arXiv subjects

Matthias Burkardt

Publications and source records attributed to Matthias Burkardt.

At least 37 records · Page 2Linked to original sources

arXiv2015

Transverse Force on Quarks in DIS

Generalized Parton Distributions (GPDs) provide information on the distribution of quarks in impact paarmeter space. For transversely polarized nucleons, these impact parameter distributions are transversely distorted and this deviation from axial symmetry leads on average to a net transverse force from the spectators on the active quark in a DIS experiment. %The strength of that force can be related to twist-3 PDFs. This force when acting along the whole trajectory of the active quark leads to a transverse single-spin asymmetries. For a longitudinally polarized nucleon target, the transverse force implies a torque acting on the quark Orbital Angular Momentum (OAM). The resulting change in OAM as the quark leaves the target equals the difference between the Jaffe-Manohar and Ji OAMs.

hep-ph↗

arXiv2014

Quark Orbital Angular Momentum and Final State Interactions

Definitions of orbital angular momentum based on Wigner distributions are used to discuss the connection between the Ji definition of the quark orbital angular momentum and that of Jaffe and Manohar. The difference between these two definitions can be interpreted as the change in the quark orbital angular momentum as it leaves the target in a DIS experiment. The mechanism responsible for that change is similar to the mechanism that causes transverse single-spin asymmetries in semi-inclusive deep-inelastic scattering.

nucl-th↗

arXiv2014

Pre-Town Meeting on Spin Physics at an Electron-Ion Collider

A polarized $ep/eA$ collider (Electron--Ion Collider, or EIC), with polarized proton and light-ion beams and unpolarized heavy-ion beams with a variable center--of--mass energy $\sqrt{s} \sim 20$ to $\sim100$~GeV (upgradable to $\sim 150$ GeV) and a luminosity up to $\sim 10^{34} \, \textrm{cm}^{-2} \textrm{s}^{-1}$, would be uniquely suited to address several outstanding questions of Quantum Chromodynamics, and thereby lead to new qualitative and quantitative information on the microscopic structure of hadrons and nuclei. During this meeting at Jefferson Lab we addressed recent theoretical and experimental developments in the spin and the three--dimensional structure of the nucleon (sea quark and gluon spatial distributions, orbital motion, polarization, and their correlations). This mini--review contains a short update on progress in these areas since the EIC White paper~\cite{Accardi:2012qut}.

hep-ph↗

arXiv2013

Transverse Force on Quarks in DIS

The interaction dependent part of the $x^2$-moment of the twist-3 polarized parton distribution $g_2(x)$ is related to the transverse force acting on the active quark in deep-inelastic scattering off a transversely polarized nucleon immediately after it has absorbed the virtual photon. Similarly, the $x^2$-moment of the chirally odd twist-3 unpolarized parton distribution $e(x)$ can be related to the transverse force experienced by a transversely polarized quark ejected from a transversely polarized nucleon.

hep-ph↗

arXiv2013

Distribution of Angular Momentum in the Transverse Plane

Fourier transforms of GPDs describe the distribution of partons in the transverse plane. The 2nd moment of GPDs has been identified by X.Ji with the angular momentum (orbital plus spin) carried by the quarks - a fundamental result that is being widely utilized in the spin decomposition of a longitudinally polarized nucleon. However, I will demonstrate that, despite the above results, the Fourier transform of the 2nd moment of GPDs does not describe the distribution of angular momentum in the transverse plane for a longitudinally polarized target.

hep-ph↗

arXiv2013

Parton Orbital Angular Momentum and Final State Interactions

Definitions of orbital angular momentum based on Wigner distributions are used as a framework to discuss the connection between the Ji definition of the quark orbital angular momentum and that of Jaffe and Manohar.We find that the difference between these two definitions can be interpreted as the change in the quark orbital angular momentum as it leaves the target in a DIS experiment. The mechanism responsible for that change is similar to the mechanism that causes transverse single-spin asymmetries in semi-inclusive deep-inelastic scattering.

hep-ph↗

arXiv2013

Magnetic properties of the nucleon in a uniform background field

We present results for the magnetic moment and magnetic polarisability of the neutron and the magnetic moment of the proton. These results are calculated using the uniform background field method on 32^3 x 64 dynamical QCD lattices provided by the PACS-CS collaboration as part of the ILDG. We use a uniform background magnetic field quantised by the periodic spatial volume. We investigate ways to improve the effective energy plots used to calculate magnetic polarisabilities, including the use of correlation matrix techniques with various source smearings.

hep-lat↗

arXiv2013

The Nucleon Spin Sum Rule

Definitions of orbital angular momentum based on Wigner distributions are used as a framework to discuss the connection between the Ji definition of the quark orbital angular momentum and that of Jaffe and Manohar. We find that the difference between these two definitions can be interpreted as the change in the quark orbital angular momentum due to final state interactions as it leaves the target in a DIS experiment.

