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Spin dependent fragmentation function analysis at Belle

Spin dependent fragmentation function analysis at Belle. Spin 2006 October 5, 京都 , 日本. M. Grosse Perdekamp (University of Illinois and RBRC) A. Ogawa (BNL/RBRC) R. Seidl (University of Illinois and RBRC). KEKB: L>1.6x10 34 cm -2 s -1 !!. Belle detector. KEKB. Asymmetric collider

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Spin dependent fragmentation function analysis at Belle

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  1. Spin dependent fragmentation function analysis at Belle Spin 2006 October 5,京都,日本 M. Grosse Perdekamp (University of Illinois and RBRC) A. Ogawa (BNL/RBRC) R. Seidl (University of Illinois and RBRC)

  2. KEKB: L>1.6x1034cm-2s-1 !! Belle detector KEKB • Asymmetric collider • 8GeV e- + 3.5GeV e+ • Ös = 10.58GeV (U(4S)) • e+e-U(4S)BB • Off-resonance: 10.52 GeV • e+e-qq (u,d,s,c) • Integrated Luminosity: >600 fb-1 • >60fb-1 => off-resonance Measurement of the Collins fragmentation function at BELLE

  3. Good tracking and particle identification! Measurement of the Collins fragmentation function at BELLE

  4. Collins fragmentation in e+e- : Angles and Cross section cos(f1+f2) method e+e- CMS frame: j2-p thrustaxis e- Q j1 j2 j1 [D.Boer: PhD thesis(1998)] e+ 2-hadron inclusive transverse momentum dependent cross section: First |kT| moments of Collins Function H1⊥ Net (anti-)alignment of transverse quark spins Measurement of the Collins fragmentation function at BELLE

  5. Collins fragmentation in e+e- : Angles and Cross section cos(2f0) method e+e- CMS frame: • Independent of thrust-axis • Convolution integralI over transverse momenta involved e- Q2 j0 [Boer,Jakob,Mulders: NPB504(1997)345] e+ 2-hadron inclusive transverse momentum dependent cross section: Net (anti-)alignment of transverse quark spins Measurement of the Collins fragmentation function at BELLE

  6. Originally off_resonance data, now also on_resonance data (29.1  547 fb-1) Track selection: pT > 0.1GeV vertex cut:dr<2cm, |dz|<4cm Acceptance cut -0.6 < cosqi< 0.9 Event selection: Ntrack  3 Thrust > 0.8 Z1, Z2>0.2 Applied cuts, binning • Hemisphere cut • QT < 3.5 GeV • Pion PID selection Measurement of the Collins fragmentation function at BELLE

  7. Examples of fits to azimuthal asymmetries Cosine modulations clearly visible N(f)/N0 2f0 (f1+f2) D1 : spin averaged fragmentation function, H1: Collins fragmentation function No change in cosine moments when including sine and higher harmonics Measurement of the Collins fragmentation function at BELLE

  8. Methods to eliminate gluon contributions: Double ratios and subtractions Double ratio method: Pros: Acceptance cancels out Cons: Works only if effects are small (both gluon radiation and signal) Pros: Gluon radiation cancels out exactly Cons: Acceptance effects remain Subtraction method: 2 methods give very small difference in the result Measurement of the Collins fragmentation function at BELLE

  9. Other Favored/Unfavored Combinations charged pions or p0 Challenge: current double ratios not very sensitive to favored to disfavored Collins function ratio  Examine other combinations: • Unlike-sign pion pairs (U): (favored x favored + unfavored x unfavored) • Like-sign pion pairs (L): (favored x unfavored + unfavored x favored) • p±p0 pairs (favored + unfavored) x (favored + unfavored) • P.Schweitzer([hep-ph/0603054]): charged pp pairs are similar (and easier to handle) (C): (favored + unfavored) x (favored + unfavored) UL UC • Build new double ratios: • Unlike-sign/ charged pp pairs (UC) Favored = up+,dp-,cc. Unfavored = dp+,up-,cc. Measurement of the Collins fragmentation function at BELLE

  10. Why is it possible to include on_resonance data?Different Thrust distributions e+e-qq̅, q∈uds e+e-cc̅ • small B contribution (<1%) in high thrust sample • >75% of X-section continuum under • U (4S) resonance • 29 fb-1 547 fb-1 • many systematic errors reduce with more • statistics • Charm-tagged Data sample also increases Measurement of the Collins fragmentation function at BELLE

  11. Improved Systematic errors (UC) • Tau contributions • PID systematics* • MC double ratios • Charged ratios (p+p+ /p-p- ) • Higher order terms • Double ratio-subtraction method A0 (cos(2f0)) moments A12 (cos(f1 +f2)) moments Reweighting asymmetries: underestimation of cos(f1+f2) asymmetries rescaled by 1.21 Correlation studies: statistical errrors rescaled by 1.02 (UL) and 0.55 (UC) Beam polarization studies consistent with zero Measurement of the Collins fragmentation function at BELLE

  12. Collins asymmetries I: 4x4 z1, z2 binning A0 (cos(2f0)) moments A12 (cos(f1 +f2)) moments • 547 fb-1 charm corrected data sample, • UL and UC double ratios PRELIMINARY PRELIMINARY Measurement of the Collins fragmentation function at BELLE

