THE PROTON SPIN PUZZLE AND DEPOLARIZATION IN p̄p → Λ̄Λ

We point out that the measurement of target spin depolarization Dnn in the p̄p → Λ̄Λ reaction may test dynamical mechanisms invoked to explain the proton spin puzzle revealed by polarized deep–inelastic scattering experiments. In particular, models with negatively polarized s̄s pairs in the proton wave function predict Dnn < 0, whereas models with positively polarized gluons would predict Dnn > 0. CERN–TH/95–47 DOE/ER/40427–04–N95 February 1995 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] The reaction p̄p → Λ̄Λ is a testing-ground for different approaches to non-perturbative QCD, in particular the quark model and meson-exchange models. In addition to total cross section and angular distribution measurements at different energies, the spin correlation of the Λ and Λ̄ have been measured [1]. These were found to be predominantly in a spin-triplet state, with the spin-singlet component very small and consistent with zero within errors. This feature could easily be understood within quark models, if the s̄s pair that carry the Λ̄ and Λ spins in the näıve constituent quark model were produced by effective vector (S1) or scalar ( P0) field exchange [2]. Spin-triplet dominance could also be accomodated in a meson-exchange model, if the relative phase of the K− and K∗− exchange amplitudes was suitably adjusted [3], but spin-singlet suppression could not be regarded as a natural prediction of this class of models. There is a plentiful evidence from other experiments at LEAR and elsewhere that baryon wave functions may be more complicated than in the näıve constituent quark model. In particular, the experimental value of the π−nucleon σ−term [4] and deep-inelastic experiments [5] provide evidence for hidden s̄s pairs in the nucleon. Most strikingly, several recent LEAR experiments [6] find clear evidence for apparent violations of the OZI quark-line rule in p̄N → φX annihilations, where X = γ, π, ππ. A natural interpretation of these data is in terms of the shake-out or rearrangement of s̄s pairs present in the p̄N initial state [7]. It recently was pointed out [8] that many features of these apparently OZI– violating hadronic processes can be understood if one assumes that the proton wave function contains an admixture of polarized s̄s pairs. This assumption is motivated by the experimental results on deep-inelastic scattering [5] which indicate that strange quarks and antiquarks in the proton indeed have a net polarization opposite to the proton spin [9]. An alternative interpretation of these deep-inelastic results ascribes them to to polarized gluons in the proton [10], a suggestion whose implications for low–energy p̄p annihilation have not yet been explored. The PS185 Collaboration is now proposing [11] an extension of its studies using a polarized target and measuring the depolarization Dnn (p → Λ polarization transfer). Quark models generally predict positive values for this quantity [12], whereas meson exchange models generally predict negative values [13]. We argue in this note that these measurements may discriminate between the polarized s̄s and gluon interpretations of the experimental results on polarized deep-inelastic scattering. Specifically, we find that the polarized s̄s model predicts negative depolarization Dnn < 0, whereas the polarized gluon model predicts positive depolarization Dnn > 0. Thus the proposed extension of the PS185 experiment could provide valuable insight into the proton spin puzzle. The mechanism which is responsible for the negative polarization of the strange sea is most probably of nonperturbative nature. Its origin can be linked to chiral