How well can we measure the jet-jet invariant mass at ISABELLE

In the design of a hadron calorimeter one must optimize (i) longitudinal length for total energy containment, (ii) sampling thickness, and (iii) material. The energy containment in the transverse direction does not become a problem in most colliding beam detectors because they tend to cover 2π in the phi direction and large coverage in the theta direction. To contain 99% of energy of a 50 GeV particle, one needs 5.5 absorption length of steel, which is 30% thicker than the requirement for 95% containment. However one should be reminded that a loss of 5% energy broadens the energy resolution by 25% compared with the value of full containment. Making the sampling thickness finer improves the energy resolution especially at lower energies, but below 2.5 cm steel, there is no improvement as long as we are dealing with particles above 10 GeV. Among the large detectors proposed so far, the dipole detector emphasizes good jet detections using an uranium calorimeter. Its anticipated mass resolution at the Z0 mass (Z0 → jet + jet) when the jets opening angle is above 600 is ΔM/M approx. 3%. The mass resolution of this level would make it possible to sort out new particles decaying into multi jets final states from QCD backgrounds