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					                 DVCS and the Recoil Detector
                        at HERMES
                                              Weilin Yu
                                                          Institut,
                                        II.Physikalisches Institut,
                               Justus-Liebig-            Gießen,
                               Justus-Liebig-Universität Gießen, Germany

                                for the HERMES Collaboration


              Spin of Nucleon, GPDs and DVCS
              DVCS Results from HERMES
              The HERMES Recoil Detector




DVCS and the Recoil Detector                                               1
                               Spin of Nucleon
                                      A very complicated object

                                       The      iti    f    l      i
                                      Th composition of nucleon spin




DVCS and the Recoil Detector                                            2
                               Spin of Nucleon
                                      A very complicated object

                                       The      iti    f    l      i
                                      Th composition of nucleon spin




                                      Spin of quarks  from DIS
                                      ( Hermes Phys. Rev. D75 (2007) 012007 )




DVCS and the Recoil Detector                                                    3
                               Spin of Nucleon
                                      A very complicated object

                                       The      iti    f    l      i
                                      Th composition of nucleon spin




                                      Spin of quarks  from DIS
                                      ( Hermes Phys. Rev. D75 (2007) 012007 )

                                      Spin of gluons G: Poor information,
                                     first results show the small contribution

                                                       h    b l       l
                                      How to measure the orbital angular
                                     momenta of partons Lq and Lg ?




DVCS and the Recoil Detector                                                     4
        Generalized Parton Distributions
                                       Proton matrix elements which are
                                      generalisations of Form Factors and PDFs

                               t   -> momentum transfer squared to the target
                                   -> longitudinal momenta fraction of struck parton
                                   -> Skewness, not zero, in LO   = xB/(2 - xB)


                                       In the forward limit, link to PDFs

4 leading twist quark GPDs


                                       Integrate over x, lead to Form Factors




DVCS and the Recoil Detector                                                      5
        Generalized Parton Distributions
                                3-D picture of nucleon structure




4 leading twist quark GPDs
                                Number density of partons with longitudinal momentum
                                fraction x at radial position r
                                                     p

                                Reduced Wigner distribution, quantum
                               phase-space „tomography“ of the nucleon




DVCS and the Recoil Detector                                                     6
        Generalized Parton Distributions
                                 3-D picture of nucleon structure


                               Ji Relation (1996)




            Moment of certain GPDs relates directly to the
4 leading twist quark GPDs
                           total angular momentum longitudinal momentum
                                  Number density of quarks with
                                 fraction x at radial position r
                                                      p

                                 Reduced Wigner distribution, quantum
                                phase-space „tomography“ of the nucleon




DVCS and the Recoil Detector                                              7
                          How to Access GPDs
       GPDs provide a unified theoretical framework for many experimental
      processes




DVCS and the Recoil Detector                                                 8
Deeply Virtual Compton Scattering (DVCS)
DVCS: hard electroproduction of a real photon
         the cleanest probe of quark GPDs; well controlled in theory; no gluons in
         the leading order
                             Factorization theorem is proven!

                                Hard scattering process (QED & QCD)
DVCS
                                    p                                    (GPDs)
                                Non-pertubative structure of the nucleon (    )




 DVCS and the Recoil Detector                                                   9
Deeply Virtual Compton Scattering (DVCS)
DVCS : hard electroproduction of a real photon
          the cleanest probe of quark GPDs; well controlled in theory; no gluons in
          the leading order
                             Factorization theorem is proven!

                                 Hard scattering process (QED & QCD)
DVCS
                                     p                                    (GPDs)
                                 Non-pertubative structure of the nucleon (    )

                                 In fixed target experiment, not distinguishable from
                                Bethe-Heitler process
       Same final state




       Bethe-Heitler                                 Interference term I
                                 Bethe-Heitler dominates the cross-section
                                 Need a way to access DVCS
 DVCS and the Recoil Detector                                                      10
                        Azimuthal Asymmetries
 Azimuthal Asymmetries possible with respect to beam charge and helicity




    el beam charge,  beam helicity, unplolarized target
                                                           F1, F2: Dirac and Pauli form
                                                           factors

                              :


 DVCS term can also be accessed:




                      f         d       lifi    both    l   d imaginary
     BH serves as a reference and an amplifier, b th real and i   i
     parts can be projected out

                       g        p            g
 GPDs accessable through other polarized targets
DVCS and the Recoil Detector                                                              11
                         HERMES Experiment


