I am a PhD in CS student in USA. I will be submitting 2 manuscripts in September to a computer science conference (ACM CHI). I have been working on one of the papers with my advisor and the other paper completely independently. The other paper is a passion project of mine which is close to my advisor's research interests but I do not want anyone else to get involved in it because it is a deeply personal subject. I wanted to ask if it is ok for me to submit a manuscript for this paper without consulting or notifying my advisor since I am not asking for his input nor am I using the resources at my school to submit this (I have been working on this paper on my own time i.e. summer vacation + other holidays, weekends etc.). I know there is a very slim chance that this submission will be successful but in the off chance that it is, will there be any awkwardness that I have to face with my advisor? I should reiterate that him and his works are in no way related to what I am submitting. We share the same research interests but my work is independent.
In my view this is asking for potential trouble. What are you going to explain when the paper gets accepted? If you can explain it later, you should also be able to explain it now.
You probably have your reasons why you want to publish as a sole author. The most sensitive part here is that you and your advisor share the same research interest. Even if the work is independent your advisor could be upset because (1) you didn’t inform him, (2) you are working in his field, (3) you did not give him a chance to contribute while he invests a lot of time in your career, (4) you may cause him to feel embarrassed, (5) you could be competing with him.
Personally, I include all my advisors on my papers and I make sure they have a contribution, just to prevent these types of problems. After your PhD you may want to maintain good relationships with your advisors.
If there are reasons you did not mention for wanting to publish as sole author without notifying your advisor, I would indentify those reasons and address them now. For example: your advisor takes advantage of you, you are afraid that he will get or take the credits, you’re afraid that he will not support your individual work, or you’re afraid you will have to adapt and restructure too much. Discuss it openly.
In the end submitting that article as a sole author is your decision (and you are entitled to it) but I would do it openly. The problems you face now are probably less than the problems you may face in the future.
While user Alice points out a lot of potential down sides let me note another point along with some disagreements.
I suspect that there is very low probability that CHI will accept both papers unless they are in different "Tracks" and on very different topics. If that is the case, there shouldn't be any conflict with your advisor. But if they are going into consideration for the same track, each paper is in competition with the other, as the committee will favor a variety of presenters/papers/viewpoints.
As to the problems suggested by Alice, I think that only point (1) has real validity and that is just a point of curtesy. On the other hand it is good to be courteous, especially to people who help and work with you.
But you are (2) supposed to be working in his field. And (4) he should feel proud that he has helped you be come independent, not embarrassed. Also (5) if you fear "competing with him" then you can never write papers in this field anymore, since it is his field too. Therefore I discount all of that.
As to (3) you explain that it is very personal, though at some point you will likely need to explain that to him. In fact, your independent paper might possibly be sent to him for review.
If you want to sooth the waters in advance, you can acknowledge the general help that your advisor has given you to make it possible to develop this work. You needn't lie and thank him for specific contribution to it, but it is true that part of what you are was aided by his guidance. You can honor that.