arXiv subjects

N. G. Stefanis

Publications and source records attributed to N. G. Stefanis.

At least 91 records · Page 5Linked to original sources

arXiv2006

Perturbative logarithms and power corrections in QCD hadronic functions. A unifying approach

I present a unifying scheme for hadronic functions that comprises logarithmic corrections due to gluon emission in perturbative QCD, as well as power-behaved corrections of nonperturbative origin. The latter are derived by demanding that perturbatively resummed partonic observables should be analytic in the whole $Q^2$-plane if they are to be related to physical observables measured in experiments. I also show phenomenological consequences of this approach. The focus is on the electromagnetic pion form factor to illustrate both effects, Sudakov logarithms and power corrections in leading order of $Λ_{\rm{QCD}}^2/Q^2$. The same approach applied to the inclusive Drell-Yan cross section enables us to perform an absolutely normalized calculation of the leading power correction in $b^2Λ_{\rm{QCD}}^2$ (b being the impact parameter), which after exponentiation, gives rise to a nonperturbative Sudakov-type contribution that provides enhancement rather than suppression, hence partly counteracting the perturbative Sudakov suppression.

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arXiv2006

Analyticity and power corrections in hard-scattering hadronic functions

Demanding the analyticity of hadronic observables (calculated in terms of power series of the running coupling) as a {\it whole}, we show that they are free of the Landau singularity. Employing resummation and dispersion-relation techniques, we compute in a unifying way power corrections to two different hard-scattering functions in perturbative QCD: the electromagnetic pion form factor to leading order and the inclusive cross section of the Drell-Yan process. In the second case, the leading nonperturbative power correction in $bΛ_{\rm QCD}$ gives rise to a Sudakov-like exponential factor in the impact parameter space which provides enhancement rather than suppression.

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arXiv2006

Tagging the pion quark structure in QCD

We combine the constraints on the pion quark structure available from perturbative QCD, nonperturbative QCD (nonlocal QCD sum rules and light cone sum rules) with the analysis of current data on F_{πγγ^*}(Q^2), including recent high-precision lattice calculations of the second moment of the pion's distribution amplitude. We supplement these constraints with those extracted from the renormalon approach by means of the twist-four contributions to the pion distribution amplitude in order to further increase stability with respect to related theoretical uncertainties. We show which regions in the space of the first two non-trivial Gegenbauer coefficients a_2 and a_4 of all these constraints overlap, tagging this way the pion structure to the highest degree possible at present.

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arXiv2006

Fractional Analytic Perturbation Theory in QCD and Exclusive Reactions

Different ``analytization'' procedures for the factorized pion form factor are discussed in comparison with the standard QCD perturbation theory at NLO. It is argued that demanding the analyticity of the exclusive amplitude as a \emph{whole}, entails insensitivity of the results on all scheme and scale-setting parameters, including the factorization scale. This enables us to develop an approach of optimized perturbation theory within the MS-bar scheme and to generalize the Analytic Perturbation Theory to non-integer (fractional) powers of the strong running coupling in the complex Q^2 plane

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arXiv2005

Improving QCD Perturbative Predictions and Transverse Momentum Effects

I elaborate on the implementation of analyticity of hadronic quantities at the amplitude level and show that it amounts to the extension of Analytic Perturbation Theory of Shirkov and Solovtsov to non-integer (fractional) powers of the strong running coupling. I give evidence that at the NLO of the factorized pion's electromagnetic form factor this framework diminishes the uncertainty due to the renormalization-scheme choice and nearly removes the dependence on both the renormalization and the factorization scale. I supplement this discussion by considering effects caused by retaining transverse momenta that give rise to Sudakov factors.

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arXiv2005

QCD Analytic Perturbation Theory. From integer powers to any power of the running coupling

We propose a new generalized version of the QCD Analytic Perturbation Theory of Shirkov and Solovtsov for the computation of higher-order corrections in inclusive and exclusive processes. We construct non-power series expansions for the analytic images of the running coupling and its powers for any fractional (real) power and complete the linear space of these solutions by constructing the index derivative. Using the Laplace transformation in conjunction with dispersion relations, we are able to derive at the one-loop order closed-form expressions for the analytic images in terms of the Lerch function. At the two-loop order we provide approximate analytic images of products of powers of the running coupling and logarithms--typical in higher-order perturbative calculations and when including evolution effects. Moreover, we supply explicit expressions for the two-loop analytic coupling and the analytic images of its powers in terms of one-loop quantities that can strongly simplify two-loop calculations. We also show how to resum powers of the running coupling while maintaining analyticity, a procedure that captures the generic features of Sudakov resummation. The algorithmic rules to obtain analytic coupling expressions within the proposed Fractional Analytic Perturbation Theory from the standard QCD power-series expansion are supplied ready for phenomenological applications and numerical comparisons are given for illustration.

