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

2 Motivation Contrary to simple pQCD predictions, f  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.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 in p↑+p → + X production are not yet fully understood.Significant developments in theory in the past few years suggest common origins for these effects, but large transverse spin asymmetries in p↑+p →  + X production 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 (S p ), proton momentum (P p ) and transverse momentum (k T ) of the unpolarized partons inside. The unpolarized parton distribution function f q (x,k T ) is modified to: Collins effect [Nucl. Phys. B396, 161 (1993)]: Correlation between the quark spin (s q ), quark momentum (p q ) and transverse momentum (k T ) of the pion. The fragmentation function of transversely polarized quark q takes the form: According to the latest theoretical developments, both mechanisms contribute to   A N

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

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

6 run/yearRun Run3Run5Run6Run detectorEEMC and FPD prototypes 6 matrices of FPD full FPD (8 matrices) East FPD West FPD++ East FPD FMS P beam, % ~15~30~45~55~ ~ ±3.3/±4.0±3.7/± / / sampled Transverse spin runs at STAR with forward calorimetry: 2001→2008

7 Single Spin Asymmetry Definition: dσ ↑(↓) – differential cross section of   when incoming proton has spin up(down) Two methods of measurements: Single arm calorimeter: R – relative luminosity (by BBC) P beam – beam polarization Two arm (left-right) calorimeter: No relative luminosity needed π 0, x F <0 π 0, x F >0 Left Right p p positive A N : more  0 going left to polarized beam

8 Runs 3-6: FPD, FPD++ Inclusive  0 cross sections A N for inclusive  0 production Detectors – from FPD to FMS Run8 and beyond: FMS FMS provides full azimuthal coverage for range 2.5    4.0 broad acceptance in x F -p T plane for inclusive , ,,K ,… production in p+p and d(p)+Au broad acceptance for   and     from forward jet pairs 20x more acceptance then previous detectors three different positions of modular detectors single monolithic detector

9 Published measurements with FPD PRL 101, (2008) Large transverse single-spin asymmetries at large x F x F dependence matches Sivers effect expectations qualitatively p T dependence at fixed x F is not consistent with 1/p T expectation of theoretical model calculations PRL 92, (2004) √s=200 GeV, = 3.8 Asymmetry revealed at lower energies persists at √s=200 GeV

10 Event selection done with: the calibration methodologies employed for the FPD have been successfully adapted to the FMS offline cell-by-cell calibration using π 0 peak position in the sorted by high tower mass distributions minbias condition hightower ADC threshold (400/200 cts. for small/large cells) (for small cells only) fiducial volume cut (0.5 cell) Run8 data analysis - calibration

11 Calorimeter stable at the level of a few percents Minimal time dependence in π 0 peak position is observed LED system: critical calibration tool LED monitoring system

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

13 First look at π 0 A N from Run8 data Octant subdivision of FMS for inclusive   spin sorting A N comparable to prior measurements Azimuthal variation appears to be as expected STAR preliminary stat.errors only75% of Run8 data

14 Summary High precision A N 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 is completed and data show good agreement with the simulated sample of events. Inclusive  0 A N (x F ) from the FMS is in agreement with the FPD measurements.

15 Outlook  Complete analysis of π 0 asymmetry  Going beyond π 0 detection: - A N for a state that contains  0 pairs - A N 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 π 0 cross section measurements PRL 97, (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, (2007)]

18 Separating Sivers and Collins effects Collins mechanism: asymmetry in the forward jet fragmentation Sivers mechanism: asymmetry in the forward jet or γ production SPSP k T,q p p SPSP p p SqSq k T, π To discriminate between the two effects we need to go beyond π 0 detection to direct photons or jet-like events Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity

19 Energy dependent corrections π 0 peak position depends on the energy linear correction extracted from π 0 peak position and being applied to photon energies works for both π 0 s and ηs, and significantly decreases shift from zero in δE γγ = E simu - E reco.

20 Association analysis comparison of generated quantities to GEANT simulations we consider Eliminating energy dependence in   mass peak gives the correct average neutral pion energy UNCORRECTED CORRECTED

21 Simulation and search algorithm for π 0 π 0 in FMS and its engineering prototype it is possible to find events where the π 0 pair originated at a significant distance from the vertex one source of such events are decays K S →π 0 π 0 (31% branching fraction) plot shows the mass distribution for displaced vertices above 100 cm from the BBC vertex; a pronounced K S mass bump is visible