<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hi Curtis,<div class=""><br class=""></div><div class="">Sorry, I couldn’t keep up with the rapid discussion on the eta/eta’ mailing list yesterday. I am fine with the proposal to quote the orientations as PARA/PERP for 0/90 and -45/+45 to be consistent with our GlueX coordinate system and it hopefully makes the notation simpler for future publications.</div><div class=""><br class=""></div><div class="">-Justin<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Nov 20, 2019, at 11:38 AM, Curtis A Meyer <<a href="mailto:cmeyer@cmu.edu" class="">cmeyer@cmu.edu</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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Dear Colleagues -
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<div class=""> a discussion ensued yesterday afternoon about the direction and orientation of the four polarizations currently used in GlueX. We currently refer to </div>
<div class="">0/90 and 45/135, where 0 and 135 are “Para” orientations and 90 and 45 are “Perp” orientations. There is no question that the 0/90 make sense, </div>
<div class="">but there was some confusion on the 45/135, which is written in a way to suggest that 45 is the para and 135 is the perp (which is backwards).</div>
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<div class="">Richard pointed out that the 45/135 are reported in the accelerator system with y up and x to beam right. If you take z along the direction of the beam, this is</div>
<div class="">a left-handed coordinate system. In GlueX, we have y up and x to beam left leading to a right handed coordinate system when z is taken along the direction </div>
<div class="">of the beam. The 45/135 are reported in the accelerator system, which Richard noted would correspond to the following in the GlueX system:</div>
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<li class="">0/90 PARA has the polarization plane at 0 degrees in Hall D coords</li><li class="">0/90 PERP has the polarization plane at 90 degrees in Hall D coords</li><li class="">45/135 PARA has the polarization plane at -45 degrees in Hall D coords</li><li class="">45/135 PERP has the polarization plane at +45 degrees in Hall D coords</li></ul>
<div class="">This suggest that it could make sense to publish our orientations in the GlueX system rather than the accelerator system, which would mean we would </div>
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<div class="">quote the 0/90 pair and the -45/+45 pair. </div>
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<div class="">If we do want to make such a change, we should probably agree to it very soon as we can still try to change the notation on the eta/eta-prime paper.</div>
<div class="">Such a notational change does not affect any of our physics results, it is just a consistent reporting of published information in the GlueX coordinate </div>
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<div class="">Curtis</div>
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A. Meyer</span><span class="Apple-converted-space"> </span>|<span class="Apple-converted-space"> </span><span style="font-size: 1.2em; line-height: 1.2em;" class="">MCS Associate Dean for Research<span class="Apple-converted-space"> </span><br class="">
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