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Probing the Protons Spin Structure with Hard Scattering, Jets and the STAR Detector at RHIC: Recent Results W. W. Jacobs for STAR Collaboration IUCF and.

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Presentation on theme: "Probing the Protons Spin Structure with Hard Scattering, Jets and the STAR Detector at RHIC: Recent Results W. W. Jacobs for STAR Collaboration IUCF and."— Presentation transcript:

1 Probing the Protons Spin Structure with Hard Scattering, Jets and the STAR Detector at RHIC: Recent Results W. W. Jacobs for STAR Collaboration IUCF and Dept of Physics, Indiana Univ. STAR

2 n Introduction n STAR longitudinal spin - Recent results n The polarized proton collider RHIC n The STAR detector Topics: Slice of STAR Spin Physics Status & Overview n STAR Transverse spin di-jets - Recent results n STAR EMC Calorimetry! n Summary n Look at near future and outlook

3 Spin Problem & DIS Gluon Helicity Preference Constraints World data (2005) on g 1 p = ½ e i 2 [ q i (x,Q 2 ) + q i (x,Q 2 )] All fixed- target data Only ~30% of proton spin arises from q and q helicity preferences ! limited info on scaling violations, on shape or integral of gluon helicity preference g(x,Q 2 ). Only valence quarks are strongly polarized ssd duu

4 RHIC p+p: Pert QCD Probe of Spin-Dep Partonic Structure pp hX soft parton distribution functions soft frag. function hard d QCD parton-parton Theory ingredients: pQCD factorization LO pQCD + large parton-level 2-spin sensitivity Prefer dominant/clean reaction mechanism w/ largea LL ( - jet) But jet and p 0 rates are sufficient to give significant DG constrain in initial RHIC polarized p data

5 Inclusive cross-section (jets,, and ± ) Good agreement with NLO pQCD over many orders of magnitude PRL 97, (2006) PLB 637, 161 (2006) STAR Hadron inclusive production 1 st RHIC inclusive Jet x-sec pQCD works! also direct photon PHENIX; forward STAR & incl chg. BRAHMS jets 2005 STAR Preliminary ±

6 Relativistic Heavy Ion Collider 100 GeV beam proton beams Each bunch filled with a distinct polarization state Spin Rotators at STAR IR allow for transverse and longitudinal spin orientation Bunch Xings every ns CNI polarimeters + Hydrogen Jet target provide run by run & absolute polarization HJT calibr to ~ 5% goal in progress …worlds 1 st Collider pp Run Year FOM=P 4 L % L max [ s -1 cm -2 ] L int [pb -1 ] at STAR (L/T)0 / / / 03.1 / / 3.4,6.8

7 jet patch size:1x1 x ,…,18 Allocated Jet Rate to tape: ~15 Hz 1x1 Jet patch E T /GeV rate ~ 2.5 Hz, sent to tape without prescaling Level 0 High Tower & Jet Triggering in STAR 2006 rate ~ 150 Hz, combine with L2 trigger to fit in limited bandwidth Trigger either on HT: 1 (of 4800 BEMC or 720 EEMC) tower E T > thresh - Or JP: 1 (of 12 BEMC or 6 EEMC) hard-wired jet patch E T > thresh. [note: also a minbias cond] BARREL EMC ENDCAP EMC

8 Theoretically allowed range in Δ g: GRSV-STD GRSV-STD: Higher order QCD analysis of polarized DIS experiments! Predicted A LL sensitivity for different G scenarios Inclusive A LL measurements (, ±, and jets) ± jets

9 Neutral pions (in Jets); Inclusive 0 A LL /ndf compared to NLO calculations (ignoring systematic errors): GRSV Std: 0.8 GRSV Max: 2.5 GRSV Min: 0.8 GRSV Zero: 0.4 GRSV max scenario disfavored use invariant mass spect. (data from HT trig) –MC 0 line shape –low inv. mass bkgrd –comb. bkgrd & residual fit assoc w/ full Jets if within a Jet cone (0.4 in, ) but typ. ~ 5 deg data

10 geant jets pythia jets Jet Reconstruction in STAR parton particle detectorDataSimulation GEANT pythia theory Search over all possible seeds (p T seed > 0.5 GeV) for stable groupings Check midpoints between jet-jet pairs for stable groupings Split/merge jets based on E overlap Add all track/tower 4-momenta Use cone radius: = 0.4 for half-BEMC = for full B+EEMC 2006 Full jet reconstruction uses midpoint-cone algorithm (hep-ex/ ): Data well described by MC

