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Л.Ногач, ИФВЭ, Протвино the STAR Collaboration

STAR. Односпиновая асимметрия в образовании π 0 -мезонов в области фрагментации поляризованного протонного пучка на установке СТАР в Брукхейвене. Л.Ногач, ИФВЭ, Протвино the STAR Collaboration. Motivation.

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Л.Ногач, ИФВЭ, Протвино the STAR Collaboration

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  1. STAR Односпиновая асимметрия в образовании π0-мезонов в области фрагментации поляризованного протонного пучка на установке СТАР в Брукхейвене Л.Ногач, ИФВЭ, Протвиноthe STAR Collaboration

  2. Motivation • Contrary to simple pQCD predictions, first measurements of  production in p↑p collisions found large transverse single-spin asymmetries. Similar large effects have recently been found in e+e- and semi-inclusive deep-inelastic scattering. • Significant developments in theory in the past few years suggest common origins for these effects, but large transverse spin asymmetries inp↑+p →  + Xproduction are not yet fully understood.

  3. Possible mechanisms • Sivers effect [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]: Flavor dependent correlation between the proton spin (Sp), proton momentum (Pp) and transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to: • Collins effect [Nucl. Phys. B396, 161 (1993)]: Correlation between the quark spin (sq), quark momentum (pq) and transverse momentum (kT) of the pion. The fragmentation function of transversely polarized quark q takes the form: According to the latest theoretical developments, both mechanisms contribute to  AN

  4. RHIC Polarized Collider RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS & PP2PP PHOBOS Siberian Snakes Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) Spin Rotators (longitudinal polarization) Pol. H- Source LINAC BOOSTER Helical Partial Siberian Snake AGS 200 MeV Polarimeter AGS pC Polarimeter Strong AGS Snake 1 MHz collision rate Polarization ~ 0.5-0.6

  5. STAR detector layout Forward Meson Spectrometer • TPC: -1.0 <  < 1.0 • FTPC: 2.8 <  < 3.8 • BBC : 2.2 <  < 5.0 • EEMC: 1 <  < 2 • BEMC: -1 <  < 1 FPD/FPD++:  ~ -3.7/3.3 FPD/FMS:  ~ -4.1/2.5-4.0 1/2 of the FMS

  6. Transverse spin runs at STAR with forward calorimetry: 2001→2008 sampled

  7. Definition: dσ↑(↓) – differential cross section of p0 when incoming proton has spin up(down) Two methods of measurements: Single arm calorimeter: R – relative luminosity (by BBC) Pbeam – beam polarization Two arm (left-right) calorimeter: No relative luminosity needed Left π0, xF<0 π0, xF>0 p  p Right Single Spin Asymmetry positive AN: more p0 going left to polarized beam

  8. Detectors – from FPD to FMS • Run8 and beyond: FMS • FMS provides full azimuthal coverage for range 2.5  h  4.0 • broad acceptance in xF-pT plane for inclusive g,p0,w,K0,… production in p+p and d(p)+Au • broad acceptance for g-p0 and p0-p0 from forward jet pairs • Runs 3-6: FPD,FPD++ • Inclusive p0 cross sections • AN for inclusive p0 production three different positions of modular detectors 20x more acceptance then previous detectors single monolithic detector

  9. Published measurements with FPD PRL 92, 171801 (2004) PRL 101, 222001 (2008) • Large transverse single-spin asymmetries at large xF • xF dependence matches Sivers effect expectations qualitatively • pT dependence at fixed xF is not consistent with 1/pT expectation of theoretical model calculations √s=200 GeV, <η> = 3.8 Asymmetry revealed at lower energies persists at √s=200 GeV

  10. Run8 data analysis -calibration • the calibration methodologies employed for the FPD have been successfully adapted to the FMS • offline cell-by-cell calibration using π0 peak position in the sorted by high tower mass distributions Event selection done with: minbias condition hightower ADC threshold (400/200 cts. for small/large cells) (for small cells only) fiducial volume cut (0.5 cell)

  11. LED monitoring system Minimal time dependence in π0peak position is observed LED system: critical calibration tool Calorimeter stable at the level of a few percents

  12. Data to simulations comparison • Full PYTHIA/GEANT simulationshave adequate statistics to reach moderate xF • Mass resolution in data is reasonable, given Run6 FPD performance • Simulations have somewhat better resolution than data DATA SIMULATION Present understanding sufficient; further investigations to be done

  13. First look atπ0AN from Run8 data STAR preliminary 75% of Run8 data stat.errors only Octant subdivision of FMS for inclusive p0 spin sorting • AN comparable to prior measurements • Azimuthal variation appears to be as expected

  14. Summary • High precision AN measurements with the FPD allows for a quantitative comparison with theoretical models. • FMS was commissioned and fully operated in Run8. It has 20x the acceptance of FPD (1264 channels compared to 98 for the pair of FPD modules). • Reconstruction and calibration procedures successfully ported from FPD to FMS. • Calibration iscompleted and data show good agreement with the simulated sample of events. • Inclusive p0 AN(xF) from the FMS is in agreementwiththe FPD measurements.

  15. Outlook • Complete analysis of π0asymmetry • Going beyond π0 detection: • - AN for a state that contains p0 pairs • - AN for states with heavier mesons • (η, ω, J/ψ…) • -direct photons, “jet-like” events – can help • to discriminate between Sivers and • Collins effects

  16. BACKUP

  17. π0 cross section measurements PRL 97, 152302 (2006) Cross-section is consistent with NLO pQCD calculations; it was used in a recent global analysis of e+e-, SIDIS and pp data to determine fragmentation functions [PR D76, 074033 (2007)]

  18. Separating Sivers and Collins effects Sivers mechanism:asymmetry in the forward jet or γ production Collins mechanism: asymmetry in the forward jet fragmentation SP SP kT,q p p p p Sq kT,π Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity To discriminate between the two effects we need to go beyond π0 detection todirect photons orjet-like events

  19. Energy dependent corrections • π0peak position depends on the energy • linear correction extracted from π0 peak position and being applied to photon energies works for both π0s and ηs, and significantly decreases shift from zero in δEγγ = Esimu - Ereco.

  20. Association analysis • comparison ofgenerated quantities to GEANT simulations • we consider CORRECTED UNCORRECTED Eliminating energy dependence in p0 mass peak gives the correct average neutral pion energy

  21. Simulation and search algorithm for π0π0 in FMS and its engineering prototype • it is possible to find events where the π0 pair originated at a significant distance from the vertex • one source of such events are decays KS→π0π0 (31% branching fraction) • plot shows the mass distribution for displaced vertices above 100 cm from the BBC vertex; a pronounced KS mass bump is visible

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