arXiv subjects

R. Jakob

Publications and source records attributed to R. Jakob.

28 records · Page 2Linked to original sources

arXiv1997

Asymmetries in polarized hadron production in e^+e^- annihilation up to order 1/Q

We present the results of the tree-level calculation of inclusive two-hadron production in electron-positron annihilation via one photon up to subleading order in 1/Q. We consider the situation where the two hadrons belong to different, back-to-back jets. We include polarization of the produced hadrons and discuss azimuthal dependences of asymmetries. New asymmetries are found, in particular there is a leading cos(2 phi) asymmetry, which is even present when hadron polarization is absent, since it arises solely due to the intrinsic transverse momenta of the quarks.

hep-ph↗

arXiv1997

Modelling quark distribution and fragmentation functions

The representation of quark distribution and fragmentation functions in terms of non-local operators is combined with a simple spectator model. This allows us to estimate these functions for the nucleon and the pion ensuring correct crossing and support properties. We give estimates for the unpolarized functions as well as for the polarized ones and for subleading (higher twist) functions. Furthermore we can study several relations that are consequences of Lorentz invariance and of C, P, and T invariance of the strong interactions.

hep-ph↗

arXiv1996

Time reversal odd fragmentation functions

The combination of transverse momentum and polarization effects in hard scattering processes, e.g polarized deep-inelastic leptoproduction, results in a rich variety of information on the hadronic structure. This information is encoded in a correspondingly large number of functions depending on both, longitudinal fractional momenta $x$ and transverse momenta $p_T$. Integration over transverse momenta establishes the connection to the usual distribution and fragmentation functions. Constraints from hermiticity and invariance under parity operation induce some relations between different functions. One specific aspect of the information on the hadronic structure is the appearance of {\em time reversal odd} fragmentation functions due to the non-applicability of time-reversal invariance for the hadronization of a quark. {\em T-odd} fragmentation functions are experimentally accessible via the measurement of, for instance, asymmetries in semi-inclusive leptoproduction.

hep-ph↗

arXiv1996

Quark transverse momentum in hard scattering processes

The role of transverse momentum of quarks in semi-inclusive leptoproduction will be discussed. It involves generalized distribution and fragmentation functions which depend on both the longitudinal lightcone momentum fraction of the quarks and on the transverse momentum. Constraints on these functions and relations between them arise as a consequence of the QCD equation of motion for the quark fields, Lorentz invariance and of C, P, and T invariance of the strong interactions. Experimentally one has access to the functions for instance in measurements of azimuthal asymmetries in hard processes.

hep-ph↗

arXiv1995

A perturbative approach to $B$ decays into two $π$ mesons

The modified perturbative approach in which transverse degrees of freedom as well as Sudakov suppressions are taken into account, is applied to $B$ decays into two $π$ mesons. The influence of various model parameters (CKM matrix elements, $B$ decay constant, mesonic wave functions) on the results as well as short distance corrections to the weak Hamiltonian are discussed in some detail. The perturbative contributions to the $B$ decays yield branching ratios of the order of $10^{-7}\;-\;10^{-6}$ which values are well below the upper limit for the $\bar{B}^0\toπ^+π^-$ branching ratio as measured by CLEO.

hep-ph↗

arXiv1994

Meson-Photon Transition Form Factors

We report results on the $π$-$γ$ transition form factor obtained within the hard scattering approach including transverse momentum effects and Sudakov corrections. The results clearly favor distribution amplitudes close to the asymptotic form, $\sim x_1x_2$, and disfavor distribution amplitudes which are strongly concentrated in the end-point regions. This observation is backed by information on the elastic form factor of the pion and on its valence quark distribution function. Applications of our approach to the $η$-$γ$ and $η^\prime$-$γ$ transition form factors are discussed as well. Combining the form factor data with the two-photon decay widths, we determine the $η$ and the $η^\prime$ decay constants and the $η$-$η^\prime$ mixing angle.

hep-ph↗

arXiv1994

Intrinsic Transverse Size Effect

Two recently proposed concepts to improve the perturbative calculation of exclusive amplitudes, gluonic radiative corrections (Sudakov factor) and confinement size effects (intrinsic transverse momentum) are combined to study the neutron magnetic form factor in the space-like region. We find that nucleon distribution amplitudes modelled on the basis of current QCD sum rules indicate overlap with the existing data at the highest measured values of momentum transfer. However, sizeable higher-order perturbative corrections (K-factor) and/or higher-twist contributions cannot be excluded, although they may be weaker than in the proton case.

hep-ph↗

arXiv1994

A Critical Analysis of the Proton Form Factor with Sudakov Suppression and Intrinsic Transverse Momentum

The behavior of the proton magnetic form factor is studied within the modified hard scattering picture, which takes into account gluonic radiative corrections in terms of transverse separations. We parallel the analysis given previously by Li and make apparent a number of serious objections. The appropriate cut-off needed to render the form-factor calculation finite is both detailed and analyzed by considering different cut-off prescriptions. The use of the maximum interquark separation as a common infrared cut-off in the Sudakov suppression factor is proposed, since it avoids difficulties with the $α_{s}$-singularities and yields a proton form factor insensitive to the inclusion of the soft region which therefore can be confidently attributed to perturbative QCD. Results are presented for a variety of proton wave functions including also their intrinsic transverse momentum. It turns out that the perturbative contribution, although theoretically self-consistent for $Q^{2}$ larger than about $6$~GeV${}^{2}$ to $10$~GeV${}^{2}$, is too small compared to the data.

hep-ph↗

arXiv1993

Octet-Baryon Form Factors in the Diquark Model

We present an alternative parameterization of the quark-diquark model of baryons which particularly takes care of the most recent proton electric form-factor data from the E136 experiment at SLAC. In addition to electromagnetic form factors of the nucleon, for which good agreement with data is achieved, we discuss the weak axial vector form factor of the nucleon as well as electromagnetic form factors of $Λ$ and $Σ$ hyperons. Technical advance in calculating the pertinent analytic expressions within perturbative quantum chromodynamics is gained by formulating the wave function of the quark-diquark system in a covariant way. Finally, we also comment on the influence of Sudakov corrections within the scope of the diquark model.

hep-ph↗

arXiv1993

The pion form factor: Sudakov suppressions and intrinsic transverse momentum

It is demonstrated that any attempt to calculate the perturbative QCD contribution to the pion form factor requires the inclusion of intrinsic transverse momentum besides Sudakov form factors. For momentum transfers of the order of a few GeV the intrinsic transverse momentum leads to a substantial suppression of the perturbative QCD contribution.

hep-ph↗