<br><br><div class="gmail_quote">On Wed, Apr 20, 2011 at 2:56 PM, Brad Sawatzky <<a href="mailto:brads@jlab.org">brads@jlab.org</a>> wrote:<br><blockquote class="gmail_quote">
Hi David,<br>
<br>
I ran the acceptance losses at each aperture by John LeRose (slide 4),<br>
and they basically make sense. Q1 rolls the distributions off at the<br>
edges, the dipole entrance has a small effect, the dipole _exit_ has a<br>
large effect, and Q3 has a fairly small effect -- all pretty consistent<br>
with what you show.<br>
<br>
Assuming reactz_gen is what we usually call z_targ (ie. the target<br>
length that the HRS can see), then that looks OK too. Nominal<br>
acceptance is +-6 cm at 90deg, or about +-8.5cm at 45deg.<br>
<br>
I don't understand the losses for very tight cuts shown on slide 6<br>
though. If you point a beam of particles right along the central axis<br>
of the spectrometer, and with p = p_0, then 100% should pass through to<br>
the focal plane.... Is this some straggling effect due to scatters<br>
between the generation point and the Q1 vacuum window? What happens if<br>
everything is vacuum (no windows, no glass target cell, etc)?<br></blockquote><div><br></div><div>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? </div>
<div> </div><blockquote class="gmail_quote">
<br>
If I understand how SIMC/SAMC works, then it is essentially a ray tracer<br>
once the particle reaches Q1. So "statistical errors" don't apply (at<br>
least the won't follow a gaussian counting statistics). If it's a<br>
cross-section weighted random walk from the generation point, through<br>
the target cell, air/He4, etc up to Q1, then you'll get some run-to-run<br>
variation in what makes it because you're "rolling the dice" in<br>
mean-free-path steps propagating the particle up to the Q1 entrance<br>
window. A simple counting-statistics uncertainty model still isn't<br>
valid though.<br>
<br>
I think what you should be doing is tightly constraining the generated<br>
particle kinematic distribution (ie. the black lines on slide 4).<br></blockquote><div><br></div><div>Do you mean slide 5? (Aperture Cut Study slide 2) </div><div><br></div><blockquote class="gmail_quote">
- narrow those up so you're producing a mono-energetic beam pointing<br>
into the middle of Q1<br>
- disable (set to vacuum) the cell walls, air, etc so there is no<br>
scattering<br>
Then every single particle had better make it to the center of the HRS<br>
acceptance. Then 'turn on' the cell walls, what happens. Turn on the<br>
air (he4), what happens?<br></blockquote><div><br></div><div>I'll get this running.</div><div> </div><blockquote class="gmail_quote">
<br>
I'm not sure I completely how the sigma_i is computed for the<br>
cross-section weighting you describe on slide 11+, but the 'weighted'<br>
distribution certainly show better agreement. Is that something built<br>
into SAMC, or does the input come from another program?</blockquote><div><br></div><div>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. </div>
<div><br></div><div>I determine the weight as follows:</div><div><br></div><div>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.</div>
<div><br></div><div>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. </div>
<div><br></div><div><br></div><div><br></div><div> </div></div><br clear="all"><br>-- <br>-----------------------------------------------------------<br>David Flay<br>Physics Department<br>Temple University<br>Philadelphia, PA 19122 <br>
<br>office: Barton Hall, BA319<br>phone: (215) 204-1331<br><br>e-mail: <a href="mailto:flay@jlab.org">flay@jlab.org</a> <br> <a href="mailto:flay@temple.edu">flay@temple.edu</a><br>
<br>website: <a href="http://www.jlab.org/%7Eflay">http://www.jlab.org/~flay</a><br> <a href="http://quarks.temple.edu">http://quarks.temple.edu</a><br>-----------------------------------------------------------<br>
<br><br>