[d2n-analysis-talk] Scintillator Time Offset Checks/Beta Correction

[Brad Sawatzky]

On Fri, 05 Mar 2010, David Flay wrote:

> I have been working on checking the time offset, timewalk, beta, and
> beta vs. track-x in the past week or so.
> 
> Today, I've made a correction to beta -- it was initially at ~ 0.93.
> 
> By putting in an arbitrary channel shift into the L and R TDCs of
> <one> plane -- I used S2m -- we can move the beta distribution to
> center on 1. 

Just so it's clear, this is beta:

             (physical flight path distance between S1, S2)    1
     beta = ------------------------------------------------ x -
                          ( t_S2m - t_S1 )                     c

OR,

   1/beta = ( t_S2m - t_S1 ) * (constant)

It is a trivial representation of the deltaT between paddles.

What you are doing is _declaring_up_front_ that
  (A) everything in the stripe are beta=1 particles
and then shuffling a global offset on one plane to make the formula come
out right.  In real-life that offset merely represents slightly
different signal paths between the electronics serving the two planes.

That's OK provided you can make a strong argument (and, one that has
_nothing_ to do with timing data) that your assumption (A) is valid.

Note that you can NOT make the later claim that a cut on beta means you
are selecting electrons.  That is circular logic -- you have already
assumed that everything in the band are (beta=1) electrons based on
/some other rationale/.  The cut on 'beta' (in the case of single-arm
LHRS data) is just a convenience, nothing more.

[ . . . ]
> Other good news is that once I place a cut on beta > 0, the mystical
> momentum dependence that we saw in the avg. time for S1 goes away --
> confirming that those were due to cosmics.  Applying the beta cut to
> my timewalk studies shows that the current timewalk polynomial factors
> in the DB are good, as there is no timewalk effect. (I sent this info
> in previous e-mails -- just wanted to consolidate the info here.)

Nope.  The logic is completely twisted on its head.  If this isn't
crystal clear to everyone, then we need to discuss it at the next
meeting.

The S2m time average in your plot is in pretty bad shape.  S1 looks
fine.

-- Brad

-- 
Brad Sawatzky, PhD <brads at jlab.org>  -<>-  Jefferson Lab / Hall C / C111
  Ph: 757-269-5947 -<>- Pager: 757-584-5947 -<>- 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