Standard Model is a theory in physics describing the particles that make up matter and explaining how those particles interact. According to the Standard Model, the matter we encounter in our daily experiences consists of atoms (see Atom ). A typical atom, in turn, is made up of negatively charged electrons and a positively charged nucleus. The nucleus contains positively charged protons and electrically neutral neutrons. And protons and neutrons are made of quarks, which can have positive or negative charges.
Particles interact by means of forces. The force that attracts electrons to nuclei in atoms is known as the electromagnetic force. The strong force holds quarks together in protons and neutrons, and it holds protons and neutrons together in a nucleus. The weak force is responsible for certain kinds of radioactive decay. For example, a free neutron—one that is not part of a nucleus—is radioactive. It will decay to a proton, an electron, and a particle known as an antineutrino. A fourth force, gravitation, is not part of the Standard Model.
The Standard Model explains that forces are transmitted by particles known as force carriers. The carrier of the electromagnetic force is the photon . In an atom, a proton interacts with an electron by means of a photon. The proton emits (sends out) a photon, which the electron then absorbs.
The carrier of the strong force is the gluon . In a proton, one quark emits a gluon, which another quark then absorbs.
There are three carriers of the weak force: two W bosons—one positively charged; the other, negatively charged—and an electrically neutral Z boson. A free neutron decays by changing into a proton and a negative W boson. The boson then breaks apart into an electron and an antineutrino.
Since the early 1970’s, scientists have subjected the Standard Model to numerous tests, such as experiments performed at the Large Hadron Collider at the CERN research center in Switzerland. Most of these experiments have produced results that are expected according to the Standard Model. However, highly precise experiments measuring the magnetic properties of a particle called the Muon have produced results that appear to contradict the Standard Model. Physicists do not yet know how to explain these measurements. Theoretical physicists may have to improve the calculations in the Standard Model, or modify or expand the Standard Model, to explain these results.
See also Atom (The parts of an atom) ; Force ; Gravitation ; Quark ; Subatomic particle .
