Re: comment on simulation section

[Matthew Shepherd <mashephe@indiana.edu>]

Hi Elton,

Thanks much for the comments.  I have also earlier this evening talked  
with Elke on the phone about her comments and we agreed on places  
where clarity can be improved.  In making revisions, I will try to  
incorporate your other comments on the text.

I want to quickly try to address a few of the substantial ones:

> page 11, bottom line, vertex errors
> Assuming the vertex position error is the 'size of the target  
> (30cm)' for
> reconstruction seems to be very pessimistic. Does this uncertainty  
> in the
> vertex position influence all the invariant mass plots? Should it be
> updated (even artificially) by assuming a charged particle z-position
> resolutions?

What matters for the sake of the invariant mass plots is whether the  
true vertices are spread in z throughout the target.  Maybe Mihajlo  
can provide a quantitative answer to how much, if any, broadening of  
the peak, gets induced by the spread in actual vertex position.

To clarify what we did:  In reconstruction all vertices are assumed to  
originate from the center of the target (of course this will be  
refined once an event vertex can be better determined from charged  
particles).  The uncertainty assumptions are needed to construct the  
error matrix which is important for kinematically fitting the the  
event -- here we went with what was the actual uncertainty currently  
in the simulation, i.e. the physical size of the target.

> page 15, figure 11, question
> Do we understand why the resolutions in the FCAL are much better  
> than the
> resolutions in the BCAL? In the introduction Alex plots pi0 and eta  
> widths
> and there seems to be little difference between FCAL and BCAL. What is
> different? If we know this we should add a note to the text. (Could  
> it be
> the vertex resolution? see above).

This is a fascinating question that we picked up on late last week and  
Mihajlo has worked on to sort out.  We think this is actually just due  
to the kinematic "bias" for the types of events FCAL and BCAL accept.   
While both have comparable energy resolution, and Alex has shown this  
is the key factor in determining pi^0 and eta width, the BCAL is  
biased towards catching softer more forward particles.  In that front  
part of the BCAL leakage is an issue and the energy resolution in  
probably worse.  The FCAL typically gets nice high energy photons that  
can be well measured.

The term that drives the mass resolution goes like:  A*sqrt( 1/E1 + 1/ 
E2 ) where E1 and E2 are photon energies and A is the statistical term  
in energy resolution (the number that is roughly the same for BCAL and  
FCAL).  For this eta pi^0 channel Mihajlo made a plot of this term in  
the BCAL and FCAL for etas and pi0s:

http://dustbunny.physics.indiana.edu/~mikornic/GlueX/CalRew08/sigmaM_Afactor_pi0.eps
http://dustbunny.physics.indiana.edu/~mikornic/GlueX/CalRew08/sigmaM_Afactor_eta.eps

In both cases the BCAL with comparable energy resolution to FCAL is  
biased towards more poorly measured pi^0's and eta^'s.  So, I think  
the interesting bottom line is that one doesn't get comparable eta and  
pi^0 widths in the BCAL and FCAL by simply matching their average  
energy resolutions.  One has to consider several other things like:   
position dependence of energy resolution in BCAL and the fact that  
BCAL is biased towards softer photons.  When these are considers one  
would like a much better energy resolution in the BCAL to match  
reconstructed pi^0 width in the FCAL.  Of course this statement  
probably also depends somewhat on the physics channel.

Mihajlo's results are rather fresh and we should compare also with  
Alex's parametric studies to try to understand  exactly what  
differences are (after all Alex is likely simulating real event  
kinematics also).  We will talk tomorrow and try to address this  
somewhat softly in the text.  It is clear we will want to have a  
prepared answer since this is kind of an obvious question.

> page 23, figure 20 (right)
> It is hard to see the data symbols. If I read them correctly there  
> seems
> to be substantial false P-wave found for masses around 0.9 GeV  
> (about 20%)
> which seems to be quite large. Symbols in different colors would  
> help see
> what is plotted, but there should be a comment on the amount of  false
> resonant P-wave.

Noted, this plot was meant to be "proof of principle" that we actually  
have amplitude fitting code in such a shape to begin to look at these  
things.  I agree there appears to be some P-wave leakage and all of  
this needs further study.  I was hoping to just demonstrate that our  
technology (both in generating and fitting amplitudes) has matured  
enough to be at a stage to begin to do these types of tests.  I can't  
say anything more substantive about P-wave leakage other than really  
what is in the caption and text: that this is the type of thing we are  
going to be looking at.  If you feel this plot is a distraction, I'm  
happy to remove it.

Cheers,

-Matt