Production of polarized baryons in polarized and unpolarizede + e − collisions

Polarized and unpolarizede ⁺ e ^? annihilation is considered in lowest order at energies 30–150 GeV taking γ andZ ⁰ exchange in the standardSU(2)×U(1) model into account. The fragmentation of polarized quarks into polarized baryons is described by two kinds of fragmentation functions; the first is based onSU(6) weights and empirical expressions for favoured and disfavoured fragmentation; the second is based upon a Monte Carlo model of the chain decay including helicities of quarks and hadrons. From the cross sections for \(e^ + e^ - \to q\bar q\) and the fragmentation functions we obtain predictions for the baryon polarization as function of the collision energy and of kinematical variables.     

Spin structure of baryon fragmentation functions

The valence part of the quark fragmentation function resulting in the octet of baryons together with their spin structure are parametrised, using a model for the hadronisation and the available experimental data on baryon products in e⁺e^? annihilation and μp deep inelastic scattering. Spin correlations are predicted.     

Observation of the orbitally excited Λ(1520) baryon in e+e− annihilation

We report the first observation of an orbitally excited baryon, the Λ(1520), in quark and gluon fragmentation. The production rate is found to be (1.15±0.21±0.16)×10⁻² and (0.80±0.17_−0.13^+0.10)×10⁻²Λ(1520) hyperons per event in direct ϒ decays and in the continuum, respectively. In contrast to the observed situation for ground state baryons, the production of the Λ(1520) in direct ϒ decays shows little or no enhancement with respect to continuum production.     

Hadronic production of doubly charmed baryons and intrinsic charm in the proton

The production of baryons involving two charmed quarks (Ξ _cc ^* or Ξ_cc) in hadron interactions at high energies and high transverse momenta is considered. It is assumed that a Ξ_cc baryon is formed in the nonperturbative fragmentation of a (cc) diquark produced in the hard partonic process of the scattering of charmed quarks from colliding hadrons: c+c → (cc)+g. It is shown that, upon the inclusion of this mechanism, the cross section for the production of doubly charmed baryons becomes approximately twice as large as that which is expected at the Tevatron and LHC colliders according to the predictions based on the model of gluon-gluon production of a (cc) diquark and obtained in the leading order of perturbative QCD.     

Spectra of charmed and bottom baryons with hyperfine interaction

Up to now,the excited charmed and bottom baryon states have still not been well studied experimentally or theoretically.In this paper,we predict the mass of ?*b,the only L=0 baryon state which has not been observed,to be 6069.2 Me V.The spectra of charmed and bottom baryons with the orbital angular momentum L = 1 are studied in two popular constituent quark models,the Goldstone boson exchange(GBE) model and the one gluon exchange(OGE) hyperfine interaction model.Inserting the latest experimental data from the "Review of Particle Physics",we find that in the GBE model,there exist some multiplets(Σc(b),Ξ c(b)and ?c(b)) in which the total spin of the three quarks in their lowest energy states is 3/2,but in the OGE model there is no such phenomenon.This is the most important difference between the GBE and OGE models.These results can be tested in the near future.We suggest more efforts to study the excited charmed and bottom baryons both theoretically and experimentally,not only for the abundance of baryon spectra,but also for determining which hyperfine interaction model best describes nature.     

Inclusive Production Λc in the e+e− - Annihilation

In the framework of the quark-diquark model of baryons, Λ_c can be considered as constituent c quark and ud diquark. In hadronizasion process baryon Λ_c can be produced from ud scalar diquark and c quark fragmentation. So in this work, in the lowest order of perturbative QCD, fragmentation functions c → Λ_c and ud → Λ_c are calculated and finally in the electron-positron annihilation in LEP, the inclusive cross section production of Λ_c is calculated about pole of z⁰.     

Wave functions of octet baryons

The leading twist baryonic wave function ?(x ₁,x ₂,x ₃) describes the distribution of three quarks inside the baryon in the longitudinal momentum fractions 0≦x _i ≦1 atP _z →∞. The properties of these wave functions for the baryons entering the nucleon octet are investigated using the QCD sum rules. The high asymmetry in the distribution of the baryon momentum between three quarks is found for all members of the octet. The properties of theSU(3)-symmetry breaking effects in baryonic wave functions are elucidated. The model wave functions are proposed which fulfil the sum rules requirements. The asymptotic behaviour of various baryon form factors (which can be measured ine ⁺ e ^?→ \(\bar {\rm B}\) B) is found out.     

Transverse energy production in the target fragmentation region in16O-nucleus reactions at 60 and 200A GeV

Charged pion yields and transverse energies of baryons are measured for the reaction¹⁶O+Cu, Ag, Au at 60 and 200A GeV bombarding energy in the target fragmentation region employing the Plastic Ball detector. Only little dependence of the measured quantities on the bombarding energy is found. The data are compared with the multi-chain fragmentation model of Ranft. As a result it turns out that a leading order formation zone cascade is not sufficient to explain the baryon yield and the transverse energies of baryons in the target fragmentation region.     

Jets in inclusive e+e− and leptoproduction

The assumption of jets with bounded p_T leads to the following consequences: (a) In e⁺e^? annihilation the coefficient of the cos²θ term (a) of single-particle inclusive distributions] exhibits a unique dependence on the observed particle momentum. This dependence provides a measure of 〈p_T²〉 in a jet. (b) In two-particle inclusive annihilation the average of the squared relative p_T is determined by the same parameter and by the momenta of the two particles. (c) The observation that in the current fragmentation region of leptoproduction the p_T distribution of fast hadrons is broader than that of show ones is explained.     

