[G12] Lambda K+ c-s update

[Johann Goetz]

Hi g12,
I have updated the diff c-s analysis for g p --> Lambda K+ on the wiki page
here<http://clasweb.jlab.org/rungroups/g12/wiki/index.php/G12_Xi_Paper_-_Lambda_Cross_Sections>.
The pertinent plot is here: Lambda K+ diff c-s
plot<http://clasweb.jlab.org/rungroups/g12/wiki/images/b/b2/Goetz_2012.09.13_lambda_dcs_1d.png>where
I have overlayed McCracken's data (orange area) on my own points
(green errorbars). All major over-all scaling factors I used to get this
results are discussed on the wiki page. Previous plots I showed did not
have a factor (1/0.64) for the BR of Lambda --> p pi-; this is now
included. However, there are factors still missing/unaccounted for --
listed here in possible order of effect on result:
  1. I did not take into account Lambda decaying outside the start counter.
      effect on diff c-s: about +15%? (cos(theta) and sqrt(s) dependent)
  2. The reconstruction inefficiency (or trigger, or whatever you want to
call this) of 16% is contrived and taken directly from g11a analyses. I
believe this value may have been for two-track events (not three which is
what I have here since I *require* the K+, p and pi- to all have a ST hit).
      effect on diff c-s: +/- 5-7%
  3. The trigger boundary at around sqrt(s) = 2.75 GeV needs to be
addressed. This should either increase acceptance below the boundary or
decrease acceptance above. This might cancel out 1. above for events below.

My current plan is to investigate 1, then 2, then 3 above.
--
Johann.
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