arXiv subjects

M. Beyer

Publications and source records attributed to M. Beyer.

At least 73 records · Page 4Linked to original sources

arXiv1999

Relativistic Quark Spin Coupling Effects in the Nucleon Electromagnetic Form Factors

We investigate the effect of different forms of relativistic spin coupling of constituent quarks in the nucleon electromagnetic form factors. The four-dimensional integrations in the two-loop Feynman diagram are reduced to the null-plane, such that the light-front wave function is introduced in the computation of the form factors. The neutron charge form factor is very sensitive to different choices of spin coupling schemes, once its magnetic moment is fitted to the experimental value. The scalar coupling between two quarks is preferred by the neutron data, when a reasonable fit of the proton magnetic momentum is found.

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arXiv1999

Few-Body States in Fermi-Systems and Condensation Phenomena

Residual interactions in many particle systems lead to strong correlations. A multitude of spectacular phenomenae in many particle systems are connected to correlation effects in such systems, e.g. pairing, superconductivity, superfluidity, Bose-Einstein condensation etc. Here we focus on few-body bound states in a many-body surrounding.

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arXiv1999

Deuteron Formation in Heavy Ion Collisions within the Faddeev Approach

We address the formation of correlations in a general nonequilibrium situation. As an example we calculate the formation of light composite particles in a heavy ion collision. In particular we study the formation of deuterons via three-body reactions in some detail. To calculate the relevant reaction rates we solve the Faddeev equation which is consistently modified to include the self energy corrections and Pauli blocking factors due to surrounding nucleons. We find that the time scales of the reaction as well as the number of emitted deuterons change as medium dependent rates are used in the calculation.

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arXiv1999

Deuteron formation in nuclear matter

We investigate deuteron formation in nuclear matter at finite temperatures within a systematic quantum statistical approach. We consider formation through three-body collisions relevant already at rather moderate densities because of the strong correlations. The three-body in-medium reaction rates driven by the break-up cross section are calculated using exact three-body equations (Alt-Grassberger-Sandhas type) that have been suitably modified to consistently include the energy shift and the Pauli blocking. Important quantities are the lifetime of deuteron fluctuations and the chemical relaxation time. We find that the respective times differ substantially while using in-medium or isolated cross sections. We expect implications for the description of heavy ion collisions in particular for the formation of light charged particles at low to intermediate energies.

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arXiv1999

Feynman versus Bakamjian-Thomas in Light Front Dynamics

We compare the Bakamjian-Thomas (BT) formulation of relativistic few-body systems with light front field theories that maintain closer contact with Feynman diagrams. We find that Feynman diagrams distinguish Melosh rotations and other kinematical quantities belonging to various composite subsystem frames that correspond to different loop integrals. The BT formalism knows only the rest frame of the whole composite system, where everything is evaluated.

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arXiv1999

Configuration Mixing in Relativistic Quark Models

Starting from the standard and commonly used relativistic nucleon wave function, we show that residual interactions based on gluon or Goldstone boson exchanges generate additional components leading to configuration mixing. This result suggests that realistic nucleon wave functions can be expected to be far more complex than seemingly successful form factor fits in relativistic quark models would have one believe.

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arXiv1999

Three-body properties in hot and dense nuclear matter

We derive three-body equations valid at finite densities and temperatures. These are based on the cluster mean field approach consistently including proper self energy corrections and the Pauli blocking. As an application we investigate the binding energies of triton and determine the Mott densities and momenta relevant for a many particle description of nuclear matter in a generalized Beth-Uhlenbeck approach. The method, however is not restricted to nuclear physics problems but may also be relevant, e.g., to treat three-particle correlations in weekly doped semiconducters or strongly coupled dense plasmas.

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arXiv1999

Deuteron formation in nuclear matter within the Faddeev approach

We consider deuteron formation in heavy ion collisions at intermediate energies. The elementary reaction rates (Nd -> NNN) in this context are calculated using rigorous Faddeev methods. To this end an in-medium Faddeev equation that consistently includes the energy shift and Pauli blocking effects has been derived and solved numerically. As a first application we have calculated the life-time of deuteron fluctuations for nuclear densities and temperatures typical for the final stage of heavy ion collisions. We find substantial differences between using the isolated and the in-medium rates.

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arXiv1999

Pionic coupling constants of heavy mesons in the quark model

We analyse pionic couplings of heavy mesons combining PCAC with the dispersion quark model to calculate the relevant transition form factors. Ground states and radial excitations are considered. For the ground state coupling constants the values $\hat g=0.5\pm0.02$ in the heavy quark limit, $g_{B^*Bπ}=40\pm 3$, $g_{D^*Dπ}=16\pm 2$ are obtained. A sizeable suppression of the coupling constants describing the pionic decays of the radial excitations is observed.

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arXiv1998

Light-front versus Bethe-Salpeter forms of two nucleon amplitudes

We discuss the relation between the two nucleon Bethe-Salpeter amplitude and the light front wave functions. Both approaches provide a covariant description for the deuteron bound state and the two nucleon scattering state. A comparison is done for the spin-orbit functions as well explicit integrals are given on the basis of the Nakanishi integral representation method.

