Experimental search for A ’

1. Experimental search forA’ Bogdan Wojtsekhowski, Jefferson Lab forAPEX collaboration

2. Approach: Aʹ Production and Background Kinematics Nucleus Production diagrams analogous to photon bremsstrahlung QED Backgrounds γ* N~αʹx branching N~α O(1) (rates before angular cuts) Aʹ products carry full beam energy! – Distinctive kinematics – Assists in background suppression Best kinematics to select events for Aʹ search

3. Hall A at Jefferson Lab • Two HRS Spectrometers • 0.3 < p < 4.0 GeV/c • -4.5% < Δp/p < 4.5% • 6 msr at 12.5° <θ<150° • 4.5 msr at θ=6° with septum • -5cm<Δy<5cm • Optics: (FWHM) • dp/p 2∙10-4 (achieved) • d=0.5 mrad, d=1 mrad • δy=1mm • Luminosity ~ 1038 cm-2s-1

4. Test Run: June 2010 Ebeam= 2.262 GeV, Ta target of 15 mg/cm2 • Signal dominated at E+ = E– = Ebeam/2 • Use septa to achieve 5° central angles (enhance e+e– rate relative to π rate) 4

5. Test Run: June 2010 e+ momentum versus e- momentum momentum sum of coincident e+e

6. Magnetic Spectrometer Optics Top view

7. Optics for APEX • Use tracking information from VDC • 2D hit position / 2D angle • Reconstruct target side • Small acceptance, large size  Fine res. • Momentum σ~0.5 × 10-3 • Angle σ~0.5 mrad (H) , 1 mrad (V) • + Multiple scattering in target (~0.4 mrad on angles) • Uncertainty contribution • Tracking precision • Optics calibration precision

8. Sieve Slit Method Before Calibration Sieve V. Pos [m] After Calibration Sieve V. Pos [m] Left HRS calibration used 35 holes, Right HRS calibration used 38 holes Sieve H. Pos [m]

9. Final Geometry Selection • 3D momentum acceptance cuts for both HRSs • Represents region for which optics was calibrated • Approximately represents acceptance • PREX collimators • Reduced solid angle acceptance from 4.3 msr to 2.8 msr • Full running  larger acceptance Sieve

10. Angular Resolutions • Use sieve data to determine angular resolution • 1D x and y projections near each sieve hole • Fitted with Gaussian on linear background • Peak positions and RMS values compared with surveyed positions and geometrical widths of holes

11. HRS optics for APEX

12. HRS optics for APEX BIG HOLE Big hole: diameter 0.105” ; observed sigma ~ 0.66 mm approx. due to the hole size Small hole: diameter 0.055” ; observed sigma ~ 0.40 mm -> angular resolution Angular resolution (horizontal, at the hole) < 0.3 mrad

13. Angular Resolutions Averages weighted according to statistics Optics calibration precision Tracking precision Final resolutions φ/θ – hor / vert angles

14. Target Multi-Scattering • When traveling through the target, e-and e+ experience many small angle deflections due to Coulumb scattering from nuclei • Resulting angular distribution: • σM.S. = (13.6 MeV/p) sqrt(x/X0) [1+0.038ln(x/X0)] • p = particle momentum • x = avg. dist. traveled through target • X0 = radiation length of target • For Tantalum target used in test run, σM.S. = 0.352 mrad

15. Mass Resolution Angular resolution averages (mrad) determined for different masses Mass resolutions (MeV) determined for different masses using 3 different methods

16. Test Run Results APEX test run found no evidence for A’ in the mass range 175-250 MeV with the coupling above ~10-6

17. PRL paper Re: LH13073 Search for a new gauge boson in electron-nucleus fixed target scattering by the APEX experiment by S.Abrahamyan, Z. Ahmed, K. Allada, et al. Dear Dr. Wojtsekhowski, We are pleased to inform you that your manuscript has been accepted for publication as a Letter in Physical Review Letters.

18. MC of HRS sieve slit Sieve slit image for 5 and 10 mm thick plates

19. MC of HRS sieve slit Positive polarity Negative polarity 10 mm W plate 20 mm W plate

20. HRS optics • Active “sieve slit”: tagging by • a Sci Fiber detector • 1 mm fibers with 1/16” pitch connected to a maPMT • Readout to via 1877s TDC • 1-3 MHz rate per fiber • Off-line time window of < 5 ns • All components are available

21. Multi foil target

22. APEX full Run