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arXiv2005

Analyticity properties of three-point functions in QCD beyond leading order

The removal of unphysical singularities in the perturbatively calculable part of the pion form factor--a classic example of a three-point function in QCD--is discussed. Different ``analytization'' procedures in the sense of Shirkov and Solovtsov re examined in comparison with standard QCD perturbation theory. We show that demanding the analyticity of the partonic amplitude as a \emph{whole}, as proposed before by Karanikas and Stefanis, one can make infrared finite not only the strong running coupling and its powers, but also cure potentially large logarithms (that first appear at next-to-leading order) containing the factorization scale and modifying the discontinuity across the cut along the negative real axis. The scheme used here generalizes the Analytic Perturbation Theory of Shirkov and Solovtsov to non-integer powers of the strong coupling and diminishes the dependence of QCD hadronic quantities on all perturbative scheme and scale-setting parameters, including the factorization scale.

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arXiv2005

Renormalization-group improved evolution of the meson distribution amplitude at the two-loop level

We discuss the two-loop evolution of the flavor-nonsinglet meson distribution amplitude in perturbative QCD. After reviewing previous two-loop computations, we outline the incompatibility of these solutions with the group property of the renormalization-group transformations. To cure this deficiency, we compute a correction factor for the non-diagonal part of the meson evolution equation and prove that with this modification the two-loop solution conforms with the group properties of the renormalization-group transformations. The special case of a fixed strong coupling (no Q^2 dependence) is also discussed and comparison is given to previously obtained results.

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arXiv2004

The gluon content of the $η$ and $η^{\prime}$ mesons and the $ηγ$, $η^{\prime}γ$ electromagnetic transition form factors

We compute power-suppressed corrections to the ηγand η^{\prime}γtransition form factors Q^2F_{η(η^{\prime})γ}(Q^2) arising from the end point regions x \to 0,1 by employing the infrared-renormalon approach. The contribution to the form factors from the quark and gluon content of the η,η^{\prime} mesons is taken into account using for the η-η^{\prime} mixing the SU_f(3) singlet η_1 and octet η_8 basis. The theoretical predictions obtained this way are compared with the corresponding CLEO data and restrictions on the input parameters (Gegenbauer coefficients) B_2^q(η_1), B_2^g(η_1), and B_2^q(η_8) in the distribution amplitudes for the η_1,η_8 states with one nonasymptotic term are deduced. Comparison is made with the results from QCD perturbation theory.

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arXiv2004

Pion form factor analysis using NLO analytic perturbation theory

I present results for the pion's electromagnetic form factor in the spacelike region, which implement the most advanced perturbative information currently available for this observable in conjunction with a pion distribution amplitude that agrees with the CLEO data on the pion-photon transition form factor at the $1σ$ level. I show that using for the running strong coupling and its powers their analytic versions in the sense of Shirkov and Solovtsov, the obtained predictions become insensitive to the renormalization scheme and scale setting adopted. Joining the hard contribution with the soft part on account of local duality and respecting the Ward identity at $Q^2=0$, the agreement with the available experimental data, including expectations from planned experiments at JLab, is remarkable both in trend and magnitude. I also comment on Sudakov resummation within the analytic approach.

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arXiv2004

Deep inside the pion. Reconciling QCD theory with data

Recent developments in the QCD description of the pion structure are reviewed. The CLEO pion-photon transition data analysis favors a distribution amplitude for the pion that is double-humped but endpoint-suppressed. After a short outline of the derivation of this amplitude from QCD sum rules with nonlocal condensates, we present the fully fledged analysis of the CLEO data prefaced by predictions for the $F^{γρπ}$ form factor and commenting on the inherent theoretical uncertainties due to higher twists and NNLO perturbative corrections. We supplement our discussion by considering within QCD factorization theory, the electromagnetic pion form factor at NLO accuracy on one hand, and diffractive di-jets production on the other, comparing our predictions with the respective experimental data from JLab and the Fermilab E791 collaboration. In all cases, the agreement is impressive.

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arXiv2004

The gluon content of the $η$ and $η^{\prime}$ mesons

We analyze power-suppressed contributions to singlet pseudoscalar $η$ and $η^{\prime}$ meson transition form factors. These corrections stem from endpoint singularities and help improve the agreement between QCD theory and the experimental data, in particular, at low-momentum transfers. Using the CLEO data, we extract information on the profile of the $η_1$ and $η_8$ distribution amplitudes in the $SU(3)_{F}$ octet-singlet basis employing both the one-angle and the two-angle mixing schemes. In the former scheme, we find good agreement with the CLEO data, while in the second case, our approach requires non-asymptotic profiles for these mesons.

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arXiv2004

Pion structure: from nonlocal condensates to NLO analytic perturbation theory

A pion distribution amplitude, derived from nonlocal QCD sum rules, has been employed to calculate $F_{γ^*γ\toπ}(Q^2)$ using light-cone sum rules, and $F_π(Q^2)$ in NLO QCD perturbation theory. Predictions are presented for both observables and found to be in good agreement with the corresponding data. Calculating the hard pion form factor by Analytic Perturbation Theory to two-loop order, it is shown that the renormalization-scheme and scale-setting dependencies are diminished.

