arXiv subjects

A. P. Bakulev

Publications and source records attributed to A. P. Bakulev.

At least 19 recordsLinked to original sources

arXiv2013

Can we understand an auxetic pion-photon transition form factor within QCD? BaBar faces Belle

A state-of-the-art analysis of the pion-photon transition form factor is presented based on an improved theoretical calculation that includes the effect of a finite virtuality of the quasi-real photon in the method of light-cone sum rules. We carry out a detailed statistical analysis of the existing experimental data using this method and by employing pion distribution amplitudes with up to three Gegenbauer coefficients $a_2, a_4, a_6$. Allowing for an error range in the coefficient $a_6\approx 0$, the theoretical predictions for $γ^*γ\toπ^0$ obtained with nonlocal QCD sum rules are found to be in good agreement with all data that support a scaling behavior of the transition form factor at higher $Q^2$, like those of the Belle Collaboration. The data on $γ^*γ\toη/η'$ from CLEO and \babar are also reproduced, while there is a strong conflict with the auxetic trend of the \babar data above 10 GeV$^2$. The broader implications of these findings are discussed.

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arXiv2013

Pion-photon transition form factor in light-cone sum rules

We extract constraints on the pion distribution amplitude from available data on the pion-photon transition form factor in the framework of light-cone sum rules. A pronounced discrepancy $(2.7-3)σ$ between the Gegenbauer expansion coefficients extracted from the CELLO, CLEO, and Belle experimental data relative to those from BaBar is found. Predictions for the pion-photon transition form factor are presented by employing a pion distribution amplitude obtained long ago from QCD sum rules with nonlocal condensates. These predictions comply with the Belle data but disagree with those of BaBar beyond 9 GeV$^2$.

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arXiv2012

Emphasizing the different trends of the existing data for the $γ^*γ\to π^0$ transition form factor

The new data on the $γ^*γ\to π^0$ transition form factor of the Belle Collaboration are analyzed in comparison with those of BaBar (including the older data of CELLO and CLEO) using an approach based on light-cone sum rules. Performing a 2-, and a 3-parametric fit to these data, we found that the Belle and the BaBar data have no overlap at the $1σ$ level. While the Belle data agree with our predictions, the Babar data are in conflict with them.

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arXiv2012

Comparing antithetic trends of data for the pion-photon transition form factor

We perform a comparative theoretical study of the data at spacelike momentum transfer for the $γ^*γ\toπ^0$ transition form factor, just reported by the Belle Collaboration, vs. those published before by BaBar, also including the older CLEO and CELLO data. Various implications for the structure of the $π^0$ distribution amplitude vis-à-vis those data are discussed and the existing theoretical predictions are classified into three distinct categories. We argue that the actual bifurcation of the data with antithetic trends is artificial and reason that the Belle data are the better option.

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arXiv2011

Pion-photon transition form factor using light-cone sum rules: theoretical results, expectations, and a global-data fit

A global fit to the data from different collaborations (CELLO, CLEO, BaBar) on the pion-photon transition form factor is carried out using light-cone sum rules. The analysis includes the next-to-leading QCD radiative corrections and the twist-four contributions, while the main next-to-next-to-leading term and the twist-six contribution are taken into account in the form of theoretical uncertainties. We use the information extracted from the data to investigate the pivotal characteristics of the pion distribution amplitude. This is done by dividing the data into two sets: one containing all data up to 9 GeV$^2$, whereas the other incorporates also the high-$Q^2$ tail of the BaBar data. We find that it is not possible to accommodate into the fit these BaBar data points with the same accuracy and conclude that it is difficult to explain these data in the standard scheme of OCD.

