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Bogdan Wojtsekhowski, Jefferson Lab

Experimental search for A’

for APEX collaboration

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Approach: Aʹ Production and Background Kinematics

Production diagrams analogous to photon bremsstrahlung

Nucleus

QED Backgrounds

Aʹ products carry full beam energy!

γ*

– Distinctive kinematics– Assists in background suppression

(rates before angular cuts)

N~ αʹ x branching N~α

Best kinematics to select events for Aʹ search

O(1)

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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

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• Signal dominated at E+ = E– = Ebeam/2

• Use septa to achieve 5° central angles

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(enhance e+e– rate relative to π rate)

Test Run: June 2010

Ebeam= 2.262 GeV, Ta target of 15 mg/cm2

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Test Run: June 2010

momentum sum of

coincident e+e

e+ momentumversus

e- momentum

Magnetic Spectrometer Optics

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Top view

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

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Sieve Slit Method

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After Calibration

Sieve H. Pos [m]

Siev

e V.

Pos

[m]

Before Calibration

Siev

e V.

Pos

[m]

Left HRS calibration used 35 holes, Right HRS calibration used 38 holes

Final Geometry Selection

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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

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

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x (mm)910111213141516

0

20

40

60

80

100

120

peak: 12 .169 +/- 0.012s igma : 0.424 +/- 0 .008

x (3,5)

y (mm)681012141618202224

0

20

40

60

80

100

peak: 14 .669 +/- 0.042s igma: 1.444 +/- 0.032

L e ft S ie ve R un 1898-1899

y (3,5)

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HRS optics for APEX

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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

Angular Resolutions

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LHRS (mrad)

RHRS (mrad)

Δφ 0.10 Δφ 0.10Δθ 0.24 Δθ 0.20

σφ_width 0.26 σφ_width 0.43σθ_width 1.81   σθ_width 1.75

σφ 0.29 σφ 0.44σθ 1.86 σθ 1.77

Optics calibration precisionTracking precision

φ/θ – hor / vert angles

Averages weighted according to statistics

Final resolutions

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

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Mass Resolution

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Mass (MeV) 180 195 210 225 240 AverageUsing different angular resolutions for each event 0.833 0.965 1.026 1.061 1.037 1.005Using angular resolutions listed in above table for all events 0.822 0.962 1.023 1.054 1.043 -Using angular resolutions from "Total" column in above table for all events 0.869 0.965 0.995 0.994 0.966 0.977

Mass (MeV) 180 195 210 225 240 AverageLeft theta (mrad) 1.95 1.87 1.89 1.93 1.88 1.86Left phi (mrad) 0.26 0.3 0.32 0.33 0.33 0.29Right theta (mrad) 1.69 1.74 1.81 1.85 1.85 1.77Right phi (mrad) 0.38 0.43 0.46 0.5 0.53 0.44

Mass resolutions (MeV) determined for different masses using 3 different methods

Angular resolution averages (mrad) determined for different masses

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Test Run Results

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

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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.

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Sieve slit image for 5 and 10 mm thick plates

MC of HRS sieve slit

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MC of HRS sieve slit

20 mm W plate

10 mm W plate

Positive polarity Negative polarity

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HRS opticsActive “sieve slit”: tagging by

a Sci Fiber detector1 mm fibers with 1/16” pitch

connected to a maPMTReadout to via 1877s TDC 1-3 MHz rate per fiberOff-line time window of < 5 nsAll components are available

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Multi foil target

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APEX full Run