Bunch coherence in parametric X-ray radiation

The amplitude of Parametric X-ray radiation emitted coherently by a bunch of electrons crossing a crystal (CPXR) is calculated. When the bunch density is modulated with a longitudinal period close to the X-ray wavelenght, constructive many-electron interferences enhance the intensity by Nb * |F(K)|^2, where Nb is the number of electrons in the bunch, F(K) the bunch form factor and K a specified wave vector. CPXR can be used to test the microbunching in a X-ray free-electron laser.Comment: 6 pages, no figure. Presented at NATO Advanced Research Workshop on Electron-Photon Interaction in Dense Media (Nor Hamberd, Armenia, 25-29 June, 2001

Considerations on the Diffraction Limitations to the Spatial Resolution of Optical Transition Radiation

The interest in using optical transition radiation (OTR) in high energy (multiGeV) beam diagnostics has motivated theoretical and experimental investigations on the limitations brought by diffraction on the attainable resolution. This paper presents calculations of the diffraction effects in an optical set-up using OTR. The OTR diffraction pattern in a telescopic system is calculated taking into account the radial polarization of OTR. The obtained diffraction pattern is compared to the patterns obtained by other authors and the effects of different parameters on the shape and on the size of the OTR diffraction pattern are studied. The major role played by the radial polarization on the shape of the diffraction pattern is outlined. An alternative method to calculate the OTR diffraction pattern is also sketchedComment: Submitted to Particle Accelerator

The Lund Model at Nonzero Impact Parameter

We extend the formulation of the longitudinal 1+1 dimensional Lund model to nonzero impact parameter using the minimal area assumption. Complete formulae for the string breaking probability and the momenta of the produced mesons are derived using the string worldsheet Minkowskian helicoid geometry. For strings stretched into the transverse dimension, we find probability distribution with slope linear in m_T similar to the statistical models but without any thermalization assumptions.Comment: 12 pages, 2 .eps figures, footnotes added, results unchanged, version to be published in Phys.Lett.

Area Decay Law Implementation for Quark String Fragmentation

We apply the Area Decay Law (ADL) straightforwardly to simulate a quark string hadronization and compare the results with the explicit analytic calculations. We show that the usual "inclusive" Monte--Carlo simulations do not correspond to the ADL because of two mistakes: not proper simulation of two--dimensional probability density and lack of an important combinatorial factor in a binary tree simulation. We also show how to simulate area decay law "inclusively" avoiding the above--mentioned mistakes.Comment: 5 pages (REVTEX) + 3 figures (available in ps format from G.G.Leptoukh , IPGAS-HE/93-3, to be published in Phys. Rev.

Direct intrahepatic portocaval shunt (DIPS) or transjugular transcaval intrahepatic portosystemic shunt (TTIPS) to treat complications of portal hypertension: Indications, technique, and outcomes beyond Budd-Chiari syndrome.

Transjugular intrahepatic portosystemic shunt (TIPS) is nowadays the benchmark treatment of severe portal hypertension complications. However, besides usual contraindication to the procedure (namely recurrent hepatic encephalopathy, severe liver dysfunction, right heart failure and/or pulmonary hypertension), TIPS appears regularly unfeasible due to abnormal and/or distorted anatomy. In this situation, the only non-surgical approaches to treat severe portal hypertension consist in the creation of an intrahepatic portocaval shunt from percutaneous (direct intrahepatic portocaval shunt - DIPS) or transjugular route (transjugular transcaval intrahepatic portosystemic shunt - TTIPS). These procedures have been rapidly adopted in patients with Budd-Chiari syndrome but are only poorly reported in patients with cirrhosis and without BCS. Considering the broadening landscape of TIPS indication in patients with cirrhosis within the last ten years, we aimed to describe the techniques, safety and efficacy of DIPS and TTIPS procedures as an alternative to TIPS in case of unfavourable anatomy

