COMPASS results: TRANSVERSITY

1. COMPASS results: TRANSVERSITY Paolo Pagano On behalf of the COMPASS collaboration Paolo Pagano (INFN - Trieste)

2. Talk outline • The physics case of transverse spin effects • Probing transversity in SIDIS • The COMPASS experiment • The results on the deuteron • for 1 hadron SSA and interpretation • for hadron pairs SSA and interpretation • 5. Conclusions and Outlook

3. q(x) q(x) Tq(x) Spin structure functions 3 distribution functions are necessary to describe the spin structure of the nucleon at LO: All of equal importance!

4. Chiral-odd: requires another chiral-odd partner epe’X ppl+l-X epe’hX Impossible in DIS Direct Measurement Convolution with fragmentation function Measuring ΔTq(x)

5. In the last ten years: Great development in the theory of transversity; Remarkable role of ΔTq(x), notably complementary to Δq(x). In the last couple of years: Role of the kT structure functions clarified (Cahn and Sivers effects, …). Tensor charge (1991 – 1992): in analogy with: Soffer inequality (1995): Leader sum rule (2004): in analogy with: Transversity • Key features of transversity: • Probes relativistic nature of quarks • No gluon analog for spin-1/2 nucleon • Different Q2 evolution and sum rule than Δq(x) • Sensitive to valence quark polarization

6. ) Not covered in this talk Transverse Spin Physics at COMPASS Possible quark polarimeters suggested using SIDIS: • Azimuthal distribution of single (leading)hadrons • Azimuthal dependence of the plane containing hadron pairs • Measurement of transverse polarization of spin ½ baryons (e.g. L hyperon) Published RESULTS 2005 Preliminary RESULTS 2005

7. Collins effect predicts an azimuthal asymmetry in the quark fragmentation; Look at leading hadron (the most sensitive to the parent struck quark); The larger z = Eh/n, the stronger the signal; An azimuthal asymmetry can also come from the un-polarised quarks; namely from an azimuthal modulation of quark transverse momentum for a transversely polarized nucleon (Sivers effect) Azimuthal asym’s of single hadrons Left Right • The quark prefers to fragment in one direction Non vanish Lq

8. Collins and Sivers asym’s Collins angle: Sivers angle: Asym’s in FC andFS extracted independently; Asym’s calculated as function of x, z and pt and for “Leading Hadrons” and for “All Hadrons”

9. Another quark polarimeter comes from interference in fragmentation The struck quark fragments in two hadrons. The fragmentation function has a component dependent on the transverse spin momentum (as for the Collins function): A Azimuthal dependence of the plane containing hadron pairs Left Right A • The hadron pair prefers to fragment in one direction

10. Hadron pair azimuthal asymmetry Azimuthal angle: A Calculated as function of x, z and Mh for all pairs of positive and negative hadrons. A. Bacchetta and M. Radici, Proceedings of DIS 2004, hep-ph/0407345

12. Physics Goals of COMPASS With the muon beam • Gluon Polarization G/G • transverse spin distribution functions Tq(x) • Flavor dependent polarized quark helicity densities q(x) • L physics • Diffractive VM-Production With hadron beams • Primakoff-Reactions • polarizability of  and K • glueballs and hybrids • charmed mesons and baryons • semi-leptonic decays • double-charmed baryons

13. Spectrometer 2002  2004 Beam:160 GeV µ+ 2 . 108 µ/spill (4.8s/16.2s) Pm ~ 76% trigger-hodoscopes DW45 straws SM2 dipole Muon-filter2,MW2 HCAL1 RICH_1 Gem_11 ECAL2,HCAL2 SM1 dipole MWPC Gems Scifi Polarised Target Muon-filter1,MW1 Veto straws,MWPC,Gems,SciFi Gems,SciFi,DCs,straws Silicon SciFi Micromegas,DC,SciFi

14. 1 2 The polarized 6LiD-Target For transversity: 3He – 4He Dilution refrigerator (T~50mK) superconductive Solenoid (2.5 T) Dipole (0.5 T) reversed once a week two 60 cm long Target-Containers with opposite polarization During data taking for transversity dipole field always  Relaxing time > 2000 hrs Polarization: 50% Dilution factor: 0.38

15. COMPASS data sample • In so far ( 3 years ) • only DIS off 6LiD • only runs with transverse polarized target being analyzed • Data sample increased in years 2003/4: • trigger system upgraded; • DAQ upgraded; • 2004 longer run. • 2003 and 2004 data production over(analysis in progress) RICH E-Calo

16. p hadrons E muons Kinematical distributions and general cuts To access DIS region and get rid of radiative corrections Use of the calorimeters Q2 1 (GeV/c)2 0.1 Y 0.9 Q2=2.4 (GeV/c)2 Y=0.33 W 5GeV/c2 xBj0.035 W=9.4GeV/c2 2002 data

