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       Forward Spin Physics at STAR
                RHIC, BNL
 Spin-dependent forward particle correlations in p+p
            collisions at s = 200GeV

                       Nikola Poljak
                    University of Zagreb
                For the STAR Collaboration

          Single Spin Asymmetry
                     d    d 
• Definition:   AN 
                     d   d 
• dσ↑(↓) – differential cross section of p0 when
  incoming proton has spin up(down)
                                    p0, xF<0           p0, xF>0
    positive AN:
     more p0 going     p                                         p
left to polarized beam

          Published measurements at STAR

   STAR: arXiv:hep-ex/0801.2990
       PRL 97, 152302 (2006)
    accepted for publication in PRL
                RUN 6
  • Large transverse single-spin
    asymmetries at large xF
  • xF dependence matches
    Sivers effect expectations
  • pT dependence at fixed xF not
    consistent with 1/pT
    expectation of pQCD-based

   At this energy the cross-section is
consistent with NLO pQCD (run2 + run3)

    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).
• Expectations that 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

              Separating Sivers and Collins effects

  Sivers mechanism: asymmetry                        Collins mechanism: asymmetry
  in the forward jet or γ production                 in the forward jet fragmentation
             Phys Rev D41 (1990) 83; 43 (1991) 261                   Nucl Phys B396 (1993) 161
   SP                                                 SP

        p                                                  p

                                    p                                           p

                                                                          Sq               kT,π
Sensitive to proton spin –                                 Sensitive to
parton transverse motion                                   transversity

            To discriminate between the two effects we need to
                 go beyond π0 detection to jet-like events

                Detectors – from FPD to FMS

Run8 and FPD,FPD++
Runs 3-6: beyond: FMS
• FMS will provide full azimuthal coverage for
  range 2.5  h  4.0
• Inclusive p cross sections
• broad acceptance in xF-pT plane for
                0    0
• AN for inclusive,… production in p+p and
   inclusive g,p ,w,K p production
                          0       0   0
• broad acceptance for gp and p p from
  forward jet pairs

       uses 3 different settings                     uses a acceptance
                                                   20x more single
        of modular detectors                      monolithic detector
                                                 than previous detectors
      Forward Meson Spectrometer (FMS)
                            Small Cell PSU Type
                                                      224 of 476
New FMS Calorimeter
                                                      Cockcroft-Walton HV bases
Lead Glass From FNAL E831
                                                      with computer control
804 cells of 5.8cm5.8cm60cm
                                                      through USB.
Schott F2 lead glass
                                                      Designed/built in house for
        QT board                                      FEU-84.
                             Readout of 1264 channels of
                             Designed and built at Penn State University
                             FMS provided by QT boards.
                             Each board has
                               Students prepare cells at test Lab at BNL
                             • 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
                           1ft       1m
                             • 12-bit ADC / channel
         Designed and built at UC Berkeley/SSL

       the calibration
 methodologies employed
  for the FPD have been
 successfully adapted to
          the FMS
  Offline calibration done
      included energy
Event selection done with:
minbias condition
Hightower ADC threshold
(400/200 cts. for small/large cells)
Ng  2
0.08  mgg  0.19 GeV /c 2
zgg  | Eg 1  Eg 2 | ( Eg 1  Eg 2 )
<0.7 (small) ; <1.0 (large cells)
fiducial volume cut (0.5 cell)

            Details of data analysis - calibration
Minimal run-by-run dependence in mass peak observed

 LED system : critical calibration tool
 MIT (LED optics)
 UC Berkeley/SSL (flasher boards)
 Texas / Protovino / BNL (assembly)
 SULI program (Stony Brook students) / BNL
            (control electronics)

                                              Calorimeter stable
                                              at level of ~1%.
    Association analysis – energy corrections
• comparison of generated quantities to reconstructed GEANT simulations
• We consider   Egg  Egg , gen.  Egg ,rec.
          UNCORRECTED                            CORRECTED

 Eliminating energy dependence in p0 mass peak gives the correct average
                               neutral pion energy

                  Distributions comparison
• Full PYTHIA/GEANT simulations have 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

                  Summary & goals
  • FMS - a new device, with many 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.
               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-
                                                           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
                Flat line fits shown                       data, also has BBC data.

                                 Relative polarimetry consistent with CNI
                                                                                Star preliminary
             First look at analysis results
                                AN cos
                                                          STAR preliminary

                P


                                      75% of run-8 data          stat.errors only

Octant subdivision of FMS for    • AN comparable to prior measurements
  inclusive p0 spin sorting.
                                 • Azimuthal variation appears to be as
                                 • Systematic errors being evaluated
                                 • First estimate tot. ≤ 1.2 stat.
                   First look at “jet-like” events

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

  • 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

        • 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


                       Single Spin Asymmetry
                                             Two methods of measurements:
                          d    d      • Single arm calorimeter:
     • Definition:   AN 
                          d   d 
                                                       1        N   RN          L
                                               AN             
                                                                N  RN  
                                                                               R
     • dσ↑(↓) – differential cross                                                   L
                                                      Pbeam                
       section of p0 when incoming
       proton has spin up(down)                 R – relative luminosity (by BBC)
                                                Pbeam – beam polarization
      π0, xF<0                  π0, xF>0
                                           • Two arm (left-right) calorimeter:
p                                         p                N  N  N  N        
                                                                                    
