Transverse momentum correlations of quarks in recursive jet models

In the symmetric string fragmentation recipe adopted by PYTHIA for jet simulations, the transverse momenta of successive quarks are uncorrelated. This is a simplification but has no theoretical reason. Transverse momentum correlations are naturally expected, for instance, in a covariant multiperipheral model of quark hadronization. We propose a simple recipe of string fragmentation which lead to such correlations. The definition of the jet axis and its relation with the primordial transverse momentum of the quark is also discussed.Comment: 7 pages, 3 figures, 1 tabl

Collins asymmetry in field ionisation of hydrogen

International audienceAn effect similar to the Collins asymmetry is found in the ionization of a hydrogen atom by a static electric field \Ev. When the initial electron possesses an orbital angular momentum $\langle\mathbf{L}\rangle$ transverse to the field, the mean transverse velocity \langle\vv_{\rm T}\rangle of the final electron points in the direction of \Ev\times\langle\mathbf{L}\rangle. However $\langle\mathbf{L}\rangle$ is oscillating in time due to the linear Stark effect, making \langle\vv_{\rm T}\rangle oscillate

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.

Single-spin Azimuthal Asymmetries in the ``Reduced Twist-3 Approximation''

We consider the single-spin azimuthal asymmetries recently measured at the HERMES experiment for charged pions produced in semi-inclusive deep inelastic scattering of leptons off longitudinally polarized protons. Guided by the experimental results and assuming a vanishing twist-2 transverse quark spin distribution in the longitudinally polarized nucleon, denoted as ``reduced twist-3 approximation'', a self-consistent description of the observed single-spin asymmetries is obtained. In addition, predictions are given for the z dependence of the single target-spin asymmetry.Comment: 8 pages, 2 figures, typos corrected, very small changes to text, reference adde

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

Lipids in liver failure syndromes: a focus on eicosanoids, specialized pro-resolvinglipid mediators and lysophospholipids.

Lipids are organic compounds insoluble in water with a variety of metabolic and non-metabolic functions. They not only represent an efficient energy substrate but can also act as key inflammatory and anti-inflammatory molecules as part of a network of soluble mediators at the interface of metabolism and the immune system. The role of endogenous bioactive lipid mediators has been demonstrated in several inflammatory diseases (rheumatoid arthritis, inflammatory bowel disease, atherosclerosis, cancer). The liver is unique in providing balanced immunotolerance to the exposure of bacterial components from the gut transiting through the portal vein and the lymphatic system. This balance is abruptly deranged in liver failure syndromes such as acute liver failure and acute-on-chronic liver failure. In these syndromes, researchers have recently focused on bioactive lipid mediators by global metabonomic profiling and uncovered the pivotal role of these mediators in the immune dysfunction observed in liver failure syndromes explaining the high occurrence of sepsis and subsequent organ failure. Among endogenous bioactive lipids, the mechanistic actions of three classes (eicosanoids, pro-resolving lipid mediators and lysophospholipids) in the pathophysiological modulation of liver failure syndromes will be the topic of this narrative review. Furthermore, the therapeutic potential of lipid-immune pathways will be described

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.

Second Stage String Fragmentation Model

A string model, advocated by Bowler, provides a physical and intuitive picture of heavy quark fragmentation. When supplemented by an ad hoc factor of (1-z), to suppress fragmentation near z=1, it supplies an excellent fit to the data. We extend Bowler's model by accounting for the further decay of the massive mesonic states produced by the initial string breaking. We find that each subsequent string break and cascade decay beyond the first, introduces a factor of (1-z). Furthermore we find that including a finite mass for the quarks, which pop out of the vacuum and split the string, forces the first string breaking to produce massive states requiring further decay. This sequence terminates at the second stage of fragmentation where only relatively "light" heavy meson systems are formed. Thus we naturally account for the phenomenologically required factor of (1-z). We also predict that the ratio of (primary) fragments-vector/(vector plus scalar) should be .61. Our second stage string fragmentation model provides an appealing picture of heavy quark fragmentation.Comment: 15 page

The Nucleon ``Tensor Charges'' and the Skyrme Model

The lowest moment of the twist-two, chiral-odd parton distribution $h_1(x)$ of the nucleon can be related to the so-called ``tensor charges'' of the nucleon. We consider the tensor charges in the Skyrme model, and find that in the large-$N_c$, SU(3)-symmetric limit, the model predicts that the octet isosinglet tensor charge, $g^8_T$, is of order $1/N_c$ with respect to the octet isovector tensor charge, $g^3_T$. The predicted $F/D$ ratio is then 1/3, in the large-$N_c$ limit. These predictions coincide with the Skyrme model predictions for the octet ${\it axial}$ charges, $g^8_A$ and $g^3_A$. (The prediction $F/D=1/3$ for the axial charges differs from the commonly quoted prediction of 5/9, which is based on an inconsistent treatment of the large-$N_c$ limit.) The model also predicts that the singlet tensor charge, $g^0_T$, is of order $1/N_c$ with respect to $g^3_T$.Comment: 9 single-spaced pages, no figures, MIT-CTP-212