Slide 1 - Jefferson Lab

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                   The G Experiment

                Allison Lung, Jefferson Lab
              representing the G0 collaboration:
 Caltech, Carnegie-Mellon, William & Mary, Hampton, IPN-Orsay,
ISN-Grenoble, JLab, Kentucky, LaTech, NMSU, TRIUMF, U Conn,
  UIUC, U Manitoba, U Maryland, U Mass, UNBC, VPI, Yerevan
                 ~ 100 scientists from 19 institutions




                            Our sponsors:
What role do strange quarks play in nucleon properties?
                 Momentum ; Spin ; Mass ; Charge and Current

proton
                                 u
                                 u             valence quarks
                                 d
         gluon                   u
                                               “non-strange” sea (u, u, d, d) quarks
                                 u
                                 s
                                              “strange” sea (s, s) quarks
                                 s


                             Main goal of G0
determine contributions of strange quark sea (s,s) to electromagnetic properties
                              of the nucleon

                        "strange form factors"
                            Charge and current:
                       N | s   s | N   ??  GE GM
                                                  s  s
    Nucleon form factors measured in elastic e-N scattering
    • well defined experimental observables
    • provide important benchmark for testing non-perturbative
      QCD structure of the nucleon


                                                              
                     N|J |N                         GE , GM
e                                          electromagnetic form factors
                 N

         Z
                       N | J | N 
                                 Z
                                                          Z
                                                        G ,G
                                                           E
                                                                  Z
                                                                  M
e                N                             neutral weak form factors


• Measured precision of EM form factors in 0.1 - 1 GeV2 Q2 range ~ 2 - 4%

• Projected precision of NW form factors in 0.1 - 1 GeV2 Q2 range ~ 10%
  from the current generation of experiments (for magnetic)
       Neutral weak form factors  strange form factors
                                              Q         QZ
                                       u     +2/3       1  8/3 sin2W
       STANDARD                                                              sin2W = 0.2312 ± 0.0002
         MODEL                         d     1/3 1 + 4/3 sin2W                weak mixing angle
       COUPLINGS                                                         key parameter of Standard Model
                                       s     1/3 1 + 4/3 sin2W




    ELECTROWEAK                            J    Qi qi  qi
                                             
                                                                           J    QiZ qi  qi
                                                                             Z

      CURRENTS                                      i                                i


                                                                      p    2 up 1 dp 1 s
Flavor decomposition of nucleon E/M             p | J  | p : GE ,, M  GE ,, M  GE ,,M  GE,,p    M
                                                                             3         3         3
              form factors:
                                                                          2 u       1 dn 1 s
                                                n | J  | n : GE ,,n  GE ,,n  GE ,,M  GE,,n
                                                                       M         M                   M
                                                                            3         3         3
                               8            up             4            dp          4          s
     p | J  | p : GE ,,M  1  sin 2 W GE ,, M    1  sin 2 W GE ,,M    1  sin 2 W GE,,p
            Z         Z p
                                                                                                        M
                               3                           3                        3         
   Invoke proton/neutron charge symmetry                                 3 equations, 3 unknowns

                   G ,p
                      E ,M
                              
                           , GE,,n , GE ,,M
                                 M
                                      Z p
                                                        G  u
                                                              E ,M
                                                                      d       s
                                                                   , GE ,M , GE ,M       
                 Parity Violating Electron Scattering -
                 Probe of Neutral Weak Form Factors
polarized electrons, unpolarized target                                 
                                                        e
    R   L   GF Q  AE  AM  AA
                                                                   p e           p
                          2
A          
    R   L  4 2  2 unpol
                                                                             2
                                                                    
                      

AE   ( ) GE (Q 2 )GE (Q 2 )
              Z        
                                                  GE
                                                    s
                                                        Strange electric and magnetic
                         
AM   (Q 2 ) GM (Q 2 )GM (Q 2 )
                Z
                                                  GM
                                                    s
                                                                form factors,
                                      
