Backward angle detectors
for the
Precision measurement of
the proton electric to
magnetic form factor ratio
experiment
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Introduction
Theory
Kinematics and BLAST geometry
Installation plan
Conclusions
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Introduction:
 These detectors will be used in the proton
form factor ratio experiment Spokespersons:
H. Gao, J.R. Calarco, H. Kolster
 For the highest Q2 measurement (0.9 GeV2)
the electrons are elastically scattered at large
angles outside of the acceptance of BLAST
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 For analysis, the scattering angle and
kinematics for an event comes from the
tracking of the proton
 Extra detectors may be needed at backward
angles for the electron to reduce background,
the detectors will be arranged for elastic
scattering
 These detectors will be fast scintillators
(BC408 2.5cm thick), the same as the existing
TOF detectors
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Theory:
 For the proposed 0.88 GeV beam energy, the
proton is scattered with momentum 1.1 GeV
 A source of background is  p   p, and this
has the same kinematics as elastic electron
scattering giving pp =1.1 GeV, however,
1. The flux of the 880MeV photon
bremsstrahlung in the beam is small
compared to electrons
2. The cross section for this scattering is small
3. The probability of generating a signal in the
scintillator is low (mean free path of
400MeV photons  60cm)
 In addition to requiring coincidence, the
backward angle detectors can further suppress
background by using timing information
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Time(TOF
p
TOF
e )
 For particles with momentum 1.1 GeV, the
flight time separation from the interaction
point to the forward TOF’s will be
Particle Flight time (ns)
12.5

p
16.3
 It will only be possible to separate these
particles using flight time information if the
backward angle detectors are installed
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 The backward angle detectors will have a top
and bottom PMT, the same as the TOF’s, and
the time difference will indicate the position of
the electron to within approximately 10cm
 During analysis, this may be a useful as check
of the backgrounds
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Kinematics and BLAST geometry:
 The simulations for the proposal were
performed with wire chamber supports
assumed to protrude 6.5cm past the inner wire
chambers
 This distance was 44cm for the outer wire
chambers
 The scintillators will need to have a width of
50-60cm (approx 4% of the events will be lost
for 50cm)
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Installation plan:
Three options (so far)
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Electronics required (John):
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Conclusions:
 The backward angle detectors can suppress
background due geometry (requiring
coincidence), flight time information (TOFpTOFe) and position information (PMT2-PMT1)
 The backward angle detectors will be
positioned to detect particles that have
scattered 6.5cm past the inner wire chambers
to allow for supports/gas pipes
 The scintillator array will need to cover a
distance of about 50-60 cm
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Pion background:
 A + with the same momentum will be
indistinguishable from the proton and will be a
source of background
 The + background will come from the
reaction with bremsstrahlung contamination
 p    n . From energy conservation
E  M p  E  En
 E
MAX
 E
 p
MAX
 869 MeV
 From 4-momentum conservation, the
momentum is lower p= 826 MeV
 This should be clearly distinguishable from
the proton
2)
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