arXiv subjects

S. V. Mikhailov

Publications and source records attributed to S. V. Mikhailov.

At least 73 records · Page 4Linked to original sources

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

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

What is this thing called pion distribution amplitude? From theory to data

We discuss the status of the pion distribution amplitude (DA) from analyzing the CLEO experimental data in the context of QCD sum-rule techniques and QCD perturbation theory at the NLO accuracy. The constraints extracted this way for the Gegenbauer coefficients a_2 and a_4 exclude Φ_{CZ} at the 4σlevel, while Φ_{asy} is outside the 3σerror ellipse. These data provide strong support for the type of endpoint-suppressed, double-humped pion DA we derived via QCD sum rules with nonlocal condensates and favor a value of the vacuum quark virtuality λ_q^2 \simeq 0.4 GeV^2. This pion DA is in agreement with the E791 data, though these experimental results should be viewed carefully and further confirmation is necessary for a more accurate judging of pion DAs from them.

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arXiv2003

CLEO and E791 data: A smoking gun for the pion distribution amplitude?

The CLEO experimental data on the $πγ$ transition are analyzed to next-to-leading order accuracy in QCD perturbation theory using light-cone QCD sum rules. By processing these data along the lines proposed by Schmedding and Yakovlev, and recently revised by us, we obtain new constraints for the Gegenbauer coefficients $a_{2}$ and $a_{4}$, as well as for the inverse moment $x^{-1}$ of the pion distribution amplitude (DA). The former determine the pion DA at low momentum scale, the latter is crucial in calculating pion form factors. From the results of our analysis we conclude that the data confirm the end-point suppressed shape of the pion DA we previously obtained with QCD sum rules and nonlocal condensates, while the exclusion of both the asymptotic and the Chernyak--Zhitnitsky DAs is reinforced at the $3σ$- and $4σ$-level, respectively. The reliability of the main results of our updated CLEO data analysis is demonstrated. Our pion DA is checked against the di-jets data from the E791 experiment, providing credible evidence for our results far more broadly.

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arXiv2003

Unbiased analysis of CLEO data at NLO and pion distribution amplitude

We discuss different QCD approaches to calculate the form factor F^{γ^*γπ}(Q^2) of the γ^*γ\toπ^{0} transition giving preference to the light-cone QCD sum rules (LCSR) approach as being the most adequate. In this context we revise the previous analysis of the CLEO experimental data on F^{γ^*γπ}(Q^{2}) by Schmedding and Yakovlev. Special attention is paid to the sensitivity of the results to the (strong radiative) α_s-corrections and to the value of the twist-four coupling δ^2. We present a full analysis of the CLEO data at the NLO level of LCSRs, focusing particular attention to the extraction of the relevant parameters to determine the pion distribution amplitude, i.e., the Gegenbauer coefficients a_2 and a_4. Our analysis confirms our previous results and also the main findings of Schmedding and Yakovlev: both the asymptotic, as well as the Chernyak--Zhitnitsky pion distribution amplitudes are completely excluded by the CLEO data. A novelty of our approach is to use the CLEO data as a means of determining the value of the QCD vacuum non-locality parameter λ^2_q = <\bar{q}D^2q>/<\bar{q}q> =0.4 GeV^2, which specifies the average virtuality of the vacuum quarks.

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arXiv2002

Lattice measurements of nonlocal quark condensates, vacuum correlation length, and pion distribution amplitude in QCD

Recent data of lattice measurements of the gauge-invariant nonlocal scalar quark condensates are analyzed to extract the short--distance correlation length, 1/λ_q, and to construct an admissible Ansatz for the condensate behaviour in a coordinate space. The correlation length values for both the quenched and full-QCD cases appear in a good agreement with the well-known QCD SR estimates of the mixed quark-gluon condensate, 2 λ_q^2 = <\bar{q}(igσ^{μν}G_{μν})q>/<\bar{q}q> = 0.8 - 1.1 GeV^2. We test two different Ansatzes for a nonlocal quark condensate and trace their influence on the twist-2 pion distribution amplitude by means of QCD SRs. The main features of the pion distribution amplitude are confirmed by the CLEO experimental results.

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arXiv2001

On a QCD-based pion distribution amplitude vs. recent experimental data

Using QCD sum rules with nonlocal condensates the twist-2 pion distribution amplitude is determined by means of its moments and their confidence intervals, including also radiative corrections. An admissible set of pion distribution amplitudes is constructed in the a_2, a_4 plane of the Gegenbauer polynomial expansion coefficients. The determined a_2, a_4 region strongly overlaps with that extracted from the CLEO data by Schmedding and Yakovlev. Comparisons are given with results from Fermilab experiment E791 and recent lattice calculations.

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arXiv2001

New shapes of light-cone distributions of the transversely polarized ρ-mesons

We re-analyze the leading twist light-cone distributions for transversely polarized ρ-, ρ'- and b_1-mesons in the framework of QCD sum rules with nonlocal condensates. Using different kinds of sum rules to obtain reliable predictions, we estimate the 2-, 4-, 6- and 8-th moments for transversely polarized ρ- and ρ'-meson distributions and re-estimate tensor couplings f^T_{ρ,ρ',b_1}. We stress that the results of standard sum rules also support our estimation of the second moment of the transversely polarized ρ-meson distribution. New models for light-cone distributions of these mesons are constructed. Phenomenological consequences from these distributions are briefly discussed. Our results are compared with those found by Ball and Braun (1996), and the latter is shown to be incomplete.

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arXiv2000

New shapes of the rho-meson light-cone distribution amplitudes: how can they influence the B --> rho e nu decay form factors

We present new models of the rho-meson leading-twist light-cone distribution amplitudes based on the QCD sum rule approach with nonlocal condensates. Their shapes differ noticeably from that known in the literature. The phenomenological consequences for physically important process B --> rho e nu are discussed in the framework of the light-cone sum rules. The results are compared with those found recently by P.Ball and V.M.Braun (1997).

