We show that using renormalization-group summation to generate the QCD radiative corrections to the π − γ transition form factor, calculated with light-cone sum rules (LCSR), renders the strong ...coupling free of Landau singularities while preserving the QCD form-factor asymptotics. This enables a reliable applicability of the LCSR method to momenta well below 1 GeV2. This way, one can use the new preliminary BESIII data with unprecedented accuracy below 1.5 GeV2 to fine tune the prefactor of the twist-six contribution. Using a combined fit to all available data below 3.1 GeV2, we are able to determine all nonperturbative scale parameters and a few Gegenbauer coefficients entering the calculation of the form factor. Employing these ingredients, we determine a pion distribution amplitude with conformal coefficients (b2, b4) that agree at the 1 σ level with the data for Q2 ≤ 3.1 GeV2 and fulfill at the same time the lattice constraints on b2 at N3 LO together with the constraints from QCD sum rules with nonlocal condensates. The form-factor prediction calculated herewith reproduces the data below 1 GeV2 significantly better than analogous predictions based on a fixed-order power-series expansion in the strong coupling constant.
We consider the calculation of the pion-photon transition form factor F super(gamma*gammapi0)(Q super(2)) within light-cone sum rules focusing attention to the low-mid region of momenta. The central ...aim is to estimate the theoretical uncertainties which originate from a wide variety of sources related to (i) the relevance of next-to-next-to-leading order radiative corrections (ii) the influence of the twist-four and the twist-six term (iii) the sensitivity of the results on auxiliary parameters, like the Borel scale M super(2), (iv) the role of the phenomenological description of resonances, and (v) the significance of a small but finite virtuality of the quasireal photon. Predictions for F super(gamma*gammapi0)(Q super(2)) are presented which include all these uncertainties and found to comply within the margin of experimental error with the existing data in the Q super(2) range between 1 and 5GeV super(2), thus justifying the reliability of the applied calculational scheme. This provides a solid basis for confronting theoretical predictions with forthcoming data bearing small statistical errors.
Using QCD sum rules with nonlocal condensates, we show that the distribution amplitude of the longitudinally polarized ρ-meson may have a shorttailed platykurtic profile in close analogy to our ...recently proposed platykurtic distribution amplitude for the pion. Such a chimera distribution de facto amalgamates the broad unimodal profile of the distribution amplitude, obtained with a Dyson–Schwinger equations-based computational scheme, with the suppressed tails characterizing the bimodal distribution amplitudes derived from QCD sum rules with nonlocal condensates. We argue that pattern formation, emerging from the collective synchronization of coupled oscillators, can provide a single theoretical scaffolding to study unimodal and bimodal distribution amplitudes of light mesons without recourse to particular computational schemes and the reasons for them.
The pion-photon transition form factor is studied by employing two types of sum rules: light cone sum rules (LCSR) and anomaly sum rules (ASR). By comparing the predictions for the pion-photon ...transition form factor, obtained from these two approaches, the applicability limit of the LCSRs at low momenta is determined. Reciprocally, the ASR threshold dependence on the momentum was extracted using our LCSR-based method in combination with two different types of pion distribution amplitudes and found that at higher Q super(2) it approaches a constant.
Using a reaction model that incorporates pion bound state effects and continuum results for proton parton distributions and the pion distribution amplitude, φπ, we deliver parameter-free predictions ...for the μ+ angular distributions in πN→μ+μ−X reactions on both unpolarised and polarised targets. The analysis indicates that such angular distributions are sensitive to the pointwise form of φπ and suggests that unpolarised targets are practically more favourable. The precision of extant data is insufficient for use in charting φπ; hence, practical tests of this approach to charting φπ must await data with improved precision from new-generation experiments. The reaction model yields a nonzero single-spin azimuthal asymmetry, without reference to T-odd parton distribution functions (DFs). This may necessitate additional care when attempting to extract such T-odd DFs from data.
We provide an in-depth analysis of the
π
distribution amplitude in terms of two different Gegenbauer representations. Detailed predictions for the
π
-
γ
transition form factor are presented, obtained ...with light-cone sum rules. Various
π
distribution amplitudes are tested and the crucial role of their endpoint behavior in the form-factor analysis is discussed. Comparison with the data is given.
The leading-twist distribution amplitude for the longitudinal rho-meson was studied using QCD Sum Rules with nonlocal condensates and a spectral density which includes next-to-leading order radiative ...corrections. The obtained profile is compared with results from standard QCD sum rules, lattice QCD, holographic QCD, a light-front quark model, and the instanton liquid model. Preliminary estimates for the first two moments of the transverse ρ-meson distribution amplitude are also given.
We consider the lightcone sum-rule (LCSR) description of the pionphoton transition form factor in combination with the renormalization group of QCD. The emerging scheme represents a certain version ...of Fractional Analytic Perturbation Theory and significantly extends the applicability domain of perturbation theory towards lower momenta Q2 ≲ 1 GeV2. We show that the predictions calculated herewith agree very well with the released preliminary data of the BESIII experiment, which have very small errors just in this region, while the agreement with other data at higher Q2 is compatible with the LCSR predictions obtained recently by one of us using fixed-order perturbation theory.
We consider the pion-photon transition form factor at low to intermediate spacelike momenta within the theoretical framework of light-cone sum rules. We derive predictions which take into account all ...currently known contributions stemming from QCD perturbation theory up to the next-to-next-to-leading order (NNLO) and by including all twist terms up to order six. In order to enable a more detailed comparison with forthcoming high-precision data, we also estimate the main systematic theoretical uncertainties, stemming from various sources, and discuss their influence on the calculations — in particular the dominant one related to the still uncalculated part of the NNLO contribution. The analysis addresses, in broad terms, also the role of the twist-two pion distribution amplitude derived with different approaches.
PDF
