Determination Of Αs Research Articles

Renormalization group improvement and QCD sum rules

We summarize the results obtained for the quark masses (u,d,s,c, and b) in Refs. [1,2] and strong coupling (αs) using renormalization group (RG) improvement of the theoretical expressions and experimental inputs that enter in the QCD sum rules. We obtain mu(2GeV)=2.00−0.40+0.33MeV, md(2GeV)=4.21−0.45+0.48MeV, and ms(2GeV)=104.34−4.24+4.32MeV using Borel Laplace sum rules for the divergence of the axial vector currents. The relativistic sum rules for the moments of the heavy quark currents lead to the determination of αs(MZ)=0.1171(7), m‾c=1281.1(3.8)MeV and m‾b=4174.3(9.5)MeV.

New analyses of event shape observables in electron-positron annihilation and the determination of αs running behavior in perturbative domain

In this paper, we give comprehensive analyses for event shape observables in electron-positron annihilation by using the Principle of Maximum Conformality (PMC) which is a rigorous scale-setting method to eliminate the renormalization scheme and scale ambiguities in perturbative QCD predictions. Conventionally the renormalization scale and theoretical uncertainties in event shape observables are often evaluated by setting the scale to the center-of-mass energy sqrt{s} . The event shape distributions using this conventional scale setting are plagued by the large renormalization scale uncertainty and underestimate the experimental data. Moreover, since the renormalization scale is simply fixed to the center-of-mass energy sqrt{s} , only one value of the coupling αs at the single scale sqrt{s} can be extracted. In contrast, the PMC renormalization scales are determined by absorbing the nonconformal β contributions that govern the behavior of the running coupling via the Renormalization Group Equation (RGE). The resulting PMC scales change with event shape kinematics, reflecting the virtuality of the underlying quark and gluon subprocess. The PMC scales thus yield the correct physical behavior of the scale and the PMC predictions agree with precise event shape distributions measured at the LEP experiment. More importantly, we can precisely determine the running of the QCD coupling constant αs(Q2) over a wide range of Q2 in perturbative domain from event shape distributions measured at a single center-of-mass energy sqrt{s} .

Violations of quark-hadron duality in low-energy determinations of αs

Using the spectral functions measured in τ decays, we investigate the actual numerical impact of duality violations on the extraction of the strong coupling. These effects are tiny in the standard αs( {m}_{tau}^2 ) determinations from integrated distributions of the hadronic spectrum with pinched weights, or from the total τ hadronic width. The pinched-weight factors suppress very efficiently the violations of duality, making their numerical effects negligible in comparison with the larger perturbative uncertainties. However, combined fits of αs and duality-violation parameters, performed with non-protected weights, are subject to large systematic errors associated with the assumed modelling of duality-violation effects. These uncertainties have not been taken into account in the published analyses, based on specific models of quark-hadron duality.

High-pT results and parton density functions from ATLAS

The ATLAS detector at the LHC is an ideal experiment to rigorously test QCD in a large variety of final states. This talk will focus on recent results over a wide range of energies, which are particularly sensitive to parton density functions. We will review results with high-pT jets, photons and vector bosons. We will then discuss fits to determine parton distribution functions (PDFs) using these diverse sets of measurements from the ATLAS experiment. These measurements are used in combination with deep-inelastic scattering data from HERA. Particular attention is paid to the correlation of systematic uncertainties within and between the various ATLAS data sets and to the impact of model, theoretical and parameterisation uncertainties. Finally, we will also present results on the determination of αs.

Precise αs determination from charmonium sum rules

The strong coupling, αs, governs perturbative Quantum Chromodynamics (QCD) and is one of the free parameters of the Standard Model. We introduce a new method that allows a precise extraction of αs(mZ) from dimensionless ratios of roots of moments of the charm-quark vector correlator. The ratios we use in our analysis have a rather weak logarithmic quark-mass dependence, starting at O(αs2), and can be obtained from experimental data with good precision, since they benefit from positive correlations among the individual experimentally determined moments. We perform a careful and conservative error analysis with special emphasis on uncertainties related to the truncation of perturbation theory, treating the renormalization scales such as to ensure order-by-order convergence. Our final result, with expressions at O(αs3), is αs(mZ)=0.1168±0.0019.