                                      Longitudinally polarized
                                     electron / positron beam from
                                     HERA
                                      Polarized / unpolarized internal
                                     gas target
                                       Data t ki
                                      D t taking summer 1995 –
                                     June 30, 2007
                                      1995 – 2000 longi. pol. target
                                       2002 – 2005 trans. pol. target
                                       2006 – 2007 unpol. target with
                                     the Recoil Detector




  12


DVCS and the Recoil Detector                                      12
                        HERMES Spectrometer



HERA:
longi.
longi. polar.
e± beams,
27.5
27 5 GeV

                                                          Tracking detectors
                                                          PID detectors


   Pure gas target: polarized H, D; unpolarized H, D, N, Ne, Kr, He,Xe
   Forward spectrometer: 40 mrad <    < 220 mrad
   p/p ~ 2%, mrad
   PID for e± : TRD, Preshower, Calorimeter
    one lepton and one photon detected in the calorimeter for DVCS!!!
  DVCS and the Recoil Detector                                                  13
                   DVCS Selection @ HERMES
 Recoil protons identified by missing mass
    [-1.52,1.72]
    [ 1.5 1.7         different for e- and e+
                                                                         Cuts :




                                      Limited by relatively
                                      low resolution
                                      and high background



  MC studies used to estimate                   12% from associated  production
 uncertainties from alignment,                  Impossible to clean without the Recoil
 acceptance, bin width                          Detector
                                                 Semi-inclusive 0 background corrected
                                                for the results
  DVCS and the Recoil Detector                                                        14
 Asymmetries from Unpolarized Target
 Beam-charge asymmetry             [BCA]


 Beam-Helicity asymmetry             [BSA]



Unable to separate from the DVCS term with single charged beam




                                                                 15
 DVCS and the Recoil Detector
 Asymmetries from Unpolarized Target
 Beam-charge asymmetry             [BCA]


 Beam-Helicity asymmetry             [BSA]



Unable to separate from the DVCS term with single charged beam

  If the beam has both charges:


 Asymmetries simultaneously extracted from data via a
 combined fitting to the collected events:

                                                        Interference and
                                                        DVCS terms
                                                        disentangled,
                                                        disentangled
                                                        unique at HERMES!

                                                                       16
 DVCS and the Recoil Detector
               Beam Helicity Asymmetries

                                               Leading asymmetry
                                              twist - 2




                                               twist – 3

                                               Overall values compatible
                                              with zero

                                               No kinematical dependence


                                                Asymmetry of  resonance,
                                               unkown


   Theoretical models overshoot the leading asymmetry
   Impact from resonance may be small

DVCS and the Recoil Detector                                            17
               Beam Charge Asymmetries

                                     ad g a y      y
                                  Leading asymmetry :



                                   Leading asymmetry
                                  L di

                                        g     p
                                  Strong t dependence

                                               twist -3
                                 GPDs

                                               gluon GPDs


                                  Theoretical models do
                                                   beam
                                 not describe the beam-
                                 helicity data, but are in
                                 good agreement with this
                                 data
                                 d
DVCS and the Recoil Detector                               18
  Asymmetries from Polarized Targets
 Transverse target-spin asymmetry                 [TTSA]




The  l             t       b     dt
Th only DVCS asymmetry can be used to access GPD E!



U: unpolarized beam, T: proton target with transversal polarization

 Interference and DVCS terms can be separated due to the
  i t      fb      h     in the i t f         t f th        ti
existence of beam-charge i th interference part of the x-section.




  Long.
 Long target-spin asymmetry                 [LTSA]



 DVCS and the Recoil Detector                                         19
                                                                        19
 HERMES: First Measurement of TTSA

                               O ya y
                               Only asymmetries
                               where GPD E is
                               not suppressed


                               Ju as input
                               parameter for the
                               model
                               sensitive to Ju




                               Can be used to
                               perform a model-
                               dependent
                               constraint for Ju
                               and Jd
                                JHEP 06 (2008) 066.


DVCS and the Recoil Detector                    20
               DVCS from Deuterium Target




                                   Low t coherent
                                   L   t:  h    t      Hi h t i    h    t
                                                       High t: incoherent

Proton and Deuterium targets results are compatible!

