Helicity amplitudes and electromagnetic decays of hyperon resonances

We present results for the helicity amplitudes of the lowest-lying hyperon resonances Y*, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model were fitted to the best known baryon masses. Accordingly, the results for the helicity amplitudes are genuine predictions. Some hyperon resonances are seen to couple more strongly to a virtual photon with finite Q^2 than to a real photon. Other Y*'s, such as the S_{01}(1670) Lambda resonance or the S_{11}(1620) Sigma resonance, couple very strongly to real photons. We present a qualitative argument for predicting the behaviour of the helicity asymmetries of baryon resonances at high Q^2.Comment: 20 pages, 26 figures, uses svjour.cls style, submitted to the European Physical Journal

The Renyi entropy H_2 as a rigorous, measurable lower bound for the entropy of the interaction region in multiparticle production processes

A model-independent lower bound on the entropy S of the multiparticle system produced in high energy collisions, provided by the Renyi entropy H_2, is shown to be very effective. Estimates show that the ratio H_2/S remains close to one half for all realistic values of the parameters.Comment: Eur. Phys. J. C in print, 17 pages, 5 figure

Bragg spectroscopy of discrete axial quasiparticle modes in a cigar-shaped degenerate Bose gas

We propose an experiment in which long wavelength discrete axial quasiparticle modes can be imprinted in a 3D cigar-shaped Bose-Einstein condensate by using two-photon Bragg scattering experiments, similar to the experiment at the Weizmann Institute [J. Steinhauer {\em et al.}, Phys. Rev. Lett. {\bf 90}, 060404 (2003)] where short wavelength axial phonons with different number of radial modes have been observed. We provide values of the momentum, energy and time duration of the two-photon Bragg pulse and also the two-body interaction strength which are needed in the Bragg scattering experiments in order to observe the long wavelength discrete axial modes. These discrete axial modes can be observed when the system is dilute and the time duration of the Bragg pulse is long enough.Comment: 5 pages, 3 figures, title, abstract, results changed, references added. to appear in The European Physical Journal

Bright solitons and soliton trains in a fermion-fermion mixture

We use a time-dependent dynamical mean-field-hydrodynamic model to predict and study bright solitons in a degenerate fermion-fermion mixture in a quasi-one-dimensional cigar-shaped geometry using variational and numerical methods. Due to a strong Pauli-blocking repulsion among identical spin-polarized fermions at short distances there cannot be bright solitons for repulsive interspecies fermion-fermion interactions. However, stable bright solitons can be formed for a sufficiently attractive interspecies interaction. We perform a numerical stability analysis of these solitons and also demonstrate the formation of soliton trains. These fermionic solitons can be formed and studied in laboratory with present technology.Comment: 5 pages, 7 figure

Dihadron fragmentation: in vacuum and in matter

Two particle correlations within a single jet produced in deeply inelastic scattering (DIS) off a large nucleus as well as in heavy-ion collisions are explored. This is performed within the framework of the medium modified dihadron fragmentation functions. The modification occurs due to gluon bremsstrahlung induced by multiple scattering. The modified fragmentation functions for dihadrons are found to follow closely that of single hadrons leading to a weak nuclear suppression of their ratios as measured by HERMES in DIS experiments. Meanwhile, a moderate medium enhancement of the near-side correlation of two high $p_T$ hadrons is found in central heavy-ion collisions, partially due to trigger bias caused by the competition between parton energy loss and the initial Cronin effect.Comment: 4 pages, 2 figures, LATEX, talk given at Hard Probes 200

The Experimental Status of the Standard Electroweak Model at the End of the LEP-SLC Era

A method is proposed to calculate the confidence level for agreement of data with the Standard Model (SM) by combining information from direct and indirect Higgs Boson searches. Good agreement with the SM is found for $m_H \simeq 120$ GeV using the observables most sensitive to $m_H$: $A_l$ and $m_W$. In particular, quantum corrections, as predicted by the SM, are observed with a statistical significance of forty-four standard deviations. However, apparent deviations from the SM of 3.7$\sigma$ and 2.8$\sigma$ are found for the Z$\nu \bar{\nu}$ and right-handed Zb$\bar{{\rm b}}$ couplings respectively. The maximum confidence level for agreement with the SM of the entire data set considered is $\simeq 0.006$ for $m_H \simeq 180$ GeV. The reason why confidence levels about an order of magnitude higher than this have been claimed for global fits to similar data sets is explained.Comment: 47 pages, 8 figures, 24 tables. An in-depth study of statistical issues related to the comparison of precision EW data to the S

Nucleon form factors, B-meson factories and the radiative return

The feasibility of a measurement of the electric and magnetic nucleon form factors at $B$-meson factories through the radiative return is studied. Angular distributions allow a separation of the contributions from the two form factors. The distributions are presented for the laboratory and the hadronic rest frame, and the advantages of different coordinate systems are investigated. It is demonstrated that $Q^2$ values up to 8 or even 9 GeV$^2$ are within reach. The Monte Carlo event generator PHOKHARA is extended to nucleon final states, and results are presented which include Next-to-Leading Order radiative corrections from initial-state radiation. The impact of angular cuts on rates and distributions is investigated and the relative importance of radiative corrections is analysed.Comment: 9 pages, 11 figures. Final version to appear in Eur. Phys. J.

Radiation induced force between two planar waveguides

We study the electromagnetic force exerted on a pair of parallel slab waveguides by the light propagating through them. We have calculated the dependence of the force on the slab separation by means of the Maxwell--Stress tensor formalism and we have discussed its main features for the different propagation modes: spatially symmetric (antisymmetric) modes give rise to an attractive (repulsive) interaction. We have derived the asymptotic behaviors of the force at small and large separation and we have quantitatively estimated the mechanical deflection induced on a realistic air-bridge structure.Comment: 10 pages, 6 figure

Measurement of the LT-asymmetry in \pi^0 electroproduction at the energy of the \Delta (1232) resonance

The reaction p(e,e'p)pi^0 has been studied at Q^2=0.2 (GeV/c)^2 in the region of W=1232 MeV. From measurements left and right of q, cross section asymmetries \rho_LT have been obtained in forward kinematics \rho_LT(\theta_\pi^0=20deg) = (-11.68 +/- 2.36_stat +/- 2.36_sys)$ and backward kinematics \rho_LT(\theta_\pi^0=160deg) =(12.18 +/- 0.27_stat +/- 0.82_sys). Multipole ratios \Re(S_1+^* M_1+)/|M_1+|^2 and \Re(S_0+^* M_1+)/|M_1+|^2 were determined in the framework of the MAID2003 model. The results are in agreement with older data. The unusally strong negative \Re(S_0+^* M_1+)/|M_1+|^2 required to bring also the result of Kalleicher et al. in accordance with the rest of the data is almost excluded.Comment: 7 pages, 7 figures, 4 tables. Changed content. Accepted for publication in EPJ

Transport of a quantum degenerate heteronuclear Bose-Fermi mixture in a harmonic trap

We report on the transport of mixed quantum degenerate gases of bosonic 87Rb and fermionic 40K in a harmonic potential provided by a modified QUIC trap. The samples are transported over a distance of 6 mm to the geometric center of the anti-Helmholtz coils of the QUIC trap. This transport mechanism was implemented by a small modification of the QUIC trap and is free of losses and heating. It allows all experiments using QUIC traps to use the highly homogeneous magnetic fields that can be created in the center of a QUIC trap and improves the optical access to the atoms, e.g., for experiments with optical lattices. This mechanism may be cascaded to cover even larger distances for applications with quantum degenerate samples.Comment: 7 pages, 8 figure