arXiv subjects
Publications and source records attributed to P. V. Pobylitsa.
New positivity bounds are derived for generalized (off-forward) parton distributions using the impact parameter representation. These inequalities are stable under the evolution to higher normalization points. The full set of inequalities is infinite. Several particular cases are considered explicitly.
Positivity constraints are derived for the generalized parton distributions (GPDs) of spin-1/2 hadrons. The analysis covers the full set of eight twist-2 GPDs. Several new inequalities are obtained which constrain GPDs by various combinations of usual (forward) unpolarized and polarized parton distributions including the transversity distribution.
Positivity bounds for twist-two parton distributions in multicolored QCD are stronger than their analogs at finite $N_c$. They include the enhanced large $N_{c}$ version of Soffer inequality. These bounds are compatible with the DGLAP evolution to higher normalization points.
New positivity bounds are derived for quark and gluon generalized parton distributions $H$ and $E$ in spin 1/2 hadrons.
We prove new soft pion theorem for the near threshold pion production by a hard electromagnetic probe. This theorem relates various near threshold pion production amplitudes to the nucleon distribution amplitudes. The new soft pion theorem is in a good agreement with the SLAC data for F_2^p(W,Q^2) for W^2 < 1.4 GeV^2 and 7 < Q^2 < 30.7 GeV^2.
We compute the quark and antiquark transversity distributions in the nucleon at a low normalization point of 600 MeV in the large-$N_c$ limit, where the nucleon can be described as a soliton of an effective chiral theory (chiral quark-soliton model). The flavor-nonsinglet distributions, $δu(x) - δd(x)$ and $δ\bar u(x) - δ\bar d(x)$, appear in leading order of the $1/N_c$-expansion, while the flavor-singlet distributions, $δu(x) + δd(x)$ and $δ\bar u(x) + δ\bar d(x)$, are non-zero only in next-to-leading order. The transversity quark and antiquark distributions are found to be significantly different from the longitudinally polarized distributions $Δu (x) \pm Δd (x)$ and $Δ\bar u (x) \pm Δ\bar d (x)$, respectively, in contrast to the prediction of the naive non-relativistic quark model. We show that this affects the predictions for the spin asymmetries in Drell-Yan pair production in transversely polarized pp and ppbar collisions.
We show that in the large N_c limit four-quark twist-4 distributions in the pion can be expressed in terms of twist-2 pion distribution amplitude. This allows us to compute the isospin-2 structure function of the pion F_2^{I=2}(x_B) in the large N_c limit. The method can be easily applied to other mesons as well.
We present the QCD analysis of the behavior of quark and gluon distributions in the limit of a large number of colors N_c. We show that the results agree qualitatively with the phenomenological data. In particular we draw attention to the fact that the polarized gluon distribution is suppressed at large N_c compared to the unpolarized one.
We derive new positivity bounds on spin-dependent parton distributions in multicolored QCD. They are stronger than Soffer inequality. We check that the new inequalities are stable under one-loop DGLAP evolution to higher normalization points.
In these short notes we present a parametrization of the results obtained in the chiral quark-soliton model for polarized antiquark distributions $Δ\bar u$, $Δ\bar d$ and $Δ\bar s$ at a low normalization point around mu=0.6 GeV.
We have calculated the leading order amplitude of hard diffractive electroproduction of two pions in lepton nucleon scattering. At the leading twist level a pion pair can be produced only in an isospin one or zero state. We have shown that isoscalar states are produced dominantly for x_{Bj}>0.3 and with an invariant mass of the two pions close to the threshold (S-wave) and in the f_2 resonance region (D-wave). These isoscalar pion pairs are dominantly produced by two collinear gluons. Comparing the production of charged and neutral pion pairs as a function of x_{Bj} and m_pipi one can get information about the gluonic component of two-pion distribution amplitudes.
In the framework of the chiral quark-soliton model of the nucleon we investigate the properties of the polarized quark distribution. In particular we analyse the so called anomalous difference between the representations of the quark distribution functions in terms of occupied and non-occupied quark states. By an explicit analytical calculation it is shown that this anomaly is absent in the polarized isoscalar distribution Δu + Δd, which is ultaviolet finite. In the case of the polarized isovector quark distribution Δu - Δd the anomaly can be cancelled by a Pauli-Villars subtraction which is also needed for the regularization of the ultraviolet divergence.
