The enhanced PHENIX muon spectrometer is shown in Figs. 1 and 2. The upgrade for spin physics consists primarily of the addition of a second muon end cap, significantly enhancing the dimuon acceptance for large pair masses. Also, in order to maximize acceptance and to extend kinematic coverage, muon identification is added to the two arms of the central magnet outside the shielding enclosure (Fig. 2).
Figure 1: Plan view showing both muon end caps of the proposed upgraded PHENIX spectrometer.
The first end cap (large triangle on the right) is the original muon arm of the PHENIX
CDR[25]. The second end cap is shown on the left as a smaller triangle subtending the same angular range,
. The muon identifiers of the two end caps are the vertical blocks of lines at
the extreme right and extreme left, representing steel absorbers and muon chambers. Also shown at
the top and bottom of the figure are muon chambers and absorbers proposed for the central magnet arms.
The central muon identifiers are located outside the concrete shielding enclosure.
Figure 2: Elevation view down the beam axis showing the proposed muon
identifier segments for the central magnet arms. The muon identifiers are
located outside the concrete shielding enclosure. Also shown is
the central magnet and selected detail of the dual detector arms.
As is shown in Sect. 2, very general considerations lead to the expectation that polarization effects
will increase with increasing 
. The qualitative increase in acceptance is
clear from Fig. 3 which compares the baseline PHENIX muon spectrometer with the enhanced versions. The
acceptance was calculated at
but is approximately valid for higher energies as well (see Appendix A.).
Figure 3: Dimuon mass acceptance for three versions of the PHENIX
muon system; dotted, PHENIX baseline (1 end cap); hatched, 2 end caps; solid, 2
end caps plus muon identification in the central detector. The
calculations were performed at 
but are approximately valid at
higher energies as well due to 
scaling of the acceptance.