1. Pb-Parity and Septum Update Presented by: Luis Mercado UMass - Amherst 12/05/2008 Thanks to Robert Michaels, Kent Pachke, Krishna Kumar, Dustin McNulty (UMASS) and the rest of the HAPPEx/PREx Collaboration for contributions to this talk.

2. Outline Introduction to PREx Review: PREx Scientific Case and motivation. Experimental Challenges. Beam test results and ongoing developments. New Septum Magnet and FOM Changes. Summary

3. Introduction – What is PREx? PREx is a Parity Violation experiment to be conducted in Hall A in 2010. Will measure PV electroweak asymmetry in elastic scattering of polarized electrons from 208 Lead. Because the Z boson couples mainly to neutrons, A LR can provide a clean measurement of R n by studying the ratio of proton and neutron form factors.

4. Motivation Proton form factor is well known, so we can extract the neutron density distribution from the neutron form factor. Measuring R n will be a fundamental test of nuclear theory. PREx aims to achieve an accuracy of 1% or less. (C. J. Horowitz)

5. Experimental Challenges Precision measurement of Q 2 – Requires high accuracy of BCMs and spectrometer angle. Precision beam polarimeter – Upgrade Compton to reach 1% error. Need sub-100 ppm pulse-to-pulse electronics noise – Can achieve this with improved Luminosity Monitors and new 18-bit ADCs.

6. Improved Monte Carlo Uses ROOT, written in C++, based on LeRose’s model of HRS transport. Some goals of new MC: – EM corrections – Syst errors: analyzing power, backgrounds, etc – Sensitivity to position, angle, energy – Determine optimal detector geometry – Strategies for transverse asymmetries – Model dependence of R n – Target optimization

7. PREx Detectors Designed by UMass and Smith College. Successfully tested in January Test Run Two were tested: “Thin” and “Thick” Small 4x4 cm quartz in Focal Plane

9. New Detector Concept We are proposing to build a new detector pair (2 x 5 cm) – 23% and 27% RMS agreed well with simulation. – Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized Al or Silver. Design and fabrication in Spring ‘09 – Monte Carlo optimization of light guides. – Detailed understanding of spectrometer optics. – Final simulation of lead target spectrum in focal plane.

10. Cavity Position and Current Monitors Bigbite 8.8 kHz uA deflected beam Target Cavity X,Y,Q Stripline X,Y Steering Coils

11. Cavity Position and Current Monitors Good position & current for 10 nA to 100 uA. – At 10 nA : Need absolute position to 0.2 mm Need resolution of ~ 0.5 mm (1-shot at 200 nsec) HRS rates ~ 200 kHz  good for Q 2 Resolution Analysis – Use 1 BPM as a measure of beam motion on the other. – Achieved 150um above 1uA and 450um at 50nA. John Musson’s group is working on electronics to reach 0.1nA and improve noise and ease of use. – Should be available Summer of 2009

12. Tests of Compton Integrating DAQ Upgrades to laser cavity and photon detector are required. Integration reduces possible errors. Must have good linearity with respect to photon energy. New GSO photon calorimeter studied at HIGS on October 13,2008. Data Analysis is under way.

14. Analysis of ’08 Test Run Used as diagnostic for experimental setup. Statistics for 4-Lumi combo is ~50 ppm When averaging Lumis, width does not scale like 1/sqrt(N). Shielding Essential. Conclusion: Excess 100ppm from BPMs. Lumi IDReg. RMS [ppm] V55.7 H62.8 D1149.6 D2451.2 C51.7 X241.1 Sum126.7 Ave126.3 H D1 V D2

15. Tests of 18-bit ADCs The latest boards showed acceptable noise floor (~8 ppm with no inputs). Need to verify these results. Qweak ADC noise levels have been verified in a realistic scenario. “In situ” tests would be highly recommended.

16. New Septum Magnet Designed by Paul Brindza and Al Gavalya New values for scattering angle (5°) and optimal energy (1.05 GeV with a tolerance of 50 MeV) improve figure-of-merit. Higher E b helps with Compton polarimetry. Will be built early 2009.

17. New Septum Magnet

18. New Septum Magnet (FOM)

19. Summary / Future Developments New detector design to be produced by Spring ’09. Cavity monitors were tested successfully and new electronics will be ready by Summer ’09. HIGS data analysis under way. Developments in reducing electronics noise have been very successful. New Septum design improves FOM

20. Lead/Diamond Target Beam Current in proposal is 50uA. Target successfully tested up to 100uA. Main improvement: good contact between foils.