1. Transversity 2005, Como Two-hadron asymmetries @ Adam Mielech INFN Trieste on behalf of COMPASS collaboration 7-10th. September 2005

2. Transversity 2005, Como Study of single spin asymmetries for 2hadron production in SIDIS ● is a way to probe transverse polarization distributions of the nucleon coupled to interference fragmentation functions ● so, it is complementary to the 1hadron case, where transversity is probed by the Collins effect.

3. Transversity 2005, Como Definitions Angles defined in Breit system: azimuthal angle of the spin of the fragmenting and initial quark -virtual photon vector -incident lepton vector -component of, perpendicular to the.

4. Transversity 2005, Como Asymmetry N (  RS ) – number of events in a given angle bin - normalization factor - depolarization factor P – target polarization (~50%) f – dilution factor (~0.4)

5. Transversity 2005, Como N Localisation SPS LHC luminosity: ~5. 10 32 cm -2 s -1 beam intensity: 2. 10 8 µ+/spill (4.8s/16.2s) beam momentum: 160 GeV/c target polarization: ~50% ~200 physicists from 12 countries longitudinally polarised muon beam longitudinally or transversely polarised deuteron ( 6 LiD) target momentum and calorimetry measurement particle identification

6. Transversity 2005, Como Collected statistics with transverse target polarization ● 2002 – 12+7 days of data taking, 1.6*10 6 SI DIS events ● 2003 – 14 days of data taking, 4*2002 ● 2004 – 14 days of data taking, 2003 data doubled Today data 2002+2003 are presented

7. Transversity 2005, Como Event selection ● Selection of events – Q 2 >1 (GeV/c) 2 – 0.9>y>0.1 – At least one +/- hadron pair 1

8. Transversity 2005, Como Hadron selection ● Selection of hadrons – Tracks which survived ”muon rejection” criteria – z h1,h2 >0.1 & x fh1,h2 >0.1 ( current fragmentation region ) – (z h1 + z h2 )<0.9 ( remove exclusive mesons )

9. Transversity 2005, Como Additional cut ● |R T |>0.05 GeV/c

10. Transversity 2005, Como Collected statistics & multiplicities h+h+ h-h- Final sample of hadron pairs after cuts: - 2002+2003: ~2.8 M - 0.22 pair / DIS event

11. Transversity 2005, Como Asymmetry   d  ' h + h - X... for the process assumming h= 

12. Transversity 2005, Como Spectator model for FF: Radici, Jakob, Bianconi, PRD 65, 074031 Jaffe, Jin, Tang Phys.Rev. Lett. 80,1166(1998) Asymmetry  0  1 strong interaction  phase shift (data):

13. Transversity 2005, Como Asymmetry Spectator model prediction: Radici, Jakob, Bianconi, PRD 65, 074031

14. Transversity 2005, Como Asymmetry z 1 +z 2 Radici, Jakob, Bianconi, PRD 65, 074031

15. Transversity 2005, Como Systematic checks ● In order to estimate systematics effects false asymmetries were investigated so far: – asymmetry calculated for the unpolarized sample created by scrambling the data of the real (polarized) sub periods – asymmetry calculated for slices of one target cell ● Width of  RS bins were changed Results shows that false asymmetries, are not larger than the statistical uncertenties of measured asymmetries. Results remains stable.

16. Transversity 2005, Como Nearest future plans ● Finalize analysis of 2004 data and systematic studies ● Look at the other ways to define oppositely charged hadron combinations e.g. h1 -> leading* hadron or, h2 -> second leading* hadron *leading in terms of p T, z,... ● Use of RICH to extract asymmetries for identified hadron pairs (so far we assuming that all hadrons are pions)

17. Transversity 2005, Como Compass RICH 5 m 6 m 3 m photondetectors: CsI MWPC mirrorwall vessel radiator: C 4 F 10 beam ● Fully operated for transverse data since 2003 ● Separation between  K and p in the momentum range 3-50 GeV/c

18. Transversity 2005, Como RICH response K  p Threshold momenta: ● p  =2 GeV/c ● p K =9 GeV/c ● p P =17 GeV/c Hadron masses calculated from the measured Cherenkov angle

19. Transversity 2005, Como Fractions of hadrons recognized by RICH For single leading (z) hadrons, with additional cut: z > 0.25

20. Transversity 2005, Como 2hadron case using RICH Pairs of hadrons to be analyzed: ,  K, KK K0K0 00  K*K*

21. Transversity 2005, Como 2hadron case using RICH Cleaner identification for more energetic particles (z h1 >0.25 & z h2 >0.25) Identification for “standard” cuts (z h1 >0.1 & z h2 >0.1) counts

22. Transversity 2005, Como Summary and outlook ● 2hadron data collected in years 2002 and 2003 with transversely polarized deuteron target were analyzed ● All measured asymmetries are small and compatible with 0. ● Performed checks indicate that systematic effects are not larger than real asymmetries and their statistical uncertainties. ● RICH studies were done in order to extract asymmetries for identified hadrons and hadron pairs ,  K and KK in near future. ● Different 2hadron combination choice methods are investigated, some results are expected in nearest future. ● Data collected in year 2004 are analyzed, results expected soon.