M. V. Polyakov

Exotic anti-decuplet of baryons: prediction from chiral solitons

We predict an exotic Z+ baryon (having spin 1/2, isospin 0 and strangeness +1) with a relatively low mass of about 1530 MeV and total width of less than 15 MeV. It seems that this regionof masses has avoided thorough searches in the past.

Hard exclusive electroproduction of two pions and their resonances

Abstract We study the hard exclusive production of two pions in the virtual photon fragmentation region with various invariant masses including the resonance region. The amplitude is expressed in terms of two-pion light cone distribution amplitudes (2πDAs). We derive dispersion relations for these amplitudes, which enable us to fix them completely in terms of ππ scattering phases and a few low-energy subtraction constants determined by the effective chiral Lagrangian. Quantitative estimates of the resonance as well as ππ background DAs at low normalization point are made. We also prove a certain new soft pion theorem relating two-pion DAs to the one-pion DA. Crossing relations between 2πDAs and parton distributions in the pion are discussed. We demonstrate that by studying the shape of the ππ mass spectra (not the absoloute cross section!) in diffractive electroproduction one can extract the deviation of the meson (π, π, etc.) wave functions from their asymptotic form 6 z (1 - z ) and hence get important information about the structure of mesons. Also we discuss how the moments of the quark distributions in the pion can be measured in the hard diffraction. We suggest (as an alternative to Sodings) a parametrization of ππ spectra which is suitable for large photon virtuality.

NUCLEON PARTON DISTRIBUTIONS AT LOW NORMALIZATION POINT IN THE LARGE NC LIMIT

Abstract At large N c the nucleon can be viewed as a soliton of the effective chiral lagrangian. This picture of nucleons allows for a consistent non-perturbative calculation of the leading-twist parton distributions at a low normalization point. We derive general formulae for the polarized and unpolarized distributions (singlet and non-singlet) in the chiral quark-soliton model. The consistency of our approach is demonstrated by checking the baryon number, isospin and total momentum sum rules, as well as the Bjorken sum rule. We present numerical estimates of the quark and antiquark distributions and find reasonable agreement with parameterizations of the data at a low normalization point. In particular, we obtain a sizeable fraction of antiquarks, in agreement with the phenomenological analysis.At large N_c the nucleon can be viewed as a soliton of the effective chiral lagrangian. This picture of nucleons allows a consistent nonperturbative calculation of the leading-twist parton distributions at a low normalization point. We derive general formulae for the polarized and unpolarized distributions (singlet and non-singlet) in the chiral quark-soliton model. The consistency of our approach is demonstrated by checking the baryon number, isospin and total momentum sum rules, as well as the Bjorken sum rule. We present numerical estimates of the quark and antiquark distributions and find reasonable agreement with parametrizations of the data at a low normalization point. In particular, we obtain a sizeable fraction of antiquarks, in agreement with the phenomenological analysis.

Hadronic matrix elements of gluon operators in the instanton vacuum

Abstract We propose a method to evaluate hadronic matrix elements of QCD gluon operators in the instanton vacuum. We construct the ground state of the interacting instanton ensemble for nonzero ϑ-angle using a variational principle. A method to study the ϑ-dependence of observables on the lattice is suggested. We then derive the effective fermion action, which allows us to calculate hadronic correlation functions in a 1/Nc expansion (Nambu-Jona-Lasinio type effective fermion theory). Gluon operators are systematically represented as effective fermion operators. Physical matrix elements are obtained after integrating the correlation functions over fluctuations of the numbers of instantons. The influence of the fermion determinant on the topological susceptibility is taken into account. Our effective description gives matrix elements fully consistent with the trace and U(1)A anomalies. The approach allows us to consistently evaluate the nucleon matrix elements of various gluon and mixed quark-gluon operators in a chiral soliton picture of the nucleon.

Generalized parton distributions and strong forces inside nucleons and nuclei

Abstract We argue that generalized parton distributions (GPDs), accessible in hard exclusive processes, carry information about the spatial distribution of forces experienced by quarks and gluons inside hadrons. This way the measurements of hard exclusive processes open a possibility for direct “measurements” of strong forces in different parts of nucleons and nuclei. Also such studies open avenue for addressing questions of the properties of the quark (gluon) matter inside hadrons and nuclei. We give a simple example of relations between GPDs and properties of “nuclear matter” in finite nuclei.

Deeply virtual Compton scattering 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. PACS : 13.60.Fz; 12.38.Bx; 13.60.-r

On the pion distribution amplitude shape

We argue that the recent BaBar data on γ → π e.m. transition form factor at large photon virtuality supports the idea that pion distribution amplitude (DA) is close to unity with ϕπ′(0)/6 ≫ 1 at a normalization point of μ = 0.6–0.8 GeV. Such pion DA can be obtained in the effective chiral quark model. The possible flat shape of the pion DA implies that the standard expansion of the DA in Gegenbauer polynomials can be divergent. On basis of chiral models we predict that the two-pion DA should exhibit anomalous endpoint behaviour for pions in the S-wave and that such feature is absent for higher partial waves. The latter implies that the ρ, f2, etc. meson DAs have no anomalous endpoint behaviour. Possible implications of such pion DA for other hard exclusive processes are shortly discussed.

Deeply virtual Compton scattering amplitude in the parton model

We compute amplitude of deeply virtual Compton scattering in the parton model. We found that the amplitude up to the accuracy O(1/Q) depends on new skewed parton distributions (SPDs). These additional contributions make the DVCS amplitude explicitly transverse.We compute amplitude of deeply virtual Compton scattering in the parton model. We found that the amplitude up to the accuracy O(1/Q) depends on new skewed parton distributions (SPD’s). These additional contributions make the DVCS amplitude explicitly transverse.

Pion and photon light cone wave functions from the instanton vacuum

The leading-twist wave functions of the pion and the photon at a low normalization point are calculated in the effective low--energy theory derived from the instanton vacuum. The pion wave function is found to be close to the asymptotic one, consistent with the recent CLEO measurements. The photon wave function is non-zero at the endpoints. This different behavior is a consequence of the momentum dependence of the dynamical quark mass suggested by the instanton vacuum. We comment on the relation of meson wave functions and off-forward parton distributions in this model.

On photoexcitation of baryon antidecuplet

Abstract.We show that the photoexcitation of the nonexotic members baryon antidecuplet, suggested by the soliton classification of low-lying baryons, is strongly suppressed on the proton target. The process occurs mostly on the neutron target. This qualitative prediction can be useful in identifying the nonexotic members of the antidecuplet in the known baryon spectrum. We also analyze the interrelation between photocouplings of various baryon multiplets in the soliton picture and in the nonrelativistic quark model.