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R. Jakob

Publications and source records attributed to R. Jakob.

At least 19 recordsLinked to original sources
arXiv2004

Generalized parton distributions from nucleon form factor data

We present a simple empirical parameterization of the x- and t-dependence of generalized parton distributions at zero skewness, using forward parton distributions as input. A fit to experimental data for the Dirac, Pauli and axial form factors of the nucleon allows us to discuss quantitatively the interplay between longitudinal and transverse partonic degrees of freedom in the nucleon ("nucleon tomography"). In particular we obtain the transverse distribution of valence quarks at given momentum fraction x. We calculate various moments of the distributions, including the form factors that appear in the handbag approximation to wide-angle Compton scattering. This allows us to estimate the minimal momentum transfer required for reliable predictions in that approach to be around |t|~3 GeV^2. We also evaluate the valence contributions to the energy-momentum form factors entering Ji's sum rule.

hep-ph↗
arXiv2004

Signatures of the handbag mechanism in wide-angle photoproduction of pseudoscalar mesons

Wide-angle photoproduction of pseudoscalar mesons is investigated under the assumption of dominance of the handbag mechanism considering both quark helicity flip and non-flip. The partonic subprocess, meson photoproduction off quarks, is analysed with the help of a covariant decomposition of the subprocess amplitudes which is independent of a specific meson generation mechanism. As examples of subprocess dynamics, however, the twist-2 as well as two-particle twist-3 contributions are explicitly calculated. Characteristic features of the handbag approach are discussed in dependence upon the relative magnitudes of the invariant functions. Differential cross sections and spin correlations are predicted to show a characteristic behaviour which allows to test the underlying assumption of handbag dominance.

hep-ph↗
arXiv2001

Accessing transversity with interference fragmentation functions

We discuss in detail the option to access the transversity distribution function $h_1(x)$ by utilizing the analyzing power of interference fragmentation functions in two-pion production inside the same current jet. The transverse polarization of the fragmenting quark is related to the transverse component of the relative momentum of the hadron pair via a new azimuthal angle. As a specific example, we spell out thoroughly the way to extract $h_1(x)$ from a measured single spin asymmetry in two-pion inclusive lepton-nucleon scattering. To estimate the sizes of observable effects we employ a spectator model for the fragmentation functions. The resulting asymmetry of our example is discussed as arising in different scenarios for the transversity.

hep-ph↗
arXiv2000

Electron Scattering with Polarized Targets at TESLA

Measurements of polarized electron-nucleon scattering can be realized at the TESLA linear collider facility with projected luminosities that are about two orders of magnitude higher than those expected of other experiments at comparable energies. Longitudinally polarized electrons, accelerated as a small fraction of the total current in the e+ arm of TESLA, can be directed onto a solid state target that may be either longitudinally or transversely polarized. A large variety of polarized parton distribution and fragmentation functions can be determined with unprecedented accuracy, many of them for the first time. A main goal of the experiment is the precise measurement of the x- and Q^2-dependence of the experimentally totally unknown quark transversity distributions that will complete the information on the nucleon's quark spin structure as relevant for high energy processes. Comparing their Q^2-evolution to that of the corresponding helicity distributions constitutes an important precision test of the predictive power of QCD in the spin sector. Measuring transversity distributions and tensor charges allows access to the hitherto unmeasured chirally odd operators in QCD which are of great importance to understand the role of chiral symmetry. The possibilities of using unpolarized targets and of experiments with a real photon beam turn TESLA-N into a versatile next-generation facility at the intersection of particle and nuclear physics.

hep-ph↗
arXiv2000

Calculation of fragmentation functions in two-hadron semi-inclusive processes

We investigate the properties of interference fragmentation functions arising from the emission of two leading hadrons inside the same jet for inclusive lepton-nucleon deep-inelastic scattering. Using an extended spectator model for the mechanism of the hadronization, we give a complete calculation and numerical estimates for the examples of a proton-pion pair produced with invariant mass on the Roper resonance, and of two pions produced with invariant mass close to the $ρ$ mass. We discuss azimuthal angular dependence of the leading order cross section to point up favourable conditions for extracting transversity from experimental data.

hep-ph↗
arXiv2000

Semi-inclusive structure functions in the spectator model

We establish the relationship between distribution and fragmentation functions and the structure functions appearing in the cross section of polarized 1-particle inclusive deep-inelastic scattering. We present spectator model evaluations of these structure functions focusing on the case of an outgoing spin-1/2 baryon. Distribution functions obtained in the spectator model are known to fairly agree at low energy scales with global parameterizations extracted, for instance, from totally inclusive DIS data. Therefore, we expect it to give good hints on the functional dependence of the structure functions on the scaling variables x(Bjorken), z and on the transverse momentum of the observed outgoing hadron, P_{h\perp}. Presently, this dependence is not very well known, but experiments are planned in the near future.

