Double pion background study and near target collimator design

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Double pion background study and near target
collimator design

• Xin Qian
• Duke University

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Double pion background study

• Dipangkar Dutta give an estimation using
  photo-pion production model.
• Not a problem, large systematic uncertainties.
• Peter Bosted looked into Hall B data.
• Is a problem, not enough statistics, other
  issues.
• Hall C Semi-Sane test run data
• On the edge, direct comparison, different angle
  for hadron arm.

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Double pion background study
Large systematics
From Dipangkar Dutta
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Double pion background study

• Hall B eg1b data
• From Peter Bosted Pion/e ratio within coincident
  timing window 30-501 at exact transversity
  condition.

All kinds of complication trigger, different
shower Counter configuration, not enough
statistics etc.
Is a problem with 1001 pion rejection
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Double pion background study

• Transversity conditions
• 5.7 GeV/c
• BigiBite 30 degree, momentum bite 0.6 GeV/c -
  2.0 GeV/c
• HRS 16 degree 2.4 GeV/c - 5

• Semi-Sane test run
• 5.76 GeV/c
• SOS 28 degree, momentum bite - 20, three
  momentum setting 0.9, 1.23 and 1.7 GeV/c
• HMS 10.8 degree, central momentum 2.7 GeV/c -
  10

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Double pion background study

• Acceptance cut, random coincident subtraction.
• Using gas Cerenkov to select electron and pion.
• Test shower (shower pre-shower) counter
  pion/rejection ability.

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Double pion background study

• Shower counter pion rejection with gas Cerenkov.
• Electron number of photo-electron gt 2.5
• Pion number of photo-electron lt 0.5

all
electrons
all
pions
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Double pion background study
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Double pion background study

• Pion contamination become smaller with increasing
  electron arm momentum.
• Pion contamination become larger with decreasing
  hadron arm momentum
• Hadron arm angle dependence is not clear

Pion contamination pions passed the shower cut
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Conclusion for two-pion background

• If neglecting hadron arm angle difference, pion
  contamination will be as large as 20 for the
  lowest x bin in transversity experiment.
• The angle dependence is not completely clear from
  this study.
• We need a gas Cerenkov for the electron
  identification.

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Near target Collimator Design

• Why we need near target collimator?
• Exclude window wall contribution.
• Near target collimator can also help to reduce
  the low energy background, enhance the beam
  current.
• Near target collimator will reduce the acceptance
• Even without the collimator (hardware cut), still
  need to exclude window wall contribution by
  software cut. Software cut will also reduce the
  acceptance (from JP) .

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Near target Collmator Design

• Using GEANT3 simulation to study the background
  rates with collimator.
• This simulation has been successful in many
  comparison with data (including GEN wire chamber
  rates).
• The model we used in the simulation is the one
  which we used to provide background rates
  estimation for transversity experiment.

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Near target Collimator Design
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Near target Collimator Design

• Shape
• height 20 cm
• angle 30 degree same as BigBite.
• Thickness can be changed.
• Length 10 cm (along beam direction).

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Near Target Collimator Design

• Acceptance cut
• 6-7 cm
• Assuming 30 degree particle.
• Completely shield target window.

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Near Target Collimator Design

• Rates on first wire chamber
• No collimator 21.3 - 2.16 MHz
• 2 cm thick (4 cm along 30 degree) 20.4 - 2
  MHz.
• 4 cm thick (8 cm along 30 degree) 13.64 - 1.67
  MHz

• Running more
• 3 cm thick
• 5 cm thick
• 6 cm thick

Acceptance cut 6-7 cm out of 40 cm.
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Conclusion

• Collimator can help reduce background from target
  window.
• 4 cm thick seems to be most useful, still need
  further study to confirm.
• With 13 MHz on the first chamber, we can enhance
  the beam current from 10 uA to 15 uA.

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Acknowledgement

• Double pion background
• Xiaodong
• Peter
• Dipangkar
• Collimator design
• JP