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Hyperon polarization in a quark-quark scattering model

Hyperon polarization in a quark-quark scattering model. Eduard De La Cruz Burelo Homer A. Neal. Outline. Hyperon polarization remains a major open question even at energies of hundreds of GeV Extraordinary behavior of hyperon polarization has no simple explanation

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Hyperon polarization in a quark-quark scattering model

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  1. Hyperon polarization in a quark-quark scattering model Eduard De La Cruz Burelo Homer A. Neal E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  2. Outline • Hyperon polarization remains a major open question even at energies of hundreds of GeV • Extraordinary behavior of hyperon polarization has no simple explanation • Polarization in pp pp and pp H+ X : could both be the result of simple quark-quark scattering? • A proposed model based on quark-quark scattering. • The qualitative explanation of the hyperon polarization for a variety of processes E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  3. Hyperon polarization • An unresolved question: • What causes the large polarization in inclusive hyperon production in p-p collisions? • Vanishingly small polarizations are predicted in QCD. • No model is now able to explain all features of Hyperon polarization at the same time. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  4. J=0 : Negative J = 1 + s quark: Positive 2 s-quarks: Negative Plans to measure it (LHC) Hyperon polarization characteristics * (Diquark)Jstate E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  5. Λ0: Lowest mass hyperon … more intriguing • Very little dependence on the initial proton energy and in the target material. • Grows negative linearly with pT until a plateau is reached around 1 GeV/c. • For constant xF (=pL/pmax) value, the plateau extends from 1 GeV/c to the highest measured pT. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  6. Intriguing questions: Why do the Λ0 and Ξ polarizations look similar? Why is the Σ polarization positive? Why is the Ω polarization almost ~zero? Why is there a plateau in the Λ0 polarization? E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  7. Features we would like to explain: • The shape and magnitude of the Λ0 polarization • The different signs in the Λ0 and Σ polarization • The similarity of the polarization for the Λ0 and Ξ • The small value of the Ω polarization E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  8. u u s u u u u u Λ0 proton proton d d d d d d d d proton proton proton proton Λ0 u u u u u u u u s q-q scattering: relating pp  pp to pp  Λ + X process • Is there any connection between the observed polarization in these two processes? • They could come from the same source: quark-quark scattering. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  9. pppp in a quark-quark scattering model • Neal et al. proposed a q-q scattering model to explain the polarization in elastic p-p scattering: • First region is due to q-q scattering • Second region is due to 2q-2q or q-2q • Subsequent data is consistent with 3q-3q scattering*. • Can we apply the same idea to pp Λ + X process? H.A. Neal et al. Phys. Lett. B 51 (1974) 79 *) H.A. Neal et al. Phys. Lett. B 439 (1998) 407 E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  10. pp Λ + X in a quark-quark scattering model • From a preexisting pair in the incoming proton, the s quark scatters, get highly polarized, and end up in the hyperon. • Hyperon polarization: PH=Pq(x,pT). Pq(xF,pT) : quark polarization effect due to a q-q scattering process. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  11. Λ0 polarization from polarization in pppp (1) • Model: PH=Pq(x,pT) • In a multiple quark scattering* P ~ mqPq(pT) Assuming ms~2mu *) J. Szwed, Phys. Lett. B 105 (1981) 403. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  12. Λ0 polarization from polarization in pppp (2) • What pT(Λ0) vs. pT(p) should we compare? • For a elastic p-p scattering in a q-q region (Neal et al.): • For pp  Λ + X in q-q region: • We should compare at: Assuming s-quark account for half of the Λ0 mass. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  13. Comparison of proton and Λ0 polarization Proton polarization multiplied by 2 Proton pT multiplied by 2/3 If we let proton data to tell us what values prefer to match Λ0 data, selected values are around to 2 and 2/3. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  14. What about the plateau? • The plateau in the Λ0 polarization can be understood as the saturation of the s quark polarization (P=100%). Polarization Polarization E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  15. Explaining the other features • Why the similar polarization for the Λ0 and Ξ? If the polarization of both hyperons is due to one s quark becoming highly polarized from the scattering process, then we expect the polarization of both hyperons to be similar. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  16. s s u d u Why the different sign in the Λ0 and Σ polarization? • Beam quarks does not contributed to the Λ0 polarization • s quark is responsible for the large polarization of the Λ0. • s quark polarization is negative. • s quark polarization is negative which force the polarization of the u and d quarks to be positive, so overall polarization is positive. • If we assume the s quark being responsible of the polarization, it can be showed that the sign of the Σ polarization is opposite to that of the Λ polarization. d E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  17. Why Ω polarization is ~0? • One of 3 s quarks get polarized in the scattering, setting a maximum of 33% to the polarization of the Ω. • Should be similar to the Λ0 polarization in the one quark scattering region. Λ0 polarization compared to the Ω polarization. Statistics too low to make strong conclusions. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  18. Future tests/Implications • Λb polarization is planned to be measured in LHC (ATLAS & LHCb) • Polarization could be huge if the q-q contribution to production is important and the Szwed mass scaling holds. • Simple relation should exist between Λ and Λb polarization is our q-q idea is correct. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

  19. Conclusions: • A model based in a simple q-q scattering process is proposed: • Polarization due the scattering process Pq(pT) is the same for all quark species. • s quark scatters from the incoming proton is the responsible for the large hyperon polarization. • Λ and proton polarization seems to have a common source. • The model is able to qualitatively address all of the main observed features of hyperon polarization. E. De La Cruz Burelo, SPIN 2006, Kyoto Japan

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