalization-scale dependence and the quark-mass scheme and we address the question of which mass scheme is more appropriate for these variables.

Measurement of αs and the β function with the DELPHI detector at LEP

Event shape distributions in e + e − annihilation are determined from the DELPHI data taken between 183 and 207 GeV. From these the strong coupling a, is extracted with several techniques. Together with the results from other LEP2 energies and at about m Z this allows both, a combined measurement of α s and a test of the scale dependence of the strong interaction. Alternatively the renormalisation group invariant (RGI) perturbation theory is applied to measure the β function of strong interaction. The results are good agreement with the QCD expectation and allow to exclude the existence of light gluinos with a mass below 30 GeV in a model independent way.

Measurements of αs from QCD event shapes at OPAL and LEP

New preliminary measurements of the strong coupling as are presented, using hadronic decays observed in the OPAL detector at LEP. Previous OPAL measurements are updated to incorporate the latest theoretical predictions. New techniques for the estimation of theoretical uncertainties are discussed. The preliminary combined measurement, from all event shape fits performed by the four LEP collaborations at all centre-of-mass energies is α s = 0.1198 ± 0.0003 (stat.) ± 0.0047 (syst.).

Measurements of αs at centre-of-mass energies between 91 and 209 GeV

Consistent measurements of event-shape variables and as using the complete set of data taken with the ALEPH detector at LEP are presented. The strong coupling constant is determined from a fit of O( α s 2) + NLLA QCD calculations to distributions of six event-shape variables at eight centre-of-mass energies. A new method is proposed to estimate theoretical systematic uncertainties. The results are compared to the expected evolution of α s .

Measurements of αs from event shapes and the four-jet rate

New results from measurements of the strong coupling constant α s ( M Z ) at LEP are presented. In particular, a new LEP combination of results based on event-shape variables has become available, where a new method for the estimation of the theoretical uncertainty has been implemented. Furthermore, two other analyses are quoted, based on power corrections and the four-jet rate.

Eleven years of QCD at LEP

Studies of hadronic final states of e+e− annihilations, observed at the Large Electron Positron Collider LEP at CERN, are reviewed. The topics included cover measurements of αs, hadronic event shapes and hadronisation studies, tests of asymptotic freedom and of the non-Abelian gauge structure of QCD, differences between quark and gluon jets, tests of power corrections and selected results of two-photon scattering processes. The improvements obtained at LEP are demonstrated by comparing to results from the pre-LEP era. This article consists of a reproduction of slides presented at the LEPFest in October 2000, supplemented by a short descriptive text and a list of relevant references.

REVIEW OF FINAL LEP RESULTS OR A TRIBUTE TO LEP

After a comment on the performance of LEP some highlights of the LEP1 and LEP2 physics programmes are reviewed. The talk concentrates on the precision measurements at the Z resonance, two fermion production above the Z, W+ W- production, ZZ production, indirect limits on the Higgs mass, LEP contributions to the exploration of the CKM matrix, and on the LEP measurements of αs.

Experimental status of αs measurements

During the period of LEP running, the precision of determinations of the strong coupling, α s ( M Z ), has improved from ~ 10% to ~ 2–3%. The contribution made by measurements of α s using the data recorded at LEP, together with progress in theoretical calculations, is reviewed. Consistent results are obtained using a variety of different techniques.

Experimental status of αs measurements

During the period of LEP running, the precision of determinations of the strong coupling, α s ( M Z ), has improved from ~ 10% to ~ 2–3%. The contribution made by measurements of α s using the data recorded at LEP, together with progress in theoretical calculations, is reviewed. Consistent results are obtained using a variety of different techniques.

A QCD Survey: 0≤Q2≤105GeV2

Selected, recent results, primarily form collider experiments but including some fixed target experiments, are presented as a survey of Quantum Chromodynamics (QCD). The concepts of leading order and next-to-leading order QCD are introduced. Inclusive [Formula: see text] jet and dijet production and deep inelastic ep scattering at very large momentum transfer are shown to be in good agreement with perturbative QCD (pQCD). Dijet, three-jet and multi-jet results from [Formula: see text], ep, and ee colliders Q2 are also compared to pQCD. BFKR searches from all three colliders are discussed. Recent measurements of structure functions and contributions to the parton distribution functions are presented. New measurements of αs are summarized, the world average is αs = 0.1184 ± 0.0031.