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arXiv2011

Pion-photon transition---the new QCD frontier

We perform a detailed analysis of all existing data (CELLO, CLEO, BaBar) on the pion-photon transition form factor by means of light-cone sum rules in which we include the NLO QCD radiative corrections and the twist-four contributions. The NNLO radiative correction together with the twist-six contribution are also taken into account in terms of theoretical uncertainties. Keeping only the first two Gegenbauer coefficients $a_2$ and $a_4$, we show that the $1σ$ error ellipse of all data up to 9 GeV$^2$ greatly overlaps with the set of pion distribution amplitudes obtained from nonlocal QCD sum rules---within the range of uncertainties due to twist-four. This remains valid also for the projection of the $1σ$ error ellipsoid on the $(a_2,a_4)$ plane when including $a_6$. We argue that it is not possible to accommodate the high-$Q^2$ tail of the BaBar data with the same accuracy, despite opposite claims by other authors, and conclude that the BaBar data still pose a challenge to QCD.

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arXiv2010

Higher-order QCD perturbation theory in different schemes: From FOPT to CIPT to FAPT

Results on the resummation of non-power-series expansions of the Adler function of a scalar, $D_S$, and a vector, $D_V$, correlator are presented within fractional analytic perturbation theory (FAPT). The first observable can be used to determine the decay width $Γ_{H\to b\bar{b}}$ of a scalar Higgs boson to a bottom-antibottom pair, while the second one is relevant for the $e^+e^-$ annihilation cross section. The obtained estimates are compared with those from fixed-order (FOPT) and contour-improved perturbation theory (CIPT), working out the differences. We prove that although FAPT and CIPT are conceptually different, they yield identical results. The convergence properties of these expansions are discussed and predictions are extracted for the resummed series of $R_S$ and $D_V$ using one- and two-loop coupling running, and making use of appropriate generating functions for the coefficients of the perturbative series.

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arXiv2009

Nonlocal condensates and pion form factor

We review a nonlocal-condensate approach and its application in QCD sum rules for the spacelike electromagnetic pion form factor. It is shown that the nonlocality of the condensates is a key point to include nonperturbative contributions to the pion form factor.

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arXiv2009

QCD sum rules with nonlocal condensates and the spacelike pion form factor

We present a detailed investigation of the spacelike pion's electromagnetic form factor using three-point QCD sum rules that exclusively involve nonlocal condensates. Our main methodological tools are a spectral density which includes $O(α_s)$ corrections, a suitably improved Gaussian ansatz to model the distribution of the average momentum of quarks in the QCD vacuum, and a perturbative scheme that avoids Landau singularities. Using this framework, we obtain predictions for the pion form factor together with error estimates originating from intrinsic theoretical uncertainties owing to the perturbative expansion and the nonperturbative method applied. We also discuss our results in comparison with other calculations, in particular, with those following from the AdS/QCD correspondence. We find good agreement of our predictions with measurements in the range of momenta covered by the existing experimental data between 1-10 GeV$^2$.

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arXiv2009

Pion Form Factor in the NLC QCD SR approach

We present results of a calculation of the electromagnetic pion form factor within a framework of QCD Sum Rules with nonlocal condensates and using a perturbative spectral density which includes \mathcal{O}(α_s) contributions.

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arXiv2008

Resummation in (F)APT

We present new results on the summation of nonpower-series expansions in QCD Analytic Perturbation Theory (APT) in the one-loop approximation. We show how to generalize the approach suggested by one of us earlier to the cases of APT and Fractional APT (FAPT) with heavy-quark thresholds. Using this approach we analyze the Higgs boson decay $H^0\to\bar{b}b$. We produce estimations of the higher-order corrections importance in (F)APT and present a very transparent interpretation of the resummation results in terms of $Λ_\text{QCD}$ shifts.

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arXiv2008

Fractional Analytic Perturbation Theory in Minkowski space and application to Higgs boson decay into a b\bar{b} pair

We work out and discuss the Minkowski version of Fractional Analytic Perturbation Theory (FAPT) for QCD observables, recently developed and presented by us for the Euclidean region. The original analytic approach to QCD, initiated by Shirkov and Solovtsov, is summarized and relations to other proposals to achieve an analytic strong coupling are pointed out. The developed framework is applied to the Higgs boson decay into a b\bar{b} pair, using recent results for the massless correlator of two quark scalar currents in the \bar{MS} scheme. We present calculations for the decay width within MFAPT including those non-power-series contributions that correspond to the O(α_{s}^{3})-terms, taking also into account evolution effects of the running coupling and the b-quark-mass renormalization. Comparisons with previous results within standard QCD perturbation theory are performed and the differences are pointed out. The interplay between effects originating from the analyticity requirement and the analytic continuation from the spacelike to the timelike region and those due to the evolution of the heavy-quark mass is addressed, highlighting the differences to the conventional QCD perturbation theory.

