  Chemistry in the ultracold regime enables fully quantum-controlled
interactions between atoms and molecules, leading to the discovery of the
hidden mechanisms in chemical reactions which are usually curtained by thermal
averaging in the high temperature. Recently a couple of diatomic molecules have
been cooled to ultracold regime based on laser cooling techniques, but the
chemistry associated with these simple molecules is highly limited. In
comparison, free radicals play a major role in many important chemical
reactions, but yet to be cooled to submillikelvin temperature. Here we propose
a novel method of decelerating \ce{CH3}, the simplest polyatomic free radical,
with lithium atoms simultaneously by travelling wave magnetic decelerator. This
scheme paves the way towards co-trapping \ce{CH3} and lithium, so that
sympathetical cooling can be used to preparing ultracold free radical sample.
