Elsevier

Nuclear Physics B

Volume 574, Issues 1–2, 15 May 2000, Pages 331-346
Nuclear Physics B

Flavor and spin structure of octet baryons at large x

https://doi.org/10.1016/S0550-3213(00)00021-3Get rights and content

Abstract

The quark flavor and spin distributions in octet baryons are calculated both in the SU(6) quark spectator diquark model and in a perturbative QCD (pQCD) based model. It is shown that the Λ has the most significant difference in flavor structure at large x between the two models, though the flavor and spin structure of other baryons can also provide tests of different models. The Drell–Yan process for Σ± beams on isoscalar targets can be used to test different predictions concerning the valence quark flavor structure of the Σ±.

Introduction

Parton distributions of hadrons and the formation of hadrons from fragmentation of partons are of considerable current interest in the community of particle and nuclear physics. There have been remarkable achievements in our knowledge of the quark–gluon structure of the nucleon from three decades of experimental and theoretical investigations in various deep-inelastic scattering (DIS) processes. However, there are still a number of unknowns concerning the detailed flavor and spin structure of the nucleon, such as the detailed origin of the proton spin (for reviews, see, e.g., Refs. [1], [2] and for a recent discussion, see Ref. [3]), the strange content of the nucleon [4], [5], [6], [7], the flavor asymmetry of the sea (for a recent review, see, e.g., Ref. [8]), the isospin symmetry breaking at small x [9], [10], and the flavor and spin structure of the valence quarks at large x [11], [12], [13], [14], [15], [16], [17], [18]. It is important to perform high-precision measurements of available physical quantities and/or to measure new quantities related to the flavor and spin structure of the nucleons, in order to have a better understanding of the quark–gluon structure of the nucleon. Nevertheless, it has recently been found [19] that the flavor and spin structure of the Λ-hyperon may serve as a new domain where the same physics that governs the structure of the nucleon can manifest itself. It was found that the flavor and spin structure of the quark distributions of the Λ differ significantly at large x from a perturbative QCD (pQCD) based analysis and a SU(6) quark–diquark model. A detailed analysis [20] of the available Λ-polarization data in e+e annihilation at the Z-pole supports the prediction that the u and d quarks inside the Λ should be positively polarized at large x, though their net helicities might be zero or negative. The most recent HERMES data [21] of spin transfer to the Λ in deep inelastic scattering of polarized leptons on the nucleon target also support the predictions of the SU(6) quark–diquark model and the pQCD based model [19].

The direct measurements of the Λ quark structure are not easy since it is a charged neutral particle which cannot be accelerated as incident beam and its short life time makes it also difficult to be used as target. The simple reciprocity relation [22], [23]qh(x)∝Dqh(z),where z=2p·q/Q2 is the momentum fraction of the produced hadron from the quark jet in the fragmentation process and x=Q2/2p·q is the Bjorken scaling variable corresponding to the momentum fraction of the quark from the hadron in the DIS process, can provide a reasonable connection between different physical quantities and lead to understandings of the Λ quark structure from the various quark to Λ fragmentations. However, such a relation is still not completely free from theoretical and experimentally uncertainties, though it may serve for our purpose as an approximate qualitative connection at a specific scale Q2, near x→1 and z→1 [24], [25]. Thus the direct measurement of the quark structure of other octet baryons other than the nucleon and Λ has a strong physical significance, and can provide a new direction to test different theories concerning the nucleon structure.

The purpose of this paper is to extend the analysis of the quark structure from the nucleon and Λ cases to the other members of octet baryons. We will calculate the quark distributions for all of the octet baryons in the SU(6) quark spectator diquark model and in a perturbative QCD based model. There are two motivations for such a study: (1) to check the difference in flavor and spin structure of all octet baryons between the two models, and see which baryon has the most significant difference; (2) some charged baryons other than nucleons, such as Σ±, may be used as beam to directly measure their own quark structure in case the structure of the target is comparatively well known. We will find that the Λ has the most significant difference at large x in the flavor and spin structure for a clean test of different predictions. We will also show that the Σ's have the most significant difference in the flavor and spin structure between the two models at medium to large x, and the measurement of Drell–Yan process for Σ± beams on the isoscalar targets can test different predictions of the quark structure of the Σ± baryons. It is more appealing that the Λ and Σ0 have complete different flavor and spin structure though they are composed of same flavor quarks.

We shall start Section 2 with the presentation of quark distributions of octet baryons in the SU(6) quark–diquark spectator model and in a perturbative QCD based model. We then compare the flavor and spin structure of all the octet baryons between the two models at large x. Then, in Section 3, we present the formulas for the Drell–Yan process using Σ± beams on isoscalar targets and show different predictions of cross-section ratios in the pQCD based model and in the quark spectator diquark model. Finally, we present conclusions in Section 4.

Section snippets

The quark spin and flavor structure of octet baryons

In this section we extend the analysis of the quark structure of nucleons to all members of octet baryons using (1) the SU(6) quark spectator diquark model and, (2) a perturbative QCD (pQCD) based model.

Drell–Yan process for Σ± beams on isoscalar targets

Although the Λ can provide a clean test of the different flavor and spin structure between the two different models, it is still not possible to make a clear distinction between the above two predictions with the available data of the Λ-polarization in the e+e process near the Z-pole [20]. We also need a connection between the quark distributions inside the Λ and the quark fragmentation into a Λ and such a connection is still not free from theoretical and experimental uncertainties. As we

Summary

We found in this paper that Λ has the most significant difference in the flavor structure at large x, compared to that of other octet baryons, between the SU(6) quark spectator diquark model and the pQCD based model, and this supports the conclusion in a previous study [19] that the Λ can provide a new domain to test different models concerning the flavor and spin structure of the nucleon. However, for the Λ we still need a connection between the quark distributions inside the Λ and a quark

Acknowledgements

This work is partially supported by Fondecyt (Chile) postdoctoral fellowship 3990048, by Fondecyt (Chile) grant 1990806 and by a Cátedra Presidencial (Chile), and by National Natural Science Foundation of China under Grant Numbers 19605006, 19875024, 19775051, and 19975052.

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    Mailing address: Department of Physics, Peking University, Beijing 100871, China.

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