Do you want an image processing solution or you are looking for a network model to detect the half of the body?
I'm not sure you will find an algorithm, which will detect a half of the human body. I think you shuld take any model (from opencv or network models), which detects human full body, and after that you will have to modify that model, to get what you want. There are a lot of implementations of person detection or person segmentation.
I'm looking for an algorithm which is able to detect half of the human body