AA  (1  4 sin 2 W )  G A (Q 2 )GM (Q 2 )
                             e
                                                  GA
                                                    e        + axial form factor


 At a given Q2 , decomposition of GsE, GsM, GeA
          requires 3 measurements:                          G0 will perform all three
                                                            measurements at three
 Forward angle e + p (elastic)                               different Q2 values -
 Backward angle e + p (elastic)                                 0.3, 0.5, 0.8 GeV
 Backward angle e + d (quasi-elastic)
          The Nucleon's e-N Axial Form Factor GAe
Z0 has axial, as well as vector couplings  we measure axial FF too

                     GA  GA  FA  Re
                      e    Z


            Z                        GAZ: neutral weak axial form
                                     factor, determined from neutron
                                      decay and neutrino scattering
     e               N                 multiplied by g e  1  4 sin 2 W ~ 0.074
                                                        v

            +
                                   FA: nucleon’s anapole moment –
                   PV                parity-violating electromagnetic
                                     moment
    e               N
            +
             Z                       Re: electroweak radiative
                                     corrections to e-N scattering
             
    e               N
      Other Aspects of G0 Physics Program


                            GAe (Q2) - e-N axial form factor
                            e + d quasielastic measurements
                            at back angles will map out
                            e - N axial form factor GAe
                            as a function of Q2



 NΔ
GA                          N Axial Transition Form Factor
                            data taken concurrently with back
                            angle e + p elastic data-taking
                            • First measurement in neutral
                              current process
                            • sensitive to hadronic radiative
                Q2 (GeV2)
                              corrections
             Strange form factors - published results
SAMPLE at MIT-Bates:                                                           D2


e  p elastic :     A p  4.92  0.61  0.73 ppm

e  d quasielastic : A d  7.55  0.70  0.60 ppm                                  H2
      (updated results, publication in progress)




                                                                 Zhu, et al.
  GM (Q 2  0.1 GeV 2 )  0.14  0.35  0.40
   s




                                        HAPPEX at Jefferson Lab:
                                  
                                  e  p elastic : Ap  15.05  0.98  0.56 ppm


                                       GE  0.39GM  0.025  0.020  0.014
                                        s        s


                                                   at Q 2  0.48 GeV 2
                G0 Experimental Program
• 1st Engineering Run
                                we are here
• 2nd Engineering Run


• Forward angle measurement
• Expected Results:
    (GEs +  GMs) for 7 values of Q2 between 0.1 – 1.0 (GeV/c)2


• 3 Backward angle measurements
• Expected Results:
 Separated GEs , GMs , and GAe for Q2 of 0.3, 0.5, and 0.8 (GeV/c)2
             N axial transition form factor
              General Experimental Requirements
     measure APV ~ -3 to -40 ppm with precision APV /APV ~ 5%

Statistics (5%      1013 - 1014 events):
   • high e- polarization (70-80%)
   • high e- current (40 Amps)
   • high luminosity (2.1 x 1038 cm-2 s-1)
   • large acceptance detector (0.5-0.9 sr)
   • long target (20 cm)
   • high count rate capability detectors/electronics (1 MHz)

Systematics (reduce false asymmetries,
              accurately measure dilution factors):
   • small helicity-correlated beam properties
   • ability to isolate elastic scattering from other processes

   statistics ~55%-75% of total error on separated GEs and GMs
The G0 Experiment in Jefferson Lab Hall C

          All New Hall C Equipment:
     Superconducting toroidal magnet – Univ of Illinois
      High power H2 /D2 targets – Caltech, UMd, JLab
  Scintillator detector array – French/Canadian/NAmerican
          Custom electronics – Orsay, Grenoble, CMU
            Jefferson Lab polarized source - JLab




        Design and Construction (1993–2001)
         Installation (Fall 2001–Fall 2002)
     G0 installed in Hall C at JLab

