STATUS REPORT ON SACLAY DRIFT CHAMBERS by rke17752

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									  Gluon polarization measurement at
                         COMPASS
   O.Kouznetsov (JINR)
on behalf of the COMPASS
      Collaboration

                 on behalf of the COMPASS Collaboration




                                       ICHEP06, Moscow 26.07-02.08 2006
     27/07/06              Gluon polarization measurement at         1
                                      COMPASS
                 Nucleon spin puzzle
                     Angular momentum sum rule:
                          ½= ½ΔΣ + ΔG + Lq + Lg


                           quarks              gluons         orb. mom.

 Measurement of ΔG is important for
          two reasons:
 - as an element of nucleon spin puzzle
  (a0 (= ΔΣ) is measured to be around
      0.25 instead of expected 0.6)
- possible role of axial anomaly in the a0
         interpretation (a0 ≠ ΔΣ)                      a0 =
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                                    COMPASS
COmmon          160 GeV polarized  beam, 6LiD polarized target
 Muon and           two stage spectrometer SAS & LAS (~50 m)
   Proton             Hcals, Ecals, RICH for particle ID,  walls
     Apparatus for
                                                        LHC
         Structure and
            Spectroscopy
                                                   SPS


                 CERN (France)
   N




                                         CERN (Suisse)

    In 2002, 2003, 2004 COMPASS has recorded
     27/07/06       Gluon polarization measurement at 3
              30×10  9 events ~ 1000 TB
                               COMPASS
           Photon Gluon Fusion (PGF)
                                                   Open Charm
                                            γ  g  cc  D X
                                                           0

                                           → clean channel;
                                      → but experimentally difficult;
                                      → perturbative scale fixed by
                                      the mass of the charm quarks

N                                         High-pT Hadron Pairs
                                              γ  g  q q  hh
              cc



                                       → easy to get a statistics;
                                      → but physical background;
                                     →perturbative scale fixed by pT
                                              or Q2 cuts
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                              COMPASS
                      Asymmetry measurement
                      two oppositely polarized 60 cm long target
                         cells: upstream (u), downstream (d)
                       cells polarization reversal every 8 hours
                      counting rate asymmetry in two target cells
                        gives the cross-section asymmetry A||
upstream downstream




                                      target polarisation PT ≈   0.50
                                         dilution factor f ≈     0.40
                                      beam polarisation PB ≈     0.76
                                    depolarisation factor D ≈    0.60
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                             COMPASS
                                               Open charm
                                               D D
                                                *      0   
                                                            soft
                                                        BR  68%

                                               D0  K  
                                                        BR  4%

                                            golden channel,
                                           weak MC dependense
                                            small statistics, no
                                           vertex detector → no
                                           primary/secondary
                                           vertex separation

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                      COMPASS
                                            Open charm

 Aexp determined from counting rate asymmetry
  no asymmetry contribution from background
  PGF/ tot = S/(S+B) determined from the fit
       MC is needed for aLL calculation

                                  aLL parametrised by pT, zD, y
                                  parametrization determined with
                                   Neural Net (NN)
                                  NN trained with AROMA
                                  good description of data
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                            COMPASS
                                               Open charm
                                                        G
                                                            0.57  0.41stat.
                                                        G

                                                         xg  0.15(RMS  0.08)

                                                          scale ~ 13 GeV2

what about another channels/mesons?
D0  K–+                     Br ~ 4 % D* tag                   this analysis
D0  K–+ 0                  Br ~ 13 % D* tag                  run 2006 (?)
D0  K*(892)+  K0- +      Br ~ 6% D* tag                     weak signal
D0  (1020)K0  K0K+K-       Br ~ 0.5% ---                      weak signal
Charged D mesons → lower production rate, no D* tag...
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                               COMPASS
                     high pT hadron pairs
PGF               considerably higher statistics
                  but physical background, last 4
                 processes are important only for low Q2

