Transversity Experiments

1. Transversity Experiments Jen-Chieh Peng University of Illinois • Experimental probes for transversity • Current experimental status on transversity and other related distribution and fragmentation functions • Outstanding issues and future prospect SIR2005, Jefferson Lab, May 18-20, 2005

2. WHY Transversity ? • Remaining frontier of kT – independent structure functions • Connections to many other kT – dependent distribution and fragmentation functions • Major experimental challenges to measure transversity. Opportunities for lepton and hadron beams. • Active interaction between theory and experiment

3. Transversity • Some characteristics of transversity: • δq(x) = Δq(x) for non-relativistic quarks • δq and gluons do not mix → Q2-evolution for δq and Δq are different • Chiral-odd → not accessible in inclusive DIS Chiral-quark soliton model Quark – diquark model (solid) and pQCD-based model (dashed) Similar to helicity distributions B. –Q. Ma, I. Schmidt and J. –J. Yang, PRD 65, 034010 (2002) hep-ph/0101300

4. How to measure transversity? Chiral-odd → not accessible in DIS Require another chiral-odd object • Transversely Polarized Drell-Yan • Semi-Inclusive DIS • Single-hadron (Collins fragmentation function) • Two hadrons (Interference fragmentation function) • Vector meson polarization • Λ - polarization

5. Polarized Drell-Yan in p-p collision (RHIC-spin) Transverse double-spin asymmetry for Drell-Yan • Well understood reaction mechanism. Clear interpretation • An unique method to extract sea-quark transversity • Small effect due to the expected small sea quark transversity PHENIX, s1/2=200GeV, 320 pb-1 hep-ph/9902250

6. GSI Polarized Antiproton eXperiment (PAX) DIS2005 talk by Lenisa Collider: L=2x1030 cm-2s-1 Fixed target: L=2.7x1031 cm-2s-1 Phase I: 2013-2017, Phase II: 2018 -

7. Simi-inclusive DIS can access all leading-twist quark distributions Leading-Twist Quark Distributions ( A total of eight distributions) No K┴ dependence K┴ - dependent, T-even K┴ - dependent, T-odd

8. All Eight Quark Distributions Are Probed in Semi-Inclusive DIS Unpolarized Transversity Polarized target Sivers Polarzied beam and target SL and ST: Target Polarizations;λe: Beam Polarization

9. Observation of Single-Spin Azimuthal Asymmetry ep → e’πx HERMES Longitudinally polarized target <ST> ~ 0.15 Origins of the azimuthal asymmetry (correlation between the target nucleon transverse spin and the pion transverse momentum)? Collins effect: Correlation between the quark’s transverse spin with pion’s pT in the fragmentation process. Sivers effect: Correlation between the transverse spin of the proton with the quark’s transverse momentum. Other higher twist effects could also contribute.

10. Proton data Comparison with HERMES longitudinal SSA data Deuteron data Efremov et al. shows that Chiral-quark soliton model can describe the SSA data (by including only the transversity / Collins term) hep-ph/0206267

11. Comparison with HERMES longitudinal SSA data proton data • Anselmino et al. showed that the Hermes SSA data for longitudinally polarized data can be explained by Sivers effect alone (without the transversity / Collins effect). hep-ph/0412316

12. AUTsin() from transv. pol. H target Simultaneous fit to sin( + s) and sin( - s) „Collins“ moments Talk by Schnell hep-ex/0408013

13. Collins asymmetryfromCOMPASS Transversely polarized 6LiD target Cover smaller x Consistent with 0 hep-ex/0503002 COMPASS 2002-2004 data: ~ factor of 4 in statistics Talk by Bressan

14. Comparison between HERMES and COMPASS Results Cancellation between p and n asymmetries?

15. JLab Hall-A proposal for 3He↑(e,e’π-)x • Beam • 6 GeV polarized e-, 15 μA, helicity flip at 60 Hz • Target • Optically pumped Rb spin-exchange 3He target, 50 mg/cm2, ~42% polarization, transversely polarized with tunable direction • Electron detection • BigBite spectrometer, Solid angle = 60 msr, θLab = 300 • Charged pion detection • HRS spectrometer, θLab = -160

