1. Recent Results from the STAR Longitudinal Spin Program Outline  Introduction  The STAR Experiment  Lambda Polarization  Neutral Pions  Charged Pions  Inclusive Jets  2006 Projections Frank Simon, MIT, for the STAR Collaboration RHIC Spin Physics Workshop, RIKEN, Japan September 29-30, 2006

2. Frank Simon: STAR Longitudinal Overview 2 09/30/2006 Introduction: Longitudinal Physics  Use polarized proton collisions to access information on the polarization of gluons in the nucleon  Longitudinal spin asymmetries are connected to parton polarizations long-rangeshort-rangelong-range f1f1 f2f2 f f2f2 f1f1

3. Frank Simon: STAR Longitudinal Overview 3 09/30/2006 Introduction: Inclusive Measurements  Longitudinal double spin asymmetry A LL for inclusive processes depends on the gluon polarization  Asymmetry also depends on particle type

4. Frank Simon: STAR Longitudinal Overview 4 09/30/2006 Introduction: Accessing Quark Polarizations  (anti)-  contains one (anti) - strange quark, the (anti)-  polarization can thus provide information on the (anti)- strange quark polarization in the nucleon  Longitudinal spin transfer in polarized pp collisions, measures the transfer of beam polarization to. Q. X, E. Sichtermann, Z. Liang, PRD 73,2006 frag. func. model

5. Frank Simon: STAR Longitudinal Overview 5 09/30/2006 The STAR Experiment Magnet  0.5 T Solenoid Triggering & Luminosity Monitor  Beam-Beam Counters  3.4 < |  | < 5.0  Zero Degree Calorimeters Central Tracking  Large-volume TPC  |  | < 1.5 Calorimetry  Barrel EMC (Pb/Scintilator)  |  | < 1.0  Shower-Maximum Detector  Endcap EMC (Pb/Scintillator)  1.0 <  < 2.0  Shower-Maximum Detector …and many other systems currently not used in the longitudinal spin analysis 2005 run

6. Frank Simon: STAR Longitudinal Overview 6 09/30/2006 Triggering and DAQ  STAR Data Acquisition currently limited to ~100 Hz, significant increase in data taking rate over the last years  Only a small fraction of all events can be recorded  The challenge: highly granular main tracking TPC (~50 M voxels)  Triggers are crucial to access rarer events  Calorimeters are the main detectors to select rarer events (e.g. hard scattering)  High Tower Triggers (HT): triggers on energy deposit in one calorimeter tower  x  = 0.05 x 0.05 in the BEMC  good trigger for  0,   Jet Patch Trigger (JP): triggers on energy deposit in a larger area if the calorimeter  x  = 1.0 x 1.0 in the BEMC  good trigger for jets

7. Frank Simon: STAR Longitudinal Overview 7 09/30/2006 2005 Data Set & Common Cuts  Total sampled luminosity ~3.1 pb -1 with HT and JP triggers  After rigorous quality cut: ~1.7 pb -1 sampled luminosity with HT and JP triggers  Endcap EMC analysis uses ~1.1 pb -1  Lambda Analysis currently only with MB triggers  ~ 0.25  Cut on the BBC timing information (corresponds to a vertex cut that restricts accepted interactions to within ±60 cm of the nominal interaction point)

8. Frank Simon: STAR Longitudinal Overview 8 09/30/2006 Lambda Reconstruction primary vertex V0_vertex V0_DCA   is reconstructed by combining TPC tracks with opposite charges after particle identification from energy loss and applying topological cuts.  Invariant mass & kinematics M=1.1157 GeV(PDG) ~1.3 GeV ~0.0075  2005 data: ~3X10 6 minimum bias events, ~19X10 3 (14X10 3 ) analyzed (after all cuts).

