1. Bogdan Wojtsekhowski, Jefferson Lab Experimental search for A’ for APEX collaboration 1

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

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) ●  p/p  2∙10 -4 (achieved) ●   0.5 mrad,    1 mrad ● δy=1mm ● Luminosity ~ 10 38 cm -2 s -1 3

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

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

6. Magnetic Spectrometer Optics 6 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 7

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

9. Final Geometry Selection 9  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

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 10

11. HRS optics for APEX 11

12. HRS optics for APEX 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 BIG HOLE 12

13. Angular Resolutions 13 LHRS (mrad)RHRS (mrad) ΔφΔφ 0.10ΔφΔφ ΔθΔθ 0.24ΔθΔθ 0.20 σ φ_width 0.26σ φ_width 0.43 σ θ_width 1.81 σ θ_width 1.75 σφσφ 0.29σφσφ 0.44 σθσθ 1.86σθσθ 1.77 Optics calibration precision Tracking precision φ/θ – hor / vert angles Averages weighted according to statistics Final resolutions

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:  σ M.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, σ M.S. = 0.352 mrad 14

15. Mass Resolution 15 Mass (MeV)180195210225240Average Using different angular resolutions for each event 0.8330.9651.0261.0611.0371.005 Using angular resolutions listed in above table for all events 0.8220.9621.0231.0541.043 - Using angular resolutions from "Total" column in above table for all events 0.8690.9650.9950.9940.9660.977 Mass (MeV)180195210225240Average Left theta (mrad)1.951.871.891.931.881.86 Left phi (mrad)0.260.30.320.33 0.29 Right theta (mrad)1.691.741.811.85 1.77 Right phi (mrad)0.380.430.460.50.530.44 Mass resolutions (MeV) determined for different masses using 3 different methods Angular resolution averages (mrad) determined for different masses

16. 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. 17

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

19. 20 mm W plate 10 mm W plate Positive polarityNegative polarity 19

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 20

21. Multi foil target 21

22. 22 APEX full Run