arXiv subjects

P. Kroll

Publications and source records attributed to P. Kroll.

120 records · Page 7Linked to original sources

arXiv1995

Virtual Compton Scattering Off Protons at Moderately Large Momentum Transfer

The amplitudes for virtual Compton scattering off protons are calculated within the framework of the diquark model in which protons are viewed as being built up by quarks and diquarks. The latter objects are treated as quasi-elementary constituents of the proton. Virtual Compton scattering, electroproduction of photons and the Bethe-Heitler contamination are discussed for various kinematical situations. We particularly emphasize the rôle of the electron asymmetry for measuring the relative phases between the virtual Compton and the Bethe-Heitler amplitudes. It is also shown that the model is able to describe very well the experimental data for real Compton scattering off protons.

hep-ph↗

arXiv1995

THE LARGE MOMENTUM TRANSFER BEHAVIOUR OF MESON-PHOTON TRANSITION FORM FACTORS

It is reported on predictions for the $π$-$γ$ transition form factor obtained within a perturbative approach which includes 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. Applications of that approach to the $η$-$γ$ and $η^\prime$-$γ$ transition form factors are discussed as well.

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

Electromagnetic Form Factors at Large Momentum Transfer

Recent improvements of the hard scattering picture for the large $p_{\perp}$ behaviour of electromagnetic form factors, namely the inclusion of both Sudakov corrections and intrinsic transverse momentum dependence of the hadronic wave function, are reviewed. On account of these improvements the perturbative contributions to the pion's and the nucleon's form factor can be calculated in a theoretically self-consistent way for momentum transfers as low as about $2$ and $3\,{\rm GeV}$, respectively. This is achieved at the expense of a substantial suppression of the perturbative contribution in the few GeV region. Eventual higher twist contributions are discussed in some detail.

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↗

arXiv1994

l=0 to l=1 Transition Form Factors

A method is proposed to extend the hard scattering picture of Brodsky and Lepage to transitions between hadrons with orbital angular momentum l=0 and l=1. The use of covariant spin wave functions turns out to be very helpful in formulating that method. As a first application we construct a light-cone wave function of the nucleon resonance $N^*(1535)$ in the quark-diquark picture. Using this wave function and the extended hard scattering picture, the $N$--$N^*$ transition form factors are calculated at large momentum transfer and the results compared to experimental data. As a further application of our method we briefly discuss the $π$--$a_1$ form factors in an appendix.

hep-ph↗

arXiv1993

Large $p_{\perp}$ Exclusive Processes:RECENT Developments

Recent improvements of the hard scattering picture for exclusive reactions, namely the inclusion of both Sudakov corrections and the intrinsic transverse momentum dependence of the hadronic wave function, are reviewed. On account of these improvements the perturbative contribution to the pion's form factor can be calculated in a theoretically self-consistent way for momentum transfers as low about $2\,{\rm GeV}$. This is achieved at the expense of a substantial suppression of the perturbative contribution in the few GeV region. Eventual higher twist contributions are also discussed.\\

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↗

arXiv1993

On Exclusive Reactions in the Time-Like Region

The electromagnetic form factors of the proton in the time-like region and two-photon annihilations into proton-antiproton are investigated. To calculate these processes at moderately large $s$ we use a variant of the Brodsky-Lepage hard-scattering formalism where diquarks are considered as quasi-elementary constituents of baryons. The proton wave function and the parameters controlling the diquark contributions are determined from fits to space-like data. We also comment on the decay $η_c \to p\bar{p}$.

hep-ph↗