Measurements of Transverse Spin Effects with the Forward Pion Detector of STAR Larisa Nogach Institute of High Energy Physics, Protvino for the STAR collaboration Outline Motivation and previous measurements Detector layout Status of the analysis of Run 6 data and results Plans for Run 7 and beyond Summary

SPIN2006, Kyoto2 Motivation Phenomenological models expect that  0 analyzing power decreases with increasing p T hep-ph/ A N for    production at RHIC at √s=200 GeV

SPIN2006, Kyoto3 Existing measurements at STAR PRL 92, (2004) √s=200 GeV, = 3.8 nucl-ex/ Cross-section is consistent with NLO pQCD calculations Asymmetry revealed at lower energies persists at √s=200 GeV

SPIN2006, Kyoto4 Transverse spin runs at STAR with forward calorimetry: 2001→2006 Run2Run3Run5Run6 detectorEEMC and FPD prototypes 6 matrices of FPD full FPD (8 matrices) East FPD West FPD++ ~15~30~45~ ±3.3/±4.0±3.7/± /3.3 sampled FOM (P 2 L) in Run 6 is ~50 times larger than from all the previous STAR runs

SPIN2006, Kyoto5 STAR detector layout TPC: -1.0 <  < 1.0 FTPC: 2.8 <  < 3.8 BBC : 2.2 <  < 5.0 EEMC: 1 <  < 2 BEMC: -1 <  < 1 FPD++/FPD:  ~ 3.3/-3.7 FPD++: engineering test of the Forward Meson Spectrometer Run 6 – FPD++

SPIN2006, Kyoto6 Detector acceptance Strong correlation between x F and p T in the individual detectors Rapid change in number of events for either increasing p T at fixed x F or increasing x F at fixed p T Broader p T range in x F bins when combining data at =3.3 and 3.7

SPIN2006, Kyoto7 Details of data analysis Event selection: Calibration: on hardware level offline cell-by-cell energy and time-dependent corrections −> is known at the level of 2%

SPIN2006, Kyoto8 Details of data analysis Analyzing power is measured by “cross-ratio” method with two-arms (left-right) detector: Run-by-run comparison with mean is consistent with statistics, except right near the threshold =3.7 =3.3

SPIN2006, Kyoto9 π 0 A N at √s=200 GeV – x F -dependence A N at positive x F grows with increasing x F A N at negative x F is consistent with zero Run 6 data at =3.7 are consistent with the existing measurements Small errors of the data points allow quantitative comparison with theory predictions Theory expects the reverse dependence on η

SPIN2006, Kyoto10 A N (p T ) at x F > 0.4 Run3+Run5 data (hep-ex/ ): Run6 data: more precise measurements consistent with the previous runs in the overlapping p T region complicated dependence on p T Online calibration of CNI polarimeter Hint of A N decrease with increasing p T at p T ~1-2 GeV/c residual x F -dependence? => A N mapping in (x F,p T ) plane is required

SPIN2006, Kyoto11 A N (p T ) in x F -bins Combined data from three runs at =3.3, 3.7 and 4.0 In each x F bin, does not significantly changes with p T Measured A N is not a smooth decreasing function of p T as predicted by theoretical models

SPIN2006, Kyoto12 Jet-like events PYTHIA simulations: - trigger on small cells; sum over entire side - vector sum of photon momenta reproduces most forward hard-scattered parton Expect 4-5 σ effect from existing Run 6 data if observed π 0 A N is due to Sivers effect hep-ex/ Is the single spin asymmetry observed for  0 also present for the jet the  0 comes from? Answer discriminates between Sivers and Collins contributions

SPIN2006, Kyoto13 Run-7 FMS as seen from STAR interaction point Forward Meson Spectrometer for Run 7 FMS will provide 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 to probe low- x gluon density in p+p and d(p)+Au collisions spacers

SPIN2006, Kyoto14 Summary  In Run 6, 6.8 pb -1 of data with average beam polarization ~60% has been acquired in p+p collisions at √s=200 GeV  (x F,p T )-mapping of the transverse single spin asymmetry of inclusive  0 production at forward rapidity has been done A N at positive x F grows with increasing x F A N at negative x F is consistent with zero Measured A N at “fixed” x F up to x F ~0.5 does not decrease with increasing p T as expected by the theory High precision of the measurements allows for a quantitative comparison with theoretical models and distinguishing between different dynamics

SPIN2006, Kyoto15 Outlook  Analysis of the data at √s=62 GeV is underway  Jet-like events: detector response simulation has been started calibration of the FPD++ outer matrices needs to be completed  Construction of the FMS is ongoing − expected to be completed by Run 7

Backup

SPIN2006, Kyoto17 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:

SPIN2006, Kyoto18 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 Need to go beyond π 0 detection to jets and direct photons Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity

SPIN2006, Kyoto19 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

SPIN2006, Kyoto20 Run6 − FPD++

SPIN2006, Kyoto21 Cell-by-cell calibration  gains are determined from π 0 peak position in 2γ invariant mass distributions sorted by high towers  accuracy of the calibration is at the level of ~2%

SPIN2006, Kyoto22 Energy-dependent corrections π 0 peak position depends on the energy – due to energy leakages and ADC granularity (dedicated MC study has been done)

SPIN2006, Kyoto23 Run-dependent corrections The FPD response vary with time/beam conditions: used for spin sorting

Correlation between FPD gains and BBC rates

(, ) p T bin, GeV/c p T bin, GeV/c <x F < <x F < <x F < <x F < <x F <0.56

distributions in p T bins

SPIN2006, Kyoto27 Cross section − separated x F and p T dependence Similar to ISR analysis: J. Singh et al., Nucl. Phys. B140, 189 (1978)

SPIN2006, Kyoto28 Jet-like events Expect that jet-like events are ~15% of   events