[d2n-analysis-talk] S1 and S2m Time Averages: Problems With the SetAlias Method?

[Brad Sawatzky]

On Wed, 22 Sep 2010, David Flay wrote:

> I zoomed out on these various plots and I got some (odd) things going
> on here.
> 
> The first plot is the sum <without> the factor of 1/6 or 1/16 (for S2m).
> 
> The second plot is <with> the factor.  This second plot seems to give
> a 'somewhat' more believable result... kind of.

Plots 1 and 2 are consistant with each other.  The 2nd plot just has the
x-axis compressed by 1/6 (1/16 for S2m), which is exactly what the code
is supposed to do.  Note that the binning is quite different in plot 1
(fine binning) vs plot 2 (bigger bins).  Neither plot makes much sense
from a physics perspective.

Plot 2 doesn't look any better than plot 1 -- all the substructure in
plot 1 is just been hidden the large bins in plot 2.

The S1 average seems in plots 1 and 2 to be all over the place: many
spikes spread over a delta-t that is completely unphysical (100+ ns).
It looks like it may be constructed using _un_corrected TDC data, which
is (obviously) not what you intend.

> The third plot uses the definitions for the time averages:
> ch->SetAlias("S1TimeAvgTPads","0.5e+09*(L.s1.lt_c[L.s1.t_pads[0]]+L.s1.rt_c[L.s1.t_pads[0]])");
> ch->SetAlias("S2mTimeAvgTPads","0.5e+09*(L.s2.lt_c[L.s2.t_pads[0]]+L.s2.rt_c[L.s2.t_pads[0]])");
> 
> This gives the most consistent and expected result of the three plots,
> at least for S2m.

Plot 3 is not plotting the same underlying data as plot 1 (or plot 2).
I don't know what is going on there, but they don't look anything alike.
The sub-structure and width of the distribution is different, even the
number of counts in the histos are different.  (As an aside, try to keep
the bin widths the same when generating comparison plots -- it'll make
it easier to see how individual peaks shift/change.)

Between plots 3 and 2, plot 3 looks "more correct", but it still has
some puzzling features.  I presume the S1 spike at at ~55ns is what you
believe to be "good" events.  What are the 3 spikes out 200ns that also
have roughly 10^5 events in them, or the spike with 10^3 events at 70ns?

More generally, it looks like several percent of the total events in
this histogram have non-physical times (ie. they can't be associated
with a real particle).  Why is that?  What happens when you require a
good track from the VDCs?

The derived plots (S1-S2m, beta, etc) will be confused as long as the
individual S1, S2m histos are strange -- no point in puzzling over those
until the later make sense.

-- Brad

-- 
Brad Sawatzky, PhD <brads at jlab.org>  -<>-  Jefferson Lab / Hall C / C111
Ph: 757-269-5947  -<>-  Fax: 757-269-5235  -<>- Pager: brads-page at jlab.org
The most exciting phrase to hear in science, the one that heralds new
  discoveries, is not "Eureka!" but "That's funny..."   -- Isaac Asimov