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-rw-r--r--chroma/generator/vertex.py2
1 files changed, 1 insertions, 1 deletions
diff --git a/chroma/generator/vertex.py b/chroma/generator/vertex.py
index 2a3bedf..513a961 100644
--- a/chroma/generator/vertex.py
+++ b/chroma/generator/vertex.py
@@ -59,7 +59,7 @@ def pi0_gun(pos_iter, dir_iter, ke_iter, t0_iter=constant(0.0), start_id=0, gamm
# In rest frame
theta_rest = np.arccos(gamma1_dir[2])
- phi_rest = np.arctan2(gamma1_dir[0], gamma1_dir[1])
+ phi_rest = np.arctan2(gamma1_dir[1], gamma1_dir[0])
# In lab frame
(gamma1_e, gamma1_dir), (gamma2_e, gamma2_dir) = \