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M. Dillig

Publications and source records attributed to M. Dillig.

17 recordsLinked to original sources
arXiv2006

Energy Dependence of the Lambda-Sigma0 Production in Proton-Proton Collisions

The energy dependence of the reactions pp -> p Lambda K+ and pp -> p Sigma0 K+ and the ratio R(Lambda/Sigma0} is studied in a constituent quark-gluon model, including the excitation of the baryon resonances N*(1650), N*(1710) and N*(1720) near the Lambda/Sigma0 thresholds. Representing the baryons as quark-diquark objects, the energy dependent ratio Lambda/Sigma0, which is qualitatively reproduced up to excess energies of 60 MeV above threshold, provides detailed information on the momentum spectrum of axial diquarks in the proton and the Sigma0.

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arXiv2006

Relativistic Covariance and Quark-Diquark Wave Functions

We derive covariant wave functions for hadrons composed of two constituents for arbitrary Lorentz boosts. Focussing explicitly on baryons as quark-diquark systems, we reduce their manifestly covariant Bethe-Salpeter equation to covariant 3-dimensional forms by projecting on the relative quark-diquark energy. Guided by a phenomenological multi gluon exchange representation of covariant confining kernels, we derive explicit solutions for harmonic confinement and for the MIT Bag Model. We briefly sketch implications of breaking the spherical symmetry of the ground state and the transition from the instant form to the light cone via the infinite momentum frame.

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arXiv2006

From Dimensional to Cut-Off Regularization

We extent the standard approach of dimensional regularization of Feynman diagrams: we replace the transition to lower dimensions by a 'natural' cut-off regulator. Introducing an external regulator of mass Lambda^(2e), we regain in the limit e -> 0 and e > 0 the results of dimensional and cut-off regularization, respectively. We demonstrate the versatility and adequacy of the different regularization schemes for practical examples (such as non covariant regularization, the axial anomaly or regularization in effective field theories).

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arXiv2006

OZI violation in low energy omega and phi production in the pp system in a quark-gluon model

We investigate OZI violation in near-threshold omega and phi production in the pp system. Assuming ideal omega/phi mixing (corrections are estimated), the energy dependence of the ratio R(omega/phi) is analyzed in a perturbative quark-gluon exchange model up to the third other in the strong coupling constant alpha(s) with the proton represented as a quark - scalar diquark system. We give a very natural explanation of the violation of the OZI rule in omega/phi production and its energy dependence near the production thresholds.

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arXiv2006

Production and Width of the Pentaquark Theta+(1540) in Strangeness Production in the Proton-Proton System

We analyze recent experimental information on the excitation of the Theta+(1540) pentaquark in the hadronic reaction pp -> p Sigma+ K0. Upon describing the conventional production process - which serves as a normalization - in a meson exchange model, we estimate the resonant Theta+ induced cross section and the width of the pentaquark via K - exchange in a meson-baryon and a quark-gluon model. From a comparison with experiment we extract information on the width of the Theta+, the p Theta+ K0 coupling constant in the meson exchange and the relative s or p wave function for a nucleon-kaon cluster in the Theta+.

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arXiv2006

Mesonic Content of the Nucleon and the Roper Resonance

We investigate colorless (mesonic) 3q-q qbar components in the nucleon and the Roper resonance N*(1440). Starting from constituent quarks and gluons we estimate the excitation of quark-antiquark pairs in a gluon exchange model with the strong coupling constant extracted from a comparison with a non-perturbative resonating group calculation of the pi, eta, rho and omega content of the nucleon. Applying the same model to the Roper resonance, we find as the most striking result a very strong scalar-isoscalar sigma content of the N*(1440) with a strength of the 3q-q qbar configurations comparable to the 3q component itself.

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arXiv2006

Quenching of the Deuteron in Flight

We investigate the Lorentz contraction of a deuteron in flight. Our starting point is the Blankenbecler-Sugar projection of the Bethe-Salpeter equation to a 3-dimensional quasi potential equation, wqhich we apply for the deuteron bound in an harmonic oscillator potential (for an analytical result) and by the Bonn NN potential for a more realistic estimate. We find substantial quenching with increasing external momenta and a significant modification of the high momentum spectrum of the deuteron.

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arXiv2006

Quark Model Estimates of the Structure of the Meson-N-N*(1535,1/2-) Transition Vertices

We address an actual problem of baryon-resonance dominated meson-exchange processes in the low GeV regime, i.e. the phase and the structure of meson-NN* transition vertices. Our starting point is a quark-diquark model for the baryons (obeying approximate covariance; the mesons are kept as elementary objects), together with the relative phases for the NN vertices, as determined from low energy NN scattering. From the explicit representation of the N and N* baryons, we exemplify the derivation of the coupling constants and form factors of the NN*(1535,1/2-)transition vertices for pseudo-scalar, scalar and vector mesons.

