[Halld-physics] [EXTERNAL] GlueX Polarization Directions

[Curtis A Meyer]

Dear Colleagues -

   a discussion ensued yesterday afternoon about the direction and orientation of the four polarizations currently used in GlueX. We currently refer to
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,
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).

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
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
of the beam.  The 45/135 are reported in the accelerator system, which Richard noted would correspond to the following in the GlueX system:


  *   0/90 PARA has the polarization plane at 0 degrees in Hall D coords
  *   0/90 PERP has the polarization plane at 90 degrees in Hall D coords
  *   45/135 PARA has the polarization plane at -45 degrees in Hall D coords
  *   45/135 PERP has the polarization plane at +45 degrees in Hall D coords

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
quote the 0/90 pair and the -45/+45 pair.

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.
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
system.

Curtis


Curtis A. Meyer | MCS Associate Dean for Research
The Otto Stern Professor of Physics
cmeyer at cmu.edu<mailto:cmeyer at cmu.edu> | 412 268 2745
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