In a recent trademark filing by Microsoft it was revealed one of their existing first-party game studios which have been around for a while. Although it took a while, it has finally received a formal name from Microsoft.
According to Neowin, the filing was made for a word mark with the description - "The mark consists of the letters LXP' superimposed over an image of an astronaut." More info about this was provided by a NeoGAF user who found that a London based design studio called Territory Studio had created the logo for LXP. It was also revealed that LXP stands for Leap Experience Pioneers.
From Territory Studio’s website further info about LXP was available, "Leap Experience Pioneers (LXP) are an in-house games developer for Microsoft based in the US. Although they had been in existence for some years they thought it was the right time to define their identity... They considered themselves pioneers (as their name suggests) and wanted to encapsulate the spirit of fronteering Americans - combining a cowboy with an astronaut with attitude. We then set about creating some short animated logo stings and brand guideline document."
For now, Microsoft is yet to officially reveal LXP or the projects they will or have been working on. Maybe this E3?