11 Inclusive Jet A LL and x-sec Analysis Issues Use Simulation (MC) to provide correction to RAW jet yield - trigger and jet inefficiencies - jet resolution & bin migration - undetected particles (n + ) - PYTHIA CDF Tune A - GEANT (Geisha) Verification of DATA/MC agreement essential The shape of the Fraction of Neutral Energy in the Jet (EMF) is sensitive to the trigger bias as well as contributions from beam background. Minbias JP2 HT2 STAR Preliminary

12 On average PARTICLE Jets are reconstructed in the DETECTOR with 20% increase in pT due to ~25% jet resolution + steep jet pT distribution Systematic offsets in pT cause dilution of the jet asymmetry which depend on the size of the asymmetry! Effects of Jet Resolution Trigger Bias The trigger biases jets toward higher neutral energy. This may change the ratios of qq+qg+gg and therefore change the asymmetries pT(GeV) Trigger Bias: JP << HT A LL /A LL larger at low pT Total Systematic:

13 g = g (max) g = -g (min) g = 0 GRSV-STD 2005 STAR preliminary Systematic error band Measured Jet P T (GeV/c) Error bars are statistical Systematic band includes 25% scale error from current polarization uncertainties due to online values Results in good agreement with 03/04 A LL data * in region of overlap but ~ 4 times more precise and p T range nearly doubled [* PRL 97, (2006)] Systematic(x ) False Asymmetries <6.5 (p T dept) Reconstructi on + Trigger Bias 2-12 (p T dept) Non-long Polarization 3 Relative Luminosity 2 Backgrounds<1< Inclusive jet A LL at mid-rapidity

14 Vogelsang and Stratmann Significant new constraints on G when compared to predictions derived from one global fit to DIS data GRSV DIS best fit= = to 0.7 PRD 63, (2001) GRSV DIS Constraint on G

15 Inclusive Jet ( )Data from > Greater Discrm Power for g Inclusive Jet ( ) Data from > Greater Discrm Power for g High-statistics (esp. at high p T ) inclusive jet and 0 A LL data from 2006 will select among g models, assuming a shape of g(x,Q 2 ). Need global analysis including these A LL results! Significant increase in sampled luminosity Polarization typically ~60% acceptance in BEMC increased by a factor of 2 significant increase in figure of merit! DG=G GRSV-std DG=-G DG=0 Projected statistical uncertainties for STAR 2006 inclusive jet A LL jet also analysis w/ Endcap EMC + vs. - analysis in Barrel

16 Di-Jet Asymmetry Measurements with STAR; Sivers Transverse Spin Asymmetries Needs ISI and/or FSI to evade time-reversal violation Assuming QCD factorization, subsume ISI/FSI contributions in gauge-invariant kT -dependent parton distribution fcns. Characterize by Sivers effect: Sensitive to parton orbital angular momentum. p proton s proton k T parton ? Do quarks and/or gluons have transverse motion preferences in a proton polarized transverse to its momentum?

17 Do we observe q Sivers consistent w/ HERMES, after inclusion of proper pQCD-calculable ISI/FSI gauge link factors for pp jets? Tests limited TMD universality. First direct measurement of gluon Sivers effects. Expect Collins and Boer-Mulders asym contributions small: no charge-sign bias from EMC-only L2 trigger jet reconstruction & small role of q-q scattering at probed p T 2006 p + p run at RHIC; 3 wks transverse STAR Motivation for pp Di-Jet Measurement z x y Colliding beams proton spin parton k T x HERMES transverse spin SIDIS asymmetries u and d quark Sivers functions of opposite sign, different magnitude. Sivers effect in pp spin-dependent sideways boost to di-jets, suggested by Boer & Vogelsang (PRD 69, (2004)) Both beams polarized, x +z x z can distinguish high-x vs. low-x (primarily gluon) Sivers effects.

18 spin bisector Jet 1 Jet 2 Reco cos( bisector ) measures sign of net k T x for event =+2 = -1 TPC EMC Barrel EMC Endcap BBC East BBC West Yellow (-z) beam Blue (+z) STAR EMC-Based (Level 0 + 2) Di-Jet Trigger in 2006 Endcap essential for hi-x vs. lo-x Sivers distinction Signed azimuthal opening angle 2006 p+p run, 1.1 pb 1 2.6M di-jet triggered events 2 localized clusters = , with E T EMC > 3.5 GeV, | | > 60 ; E T wted centroid Broad 1,2 coverage Full, symmetric 1,2 coverage