                                                AN            L     R     R     L

                                                     PBeam  N   N   N   N     
                                                              L     R     R     L    
                                                 No relative luminosity needed
       positive AN: more p0 going
         left to polarized beam

                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:
     How can the p0 cross section depend on the
                proton transversity?
1.   Proton  quark scattering is insensitive to transverse spin. However, the
     quark retains its initial spin after a hard scattering, and the quark  π0
     fragmentation can have azimuthal dependence on the transverse spin of
     the quark. This process is referred to as the Collins Effect. [Nucl. Phys.
     B396, 161 (1993)]

2.   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  quark scattering is
     sensitive to the transverse spin of the proton. This process is referred to
     as the Sivers Effect. [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]
                                          Collins Effect

          sq = Spin of the struck quark

          pq = Momentum of the struck quark

          kTπ = Transverse momentum of the neutral pion

y                                                 The spin of the scattered quark is
                     SP                          correlated with the spin of the proton

                          p                 sq                             The fragmentation of the quark to
                                                                                 p0 has sq dependence

               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
                                          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
y                    SP                    with the spin of the proton
               P                                        Quark Parton Distribution
               p                                      Function has kTq dependence

                                                       P (any polarization)
            p + p  p0 + X

Spin of the proton affects the scattering angle
  through the quark transverse momentum                                  π0
                       High Voltage Systems                                                             24

    PC                          Light-tight, ventilated enclosure (half of FMS)
                                                  Large cells        / 788 in total              +9V/2.4A
                                Up to16 controllers of either type
                 USB to I2C                                        Zener-diode-stabilized
                                XP2202 phototube powered byResistive bases          +30V/1.2A
                                resistive voltage divider, with high-voltage delivered by
                                      four 256-channel LeCroy 1440 main frames -6/0.5A

                                                         Small cells viewed by FEU-84
       Up to16 PSU bases
                                                                   224 in total
                                                              Yale bases          PSU bases
                                            Up to16 Yale bases          system for FEU-84
              PSU controllers
                                                      designed/built by Steve Heppelman,
                                                         Len Eun, et al. at Penn State

                                                                                as in E864
                                                                              Yale controllers
             Small cells view by XP2972
Two PC-controlled 256-channel Cockcroft-
              Master controller
                     252 in total
Walton control systems designed/built by Steve
    Existing Len Eun, et al. bases courtesy
Heppelmann,phototubes and (Penn State) forof
           inner calorimeter HV control as
                  High-voltage systems
small-cell Yale University, from AGS-E864 implemented in north FMS half
                 Electronics and Trigger                                           25

 Hank Crawford, Fred Bieser, Jack Engelage, Eleanor Judd, Chris Perkins, et al.
                              (UC Berkeley/SSL)

   North FMS rack,                                   Readout of 1264 channels of
servicing 632 detectors                              FMS provided by QT boards.
                          Present Status             Each board has
                                                  • four 9U VME in
                          • 48 QT boards mounted in32 analog inputsSTAR Wide
                          Angle Hall;             • 12-bit ADC / channel
                          • all QT boards operational,5-bit TDC / channel bad
                                                     • with essentially zero
                                                     • five FPGA for data and trigger
  QT3 Crate               • QT1,QT2,QT3,QT4 crates connected to phototubes via
 12 QT boards             a “straight map”;          • operates at 9.38 MHz and
                                                     higher harmonics
                          • Trigger presently a high-tower trigger
                                                     • produces 32 bits for each
                                                     RHIC crossing for trigger
                          Plans for run-9
                          QT32 with 4 QT8
QT8 daughter card
  QT4 Crate
 12 QT boards             daughter cards
                          • Remap from patch panels to QT boards for cluster

                         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

 S and B are spin dependent
              Energy-dependent corrections

p0 peak position depends on the

• 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 Egg = Esimu - Ereco.

                     Resolution smearing

• A data-driven model is applied to
introduce irresolution to the
• 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

             p0 pair finding algorithm

• An algorithm that will provide
a mean to find p0 pairs and
investigate their asymmetries
has been developed and
studied in simulation
• The combinatoric background
appears predominantly at the
low mass region, while the
high mass region is dominated
by pions from two different
away side jets

                              K_S events

• 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

• Plot shows the mass distribution
for displaced vertices above 100
cm from the BBC vertex. A
pronounced KS mass bump is

F.Bieser2, L.Bland1, T. Bakowski5, E. Braidot7, R.Brown1, H.Crawford2,
A.Derevshchikov4, J.Drachenberg6, J.Engelage2, L.Eun3, M.Evans3,
D.Fein3, C.Gagliardi6, A. Gordon1, S.Hepplemann3, E.Judd2, V.Kravtsov4, J.
Langdon5, Yu.Matulenko4, A.Meschanin4, C.Miller5, D.Morozov4, M.Ng2,
L.Nogach4, S.Nurushev4, A.Ogawa1, H. Okada1, J. Palmatier3,
T.Peitzmann7, S. Perez5, C.Perkins2, M.Planinic8, N.Poljak8, G.Rakness1,3,
J. Tatarowicz3, A.Vasiliev4, N.Zachariou5

1Brookhaven   National Laboratory
2University of California- Berkeley
3Pennsylvania State University
4IHEP, Protvino                    total number of undergraduate students =   10
5Stony Brook University
                                    total number of   graduate students =     5
6Texas A&M University
7Utrecht, the Netherlands

8Zagreb   University

Run6 − FPD++

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