[d2n-analysis-talk] T2,T6 time walk

[Brad Sawatzky]

On Mon, 22 Mar 2010, posik at jlab.org wrote:

> I also wanted to see what the un-prescaled t2 and t6 look like
> (t2vst6.png) where T2 is the y-axis and T6 is the x-axis. I also tried
> to plot the variable DBB.l1a , but I just get a white canvas and zero
> entries. I was wondering is this because DBB.l1a is an un-prescaled
> variable and the l1a triggeres only for prescaled triggers? Or is this
> a crate map type error?  There is also a variable called DBB.f1l1a
> which I assume is the l1a going to f1 TDC?

The L1A *is* the trigger -- if you read out an event, then there was an
L1A.  (The L1A is always after prescaling, but that doesn't matter
here).

>  I am a little confused on how to actually go about correcting for
>  this time walk effect. Do I apply corrections to the t2 vs TDC or do
>  I really want to correct the t2 vs t6 since t2 is our primary
>  trigger?

A hit in a TDC is the time difference between the trigger and the
individual hit coming into the TDC.  For example:
  dT = T_hit - Trigger
For our "standard" events, the trigger is T2, so 
  dT = T_hit - T2
However, we also have a class of events where we get a T2 and a T6, but
the T6 carries the timing (due prescaling and the way the Cerenkov is
ANDed with the T6 to form T2).  For this case you have
  dT = T_hit - T6
You want your histogram to always reflect the dT between the hit and T2
so you can do this
  dT = T_hit - T6 + (T2 - T6) + const
where (T2 - T6) is just DBB.t2.  The 'const' is the value of the self
timing peak in DBB.t2

>  Also I am a bit concerned with ~95% of my statistics being lost with
>  a one track cut(BB.tr.n==1). Could this be a problem with the optics
>  or mwdc data files? Diana are you also seeing a large decrease in
>  statistics with this cut?

You had this problem before -- are you certain you're using the standard
analyzer build?  (ie. Environment variables are all correct?)

-- Brad

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