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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.
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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
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
Introduction: Inclusive Measurements • Longitudinal double spin asymmetry ALL for inclusive processes depends on the gluon polarization • Asymmetry also depends on particle type
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 .
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
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 towerx = 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
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)
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).
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
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 (0.6420.013). • 2% from transverse beam polarization components at STAR. • + overall scale uncertainty from RHIC beam polarization measurement. Cross check with K0s: LL =0.010.01 • K0s are spin-0 mesons -> null measurement • reconstruction/analysis similar to (anti)Lambda • Statistical error is ~1/5 of (anti)Lambda’s DLL
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
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 ~
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
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
EEMC: Neutral Pion Reconstruction • 0 + MC • cluster splitting • beam background • combinatoric background
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
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
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
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
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
Comparisons to Published Results • Comparison with published results: • STAR 2003/2004 Jets, pt divided by 2 • PHENIX 2003/2004 0
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 %
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
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
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
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
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
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
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
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
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)
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
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
Looking ahead: Neutral Pions in the Endcap • significant reduction of beam background due to newly installed shielding in the accelerator tunnel
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
DG=G GRSV-std DG=-G DG=0 Looking ahead: Inclusive Jets • triggers place greater emphasis on high pt jets and di-jets
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