9. Frank Simon: STAR Longitudinal Overview 9 09/30/2006 Lambda Asymmetry D LL Subtracting bg. contribution to DLL r: fraction of background under the peak  : decay parameter 0.642  0.013  : angle between the momentum of decay proton in  ’s rest frame and  ’s momentum at the lab frame

10. Frank Simon: STAR Longitudinal Overview 10 09/30/2006 Lambda Asymmetry D LL Systematics Cross check with K0s:  LL =0.01  0.01  4X10 -3 from relative luminosity measurement.  2% from decay-parameter (0.642  0.013).  2% from transverse beam polarization components at STAR.  + overall scale uncertainty from RHIC beam polarization measurement.  K0s are spin-0 mesons -> null measurement  reconstruction/analysis similar to (anti)Lambda  Statistical error is ~1/5 of (anti)Lambda’s D LL  proof of principle measurement  high pt data needed to achieve physics result

11. Frank Simon: STAR Longitudinal Overview 11 09/30/2006 Lambda Asymmetry: Outlook  Triggering is needed to reach high p t efficiently  The biggest data sample in 2005 is the jet-patch trigger (triggers on electromagnetic energy deposit)  Select events with hard scattering projected sensitifity for 2005 JP trigger  higher luminosity and maybe dedicated triggers needed

12. Frank Simon: STAR Longitudinal Overview 12 09/30/2006 Double Longitudinal Asymmetry: Overview Ingredients:  Polarization: measured by RHIC polarimeters  Relative Luminosity R measured with the STAR BBC & scaler system (relative luminosities for each bunch crossing available)  Systematic studies by comparing BBC with ZDC measurements (limited by ZDC statistics): systematic error on R ~10 -3  Spin dependent yields N ++, N +- :  Spin direction in the interaction region verified by the STAR BBCs, significance ~

13. Frank Simon: STAR Longitudinal Overview 13 09/30/2006 Neutral Pion Reconstruction in STAR  BEMC and EEMC provide calorimetric coverage for -1 <  < 2 (2005: 0 <  < 2)  Both calorimeters have a Shower Maximum Detector that provides increased spatial resolution to separate photons  Neutral pions are reconstructed via their decay into two photons:  0 invariant mass: BEMC SMD EEMC SMD

14. Frank Simon: STAR Longitudinal Overview 14 09/30/2006 EEMC: Beam Background  Beam background (asymmetric in  ) observed in all layers of the endcap calorimeter  Consistent with tracks parallel to the beam observed in the TPC  Also affects  pairs sector ~  7.0 < pT < 8.0 GeV towers pre-1 pre-2 post sector ~ 

15. Frank Simon: STAR Longitudinal Overview 15 09/30/2006 EEMC: Neutral Pion Reconstruction   0 +  MC  cluster splitting  beam background  combinatoric background

16. Frank Simon: STAR Longitudinal Overview 16 09/30/2006 EEMC  0 : Double Longitudinal Asymmetry A LL  2005 result (using 1.1 pb -1 ) limited by systematic uncertainties due to beam background and by statistics  beam background systematics also limited in precision by available statistics  Currently no resolving power between different  g parameterizations  Shielding installed in accelerator tunnel to reduce background in 2006 run  Important baseline measurement for future prompt photon measurements expected

17. Frank Simon: STAR Longitudinal Overview 17 09/30/2006 BEMC: Neutral Pion Reconstruction   invariant mass spectrum in the signal region described by:  MC  0 line shape  low invariant mass background (caused by cluster splitting in the SMD)  combinatoric background & residual fit  Correction factor for cross section determination obtained from PYTHIA & HERWIG simulations different cuts used for MB and HT triggers HT1 HT2

18. Frank Simon: STAR Longitudinal Overview 18 09/30/2006 BEMC  0 : Inclusive Cross Section  L sampled : 0.4 pb -1 (HT triggers), 44  b -1 (MB)  Point-to-point systematics from yield extraction  Total systematics dominated by 5% uncertainty in BEMC energy scale, significant contribution from correction factor uncertainty  Compared to NLO pQCD (CTEQ6M pdfs) using KKP and Kretzer fragmentation func.  better agreement with KKP  large scale uncertainties in pQCD calculations, indicated by choosing different scales for KKP calculations Invariant cross section: consistent with previous results from PHENIX

19. Frank Simon: STAR Longitudinal Overview 19 09/30/2006 BEMC  0 : Double Longitudinal Asymmetry A LL constant fit (assumes no p t dependence):  ALL = -0.017 +- 0.021    NDF compared to NLO calculations (ignoring systematic errors):  GRSV Std: 0.8  GRSV Max: 2.4  GRSV Min: 0.8  GRSV Zero: 0.5  GRSV max scenario disfavored overall scale uncertainty from polarization measurement not included

