1 / 38

Recent Results from the STAR Longitudinal Spin Program

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

questa
Download Presentation

Recent Results from the STAR Longitudinal Spin Program

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. f f1 f2 long-range short-range long-range f1 f2 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

  3. Introduction: Inclusive Measurements • Longitudinal double spin asymmetry ALL for inclusive processes depends on the gluon polarization • Asymmetry also depends on particle type

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

  5. 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. 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. 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 • <PYPB> ~ 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. Lambda Reconstruction  • 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 • Invariant mass & kinematics <pT>~1.3 GeV <|xF|>~0.0075 M=1.1157 GeV(PDG) • 2005 data: ~3X106 minimum bias events, • ~19X103 (14X103) analyzed (after all cuts).

  9. Lambda Asymmetry DLL  : decay parameter 0.642 0.013  : angle between the momentum of decay proton in ’s rest frame and ’s momentum at the lab frame • Subtracting bg. contribution to DLL r: fraction of background under the peak

  10. Lambda Asymmetry DLL • proof of principle measurement • high pt data needed to achieve physics result Systematics • 4X10-3from 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. Cross check with K0s: LL =0.010.01 • K0s are spin-0 mesons -> null  measurement • reconstruction/analysis similar to (anti)Lambda • Statistical error is ~1/5 of (anti)Lambda’s DLL

  11. Lambda Asymmetry: Outlook • Triggering is needed to reach high pt 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. 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. 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. towers pre-1 pre-2 7.0 < pT < 8.0 GeV post sector ~ f sector ~ f 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

  15. EEMC: Neutral Pion Reconstruction • 0 +  MC • cluster splitting • beam background • combinatoric background

  16. EEMC 0: Double Longitudinal Asymmetry ALL • 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. 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 HT2 HT1 different cuts used for MB and HT triggers

  18. consistent with previous results from PHENIX BEMC 0: Inclusive Cross Section Invariant cross section: • Lsampled: 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

  19. BEMC 0: Double Longitudinal Asymmetry ALL • 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 constant fit (assumes no pt dependence): • ALL = -0.017 +- 0.021

  20. BEMC 0: ALL 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) pt 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. Comparisons to Published Results • Comparison with published results: • STAR 2003/2004 Jets, pt divided by 2 • PHENIX 2003/2004 0

  22. 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 cross section well described by NLO pQCD calculations identification of charged pions with purity > 90 %

  23. 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. Charged Pions: Trigger Bias • 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 ALL 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 pT and charge sign Other Cross-Checks • Charge-summed asymmetry consistent with neutral pions • “Near-side” and “away-side” asymmetries consistent with each other

  25. 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 detector particle • E.M. Triggers are used to access jets at high pt • JetPatch (JP) trigger new in 2005! parton • Trigger efficiency: • Minbias • JP2 • HT2

  26. 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 energyfor PYTHIA (black) and data(red)Left: JP2Right: 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. 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. 2004 STAR Jet ALL 2005 STAR Jet ALL 2004 STAR Jet ALL preliminary Jet ALL: Published 2003/4 vs Preliminary 2005 • 2003/4 Result and new preliminary 2005 result consistent

  29. Inclusive Jet ALL: 2005 Result 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 Error bars are statistical Systematic band does not include 25% scale error from polarization

  30. Alike-sign AL yellow x axis: jet neutral energy fraction 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 observedcut 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 Systematic error assigned to preliminary result • ALL Al.s./2 • Al.s. = 7.9 ± 5.2 x10-3 ALL = 0.0065

  31. 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 ALL constrained via the transverse asymmetry A (consistent with zero, limited by statistics, A < 0.093) assigned systematic error on ALL 2 x 10-3 to 12 x 10-3 (pt dependent)

  32. 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 RMS in ,  ~ 0.055 • <z> defined as the mean ratio of 0 pt to Jet pt • Affected by trigger, depends on pt evolution of cross section and z dependence of the fragmentation function

  33. 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. Looking ahead: Neutral Pions in the Endcap • significant reduction of beam background due to newly installed shielding in the accelerator tunnel

  35. Looking ahead: Neutral Pions at mid-rapidity

  36. 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. DG=G GRSV-std DG=-G DG=0 Looking ahead: Inclusive Jets • triggers place greater emphasis on high pt jets and di-jets

  38. 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 pt range • Inclusive cross sections for jet and 0 production consistent with NLO pQCD calculations over a wide range in pt • 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

More Related