References - Bogoliubov Laboratory of Theoretical Physics - JINR

(a) (b)
Figure 3: (a) Selection of elastic events using elliptical (Δx, Δy) cutforQ 2 =8.5GeV 2 .
(b) Δp spectrum for Q 2 =8.5GeV 2 after Δp − Δθ and (Δx, Δy) cut.
With significant background, kinematic correlation with the scattered electron becomes
crucial for elastic ep scattering. A drastic background reduction is achieved by
calculating the electron impact point (x, y) in BigCal from the proton momentum, angle
and the beam energy assuming two body ep scattering, and comparing it with the observed
impact point. Elastic events are found at (Δx, Δy) =(0, 0) as shown in figure 3a.
Figure 3b shows the momentum spectrum after Δp and (Δx, Δy) cuts.
Figure 4 shows the variation
of the extracted form factor ratio
as a function of the estimated
background within the cuts for the
Q 2 of 8.5 GeV 2 point. For this
setting, the background increases
much more rapidly as a function
of the cut width, which is apparent
in the rapid decrease of the uncorrected
form factor ratio. The corrected
form factor ratio in this case
is still independent of the background
correction up to statistical
fluctuations in the polarization of
the extra events accepted by wider
cuts and the uncertainty in the
background polarization used for
Figure 4: Cut width dependence of the extracted form factor
ratio, at Q 2 =8.5 GeV 2 .
the correction, demonstrating that the background subtraction is handled correctly.
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