19 Fast MC Simulations Illustrate Di-Jet Sivers Effects 2-parton events, transverse plane match full jet reco. p T distribution Gaussian + expl tail k T distribution fits distribution random k T x,y (rms = 1.27 GeV/c) for each parton Sivers spin-dep. k T x offset shift, L-R di-jet bisector asymetry 1-spin effects vary linearly with k T x offset f = 0.85 dilution corrected in data

20 Error-weighted average of 16 independent A N ( > ) values for |cos( bisector )| slices effective beam poln for each slice = P beam |cos( bisector )| rotation samples k T y, parity-violating s p k T correlation STAR data - both jets rotated by 90 Null Tests STAR Results Integrated Over Pseudorapidity Sivers asymmetries consistent with zero with stat. unc. = Fast MC sensitivity to Sivers k T x offset few MeV/c (k T x ) 2 1/2 Systematic uncertainties smaller than statistics All null tests, including forbidden 2-spin asym. cos( bisector ), consistent with zero, as are physics asymmetries for all polarization fill patterns Note: P_beam from online CNI analysis, with 20% calibr. uncertainty A N +z A N -z 2-spinA N +z A N -z 2-spin STAR data

21 What Did We Expect? Constraints from SIDIS Results Fits to HERMES SIDIS Sivers asym constrain u and d quark Sivers functions, for use in pp dijet + X predictions. Bomhof, Mulders, Vogelsang & Yuan, hep-ph/ < pTparton < 10 GeV/c Initial State Interns only (à la Drell-Yan) Trento sign conv. (opposite Madison) ISI only FSI only ISI+FSI VY 2 SIDIS Sivers fit Theory of Transverse SSA Developing Very Rapidly! Bacchetta et al. [PRD 72, (2005)] deduce gauge link struct for pp jets, hadrons: A N (ISI+FSI) 0.5 A N (ISI) Gauge links more robust for SSA wted by p T or |sin |, due to k T - factorization breakdown (Collins & Qiu, arXiv: ) models (2) of Sivers fcn. x- dep W. Vogelsang and F. Yuan, PRD 72, (05) Jet 1 rapidityJet p T (GeV/c) no hadronization no gluon Sivers functions

22 STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum I Trans Spin Asym Emphasizes (50%+ ) quark Sivers Spin Pol quark Unpol qluon +z y x Blue beam Yellow beam Extract analyzing powers averaged over and b w/ fit to asyms in |cos b | using cross ratio: Typical x T ~ ; range 0.01 < x Bj < 0.4

23 STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum STAR A N all consistent with zero both net high-x parton and low-x gluon Sivers effects ~10x smaller in pp di-jets than SIDIS quark Sivers asym.! Blue beam Yellow beam All calcs. for STAR acceptance Reverse calc. A N signs for Madison convention Scale Bomhof calcs by 1/ |sin | 3.0 to get A N of unit max. magnitude u vs d and FSI vs ISI cancellations sizable SSA in inclusive fwd. h prodn and SIDIS (weighted SSA) compatible with small weighted di- jet SSA -- test via LCP flavor select

24 Near Term future, and RHIC run 8 & 9 Inclusive channels for longitudinal spin prog suffer from broad integration over x model-dep. G extraction as well as other systematic issues With improved beam & detector performance, focus will now shift to jet-jet and -jet coincidences for event-by-event constraints on colliding parton x 1, p T (GeV) x gluon GeV Inclusive 0 N.B. x-range sampled depends on g(x,Q 2 ) ! -- M. Stratmann Sivers -> Ongoing analysis incorporates TPC tracks for full jet reconstruction allowance for cuts on jet pT, u vs. d filtering via leading hadron charge sign, etc. (w/o cuts consistent w/ EMC only) Presently in detailed planning stages for RHIC run 8 (accelerator cool- down ~1 Nov. 2007) … as per the Beam Use Request (BUR) there will be a significant amount of polarized pp running (divided among longitudinal and transverse spin orientations) … as we also work on run 6 analysis! Similarly we expect a signifcant amount of polarized pp beam in run 9

25 : Coincidence Measurements to Map g(x) Fully For example, simulations (L. Bland) of STAR capabilities for - jet coincidences give rough indication of g discriminating power for various models of input gluon polarization. Simplified LO analysis used for simulations here to illustrate sensitivity - jet and di-jet s = 200 & 500 GeV, will map g(x ,high Q 2 ), when included in NLO treatments of entire spin structure database.

