@ManyToMany(mappedBy="papers")   
   private List<User> authors;


   @ManyToMany(fetch=FetchType.EAGER)
   @JoinTable(name="t_authors_papers",
         joinColumns=@JoinColumn(name="user_id", referencedColumnName="login"),
         inverseJoinColumns=@JoinColumn(name="paper_id", referencedColumnName="id")
   )
   private List<Paper> papers;

public void remove author(String login){

Paper paper = em.find(Paper.class, paperId);
      List<User> authors=paper.getAuthors();
      for(User user:authors){
         user.removePaper(paper);
      }
      em.remove(paper);
}