Fermi, in Chicago, focuses on subnuclear physics thanks to accelerating machines of gradually increasing energy particles. He contributed to the development of the cyclotron, then the most powerful in the world, and in particular to the creation of the great magnet of the machine. Studying the collisions with a hydrogen target of the pions produced by the cyclotron, Fermi in 1952 discovered the first example of a new category of particles, with a very short life, called “resonances”. This is the Δ ++ particle which will acquire a crucial role for understanding the quark structure of particles and the strong interaction between quarks, called subnuclear “color” interaction.