[d2n-analysis-talk] LHRS Trigger Efficiency Study

[Brad Sawatzky]

On Tue, 13 Jul 2010, David Flay wrote:

> On Tue, Jul 13, 2010 at 2:06 PM, Brad Sawatzky <brads at jlab.org> wrote:
> 
> > The 'eff' value for T4 doesn't really mean much.  (It isn't really an
> > efficiency by any standard definition -- I wouldn't worry about it.)
> >
> > Now that I think about it a bit more (and I'm suspicious that the
> > efficiency seems too good to be real), I forgot a some factors in the
> > expression I gave you.  The trigger counts really ought to be multiplied
> > by the prescale factors
> >  eff_3 = bit3*ps1/(bit3*ps1 + bit4*ps4)
> >
> > You'll need to grab the prescales from the ROOT file on every run since
> > they can change, of course.
> >
> 
> Wouldn't including the prescales give the latch pattern variable -- LTN =
> ps*bitN?  Since bitN is after the prescale.

That's correct, but I don't think the LTN are the latched triggers.  We
still need to use the prescales though.  Imagine that the T3 trigger was
only 50% efficient, but ps4 was set to be 10^6; The calculation would
give eff_3 = 1 because all the T4's got prescaled away.

The db_DL.dat file claims the LTN variables are 'trigger input LHRS copy
in 1877', so it sounds like they are before prescale (which could
explain the T4 discrepancy you saw).

The 'LHRS trigger latch pattern' comment is attached to the lbitN
variables.

> I know that we're missing events in that despite ps = 1 for T3 and T4 for
> run 20676 (from the
> logbook<http://www.jlab.org/%7Eadaq/halog/html/0903_archive/090314032236.html>):
> the LT3 doesn't match bit3 (and more noticeable is the discrepancy of
> the T4 values) :
> 
> Trigger 3 [Bit]: 13870
> Trigger 4 [Bit]: 10
> Trigger 3 [Latch]: 13885
> Trigger 4 [Latch]: 314
> T3 Trigger Efficiency [using bit variables]:
> Trigger 3: 99.9280 +/- 0.0228%
> 
> (where I count the number of entries in the histos for 'DL.bit3', etc.)

Just a word of caution here.  Might not matter for the work you're
doing, but you should be aware of this:

You need to be a bit careful with the evtypebits, DL.bit*, DL.lbit* stuff.
The .bit* and .lbit* are (iirc) just raw TDC data.  The evtypebits is
constructed from the DL.bit* TDC data by applying a _very_ loose cut.

A 'real' bit3 will be a hit in the DL.bit3 histogram that lands in a
self timing spike (ie. a fairly narrow cut).  In practice, a lot of
folks forget about that and just do stuff like (DL.bit3 > 0) to see if
there was a hit on T3 -- that will catch anything in that TDC channel
within a microsecond of the L1A.  Not what you usually want.

The cut on the .bit* histos that is used to make evtypebits is also
pretty loose (like 200 < DL.bit3 < 2000) -- which is OK if you have a
low rate trigger, but not so good if your trigger rate is high and
you're trying to be precise.

The trigger latch pattern is also a bit arbitrary.  The way that works
is that the Trigger supervisor sees N triggers on its inputs, the
earliest trigger that passes prescale generates the L1A.  Any triggers
that arrive within XXX ns of the 'L1A' trigger get latched on the output
that goes to the DL.lbit* TDC channels.   (The XXX is programmable in the
TS and is typically set to 10ns.)  This is fine, but is only useful if
all of the interesting triggers are tweaked so they will arrive within
the 10ns of each other at the TS.  That is often NOT the case, which
limits the utility of the 'official' TS latch pattern (lbitN).

> cuts used (applied to all histos):
> 
>     //one track from the target
>     cut0 = Form("(L.tr.n==1)");
>     //cut out the pulser, require either a T3 or a T4 trigger
>     cut0 +=
> Form("&&(DL.edtpl==0)&&(((DL.evtypebits&(1<<3))==(1<<3))||((DL.evtypebits&(1<<4))==(1<<4)))");
> 
>     cut1 = cut0;
>     //VDC cuts -- requirement of one track reconstruction
>     cut1 += Form("&&(L.vdc.u1.nclust==1)&&(L.vdc.v1.nclust==1)");
>     cut1 += Form("&&(L.vdc.u2.nclust==1)&&(L.vdc.v2.nclust==1)");
> 
>     cut2 = cut1;
>     //PID cuts
>     cut2 += Form("&&(L.cer.asum_c>300)&&(prl_E_P>0.54)&&(L.prl1.e>200)");
> 
> PS -- should Cherenkov TDC cuts be used here?  I tried it out, and I get
> eff_T3 ~ 99.96%, as compared to above, at 99.92%.
>         I tacked on the TDC cuts since I do use them to define good
> electrons in the Cherenkov.

It is best to tack them on just to be consistent -- I'm surprised it
makes any difference since you're already cutting on the cerenkov ADC
sum.  Mind you, 4 parts in 10000 isn't really significant for that
quantity.

>         Also -- I was looking at Patricia's work on the trigger efficiency
> -- her left arm results are comparable to mine (99.98%):
> http://hallaweb.jlab.org/experiment/E01-012/reports/trig_vdc.pdf
>         However, she is fairly brief on her description of the calculation.

Glad to see the consistency with older work.

-- 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