hep-ph↗

arXiv2012

Magnetic properties of the neutron in a uniform background field

We present calculations of the magnetic moment and magnetic polarisability of the neutron from the background field method. The calculations are performed on $32^3\times64$ dynamical lattices generated by the PACS-CS collaboration and made available via the ILDG. We consider uniform fields quantised by the periodic spatial volume. We explore different approaches for improving the quality of the fits used in the results. Also included are initial results for the magnetic moment of the lowest lying negative parity nucleon states.

hep-lat↗

arXiv2011

Orbital Angular Momentum in Scalar Diquark Model and QED

We compare the orbital angular momentum of the 'quark' in the scalar diquark model as well as that of the electron in QED (to order α) obtained from the Jaffe-Manohar de- composition to that obtained from the Ji relation. We estimate the importance of the vector potential in the definition of orbital angular momentum.

hep-ph↗

arXiv2010

Quark and Gluon Angular Momentum in the Nucleon

Parton distributions in impact parameter space, which are obtained by Fourier transforming GPDs, are exhibit a significant deviation from axial symmetry when the target and/or quark is transversely polarized. From this deformation, we present an intuitive derivation of the Ji relation. In a scalar diquark model and in QED, we compare the Ji and Jaffe-Manohar decompositions of the nucleon spin. Using the MIT bag model, we estimate spectator effects through the presence of the gluon vector potential in the definitions of the quark orbital angular momentum.

hep-ph↗

arXiv2010

Transverse (Spin) Structure of Hadrons

Parton distributions in impact parameter space, which are obtained by Fourier transforming GPDs, exhibit a significant deviation from axial symmetry when the target and/or quark are transversely polarized. Connections between this deformation and transverse single-spin asymmetries as well as with quark-gluon correlations are discussed. The sign of transverse deformation of impact parameter dependent parton distributions in a transversely polarized target can be related to the sign of the contribution from that quark flavor to the nucleon anomalous magnetic moment. Therefore, the signs of the Sivers function for $u$ and $d$ quarks, as well as the signs of quark-gluon correlations embodied in the polarized structure function $g_2$ can be understood in terms of the proton and neutron anomalous magnetic moments.

hep-ph↗

arXiv2010

Quark Orbital Angular Momentum in the MIT Bag Model

Using the MIT bag model, we study the contribution from the gluon vector potential due to the spectators to the orbital angular momentum of a quark in the bag model. For $α_s = {\cal O}(1)$, this spectator contribution to the quark orbital angular momentum in the gauge-covariant Ji decomposition is of the same order as the non gauge-covariant quark orbital angular momentum and its magnitude is larger for $d$ than for $u$ quarks and negative for both.

hep-ph↗

arXiv2010

Transverse Structure of Hadrons

hep-ph↗

arXiv2009

Generalized Parton Distributions, Deeply Virtual Compton Scattering and TMDs

Parton distributions in impact parameter space, which are obtained by Fourier transforming GPDs, exhibit a significant deviation from axial symmetry when target and/or quark are transversely polarized. In combination with the final state interactions, this transverse deformation provides a natural mechanism for naive-T odd transverse single-spin asymmetries in semi-inclusive DIS. The deformation of PDFs in impact parameter space can also be related to the transverse force acting on the active quark in polarized DIS at higher twist.

hep-ph↗

arXiv2009

The $g_2$ Structure Function

Polarized structure functions at low $Q^2$ have the physical interpretation of (generalized) spin polarizabilities. At high $Q^2$, the polarized parton distribution $g_2(x)$ provides access to quark-gluon correlations in the nucleon. We discuss the interpretation of the $x^2$ moment of $\bar{g}_2(x)$ as an average transverse force on quarks in deep-inelastic scattering from a transversely polarized target. Qualitative connections with generalized parton distributions are emphasized. The $x^2$ moment of the chirally-odd twist-3 parton distribution $e(x)$ provides information on the dependence of the average transverse force on the transversity of the quark.

hep-ph↗

arXiv2009

GPDs and DVCS with Positrons

The beam charge asymmetry helps to isolate the real part of the deeply virtual Compton scattering (DVCS) amplitude. It is discussed what information can be gained both from the real and imaginary part of the DVCS amplitude.

hep-ph↗

arXiv2009

Parton Distributions in the Impact Parameter Space

Parton distributions in impact parameter space, which are obtained by Fourier transforming GPDs, exhibit a significant deviation from axial symmetry when the target and/or quark is transversely polarized. In combination with the final state interactions, this transverse deformation provides a natural mechanism for naive-T odd transverse single-spin asymmetries in semi-inclusive DIS. The deformation can also be related to the transverse force acting on the active quark in polarized DIS at higher twist.

hep-ph↗