Should I mention an informal research intern in my CV? I was doing an informal research intern in a research institution more than a year ago, and it's probably the only experience related to the scientific research during my undergraduate time. Basically it's on a specific topic on AI, and I read papers and did the experiments under the instructions of graduate students of the institution. We submitted a paper where I was one of the co-author to a journal, but it was not accepted.
The word informal means:

I didn't sign an agreement with the institution and thus I didn't get the certificate from the instutution
I was doing the work with graduate students instead of the faculties of the institution. I even didn't see the professor (the supervisor of the graduate students who guided me). And it's almost impossible to get a letter of recommendation from the professor of the institution)

If I don't mention it in the CV, I will be the applicant with bare research experience.
If I mention it, without a letter of recommendation from a professor related, I will probably lose the credibility of the research experience.
PS: I am applying for a Master Program of Computer Science. I have other interns in tech companies but they are not for scientific research.
What should I do?
You have scientific research experience relevant to the position you're applying for, so explain that experience in your application.
You can mention that you worked with graduate students and you could even ask one of them for a letter of recommendation. You cannot mention working with a professor, because you didn't, nonetheless, they may be willing to write a letter of recommendation that mentions that you worked with their graduate students and they could perhaps quote one of them for the details.