[Halld-tagger] End user polarization

[Ken Livingston]

Hi Michael and all,
That sounds good to me. It would at least give the option of asking for 
the polarisation of every photon in every event - even if it's coming 
from a single lookup table that's been taken from the triplet 
polarimiter analysis averaged over the whole beamtime. It would make 
analysis easier.

It's worth considering the experience from CLAS and Mainz.

Here's the kind of function we use:

double GetPol(int plane, double edge, double Eg,   int poltype = 
PSMOOTH, double lowThresh=0.2, Double highThresh=0.3)
double GetPol(int plane, double edge, int TagChan, int poltype = 
PSMOOTH, double lowThresh=0.2, Double highThresh=0.3)

where:
plane      = polarisation plane (PARA=0,PERP=1)
edge       = coherent edge in GeV
Eg         = photon energy in GeV (or TagChan for faster performance)
poltype    = selects ther required column in the lookup tables.
lowThresh  = minimum pol required for events on left of edge
highThresh = minimum pol required for events on right of edge

This function returns the polarisation, or -1 if it is unknown or fails 
the theshold criteria.

The lookup tables which are used by the function can be pulled in as 
required - eg. on a run by run basis from a calibration database - or on 
an "edge by edge" basis as described below.

In CLAS, where the peak drifted around a lot, the CBSA (Coherent Brem 
Shape Analysis) was used to fit as many enhancements as possible to 
cover the range of edges for each nominal setting. The parameters from 
the CBSA fits were then used to generate different lookup tables for 
each edge position, in 2MeV steps. In the anaysis, each scaler read from 
the hodoscope (2s intervals) was used to make an enhancement and 
determine the current edge position, and a lookup table was made by 
interpolating between the 2 nearsest tables.
The current CBSA fit was developed (by me) specifically for this 
puspose. It has been hacked, fudged and phenomenologised directly from 
the equations outlined in the Timm paper, with some simplifications, and 
some assumptions about the beam and collimator, to make something that 
could be fitted with minuit. Sometimes it even converges! The aim was to 
improve on the previous method (anb, for example), where a bunch of 
dubious parameters was adjusted by hand until the user was happy that 
the enhancement agreed with the data.

In Mainz, where things are very stable and more analagous to GlueX, we 
generate the lookup tables dynamically using the parameters from the 
CBSA fit every time the coherent edge changes by more than some 
threshold amount (eg 1MeV with edge at 450MeV). I think the optimum for 
GlueX in the longer term would be to do something similar, with tables 
generated from TPOL results binned into edge positions. Small changes in 
the edge position can make a big difference to the polarisation in those 
photon energy bins - this has to be carefully handled somehow.

Regards,
Ken




On 05/06/17 18:35, dugger at jlab.org wrote:
> Hi,
>
> I was thinking about event-by-event polarizations.
>
> While TPOL can not give an event-by-event polarization, we could provide
> some sort of subroutine that gives the average run polarization on an
> event-by-event basis. The subroutine would take as arguments the run
> number and photon energy for an event, with the return value being the
> polarization (based off a polarization binning of 100 MeV).
>
> Does this sound like the sort of thing that is desired?
>
> My plan is to go ahead with making the code that gives the polarization
> for a set of runs (provided by the end user), and then have our grad
> student (Sebastian Cole) create the final code that takes into account the
> feedback we get from our fellow GlueX collaborators.
>
> Take care,
> Michael
>
> _______________________________________________
> Halld-tagger mailing list
> Halld-tagger at jlab.org
> https://mailman.jlab.org/mailman/listinfo/halld-tagger


-- 
=======================================================
Ken Livingston

Dept. of Physics & Astronomy,        Tel: +44 141 330 6428
University of Glasgow,               Fax: +44 141 330 5889
Glasgow G12 8QQ.
Scotland. UK.
=======================================================