Polarized Fragmentation Functions Research Articles

OVERVIEW OF LAMBDA AND HYPERON PHYSICS

Some aspects of hyperon polarization phenomena in inclusive production in several high energy collision processes are reviewed. We concentrate on ways to achieve an accurate determination of the unpolarized and polarized fragmentation functions of a quark into a [Formula: see text]. A possible extension to the production of the other hyperons (Σ±,0, Ξ-,0), will be also briefly discussed.

Polarized hyperons from pA scattering in the gluon saturation regime

We study the production of transversely polarized Λ hyperons in high-energy collisions of protons with large nuclei. The large gluon density of the target at saturation provides an intrinsic semi-hard scale which should naturally allow for a weak-coupling QCD description of the process in terms of a convolution of the quark distribution of the proton with the elementary quark–nucleus scattering cross section (resummed to all twists) and a fragmentation function. In this case of transversely polarized Λ production we employ a so-called polarizing fragmentation function, which is an odd function of the transverse momentum of the Λ relative to the fragmenting quark. Due to this kt-odd nature, the resulting Λ polarization is essentially proportional to the derivative of the quark–nucleus cross section with respect to transverse momentum, which peaks near the saturation momentum scale. Such processes might therefore provide generic signatures for high parton density effects and for the approach to the “black-body” (unitarity) limit of hadronic scattering.

Flavor and spin structure of quark fragmentation functions in a diquark model for octet baryons

We use a simple quark-diquark model to describe unpolarized, longitudinally polarized, and transversely polarized fragmentation functions for the octet baryons. All free parameters of the model are determined based on the experimental data on octet baryon B production in the ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{BX}$ process. The SU(3) flavor symmetry breaking effect in the octet baryon fragmentation functions is investigated. The strangeness suppression effect in hyperon production of ${e}^{+}{e}^{\ensuremath{-}}$ annihilation is found to be significant, and its relevant consequences for the cross sections of octet hyperon production in $\mathrm{pp}$ collisions are estimated. In order to check the flavor structure of the octet hyperon fragmentation functions, hyperon-antihyperon asymmetries in ${k}^{+}p$ collisions are provided. Furthermore, we find that the spin observables calculated with the $\ensuremath{\Lambda}$ fragmentation functions are compatible with available experimental data on the longitudinally polarized $\ensuremath{\Lambda}$ production. Spin observables for other octet baryons produced in various inclusive processes are also predicted for future experiments.

Spin transfers for baryon production in polarized pp collisions at RHIC-BNL

We consider the inclusive production of longitudinally polarized baryons in p → p collisions at RHIC-BNL, with one longitudinally polarized proton. We study the spin transfer between the initial proton and the produced baryon as a function of its rapidity and we elucidate its sensitivity to the quark helicity distributions of the proton and to the polarized fragmentation functions of the quark into the baryon. We make predictions using an SU(6) quark spectator model and a perturbative QCD (pQCD) based model. We discuss these different predictions, and what can be learned from them, in view of the forthcoming experiments at RHIC-BNL.

New prospects for polarized hyperon fragmentation functions

We study the inclusive production of Λ ( Λ ) in several high energy collision processes ( e + e −, e ± p, ν( ν ) p, pp), in view of an accurate determination of the unpolarized and polarized fragmentation functions of a quark into a Λ ( Λ ). For polarized fragmentation functions the inaccuracy and the scarcity of present data do not allow to distinguish between various theoretical models. We will indicate how future measurements will provide ways to discriminate between them and also how to achieve a necessary quark flavor separation. A possible extension to the production of the other hyperons (Σ ±,0, Ξ −,0), will be also briefly discussed.

Q→ Λ fragmentation function and nucleon transversity distribution in a diquark model

Based on a simple quark–diquark model, we propose a set of unpolarized, longitudinally polarized and transversely polarized fragmentation functions for the Λ by fitting the unpolarized Λ production data in e +e − annihilation. It is found that the helicity structure of the obtained Λ fragmentation functions is supported by the all available experimental data on the longitudinal Λ polarization. Within the same framework of the diquark model, the nucleon transversity distributions are presented and consistent descriptions of the available HERMES data on the azimuthal spin asymmetries in pion electroproduction are obtained. Furthermore, the spin transfers to the transversely polarized Λ in the charged-lepton DIS on a transversely polarized nucleon target are predicted for future experiments.

Quark fragmentation functions in a diquark model forΛproduction

Using a simple quark-diquark model, we extract a set of unpolarized and polarized fragmentation functions for the $\Lambda$ based on the available unpolarized $\Lambda$ production data in $e^+ e ^- $ annihilation. It is found that there is a strong SU(3) flavor symmetry breaking in the unpolarized $\Lambda$ fragmentation functions and that the polarized $u$ and $d$ quark contributions to the polarized $\Lambda$ fragmentation are positive in medium and large $z$ region. The predictions of $\Lambda$-polarization with the obtained quark fragmentation functions are compatible with the available data on the longitudinally polarized $\Lambda$ produced in $e^+e^-$-annihilation, polarized charged lepton deep inelastic scattering (DIS), and neutrino DIS. The spin asymmetry for the $\Lambda$ production in $p\vec{p}$ collisions is also predicted for a future test in experiments at RHIC-BNL.

Transversity distribution and polarized fragmentation function from semi-inclusive pion electroproduction

A method is discussed to determine the hitherto unknown u-quark transversity distribution from a planned HERMES measurement of a single-spin asymmetry in semi-inclusive pion electroproduction off a transversely polarized target. Assuming u-quark dominance, the measurement yields the shapes of the transversity distribution and of the ratio of a polarized and the unpolarized u-quark fragmentation functions. The unknown relative normalization can be obtained by identifying the transversity distribution with the well-known helicity distribution at large x. The systematic uncertainty of the method is dominated by the assumption of u-quark dominance.

