From: John Sullivan <sullivan@p2hp4.lanl.gov>
Subject: Re: comments on MVD multiplicity at L1
To: wohn@iastate.edu (Fred K Wohn)
Date: Fri, 04 Dec 1998 17:13:13 MST
Cc: hubert@lanl.gov (Hubert van Hecke), jsimon@lanl.gov (Jehanne Simon-Gillo),
        schlei@lanl.gov (Bernd Schlei), mjbennett@lanl.gov (Mike Bennett),
        young@orph01.phy.ornl.gov (Glenn Young),
        petridis@iastate.edu (Athan Petridis),
        lajoie@iastate.edu (John Lajoie), lwood@iastate.edu (Lynn Wood),
        jhill@iastate.edu (John Hill), mrosati@iastate.edu (Marzia Rosati),
        lebedev@iastate.edu (Sasha Lebedev), wohn@iastate.edu (Fred Wohn),
        Soren-Sorensen@utk.edu (= Soren Sorensen)
In-Reply-To: <199812032059.OAA19835@pv7453.vincent.iastate.edu>; from "Fred K Wohn" at Dec 03, 98 2:59 pm
X-Mailer: Elm [revision: 212.2]

Hi,
   This is a response to Fred's note 1 or 2 days ago.

   I have not done any new calculations, but thought I would at least
answer Fred's direct question.

> 
>    Since I have plots from Athan on his work, one of the first things
> that struck me was the similarity between his plots of Nstrip vs z
> and John's. However Athan did one thing that I think (and I'm asking
> John with this e-mail) is different from what John plotted. The data
> points that Athan showed were mean values (averages over the events
> and the 6 L1 strip sums with the same value of z), but his error bars
> were not the uncertainty in the mean but the rms deviations. We wanted
> the rms since it gives the event-to-event fluctuations. If all is
> gaussian, the the error in the mean is the rms/N^(1/2) - this roughly
> agrees to my eyeball since John's N = 150, so the rms should be about
> 12 times larger than the error bars in John's plots. With this 
> difference in mind John's and Athan's plots look quite similar.

   Fred is correct about the definition of my error bars. However,
I like the way they are defined. A sufficiently ambitious person could
have figured out the defintion of the error bars out from my web page
because the root commands to make the plot are on the page. However,
root seems to be closely related to Welsh as far as I'm concerned, so you
have to be very ambitious. My error bars are rms/N^(1/2), which means
they are the estimated uncertainty on the mean. I say estimated 
uncertainty because it is relevant in this case -- the number of data
points is too small to determine the rms in some cases. I think this is
the correct approach for estimating this correction, because we can not
do much better than make a correction so that the mean value is correct.
When attempting to correct the mean value vs. the vertex position, the
uncertainty on the mean seems like the correct error bar to use in the
fit.

   I had not thought about the multiplicity fluctuations much. I still
believe that it would be unfortuate to design away the ability to make
a vertex correction of the MVD's signal to level-1. I think this because
I can imagine a dN/deta distribution (perhaps unrealistic) for which
this effect would be much larger. In a year or so we will no longer need
to speculate.

   The good news seems to be that there is no discrepency in the
calculations themselves (Athan and myself). Therefore, I think you
guys can safely argue about the consequences of the calculations
at BNL next week. I will not be there. My opinion is summarized in
the paragraph above and I am content to let Mike Bennett represent
me in this matter.

My web page on this subject is at:
http://p25ext.lanl.gov/people/sullivan/notes/phenix/trigger/trig_vs_zvert.html

Regards,
John