  We examine the East-West asymmetry of the peak intensity in energetic storm
particle (ESP) events using the improved Particle Acceleration and Transport in
the Heliosphere (iPATH) model. We find that injection efficiency peaks east of
the nose of coronal mass ejection shock where the shock exhibits a
quasi-parallel geometry. We show that the peak intensity at the eastern flank
is generally larger than that at the western flank and it positively correlates
with the injection efficiency. We also examine this asymmetry for heavy ions,
which depends sensitively on the ion energy. Comparison between the modelling
results with the measurements of ESP events at 1 au shows a reasonable
agreement. We suggest that the injection efficiency can be a primary factor
leading to the East-West asymmetry of the peak intensity in ESP events.
Additionally, the charge-to-mass (Q/A) dependence of the maximum particle
energy affects this asymmetry for heavy ions.
