* High energy neutrons of about 70 MeV would be produced when a p - is captured by aluminium or other material. They can be identified by the time-of-flight of the particles entering the calorimeter. In the present experiment p - beam particles pass only through low Z material, mylar for instance. Hence, only scattered p - that would be captured by the LH2-target housing or the cryostat rods would contribute to that background
* The presence of deuterium in the LH2-target would result into an excess of RC-events and low energy neutrons. For this reason high purity hydrogen was used in the target.
* A low energy background resulting from pair conversion in the target vessel or in air can not be avoided entirely. The electrons and positrons are vetoed by using the plastic scintillator hodoscope array. Since they also can undergo bremsstrahlung low energy photons could enter the calorimeters. This source of background can only be reduced by comparing the low-energy lineshape with the simulation.
* After the magnet triplet the p - travels about 30 cm through air. This can give rise to in-flight SCX. The contribution of in-flight reactions would result in an error for the determination of the detector acceptances, since the reaction points are unknown. Hence, the B1-counter was positioned directly in front of the target's entrance window[29].
* Conversion can take place resulting in the modified reactions p -p->e-e+n, p 0->e-e+ g , respectively. Whether this results from structural effects or photo reaction with H2 is indistinguishable; however this leads to a multiplicative correction of 0.999 for both reactions [Coc61].
* Scattered beam pions have a change to enter the CsI array with a kinetic energy of 40 MeV at most.
* Background from decaying muons succeeding p -->µ- n µ decay can be neglected, since the slowdown and capture process of the p - in H2 is three orders of magnitudes faster (see section 6.3 and [Pan50]).