  The anomalous response of glasses in the echo amplitude experiment is
explained in the presence of a magnetic field. We have considered the low
energy excitations in terms of an effective two level system. The effective
model is constructed on the flip-flop configuration of two interacting two
level systems. The magnetic field affects the tunneling amplitude through the
Aharanov-Bohm effect. The effective model has a lower scale of energy in
addition to the new distribution of tunneling parameters which depend on the
interaction. We are able to explain some features of echo amplitude versus a
magnetic field, namely, the dephasing effect at low magnetic fields, dependence
on the strength of the electric field, pulse separation effect and the
influence of temperature. However this model fails to explain the isotope
effects which essentially can be explained by the nuclear quadrupole moment. We
will finally discuss the features of our results.
