Parity Quality Beam

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PQB reflection.jpg

A significant portion of the experiments performed at Jefferson Lab require a polarized electron beam. A subset of these are parity violation experiments which have exceptionally stringent requirements on the quality of the electron beam. As the sign of the electron beam helicity reverses at the target of the experiment no other measurable quantity of the electron beam may change. This includes the beam intensity, polarization, position, angle, energy or spot size.

To ensure this beam quality, a significant effort occurs at the polarized electron source, where the laser beam and electron photogun combine to produce the intended electron beam. Also significant is the ability to accelerate the beam to high energy while transporting it to the experimental target, all while maintaining a parity quality electron beam (PQB).



Parity Violation

Discussion of parity violation can be found at NIST: The Fall of Parity: [1]



Accelerator Setup to Run Parity Violation

An overview of how parity-violating electron scattering experiments are performed at CEBAF, P.A. Adderley et al., Nuclear Inst. and Methods in Physics Research, A 1046 (2023) 167710. [2]



Parity Violation Experiments

  1. JLab Hall A Parity Experiments (HAPPEx, PREx, C-REx, PVDIS): [3]
  2. JLab Hall C G0 Experiment
  3. JLab Hall C QWeak Experiment: [4]
  4. JLab Hall A Møller Experiment: [5]
  5. Mainz A4 Experiment



PV Experiments Dec2022.jpg
  • Table References:
  1. Measurement of the neutral weak form factors of the proton, K. A. Aniol et al., Phys. Rev. Lett. 82, 1096 (1999). [6]
  2. New Measurement of Parity Violation in Elastic Electron-Proton Scattering and Implications for Strange Form Factors. K. A. Aniol et al., Phys. Lett. B 509, 211 (2001). [7]
  3. Parity-violating Electroweak Asymmetry in e ⃗p Scattering. K. A. Aniol et al., Phys. Rev. C 69, 065501 (2004). [8]
  4. Strange-Quark Contributions to Parity-Violating Asymmetries in the Forward G0 Electron-Proton Scattering Experiment, D. S. Armstrong et al. Phys. Rev. Lett. 95, 092001 (2005). [9]
  5. Parity-Violating Electron Scattering from 4He and the Strange Electric Form Factor of the Nucleon, K. A. Aniol et al., Phys. Rev. Lett. 96, 022003 (2006). [10]
  6. Constraints on the nucleon strange form factors at Q2 ∼ 0.1 GeV2. K. A. Aniol et al., Phys. Lett. B 635, 275 (2006). [11]
  7. Precision Measurements of the Nucleon Strange Form Factors at Q2∼0.1 GeV2 , A. Acha et al., Phys. Rev. Lett. 98, 032301 (2007). [12]
  8. Strange Quark Contributions to Parity-Violating Asymmetries in the Backward Angle G0 Electron Scattering Experiment, D. Androić et al., Phys. Rev. Lett. 104, 012001 (2010). [13]
  9. New Precision Limit on the Strange Vector Form Factors of the Proton, Z. Ahmed et al., Phys. Rev. Lett. 108, 102001 (2012). [14]
  10. Measurements of Parity-Violating Asymmetries in Electron-Deuteron Scattering in the Nucleon Resonance Region. D. Wang et al., Phys. Rev. Lett. 111, 082501 (2013). [15]
  11. Measurement of parity violation in electron-quark scattering. D. Wang et al., Nature 506, 67 (2014). [16]
  12. Measurement of parity-violating asymmetry in electron-deuteron inelastic scattering. D. Wang et al., Phys. Rev. C 91, 045506 (2015). [17]
  13. Measurement of the Neutron Radius of 208Pb through Parity Violation in Electron Scattering, S. Abrahamyan et al., Phys. Rev. Lett. 108, 112502 (2012). [18]
  14. First Determination of the Weak Charge of the Proton, D. Androic et al., Phys. Rev. Lett. 111, 141803 (2013). [19]
  15. Precision measurement of the weak charge of the proton. D. Androi´c et al., Nature 557, 207 (2018). [20]
  16. Parity-violating inelastic electron-proton scattering at low Q2 above the resonance region, D. Androić et al., Phys. Rev. C 101, 055503 (2020). [21]
  17. Determination of the 27Al Neutron Distribution Radius from a Parity-Violating Electron Scattering Measurement, D. Androić et al., Phys. Rev. Lett. 128, 132501 (2022). [22]
  18. Accurate Determination of the Neutron Skin Thickness of 208Pb through Parity-Violation in Electron Scattering, D. Adhikari et al., Phys. Rev. Lett. 126, 172502 (2021). [23]
  19. Precision Determination of the Neutral Weak Form Factor of 48Ca, D. Adhikari et al., Phys. Rev. Lett. 129, 042501 (2022). [24]
  20. The MOLLER Experiment: An Ultra-Precise Measurement of the Weak Mixing Angle Using Møller Scattering, J. Benesch et al., arXiv:1411.4088. [25]
  21. SoLID (Solenoidal Large Intensity Device), Updated Preliminary Conceptual Design Report, The SoLID Collaboration, November 2019. [26]



Selected References

  • Experimental Test of Parity Conservation in Beta Decay.