17. Cuts for Collins and Sivers SSA 2002 data pT0.1GeV/c z 0.25 (0.2 ah) pT0.51GeV/c z0.45

18. Extraction of Collins and Sivers asymmetries • Evaluating the population as a function of Φ, independently in both target cells; • Extracting 2 asymmetries and calculating the weighted mean. j = C, S (Φj calculated always with spin = ↑) F is the muon flux n the number of target particles σ the spin averaged cross-section aj the product of angular acceptance and efficiency of the spectrometer;

19. Evaluation of systematics • Stability of the asymmetries: i) in time; ii) in two halves of the target cells; iii) according to the hadron momentum. • Evaluated 3 different estimators and compatibility in the results checked. Look at the paper: “First Measurement of the Transverse Spin Asymmetries of the Deuteron in Semi-Inclusive Deep Inelastic Scattering” (The COMPASS Collaboration) Phys. Rev. Lett. 94, 202002 (2005)

20. Collins and Sivers effects (1) • Asymmetries as a function of x, z, pt • Only statistical errors shown Phys. Rev. Lett. 94, 202002 (2005) Systematic errors are smaller than the quoted statistical errors.

21. Collins and Sivers effects (2) • Asymmetries as a function of x, z, pt • Only statistical errors shown Phys. Rev. Lett. 94, 202002 (2005) Systematic errors are smaller than the quoted statistical errors.

22. Interpretation of Collins SSA • Small asymmetries. Two explanations: • Cancellation between proton and neutron; • Too small Collins mechanism. • If and large as from preliminary measurement by BELLE (see hep-ex/0507063), this is evidence for cancellation in iso-scalar target; • Also a new phenomenological fit of the data by Vogelsang and Yuan (see hep-ph/0507266).

23. COMPASS (deut.) vs. HERMES (protons) Positive hadrons • HERMES: • Negative Collins asymmetries; • Positive Sivers asymmetries. HERMES data points for Collins effect flipped by a phase of π to match COMPASS notation ! HERMES data points from: A. Airapetian et al, Phys. Rev. Lett. 94 (2005) 012002[DC53] (hep-ex/0408013)

24. COMPASS (deut.) vs. HERMES (protons) Negative hadrons • HERMES: • Large Positive Collins asymmetries; • No Sivers effect. HERMES data points for Collins effect flipped by a phase of π to match COMPASS notation ! HERMES data points from:A. Airapetian et al, Phys. Rev. Lett. 94 (2005) 012002[DC53] (hep-ex/0408013)

25. Theoretical work on Sivers SSA • M. Anselmino et al. • hep-ph/057181 • (including HERMES • Prel. Results shown at DIS05) • “Extracting the Sivers function from polarized SIDIS data and making predictions” • Phenomelogical model whose parameters are constrained by HERMES proton measurements; • COMPASS 2002 preliminary results for Sivers effect are in agreement with the model.

26. Prediciton of Sivers asymmetries on a proton target • M. Anselmino et al. • hep-ph/057181

27. Expected accuracy for Collins asymmetries • Accuracy from total statistics accumulated with LiD (deuteron) target; • Projections from 30 days of data taking (included in COMPASS schedule for 2006) with NH3 (proton) target : Taking into account: • Variation of statistical errors: • taking into account the variation of:

28. Kinematics from the analysis on hadron pairs 2002-2003: 2.8 106 combinations Select all combinations of positive (h1) and negative (h2)

29. Extraction of asymmetries for hadron pairs • Evaluating the population as a function of ΦRS, independently in both target cells; • Extracting 2 asymmetries and calculating the weighted mean. ΦRS calculated always with spin = ↑; F is the muon flux n the number of target particles σ the spin averaged cross-section a the product of angular acceptance and efficiency of the spectrometer;

30. Effects on hadron pairs • Asymmetries as a function of x, z, Mh • Only statistical errors shown Systematic errors are smaller than the quoted statistical errors. Presented for the first time @ DIS2005

31. sinf AMh Theoretical work on hadron pair (asym vs. invariant mass) R. L. Jaffe, X. Jin and J. Tang, Phys. Rev. Lett. 80, 1166 (1998) Radici, Jakob, Bianconi, PRD 65, 074031 H∢(z,M2p+p-)∼sind0 sind1 sin(d0-d1)

32. sinf Ax H (z) ∢ Theoretical work on hadron pairs (asym vs. xbj and z) Radici, Jakob, Bianconi, PRD 65, 074031

33. Summary and Outlook • Collins and Sivers SSA shown from 2002 data published by PRL: • The asymmetries are small and compatible with zero; • Making use of all the deuteron data, accuracy will improve by a factor ~ 3. • SSA from hadron pairs calculated from 2002 + 2003 statistics: • The asymmetries are small and compatible with zero; • Using 2004 data, accuracy improves by a factor 1.4. • Stay tuned because: • New channels in hadron pair production are under study. • PID is being included in all the analyses; • New results on Lambda transverse polarization are coming. • Data (of comparable statistics) will be collected on a transversely polarized proton target (NH3) in 2006.

34. Thank you!

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36. Estimators Weaker assumptions on acceptance effects less sensitive to distorsions set by Cahn effect Reconstruction time independent in the solid angle

37. Why a proton run? To measure the shape of