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arXiv2000

A multiloop improvement of non-singlet QCD evolution equations

An approach is elaborated for calculation of "all loop" contributions to the non-singlet evolution kernels from the diagrams with renormalon chain insertions. Closed expressions are obtained for sums of contributions to kernels $P(z)$ for the DGLAP equation and $V(x,y)$ for the "nonforward" ER-BL equation from these diagrams that dominate for a large value of $b_0$, the first $β$-function coefficient. Calculations are performed in the covariant $ξ$-gauge in a MS-like scheme. It is established that a special choice of the gauge parameter $ξ=-3$ generalizes the standard "naive nonabelianization" approximation. The solutions are obtained to the ER-BL evolution equation (taken at the "all loop" improved kernel), which are in form similar to one-loop solutions. A consequence for QCD descriptions of hard processes and the benefits and incompleteness of the approach are briefly discussed.

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arXiv1999

QCD vacuum tensor susceptibility and properties of transversely polarized mesons

We re-estimate the tensor susceptibility of QCD vacuum, $χ$, and to this end, we re-estimate the leptonic decay constants for transversely polarized $ρ$- ,~$ρ'$- and b_1-mesons. The origin of the susceptibility is analyzed using duality between $ρ$- and b_1- channels in a 2-point correlator of tensor currents. We confirm the results in Nucl.Phys. B283(1987)706 for the 2-point correlator of tensor currents and disagree with Phys.Lett. B395(1997)307 on both OPE expansion and the value of QCD vacuum tensor susceptibility. Using our value for the latter we determine new estimations of nucleon tensor charges related to the first moment of the transverse structure function h_1 of a nucleon.

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arXiv1999

Gluon Field Strength Correlation Functions within a Constrained Instanton Model

We suggest a constrained instanton (CI) solution in the physical QCD vacuum which is described by large-scale vacuum field fluctuations. This solution decays exponentially at large distances. It is stable only if the interaction of the instanton with the background vacuum field is small and additional constraints are introduced. The CI solution is explicitly constructed in the ansatz form, and the two-point vacuum correlator of gluon field strengths is calculated in the framework of the effective instanton vacuum model. At small distances the results are qualitatively similar to the single instanton (SI) case, in particular, the form factor $D_{1}$ is small, which is in agreement with the lattice calculations.

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arXiv1998

The $ρ$-meson and related meson wave functions in QCD sum rules with nonlocal condensates

We apply the nonlocal condensate formalism to construct generalized sum rules (including $O(\as)$-radiative corrections) for $π$-, $ρ$- and $ρ'$-meson wave functions of twist 2. Besides, we predict the lepton decay constant $f_{ρ'}$ and estimate the mass of $ρ'$-meson. For all these mesons we obtain the first 10 moments of longitudinal wave functions, suggest the models for them and discuss their properties. We consider the peculiarities of the QCD sum rules with nonlocal condensates for transverse $ρ$- and $b_1$-meson wave functions. These results are compared with those found by Ball and Braun and by Chernyak and Zhitnitsky.

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arXiv1998

The non-singlet QCD evolution kernels improved by renormalon chain contributions

Closed expressions are presented for the contributions to QCD non-singlet forward evolution kernels $P(z)$ for the DGLAP equation and to $V(x,y)$ for non-forward (ER-BL) evolution equation for a certain class of diagrams which include renormalon chains. Calculations are performed in covariant $ξ$-gauge, in the \MSbar scheme. The assumption of "naive nonabelianization" approximation for kernel calculations is discussed, and a special choice of the gauge parameter $ξ=-3$ is analyzed in this context. Partial solutions to the ER-BL evolution equation are obtained.

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arXiv1998

Renormalon chains contributions to the non-singlet evolutional kernels in QCD

Contributions to QCD non-singlet evolution kernels $P(z)$ for the DGLAP equation and $V(x,y)$ for the Brodsky-Lepage evolution equation are calculated for a certain class of diagrams which include renormalon chains. Closed expressions are presented for the sums of these contributions that dominate for a large number of flavors $N_f \gg 1$. Calculations are performed in covariant $ξ$-gauge, in the \MSbar scheme. The assumption of "naive nonabelianization" approximation for kernel calculations is discussed. The partial solution to the Brodsky-Lepage evolution equation is obtained.

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arXiv1997

Renormalon chains contributions to the non-singlet evolutional kernels in $ϕ^3_{6}$ and QCD

The contributions to non-singlet evolution kernels $P(z)$ for the DGLAP equation and $V(x,y)$ for the Brodsky-Lepage evolution equation are calculated for certain classes of diagrams which include the renormalon chains. Closed expressions are obtained for the sums of contributions associated with these diagram classes. Calculations are performed in the $[ϕ^3]_6$ model and QCD in the \MSbar scheme. The contribution for one of the classes of diagrams dominates for a number of flavors $N_f \gg 1$. For the latter case, a simple solution to the Brodsky-Lepage evolution equation is obtained.

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arXiv1997

Continuous mass-dependent analysis of the non-singlet $xF_3$ CCFR data

We consider the issue of an accurate description of the evolution of the non-singlet structure function moments $M_n(Q)$ near heavy quark threshold. To this aim we propose a simple modification of the standard massless \MSbar scheme approach to the next-to-leading QCD analysis of DIS data. We apply it to the processing of the modern CCFR'97 data for $xF_3$ structure function and extract the value of $α_s(M_z) \approx 0.122 \pm 0.004$ We check also the consistency of light gluino hypothesis with CCFR'97 data.

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