On asimuthal anisotropy in fragmentation of classical relativistic string

A fragmenting relativistic string is widely used for modelling particle production via quark-gluon strings formed in hadron inelastic interactions of high energies. In this note we focus on motion and fragmentation of relativistic string with non-zero transverse separation of its ends and study this scenario as a possible mechanism bringing anisotropy into the asimuthal angle disribution of produced particles in inelastic interactions of hadrons.Comment: 12 pages, 6 figure

Monte-Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions

The complete knowledge of the nucleon spin structure at leading twist requires also addressing the transverse spin distribution of quarks, or transversity, which is yet unexplored because of its chiral-odd nature. Transversity can be best extracted from single-spin asymmetries in fully polarized Drell-Yan processes with antiprotons, where valence contributions are involved anyway. Alternatively, in single-polarized Drell-Yan the transversity happens convoluted with another chiral-odd function, which is likely to be responsible for the well known (and yet unexplained) violation of the Lam-Tung sum rule in the corresponding unpolarized cross section. We present Monte-Carlo simulations for the unpolarized and single-polarized Drell-Yan $\bar{p} p^{(\uparrow)} \to \mu^+ \mu^- X$ at different center-of-mass energies in both configurations where the antiproton beam hits a fixed proton target or it collides on another proton beam. The goal is to estimate the minimum number of events needed to extract the above chiral-odd distributions from future measurements at the HESR ring at GSI. It is important to study the feasibility of such experiments at HESR in order to demonstrate that interesting spin physics can be explored already using unpolarized antiprotons.Comment: Deeply revised text with improved discussion of kinematics and results; added one table; 12 figures. Accepted for publication in Phys. Rev.

Effects of Quark Spin Flip on the Collins Fragmentation Function in a Toy Model

The recent extension of the NJL-jet model to hadronization of transversely polarized quarks allowed the study of the Collins fragmentation function. Both favored and unfavored Collins fragmentation functions were generated, the latter purely by multiple hadron emissions, with 1/2 moments of opposite sign in the region of the light-cone momentum fraction $z$ accessible in current experiments. Hints of such behavior has been seen in the measurements in several experiments. Also, in the transverse momentum dependent (TMD) hadron emission probabilities, modulations of up to fourth order in sine of the polar angle were observed, while the Collins effect describes just the linear modulations. A crucial part of the extended model was the calculation of the quark spin flip probability after each hadron emission in the jet. Here we study the effects of this probability on the resulting unfavored and favored Collins functions by setting it as a constant and use a toy model for the elementary single hadron emission probabilities. The results of the Monte Carlo simulations showed that preferential quark spin flip in the elementary hadron emission is needed to generate the favored and unfavored Collins functions with opposite sign 1/2 moments. For the TMD hadron emission modulations, we showed that the model quark spin flip probabilities are a partial source of the higher rode modulations, while the other source is the Collins modulation of the remnant quark from the hadron emission recoil.Comment: 7 pages, 6 figures. To appear in proceedings of HITES 2012, Conference in Honor of Jerry P. Draayer, Horizons of Innovative Theories, Experiments, and Supercomputing in Nuclear Physics, New Orleans, Louisiana, June 4-7, 201

Shadowing of the electromagnetic field of a relativistic electron

In coherent radiation sources (diffraction radiation, Smith-Purcell effect, etc.) based on relativistic electrons passing by a material radiator, the electron self-field is partly shadowed after each part of the radiator over a distance of the order of the formation length g2l. This effect has been investigated on coherent diffraction radiation (DR) by electron bunches. An absorbing half-plane screen was placed at various distances L before a standard DR source (inclined half-plane mirror). The DR intensity was reduced when the screen was at small L and on the same side as the mirror. No reduction was observed when the screen was on the opposite side. The shadowing effect can significantly reduce the total energy radiated in a long radiator.Comment: Was Presented at the symposium Channeling 2008 (Charged and Neutral Particles Channeling Phenomena), October 25 - November 1, 2008 Erice (Trapani - Sicily), Italy, and was accepted for publication in proceedings of symposiu