Determination of \u03b1s from static QCD potential: OPE with renormalon subtraction and lattice QCD

We determine the strong coupling constant αs from the static QCD potential by matching a theoretical calculation with a lattice QCD computation. We employ a new theoretical formulation based on the operator product expansion, in which renormalons are subtracted from the leading Wilson coefficient. We remove not only the leading renormalon uncertainty of mathcal{O} (ΛQCD) but also the first r-dependent uncertainty of mathcal{O}left({Lambda}_{mathrm{QCD}}^3{r}^2right) . The theoretical prediction for the potential turns out to be valid at the static color charge distance {Lambda}_{overline{mathrm{MS}}}rlesssim 0.8 (r ≲ 0.4 fm), which is significantly larger than ordinary perturbation theory. With lattice data down to {Lambda}_{overline{mathrm{MS}}}rsim 0.09 (r ∼ 0.05 fm), we perform the matching in a wide region of r, which has been difficult in previous determinations of αs from the potential. Our final result is αs(MZ2) = 0.1179− 0.0014+ 0.0015 with 1.3% accuracy. The dominant uncertainty comes from higher order corrections to the perturbative prediction and can be straightforwardly reduced by simulating finer lattices.

Determination of αs from static QCD potential with renormalon subtraction

We determine the strong coupling constant αs(MZ) from the static QCD potential by matching a lattice result and a theoretical calculation. We use a new theoretical framework based on operator product expansion (OPE), where renormalons are subtracted from the leading Wilson coefficient. We find that our OPE prediction can explain the lattice data at ΛQCDr≲0.8. This allows us to use a larger window in matching, which leads to a more reliable determination. We obtain αs(MZ)=0.1179−0.0014+0.0015.

Precision Determination of αs from Lattice QCD

We present an overview of the recent lattice determination of the QCD coupling α s by the ALPHA Collaboration. The computation is based on the non-perturbative determination of the Λ -parameter of = 3 QCD, and the perturbative matching of the = 3 and = 5 theories. The final result: α s ( m Z ) = 0 . 11852 ( 84 ) , reaches sub-percent accuracy.

Features of measurements of thermoradiation characteristics of αs, ε materials on the integrated photometer FM-59M and thermoradiometer TRM-I

The paper describes the features of the measurements of the thermoradiation characteristics of materials: the absorption coefficient of solar radiation-αs and the emission coefficient – ε on compact integral devices: the FM 59M photometer and the TPM-I thermoradiometer. The results of the determination of αs, ε are compared. on the FM-59M photometer and TPM-I thermoradiometer with the results obtained on the basis of recording the reflection spectra on highprecision spectral devices. The main errors in the determination of αs, ε on the integral photometer FM-59M and thermoradiometer TRM-I are indicated, and methods for their elimination are proposed.

Precise determination of αS(MZ) from global fits of e+e− data to NNLO+NNLL predictions

We present a comparison of the computation of energy-energy correlations and Durham algorithm jet rates in e+e− collisions at next-to-next-to-leading logarithmic accuracy matched with the next-to-next-to-leading order perturbative prediction to LEP, PEP, PETRA, SLC and TRISTAN data. With these predictions we perform global extractions of the strong coupling constant taking into account non-perturbative effects modelled with modern Monte Carlo event generators that simulate NLO QCD corrections.

The strong coupling from ALEPH tau decays

We analyze some of the different determinations of αs(mτ2) that can be found in the literature and the numerical impact of their possible weaknesses with the publicly available non-strange spectral function from ALEPH tau decays. We also introduce some novel approaches. We find very stable results and uncertainties dominated by the perturbative part. Our final value is αs(mτ2)=0.328±0.013.

The determination of αs by the ALPHA collaboration

We review the ALPHA collaboration strategy for obtaining the QCD coupling at high scale. In the three-flavor effective theory it avoids the use of perturbation theory at α∼>0.2 and at the same time has the physical scales small compared to the cutoff 1/a in all stages of the computation. The result ΛMS‾(3)=332(14)MeV is translated to αMS‾(mZ)=0.1179(10)(2) by use of (high order) perturbative relations between the effective theory couplings at the charm and beauty quark “thresholds”. The error of this perturbative step is discussed and estimated as 0.0002.

