recently i am reading chiral symmetry in QFT. Almost all textbooks define γ5 as a chiral operator without saying some reasons. i am very confused why γ5 has something to do with chiral symmetry, can somebody explain it more intuitively and physically? who first introduce γ5 as a chiral operator? and i also want to know what is a chiral transformation? is it a transformation for example which can transform a left-handed to a right-handed? Thanks!