The amplitude for hard exclusive pseudoscalar meson electroproduction off nucleon (nuclear) targets is computed in QCD within the leading $α_s \ln {Q^2/λ_{QCD}^{2}}$ approximation. We show that the distribution of recoil nucleons depends strongly on the angle between the momentum of the recoil nucleon and the polarization vector of the target (or outgoing nucleon). This dependence is especially sensitive to the spin flip skewed parton distribution (SPD) $\widetilde E$. We argue also that the scaling for this spin asymmetry sets in at lower Q^2 than that for the absolute cross section. Basing on the chiral quark-soliton model of the nucleon we estimate quantitatively the spin asymmetry. In the case of pi+ production this asymmetry is dominated at small t by the contribution of the pion pole in the isovector SPD $\widetilde E$ as required by PCAC. In the case of K0 production off a proton we find a large enhancement of the cross section as compared to the case of pi0 production. For the forward production of neutral pseudoscalar mesons off a deuteron target we find the cross section should be zero for the zero deuteron helicity (along the $γ^*D$ direction). We consider also cross sections of quasielastic processes off nuclei including the feasibility to implant K+,rho-mesons into nuclear volume.
Recently in the context of studies of the intrinsic charm content of the nucleon and of the eta' meson two groups have arrived at different results for the 1/m^3 term of the heavy quark expansion for operator $\bar Qγ_5Q$ differing by the factor of six. We show that the form of both results violates certain general conditions. Using the expression for the axial anomaly with the finite Pauli-Villars regularization we obtain a new expression for 1/m^3 term of the heavy quark expansion for $\bar Qγ_5 Q$. With this new result we obtain an estimate for the constant f_{η'}^{(c)}=-2 MeV.
We summarize recent attempts to calculate the flavor asymmetry of the nucleon's sea quark distributions in the large-$N_c$ limit, where the nucleon can be described as a soliton of an effective chiral theory. We discuss the leading-twist longitudinally polarized and transversity antiquark distributions, $Δ\bar u (x) - Δ\bar d (x)$ and $δ\bar u (x) - δ\bar d (x)$, as well as the unpolarized one, $\bar u (x) - \bar d (x)$, which appears only in the next-to-leading order of the $1/N_c$-expansion. Results for $\bar u (x) - \bar d (x)$ are in good agreement with the recent Drell-Yan data from the FNAL E866 experiment. The longitudinally polarized antiquark asymmetry, $Δ\bar u (x) - Δ\bar d (x)$, is found to be larger than the unpolarized one.
We calculate the isovector (flavor-nonsinglet) unpolarized quark- and antiquark distributions in the nucleon at a low normalization point in the large-N_c limit. The nucleon is described as a soliton of the effective chiral theory. The isovector distribution appears in the next-to-leading order of the 1/N_c-expansion. Numerical results for the quark- and antiquark distributions compare well with the parametrizations of the data at a low normalization point. This large-N_c approach gives a flavor asymmetry of the antiquark distribution (violation of the Gottfried sum rule) in good agreement with the measurements.
The pion wave function is computed in the low energy effective theory inspired by the instanton vacuum model. The resultat is numerically rather close to the asymptotical wave function.
We study the off-forward quark distributions (OFQD's) in the chiral quark-soliton model of the nucleon. This model is based on the large-N_c picture of the nucleon as a soliton of the effective chiral lagrangian and allows to calculate the leading twist quark- and antiquark distributions at a low normalization point. We demonstrate the consistency of the approach by checking various sum rules for the OFQD's. We present numerical estimates of the isosinglet distribution H(x,ξ,Δ^2). In contrast to other approaches we find a strong qualitative dependence on the longitudinal momentum transfer, ξ. In particular, H(x,ξ,Δ^2) as a function of x exhibits fast crossovers at |x| = ξ/2. Such behaviour could lead to a considerable enhancement of the deeply-virtual Compton scattering cross section.