25.5. The Standard Model http://www.ck12.org
25.5 The Standard Model
Vocabulary
- electroweak force:Force composed of nuclear weak force and electromagnetism.
- quantum chromodynamics:Theoretical frameork.
- quarks:Small particles with a fractional charge that make up nucleons.
- standard model:Model combining everything that was known in the 1970s about the myriad of subatomic
particles of the time. - weak nuclear force
The weak nuclear force and the standard model
We have mentioned several times that there are four fundamental forces in nature:
- Gravity
- Electromagnetism
- The strong nuclear force
- The weak nuclear force
It would not be proper to end this chapter without at least a passing reference to the weak nuclear force and the
ultimate goal of theoretical physics.
Physicists believe today that every particle has an associated “antiparticle.” For example, the antiparticle of the
electron is called the positron. It is identical to an electron in every way except for the fact that its charge is positive.
When they combine, they annihilate each other A neutral particle like the neutron also has an antiparticle and a
neutron and its antiparticle also annihilate each other when they combine. The realization that all particles have
antiparticles is instrumental in developing a complete theory of subatomic particles, which is why we mention it
here.
We stated earlier that a free neutron (one not confined to the nucleus of an atom) decays into a proton, an electron,
and an antineutrino. The net charge remains zero, the antineutrino having no charge. An antineutron decays into
an antiproton (negative charge), a positron, and a neutrino (again, the net charge is zero.) How, you may wonder,
can a neutron be considered a fundamental particle if it can decompose into other particles? Indeed, this is not the
case. Physicists no longer accept that nucleons are fundamental particles. The electron is still believed to be a
fundamental particle, though.
If there are particles more fundamental than nucleons, then the nucleons must be composed of yet smaller particles.
Physicists recognize these particles as having fractional charge and call themquarks. It is the weak nuclear force
which allows quarks to transform a proton into a neutron (and vice versa). It is also known that the weak nuclear
force creates neutrinos and facilitates certain types of radioactive decay and plays an important role in solar fusion.