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arXiv1998

Test of Time Reversal Invariance in Polarized Proton-Deuteron Scattering

A novel test of time-reversal invariance in proton-deuteron scattering is planned as an internal target transmission experiment at the cooler synchrotron COSY. The P-even, T-odd observable is the polarization correlation $A_{y,xz}$ of the total cross section measured using a polarized internal proton beam (polarization $p_y$) and an internal polarized deuterium target (tensor polarization $p_{xz}$). Measuring this observable is a true null test of time reversal invariance and therefore allows to reach a high accuracy. Sufficient luminosity can be obtained using a window-less storage cell placed on the axis of the proton beam. Tensor polarized atoms are produced in an atomic beam source based on Stern-Gerlach separation in permanent sextupole magnets and adiabatic high frequency transitions. The total cross section correlation is measured by monitoring the beam transmission in the COSY storage ring mode of operation. The proton beam momentum will be in the range 2-3 GeV/c. This momentum is ideally suited to test possible short range contributions, i.e. natural parity charged $ρ$-type and unnatural parity $a_1$-type meson exchange contributions. The feasibility of the experiment, systematic errors and the expected accuracy are discussed.

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arXiv1998

On isovector meson exchange currents in the Bethe-Salpeter approach

We investigate the nonrelativistic reduction of the Bethe-Salpeter amplitude for the deuteron electrodisintegration near threshold energies. To this end, two assumptions have been used in the calculations: 1) the static approximation and 2) the one iteration approximation. Within these assumptions it is possible to recover the nonrelativistic result including a systematic extension to relativistic corrections. We find that the so-called pair current term can be constructed from the $P$-wave contribution of the deuteron Bethe-Salpeter amplitude. The form factor that enters into the calculation of the pair current is constrained by the manifestly gauge independent matrix elements.

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arXiv1998

Form factors of exclusive $b\to u$ transitions

We present the form factors of the $B\toπ,ρ$ transitions induced by the $b\to u$ quark currents at all kinematically accessible $q^2$. Our analysis is based on the spectral representations of the form factors within the constituent quark picture: we fix the soft meson wave functions and the constituent quark masses by fitting $A_1(q^2)$ and $T_2(q^2)$ to the lattice results at small recoils ($17\lesssim q^2\lesssim 20 GeV^2$). We then calculate the $B\toπ,ρ$ transition form factors down to $q^2=0$. For the $B\toπ$ case the region $q^2\lesssim 20 GeV^2$ however does not cover the whole kinematically accessible range. Due to the smallness of the pion mass the region of small recoils is close to the nearby $B^*(5234)$ resonance. We develop a parametrization which includes the $B^*$ dominance of the form factors $f_+$ and $f_-$ at small recoils and numerically reproduces the results of calculations at $q^2\lesssim 20 GeV^2$. We find $Γ(B\toπ\ellν)=8.0^{+0.8}_{-0.2} |V_{ub}|^2 $ps$^{-1}$ and $Γ(B\toρ\ellν)=15.8\pm 2.3 |V_{ub}|^2 $ps$^{-1}$.

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arXiv1998

Relativistic Spin-Flavor States in Light Front Dynamics

Orthonormal spin-flavor wave functions of Lorentz covariant quark models of the Bakamjian-Thomas type are constructed for nucleon resonances. Three different bases are presented. The manifestly Lorentz covariant Dirac-Melosh basis is related to the Pauli-Melosh basis and the symmetrized Bargmann-Wigner basis that are manifestly orthogonal.

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arXiv1997

Determination of V_cb from exclusive decays in a relativistic quark model

In the framework of a relativistic covariant Bethe-Salpeter model for the quark-antiquark system we present a renewed determination of the Cabbibo-Kobayashi-Maskawa matrix element $|V_{cb}|$. Complementing an earlier analysis applied to the whole decay spectrum for $B\to D^* eν$ we now also employ the ``zero-recoil method'' that uses the end point of the decay spectrum ($ω=1$) and is suited for heavy-to-heavy transitions. The averaged experimental value extracted from the data at zero recoil, $|V_{cb}|{\cal F}(ω=1)=0.0343\pm0.0015$, then leads to $|V_{cb}|=0.0360\pm 0.0016$. This value is somewhat larger than the one that uses the whole decay spectrum for the model analysis. We also contrast this result to a nonrelativistic model and to recent experiments on the $B\to D eν$ semileptonic decay.

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arXiv1997

Deuteron electrodisintegration near threshold in the Bethe-Salpeter approach

In the framework of the covariant Bethe-Salpeter formalism we calculate the amplitude of electrodisintegration of the deuteron near the threshold energy. Due to the small relative momentum of the final np-pair, the deuteron break-up process may be sufficiently described by a transition to the dominant 1S0 channel into the final state. Therefore only one unknown structure function (form factor) exists. The explicit formula for this form factor is given in terms of partial Bethe-Salpeter amplitudes. We compare our result to the usual non-relativistic calculations. The essential role of P-states as well as their possible link to mesonic exchange currents is discussed.

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arXiv1997

Deuteron life-time in hot and dense nuclear matter near equilibrium

We consider deuteron formation in hot and dense nuclear matter close to equilibrium and evaluate the life-time of the deuteron fluctuations within the linear response theory. To this end we derive a generalized linear Boltzmann equation where the collision integral is related to equilibrium correlation functions. In this framework we then utilize finite temperature Green functions to evaluate the collision integrals. The elementary reaction cross section is evaluated within the Faddeev approach that is suitably modified to reflect the properties of the surrounding hot and dense matter.

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arXiv1996

Can we learn anything new from polarization observables of the deuteron disintegration near threshold?

We discuss polarization characteristics of the deuteron disintegration near the threshold energy. Due to the small relative energy of the outgoing $np$ pair, the dominant amplitude for this process is a 1+ -> 0^+ transition. Relativistic covariance requires only one electromagnetic transition form factor for this amplitude. Subsequently this leads to substantial simplifications in the formulas of the polarization observables and allows to draw conclusions independent of the details of the interaction.

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