\Lambda, \bar{\Lambda} polarization and spin transfer in lepton deep-inelastic scattering

The flavor and helicity distributions of the and hyperons for both valence and sea quarks are calculated in a perturbative QCD (pQCD) based model. We relate these quark distributions to the fragmentation functions of and , and calculate the z-dependence of the longitudinal spin transfer to and in lepton deep-inelastic scattering (DIS). It is shown that the spin transfer to the is compatible with the first HERMES results at DESY, and further tests are suggested. We also make predictions for the z-dependence of the and longitudinal polarizations in neutrino (antineutrino) DIS processes. We investigate the sea contribution to the fragmentation functions, and we test a possible scenario where the sea quarks in (or the sea antiquarks in ) are negatively polarized, whereas the sea antiquarks in the (or the sea quarks in ) are positively polarized. The asymmetry of the polarized fragmentation functions of the sea quarks and antiquarks to and provides a way to understand the different behavior between the and spin transfers observed in the recent E665 experiment at FNAL. Received: 5 April 2000 / Published online: 26 July 2000     

Three-loop on-shell charge renormalization without integration:\Lambda _{QED}^{\overline {MS} } to four loops

Using algebraic methods, we find the three-loop relation between the bare and physical couplings of one-flavourD-dimensional QED, in terms of Γ functions and a singleF ₃₂ series, whose expansion nearD=4 is obtained, by wreath-product transformations, to the order required for five-loop calculations. Taking the limitD→4, we find that the \(\overline {MS} \) coupling \(\bar \alpha (\mu )\) satisfies the boundary condition $$\begin{gathered} \frac{{\bar \alpha (m)}}{\pi } = \frac{\alpha }{\pi } + \frac{{15}}{{16}}\frac{{\alpha ^3 }}{{\pi ^3 }} + \left\{ {\frac{{11}}{{96}}\zeta (3) - \frac{1}{3}\pi ^2 \log 2} \right. \hfill \\ \left. { + \frac{{23}}{{72}}\pi ^2 - \frac{{4867}}{{5184}}} \right\}\frac{{\alpha ^4 }}{{\pi ^4 }} + \mathcal{O}(\alpha ^5 ), \hfill \\ \end{gathered} $$ wherem is the physical lepton mass and α is the physical fine structure constant. Combining this new result for the finite part of three-loop on-shell charge renormalization with the recently revised four-loop term in the \(\overline {MS} \) β-function, we obtain $$\begin{gathered} \Lambda _{QED}^{\overline {MS} } \approx \frac{{me^{3\pi /2\alpha } }}{{(3\pi /\alpha )^{9/8} }}\left( {1 - \frac{{175}}{{64}}\frac{\alpha }{\pi } + \left\{ { - \frac{{63}}{{64}}\zeta (3)} \right.} \right. \hfill \\ \left. { + \frac{1}{2}\pi ^2 \log 2 - \frac{{23}}{{48}}\pi ^2 + \frac{{492473}}{{73728}}} \right\}\left. {\frac{{\alpha ^2 }}{{\pi ^2 }}} \right), \hfill \\ \end{gathered} $$ at the four-loop level of one-flavour QED.     

The \overline{\mbox{\sf P}}{\sf ANDA} detector at FAIR

Selected problems of collisional processes in exotic atoms are discussed. The emphasis is on the collisional effects in antiprotonic helium including formation of antiprotonic atoms, collisional quenching of hot and thermalized metastable antiprotonic helium, shift and broadening of E1 and M1 spectral lines.     

