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K. Goeke

Publications and source records attributed to K. Goeke.

At least 19 recordsLinked to original sources
arXiv2010

QCD isospin breaking ChPT low-energy constants from the instanton vacuum

In the framework of the instanton vacuum model we evaluate the Chiral Perturbation Theory (ChPT) low-energy constants h_3,l_7. We found that in the instanton vacuum model the constant l_7 is very sensitive to the shape of the instanton and the instanton vacuum parameters. We evaluated the constant l_7 for two different zero-mode profiles and as a function of the average instanton size ρand inter-instanton distance R. Our result agrees with an old "order of magnitude" estimate of this constant from [1]. The obtained value of l_7 implies that the pure QCD contribution to the pion mass difference is small, ~1% of the observed experimental value.

hep-ph↗
arXiv2009

Leading twist nuclear shadowing, nuclear generalized parton distributions and nuclear DVCS at small x

We generalize the leading twist theory of nuclear shadowing and calculate quark and gluon generalized parton distributions (GPDs) of spinless nuclei. We predict very large nuclear shadowing for nuclear GPDs. In the limit of the purely transverse momentum transfer, our nuclear GPDs become impact-parameter-dependent nuclear parton distributions (PDFs). Nuclear shadowing induces nontrivial correlations between the impact parameter b and the light-cone fraction x. We make predictions for the deeply virtual Compton scattering (DVCS) amplitude and the DVCS cross section on Pb-208 at high energies. We calculate the cross section of the Bethe-Heitler (BH) process and address the issue of the extraction of the DVCS signal from the e A \to e γA cross section. We find that the e A \to e γA differential cross section is dominated by DVCS at the momentum transfer t near the minima of the nuclear form factor. We also find that nuclear shadowing leads to dramatic oscillations of the DVCS beam-spin asymmetry, A_{LU}, as a function of t. The position of the points where A_{LU} changes sign is directly related to the magnitude of nuclear shadowing.

hep-ph↗
arXiv2008

Generalized parton correlation functions for a spin-0 hadron

The fully unintegrated, off-diagonal quark-quark correlator for a spin-0 hadron is parameterized in terms of so-called generalized parton correlation functions. Such objects are of relevance for the phenomenology of certain hard exclusive reactions. In particular, they can be considered as {\it mother distributions} of generalized parton distributions on the one hand and transverse momentum dependent parton distributions on the other. Therefore, our study provides new, model-independent insights into the recently proposed nontrivial relations between generalized and transverse momentum dependent parton distributions. As a by-product we obtain the first complete classification of generalized parton distributions beyond leading twist.

hep-ph↗
arXiv2008

Generalized parton distributions and Deeply Virtual Compton Scattering in Color Glass Condensate model

Within the framework of the Color Glass Condensate model, we evaluate quark and gluon Generalized Parton Distributions (GPDs) and the cross section of Deeply Virtual Compton Scattering (DVCS) in the small-$x_{B}$ region. We demonstrate that the DVCS cross section becomes independent of energy in the limit of very small $x_{B}$, which clearly indicates saturation of the DVCS cross section. Our predictions for the GPDs and the DVCS cross section at high-energies can be tested at the future Electron-Ion Collider and in ultra-peripheral nucleus-nucleus collisions at the LHC.

hep-ph↗
arXiv2008

Sivers effect at Hermes, Compass and Clas12

Single spin asymmetries in semi-inclusive deep-inelastic scattering off transversely polarized targets give information on, among other fascinating effects, a pseudo time-reversal odd parton distribution function, the 'Sivers function'. In this proceeding we review the extractions of this function from HERMES and COMPASS data. In particular, the HERMES pion and kaon data suggest significant sea-quarks contributions at $x \simeq 0.15$ to the Sivers effect. We present a new fit that includes all relevant sea quark distributions and gives a statistically satisfactory overall description of the data, but does not describe ideally the $K^+$ data from HERMES. We argue that measurements of the pion- and kaon Sivers effect at CLAS12, and COMPASS, will clarify the situation.

hep-ph↗
arXiv2008

Deeply Virtual Compton Scattering on nucleons and nuclei in generalized vector meson dominance model

We consider Deeply Virtual Compton Scattering (DVCS) on nucleons and nuclei in the framework of generalized vector meson dominance (GVMD) model. We demonstrate that the GVMD model provides a good description of the HERA data on the dependence of the proton DVCS cross section on Q^2, W (at Q^2=4 GeV^2) and t. At Q^2 = 8 GeV^2, the soft W-behavior of the GVMD model somewhat underestimates the W-dependence of the DVCS cross section due to the hard contribution not present in the GVMD model. We estimate 1/Q^2 power-suppressed corrections to the DVCS amplitude and the DVCS cross section and find them large. We also make predictions for the nuclear DVCS amplitude and cross section in the kinematics of the future Electron-Ion Collider. We predict significant nuclear shadowing, which matches well predictions of the leading-twist nuclear shadowing in DIS on nuclei.

hep-ph↗
arXiv2008

Sivers and Collins Single Spin Asymmetries

The Sivers and Collins asymmetries are the most prominent Single Spin Asymmetries (SSA)in Semi-Inclusive Deeply Inelastic Scattering (SIDIS)with transverse target polarization. In this talk we present our understanding of these phenomena.

hep-ph↗
arXiv2007

How is Exotics Produced ? Where to Search for it ?

