  Symmetry protected Dirac semimetals can be transformed into Weyl semimetals
by breaking the protecting symmetry, leading to many exotic quantum phenomena
such as chiral anomaly and anomalous Hall effect. Here we show that, due to the
large Zeeman g factor and small band width along b-axis in Dirac semimetal
ZrTe5, a magnetic field of about 8 T along b-axis direction may annihilate the
Weyl points and open up a two-dimensional (2D) Dirac mass gap, when the Zeeman
splitting exceeds the band width along b-axis. This is manifested by a sharp
drop of magnetoresistance (MR) above 8 T, which is probably due to additional
carriers induced by the orbital splitting of the zeroth Landau level associated
with the 2D Dirac point, which is descendant of the original Weyl points.
Further evidence of the additional carriers is provided by the Hall effect and
different anisotropic magnetoresistance (AMR) in low and high field regions.
Our experiment reveals a probable topological quantum phase transition of field
induced Weyl points annihilation in Dirac semimetal ZrTe5 and gives an
alternative explanation for the drop of MR at high field.