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arXiv2004

Accessing the pion distribution amplitude through the CLEO and E791 data

Using QCD perturbation theory in NLO and light-cone QCD sum rules, we extract from the CLEO experimental data on the F^{γ^*γπ}(Q^{2} transition form factor constraints on the Gegenbauer coefficients a_2 and a_4, as well as on the inverse moment _π of the pion distribution amplitude. We show that both the asymptotic and the Chernyak--Zhitnitsky pion distribution amplitudes are excluded at the 3σ- and 4σ-level, respectively, while the data confirms the end-point suppressed shape of the pion DA we previously obtained with QCD sum rules and nonlocal condensates. These findings are also supported by the data of the Fermilab E791 experiment on diffractive dijet production.

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arXiv2004

Pion form factor in QCD: From nonlocal condensates to NLO analytic perturbation theory

We present an investigation of the pion's electromagnetic form factor F_π(Q^2) in the spacelike region utilizing two new ingredients: (i) a double-humped, endpoint-suppressed pion distribution amplitude derived before via QCD sum rules with nonlocal condensates--found to comply at the 1σlevel with the CLEO data on the $πγ$ transition--and (ii) analytic perturbation theory at the level of parton amplitudes for hadronic reactions. The computation of F_π(Q^2) within this approach is performed at NLO of QCD perturbation theory (standard and analytic), including the evolution of the pion distribution amplitude at the same order. We consider the NLO corrections to the form factor in the \bar{MS} scheme with various renormalization scale settings and also in the α_V-scheme. We find that using standard perturbation theory, the size of the NLO corrections is quite sensitive to the adopted renormalization scheme and scale setting. The main results of our analysis are the following: (i) Replacing the QCD coupling and its powers by their analytic images, both dependencies are diminished and the predictions for the pion form factor are quasi scheme and scale-setting independent. (ii) The magnitude of the factorized pion form factor, calculated with the aforementioned pion distribution amplitude, is only slightly larger than the result obtained with the asymptotic one in all considered schemes. (iii) Including the soft pion form factor via local duality and ensuring the Ward identity at Q^2=0, we present predictions that are in remarkably good agreement with the existing experimental data both in trend and magnitude.

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arXiv2004

QCD-based pion distribution amplitudes confronting experimental data

We use QCD sum rules with nonlocal condensates to re-calculate more accurately the moments and their confidence intervals of the twist-2 pion distribution amplitude including radiative corrections. We are thus able to construct an admissible set of pion distribution amplitudes which define a reliability region in the $a_{2}$, $a_{4}$ plane of the Gegenbauer polynomial expansion coefficients. We emphasize that models like that of Chernyak and Zhitnitsky, as well as the asymptotic solution, are excluded from this set. We show that the determined $a_{2}$, $a_{4}$ region strongly overlaps with that extracted from the CLEO data by Schmedding and Yakovlev and that this region is also not far from the results of the first direct measurement of the pion valence quark momentum distribution by the Fermilab E791 collaboration. Comparisons with recent lattice calculations and instanton-based models are briefly discussed.

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arXiv2003

Mapping out the quark structure of hadrons in QCD

In the context of QCD sum rules with non-local condensates we present a pion distribution amplitude, which is double-humped with its end-points x\to(0,1) strongly suppressed, and show that it matches the CLEO experimental data on the pion-photon transition at the 1σlevel accuracy, being also in compliance with the CELLO data. We also include some comments on the nucleon distribution amplitude and the nucleon evolution equation.

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arXiv2003

Power corrections to the space-like transition form factor F_{η^{\prime}g^{}g^{}}(Q^2,ω)

Employing the standard hard-scattering approach (HSA) in conjunction with the running-coupling (RC) method, the latter joined with the infrared-renormalon calculus, we compute power-suppressed corrections $\sim 1/Q^{2n}, n=1, 2,...$ to the massless $η^{\prime}$ meson - virtual gluon transition form factor (FF) $Q^2F_{η^{\prime}g^{*}g^{*}}(Q^2,ω)$. Contributions to the form factor from the quark and gluon components of the $η^{\prime}$ meson are taken into account. Analytic expressions for the FF's $F_{η^{\prime}gg^{*}}(Q^2,ω=\pm 1)$ and $F_{η^{\prime}g^{*}g^{*}}(Q^2,ω=0)$ are also presented, as well as Borel transforms $B[Q^{2}F_{η^{\prime}g^{*}g^{*}}](u)$ and resummed expressions. It is shown that except for $ω=\pm 1, 0$, the Borel transform contains an infinite number of infrared renormalon poles. It is demonstrated that in the explored range of the total gluon virtuality $1 \rm {GeV}^{2} \leq Q^2 \leq 25 \rm {GeV}^{2}$, power corrections found with the RC method considerably enhance the FF $F_{η^{\prime}g^{*}g^{*}}(Q^{2}, ω)$ relative to results obtained only in the context of the standard HSA with a ``frozen'' coupling.

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