NEW PROSPECTS FOR POLARIZED FRAGMENTATION FUNCTIONS(3rd Session: New Idea on Polarization Phenomena,Polarized Partons at High Q^2 Region)

NEW PROSPECTS FOR POLARIZED FRAGMENTATION FUNCTIONS(3rd Session: New Idea on Polarization Phenomena,Polarized Partons at High Q^2 Region)

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

The flavor and helicity distributions of the $\Lambda$ and $\bar{\Lambda}$ 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 the $\Lambda$ and $\bar{\Lambda}$, and calculate the $z$-dependence of the longitudinal spin transfer to the $\Lambda$ and $\bar{\Lambda}$ in lepton deep-inelastic scattering (DIS). It is shown that the spin transfer to the $\Lambda$ is compatible with the first HERMES results at DESY and further tests are suggested. We also make predictions for the $z$-dependence of the $\Lambda$ and $\bar{\Lambda}$ longitudinal polarizations in neutrino (antineutrino) DIS processes. We investigate the sea contribution to the fragmentation functions, and we test a possible scenario where sea quarks in $\Lambda$ (or sea antiquarks in $\bar{\Lambda}$) are negatively polarized, whereas sea antiquarks in the $\Lambda$ (or sea quarks in $\bar{\Lambda}$) are positively polarized. The asymmetry of the polarized fragmentation functions of sea quarks and antiquarks to $\Lambda$ and $\bar{\Lambda}$ provides a way to understand the different behaviour between the $\Lambda$ and $\bar{\Lambda}$ spin transfers observed in the recent E665 experiment at FNAL.

How to learn about polarized lambda-baryon fragmentation functions?

We study the inclusive production of Λ ( Λ ) in several high energy collision processes ( e +e −, e ±p, ν( ν)p, pp ), in view of an accurate determination of the polarized fragmentation functions of a quark into a Λ ( Λ ). We will recall that present data are unable to distinguish between various theoretical models, in particular for the spin transfer mechanisms and we will indicate how future measurements will provide ways to discriminate between them and also how to achieve a necessary quark flavor separation.

Quark Flavor Separation inΛ-Baryon Fragmentation

It is shown that unpolarized and polarized $\ensuremath{\Lambda}$ and $\overline{\ensuremath{\Lambda}}$ production in neutrino and antineutrino deep inelastic scattering, can provide a clean separation of unpolarized and polarized fragmentation functions of a quark into a $\ensuremath{\Lambda}$, for both light-flavor quarks and antiquarks and also for strange quarks. Combining with $\ensuremath{\Lambda}$ and $\overline{\ensuremath{\Lambda}}$ production in polarized electron deep inelastic scattering, one can systematically measure or check the various flavor and spin dependent fragmentation functions. Such measurements can provide crucial tests of different predictions concerning the spin structure of hadrons and the quark-antiquark asymmetry of the nucleon sea.

Quark fragmentation into vector and pseudoscalar mesons at LEP

Some data on the ratio of vector to vector + pseudoscalar mesons, V/(V+P), and the probability of helicity zero vector states, rho_00, are now available from LEP. A possible relation between these two quantities and their interpretation in terms of polarized fragmentation functions are discussed; numerical estimates are given for the relative occupancies of K and K*, D and D*, B and B* states.

Next to leading order QCD corrections to polarized Λ production in DIS

We calculate next to leading order QCD corrections to semi-inclusive polarized deep inelastic scattering and e + e −- annihilation cross sections for processes where the polarization of the identified final-state hadron can also be determined. Using dimensional regularization and the HVBM prescription for the γ 5 matrix, we compute corrections for different spin-dependent observables, both in the MS and MS p factorization schemes, and analyse their structure. In addition to the well-known corrections to polarized parton distributions, we also present those for final-state polarized fracture functions and polarized fragmentation functions, in a consistent factorization scheme.

Gluon fragmentation to 1D2 quarkonia.

Gluon fragmentation to heavy $J^{PC}=2^{-+}$ quarkonia is studied herein. We compute these D-wave states' polarized fragmentation functions and find that they are enhanced by large numerical prefactors. The prospects for detecting the lowest lying $^1D_2$ charmonium state at the Tevatron are discussed.

Spin alignment in the production of charm and/or bottom vector mesons via heavy quark fragmentation.

We calculate the process-independent fragmentation functions for a $\bar b$ antiquark to fragment into longitudinally and transversely polarized $B_c^*$ ($^3S_1$) mesons to leading order in the QCD strong coupling constant. In the special case of equal quark mass we recover previous results for the fragmentation of $c\to \psi$ and $b\to\Upsilon$. Various spin asymmetry parameters are defined as measures of the relative population of the longitudinally and transversely polarized vector meson states. In the heavy quark mass limit $m_b \to \infty$ our polarized fragmentation functions obey heavy quark spin symmetry, we therefore apply them as a model to describe the fragmentation of charm and bottom into heavy-light mesons like $D^*$ and $B^*$. The spin asymmetry parameter, $\alpha(z)$, is consistent with the existing CLEO data for $D^*$. The scaling behavior of $\langle z \rangle$ is studied in detail. We find excellent agreement between the predictions of $\langle z \rangle$ from our fragmentation functions and the experimental data for $D^*$ and $B^*$ from the LEP, CLEO, and ARGUS detectors. Finally, we also point out that the spin asymmetry depends significantly on the transverse momentum $p_\perp$ of the vector mesons relative to the fragmentation axis.