26 Summary NLO pQCD describes hadron x-sections at RHIC for inclusive jets, and ± ) allowing spin program to access G directly Longitudinal spin: 2005 inclusive jet data provide significant new constraint on G when compared to predictions derived from one global fit to DIS data ( GRSV-max scenario ruled out w/ 03/04 data); 2006 data should provide sizable addl constraint (as will global fits!) STAR longitudinal spin program entering phase of correlation and direct measurements, while continuing to expand the p T reach of the incl. channels; , focus on A LL for di-jet and +jet production Transverse spin: spin asyms for pp di-jet production Sivers asyms consistent w/ zero, whether dominated by valence or sea partons … data will constrain unified theoretical accounts of SSA in hard trans spin pQCD, and connection to parton orbital momentum. Present pQCD calcs. reconcile small observed asyms with larger effects seen in SIDIS (& pp forward hadron), via cancelling ISI vs. FSI and u vs. d contr. p+p in pQCD regime viable complement to DIS more data coming!


28 2005 Inclusive Jet A LL GRSV curves* G = G G = -G G = 0 G = STD Phys.Rev.Lett (2006) 2005 A LL is consistent with previous 2003/2004 results.

29 PYTHIA+GEANT full jet reconstruction vs. parton-level resolution: EMC-Only Information OK For 1 st Dijet Sivers Asymmetry Jet finder TPC+EMC jet cone radius 0.6 Full offline di-jet reconstruction for ~2% of all runs shows triggered jet p T spectrum: Typical x T ~ ; range 0.01 < x Bj < 0.4 and angle resolution L2 OK: [ ( )=5.0, ( )=0.10] full reco. jet vs. parton angles Net L2-to-parton ( jet ) = 6.3, ( di-jet ) = 9.0 (full reco) – (L2) [deg] [ ( )=3.9, ( )=5.8 ] L2 vs. full jet << observed ( ) 20, mostly from k T

30 Distinguishing Sivers from Collins Asymmetries In SIDIS, can distinguish transverse motion preferences in PDFs (Sivers) vs. in fragmentation fcns. (Collins) via asym. dependence on 2 azimuthal angles: HERMES results both non-zero, but + vs. – difference suggests Sivers functions opposite for u and d quarks. Collins Sivers

31 Theory of Transverse SSA Developing Very Rapidly! Bacchetta, Bomhof, Mulders & Pijlman [PRD 72, (2005)] deduce gauge link structure for pp jets, hadrons: A N (ISI+FSI) 0.5 A N (ISI) Gauge links more robust for SSA weighted by p T or |sin |, due to k T - factorization breakdown (Collins & Qiu, arXiv: ) ISI only FSI only ISI+FSI Bomhof, Mulders, Vogelsang & Yuan, hep-ph/ VY 2 SIDIS Sivers fit Bomhof, Mulders, Vogelsang & Yuan, hep-ph/ u quark d quark u+d Sivers fcns. from twist-3 qg correln fits to pp forward hadron Ji, Qiu, Vogelsang & Yuan [PRL 97, (2006)] show strong overlap between Sivers effects & twist-3 quark- gluon (Qiu-Sterman) correlations: twist-3 fits to A N (p+p fwd. h) can constrain Sivers fcn. moment relevant to weighted di-jet SSA Kouvaris et al. [PRD 74, (2006)] fits give nearly complete u vs. d cancellation in weighted di-jet SSA

32 Future: W 500 GeV u/ u and d/d to Illuminate Origin of the Nucleons qq Sea ¯¯¯¯¯ 2 asyms. 2 charges poln of valence q, sea q separately for u,d. Detect W ± via isolated high-p T daughter e ± or ±, no away-side jet ¯ Projected uncertainties for quark and antiquark polarizations Many non-perturbative models of nucleon structure predict sign & magni- tude difference between u and d polar- izations in nucleon sea, not yet seen. Probe via single-spin parity-violating asym. A L for p + p W + X with respect to helicity flip of each beam.

33 Detector at RHIC STAR = -ln[tan( /2)] TPC | |<1.4 Charged particle momentum BEMC | |<1.0 Neutral Energy High pT Trigger EEMC 1< <2 Neutral Energy High pT Trigger BBC 3.4< <5 MinBias Trigger Relative Lumi (also ZDC) EndCap EMC BBC East Barrel EMC BBC West TPC Yellow beamBlue beam

34 STAR FMS, EEMC and BEMC provides nearly complete EM coverage from -1 η +4 STAR Calorimeter Coverage

35 proton spin proton momentum proton spin proton momentum + Blue beam asymmetry Yellow beam asymmetry 2006 p+p run STAR measurement of Sivers transverse single-spin asymmetry for di-jets -- shows smaller effects than predicted for observable sensitive to orbital components of parton motion in proton.

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