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Transverse Spin at STAR

Transverse Spin at STAR. Andrew Gordon RHIC & AGS Annual Users Meeting Workshop 5 May 27, 2008. Outline of report on transverse data at STAR. Part I: Large A N can be gainfully employed to track beam polarization. Part II: Recent measurements at STAR.

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Transverse Spin at STAR

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  1. Transverse Spin at STAR Andrew Gordon RHIC & AGS Annual Users Meeting Workshop 5 May 27, 2008

  2. Outline of report on transverse data at STAR Part I: Large AN can be gainfully employed to track beam polarization Part II: Recent measurements at STAR Part III: Status of Run 8 data and measurements

  3. Part I: Sizeable asymmetries can be used to measure beam polarization STAR BBC Inner tiles cover ~3.5<h<5 Can use large asymmetries to measure (relative) bunch-by-bunch polarization See also J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1, 28 Jan 2005

  4. Inner tiles of BBC accumulated every clock cycle { Scaler Boards } Discriminated phototube outputs 24-bit word is histogrammed every clock cycle { East-West Coincidence } Bunch Crossing (7-bits)

  5. Bunch-by-bunch polarization from colliding beams Width=1.220.15 Yellow (east BBC) Expected spin up Expected spin down sig Width=1.050.12 Blue (west BBC) sig ~3.5 s (statistical) measurement of polarization per bunch per hour (|Pbx|-<|Pbx|>) Sigbx= sbx Statistical uncertainties only

  6. <z> <xq> <xg> Part II: Recent asymmetry measurements at STAR Polarization here: valence quark spin effects p0 p p xgp xqp Polarization here: low x-gluons and other partons High rapidity p’s (hp~4) from asymmetric partonic collisions p+p p0, hp=3.8, √s=200GeV Mostly high-x valence quark on low-x gluon • (0.3 < xq< 0.7, 0.001< xg < 0.1) Fragmentation z nearly constant and high 0.7 ~ 0.8 NLO pQCD Jaeger, Stratmann, Vogelsang, Kretzer

  7. Two examples of sources of transverse spin effects Sivers mechanism: correlation between proton spin and quark KT (implies orbital angular momentum). Collins mechanism: correlation between quark polarization (transversity) spin and asymmetry in jet fragmentation Initial state transversely polarized quark Final state transversely polarized quark Asymmetry in fragmentation Depends on transversity distribution. Potential asymmetry in fragmentation provides ability to see transversity, (and vice versa). One important goal: To separate these experimentally.

  8. Cross sections at s=200 GeV are consistent with pQCD …but well described at 200 GeV Cross sections significantly under-predicted at lower √s… STAR √s=52.8GeV √s=200 GeV √s=23.3GeV ~ 2 NLO collinear calculations with different scale: pT and pT/2 ISR data (STAR), PRL 92 (2004) 171801 Bourrely and Soffer [hep-ph/0311110] (Eur. Phys. J C36 (2004) 371) data references therein Suggests that asymmetry data can be described within the context of pQCD.

  9. Large asymmetries persist at high s Examples: s = 20 GeV s = 62 GeV s = 200 GeV p + p  p+ X, s = 20 GeV p + p  p0 + X, s = 200 GeV pT=0.5-2.0 GeV/c p + p  p± + X, s = 62 GeV Arsene et al. (BRAHMS), submitted to Phys. Rev. Lett. [arXiv:nucl-ex/0801.1078] RHIC, Brahms, 2007 (STAR) Phys. Rev. Lett. 92 (2004) 171801 0: E704, Phys.Lett. B261 (1991) 201. +/-: E704, Phys.Lett. B264 (1991) 462. RHIC, STAR, 2004 Fermilab, Fixed target, E704, 1991 Perturbative cross section Non-Perturbative cross section

  10. Runs 3, 5, and 6 forward p0 asymmetry PT vs XF coverage of data Steeply falling with PT and XF. FPD set at different distances from beam for the different <h> ranges. arXiv:0801.2990v1, submitted to PRL

  11. Run 3, 5, and 6 asymmetry data (cont): Theory can predict XF dependence based on Sivers function fits to p+/p- asymmetries… Data: B.I. Abelev et al. (STAR), submitted to PRL [arXiv:hep-ex/0801.2990v1], submitted to PRL Theory (red): M. Boglione, U. D’Alesio, F. Murgia [arXiv:hep-ph/0712.4240] Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006) 114013

  12. …but rising PT dependence is not predicted by the same fits Admixture of Collins and Sivers? XF>0.4 Current data can extend PT reach of measurements B.I. Abelev et al. (STAR) [arXiv:hep-ex/0801.2990v1], ), submitted to PRL Data broken out in XF bins

  13. STAR Results vs. Di-Jet Pseudorapidity Sum Run-6 Result VY 1, VY 2 are calculations by Vogelsang & Yuan, PRD 72 (2005) 054028 AN pbeam  (kT  ST) jet Emphasizes (50%+ ) quark Sivers Boer & Vogelsang, PRD 69 (2004) 094025 pbeam into page jet Idea: directly measure kT by observing momentum imbalance of a pair of jets produced in p+p collision and attempt to measure if kT is correlated with incoming proton spin • AN consistent with zero • ~order of magnitude smaller in pp  di-jets than in semi-inclusive DIS quark Sivers asymmetry! PRL 99 (2007) 142003 STAR

  14. Part III: Run 8 data Because of lower polarization than expected, figure of Merit (P2L)fell short by roughly a factor of two. Run 8 Integrated FMS transverse figure-of-merit. Only 43% of goal, after calibration from jet Fast detectors only. However, the FMS provides roughly 20 times the coverage of previous runs in the forward region.