C. S. Wu et al., Phys. Rev. 105 (1957) 1413 [27] media:PhysRev.105.1413.pdf

  • Parity-Violating Electron Scattering and the Electric and Magnetic Strange Form Factors of the Nucleon.

D. S. Armstrong and R. D. McKeown, Annu. Rev. Nucl. Part. 62 (2012) 337 [28] media:annurev-nucl-102010-130419.pdf

  • The Qweak experimental apparatus.

T. Allison et al., Nucl. Instr. and Meth. A 781 (2015) 105 [29] media:NuclInstrumMethA.781.105.pdf


Injector PQB Setup

  • SLAC's polarized electron source laser system and minimization of electron beam helicity correlations for the E-158 parity violation experiment.

T. B. Humensky et al., Nucl. Instr. and Meth. A 521 (2004) 261 [30] media:NuclInstrMethA.521.261.pdf

  • Helicity correlated asymmetries caused by optical imperfections.

K. Aulenbacher, Euro. Phys. J. A 32 (2007) 543 [31] media:EuroPhysJA.32.543.pdf



Mailing List

  • Accelerator Parity Quality Beam mailing list: acc_parity@jlab.org
  • Web page for users of mailing list is: [32]



MOLLER PQB


Helicity Decoder Board


Synchrotron Radiation


Collaboration Presentations

  1. Beam Monitoring: media:Moller_23Jan15_pitt.pdf


Other Presentations





PREx-II PQB




QWeak PQB

Accelerator Meetings


B-Team Presentations


Collaboration Presentations


Other Presentations


Technical Developments


  • Discussion of technical developments in preparation for the QWeak experiment:

Jefferson Lab injector development for next generation parity violation experiments. J. Grames, J. Hansknect, M. Poelker, R. Suleiman, Hyperfine Interactions 201 (2011) 69 media:HyperfineInter.201.69.pdf


Jlab source PV2.jpg


Two-Wien Slow Helicity Reversal

  • Two Wien Filter Spin Flipper. J. Grames et al., Part. Accel. Conference (PAC’11), New York, NY, (2011) media:tup025.pdf


New Helicity Board

  • Helicity Board Configuration Quick Reference:
Mode T_Settle T_Stable Helicity Pattern Reporting Delay Helicity Board Frequency (Hz)
Default (30 Hz) Free Clock 500 µs 33330 µs Quartet 8 windows 29.56
5 MeV Mott Free Clock 500 µs 33330 µs Quartet No delay 29.56
Hall A Moller Free Clock 100 µs 16667 µs Quartet 8 windows 60.0
Hall A Moller Free Clock 100 µs 8233.35 µs Quartet 8 windows 120.0
PREx Free Clock 100 µs 4066.65 µs Octet 16 windows 240.0
QWeak Free Clock 70 µs 971.65 µs Quartet 8 windows 960.0


  • The pseudo-random 30-bit Shift Register
  1. root macro is shown here (change .txt to .C): media:NewPseudoRanBitGenR.txt media:NewPseudoRanBitGenR.gif
  2. QWeak c-code:
C-code.Randbit30.jpg


New Pockels Cell HV Switch


Electronic Cross-talk & Ground Loop Elimination


Helicity Magnets Control Upgrade

  • On January 4, 2012, the power supply cards were modified to reduce the slopes during QWeak. Added 220 milli-ohm resistor in parallel with each channel of magnet and series load resistors (approx 3.8 ohms). Current through a given magnet should now be 2.5 - 3% of the total amplifier output current.
  • Details of the plan to upgrade the controls of the Helicity Magnets (to be completed before Moller experiment): media:HMUpgrade_plan_27Dec11.pdf media:HMUpgrade_plan_27Dec11.pptx




G0 PQB

  • The G0 experiment: Apparatus for parity-violating electron scattering measurements at forward and backward angles.

