arXiv subjects

N. Kivel

Publications and source records attributed to N. Kivel.

51 records · Page 3Linked to original sources

arXiv2002

On hard electroproduction of mesons: helicity flip amplitudes with tensor gluon contributions

We consider hard electroproduction of mesons at twist-3 level and focus our attention on helicity flip amplitudes corresponding to the production of transversely polarized mesons by transverse photons. We demonstrate that helicity-flip amplitude for the transversely polarized particles are closely related to the contributions of the matrix elements of the tensor gluon operator which can serve as pure gluonic probe of the hadrons. Using standard factorisation approach and Wanzura-Wilczek approximation for the twist-3 matrix elements we find that such amplitudes do not have divergencies from the endpoint regions which appear in the other twist-3 amplitudes.

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arXiv2001

Twist-4 photon helicity-flip amplitude in DVCS on a nucleon in the Wandzura-Wilczek approximation

We computed twist-4 part of the photon spin-flip amplitude in deeply virtual Compton scattering on a nucleon in the Wandzura-Wilczek approximation. We found a factorizable contribution, which arises from photon scattering on quarks with non-zero angular momentum along the collision axis. As the genuine twist-2 amplitude arises at the NLO, for moderate virtualities of the hard photon, $Q^2 \le 10$ GeV$^2$, kinematical twist-4 correction can give numerically important contribution to the photon helicity-flip amplitude.

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arXiv2001

DVCS on the nucleon to the twist-3 accuracy

The amplitude of the deeply virtual Compton scattering off nucleon is computed to the twist-3 accuracy in the Wandzura-Wilczek (WW) approximation. The result is presented in the form which can be easily used for the analysis of DVCS observables.

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arXiv2000

Hard Exclusive Production of Tensor Mesons

We point out that hard exclusive production of tensor mesons $f_2(1270)$ with helicity $λ=\pm 2$ is dominated by the gluon component in the meson wave function and can be used to determine gluon admixture in tensor mesons in a theoretically clean manner. We present a detailed analysis of the tensor meson distribution amplitudes and calculate the transition form factor $γ+γ^*\to f_2(1270)$ for one real and one virtual photon.

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arXiv2000

DVCS on the nucleon : study of the twist-3 effects

We estimate the size of the twist-3 effects on deeply virtual Compton scattering (DVCS) observables, in the Wandzura-Wilczek approximation. We present results in the valence region for the DVCS cross sections, charge asymmetries and single spin asymmetries, to twist-3 accuracy.

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arXiv2000

Wandzura-Wilczek approximation for the twist-3 DVCS amplitude

We present a derivation of Wandzura-Wilczek (WW) like relations for skewed parton distributions. It is demonstrated for photon-pion scattering that the skewed twist-3 parton distributions contributing to the DVCS amplitude have discontinuities at the points $x=\pmξ$ in the WW approximation. This may lead to a violation of factorisation for the twist-3 DVCS amplitude with transverse polarization of the virtual photon. We show, however, that the contribution of the divergencies to the scattering of a transversely polarized virtual photon affects DVCS observables only at order $1/Q^2$ and can be neglected at twist-3 accuracy. For the scattering of a longitudinally polarized photon the twist-3 amplitude is free of such divergencies.

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arXiv2000

Twist-3 contribution to the $γ^*γ\to ππ$ amplitude in the Wandzura-Wilczek approximation

We have calculated the Wandzura-Wilczek contribution to the twist-3 part of $γ^*γ\to 2π$ amplitude. It describes interaction of the longitudinally polarized virtual photon with the real one, and it is suppressed by 1/Q, where $Q^2$ is the virtuality of the $γ^*$, as compared to the twist-2 contribution. We have found that, in the Wandzura-Wilczek approximation, factorization applies to the twist-3 amplitude.

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arXiv2000

Power corrections to the process $γγ^*\to ππ$ in the Light-Cone Sum Rules approach

We applied QCD Light Cone Sum Rules to estimate power corrections to the helicity-conserving amplitude in the process $γ^*γ\to ππ$. We found that above $Q^2 \sim 4$ GeV$^2$ power corrections are numerically small and the twist-2 part dominates.The amplitude can be reliably calculated in this region using models of $2 π$ distribution amplitudes as an input. We found that the magnitude of the NLO corrections depends rather strongly on the normalization of the gluonic distribution amplitude.

