[Halld-tagger] Fwd: polarization update

[Richard Jones]

Message from Ken regarding comparison between different methods for beam
polarization estimation.
-forwarded by Richard Jones

---------- Forwarded message ----------
From: Ken Livingston <Kenneth.Livingston at glasgow.ac.uk>
Date: Mon, Mar 14, 2016 at 6:35 AM
Subject: Re: polarization update
To: Justin Stevens <jrsteven at jlab.org>, Nathan Sparks <nsparks at jlab.org>,
Richard Jones <richard.t.jones at uconn.edu>


Hi Justin,
That a very nice study. It's exactly what's needed. If we believe the
photon asymmetry for the rho in this energy range is really 1, and are
convinced that it's very cleanly extracted signal, that's the best
polarimiter we can have. It has an event rate that's better than the triple
polarimiter and doesn't need an analysing power,  and it does not need all
the vaguely known parameters that are required to match the enhancement
with a coherent brem calculation.

For the 20 um radiator it's also good to see that, as we'd expect, the
shape of the enhancement (and hence polarization) is much better with
collimated data rather than raw scalers. The similarity between the
polarization for the 20um and 50um is borne out by Nathan's studies
although the energy range is different (8-9GeV).

It looks like both your studies are assuming that Ppara = Pperp.
I got the impression from the raw spectra that this might not be true for
the 20um diamond. It would be good to check this out.
A quick and check would come from using the more complicated fit function I
sent you earlier, where the flux ratio and pol ratio are free parameters.
>From this, you could plot the pol ratio as a function of E_gammma, as you
have for Sigma.

My "phenomenological fit" is clearly not as phenomenal as I'd hoped. It
doesn't cope with the ~200MeV spread in the coherent edge for the 20um
diamond. I believed it would. It can fit almost any enhancement, but is not
much use if it doesn't get the corresponsing polarization correct.  Either
I've made a silly assumption somewhere, screwed up the algebra or screwed
up the code. Maybe all three together - although it generally gives good
agreement when used to extract asymmetries and compare with previous
measurements.

I'll need to scrutinise it as soon as I get time.


Regards,
Ken







On 11/03/16 17:06, Justin Stevens wrote:

> FYI, from this mornings meeting: rho and TPOL asymmetries comparing 20 and
> 50 um diamonds.
>
> https://logbooks.jlab.org/entry/3390212
> https://logbooks.jlab.org/entry/3390218
>


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

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