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arXiv2007

Pion structure in QCD: From theory to lattice to experimental data

We describe the present status of the pion distribution amplitude (DA) as it originates from several sources: (i) a nonperturbative approach based on QCD sum rules with nonlocal condensates, (ii) an $O(α_s)$ QCD analysis of the CLEO data on F^{γγ^*π}(Q^2) with asymptotic and renormalon models for higher twists, and (iii) recent high-precision lattice QCD calculations of the second moment of the pion DA. We show predictions for the pion electromagnetic form factor, obtained in analytic QCD perturbation theory, and compare it with the JLab data on F_π(Q^2). We also discuss in this context an improved model for nonlocal condensates in QCD and show its consequences for the pion DA and the γγ^*\toπtransition form factor. We include a brief analysis of meson-induced massive lepton (muon) Drell--Yan production for the process π^{-}N\toμ^{+}μ^{-}X, considering both an unpolarized nucleon target and longitudinally polarized protons.

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arXiv2007

Polarized and unpolarized $μ$-pair meson-induced Drell--Yan production and the pion distribution amplitude

We present a detailed analysis of meson-induced massive lepton (muon) Drell--Yan production for the process $π^{-}N\toμ^{+}μ^{-}X$, considering both an unpolarized nucleon target and longitudinally polarized protons. Using a QCD framework, we focus on the angular distribution of $μ^+$, which is sensitive to the shape of the pion distribution amplitude, the goal being to test corresponding results against available experimental data. Predictions are made, employing various pion distribution amplitudes, for the azimuthal angle dependence of the $μ^{+}$ distribution in the polarized case, relevant for the planned COMPASS experiment. QCD evolution is given particular attention in both considered cases.

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arXiv2006

Pion distribution amplitude and form-factors: Improved Gaussian model of QCD vacuum

We describe the present status of the pion distribution amplitude (DA) as it originated from several sources: (i) a nonperturbative approach, based on QCD sum rules with nonlocal condensates, (ii) an O(α_s) QCD analysis of the CLEO data on F^{γγ^*π}(Q^2) with asymptotic and renormalon models for higher twists, and (iii) recent high-precision lattice QCD calculations of the second moment of the pion DA. Then we show the comparison of the results for the pion electromagnetic form factor, obtained in analytic perturbation theory, with JLab data on F_π(Q^2). After that we introduce the improved model for nonlocal condensates in QCD and show its consequences for the pion DA and γγ^*\toπtransition form factor. In order to facilitate possible applications of BMS and improved ``bunches'' we suggest approximate analytic descriptions of their boundaries.

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arXiv2006

Pion quark structure in QCD

We describe the present status of the pion distribution amplitude as it originated from two sources: (i) a nonperturbative approach, based on QCD sum rules with nonlocal condensates and (ii) a NLO QCD analysis of the CLEO data on F^{γγ^*π}(Q^2), supplemented by the recent high-precision lattice calculations of the second moment of the pion distribution amplitude.

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arXiv2006

Self-consistent Gaussian model of nonperturbative QCD vacuum

We show that the minimal Gaussian model of nonlocal vacuum quark and quark-gluon condensates in QCD generates the non-transversity of vector current correlators. We suggest the improved Gaussian model of the nonperturbative QCD vacuum, which respects QCD equations of motion and minimizes the revealed gauge-invariance breakdown. We obtain the refined values of pion distribution amplitude (DA) conformal moments using the improved QCD vacuum model and construct the allowed region for Gegenbauer coefficients a_2 and a_4 of the pion DA.

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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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