1 / 26

Spin-dependent forward particle correlations in p+p collisions at = 200GeV

STAR. Forward Spin Physics at STAR RHIC, BNL. Spin-dependent forward particle correlations in p+p collisions at = 200GeV. Nikola Poljak University of Zagreb (For the STAR Collaboration). Definition: d σ ↑ ( ↓ ) – differential cross section of p 0 when incoming proton has spin up(down) .

sona
Download Presentation

Spin-dependent forward particle correlations in p+p collisions at = 200GeV

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. STAR Forward Spin Physics at STARRHIC, BNL Spin-dependent forward particle correlations in p+p collisions at= 200GeV Nikola PoljakUniversity of Zagreb(For the STAR Collaboration)

  2. Definition: dσ↑(↓) – differential cross section of p0 when incoming proton has spin up(down) Left p0, xF<0 p0, xF>0 p  p Right Single Spin Asymmetry positive AN: more p0 going left to polarized beam

  3. PRL 97, 152302 (2006) nucl-ex/0602011 At this energy the cross-section is consistent with NLO pQCD (run2 + run3) Published measurements at STAR STAR: arXiv:hep-ex/0801.2990 accepted for publication in PRL RUN 6 • Large transverse single-spin asymmetries at large xF • xF dependence matches Sivers effect expectations qualitatively • pT dependence at fixed xF not consistent with 1/pT expectation of pQCD-based calculations

  4. Developments in theory and experiment • new phenomenological analyses within a generalized parton model can explain both Sivers moments in semi-inclusive deep inelastic scattering and many features of p↑+p → p +X. (PRD 77 051502(R)) • ~20% of the COMPASS transversely polarized proton data has been analyzed and reported.  COMPASS finds non-zero Collins moments and Sivers moments compatible with zero, although expected Sivers moments are small in the x,Q2 range of their experiment. (Levorato, for COMPASS; Ferrara, 2008). • Expectationsthat the Collins effect is suppressed in p↑+p → p +X (PRD 71 014002) were found incorrect due to a sign error (arXiv:0804.3047) The need remains to separate Collins and Sivers effects in p↑+p → p +X

  5. 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 tojet-like events

  6. Detectors – from FPD to FMS • Run8 and beyond: FMS • FMS will provide 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 uses 3 different settings of modular detectors 20x more acceptance than previous detectors uses a single monolithic detector

  7. Students prepare cells at test Lab at BNL Forward Meson Spectrometer (FMS) New FMS Calorimeter Lead Glass From FNAL E831 804 cells of 5.8cm5.8cm60cm Schott F2 lead glass Small Cell PSU Type 224 of 476 QT board Cockcroft-Walton HV bases with computer control through USB. Designed/built in house for FEU-84. • Readout of 1264 channels of FMS provided by QT boards. Each board has • 32 analog inputs • 5-bit TDC / channel • Five FPGA for data and trigger • Operates at 9.38 MHz and higher harmonics • Produces 32 bits for each RHIC crossing for trigger • 12-bit ADC / channel Designed and built at Penn State University Designed and built at UC Berkeley/SSL

  8. Calibration the calibration methodologies employed for the FPD have been successfully adapted to the FMS Event selection done with: Offline calibration donecell-by-cell included energy corrections minbias condition Hightower ADC threshold (400/200 cts. for small/large cells) <0.7 (small) ; <1.0 (large cells) fiducial volume cut (0.5 cell)

  9. Details of data analysis - calibration Minimal run-by-run dependence in mass peak observed LED system : critical calibration tool Calorimeter stable at level of ~1%. MIT (LED optics) UC Berkeley/SSL (flasher boards) Texas / Protovino / BNL (assembly) SULI program (Stony Brook students) / BNL (control electronics)

  10. Association analysis – energy corrections • comparison ofgenerated quantities to reconstructed GEANT simulations • We consider UNCORRECTED CORRECTED Eliminating energy dependence in p0 mass peak gives the correct average neutral pion energy

  11. Distributions comparison • Full PYTHIA/GEANT simulationshave adequate statistics to reach moderate xF at large pT • Cell mass resolution in data is reasonable, given run-6 FPD performance • Simulations have somewhat better resolution than data DATA SIMULATION Present understanding sufficient; further investigations to be done

  12. Summary & goals • FMS- a new device, withmany more channels (1264 detectors compared to 98 for north/south FPD modules). • FMS has 20x more acceptance than the previous modular detectors • the FMS involves the large cells, not used in the FPD • methodologies used in FPD successfully adapted to FMS • intercompare reconstructed PYTHIA+GSTAR events against reconstructed data • verify spin information from STAR local polarimeter • extract transverse single spin asymmetries from FMS from • data for p↑+ p -> p0 + X as a point of contact with previous work • extract transverse single spin asymmetries from FMS from • data for p↑+ p ->“jet-like”+ X final state.