hep-ph↗
arXiv1999

Angular dependences in electroweak semi-inclusive leptoproduction

We present the leading order unpolarized and polarized cross sections in electroweak semi-inclusive deep inelastic leptoproduction. The azimuthal dependences in the cross section differential in the transverse momentum of the vector boson arise due to intrinsic transverse momenta of the quarks. However, the presented asymmetries are not suppressed by inverse powers of the hard scale. We discuss the different opportunities to measure specific asymmetries as offered by neutral compared to charged current processes and point out the optimal kinematical regions. The present and (proposed) future HERA collider experiments would be most suitable for measuring some of the asymmetries discussed here, especially in case of Lambda production.

hep-ph↗
arXiv1999

Two-hadron interference fragmentation functions. Part II: a model calculation

We present a model calculation of leading order interference fragmentation functions arising from the distribution of two hadrons produced in the same jet of a fragmenting quark in a hard process. Using a simple spectator model for the mechanism of the hadronization process, for the first time we give a complete calculation and numerical estimates for the example of a proton-pion pair production with its invariant mass on the Roper resonance.

hep-ph↗
arXiv1999

Two-hadron interference fragmentation functions. Part I: general framework

We investigate the properties of interference fragmentation functions measurable from the distribution of two hadrons produced in the same jet in the current fragmentation region of a hard process. We discuss the azimuthal angular dependences in the leading order cross section of two-hadron inclusive lepton-nucleon scattering as an example how these interference fragmentation functions can be addressed separately.

hep-ph↗
arXiv1999

Interference Fragmentation Functions in Deep Inelastic Scattering

We give a general overview of the role and interpretation of fragmentation functions in hard processes. Transverse momentum dependence gives rise to time-reversal odd fragmentation functions contributing at leading order. Final state interactions are necessary to have non-zero time reversal odd functions. We present a model calculation for the special case of interference fragmentation functions in two-hadron inclusive lepton hadron scattering.

hep-ph↗
arXiv1999

Skewed Parton Distributions in Real and Virtual Compton Scattering

The handbag contribution to Compton scattering at moderately large momentum transfer factorises into parton-photon subprocess amplitudes and new form factors representing 1/x-moments of skewed parton distributions. A detailed phenomenological study for polarised and unpolarised real and virtual Compton scattering is presented

hep-ph↗
arXiv1998

Linking Parton Distributions to Form Factors and Compton Scattering

We relate ordinary and skewed parton distributions to soft overlap contributions to elastic form factors and large angle Compton scattering, using light-cone wave functions in a Fock state expansion of the nucleon. With a simple ansatz for the wave functions of the three lowest Fock states we achieve a good description of unpolarised and polarised parton distributions at large x, and of the data for the Dirac form factor and for Compton scattering, both of which can be saturated with soft contributions only. Large angle Compton scattering appears as a good case to investigate the relative importance of soft and hard contributions in exclusive processes which are sensitive to the end point regions of the nucleon wave function.

hep-ph↗
arXiv1998

Spin physics with spin-0 hadrons

We discuss various azimuthal asymmetries in semi-inclusive DIS and e^+e^- ==> h1 h2 X which involve chiral odd quantities like the transversity distribution h_1 and a fragmentation function H_1^\perp. For the fragmentation described by H_1^\perp azimuthal angular dependence has to be measured, but no polarization vector of a final state hadron. We present first results on asymmetries including the electroweak currents in one-hadron inclusive DIS.

hep-ph↗
arXiv1998

Leading asymmetries in two-hadron production in e^+e^- annihilation at the Z pole

We present the leading unpolarized and single spin asymmetries in inclusive two-hadron production in electron-positron annihilation at the Z pole. The azimuthal dependence in the unpolarized differential cross section of almost back-to-back hadrons is a leading cos(2 phi) asymmetry, which arises solely due to the intrinsic transverse momenta of the quarks. An extensive discussion on how to measure this asymmetry and the accompanying time-reversal odd fragmentation functions is given. A simple estimate indicates that the asymmetry could be of the order of a percent.

hep-ph↗
arXiv1997

Distribution and fragmentation functions in a spectator model

Quark distribution and spectator functions are estimated in a diquark spectator model. The representation of the functions in terms of non-local operators together with the rather simple model allow estimates for the yet experimentally undetermined functions, like the polarized fragmentation functions or higher twist functions.

hep-ph↗