Measurement of theϒmass

We have detected the polarization of the positron beam in the Cornell Electron Storage Ring (CESR) with a polarimeter which detects back-scattered laser light. The beam was resonantly depolarized using a vertical shaker magnet with a rf field, thus measuring the $g-2\mathrm{}$ frequency. This gave us a precise value for the beam energy in CESR. By scanning the $\Upsilon$ resonance with the CUSB detector, we obtained a mass $M_{\Upsilon }=9459.97\mathrm{}\pm 0.11\pm 0.07$ MeV. We also give rescaled values for the excited $\Upsilon$ states.