IX International Conference on Hypernuclear and Strange Particle Physics

1. IX International Conference on Hypernuclear and Strange Particle Physics Johannes Gutenberg-Universität Mainz, October 10 - 14, 2006 SPIN OBSERVABLES IN BARYON-BARYON INTERACTIONS @ DISTO Marco Maggiora Dipartimento di Fisica ``A. Avogadro'' and INFN - Torino, Italy

2. DISTO @ Saturne: polarised proton beam up to T = 2.9 Gev Acceptance: • 2-cm thick unpolarised LH2 target • S170 magnet (<14.7 KGauss,  =  120 , =  20) • semi-cylindrical 1mm-square scintillating fibers triplets inside magnet • MWPC planar triplets outside magnet • scintillator hodoscopes vertically and horizontally segmented • scintillator hodoscopes as polarimeter slabs • doped water Cerenkov counters

3. Hyperon production @ DISTO GOAL:first exclusive (kinematically complete) measurements with a polarised beam for Reaction Detected Prongs 1.58 1.79 2.34 2.40

4. Hyperon events’ topology – Data @ 2.94 , 3.31 and 3.67 GeV complete reconstruction  missing 0 and/or  missing • DECAY VERTEX: • REACTION VERTEX: • 1 positive track • 1 negative track • 2 positives tracks

5. Spin observables Y dependence on the spin of the proton from the beam transfer of the transverse component of to Depolarisation Analysing power

6. Spin observables: experimental asymmetries •  =  15.5 • most of events are acquired at high cos() Self-analysing  weak decay:

7. pp elastic scattering: ping pong trigger Poor data from literature In particular @ 3.67 GeV/c Errors for spin obs do not include contribution from Elastic proton-proton scattering p" p ! pp pp elastic scattering acquired for some spills every run • beam polarisation continuos evaluation • data validation L and R on one side only free from L/R acceptance PB↑and PB↓ simultaneously acquired

8. Hyperon production: event reconstruction . Unrefitted M- p Reffitted MpK

9. DYY evaluation: accounting for hyperon contamination • select exclusive  and ‘s from 0 decay through a MpK cut • evaluate separately and • estimate relative contaminations , the fraction of  and 0 in • each gate, evaluated throught fit functions integration in each gate. Of course is • extract the corrected DYY values from the linear combinations: • no cos dependence ) direct evaluation of  and 0 yealds DYY EVALUATION: 0.7 · |cos |· 1. AY EVALUATION: 0.7 · |cos |· 1.

10. DYY @ 2.94 , 3.31 and 3.67 GeV/c – Exclusive Λ production - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7 • Exclusive Λproduction: • DYYsizeable and negative • smooth energy dependence

11. DYY @ 2.94 , 3.31 and 3.67 GeV/c – Λ from Σ0 decay - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7 • Λ from decay: • DYYstrongly different from the one of exclusive Λ • positive and negative values • smooth energy dependence

12. AY @ 2.94 , 3.31 and 3.67 GeV/c – Exclusive Λ production - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7 • Exclusive Λproduction: • effects less important vs DYY • smooth energy dependence

13. AY @ 2.94 , 3.31 and 3.67 GeV/c – Λ from Σ0 decay - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7 • Λ from decay: • differences from exclusive Λ AY • smooth energy dependence

14. DYY @ 2.94 , 3.31 and 3.67 GeV/c – Exclusive Λ production FIRST EXCLUSIVE DATA AVAILABLE, FIRST DATA IN THE TFR (xF < 0 ) • Inclusive Λproduction: • high energy )DYY > 0 • Exclusive Λproduction: • lower energy) DYY < 0 • Exclusive Λ from Σ0 decay: • lower energy)≠ DYY Different reaction mechanism?

15. DYY theoretical predictions – Exclusive Λ production . DIRECT FUSION MODEL[1] DYYSIZEABLE AND POSITIVE Polarisation transfer mechanism: [1] C. Boros and Liang Zuo-tang, Phys. Rev. D53 (1996) 2279

16. DYY theoretical predictions – Exclusive Λ production LAGET’S OBE MODEL[1] NEGATIVE DYY CAN BE INTERPRETED xF << 0 (TFR): DNN = 0 xF >> 0 (BFR): kaon) DNN = -1 pion) DNN = +1 [1]J. M. Laget, Phys. Lett. B259 (1991) 24.

17. Conclusions • Λ from decay: • DYYstrongly different from the one of exclusive Λ • positive and negative values • smooth energy dependence • first exclusive data available • first data in the TFR (xF < 0 ) region • first data for the exclusive selection of the Λ from Σ0 decay • Exclusive Λproduction: • DYYsizeable and negative • smooth energy dependence • DYY not easily interpreted in a quark-model framework • OBE approach can account with FSI for DYY < 0 • Both Λproduction: • effects less important vs DYY • smooth energy dependence

18. Question time QUESTION TIME

19. 4body event reconstruction @ DISTO pattern reconstruction and track fitting iteration: pattern recognition provides candidate for the fitting stage; input is the 12D coordinates vector track fitting: 5D parameter vector (x,y): coordinates of the intersection point with z = 0 a: inclination of track at z = 0 : starting angle in (x-z) plane : inverse momentum perpendicolar to B detector coordinated depend smootly on all parameters

20. 4body event reconstruction @ DISTO track fitting by lookup table: goal is inverting in look-up table: 5D lattice that provide for tracks track coordinates consist in linear interpolation of the lattice to obtain 2 minimisation: F inversion is performed minimizing: iteration stops if do not change to nearest grid point

21. 4body event reconstruction @ DISTO kinematically constrained refit: • global 4 tracks – 2 vertices refit • same approach, inversion of • more complex parameter 18D • 3 (x12,y12,z12) for the reaction vertex • six (1, a1, pxz,1, 2, a2, pxz,2 to describe momenta of the two track emerging from the reaction vertex • 3 (x12,y12,z12) for decay vertex • six (3, a3, pxz,3, 4, a4, pxz,4 to describe momenta of the two track emerging from the decay vertex • 4 degrees of freedom are constrained kinematically

22. Hyperon production: reconstruction @ DISTO kinematic constrains: • reconstructed  momentun parallel to the joiner of the reaction and decay verteces ) 2 parameters • reconstructed M- p at decay vertex is M = 1.115 GeV/c2) 1 parameter • M4B= 0 ( or ) or M4B= M ( ) ) 1 parameter 14D “soft” constraint on reaction vertex along the beam

23. Hyperon production: event selection • One of the most effective cuts in event selection is • the kinematically constrained refit itself! • M- p = M • M4B = 0 or M4B = M • Additional cuts: • + veto • |p,| < 0.15 rad, decay proton momentun in LF • p ID for positive track from decay vertex • ptot< 1 GeV/c ) missing a  at most • |vz| < 3.5 cm ) Klegecell veto Kinematic region restricted to: -0.7 ≤ xF ≤ 0.9 pT ≤ 750 MeV/c |cos | < 0.7

24. Particle identification @ DISTO particle identication: iterative process for tagging candidates + veto: pp+- is major background

25. Particle identification @ DISTO particle identication: combined Cerenkon and hodoscopes tagging very small + contamination in hyperon sample

26. Spin observables: inferring  polarisation sign of PB Self-analysing  weak decay: Geometrical interpretation of AN:

27. DYY theoretical predictions – Exclusive Λ production LAGET’S OBE MODEL[1] NEGATIVE DYY CAN BE INTERPRETED [1]J. M. Laget, Phys. Lett. B259 (1991) 24.