  All results of the neutron lifetime measurements performed in the last 20
years with the UCN storage method are in a good agreement. These results are
consistent with the latest most accurate measurements of the neutron decay
asymmetry within the Standard Model. However, there is a significant
discrepancy at $3.6\sigma$ (1% of the decay probability) level between the
averaged result of the storage method experiments and the most accurate
measurements performed with the beam method. This article addresses the
possible causes of such discrepancy. We focused on finding the spectrum of
possible systematic corrections in the beam experiment. Four separate sources
of the systematic errors which had not been properly addressed in articles
dedicated to the beam technique were considered. Two of those sources are
related with the motion of protons in an electromagnetic field and the elastic
scattering by the residual gas. The third error concerns proton loss in the
dead layer of the detector. It is shown the corresponding correction requires a
more detailed analysis than that given in the papers describing the beam
measurement method. The fourth source of the systematic error is the charge
exchange process on the residual gas. The influence of that process was
neglected in the beam method experiments. We present arguments that careful
analysis of this issue is necessary since the proposed proton losses correction
decreases the measured lifetime. For the problem of proton charge exchange on a
residual gas, the spectrum of possible corrections is investigated. It is shown
that for an accurate calculation of the correction, it is necessary to directly
measure the concentration and composition of the residual gas inside the proton
trap. The analysis reveals that even presence of only $H_2$ molecules can lead
to the significant correction which is the most probable explanation of the
neutron anomaly.