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arXiv2006

Multinucleon Mechanisms in Inclusive Subthreshold K+ Production in p+C12 Collisions at 1 GeV

We analyze recent inclusive data on K+ production in proton induced Lambda-multi nucleon knock out on Carbon 12. We describe collective effects among the incoming p and bound nucleons in a pi, rho rescattering model via the excitation of various baryon resonances. We find the dominance of Lambda N and Lambda NN final states for K+ momenta below 200 MeV/c, while for momenta above 400 MeV/c the data reflect a continuum with an unbound Lambda together with the bound residual nucleus C 12.

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arXiv2002

Systematics of the Exclusive Meson Production in the Proton-Proton System in Relativistic Quark-Models

We investigate the exclusive production of the pseudoscalar mesons pi0, eta, eta', K+ and of the vector mesons omega, phi in a microscopic gluon-exchange or instanton model. We describe the baryons as covariant quark - scalar diquark systems with harmonic confinement, thus taking into account center-of-mass corrections and Lorentz contraction in different frames. The excitation of intermediate baryon resonances is accounted by colorless 2-gluon (soft Pomeron) exchange. We find that our model accounts for the systematics of the high precision data on exclusive meson production from various modern proton factories.

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arXiv2002

Baryon Wave Functions in Covariant Relativistic Quark Models

We derive covariant baryon wave functions for arbitrary Lorentz boosts. Modeling baryons as quark-diquark systems, we reduce their manifestly covariant Bethe-Salpeter equation to a covariant 3-dimensional form by projecting on the relative quark-diquark energy. Guided by a phenomenological multigluon exchange representation of a covariant confining kernel, we derive for practical applications explicit solutions for harmonic confinement and for the MIT Bag Model. We briefly comment on the interplay of boosts and center-of-mass corrections in relativistic quark models.

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arXiv1998

Protoninduced Pion-, Eta- and Kaon-Production at Threshold

We present some details of our recent theoretical investigations on the total cross section of the reactions $pp\to pp π^0$, $pp\to pp η$ and $pp\to pΛK^+$ at threshold. Applying standard nonrelativistic perturbation theory within the framework of meson exchange models (MEC) to all of the investigated processes we observe that theory is still far--off from a quantitative understanding of the existing data, although the qualitative behaviour of the threshold cross sections can be reproduced. The often quoted $η^2$--dependence of the $pp\to pp π^0$ cross section at threshold is due to a calculational mistake of Koltun and Reitan (1966) and not based on theoretical grounds, where threebody phasespace behaviour would yield $η^4$, which can be affected by final state and short range effects. To respect the high momentum transfers for $pp\to pp η$ and $pp\to pΛK^+$ relativistic models were developed. Hereby the $η$--production still is based on MEC, while the $K^+$ production was investigated in the quark-gluon-picture based on the Bethe--Salpeter formalism.

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arXiv1997

Photoproduction of neutral pion pairs in the Coulomb field of the nucleus

The total cross section for Coulomb photoproduction of neutral pion pairs in the reaction gamma A --> pi0 pi0 A is estimated within the effective chiral lagrangian approach. The amplitude of gamma gamma* --> pi0 pi0 with one off-shell photon is calculated at O(p^6) in the momentum expansion; in addition, nuclear absorption is taken into account. Besides its experimental feasibility, the results of the calculation demonstrate that the reaction gamma A --> pi0 pi0 A is a powerful source of information on the process gamma gamma --> pi0 pi0 close to threshold.

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arXiv1996

Spin-3/2 fields in the heavy-baryon effective theory

In the framework of the path integral approach we develop the velocity component formalism for spin-1/2 and -3/2 baryons coupled with pseudoscalar mesons in the limit of heavy baryons. Starting from the most general chiral meson-baryon lagrangian including the invariance of spin-3/2 fields under point transformation we detail various problems in integrating out the heavy velocity components in the baryon fields and derive the effective lagrangian for the light components up to O(1/M_{baryon}).

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arXiv1996

Chiral Symmetry in algebraic and analytic approaches

We compare among themselves two different methods for the derivation of results following from the requirement of polynomial boundedness of tree-level chiral amplitudes. It is shown that the results of the algebraic approach are valid also in the framework of the analytical one. This means that the system of Sum Rules and Bootstrap equations previously obtained with the help of the latter approach can be analyzed in terms of reducible representations of the unbroken Chiral group with the known structure of the mass matrix.

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arXiv1995

Analysis of data on low energy piN-->pipiN reaction. I. Total cross sections

This is the first of a series of papers on a consistent model independent analysis of the complete experimental information on the reaction $πN \rightarrow ππN$ at pion momenta up to 500 MeV/c. The paper summarizes the theoretical approach and details of the computational procedure. The complete database on total cross sections in 5 $ππN$ channels is given together with a critical discussion of their model independent analysis.

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