What is the Standard Model of particle physics and why might it be broken?

Fundamentals of particle physics

The entities described by the Standard Model include 12 particles called fermions and 5 called bosons.

fermion are particles that you can think of as forms of matter, like the familiar electron. Other types of fermions include quarks – which bond to form protons and neutrons – as well as the ghostly particles known as neutrinos. For reasons we don’t yet understand, fermions come in groups of three. For example, there are three electron-like particles: electrons, muons, and tau. Muons and tau are much like electrons, except they are heavier and unstable, which means they are susceptible to rapid decay into other particles. Similarly, there are also three types of neutrinos (electron neutrino, muon neutrino and tau neutrino) and three pairs of quarks (up and down, strange and charm, down and up).

The bosons of the Standard Model are responsible for the forces that act on matter and energy in our universe. The best known boson is the photon, which composes the light. The electromagnetic force arises through interactions between photons and electrically charged particles. In other words, if the photon did not exist, there would be no electromagnetic force. Similarly, the bosons called gluons are responsible for the strong nuclear force, which binds quarks to form protons and neutrons. It also holds protons and neutrons together inside atomic nuclei. A third force, the weak nuclear force, which is responsible for the radioactive decay of atoms, results from the interactions of particles known as O and Z bosons. Meanwhile, the strength of the fourth force, gravity, is believed to be correlated to the Higg boson.


Cracks in the foundation

By any reasonable measure, the standard model was a resounding success. It has been used to predict that particles such as the Higgs boson and the top quark must exist – long before they were observed in an experiment. Simply put, it is the most empirically successful theory in the history of science.

That said, I’ve never met a physicist who thinks this theory is complete. There are several known facets of our universe that the Standard Model simply does not and cannot address.

Basically, the Standard Model ignores the existence of gravity. Despite considerable effort, we still do not know how to consistently include gravity in our theories of particle physics. To begin with, we were unable to find a force-carrying particle for gravity, or a so-called graviton. Fortunately, gravity has a negligible effect on the microworld explored by particle physicists. But this means that the macro-effects of gravity are not covered by the standard model.

Additionally, cosmologists have discovered that our universe contains vast amounts of dark matter and dark energy, none of which can be explained by anything in the Standard Model. Nor does it explain why we exist in a matter-dominated universe. In theory, the Big Bang should have created equal amounts of matter and antimatter – yet almost everything we see today is made of matter. Something must have tipped the scales, but we don’t know what.

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