  13. BBC polarization time dependence samples Correlation of multiplicity topology in beam-beam counter (BBC) with polarization direction turns out to be good polarimeter for s = 200 GeV see J. Kiryluk (STAR) ArXiv:hep-ex/0501072v1 For run-8 data, analysis of BBC asymmetries, using effective analyzing powers from run-6, is effective quality assurance for the FMS analysis Every run for which there is FMS data, also has BBC data. Polarization Flat line fits shown Relative polarimetry consistent with CNI Star preliminary

  14. First look atanalysis results STAR preliminary 75% of run-8 data stat.errors only • AN comparable to prior measurements • Azimuthal variation appears to be as expected • Systematic errors being evaluated • First estimate stot. 1.2 sstat. Octant subdivision of FMS for inclusive p0 spin sorting.

  15. First look at “jet-like” events Event selection done with: • >15 cells with energy > 0.4GeV in the event (no single pions in the event sample) • cone radius = 0.5 (eta-phi space) • “Jet-like” pT > 1 GeV/c ; xF > 0.2 • 2 perimeter fiducial volume cut (small/large cells) The agreement between data and simulation looks convincing

  16. Conclusions • FMS is complete and in place. Commissioned and operated in run-8. It has 20x the acceptance of FPD • Reconstruction and calibration procedures successfully ported from FPD to FMS • Calibration is mostly complete and data shows good agreement with the simulated sample of events • Inclusive p0 AN(xF) from FMS is comparable to FPD precision measurements • analysis of jet-like events is under way

  17. Outlook • Complete analysis of “jet-like” events • Determine AN(pT) for p↑+ p -> p0 + X • Determine AN for final state that contains p0 pairs • Determine AN for final states with heavier mesons • Run-9 - Go beyond p0 detection to direct photons + jet final state AN THANK YOU

  18. BACKUP

  19. 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:

  20. How can the p0 cross section depend on the proton transversity? • Proton  quark scattering is insensitive to transverse spin. However, the quark retains its initial spin after a hard scattering, and thequark π0fragmentation can have azimuthal dependence on thetransverse spin of thequark. This process is referred to as theCollins Effect. [Nucl. Phys. B396, 161 (1993)] • A quark inside a proton may have orbital angular momentum that is correlated to the spin of the proton. If two quarks with opposite transverse momentum contribute different scattering amplitudes to the same final state, a case can be made where the proton  quarkscattering is sensitive to thetransverse spin of the proton.This process is referred to as theSivers Effect. [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]

  21. Collins Effect sq = Spin of the struck quark pq = Momentum of the struck quark kTπ= Transverse momentum of the neutral pion y SP The spin of the scattered quark is correlated with the spin of the proton x sq The fragmentation of the quark to p0 has sq dependence p z kTπ pq P (any polarization) p + p  p0 + X π0 Spin of the proton affects the scattering angle through the spin of the large x quark π0

  22. Sivers Effect Sp = Spin of the proton Pp = Momentum of the proton kTq = Transverse momentum of the quark inside the proton Quark transverse momentum is correlated with the spin of the proton y SP x kTq Pp Quark Parton Distribution Function has kTq dependence z pq P (any polarization) π0 p + p  p0 + X Spin of the proton affects the scattering angle through the quark transverse momentum π0

  23. Background fitting df/dx = S / [σ(2π)½] exp[-(x-μ)2/2σ2] + B[β2 / (t1 - t2)](x - x0)exp[-β(x - x0)]; Tuned 2-γ fit, especially for Large cells.  Reduced from 6 to 5 parameters by fixing Eγγ S: Gaussian peak integral,μ: Gaussian peak centroid,σ: Gaussian width,B:  integral of background function for |xi - μ| < 3σ,xP:  background peak position,β:  background exponential falloff parameter. S and B are spin dependent

  24. Energy-dependent corrections • p0peak position depends on the energy • Linear correction extracted from p0 peak position and being applied to photon energies • works for both p0s and ηs, and significantly decreases shift from zero in dEgg = Esimu - Ereco.

  25. Resolution smearing • A data-driven model is applied to introduce irresolution to the simulation • This smearing is taken from the individual detector performance, as measured from high-tower associated invariant mass • Applying this to the full PYTHIA/GSTAR simulations of the small cells results in a better match between simulation and data

  26. Simulation and search algorithm for p0p0 in FMS and its engineering protoype • Without Z vertex information in the calculation above, it is possible to find events where the p0 pair originated at a significant distance from the origin • One source of such events are decays KS→p0p0 (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

More Related