A neutron is made of two down quarks and one up quark . Under the Standard Model of particle physics , because the neutron consists of three quarks , the only possible decay mode without a change of baryon number is for one of the quarks to change flavor via the weak interaction . The neutron consists of two down quarks with charge â ' e and one up quark with charge + e , and the decay of one of the down quarks into a lighter up quark can be achieved by the emission of a W boson .