[Halld-physics] Random subtraction and uniqueness tracking

[Richard Jones]

Justin,

I wonder if it might make sense to devote 10 minutes at the beginning of
each physics meeting to a "How-To" presentation on basics of our data
analysis. Accidentals subtraction is something that seems to keep coming up
over and over. An alternative to a "basics of physics analysis" series, we
could also commission a series of short Tech Notes. Maybe the first one
could be, "How photon tagging works", just a couple of pages with a
worked-out example would do.

-Richard J.

On Tue, Mar 20, 2018 at 1:18 PM, Peter Pauli <ppauli at jlab.org> wrote:

> Hello everybody,
>
> I apologise in advance for this lengthy email.
> I am trying to understand how to properly do the random subtraction while
> avoiding double counting. After talking to a couple of people at the
> collaboration meeting and during shifts I found that this seems to be non
> trivial and it seemed that some people where not to sure about it either.
>
> According to Paul's talk that he gave at the 2016 software workshop the
> rule is to always include all the variables into your uniqueness tracking
> that are used to calculate the histogrammed quantity.
> Let's say I look at gp->p pip pim. I want to histogram the pip pim
> invariant mass and do the random subtraction. Although I only use the pip
> and pim tracks to calculate the invariant mass I have to include the beam
> photon in my uniqueness tracking because I also use that quantity (timing)
> in the following step to get the random subtracted histogram. But as far as
> I understand in the voting on the best RF bunch each track gets a vote. So
> do I need to include the proton in the uniqueness tracking as well?
>
> What happens if I use the track vectors coming out of the kin fit? The
> standard 4-mom fit that is usually used will include information from all
> tracks in the combo. Does that mean as soon as I plot e.g. the invariant
> mass using kin fitted tracks I need to include everything in the uniqueness
> tracking?
>
> What about binned quantities. Many results are shown in bins of momentum
> transfer squared. In my analysis I look at gp->kp Lambda(->km proton) and
> calculate the momentum transfer as t^2 = (g_P4-kp_P4)^2. If I now want to
> bin the proton Kminus inv mass in t-bins does that mean I need to include
> the beam particle and Kplus in my uniqueness tracking? Aren't bins in
> principle like a 2D plot? Binning means effectively histogramming a 2D plot
> I would say, therefore I need to include them. But what is if now make my
> bins really large (e.g. 0 to -inf)? This would effectively be unbinned and
> I would just track the beam particle, proton and km. Does that mean I need
> uniqueness tracking of beam, km, proton for each of my bins individually
> but not include kp in the tracking?
>
> If there is a document somewhere that clarifies those issues I would be
> happy if you could point me to it. I didn't find anything but maybe I
> missed it.
>
> These are only a couple of questions I have about this but I think it is
> enough for one email. I thought about this stuff for quite some time now
> and feel like I start to confuse myself about some of the issues. Therefore
> I thought I just send it to the physics mailing list. Maybe there are
> straight forward answers and I just don't see them or maybe this is worth a
> discussion at an analysis meeting. Either way I am sure this is interesting
> for more people than just me.
>
> Cheers,
> Peter
>
> --
> ================================
> Peter Pauli
>
> Dept. of Physics & Astronomy
> University of Glasgow,
> Glasgow G12 8QQ.
> Scotland. UK.
>
> Tel: +44 (0)141 330 6398
> ================================
>
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