Longitudinal $\bar{\Lambda}_0$ polarization in heavy-ion collisions as a probe for QGP formation

We present an analysis of the longitudinal polarization in ultrarelativistic heavy-ion collisions. The polarization of s coming from the decay chain exhibits a very well differentiated behavior depending on the production region of the primordial s. This effect reflects the different values of the ratio in the QGP region, where nucleon-nucleon interactions take place in a hot and dense environment, and the peripherical region, in which ordinary nucleon-nucleon interactions occur. An increase in the longitudinal polarization signals a strangeness enhancement which is thought of as a property of the QGP phase.Received: 21 September 2004, Revised: 25 October 2004, Published online: 17 December 2004     

{^6_{\Lambda} \rm{H}} Modeled as {^4_{\Lambda} \rm{H}} + n + n

A three-body calculation for the \({^4_{\Lambda} \rm{He}}\) and \({^6_{\Lambda}{\rm H}}\) hypernuclei has been undertaken. The respective cores are \({^4_{\Lambda}{\rm H}}\) . The interactions in the \({^6_{\Lambda}{\rm He}}\) system, modeled as \({^4_{\Lambda} {\rm H+p+n}}\) , are reasonably well known. For example, the p n interaction is well determined by the p n scattering data, the \({^4_{\Lambda}{\rm H}}\) –p interaction can be fitted to the \({^5_{\Lambda}{\rm He}}\) binding energy. The \({^4_{\Lambda}{\rm He}}\) –n interaction can be fitted to α–n scattering data. For the ⁴He–n system the s-wave can be modeled alternatively as a repulsive potential or as an attractive potential with a forbidden bound state. We explore these alternatives in ⁶He, because the interaction comes into play in modeling \({^6_{\Lambda}{\rm He}}\) as well as in our \({^4_{\Lambda}{\rm H}}\) + n + n model of \({^6_{\Lambda}{\rm H}}\) , where the valence neutrons are Pauli blocked from the s-shell of the core nucleus.     

Exploring the nuclear potential of antihyperons with antiprotons at {\overline{\rm\bf P}}{\rm\bf ANDA}

A schematic Monte Carlo simulation is used to examine the potential of the ${\overline{\rm P}} {\rm ANDA}$ experiment to extract information on the interaction of antihyperons in nuclei by exclusive hyperon-antihyperon pair production close to threshold in antiproton nucleus interactions. Due to energy and momentum conservation event-by-event transverse momentum correlations of the produced hyperon and antihyperons contain information on the difference between their potentials. It is demonstrated that for ${{\Lambda}{\overline{\Lambda}}}$ and ${{{\Xi}}{{\overline{\Xi}}}}$ pairs produced at antiproton momenta of 1.66 GeV/c and 2.9 GeV/c, respectively, the asymmetry is sufficiently sensitive even if the density as well as the momentum dependencies of the potentials are considered.     

Antibaryon\left( {\bar p,\bar \Lambda } \right) production in relativistic nuclear collisions

The relativistic meson field theory is used to study the effects of the in-medium interaction on the predicted antibaryon abundancy in hot hadronic matter. It is demonstrated that subtreshold production of antiprotons in high energy heavy ion collisions at E_lab =1–2 GeV/nucleon is enhanced by 2–3 orders of magnitude as compared to a standard fireball model estimate. Furthermore, we show that after the inclusion of interactions the anti-hyperon yields, e.g. \(\bar \Lambda \) /π ^? are enhanced by about a factor ten. Predicted yields are in excess of the data measured by the NA35 and WA85 collaborations at CERN. The annihilation of antibaryons in surrounding matter at the final stage of the reaction may essentially reduce their abundancy.     

Three-body baryonic $\overline B\to \Lambda \bar p \pi$ decays and such

We study the decay rates and spectra of , , , , and modes in the factorization approach. The baryon pairs are produced through vector, axial vector, scalar and pseudoscalar operators. Previous predictions, including ours, are an order of magnitude too small compared to experiment. By incorporating QCD counting rules and studying the asymptotic behavior, we find an earlier relation between the pseudoscalar and axial vector form factors to be too restrictive. Instead, the pseudoscalar and scalar form factors are related asymptotically. Following this approach, the measured rate () and spectrum can be understood, and should be dominantly left-hand polarized, while we expect . These results and other predictions can be checked soon. Received: 27 February 2003 / Published online: 5 May 2003