On the basis of existing data, we suggest such a mechanism of production for exotic hadrons that can explain, at least qualitatively, why the Theta+-baryon is seen in some experiments and not in others. With our hypothesis, production of exotic hadrons is a new kind of hard processes. We also can propose new experiments to check (and confirm?) existence of exotics and to provide new important information about both exotic and conventional hadrons [The talk is based on the papers by Ya. Azimov, K. Goeke, and I. Strakovsky, Phys Rev D 76, 074013 (2007)]

hep-ph↗
arXiv2007

Relations between GPDs and TMDs: model results and beyond

Comparing the respective structures of the correlators defining generalized and transverse momentum dependent parton distributions, one finds possible relations between these two objects. Although it looks like the relations found do not hold in general, we show that they can be established at least in simple spectator model calculations. In addition, we discuss these relations in the context of generalized transverse momentum dependent parton distributions.

hep-ph↗
arXiv2007

Are there approximate relations among transverse momentum dependent distribution functions?

Certain exact relations among transverse momentum dependent parton distribution functions due to QCD equations of motion turn into approximate ones upon the neglect of pure twist-3 terms. On the basis of available data from HERMES we test the practical usefulness of one such ``Wandzura-Wilczek-type approximation'', namely of that connecting h_{1L}^{\perp(1)a}(x) to h_L^a(x), and discuss how it can be further tested by future CLAS and COMPASS data.

hep-ph↗
arXiv2007

Relations between generalized and transverse momentum dependent parton distributions

Recent work suggests non-trivial relations between generalized parton distributions on the one hand and (naive time-reversal odd) transverse momentum dependent distributions on the other. Here we review the present knowledge on such type of relations. Moreover, as far as spectator model calculations are concerned, the existing results are considerably extended. While various relations between the two types of parton distributions can be found in the framework of spectator models, so far no non-trivial model-independent relations have been established.

hep-ph↗
arXiv2007

Low energy constants of \chiPT from the instanton vacuum model

In the framework of the instanton vacuum model we make expansion over the current mass m and number of colors N_c and evaluate ${\cal O}(1/N_c, m, m/N_c, m \ln m/N_c)$-corrections to the dynamical quark mass M, the quark condensate $<\bar qq>$, the pion mass M_πand decay constant F_π. There are several sources of these corrections: meson loops, finite size of the instanton distribution and the quark-quark "tensor" interaction terms. In contrast to the expectations, we found that numerically the 1/N_c-corrections to dynamical mass are large and mostly come from meson loops. As a consequence, we have large 1/N_c-corrections to all the other quantities. To provide the values of $F_π(m=0), \ave{\bar qq(m=0)}$ in agreement with \chiPT, we offer a new set of parameters ρ, R. Finally, we find the low-energy SU(2)_f chiral lagrangian constants $\bar l_3, \bar l_4$ in a rather good correspondence with the phenomenology.

hep-ph↗
arXiv2007

A possible explanation why the Theta+ is seen in some experiments and not in others

To understand the whole set of positive and null data on the Theta+(1530)-production, we suggest the hypothesis that multiquark hadrons are mainly generated from many-quark states, which emerge either as short-term hadron fluctuations, or as hadron remnants in hard processes. This approach allows us to describe both non-observation of the Theta+ in current null experiments and peculiar features of its production in positive experiments. Further, we are able to propose new experiments that might be decisive for the problem of the Theta+ existence. Distributions of the Theta+ in such experiments can give important information both on higher Fock components of conventional hadrons and about structure and hadronization properties of hadron remnants produced in hard processes. We also explain that description of multiquark hadrons may require a modified form of the constituent quark model, with quark masses and couplings being intermediate between their values for the familiar constituent quarks and the current ones.

hep-ph↗
arXiv2007

Are generalized and transverse momentum dependent parton distributions related ?

The present knowledge on non-trivial relations between generalized parton distributions on the one hand and transverse momentum dependent distributions on the other is reviewed. While various relations can be found in the framework of spectator models, so far no model-independent non-trivial relations have been established.

hep-ph↗
arXiv2007

Magnetic and axial-vector transitions of the baryon antidecuplet

We report the recent results of the magnetic transitions and axial-vector transitions of the baryon antidecuplet within the framework of the chiral quark-soliton model. The dynamical model parameters are fixed by experimental data for the magnetic moments of the baryon octet, for the hyperon semileptonic decay constants, and for the singlet axial-vector constant. The transition magnetic moments $μ_{ΛΣ}$ and $μ_{NΔ}$ are well reproduced and other octet-decuplet and octet-antidecuplet transitions are predicted. In particular, the present calculation of $μ_{ΣΣ^*}$ is found to be below the upper bound $0.82μ_N$ that the SELEX collaboration measured very recently. The results explains consistently the recent findings of a new $N^*$ resonance from the GRAAL and Tohoku LNS group. We also obtain the transition axial-vector constants for the $Θ^+\to KN$ from which the decay width of the $Θ^{+}$ pentaquark baryon is determined as a function of the pion-nucleon sigma term $Σ_{πN}$. We investigate the dependence of the decay width of the $Θ^{+}$ on the $g_{A}^{(0)}$, with the $g_{A}^{(0)}$ varied within the range of the experimental uncertainty. We show that a small decay width of the $Θ^{+}\to KN$, i.e. $Γ_{ΘKN} \leq 1$ MeV, is compatible with the values of all known semileptonic decays with the generally accepted value of $g_{A}^{(0)} \approx 0.3$ for the proton.

hep-ph↗
arXiv2007

Status of Sivers and Collins Single Spin Asymmetries

The Sivers and Collins asymmetries are the most interesting Single Spin Asymmetries in semi-inclusive deeply inelastic scattering with transverse target polarization. In this talk we present our understanding of these phenomena.

hep-ph↗