[d2n-analysis-talk] BB optics

[posik at jlab.org]

Hi all,

  I figured I would send out a BigBite optics update and see what you think.

  So I attached a plot of the dp/p form last week (before_mom.png) and
another from yesterday (after_mom.png) , where dp is the expected
momentum - reconstructed momentum and p is the expected momentum.

  I define the expected momentum as the elastic scattering energy:

     expected momentum = 0.93827*Ebeam_real/(0.93827 +
Ebeam_real*(1-cos(theta)))

To correct the momentum shift, I took Xin's advice and added an overall
correction the the radius which goes into reconstructing the momentum. As
you can see with this shift our dp/p plot is alighned at 0 and has a
resolution of ~1%. This correction also fixes the ~10Mev shift seen in the
invariant mass. The 10 Mev shift can be seen in  before_res.png and the
fixed invariant mass can be seen in after_res.png, in these plots the
right shows momentum  vs. W.

I also included plots of our current vertex. The red lines are the
positions of the carbon foils.

There is still some corrections that can be done on the vertex, mostly in
the bottom BB magnet region where the field is weaker,see deviation.png,
this shows the vertex vs the bendy and you see as you move left you have a
down bending behavior.

I was wondering if our current optics situation is good enough or should I
try to correct the vertex any further?

Thanks
Matt
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