I am looking for some recommendations / papers that claim that a keyboard and a mouse should be placed next to each other. What about a case where the mouse is a used as an input device and the keyboard is on screen? To me it seems a bit inefficient.
The reason to place the mouse next to the keyboard is to make it fast, easy, and accurate to switch between pointing (with the mouse) and keying (with keyboard), as is typically necessary for computer input. Not surprisingly, the further the mouse is from the keyboard, the longer it takes to reach for it.
The difference in reaching time can be modeled using Fitt’s Law to determine how much of an impact various separation distances will have. I can say that several inches makes a noticeable difference. At my personal workstation, I have the cursor and number keys on the left side of the keyboard, allowing the mouse to be six inches closer to the touch-type home keys. It feels awkward using other keyboards. If you’re designing a custom-built workstation, and your users don’t use the keypad or cursor keys, you may want to specify a laptop-style mini-keyboard with no dedicated number pad or complete set of dedicated cursor keys in order to place the mouse closer.
Minimizing mouse-to-keyboard transition times applies as much to keyboards on the screen as to physically separate keyboards. However, it may be less important because if you have a keyboard on the screen, then you probably have a touchscreen. This means the users can point and key with the screen, which allows shortest reach (0 in some cases). The users can ignore the mouse except in relatively rare situations when they need to point precisely to something small on the screen. In this design case, it may pay off for the mouse to be relatively far away to address other usability issues (e.g. optimal screen viewing)
It’s so obvious that short mouse reaches are preferred that it may be hard to find such a recommendation in writing. Whenever I need something obvious in writing, I reach for MIL-STD 1472 (p18):
126.96.36.199.1 Grouping. Controls which are operated in a task-driven sequence or which are operated together shall be grouped together along with their associated displays