The contamination in the beam, mostly due to the prompt tauonic decay of the produced from proton interactions in the primary target and in the hadron stopper, is the ultimate irreducible background for a oscillation search.
In the NGS, the prompt problem appears less severe than in the WANF [9, 10]. Compared to the WANF, the NGS near detector site is about twice as far from both the primary target and the hadron dump. As the angular distribution of 's is rather broad and approximately flat, a yield lower by about a factor 4 is qualitatively expected just from the reduction of the solid angle.
Quantitative estimates of R in the NGS have also to take into account the different material, graphite, used here for both the target and the beam dump. This implies a larger yield of (and ) than that from the WANF beryllium target and a lower yield than from the WANF iron beam dump.
Table 2: Predicted Prompt Background at the Location of TOSCA.
Results from detailed estimates of R ( the ratio of -CC to -CC events) in the NGS are given in Table 2. The background level appears, therefore, tolerable for the NGS reference proton beam momentum of 400 GeV/c. The value of R at 400 GeV/c proton momentum is lower than that expected for the operation of the WANF at 350 GeV/c ( )