  15. New forward detector for Run 8: FMS North-half, view from the hall Nearly contiguous coverage for 2.5<h<4.0. Run 8 FMS FMS provides nearly 20x the coverage of previous forward detectors Run 5 FPD

  16. FMS Physical Location Far West side of Hall, at the opening to RHIC tunnel. 7.5 meters from interaction point. FMS

  17. Run8 expectations for forward p0 asymmetry Projections for 9 pb-1 recorded in FMS with 70% polarization. With actual figure of merit, error bars will increase by roughly factor of 2. Run 8 data should be able to extend kinematic reach for inclusive p0 and heavy mesons.

  18. Run 8 data current status 1) p0 reconstructions are readily available RHIC News, April 22, 2008 First reconstructions of events from the STAR FMS. Clustering and shower shape analysis codes used for the forward pion detector were adapted to the FMS. Although final calibrations and efficiency corrections are still required, clear evidence for p0 events is seen from the diphoton invariant mass distribution for both the large cell outer calorimeter and the small cell inner calorimeter.

  19. 2) Calibrations ongoing Adjust gain for each detector to set most probable Mgg = Mp Run 6 resolution of d(Mp)/Mp~10% should be possible. Need multiple passes through the data since pion and photon energy get spread over several towers. Total of 1264 towers 3) Minimal run-by-run dependence in mass peak observed Calorimeter stable at order of 1% 4) LED pulser provides continuous gain monitoring Entire Run 8 data set should become quickly available with final calibration. Run 6 resolution of d(Mp)/Mp~10% should be possible. Calorimeter stable at order of ~1%.

  20. More run 8 opportunities with FMS Near term: Increase reach of inclusive p0 asymmetry (above) Spin-dependent jet-like events and spin-dependent p0p0 correlations Far term: Measurements of spin-dependent inclusivedirectgandg+jet Benchmarking of abilities to measure spin-dependent Drell-Yan

  21. Spin-dependent p0p0 correlations Look for 4-photon events in FMS consistent with two p0 decays Two azimuthal angles Angle of p0p0 system relative to spin direction Angle of leading p0 relative to p0p0 system Can provide information about Collins/Sivers separation

  22. Strategy to find p0p0 events in FMS Look for all events with >4 photons candidates in FMS. For all photon pairs, calculate a vertex location such that the two photons combine to the pion mass. Candidate p0p0 events are events where two photon pairs have consistent vertices. Pythia+fast FMS simulation Caveat 1: FMS at ideal resolution in simulation DZ>175 cm All found pairs Combinatoric background Caveat 2: FMS at ideal hadronic response in simulation (ie, hadrons ignored) M(p0p0) (GeV) KS decays to p0p0 Caveat 3: Photon reconstruction efficiency at ideal level in simulation (100%). No cluster merging, etc. DZ=ZFIT-ZBBC (cm) KSp0p0 show up at high displaced vertex ZFIT=error-weighted average of two pion condidates

  23. Can extend to g(forward)+jet Theory (A. Bacchetta et al. PRL 99 (2007) 212002) predicts a sign flip in left/right asymmetry of vector sum of photon+jet relative to SIDIS. Measurement thus provides ability to test QCD at the fundamental level of its color structure. Current estimate: LINT=30 pb-1 at P=65% with tracking detectors to test this prediction. Future measurement of spin-dependent direct g Transversely polarized proton q photon Quark jet g Unpolarized proton Lack of photon fragmentation provides clear access to Sivers function Theory (Kouvaris, Qiu, Vogelsang, Yuan, PRD 74 (2006) 114013) predicts a sign flip in left/right asymmetry for inclusive direct photon. (Background from fragmenting quarks predicted to have opposite sign.)

  24. Large FMS acceptance allows rejection of p0 background Search for photons in yellow band and reject events with nearby photon to reduce background from p0 and h decays. Large acceptance of FMS makes this possible. For Eg>25, 95% of second photon from p0 decays occur within radius of ~4 large cells (~23 cm). Example p0 rejection region. Large acceptance also allows calorimeter isolation criterion to help reduce background from photons that result from fragmentation.

  25. Pythia + fast-FMS simulation Restricting the measurement of the forward photon to E>35 GeV at <h>=3.2 produces a signal:background ratio of 2.1.

  26. Benchmarking Drell-Yan As a test of our ability to do future spin-dependent Drell-Yan measurements, we can look for lepton pairs in the Run 8 data. Mass distribution of isolated clusters using subset of Run 8 data and initial FMS calibrations. Widths and signal to background ratios will be strongly dependent on ongoing calibration efforts. p0 mass region Can be used to calibrate simulation of ability to identify e+e- pairs with FMS, an important first step towards a Drell-Yan measurement.

  27. Conclusions Forward pion asymmetries from previous runs have been completed and submitted for publication. Run 8 FMS calibrations underway. All of run 8 should be available soon thereafter. 20x increase in detector size with FMS should allow significant increase in physics reach.

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