Lepton-hadron processes beyond leading order in quantum chromodynamics

We present a summary of QCD formulae describing the effects of scaling violation in leptonhadron processes, with the inclusion of recently derived higher order corrections. Deep inelastic leptoproduction, one-hadron inclusive distributions in leptoproduction and ine ⁺ e ^? annihilation and Drell-Yan processes are discussed in detail. Higher order corrections to parton densities, fragmentation functions and to lepton-parton cross-sections in the above processes are presented in a common factorization scheme, so that a comparative analysis of various processes as well as an independent analysis of each of them is possible. A discussion of the various scheme dependences at next-to-leading level is also included.     

String effects on Fermi–Dirac correlation measurements

We investigate some recent measurements of Fermi–Dirac correlations by the LEP collaborations indicating surprisingly small source radii for the production of baryons in e⁺e^- annihilation at the Z⁰ peak. In hadronization models there is besides the Fermi–Dirac correlation effect also a strong dynamical (anti-) correlation. We demonstrate that the extraction of the pure FD effect is highly dependent on a realistic Monte Carlo event generator, both for separation of those dynamical correlations that are not related to Fermi–Dirac statistics, and for corrections of the data and background subtractions. Although the model can be tuned to well reproduce single particle distributions, there are large model uncertainties when it comes to correlations between identical baryons. We therefore, unfortunately, have to conclude that it is at present not possible to draw any firm conclusion about the source radii relevant for baryon production at LEP. PACS 13.60.Rj; 13.66.Bc     

Diquark Fragmentation Contribution in Λ<Subscript><Emphasis Type="Italic">b</Emphasis></Subscript> Production

In the framework of the quark-diquark model of baryons, Λ _b can be considered as b constituent quark an ud constituent diquark. In this study, we investigate the effect ud scalar diquark fragmentation into Λ _b , therefor we calculate frgmentation functions of b quark and ud diquark into Λ _b baryon through the use of perturbative QCD. In the next stage, throuth the use of the obtained fragmentation functions, we calculate the total fragmentation probabilities and average fragmentation parameters for b→Λ _b and u d→Λ _b . Finally, the inclusive cross section of Λ _b baryon in electron-positron annihilation in ALEPH experiment is calculated with regard to ud diquark fragmentation contribution.     

The Gribov-Lipatov relation in QCD

We analyse and calculate the structure functions of both deep inelastic leptoproduction ande ⁺ e ^? annihilation in QCD and in the parton model in a way which brings out the similarities and differences between the two situations. In particular we give a probability interpretation of the Gribov-Lipatov relation for quarks in QCD. The relation does not hold for hadronic structure functions but we determine its modified consequences for this case. A generalised Drell-Yan conjecture is also discussed and verified to lowest order in QCD.     

Spin effects and the production of heavy baryons in Z0 boson decays

Predictions are made within the scope of perturbative quantum chromodynamics and the nonrelativistic two-quark baryon model of the magnitude and dependence uponz (the fraction of the momentum of the heavy quark carried off by the baryon) of the longitudinal spin asymmetry parameterA _RL in the direct creation of Λ _b and Λ _c baryons in theZ ⁰-boson channel in the LEPe ⁺ e ⁻ collider. An estimate is found for the value of in the cascade production of Λ _b baryons. Samara State University, Samara. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 2, pp. 66–75, February, 1999.     

A simple approach to describe hadron production rates in e+e− annihilation

We show that, based on the idea of string fragmentation, the production rates of light flavored mesons and baryons originating from fragmentation can be described by the spin, the binding energy of the particle, and a strangeness suppression factor. Apart from a normalization factor, e⁺e^? data at different center-of-mass energies can be described simultaneously. Applying to the heavy flavor production, we find that our predictions are in good agreement with data.     

Charge radii and magnetic moments of odd- A 183-189Pb isotopes

Unexpected features of the BaBar data on e ⁺ e ^- $ \rightarrow$ B $ \overline{{B}}$ cross-sections (B stands for baryon) are discussed. These data have been collected, with unprecedented accuracy, by means of the initial-state radiation technique, which is particularly suitable in giving good acceptance and energy resolution at threshold. A striking feature observed in the BaBar data is the non-vanishing cross-section at threshold for all these processes. This is the expectation due to the Coulomb enhancement factor acting on a charged fermion pair. In the case of e ⁺ e ^- $ \rightarrow$ p $ \overline{{p}}$ it is found that Coulomb final-state interactions largely dominate the cross-section and the form factor is | G ^p(4M ² _p)| ～ 1 , which could be a general feature for baryons. In the case of neutral baryons an interpretation of the non-vanishing cross-section at threshold is suggested, based on quark electromagnetic interaction and taking into account the asymmetry between attractive and repulsive Coulomb factors. Besides strange baryon cross-sections are compared to U -spin invariance predictions.     

Polarization of baryons and vector mesons in quark jets ine + e −-annihilation

The cross section and density matrix elements fore ⁺ e ^?→V+X ande ⁺ e ^?→B+X are calculated in the γ-Z ⁰ interference region. The Feynman-Field model for the quark fragmentation functions is extended to include baryons and spin of the particles. Possibilities for obtaining more specific information about the spin dependence of the fragmentation mechanism and the weak coupling of quarks are discussed.