Expected Polarization of Λ particles produced in deep inelastic polarized lepton scattering

We calculate the polarization of Λ and Λ̄ particles produced in deep inelastic polarized lepton scattering. We use two models: the näıve quark model and a model in which SU(3)F symmetry is used to deduce the spin structure of SU(3) octet hyperons from that of the proton. We perform the calculations for Λ and Λ̄ produced directly or as decay products of Σ and Σ. D r a f t : F e b r u a r y 1 , 2 0 0 8 , 1 1 : 4 8 A M Supported in part by grants from US-Israel Bi-National Science Foundation and from the Israeli Science Foundation. The spin structure function of the nucleon has been studied extensively during the past several years. Very different experimental systems utilizing polarized muon [1], electron [2] and positron [3] beams and covering different kinematic regions obtained results which are consistent with each other. This effort led to precise measurements of the nucleon spin structure functions g 1 (x) (N = n,p) and their integrals Γ1 = ∫ g 1 (x)dx. Here x ≡ xBj = Q /2Mν is the Bjorken scaling variable, −Q is the four-momentum transfer to the target nucleon squared, ν is the energy loss of the lepton in the laboratory frame, and M is the nucleon mass. The interpretation of these results is that quarks in the nucleon carry only ∼30% of the nucleon spin and that the strange (and non-strange) sea is polarized opposite to the polarization of the valence quarks. This interpretation leads to the question of where is the rest of the nucleon spin is coming from which will not be addressed here. Other open questions are: 1) how sensitive are the conclusions to the SU(3) symmetry assumed in the process of interpretation of the experimental data? 2) what is the mechanism that polarizes the strange sea? 3) how well do we understand the spin structure of other hadrons? 4) are the quarks and antiquarks in the sea equally polarized? These questions may be addressed through measurement of the polarization of Λ and Λ̄ produced in polarized lepton Deep Inelastic Scattering (pDIS) on unpolarized targets. Having the strange quark as valence rather than sea quark, the polarization of Λ particles is sensitive to its contribution to the Λ spin. By measurements of both Λ and Λ̄ polarisation both s and s̄ roles can be studied. Recently some experimental results became available [4] and more are expected in the future [3, 5]. As an illustration, a simple mechanism that can produce Λ polarization in pDIS is when the polarized virtual photon is absorbed by a strange quark in the target nucleon sea. The struck quark emerges with helicity in the direction of that of the photon. When it hadronizes into a Λ (in the current fragmentation region), it is likely to become a valence quark in the Λ and the näıve quark model predicts that the polarization of the Λ will be the same as that of the strange quark. An experiment that can probe the low xBj region will have a significant fraction of the Λ and Λ̄ originating from this mechanism and will have sensitivity to the strange sea in the target nucleon. The transfer of polarization from the beam to the produced Λ and Λ̄ is controlled by their helicity difference fragmentation functions ∆q̂ [6]. In recent measurements at LEP ∆q̂ was measured from Λ polarization near the Z pole and was found to be consistent with the näıve quark model expectations [7, 8]. It is not unreasonable, therefore, to expect that in the current fragmentation region of pDIS the polarization will follow the same expectations. An alternative approach is presented in [6] where SU(3)F symmetry is assumed and the spin structure of SU(3) octet hyperons is deduced from that of the proton. Other mechanisms exist and are discussed in [9, 10, 11, 12].