Study of Calorimeter Calibration with Tau's in CMS.

We propose to calibrate in situ the CMS calorimetry using the single, isolated pions from tau-> pi nu in W -> tau nu and Z, gamma^* -> tau tau processes applying the p/E method. In case of pions non-interacting in the ECAL the method is straightforward, but for pions interacting in the ECAL care is needed to suppress and keep under control pi+- pi0's from tau's or QCS jets, which could vitiate the method. This can be achieved exploiting the ECAL granularity and tracker-calorimetry special matching. The momentum of the isolated high pt pion can be directly compared to the calorimeter measurement. Triggering of the W -> tau nu events is envisaged with a special tau-jet trigger combined with a missing transverse energy trigger. The Z gamma^* -> tau tau events could be triggered by lepton + tau-jet and double tau-jet trigger. The event rate for pt of pion > 15 GeV is e nough to calibrate each HCAL cell at a 1% precision after collection of 10^4 pb-1 of data.