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Longitudinal Spin Transfer of in Polarized pp Collisions at 200 GeV

Longitudinal Spin Transfer of in Polarized pp Collisions at 200 GeV. Qinghua Xu, LBNL for the STAR Collaboration SPIN 2006, Oct 6, 2006. Motivation Proof-of-principle analysis at low p T with STAR Preliminary results for D LL in inclusive production

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Longitudinal Spin Transfer of in Polarized pp Collisions at 200 GeV

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  1. Longitudinal Spin Transfer of in Polarized pp Collisions at 200 GeV Qinghua Xu, LBNL for the STAR Collaboration SPIN 2006, Oct 6, 2006 • Motivation • Proof-of-principle analysis at low pT with STAR • Preliminary results for DLL in inclusive production • Summary and outlook Qinghua Xu, LBNL

  2. Why study ? • Due to its self-analyzing weak decay,  polarization • has been widely studied in different processes with • many interesting results (following talks). Unit vector along proton momentum in rest frame. polarization vector decay parameter 0.642 0.013 • What can we learn from polarization with • polarized pp collider at RHIC? Qinghua Xu, LBNL

  3. What can we learn with in polarized pp collisions ? • Longitudinal spin transfer in • polarized proton-proton collisions: measures the transfer of beam polarization to hyperon. • contains a strange quark, can it • provide sensitivity to  production is expected to be dominated by u quark, while s_bar plays a dominate role for production at large pT! How sensitive is anti-Lambda DLL to Qinghua Xu, LBNL

  4. Spin transfer of in pp collision Q. X, E. Sichtermann, Z. Liang, PRD 73,2006 Q2=5 GeV Pol. frag. func. model -0.090.02 (Inclusive DIS) 0.010.03 (Semi-inclusive DIS-HERMES) • Anti-Lambda DLL may give new insights into at high pT! Qinghua Xu, LBNL

  5. Spin transfer versus double spin asymmetry 0<<1 for DLL -1<<1 for ALL [GeV] • DLL is ~ 4X more sensitive to than ALL. • ~0.01 measurement will distinguish parameterizations. Qinghua Xu, LBNL

  6. RHIC- the first polarized pp collider Stable polarization direction - transverse Longitudinal polarization at STAR/Phenix 4 spin orientations: ++,+-,-+,-- AGS Heclical Partial Snake Qinghua Xu, LBNL

  7. STAR Detector TPC (-1.6<<1.6) p  • is reconstructed by combining • TPC tracks with opposite charges after • particle identification from energy • loss and applying topological cuts. V0_vertex V0_DCA • primary vertex Qinghua Xu, LBNL

  8. 2005 data: ~3 pb-1, ~50% beam polarization • ~3X106 minimum bias events, • ~30X103 (24X103) analyzed. • Invariant mass & kinematics <pT>~1.3 GeV <|xF|>~0.0075 M=1.1157 GeV(PDG) • Cross section of reasonably described by pQCD. nucl-ex/0607033, submitted to PRC Qinghua Xu, LBNL

  9. Method to extract DLL •  polarization is usually extracted from the momentum distribution of its • weak decay ( ): : angle between the momentum of decay proton in ’s rest frame and ’s momentum at the lab frame : decay parameter: 0.642 A(cos): detector acceptanceafter integrating over momentum space • P() in inclusive pp reaction change its sign when flipping beam helicity, • due to parity conservation: • Asymmetry between  counts with opposite polarization within : [1,2]: • Equation to extract DLL: Qinghua Xu, LBNL

  10. Signal and Background • Background from K0s: • --- a cut of cos<-0.2 applied. • Subtracting bg. contribution to DLL r: fraction of background under the peak Qinghua Xu, LBNL

  11. Comparison of DLL with two methods: • Time stability of DLL: # fill Qinghua Xu, LBNL

  12. Results of DLL : <pT>~1.3 GeV <|xF|>~0.0075 Systematic error ---scale uncertainty from beam polarization measurement not included. • Positive is along the polarized beam direction. • Statistical uncertainty is ~0.05, systematic < 0.01. Qinghua Xu, LBNL

  13. Systematics: • 4X10-3from relative luminosity measurement. • 2% from decay-parameter (0.6420.013). • 2% from transverse beam polarization components at STAR. • +17% scale uncertainty from RHIC beam polarization measurement. Cross check with K0s: LL/Pbeam K0s: 0.010.01 # fill • K0s are spin-0 meson -> null  measurement. • reconstruction/analysis similar to (anti)Lambda. • Statistical error is ~1/5 of (anti)Lambda’s DLL. Qinghua Xu, LBNL

  14. Summary • Spin transfer DLL for anti-Lambda in proton collisions is sensitive to at large pT (pT > ~6 GeV/c) • Proof of principle analysis from 2005 minimum bias data <pT>~1.3 GeV <|xF|>~0.0075 Systematic error ---scale uncertainty from beam polarization measurement not included. Qinghua Xu, LBNL

  15. Outlook • Triggering is needed to efficiently reach high pT. • In 2005, we collected most of the data with a jet-patch • trigger and the projected precision is: • Considering dedicated trigger development, andlooking • forward to high-luminosity running periods. 1.6 pb-1 of BJP2 0<<1 Qinghua Xu, LBNL

  16. Backup slides • Possible dedicated trigger for high pT anti-Lambda: • Different pre-shower response to anti-proton in • electromagnetic calorimeter than photon • Triggering on high pT pion with muon trigger • Higher level trigger on distribution of invariant mass • versus momentum ratio of proton and pion Qinghua Xu, LBNL

  17. Invariant mass distribution of  in different pT range (BJP) Qinghua Xu, LBNL

  18. (anti)Lambda counts versus pT with BJP2 trigger of 2005 Qinghua Xu, LBNL

  19. Decay contribution to anti_Lambda production in pp Qinghua Xu, LBNL

  20. Qinghua Xu, LBNL

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