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Magnet
Target
Detectors
Beam Beam
(Phase I) (Phase II)
The G0 experiment : Doug Beck Spokesperson. Collaboration : Caltech, Carnegie-Mellon, W&M, Hampton, IPN-Orsay, Kentucky, LPSC-
Grenoble, LaTech, NMSU, JLab, TRIUMF, U. Conn, U. Illinois, Manitoba, Maryland, U. Mass., UNBC, VPI, Yerevan, Zagreb
Outline of the talk
1/ G0 backangle phase
(intro)
2/ Report on 2006 data
taking
3/ Summary and 2006 plans
PAVI06 Workshop, Milos (Greece), May 17th, 2006
Serge Kox, LPSC Grenoble
G0 Experiments : Backangle running
EM and Weak form factors Elastic scattering of electron probes the nucleon via (EM) or Z0 (Weak) exchange
Flavor decomposition (assumption : 3 lightest quarks and charge symmetry)
MZ << M in the GeV region PV asymmetries in polarized elastic e-N scattering to extract the weak form factors
Nucleon structure and EW probes
(Q2)
e
N N
e jμW ,e
Jμ
W ,N
(Q2)
N N
e e jμEM,e
Jμ
EM,N
< N |J
μEM |N > → GE
,N ,GM,N
< N |J μW |N > → GE
(Z,N )
,GM
(Z,N )
< N |J μ5W |N > → GA
(Z,N)
GE,M
s = (1 - 4sin2θW )GE,M,p -GE,M
,n -GE,MZ,p
Role of the strange quark in the EM nucleon structure Strange quarks are the lightest non valence quarks of the sea
Excellent candidate to study the dynamics and influence of the sea
Signal investigated in PV experiments Charge and magnetization distributions
Static properties (μs, s) near Q2 = 0
Wide Q2 range to scan spatial distributions
G0 program : Physics case
s
Proton
gluon
uudu
u
s(ms < QCD)
The axial part in e-N scattering Tree-level + EW radiative corrections Parity-violating electromagnetic moment (FA)
Difficult to predict (multi-quarks processes) Q2 dependence
N- transition (dependence with Q2 MA) and pion production (EW rad.
corrections)
+ +
CEBAF and Hall C Polarized beam properties
Polarization 75 85 % Beam parity quality
G0 backangle (E, M separation) 2 ns beam structure, Ibeam up to 80 μA Non-standard energies (< 800 MeV) Shielding and Moeller settings
G0 experiments @ Jefferson Laboratory
G0 backward : 2 measurements scheduled in 2006 PAC 28 : common strategy with HAPPEX III (0.8 0.6 GeV2) Small Q2 acceptance : 2 different beam energies Measurements on LH2 and LD2 targets (M, A separation) mid-March to May and mid-July to December !!
Ee (MeV) Q2 (GeV2)
362 0.23
686 0.62
G0 Backward angle configuration
Particle detection and identification Turn-around of the magnet
Detection of the electrons (θe ~ 110°)
Extensive simulation of background Need for new shielding around the target cell and at the magnet exit window
Cryostat Exit Detectors (CED) to separate elastic and inelastic electrons
Cherenkov detector /e separation (both are recorded)
5 cm aerogel, n= 1.03 (OK for p < 380 MeV/c), 90% efficient for e- and rejection factor 100 From cosmic muons and test beam : 5-8 p.e. Special design for Magnetic shielding (SMS fringe field)
e- beamtarget
CED + Cherenkov
FPD
G0 beammonitorin
g
Superconducting Magnet (SMS)
Detectors (Ferris wheel)
FPD
Spokesman
Target service module
G0 in Hall C : The key elements
Detectors (Mini-Ferris wheel)CED+Cherenkov
Polarized source 85% polarization has been reached routinely using superlattice GaAs cathodes New Fiber laser for Hall C (adjustable pulse repetition rate)
Allows flexible time structure (1-2h for setting) : 32 ns used for Cherenkov study 780 nm is at polarization peak (P ~ 85%) for superlattice GaAs
Beam properties Hall C instrumentation OK 60 μA of low energy beam
New optics, beam dump and halo handled Parity quality beam properties
Adiabatic damping, PITA, RWHP, IA 35 h IN and 42 h OUT at 60 μA (LH2) Room for improvement (position feedback) Halo within a 6 mm diameter was determined to be < 0.3 x 10-6 (spec : 10-
6 )
2006 running : polarized electron beam
Beam Param. Achieved in G0
(IN-OUT)Specs
Charge asym. -0.4 ± 0.24 ppm 2 ppm
X-Y position diff. 20-24 ± 5 nm 40 nm
X-Y angle diff. -2 to -4 ± 2 nrad 4 nrad
Energy diff. 2 ± 4 eV 30 eV
Polarimetry and target
Moeller polarimeter in Hall C Energy smaller than 800 MeV (design)
Need to move quadrupoles closer to target Difficult tune (beam position, magnet settings)
Finally successful at 686 MeV 1 um foil = -86.36 +/- 0.36% (stat) 4 um foil = -85.94 +/- 0.33% (stat) Systematic error 2 %, expected to be reduced
Target and Lumi detectors LH2 and LD2 target
(“Flyswatter” and gas target for cell contribution) Target boiling test with Lumi detectors
Intensity up to 60 μA Very flat behavior (rates/beam current) Ratio LD2:LH2:12C are the ones expected
Summary of first data taking in 2006
First period of running at 682 MeV Commissioning and data taking … in a row !!
As usual a risky business and a scary/tough period !!
Many new features handled successfully Beam : low energy … but no compromise on intensity and Parity Quality New settings (polarimeter, target, …) New set-up (CED, Cherenkov, electronics …)
Analysis underway (remember this ended … 15 days ago !!)
Remained to be fixed for running in the Fall
Work/tests underway to reach 60 μA with LD2 Cherenkov (anode current and random coincidences)
Gas flow in diffusion box (CO2), gain/HV reduction, M > 2 CED-FPD (random/loss)
Use backplane scintillators of FPD counters (factor 5-10 reduction)
G0 Backward angle … What’s next in 2006
Still a long way to go … and maybe some new challenges at 362 MeV Halo issue (if due to processes in residual gas)
Test run underway this week at Jlab. May 16th : 60 μA delivered with small halo !!
Adiabatic damping More work on Moeller polarimeter
Hopefully by the end of 2006 …
+
• Careful setup procedures for the Pockels cell using “HAPPEx method” were used• Special accelerator tuning techniques to achieve “adiabatic damping” were used• Position differences and charge asymmetries were minimized using the rotating half wave plate (RHWP) and adjusting Pockels cell voltages (PITA)• Further reduction in charge asymmetries was done with IA (intensity attenuator) cell• No position feedback was done; a “slow” feedback using PITA will be implemented in next run
• Beam “halo”: The fraction of the beam outside a diameter of 6 mm was determined to be < .3 x 10-6 under good running conditions.
Parity Quality Beam During G0 Backangle Run
Uses 1560 nm seed laser and amplifier commonly used in the telecommunications industry Electrical gain-switching avoids phase lock problems experienced with earlier optically modelocked systems Second harmonic generation device yields some 780 nm light from the 1560 nm light 780 nm is at polarization peak (P ~ 85%) for superlattice GaAs
slide courtesy of M. Poelker
New Fiber-Based Laser Used During G0 Backangle Running