DVCS and the Recoil Detector                                         21
Asymmetries Hightlight (1996 ~ 2005)


                                   Beam-Charge asymmetry



                                               y y       y
                                   Beam-Helicity asymmetry



                                   Trans. Pol
                                   Trans Pol. target asymmetry



                                   Longi. Pol. target asymmetry

                               ~   No     l di           t h
                                   N non-leading asymmetry has
                                   been measured with large
                                   value.

DVCS and the Recoil Detector                                      22
                  DVCS from Nuclear Targets




No nuclear mass dependence observed within uncertainties!



 DVCS and the Recoil Detector                               23
                          The Recoil Detector
   A Recoil Detector was installed to identify the recoiling protons
   Dedicated high luminosity run between 2006 - 2007 with unpolarized
  hydrogen and deuterium targets


                                                                PID detectors
                                                                Tracking detectors



                                                                  

                                                                 e
                                                                 e'

     Hydrogen
     Deuterium




DVCS and the Recoil Detector                                                     24
                          The Recoil Detector


                                       Photon detector

                                       Scintillating fiber tracker

                                       Silicon detector


                                       Detects recoil protons / pions
                                       Improves resolution of momentum
                                      measurement
                                       Cleans the background



1 tesla magnetic field

DVCS and the Recoil Detector                                             25
                               The Silicon Detector
                                            2 layers of 16 double-sided
                                           sensors
                                               10 cm x 10 cm active area
                                               300 um thickness

                                            Inside accelerator vacuum,
                                           5 cm close to the lepton beam
                                            Readout by HELIX 3.0 chips,
                                           signals divided to high and low
                                           g                     y
                                           gain to increase the dynamic
                                           range


                                            Momentum reconstruction >
                                           125 MeV/c
                                                 for l    d   di
                                            PID f low- and medium-
                                           energy protons


DVCS and the Recoil Detector                                                 26
                  The Scintillating Fiber Tracker
                                        2 barrels with each 4 layers of 1
                                       mm Kuraray scintillating fibres
                                        Each barrel has 2 parallel and 2
                                       stereo layers tilted by 100 to form
                                       spacepoint
                                        Fibers connected to multi-channel
                                       PMTs through lightguides, signals
                                          d     by             h
                                       read out b GASSIPLEX chips
                                        Last dynode signal used for timing



 Track reconstruction via deflection of the
charged particles inside the magnetic field
 PID for medium and high-energy particles
     d       l ti
 good  resolution



  DVCS and the Recoil Detector                                                27
                               The Photon Detector
                                         3 layers of tungsten/scintillator
                                        sandwich
                                         1 layer parallel to beam axis while
                                        the other 2 layers under +450 /-450


                                         Detection of photons from decay of
                                        resonance:  -> p 0
                                         PID for p > 600 MeV/c




DVCS and the Recoil Detector                                                    28
           Recoil Momentum Reconstruction




  Low-energy protons                      125 MeV/c < p < 145 MeV/c
    Momentum via sum of deposited energies
  Medium-energy protons                   145 MeV/c < p < 250 MeV/c
    Momentum via dE/dx
  High-energy particles (protons/pions) p >200 MeV/c
    Momentum via bending in magnetic field
DVCS and the Recoil Detector                                     29
                  Recoil Detector Performance

                                      Energy deposition in the sub-
                                     detectos




  Good     t       l ti
 G d momentum resolution
 Improvement via accounting E in
the track fitting
                g




DVCS and the Recoil Detector                                           30
                               Recoil Events Selection

                                                w.o recoil




C      l t d     t from recoil
  Correlated events f        il
detector and forward spectrometer                with recoil
 Background reduced to a level
of 3%, photon detector will help
more
 Background contribution only
from the resonant  process


DVCS and the Recoil Detector                                   31
                 BSA with the Recoil Detector




C      l t d     t from recoil
  Correlated events f        il
detector and forward spectrometer
 Background reduced to a level
of 3%, photon detector will help
more
 Background contribution only
from the resonant  process
 Very preliminary BSA from the
recoil detector
DVCS and the Recoil Detector        Indication of small Aass.?   32
                               Conclusions

         Hermes provides a very useful framework to
       study DVCS and GPDs.
         Several DVCS azimuthal asymmetries were
       measured with respect to beam helicity and charge,
                 g    p                   p
       and to target spin in the HERMES experiment.
         The recoil detector took data during 2006 and
            ,                                y
       2007, with which DVCS can be directly measured
       and background can be rejected.




DVCS and the Recoil Detector                                33

				
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