D. Androić et al., Nucl. Instr. and Meth. A 646 (2011) 59 [35] media:NuclInstrumMethA.646.59.pdf


Lessons Learned

Lessons learned for parity violation experiments at Jefferson Laboratory is an on-going series of meetings between scientists and students from the accelerator and physics division to learn from past experiences and meet new challenges in the field of parity violation experiments:


  • Supperlattice SVT4722#2 in Gun3, January 2005:
  1. G0 beam studies summary to the MCC 8 AM meeting on February 1, 2005: media:svt4722-2_8am_050201.ppt


  • Parity Beam Quality Specifications:
  1. Specifications for Beam Parameters for HAPPEX-II and HAPPEX-He from the HAPPEX Collaboration: media:HAPPEX_Beam_Requirements.ps
  2. Beam Parameter Specifications for the G0 Experiment from the G0 Collaboration: media:g0_beamspec.ps
  3. Preliminary Beam Specification Parameters for the Qweak Experiment from Roger Carlini: media:Qweak_Beam_Parameters_Prem.pdf media:Qweak_Beam_Parameters_Prem.doc


  1. Phase Trombone for HAPPEX-2 from Alex: media:Phase_Tromb_HAPPEX_2004.pdf
  2. Parity Lessons Learned from Chao: media:Parity_Lessons_Learned_chao_2004.pdf
  3. Parity Quality Beam Development and Experiment Timeline from Matt: media:Parity_Exp_Timeline_Poelker_2004.doc
  4. Aligning the Pockels cell - the HAPPEX procedure: media:Aligning_the_pockels_cell_2004.doc
  5. Comments about Helicity Effects for G0 and HAPPEX from Mark: media:pitt_parity_lessons_learned_meeting_2004.pdf media:pitt_parity_lessons_learned_meeting_2004.ppt
  6. Injector Setup for G0 and HAPPEX from Reza: media:lessons_learned_G0_and_HAPPEx_reza_2004.ppt


  1. G0 slide regarding beam loading from Kaz: media:beam_loading_kaz_2004.pdf media:beam_loading_kaz_2004.ppt
  2. Augmented version of slides from Mark for Part 1 meeting: media:pitt_parity_lessons_learned_meeting_dec_09_04.pdf media:pitt_parity_lessons_learned_meeting_dec_09_04.ppt
  3. HAPPEX presentation from Kent: media:HAPPEX_LessonsLearned_kent_2004.pdf media:HAPPEX_LessonsLearned_kent_2004.ppt
  4. Here is Chao's report on recent beam studies: media:Parity_meeting_120904_chao.pdf


  • Lessons Learned Part 3 January 13, 2005:
  1. Matt's meeting outline: media:Parity_Tests 1_05_matt.doc
  2. Chao showed injector matching results from December: media:Test_Result_122004_chao.pdf
  3. Riad's summary of Injector Studies Using Superlattice Photocathode, January 13, 2005: media:inj_studies_011005_riad.ppt


G0 Backward Angle Run 2006

  • G0 Accelerator Planning Meeting, November 1, 2005:
  1. Introduction & Proposed Beam Specifications, Grames: media:G0Planning_051101_grames.ppt
  2. Beam Halo Considerations (from Beam Specs document), Pitt: media:beam_halo_pitt_051101.ppt
  3. Beam Halo Forward Angle Example, Pitt: media:g0_halo_profile_pitt_051101.pdf
  4. Proposed Accelerator Configuration, Freyberger: media:G0_prep_freyberger_051101.pdf
  5. Source & Beam Issues, Suleiman: media:G0_acc_suleiman_051101.ppt
  6. Injector Matching, Chao: media:G0_Preparation_chao_051101.pdf
  7. Engineering Issues, Lauze: media:EEConcerns_lauze_051101.doc



  • Summary of G0 Pockels Cell Studies:
  1. Presentation in January 2006: media:pc_install_beam_studies_jan_2006.pdf media:pc_install_beam_studies_jan_2006.ppt
  2. Presentation in March 2006: media:pc_install_beam_studies_march_2006.pdf media:pc_install_beam_studies_march_2006.ppt
  3. Presentation in July 2006: media:pc_install_beam_studies_july_2006.pdf media:pc_install_beam_studies_july_2006.ppt
  4. Presentation in September 2006: media:pc_install_beam_studies_sept_2006.pdf media:pc_install_beam_studies_sept_2006.ppt
  5. Presentation in January 2007: media:pc_install_beam_studies_jan_2007.pdf media:pc_install_beam_studies_jan_2007.ppt



Other Presentations and Tech Notes


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