I am new in this group and yet to know the rules about posting doubts/ questions. So first of all if anybody helps me with the guidelines (which may be some link) to be followed, I will appreciate that. Second, my main question is: what is meant by higher dimensional proton decay operator? I have seen this term in some titles of papers and also in Wikipedia article on proton decay, but I have not found any explanation about its meaning. I have not come across such terms in my elementary quantum field theory course. In wiki article, qqql/ (Lambda)^2 is said to be a dimension-6 proton decay operator. I do not understand how that is so. can anybody explain?