[Halld-tagger] Fwd: polarization update

[Nathan Sparks]

The reported degree of polarization from the TPOL is a rough estimate, based on the information I had available at the time.
I have no reason to believe it is not in the right ballpark, and it was not presented as a final result.

-Nathan
> On Mar 14, 2016, at 7:48 AM, dugger at jlab.org wrote:
> 
> Hi,
> 
> The reported polarizations for the polarimeter are not correct. It was too
> early for the degree of polarization to have been reported.
> 
> Take care,
> Michael
> 
>> 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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