April ‘02


                         detector


                                     August ‘02

                                                             magnet




                                                            beamline
            cryogenic supply                                monitors


                    detectors
                  (Ferris wheel)

                                    target service vessel
                 G0 Forward Angle Measurement
• Electron beam energy = 3 GeV on 20 cm LH2 target
• Detect recoil protons ( ~ 62 - 78o corresponding to 15 - 5o electrons)
• Magnet sorts protons by Q2 in focal plane detectors
• Full desired range of Q2 (0.16 - 1.0 GeV2) obtained in one setting
• Beam bunches 32 nsec apart (31.25 MHz = 499 MHz/16)
• Flight time separates p (about 20 ns) and + (about 8 ns)
                                                                       highest Q2




                                                 French octant     lowest Q2
                G0 Superconducting Magnet System
Superconducting toroidal magnet:
   8 coils ; common cryostat

          B  dl  1.6 Tm
         35  bend  
     acceptance~ 0.44 (2



• Initial manufacturing defects
   repaired in early 2002
• Ran at 4500 A initially
          (Aug. - Dec. 2002)
• Ran at full design current
  (5000 A) during Jan 2003 run
                              G0 Target
• 20 cm LH2 cell, 250 W heat load from beam at 40 A
• High flow rate to minimize target density fluctuations
• Observed target density fluctuations at 40 A negligible
                                                                  Heat
                                                                  Exchanger




                                                                 Target cell
                                                Cryogenic pump
                                       High power heater




                                               Normal running
                    G0 Focal Plane Detectors (FPD)
• 8 octants ; 256 signals total
• 16 pairs of arc-shaped scintillators each
• back and front coincidences to eliminate neutrals
                                                        Detector
• 4 PMTs (one at each end of each scintillator)
                                                      “Ferris Wheel"
• long light guides (PMT in low B field)

                               Detector
                             “Ferris Wheel"




North American octant




                                                          back
                                  front
                           G0 Forward Angle Electronics
• Custom electronics designed to provide high-rate histogramming
• NA: mean timer  latching time digitizer  scalers (1 ns)
• French: mean timer  flash TDCs (0.25 ns)
• Time histograms read out by DAQ system every 33 msec




           NA LTD crate (1/2)                 French DMCH16 Module 1/8

                     Time of Flight measurement
                PMT Left
                              Mean
        Front                 Timer
                PMT Right                      TDC
                                      Coinc     /      Time histogramming
                                               LTD
                PMT Left
                              Mean
        Back                  Timer                       Time resolution
                PMT Right
                                                          250 ps / 1ns
                                        G0 Beam
• Requires unusual time structure: 31 MHz (32 nsec between pulses)
 (1/16 of usual CEBAF time structure of 499 MHz (2 nsec between pulses)

• Required new Ti:Sapphire laser in polarized electron gun

• Higher charge per bunch  space charge effects complicated
 beam transport in injector       (challenging beam optics problem)


• Beam quality closest to operating specs during Jan 2003 delivery
    • Beam current 40 A
    • Beam fluctuations at (30 Hz/4) ~ X, Y < 20 m                 I/I < 2000 ppm

• Ongoing Beam Work                              CEBAF polarized injector
    • multiple Hall delivery
    • beam position feedback
    • reliable control of
       helicity-correlated properties
              Data-taking and polarization flip sequence
  Data readout interval = ( 1 / 30 Hz ) = 33 msec
   detector TOF histograms recorded
     integrated values of beam monitors (charge and position) recorded
 electron beam polarizaton flip sequence(pseudo- random pattern)
                                     
                                 
                              
                          1quartet



 asymmetry(detector yield or intensity) :        differences (beam position differences)
            Y1  Y2  Y1  Y2
       A                                           x  X1  X 2  X 1  X 2
            Y1  Y2  Y1  Y2