20. Frank Simon: STAR Longitudinal Overview 20 09/30/2006 BEMC  0 : A LL Systematic Studies & Errors  Parity-Violating Single Spin Asymmetries  Good tool to investigate possible spin dependent background effects or issues with relative luminosities  For  0 all observed single spin asymmetries are within 1  of 0, no systematic assigned  Random Pattern Analysis  Asymmetries calculated with randomized bunch patterns  no indication of non-statistical effects found  Systematic Errors assigned for  remaining Background (from beam background, not removed invariant mass background) p t dependent from 5 x 10 -3 to 11 x 10 -3  yield extraction (normalization of background model) from 3 x 10 -3 to 7 x 10 -3  non-longitudinal spin components in beams 3 x 10 -3 (taken from jet analysis)  relative luminosities 2 x 10 -3

21. Frank Simon: STAR Longitudinal Overview 21 09/30/2006 Comparisons to Published Results  Comparison with published results:  STAR 2003/2004 Jets, p t divided by 2  PHENIX 2003/2004  0

22. Frank Simon: STAR Longitudinal Overview 22 09/30/2006 Charged Pions: Detection & Cross Section Charged hadron tracking & identification in the main TPC  Efficient reconstruction of charged tracks to 20 GeV; particle ID via dE/dx up to 10 GeV  For asymmetry analysis: High pt reach using e.m. JP trigger identification of charged pions with purity > 90 % cross section well described by NLO pQCD calculations

23. Frank Simon: STAR Longitudinal Overview 23 09/30/2006 Charged Pions: Asymmetry  Calculations by W. Vogelsang using KKP fragmentation functions  Charge-separated versions of KKP pion fragmentation functions obtained by multiplying favored partons by (1+z) and unfavored by (1-z).  Maximal positive gluon polarization disfavored

24. Frank Simon: STAR Longitudinal Overview 24 09/30/2006 Charged Pions: Trigger Bias Other Cross-Checks  Charge-summed asymmetry consistent with neutral pions  “Near-side” and “away-side” asymmetries consistent with each other  Majority of pions are sub-leading particles in e.m. triggered jet  Significant statistics from “away- side”, untriggered jet as well  PYTHIA afterburner used to construct “polarized” event generator  Calculate A LL in simulation with and without trigger requirement  Bias estimated using average of GRSV-min and GRSV-std scenarios  3.0 - 7.3 x 10 -3 as a function of p T and charge sign

25. Frank Simon: STAR Longitudinal Overview 25 09/30/2006 Reconstructing Jets  Midpoint Cone Algorithm (Tevatron II)  TPC pt for charged hadrons, EMC energy for e.m. showers  Cone radius of 0.4 in ,  seed energy 0.5 GeV  restricted to 0 <  < 1 for e.m energy  Jet axis for accepted jets restricted to 0.2 <  < 0.8 parton particle detector Trigger efficiency: Minbias JP2 HT2  E.M. Triggers are used to access jets at high p t  JetPatch (JP) trigger new in 2005!

26. Frank Simon: STAR Longitudinal Overview 26 09/30/2006 Jet Cuts  Software trigger requirement: Accepted Jets have to be able to fire the trigger  Neutral to total energy fraction < 0.8 to reject beam background that manifests itself in all-neutral jets (same cut as used in 2003/4) Jet neutral energy for PYTHIA (black) and data (red) Left: JP2 Right: HT2  1.97 M jet events post cuts  1.39 M in JP2 trigger => Workhorse trigger for the jet analysis  ~2% of all jet events contain multiple jets that satisfy the trigger requirements

27. Frank Simon: STAR Longitudinal Overview 27 09/30/2006 Inclusive Jet Cross Section First inclusive jet cross section result at RHIC (2004 data) Sampled luminosity: ~0.18 pb -1 Good agreement with NLO over 7 orders of magnitude (within systematic error) Leading systematic uncertainty: 10% E-scale uncertainty  50% uncertainty on yield

28. Frank Simon: STAR Longitudinal Overview 28 09/30/2006 Jet ALL: Published 2003/4 vs Preliminary 2005  2003/4 Result and new preliminary 2005 result consistent 2004 STAR Jet A LL 2005 STAR Jet A LL preliminary

29. Frank Simon: STAR Longitudinal Overview 29 09/30/2006 Inclusive Jet A LL : 2005 Result Error bars are statistical Systematic band does not include 25% scale error from polarization  2 / NDF to curves: (stat+syst error in quadrature) GRSV-STD:1.1  G = G:12  G = 0:0.7  G = -G:1.4 Rules out  G=G

