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GEp-III in Hall C

Andrew Puckett, MIT On behalf of the Jefferson Lab Hall C GEp-III Collaboration April 15, 2008. GEp-III in Hall C. e'. e. Introduction. Form Factors are fundamental properties of the nucleon Characterize nucleon response to electromagnetic probe—quark structure and dynamics

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GEp-III in Hall C

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  1. Andrew Puckett, MIT On behalf of the Jefferson Lab Hall C GEp-III Collaboration April 15, 2008 GEp-III in Hall C

  2. e' e Introduction • Form Factors are fundamental properties of the nucleon • Characterize nucleon response to electromagnetic probe—quark structure and dynamics • Two ways to measure FF: • Cross section (Rosenbluth separation) • Polarization Observables • Beam-target asymmetry • Polarization transfer • Rosenbluth and Polarization results incompatible at high momentum transfer

  3. Rosenbluth Separation Elastic scattering cross section in Born approximation: Measure cross section at fixed momentum transfer and vary scattering angle: Example Rosenbluth measurement—Qattan et al, PRL 94, 142301 (2005) (JLab Hall A).

  4. Recoil Polarization • Transferred Polarization components related to FF • Electric-magnetic FF ratio proportional to ratio of transferred polarization components • Polarization only viable way to measure the electric FF at high momentum transfer Example—Gayou et al. PRL 88, 092301 (2002), JLab Hall A

  5. Physics Motivation • Measure proton FF to highest possible Q2—onset of pQCD scaling behavior? • Constrain parametrizations of GPDs—Dirac and Pauli FF are first moments of GPDs H and E • Understanding the nucleon spin—quark orbital angular momentum • Understand the disagreement between Rosenbluth and polarization results • Knowledge of nucleon FF is a vital input in the interpretation of many experiments in nuclear and hadronic physics!

  6. Experiments E04-108 and E04-019 New experiments carried out from Oct. 2007-Jun. 2008 in Hall C at JLab—New detectors BigCal and FPP

  7. Kinematics • Production kinematics—two new high Q2 points at 7.1 and 8.5 GeV2. Precession angle near optimal 270 degrees for higest Q2 • One high Q2 point overlapping Hall A data at 5.2 GeV2—spin precession angle chosen near 180 degrees to study systematics of spin transport • GEp-2g measures three different epsilon values at fixed Q2 with high precision, investigate TPEX contribution to elastic ep scattering

  8. FPP • Two polarimeters in series—improves efficiency • Each CH2 Analyzer followed by pair of drift chambers • Each chamber pair contains a total of six planes—2 X, 2 U, 2 V • 2 cm between sense wires—size of drift cell • Both polarimeters supported on one rigid frame

  9. Polarimetry-FPP • HMS measures proton momentum and rotates target Pt, Pl into focal plane Pn, Pt components which are measured by FPP—also defines incident track for FPP • FPP measures focal plane asymmetry • Spin transport matrix calculated event by event—dominant source of systematic uncertainty • Likelihood analysis determines target polarization from FP polarization • Beam polarization and analyzing power cancel in the FF ratio—no systematic uncertainty

  10. Focal Plane Angular Asymmetries Example helicity difference and helicity sum angular distributions from the lowest-epsilon point of GEp-2g: Ebeam=1.867 GeV, ε = .15 • After optimizing tracking and alignment in software, instrumental asymmetries are found to be small • False asymmetries are still a work in progress—much room for improvement

  11. Polarimetry—Spin Precession • Nominal bend angle of HMS is 25 degrees • Nominal spin precession angle: • FPP measures Pn' and Pt' after precession in HMS magnets • Want to measure target Pl and Pt • In ideal dipole approx: • Pt' = Pt • Pn' = Pl sin χ + Pn cos χ • Pl' = Pl cos χ– Pn sin χ Generally speaking, sin χ= 0 is to be avoided since FPP is not sensitive to Pl'.

  12. BigCal • 1744 lead-glass bars 4x4x40 cm3, coupled to PMTs via optical 'cookies' • ADC of each PMT, TDC of sums of 8 • Overlapping sums of 64 with high threshold for trigger • 4” Al absorber in front, protects against rad. damage, degrades resolution • Position resolution ~5 mm • 135 msr acceptance at 4.35 m from target • Set at a distance to match acceptances of electron and proton arm

  13. BigCal Kinematic correlation crucial to suppress inelastic backgrounds, especially at high Q2

  14. Summary—Preliminary Results • Initial analysis of the data shows strikingly good agreement with the Hall A polarization data. • The large error bar at Q2 = 5.2 GeV2 is a result of the unfavorable precession angle—deliberately chosen • New detectors work as advertised • Data taking for Q2=8.5 GeV2 is ongoing PRELIMINARY Will spend roughly last 2 weeks of allotted time at 7.1 GeV2

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