If you sterilized the soil like that it was something you brought home with the plant. Is it inside or ouyside.

If you can't see it in the daytime maybe it's a night feeder and is hiding away in the day. Take a flashlight and examine it after dark. I'll bet you find the culprit

Hi, What do you mean by 'Is it inside or ouyside'? I have kept the plant in my balcony/patio. Today, I noticed that whatever is eating has actually eaten and broken a small branch of the cosmos. All other plants beside it do not show any such signs.
I will try examining it in the dark. Can squirrels/chipmunks eat it? My neighbour often complains that the chipmunks eat her rose buds.

I meant indoors or outdoors--I wouldn't be surprised if it were a squirrel or chipmunk especially if a branch was broken. Bugs would not do that.

Something that I didn't know until a few years ago when I saw it with my own eyes is that squirrels eat baby birds. In the evening so the plant won't burn from being wet--give it a spray with water and then sprinkle on cayenne pepper. The water will make the cayenne stick. One mouthful of that and he won't be back

WOW! Great idea. I am going to try it now. I will let you know how it goes. But why do chipmunks eat only cosmos leaves and that too only at night? I have a dozen plants beside it. Everything goes untouched.