Searcharxiv⌕ Search

arXiv subjects

P. Kroll

Publications and source records attributed to P. Kroll.

At least 109 records · Page 6Linked to original sources

Mixing and Decay Constants of Pseudoscalar Mesons

We propose a new eta-eta' mixing scheme where we start from the quark flavor basis and assume that the decay constants in that basis follow the pattern of particle state mixing. On exploiting the divergences of the axial vector currents - which embody the axial vector anomaly - all basic parameters are fixed to first order of flavor symmetry breaking. That approach naturally leads to a mass matrix, quadratic in the masses, with specified elements. We also test our mixing scheme against experiment and determine corrections to the first order values of the basic parameters from phenomenology. Finally, we generalize the mixing scheme to include the eta(c). Again the divergences of the axial vector currents fix the mass matrix and, hence, mixing angles and the charm content of the eta and eta'.

hep-ph↗

A New Cataclysmic Variable RX J0757.0+6306: Candidate for the Shortest Period Intermediate Polar

A new cataclysmic variable is identified as the optical counterpart of the faint and hard X-ray source RX J0757.0+6306 discovered during the ROSAT all-sky survey. Strong double-peaked emission lines bear evidence of an accretion disc via an S-wave which varies with a period of 81 +/- 5 min. We identify this period as the orbital period of the binary system. CCD photometry reveals an additional period of 8.52 +/- 0.15 min. which was stable over four nights. We suggest that RX J0757.0+6306 is possibly an intermediate polar, but we cannot exclude the possibility that it is a member of the SU UMa group of dwarf novae.

astro-ph↗

Colour Octet Contribution in Exclusive Charmonium Decay into Baryon-Antibaryon

It is argued that colour octet contribution in exclusive charmonium decays is very important even though the same infrared divergence found in P-wave colour singlet inclusive decay is absent. This comes about because of the suppression at the level of the wavefunction of P-wave quarkonia. Using the modified hard scattering approach of Brodsky, Lepage, Botts, Li and Sterman, we use charmonium decay into baryon-antibaryon pair as an example and show that the colour singlet contribution alone is clearly insufficient to explain the experimental decay widths.

hep-ph↗

Infinite Momentum Frame Calculation of Semileptonic Heavy Λ_b\toΛ_c Transitions Including HQET Improvements

We calculate the transition form factors that occur in heavy $Λ$-type baryon semileptonic decays as e.g. in $Λ_{b} \to Λ_{c}^{+} + l^{-} + \barν_{l} $. We use Bauer-Stech-Wirbel type infinite momentum frame wave functions for the heavy $Λ$-type baryons which we assume to consist of a heavy quark and a light spin-isospin zero diquark system. The form factors at $ q^{2} = 0 $ are calculated from the overlap integrals of the initial and final $Λ$-type baryon states. To leading order in the heavy mass scale the structure of the form factors agrees with the HQET predictions including the normalization at zero recoil. The leading order $ω$-dependence of the form factors is extracted by scaling arguments. By comparing the model form factors with the HQET predictions at ${\cal O}(1/m_{Q})$ we obtain a consistent set of model form factors up to ${\cal O}(1/m_{Q})$. With our preferred choice of parameter values we find that the contribution of the non-leading form factor is practically negligible. We use our form factor predictions to compute rates, spectra and various asymmetry parameters for the semi-leptonic decay $Λ_{b} \to Λ_{c}^{+} + l^{-} + \barν_{l} $.

hep-ph↗

RX J0719.2+6557: A new eclipsing polar

A new magnetic cataclysmic variable is identified as the counterpart of the X-ray source RX J0719.2+6557. The emission lines show radial velocity variations with a period of 98.2 min. This coincides with the period of deep eclipses (up to 4 mag) in the photometric light curve. The phase of the eclipse relative to the spectroscopic phase, and its structure indicates that the dominant source of emission is located on the stream of accreting matter, which is eclipsed by the secondary companion. The emission lines bear evidence of a weaker component, most probably the contribution from the heated side of the secondary star. These features define this object as a probable polar in a high state. NIR spectroscopy revealed some unusual, strong emission features at 8200 Åand 8660 Åpossibly originating on the heated side of the secondary.

astro-ph↗

Predictions for the eta_c-gamma transition form factor

The eta_c-gamma transition form factor is calculated in a model based on a modified hard scattering approach to exclusive reactions, in which transverse degrees of freedom are taken into account. For the eta_c-meson a distribution amplitude of the Bauer-Stech-Wirbel type is used, where the two free parameters, namely the decay constant f(eta_c) and the transverse size of the wave function, are related to the Fock state probability and the width for the two-photon decay Gamma[eta_c -> gamma gamma].

hep-ph↗

Heavy Baryon Transitions in a Relativistic Three-Quark Model

Exclusive semileptonic decays of bottom and charm baryons are considered within a relativistic three-quark model with a Gaussian shape for the baryon-three-quark vertex and standard quark propagators. We calculate the baryonic Isgur-Wise functions, decay rates and asymmetry parameters.

hep-ph↗

On hard exclusive reactions in the time-like region

The proton form factor, two-photon annihilations into $p\bar{p}$ as well as exclusive charmonium decays are critically examined. It will be argued that the standard perturbative QCD analysis of these reactions fails, i.e. the need for additional contributions can convincingly be demonstrated. Possible dynamical mechanisms such as colour-octet admixtures to the charmonium states or diquarks inside baryons, will be discussed and compared to the data.

