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

2. DISTO @ Saturne: polarised proton beam up to T = 2.9 Gev Acceptance: 2-cm thick unpolarised LH 2 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. 2.40 2.34 1.79 1.58 Detected ProngsReaction GOAL: first exclusive (kinematically complete) measurements with a polarised beam for Hyperon production @ DISTO

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

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

7. pp elastic scattering: ping pong trigger 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 P B ↑ and P B ↓ simultaneously acquired Poor data from literature In particular @ 3.67 GeV/c Errors for spin obs do not include contribution from

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

9. D YY evaluation: accounting for hyperon contamination 1.select exclusive  and  ‘s from  0 decay through a  M pK cut 2.evaluate separately and 3.estimate relative contaminations, the fraction of  and  0 in each gate, evaluated throught fit functions integration in each gate. Of course is 4.extract the corrected D YY values from the linear combinations: 1.no cos   dependence ) direct evaluation of  and  0 yealds D YY EVALUATION: 0.7 · |cos   |  · 1. A Y EVALUATION: 0.7 · |cos   |  · 1.

10. D YY @ 2.94, 3.31 and 3.67 GeV/c – Exclusive Λ production Exclusive Λ production: 1.D YY sizeable and negative 2. smooth energy dependence - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ p T ≤ 750 Mev/c ; |cos φ Λ | ≤ 0.7

11. Λ from decay: 1.D YY strongly different from the one of exclusive Λ 2.positive and negative values 3. smooth energy dependence D YY @ 2.94, 3.31 and 3.67 GeV/c – Λ from Σ 0 decay - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ p T ≤ 750 Mev/c ; |cos φ Λ | ≤ 0.7

12. A Y @ 2.94, 3.31 and 3.67 GeV/c – Exclusive Λ production Exclusive Λ production: 1.effects less important vs D YY 2. smooth energy dependence - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ p T ≤ 750 Mev/c ; |cos φ Λ | ≤ 0.7

13. Λ from decay: 1. differences from exclusive Λ A Y 2. smooth energy dependence A Y @ 2.94, 3.31 and 3.67 GeV/c – Λ from Σ 0 decay - 0.7 ≤ xF ≤ 0.9 ; 0. ≤ p T ≤ 750 Mev/c ; |cos φ Λ | ≤ 0.7

14. D YY @ 2.94, 3.31 and 3.67 GeV/c – Exclusive Λ production Inclusive Λ production: high energy ) D YY > 0 Exclusive Λ production: lower energy ) D YY < 0 Exclusive Λ from Σ 0 decay: lower energy ) ≠ D YY Different reaction mechanism? FIRST EXCLUSIVE DATA AVAILABLE, FIRST DATA IN THE TFR (x F < 0 )

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

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

17. Conclusions first exclusive data available first data in the TFR (x F < 0 ) region first data for the exclusive selection of the Λ from Σ 0 decay Exclusive Λ production: 1.D YY sizeable and negative 2. smooth energy dependence 3.D YY not easily interpreted in a quark-model framework 4.OBE approach can account with FSI for D YY < 0 Λ from decay: 1.D YY strongly different from the one of exclusive Λ 2.positive and negative values 3. smooth energy dependence Both Λ production: 1.effects less important vs D YY 2. 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: 1.global 4 tracks – 2 vertices refit 2.same approach, inversion of 3.more complex parameter 18D 3 (x 12, y 12, z 12 ) for the reaction vertex six (  1, a 1, p xz,1,  2, a 2, p xz,2 to describe momenta of the two track emerging from the reaction vertex 3 (x 12, y 12, z 12 ) for decay vertex six (  3, a 3, p xz,3,  4, a 4, p xz,4 to describe momenta of the two track emerging from the decay vertex 4.4 degrees of freedom are constrained kinematically

22. Hyperon production: reconstruction @ DISTO kinematic constrains: 1.reconstructed  momentun parallel to the joiner of the reaction and decay verteces ) 2 parameters 2.reconstructed M  - p at decay vertex is M   1.115 GeV/c 2 ) 1 parameter  M 4B = 0 ( or ) or  M 4B = 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! 1.M  - p = M  2.  M 4B = 0 or  M 4B = M  Additional cuts: 1.   veto 2. |    | < 0.15 rad, decay proton momentun in LF 3.p ID for positive track from decay vertex  p  < 1 GeV/c ) missing a  at most 5.|v z | < 3.5 cm ) Klegecell veto Kinematic region restricted to: -0.7 ≤ x F ≤ 0.9 p T ≤ 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 Self-analysing  weak decay: sign of P B Geometrical interpretation of A N :

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