How can i detect that an asset has been published from a plugin? So, i must write this Craft plugin to index documents as soon as they are published, though a special publication field used amongst different pages templates (including a matrix field !).

How can i use the publish event to detect that this special kind of assets (assets belonging to a special directory) have been used ?
You could use the entries.onSaveEntry event to get entries as they're saved, check the publish date, then loop through its properties, and check the paths on any assets.

You'd probably also want to hook into entries.onDeleteEntry to de-index any assets from entries that get deleted.

Please note: this won't auto-index entries that were scheduled to publish at a later date.