  13. Collins asymmetries II: sin2q/(1+cos2q) binning (UL) • Nonzero quark polarization ~ sin2 q • Unpolarized de-nominator ~ 1+cos2q • Clear linear behavior seen when using either thrustz or 2nd hadron as polar angle • Better agreement for thrust axis (~approximate quark axis) • UC plots similar PRELIMINARY thrustz q2 Measurement of the Collins fragmentation function at BELLE

  14. e+e-(p+p-)jet1(p-p+)jet2X Stay in the mass region around r-mass Find pion pairs in opposite hemispheres Observe angles j1+j2 between the event-plane (beam, jet-axis) and the two two-pion planes. Transverse momentum is integrated (universal function, evolution easy  directly applicable to semi-inclusive DIS and pp) Theoretical guidance by papers of Boer,Jakob,Radici and Artru,Collins Early work by Collins, Heppelman, Ladinski Interference Fragmentation – thrust method j2-p p-j1 Measurement of the Collins fragmentation function at BELLE

  15. Unpolarized FFs MC simulation, ~1.4 fb-1 • Performed Systematic studies: • Smearing • Tracking efficiency • Acceptance • Energy scale • Particle ID study not yet finished • AKK: Try flavor tagged/enhanced analysis? • How about 2 hadron analysis  favored,disfavored FF 1.4 fb-1, 4.5·106 events 2.5 % of off_resonance data 2. Measurement of the Collins fragmentation function at BELLE

  16. Summary and outlook • Analysis almost finalized, smalller additions planned (kt dependence of Collins effect, parameterizations) • Interference fragmentation function analysis started • Many more QCD/Spin related studies possible (unpol FFs, timelike DVCS, L Polarimetry) • Double ratios: double ratios from data most systematic errors cancel • Analysis procedure passes all null tests • Systematic uncertainties understood • Statistics increased from 29  547 fb-1 •  Significant nonzero asymmetry with double ratios are observed • UL/C about half as large UL/L double ratios • Data can be used for more sophisticatedanalysis • Paper with 29 fb-1 U/L double ratios published (PRL96:232002,2006) Measurement of the Collins fragmentation function at BELLE

  17. Backup Slides Measurement of the Collins fragmentation function at BELLE

  18. Why correction factors of 1.2 and 0.55? • Charged pion pairs are the sum of U and L • Errors are correlated when building double ratios UC or LC • Division of histograms takes correlation not into account • Proper error calculation confirms correction factors found in correlation studies and c2 distributions Measurement of the Collins fragmentation function at BELLE

  19. Collins asymmetries IIa: sin2q/(1+cos2q) binning (UC) • Nozeron quark polarization ~ sin2 q • Unpolarized de-nominator ~ 1+cos2q • Clear linear behavior seen when using either thrustz or 2nd hadron as polar angle • Better agreement for thrust axis (~approximate quark axis) • Similar behavior, smaller magnitude as UL asymmetries PRELIMINARY thrustz q2 Measurement of the Collins fragmentation function at BELLE

  20. Collins asymmetries III: QT binning (UL) PRELIMINARY • Reduced asymmetries in low thrust sample • At low thrust significant B contribution (for t<0.8 ~20 % B for t>0.8 < 1 % B) • A12 thrust axis dependent • High QT (>3.5 GeV) asymmetries from beam related BG • UC plots similar Thrust>0.8 Thrust<0.8 (not corrected for heavy quark contributions) Measurement of the Collins fragmentation function at BELLE

  21. Collins asymmetries IIIa: QT binning (UC) PRELIMINARY • Reduced asymmetries in low thrust sample • At low thrust significant heavy quark contribution • High QT (>3.5 GeV) asymmetries from beam related BG • similar behavior, but smaller magnitude as UL asymmetries Thrust>0.8 Thrust<0.8 (not corrected for heavy quark contributions) Measurement of the Collins fragmentation function at BELLE

  22. Final charm corrected results for e+ e-p p X (29fb-1 of continuum data) Final results Preliminary results • Significant non-zero asymmetries • Rising behavior vs. z • UL/C asymmetries about 40-50% of UL/L asymmetries • First direct measurements of the Collins function • UL/L data published Measurement of the Collins fragmentation function at BELLE

  23. Final charm corrected results for e+ e-p p X (547 fb-1) • Significance largely increased • Behavior unchanged • Reduced systematics • Precise measurements of the Collins function Measurement of the Collins fragmentation function at BELLE

  24. B, charm and uds contributions in the data thrust>0.8 thrust<0.8 uds quarks charm quarks Charged B-mesons Netral B-mesons Open symbols: charm enhanced data sample used for charm correction Full symbols: main data sample Measurement of the Collins fragmentation function at BELLE

  25. Collins effect in quark fragmentation J.C. Collins, Nucl. Phys. B396, 161(1993) q Collins Effect: Fragmentation with of a quark qwith spin sqinto a spinless hadron h carries an azimuthal dependence: Measurement of the Collins fragmentation function at BELLE

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