16. Transversity measurements at JLab 12 GeV upgrade Projected sensitivity at CLAS12

17. Measuring transversity using two-hadron production in SIDIS SSA for π+π- production in SIDIS

18. Mass dependence of the π+π- interference FF Hermes data for longitudiannly polarized deuterium target Jaffe et al. hep-ph/9709322 hep-ex/0501009 New results from Hermes and Compass for transversely polarized target (talks by Schnell and Bressan)

19. “Byproduct” of the transversity experiments AUTsin() from Hermes transv. pol. H target “Sivers“ moments First measurement of Siversasymmetry Sivers function nonzero  orbital angular momentum ofquarks Sivers moments from HERMES and COMPASS (Talks by Schnell and Bressan)

20. Extraction of Sivers functions from the Sivers moment measurements Fits to the Hermes data “Prediction” of the Compass data Anselmino et al. hep/ph/0501196

21. Is the Hermes AUL data consistent with AUT data? A combined analysis of the AUL and the AUT data Data Higher-twist contribution Contributions from Collins and Sivers Elschenbroich et al. hep-ex/0504025

22. Transverse SSA in p-p collision Result from STAR p+ p0 p- New data at forward and backward XF presented at DIS2005 (talk by Perdekamp)

23. New AN data from BRAHMS Comparing An for p+ and p- Polarization was ~42% for p+measurements and ~38% for p-. syst error on P ~ 20-30%. Improve in final analysis of CNI and Gas Jet data. AN= +0.05 +- 0.005 +- [0.015] in 0.17 < xF < 0.32 AN= -0.08 +- 0.005 +- [0.02] in 0.17 < xF < 0.32 DIS2005 Talk by Videbaek

24. Negative XF data from BRAHMS BRAHMS preliminary This corresponds to negative xF, and is consistent with 0 as expected. A unified picture for SSA with hadron and lepton beams?

25. Analysing power (AN) is sensitive to Sivers function SSA with Transversely Polarized Drell-Yan Sivers function in Drell-Yan is expected to have a sign opposite to that in DIS! • Prediction by Anselmino, D’Alesio, Murgia (hep-ph/0210371) for a negative AN. • |AN| increases with rapidity, y, and with dilepton mass, M. (Brodsky, Hwang, Schmidt, hep-ph/0206259; Collins, hep-ph/0204004) p↑ + p → l+ l- + X √s = 200 GeV AN Is this measurement feasible at RHIC? y

26. Expected statistical sensitivity for Drell-Yan AN Assuming 400 pb-1 50% polarization p↑ + p → l+ l- + x • Might be feasible to determine the sign of the Sivers function at RHIC • Should consider fixed-target polarized Drell-Yan too √s = 200 GeV 6 < M < 10 GeV

27. Cos2Ф Dependence in Unpolarized Drell-Yan Large cos2Ф dependences have been observed in π – induced Drell-Yan • RHIC would provide unpolarized p-p Drell-Yan data too • Fixed-target unpolarized p-p Drell-Yan data also exist This azimuthal dependence could arise from a product of KT-dependent distribution function h1┴ ( Boer, hep-ph/9902255; Boer, Brodsky, Hwang, hep-ph/0211110) In quark-diquark model, h1┴ is identical to Sivers function No Cos2Ф depenence for unpolarized p-p Drell-Yan has been reported yet (The effect from h1┴ is expected to be smaller)

28. Unpolarized p-p and p-d dimuon production Fermilab E866, √s = 38.8 GeV J/Ψ Ψ’ Talk by Reimer Υ ~ 2.5 x 105 Drell-Yan events

29. Sin2Φmoment of single-spin asymmetry (Probing Collins function) CLAS EG1b preliminary Talk by Bosted

30. Summary • Exciting time for physics of transversity and related topics. • Still a long way from measuring the transversity. However, the journey might be as interesting as the destination. • Look forward to the talks on new experimental results and the discussion on theoretical interpretations.