Selected topics on Low Energy Antiproton Physics

Some of the last results on low energy antiproton physics are reviewed. First Faddeev calculations for \={n}d scattering length are presented.Comment: 10 pages, 11 figures Invited talk to the Biennial Conference on Low Energy Antiproton Physics (LEAP 2000), Venice 20-26 August (2000). To appear in Nuclear Physics

Stability of two-fermion bound states in the explicitly covariant Light-Front Dynamics

The covariant light-front equations have been solved exactly for a two fermion system with different boson exchange ladder kernels. We present a method to study the cutoff dependence of these equations and to determine whether they need to be regularized or not. Results are presented for scalar and pseudo-scalar exchange. This latter furthermore exhibits some strange particularities which will be discussed.Comment: 5 pages, 8 figures, to be published in Nucl. Phys. B (Proc. Suppl.), contribution to the XIth Light-cone Meeting at ECT* in Trento, Sep 3-11, 200

Relativistic wave functions and energies for nonzero angular momentum states in light-front dynamics

Light-front dynamics (LFD) is a powerful approach to the theory of relativistic composite systems (hadrons in the quark models and relativistic nucleons in nuclei). Its explicitly covariant version has been recently applied with success to describe the new CEBAF/TJNAF data on the deuteron electromagnetic form factors. The solutions used in were however not obtained from solving exactly the LFD equations but by means of a perturbative calculation with respect to the non relativistic wave function. Since, a consequent effort has been made to obtain exact solutions of LFD equations. The first results concerning J=0 states in a scalar model have been published in nucl-th/9912050. The construction of $J \ne 0$ states in LFD is complicated by the two following facts. First, the generators of the spatial rotations contain interaction and are thus difficult to handle. Second, one is always forced to work in a truncated Fock space, and consequently, the Poincar\'e group commutation relations between the generators -- ensuring the correct properties of the state vector under rotation -- are in practice destroyed. In the standard approach, with the light-front plane defined as $t+z=0$, this violation of rotational invariance manifests by the fact that the energy depends on the angular momentum projection on $z$-axis. We present here a method to construct $J\ne0$ states in the explicitly covariant formulation of LFD and show how it leads to a restoration of rotational invariance.Comment: To appear in Nucl.Phys.B, 3 pages, 2 figures, .tar.gz fil

Two-Body Bound States in Light-Front Dynamics

We present the main features of the explicitly covariant Light-Front Dynamics formalism and a summary of our recent works on this topic. They concern the bound states of two scalar particles in the Wick-Cutkosky model and of two fermions interacting via the usual OBEP ladder kernels.Comment: Invited contribution to the XIVth International School on Nuclear Physics, Varna, Bulgaria, Sept 25-30. 20 pages, 23 figure

Stability of bound states in the light-front Yukawa model

We show that in the system of two fermions interacting by scalar exchange, the solutions for J$^{\pi}$=$0^+$ bound states are stable without any cutoff regularization for coupling constant below some critical value.Comment: 5 pages, 2 figures, submitted to publicatio

Electromagnetic form factor via Bethe-Salpeter amplitude in Minkowski space

For a relativistic system of two scalar particles, we find the Bethe-Salpeter amplitude in Minkowski space and use it to compute the electromagnetic form factor. The comparison with Euclidean space calculation shows that the Wick rotation in the form factor integral induces errors which increase with the momentum transfer Q^2. At JLab domain (Q^2=10 GeV^2/c^2), they are about 30%. Static approximation results in an additional and more significant error. On the contrary, the form factor calculated in light-front dynamics is almost indistinguishable from the Minkowski space one.Comment: 8 pages, 7 figures, to be published in Eur. Phys. J. A; Reference [15] is adde

Two-fermion bound states in the explicitly covariant Light-Front Dynamics

We develop a general method to construct two relativistic fermions bound states with a given $J^{\pi}$, in the framework of the explicitly covariant light-front dynamics

Relativistic bound-state calculations in Light Front Dynamics

We calculated bound states in the quantum field theoretical approach. Using the Wick-Cutkosky model and an extended version of this model (in which a particle with finite mass is exchanged) we have calculated the bound states in the scalar case.Comment: 3 pages, proceedings of the Light Cone Meeting Trento 2001, to be published in Nucl. Phys. B - Proceedings Supplement

Relativistic bound states in Yukawa model

The bound state solutions of two fermions interacting by a scalar exchange are obtained in the framework of the explicitly covariant light-front dynamics. The stability with respect to cutoff of the J$^{\pi}$=$0^+$ and J$^{\pi}$=$1^+$ states is studied. The solutions for J$^{\pi}$=$0^+$ are found to be stable for coupling constants $\alpha={g^2\over4\pi}$ below the critical value $\alpha_c\approx 3.72$ and unstable above it. The asymptotic behavior of the wave functions is found to follow a ${1\over k^{2+\beta}}$ law. The coefficient $\beta$ and the critical coupling constant $\alpha_c$ are calculated from an eigenvalue equation. The binding energies for the J$^{\pi}$=$1^+$ solutions diverge logarithmically with the cutoff for any value of the coupling constant. For a wide range of cutoff, the states with different angular momentum projections are weakly split.Comment: 22 pages, 13 figures, .tar.gz fil

Electromagnetic form factor via Minkowski and Euclidean Bethe-Salpeter amplitudes

The electromagnetic form factors calculated through Euclidean Bethe-Salpeter amplitude and through the light-front wave function are compared with the one found using the Bethe-Salpeter amplitude in Minkowski space. The form factor expressed through the Euclidean Bethe-Salpeter amplitude (both within and without static approximation) considerably differs from the Minkowski one, whereas form factor found in the light-front approach is almost indistinguishable from it.Comment: 3 pages, 2 figures. Contribution to the proceedings of the 20th International Conference on Few-Body Problems in Physics (FB20), Pisa, Italy, September 10-14, 2007. To be published in "Few-Body Systems