[d2n-analysis-talk] SAMC and Acceptance Cut Studies

[David Flay]

On Wed, Apr 20, 2011 at 2:56 PM, Brad Sawatzky <brads at jlab.org> wrote:

> Hi David,
>
> I ran the acceptance losses at each aperture by John LeRose (slide 4),
> and they basically make sense.  Q1 rolls the distributions off at the
> edges, the dipole entrance has a small effect, the dipole _exit_ has a
> large effect, and Q3 has a fairly small effect -- all pretty consistent
> with what you show.
>
> Assuming reactz_gen is what we usually call z_targ (ie. the target
> length that the HRS can see), then that looks OK too.  Nominal
> acceptance is +-6 cm at 90deg, or about +-8.5cm at 45deg.
>
> I don't understand the losses for very tight cuts shown on slide 6
> though.  If you point a beam of particles right along the central axis
> of the spectrometer, and with p = p_0, then 100% should pass through to
> the focal plane....   Is this some straggling effect due to scatters
> between the generation point and the Q1 vacuum window?  What happens if
> everything is vacuum (no windows, no glass target cell, etc)?
>

On slide 6, it shows that the losses aren't occurring until the dipole exit,
not Q1 -- maybe I'm not following concerning the straggling effect before Q1
in this case?


>
> If I understand how SIMC/SAMC works, then it is essentially a ray tracer
> once the particle reaches Q1.  So "statistical errors" don't apply (at
> least the won't follow a gaussian counting statistics).  If it's a
> cross-section weighted random walk from the generation point, through
> the target cell, air/He4, etc up to Q1, then you'll get some run-to-run
> variation in what makes it because you're "rolling the dice" in
> mean-free-path steps propagating the particle up to the Q1 entrance
> window.  A simple counting-statistics uncertainty model still isn't
> valid though.
>
> I think what you should be doing is tightly constraining the generated
> particle kinematic distribution (ie. the black lines on slide 4).
>

Do you mean slide 5? (Aperture Cut Study slide 2)

 - narrow those up so you're producing a mono-energetic beam pointing
>   into the middle of Q1
>  - disable (set to vacuum) the cell walls, air, etc so there is no
>   scattering
> Then every single particle had better make it to the center of the HRS
> acceptance.  Then 'turn on' the cell walls, what happens.  Turn on the
> air (he4), what happens?
>

I'll get this running.


>
> I'm not sure I completely how the sigma_i is computed for the
> cross-section weighting you describe on slide 11+, but the 'weighted'
> distribution certainly show better agreement.  Is that something built
> into SAMC, or does the input come from another program?


The cross-section is computed for each event in SAMC, using the methodology
laid out in  Phys. Rev. D 12, 1884 (1975) for the radiative corrections.  So
there is no input from external programs.

I determine the weight as follows:

1. Determine the average cross section for the run: Plot the 1-D histogram
of the cross section (see attached).  The mean of this histogram I take as
the 'average'.  The biggest contribution to this curve is due to Eq. A83 of
that paper I mentioned, which is the calculation of the radiative tail
associated with quasi-elastic scattering using the peaking approximation.

2. When I make these plots (of the target and focal plane vars), I loop over
event number, and get the cross section that was calculated for that event,
and take the ratio sig_i/sig_avg as its weight when I fill each respective
histogram.






-- 
-----------------------------------------------------------
David Flay
Physics Department
Temple University
Philadelphia, PA 19122

office: Barton Hall, BA319
phone: (215) 204-1331

e-mail: flay at jlab.org
           flay at temple.edu

website: http://www.jlab.org/~flay
             http://quarks.temple.edu
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