Various experimental/theoretical researches and discoveries reveal that everything visible in the universe is made up of six quark (up, down, charm, strange, top and bottom) and six lepton (electron, electronneutrino, muon, muon- neutrino, tau and tau- neutrino) particles and these twelve particles interact with each other through strong, electromagnetic and weak interactions. According to modern theory, the particles exchange the mediator/carrier particles of their respective interactions. Intermediate bosons are the mediator for weak interaction, gluon is for strong interaction and photon is a mediator for electromagnetic interaction. This whole information related with the twelve particles and three mediators are summarized in a model, which is popularly known as Standard Model. This model is well tested through a variety of outcomes based on theoretical and experimental researches and it is competent to explain the atomic and subatomic world. Moreover, this model is of very simple and comprehensive nature. However, model demands that the mediator particles should have zero mass, whereas experimental observations reveal that intermediate bosons are massive in nature. This is the major complexity of the model, in context of atomic/sub-atomic world.
