Precise measurement of the toponium mass from the observation of its two-photon decay at the LHC

We propose to measure the toponium mass using an electromagnetic calorimeter of the 'Crystal Ball' type at the LHC to detect tanti t bound states (toponium). If such a heavy system exists, the JPC=0-+ state will dominate production at the LHC and could be observed above a relatively low background in the two-photon decay channel. The toponium mass could be measured for instance with a precision of 3x10-4 for msub(anti t)=200 GeV/c2. This method provides an accuracy comparable with what will ultimately be obtained with future dedicated e+e- top-quark factories. (orig.)