The strong coupling from ALEPH tau decays

The strong coupling from ALEPH tau decays. We use the publically available non-strange spectral function from ALEPH tau decays to critically analyze the different determinations of αs(mτ2) that can be found in the literature and the numerical impact of their possible weaknesses. We also introduce some novel approaches. We find that perturbative uncertainties dominate. Our results with different approaches are very stable. Our final value is αs(mτ2)=0.328±0.013.

αs from the top quark pair production cross section

Five measurements of the top quark pair production cross section from hadron colliders are compared with the theoretical prediction (up the next-to-next-to-leading order in QCD) in order to determine the strong coupling constant αs. The determination is performed for several PDF sets. The individual determinations of αs are combined to form a single best estimate using the Best Linear Unbiased Estimate (BLUE) procedure. Preliminary combined results indicate a higher value of αs than that determined in a previous extraction of αs from the top quark pair production cross section.

Updated determination of αs(mτ2) from τ decays

Using the most recent release of the ALEPH [Formula: see text] decay data, we present a very detailed phenomenological update of the [Formula: see text] determination. We have exploited the sensitivity to the strong coupling in many different ways, exploring several complementary methodologies. All determinations turn out to be in excellent agreement, allowing us to extract a very reliable value of the strong coupling. We find [Formula: see text] which implies [Formula: see text]. We critically revise previous work, and point out the problems flawing some recent analyses which claim slightly smaller values.

State-of-the-Art Predictions for C-parameter and a Determination of αs

The C-parameter event-shape distribution for e+e− annihilation into hadrons is computed in the framework of SCET including input from fixed-order perturbation theory. We calculate all missing ingredients for achieving N3LL resummation accuracy in the cross section, which is then matched onto O(αs3) fixed-order results. Hadronization power corrections are incorporated as a convolution with a nonperturbative shape function. Wide-angle soft radiation effects introduce an O(ΛQCD) renormalon ambiguity in the cross section, which we cure by switching to the Rgap short-distance scheme. We also include hadron mass effects, but find their effect is rather small. Performing fits to the tail of the C-parameter distribution for many center of mass energies we find that the strong coupling constant is αs(mZ)=0.1121±0.0015, with χ2/dof=0.99.

On the electroweak contribution to the matching of the strong coupling constant in the SM

The effective renormalizable theory describing electromagnetic and strong interactions of quarks of five light flavors (nf=5QCD×QED) is considered as a low-energy limit of the full Standard Model. Two-loop relation between the running strong coupling constants αs defined in either theories is found by simultaneous decoupling of electroweak gauge and Higgs bosons in addition to the top-quark. The relation potentially allows one to confront “low-energy” determination of αs with a high-energy one with increased accuracy. Numerical impact of new O(αsα) terms is studied at the MZ scale. It is shown that the corresponding contribution, although being suppressed with respect to O(αs2) terms, is an order of magnitude larger than the three-loop QCD corrections O(αs3) usually taken into account in four-loop renormalization group evolution of αs. The dependence on the matching scale is also analyzed numerically.

Determination of the top-quark pole mass and strong coupling constant from the [formula omitted] production cross section in pp collisions at [formula omitted

The inclusive cross section for top-quark pair production measured by the CMS experiment in proton–proton collisions at a center-of-mass energy of 7 TeV is compared to the QCD prediction at next-to-next-to-leading order with various parton distribution functions to determine the top-quark pole mass, mtpole, or the strong coupling constant, αS. With the parton distribution function set NNPDF2.3, a pole mass of 176.7−3.4+3.8 GeV is obtained when constraining αS at the scale of the Z boson mass, mZ, to the current world average. Alternatively, by constraining mtpole to the latest average from direct mass measurements, a value of αS(mZ)=0.1151−0.0032+0.0033 is extracted. This is the first determination of αS using events from top-quark production.

Status of the lattice and τ decay determinations of αs

The two highest precision determinations of αs(MZ2), that based on the analysis of short-distance-sensitive lattice observables, and that based on an analysis of hadronic τ decay data, have, until very recently, given results which are not in good agreement. I review new versions of these analyses which bring the two determinations into excellent agreement, and discuss prospects for additional future improvements.

Jets and [formula omitted] measurements in DIS at HERA

Recent measurements of jet production at HERA provide an excellent testing ground for perturbative QCD (pQCD) and allow improved determinations of the strong coupling, αs. A review of recent measurements of dijet and inclusive-jet cross sections in neutral current DIS (NC DIS) at HERA is presented, and the latest determinations of αs are shown.