QCD-
Compton           LO DIS                             qg




 gq                    qq                            gg


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                          COMPASS
                                  high pT hadron pairs
                             PGF contribution enhancement
                                               pT > 0.7 GeV/c
             10% of
             statistics
                                     pT12 + pT22 > 2.5 (GeV/c)2
                                             xF, z > 0.1
                                      m(h1,h2) > 1.5 GeV/c2
      Q2 < 1 GeV2 ← analysis → Q2 > 1 GeV2
90% (500K evt) ← statistics from total → 10%
     50% ← resolved photons contribution → ----
QCD-C, LO ← another background → QCD-C, LO
   PYTHIA ← MC generator used → LEPTO
 27/07/06         Gluon polarization measurement at             10
                             COMPASS
                      high pT hadron pairs Q2>1 GeV2
 A||   G  a   PGF
                   d a   
                       PGF
                                 a          LO       LO     QCD C
                                                                        QCD C
                A1 (
                LL
                                           LL              LL
                                                                                    )
 D   D G   tot
                       D  tot
                                   D                                        tot


- cut on xB < 0.05 → A1d ≈ 0 → LO & QCD-C can be neglected
- but they are still important for PGF event fraction estimation




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                                   COMPASS
                                                    high pT hadron pairs
  Results from Monte Carlo:                             Q2>1 GeV2
                                                 PGF
              Ratio of PGF events                       0.34  0.07
                                                  tot


                                             aLL D  0.75  0.05
                                              PGF



A||/D = -0.015 ± 0.080 (stat.)± 0.013 (syst.)

G
                                    
    0.06  0.31stat.  0.06syst.
G
                                                                 xg  0.13
              scale ~ 3 GeV2
main contribution to systematic error
  comes from false asymmetries
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                                        COMPASS
                                            high pT hadron pairs
 Results from Monte Carlo                        Q2<1 GeV2
     A||/D = Rpgf G/G aLLpgf
                                                               30%(PGF)
              + Rqcdc q/q aLLqcdc

              + Rqq q/q aLLgq (G/G)
                                                               50%(all)
Resolved
              + Rqg G/G aLLqg (q/q) 

              +…

 7% in total, moreover the asymmetry for this kind of events
   is small (from A1 measurements at low Q2) : neglected
   27/07/06                Gluon polarization measurement at              13
                                      COMPASS
MC /Real Data (blue                      high pT hadron
points) comparison
                                         pairs Q2<1 GeV2




                      resolved photon contribution
                      the polarized PDFs of the photon is unknown
                      unpolarized PDFs used to constrain polarized
                       q ( x, Q 2 )  q ( x, Q 2 )  q ( x, Q 2 )

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                             COMPASS
                                      high pT hadron pairs
                                           Q2<1 GeV2
                               2002-2003 published:PLB 633 (2006) 25
                                           2002-2004 data
                              A||/D= 0.004 ± 0.013 (stat.)± 0.003 (syst.)


    G
                                           
        0.016  0.058stat.  0.055syst.
    G
     xg  0.0850..035
               
                 0 071
                                 Syst. error: 0.052 (MC), 0.014 (false
   scale ~ 3 GeV2               asymmetries), 0.013 (resolved photons)
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                                    COMPASS
             QCD analysis of g1 data
 G>0 and G<0 solutions describe data equally well
 could direct measurements discriminate between them?

                                                        the first moment
                                                         of G is of the
                                                               order
                                                       0.2-0.3 in absolute
                                                             value at
                                                           Q2=3 GeV2


  27/07/06         Gluon polarization measurement at                16
                              COMPASS
           Direct G/G measurements




                                                      XG
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                             COMPASS
                       Conclusion
  2002-2004 COMPASS data                                   + 2006 (estim.)

                       G
Open charm                 0.57  0.41stat.               0.28(stat.)
                       G
 Q2 >1 GeV2       G
hadron pairs          0.06  0.31stat.  0.06syst.      0.14(stat.)
                  G

  Q2 <1 GeV2 G
                      0.016  0.058stat.  0.055syst. 0.045(stat.)
hadron pairs G
             Results indicate small ΔG around xg ≈ 0.1
QCD analysis of g1 shows small value of the first moment of G
In contrast G ~3 is needed to solve the
                                                             1988-20??
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                                  COMPASS
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                      COMPASS
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                      COMPASS

								
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