[Halld-tagger] Fwd: polarization update

[dugger at jlab.org]

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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