Research experience not backed up by recommendation letter I have done a three months independent research at a national lab in US with guidance from a scientist who was working there, but he is very busy and not much time is left to ask him to write a recommendation letter (RL) since the application deadline is coming. 
I want to include this experience in my SOP, and wonder, in general, how is the reaction of graduate admission committees to research experience which is not backed up by a RL?
One of the duties of a researcher who takes on a mentor role is to write letters of recommendation for his students. Your advisor will understand this responsibility. You may want to read the answers to this question regarding writing your own recommendation letter, as this may be relevant to your situation, but you should always be willing to ask for a letter.