[G8b_run] g8 meeting this week

[Michael Dugger]

Brian,

The phase space occupancy is going to be very different for the two 
reactions (3pi p and 2pi p). Especially at threshold.

One thing you can do to mitigate the differences in the thrown 
distributions is to make efficiency plots:
1) MC efficiency: for the p, pi+, and pi- look at the efficiencies the 
typical way (N_seen/N_thrown)
2) Empirical efficiency example: Form a set of events with pi+ pi- X, 
where X has the mass near that of the proton, and call these events 
N_thrownEmp. Out of the N_thrownEmp set, count how many times you saw the 
proton, and call these events N_seenEmp. Create the empirical efficiency 
as N_seenEmp/N_thrownEmp.
3) Compare the MC to the empirical efficiencies.

Note: To get the best results for the empirical efficiency you should fit 
the MassX distribution for the pi+ pi- X reaction and take the background 
subtracted yield as N_ThrownEmp. However, while it is best to strip the 
background, you can get a decent estimate of the empirical efficiency 
without the background stripping. It should be good enough to see the big 
dips.

Just my 2 cents.

Take care,
Michael


On Tue, 1 Nov 2011, Brian Vernarsky wrote:

> Eugene,
>
> Yes the momentum distribution is quite different in the MC vs the
> data, which concerned me as well, however when I look at the momentum
> distribution in the g11a MC I see the same type of spectrum so I have
> just assumed that it is normal.  I suspect that it has to do with the
> fact that the data contains all p,pi+,pi- events whereas the MC was
> thrown as p,omega->p,pi+,pi-(pi0).  That still doesn't explain it
> completely, but again since g11's MC looks the same I have assumed
> that part is correct.  What I find interesting in those plots however
> is the fact that the g11 data is quite smooth whereas the g8b data has
> so many holes, especially towards the higher cos(theta) values.
>
> I have checked that the events produced are correct events, they check
> out just fine.  We have also fed the events through g11a's gsim and
> they come out fine.
>
> We did consider the torus field because there is clearly a difference
> between the positive and negative particles.  The torus current was
> only 8 A different from g11a however and was found to be input
> correctly.  To be sure though we wildly varied the current and saw a
> change, however not significant enough to be our problem, but it was
> enough to prove that the current we were inputting was being read
> properly.
>
> I have not yet checked event by event to ensure the thrown and
> reconstucted momenta are the same however.  I will look into it and
> see what I can find.
>
> Thanks,
>
> Brian
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