hep-ph↗

Colour-octet contributions to exclusive charmonium decays

We investigate the theoretical uncertainties of $P$-wave charmonium decays into two pions, $χ_{\c J}\rightarrow π^+π^-$, $π^0π^0$. Constraining the pion distribution-amplitude from the recent precise data on $F_{πγ}(Q^2)$, we find an order-of-magnitude discrepancy between data and prediction. The disagreement persists even after inclusion of transverse degrees of freedom and Sudakov suppressions. We propose the colour-octet mechanism as the solution to the puzzle. The colour-octet decay contribution arising from the higher Fock component $|\c \cbar \g >$ of the $χ_{\c J}$ wave function is actually not power suppressed with respect to the usual colour-singlet decay arising from the dominant $|\c \cbar >$ Fock state. An explicit calculation yields an agreement with the data for a very reasonable value for the single extra non-perturbative parameter.

hep-ph↗

RX J0719.2+6557: A New Eclipsing Polar

A new magnetic, eclipsing cataclysmic variable is identified as the counterpart of the X-ray source RX J0719.2+6557 detected during the ROSAT all-sky survey. The relative phasing of photometric and spectroscopic periods indicates a self-eclipsing system. Doppler tomography points to the heated surface of the secondary as a strong source of emission and of diskless accretion. Near-infrared spectroscopy revealed two unusual strong emission features originating on the heated side of the secondary.

astro-ph↗

Diquark model predictions for photon induced exclusive reactions

The present status of the diquark model for exclusive reactions at moderately large momentum transfer is reviewed. That model is a variant of the Brodsky-Lepage approach in which diquarks are considered as quasi-elementary constituents of baryons. Recent applications of the diquark model, relevant to high energy physics with electromagnetic probes, are discussed: electromagnetic form factors of baryons in both the space-like and the time-like region, photoproduction of mesons, two-photon annihilations into proton-antiproton pairs as well as real and virtual Compton scattering on which the main emphasis is laid.

hep-ph↗

The $πγ$ transition form factor and the pion wave function

The pion wave function is discussed in the light of the recent CLEO data on the pion gamma transition form factor. It turns out that the wave function is close to the asymptotic form whereas wave functions strongly concentrated in the end-point regions are disfavoured. Consequences for other exclusive quantities, as for instance the pion's electromagnetic form factor, are also discussed.

hep-ph↗

Exclusive Photoproduction of Large Momentum-Transfer K and K* Mesons

The reactions gamma p -> K+ Lambda and gamma p -> K* Lambda are analyzed within perturbative QCD, allowing for diquarks as quasi-elementary constituents of baryons. The diquark-model parameters and the quark-diquark distribution amplitudes of proton and Lambda are taken from previous investigations of electromagnetic baryon form factors and Compton-scattering off protons. Unpolarized differential cross sections and polarization observables are computed for different choices of the K and K* distribution amplitudes. The asymptotic form of the K distribution amplitude (proportional to x1 x2) is found to provide a satisfactory description of the K photoproduction data.

hep-ph↗

Modelling the nucleon wave function from soft and hard processes

Current light-cone wave functions for the nucleon are unsatisfactory since they are in conflict with the data of the nucleon's Dirac form factor at large momentum transfer. Therefore, we attempt a determination of a new wave function respecting theoretical ideas on its parameterization and satisfying the following constraints: It should provide a soft Feynman contribution to the proton's form factor in agreement with data; it should be consistent with current parameterizations of the valence quark distribution functions and lastly it should provide an acceptable value for the $\jp \to N \bar N$ decay width. The latter process is calculated within the modified perturbative approach to hard exclusive reactions. A simultaneous fit to the three sets of data leads to a wave function whose $x$-dependent part, the distribution amplitude, shows the same type of asymmetry as those distribution amplitudes constrained by QCD sum rules. The asymmetry is however much more moderate as in those amplitudes. Our distribution amplitude resembles the asymptotic one in shape but the position of the maximum is somewhat shifted.

hep-ph↗

Gamma-Gamma Physics at LEP2

This report is an overview of the gamma-gamma physics capabilities of LEP2, and covers the following topics: structure functions, equivalent photon approximation, tagging conditions etc, soft and semihard physics, large-$p_t$ processes, heavy-quark physics, and exclusive channels.

hep-ph↗

Four new active galaxies with steep soft X-ray spectra

We have discovered four AGN in the ROSAT all-sky-survey data with very steep X-ray spectra. We apply several models to these X-ray spectra with emphasis on warm absorber models which give an adequate description of the data. We report on the follow-up optical and radio observations which allow the identification of three of these objects as Narrow Line Seyfert 1 galaxies, and the fourth as BL Lac object. We have measured small-FWHM Hbeta lines, strong FeII emission and weak [OIII] emission in the three Narrow Line Seyfert 1 galaxies, in line with known correlations with respect to the steepness of the X-ray spectra. We have discovered strong optical variability in the BL Lac object and two of the Seyfert galaxies using photographic plates of the Sonneberg Observatory field patrol. We finally discuss the statistical implications of our search algorithm on the expected number density of soft X-ray selected AGN and conclude that up to 30% of X-ray selected AGN might have supersoft X-ray spectra.

astro-ph↗

Spin effects in hard exclusive reactions

The present status of applications of perturbative QCD to large momentum transfer exclusive reactions is discussed. It is argued that in the region of momentum transfer accessible to present day experiments soft contributions dominate the exclusive processes.

hep-ph↗