Typical difference
and charge                           x:               y:                          AI:
asymmetry                            x~ 9 m         y~ 10 m                   AI~ 870 ppm
histograms
    Systematics: from raw asymmetry to physics results
Form raw measured asymmetry from the detector yields: A      Y Y Y Y
                                                             1 2 1 2
                                                                     Y1  Y2  Y1  Y2
                                                       meas



Correct for false asymmetries from helicity-correlated beam properties:

                              P
                     N
  Acorr  Ameas   21Y      Y
                             Pi       i       • helicity-correlated beam properties
                     i 1                      • deadtime corrections
     wherePi  P  P

Correct for background and its asymmetry:

               Acorr  Aback f back
      Asig                                    • background dilution factor correction
                       f sig
Correct for beam polarization and radiative corrections:
                      Acorr                    • electron beam polarization
       Aphys 
                   Pbeam Rrad                  • electromagnetic radiative corrections
Correct for measured Q2 and EM form factors:
                                             • <Q2> determination
  Aphys  Q f (GE , GM , G , G )
               2                   s       s
                                   E       M
                                               • electromagnetic form factors
      Time of Flight Spectra from G0 Engineering Run

16 detectors of a single octant                                   Det 8
                                                      pions
   -t.o.f recorded every 33 msec
   -good spectra for all octants, all detectors



      1           2           3            4

                                                          elastic protons

      5           6           7            8      inelastic protons



      9           10          11          12




     13           14          15          16
             Behavior of raw asymmetry results under
                   slow half wave plate reversal
                        North American detectors and electronics

Based on
51 hours
of data at
40 A


Jan 2003
Engineering             French detectors and electronics
Run


No unexpected
false
asymmetries
seen
       Asymmetry results from Jan. 2003 Engineering Run

• Based on 51 hours
 of data at 40 A
 (note: full production run
  will be 700 hours)

• Includes
• false asymmetry corrections
• deadtime corrections
• background corrections
• beam polarization correction




                                 increasing Q2
1st Engineering Run - successfully completed
                       Current Work
– False Asymmetries
      – Helicity correlated beam charge effects (goal <1ppm over 700 hours)
      – Helicity correlated beam position effects (goal <20 nm over 700 hours)


– Deadtime
      – typical deadtime ~10% with rates of ~1-2 MHz/detector
      – causes false asymmetries when combined w/nonzero charge asymmetry
      – uncorrected effect ~15% ; after correction ~1% ; Afalse ~0.01ppm


– Background Determination
      – requires both yield and asymmetry of background
      – yield ~10-25% depending on detector
      – |Aback| ~ |Aelastic| near elastic peak (preliminary)
                     G0 Backward Angle Measurement
              • detect scattered electrons at e ~ 110o
              • need three runs each for LH2 and LD2
                     at E = 424, 576, 799 MeV
                   for Q2 = 0.3, 0.5, 0.8 (GeV/c)2
                     (total of 6 runs x 700 hours each)

Requires additional hardware:
• Cryostat Exit Detectors (CED) to separate elastic and inelastic electrons
  used in coincidence with FPDs
• Cerenkov detector for pion rejection (primarily for LD2 target)
• additional electronics
• LD2 target                                    Detectors

                                                                Magnet
                      Electron beam




Requires physical turn-around:
                         Near Future
             • 2nd G0 engineering run in Oct-Dec 2003
             • Forward angle production run in Feb 2004

Hopefully by late 2004 we can compare world's forward angle data….
                      Future
       Back angle running starting early in 2005
               …..and a few years later,
we can present a wide Q2 range of separated form factors




                                       + MAMI A4 data
                             SUMMARY
• G0 has a broad experimental program that will result
  in the first separated values of GEs, GMs, and GAe over
  a wide range of Q2

• 1st G0 Engineering Run successfully completed
   –   all hardware operational
   –   obtained ~2 days of test asymmetry data
   –   clearly see weak interaction
   –   no unexpected false asymmetries seen


• On track to resume running in October 2003

• Look forward to first physics results late 2004

				
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posted:5/9/2013
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