30. Frank Simon: STAR Longitudinal Overview 30 09/30/2006 Jet Systematic Studies: False Asymmetries  False asymmetries (Single Spin asymmetries)  Should be zero with the present level of statistics (single spin asymmetries are caused by party violating weak interaction, significantly smaller than 10 -4 )  Non-zero single spin asymmetries (yellow beam) & like sign asymmetries observed cut dependent from 1 to 3   Probably caused by one anomalous spin state, source so far unclear  no significant single spin asymmetries observed in  0 and charged pion analyses A like- sign A L yellow x axis: jet neutral energy fraction   A LL  A l.s. /2  A l.s. = 7.9 ± 5.2 x10 -3   A LL = 0.0065 Systematic error assigned to preliminary result

31. Frank Simon: STAR Longitudinal Overview 31 09/30/2006 Jet Systematic Studies: Others  Trigger & Reconstruction Bias  Finite momentum resolution for jets leads to smearing => Reconstruction Bias  Jet trigger based on electromagnetic energy only => Trigger Bias  PYTHIA with afterburner for polarized events used to investigate bias  Relative Luminosities  Independent measurement with the zero degree calorimeters (statistics limited)  systematic from the difference between BBC and ZDC (stat. limited) ~ 0.002  Non-longitudinal beam polarization:  Transverse component determined via left-right and up-down BBC asymmetries  Effect on A LL constrained via the transverse asymmetry A  (consistent with zero, limited by statistics, A  < 0.093) assigned systematic error on A LL 2 x 10 -3 to 12 x 10 -3 (p t dependent)

32. Frank Simon: STAR Longitudinal Overview 32 09/30/2006 Neutral Pions in Jets  STAR is capable of full Jet reconstruction  reconstructed   are associated with Jets (HT triggered) if the  0 lies within the Jet cone (0.4 in ,  )   0 direction is strongly correlated with the Jet axis:  leading  0 typically within 5° of the Jet axis  defined as the mean ratio of  0 p t to Jet p t  Affected by trigger, depends on p t evolution of cross section and z dependence of the fragmentation function RMS in ,  ~ 0.055

33. Frank Simon: STAR Longitudinal Overview 33 09/30/2006 Looking ahead… Expectations for 2006 Data  Sampled luminosity in 2006 spin run significantly increased over 2005  Polarization typically ~60%  Sizeable increase in figure of merit!  Triggering available over the full acceptance of the STAR BEMC:  -1 <  < 1, 2  in   Shower-Maximum Detector operational over the full BEMC acceptance  Factor of 2 increase in acceptance for inclusive jet and mid-rapidity  0 measurements  Will enable di-jet and jet-hadron correlation measurements

34. Frank Simon: STAR Longitudinal Overview 34 09/30/2006 Looking ahead: Neutral Pions in the Endcap  significant reduction of beam background due to newly installed shielding in the accelerator tunnel

35. Frank Simon: STAR Longitudinal Overview 35 09/30/2006 Looking ahead: Neutral Pions at mid-rapidity

36. Frank Simon: STAR Longitudinal Overview 36 09/30/2006 Looking ahead: Charged Pions  Increased statistics will offer the possibility to reduce the trigger bias by studying charged pions on the away side of a e.m. triggered jet  will require dedicated NLO calculations

37. Frank Simon: STAR Longitudinal Overview 37 09/30/2006 Looking ahead: Inclusive Jets  triggers place greater emphasis on high p t jets and di-jets DG=G GRSV-std DG=-G DG=0

38. Frank Simon: STAR Longitudinal Overview 38 09/30/2006 Summary  With the longitudinal 2005 p+p data STAR measures spin asymmetries in a variety of channels  Proof-of-Principle for  polarization measurements  First result for inclusive  0 at mid- and forward rapidity  First result for charged pions at mid-rapidity  Inclusive Jets with significantly increased statistics and p t range  Inclusive cross sections for jet and  0 production consistent with NLO pQCD calculations over a wide range in p t  Neutral pion cross section favors KKP fragmentation functions over Kretzer set  Double longitudinal spin asymmetry in jets, neutral and charged pions is consistent and disfavors large positive gluon polarization  Jet result can significantly constrain the allowed  G values  significant contribution to global understanding of the gluon polarization  Significant increase in figure of merit with the already recorded 2006 data set