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arXiv1999

NLO corrections and contribution of a tensor gluon operator to the process $γ^*γ\to ππ$

We have calculated the NLO corrections for the leading twist amplitude of the reaction $γ^*γ\toππ$ at large virtuality of the photon and at $W^2\ll Q^2$. With a simple model for the two-pion quark and gluon distribution amplitudes we have estimated that the NLO effects may reduce the leading-order amplitude by as much as 30%. We have found that besides the usual twist-2 quark and gluon operators the NLO amplitude receives contribution from a distribution associated with twist-2 tensor gluon operator. The new gluon distribution can be investigated by studying the azimuthal asymmetry of the cross section for $e+γ\to e'+ππ$.

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arXiv1999

Conformal string operators and skewed parton distributions

We discuss skewed parton distributions in the coordinate space. Solution of the corresponding LO evolution equation is constructed in terms of eigenfunctions of the evolution kernel and its relation to the conformal symmetry is explained.

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arXiv1999

On scale dependence of QCD string operators

We have obtained a general solution of evolution equations for QCD twist-2 string operators in form of expansion over complete set of orthogonal eigenfunctions of evolution kernels in coordinate-space representation. In the leading logarithmic approximation the eigenfunctions can be determined using constraints imposed by conformal symmetry. Explicit formulae for the LO scale-dependence of quark and gluon twist-2 string operators are given.

hep-ph↗

arXiv1999

Conformal string operators and evolution of skewed parton distributions

We have investigated skewed parton distributions in coordinate space. We found that their evolution can be described in a simple manner in terms of non-local, conformal operators introduced by Balitsky and Braun. The resulting formula is given by a Neumann series expansion. Its structure resembles, for all values of the asymmetry parameter, the well-known solution of the ERBL equation in the momentum space. Performing Fourier transformation we have reproduced known results for evolution of momentum-space distributions.

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arXiv1998

The Operator Product Expansion, Non-perturbative Couplings and the Landau Pole: Lessons from the O(N) $σ$-Model

We obtain the operator product expansion of the self-energy in the O(N) non-linear $σ$-model to all orders in the coupling and the large momentum, and to next-to-leading order in 1/N. In the light of this result we discuss recent suggestions that there may be additional power corrections from short distances, associated with defining the coupling constant non-perturbatively. The non-linear $σ$-model provides no evidence for such `non-standard' power corrections. We also find that the OPE converges for sufficiently large external momentum, presumably because there are no multi-particle thresholds at arbitrarily high energies in the 1/N expansion.

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arXiv1997

Resummation of (-B(0) Alpha-s)**N Corrections to the Photon - Meson Transition Form-Factor Gamma* + Gamma ---> Pi0

We have resummed all the (-b_0 alpha_s)^n contributions to the photon-meson transition form factor F_{gamma pi}. To do this, we have used the assumption of `naive nonabelianization' (NNA). Within NNA, a series in (N_f alfa_s)^n is interpreted as a series in (-b_0 alpha_S)^n by means of the restoration of the full first QCD beta-function coefficient -b_0 by hand. We have taken into account corrections to the leading order coefficient function and to the evolution of the distribution function. Due to conformal constraints, it is possible to find the eigenfunctions of the evolution kernel. It turns out that the nondiagonal corrections are small, and neglecting them we obtained a representation for the distribution function with multiplicatively renormalized moments. For a simple shape of the distribution function, which is close to the asymptotic shape, we find that the radiative correction decrease the LO by 30 % and the uncertainty in the resummation lies between 10 % and 2 % for Q^2 between 2 and 10 GeV^2.

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arXiv1997

Large-order Behaviour due to Ultraviolet Renormalons in QCD

Ultraviolet renormalons, contrary to their infrared counterparts, lead to a universal contribution to the large-order behaviour of perturbative expansions in QCD. In this letter, we determine nature of the leading ultraviolet renormalon singularity for the inclusive hadroproduction cross section in $e^+ e^-$ annihilation, for hadronic $τ$ decays and the moments of deep-inelastic scattering structure functions. We comment on the relevance of ultraviolet renormalons to estimates in low orders of perturbation theory.

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