2.4 The Standard Model in Particle Physics
2.4.1 The Four Fundamental Forces in Nature
In the 20th century, physicists used radioactive decay, cosmic rays, and more and more powerful particle accelerators in order to get a fairly complete picture of the structure of matter. Nowadays physicists assume that there exist the following four fundamental forces in nature.
(i) Strong force(e.g., the proton as a bound state of three quarks).
(ii) Electromagnetic force (e.g., the chemical binding of molecules).
(iii) Weak force(e.g., theβ-decay of the neutron).
(iv) Gravitational force(e.g., the motion of planets around the sun, the ex- pansion of the universe, and black holes).
Some important properties of these forces are summarized in Table 2.1. As we will see below, the electromagnetic force and the weak force are part of a unified force called the electroweak force.
Strong force. The nuclear force is responsible for the stability of the proton and for the relative stability of the neutron and the atomic nucleus, which consists of protons and neutrons (nucleons). The range of the strong force is equal to the diameter of the proton, namely, 1 fermi = 10−15m.
Table 2.2. Typical experimental data
interaction
typical cross section in millibarn
(10−31m2)
typical mean energy of resonances
in MeV
typical mean lifetime of resonances
in seconds
strong 1 102 10−23
electromagnetic 10−3 10−3 10−18
weak 10−9 10−14 10−7
The proton consists of twouquarks and one dquark, whereas the neutron consists of twodquarks and oneuquark. The protonpis the nucleus of the hydrogen atom. The neutron was discovered experimentally by Chadwick in 1932 (Nobel prize in physics in 1935). In 1932 Ivanenko predicted that the nucleus consists of protons and neutrons.
Electromagnetic force.The electromagnetic force is responsible for the stability of atoms and molecules by acting on the protons and electrons of the atoms. All physical and chemical properties of solid states, liquids, and gases are based on the electromagnetic force. The range of the electromag- netic force is infinite. The electron was discovered by Thomson in 1895 who investigated cathode rays (Nobel prize in physics in 1906). If electrons hit a metal, electromagnetic rays of high energy are generated. These so-called X-rays were discovered by R¨ontgen in 1895 (first Nobel prize in physics in 1901). A semiclassical model of the atom was formulated by Bohr in 1913 (Nobel prize in physics in 1922). Bohr was strongly influenced by Ruther- ford’s scattering experiments performed in the years 1909–1911. The final model of the atom was based on quantum mechanics invented by Heisenberg in 1925 (Nobel prize in physics in 1932) and Schr¨odinger in 1926 (Nobel prize in physics together with Dirac in 1933). The shell structure of the atom can only be understood by using the electron spin and Pauli’s exclusion principle for fermions (e.g., electrons) from 1926. Pauli was awarded the Nobel prize in physics in 1945.
Weak force.This force is responsible for the radioactive decay of atoms discovered by Bequerel in 1892 (Nobel prize in physics together with Marie and Pierre Curie in 1903). The basic reaction is theβ-decay of the neutron,
n→p+e−+νe.
That is, the neutron n decays into one protonp, one electron e−, and one anti-electron neutrino νe. The mean lifetime of the neutron is 15 minutes.
Experiments show that the mean lifetime of the proton is larger than 1032 years. This is an incredibly huge number.26The existence of the neutrino was
26Note that the age of our universe is 13.7ã109 years.
predicted by Pauli in 1933 in order to guarantee momentum conservation in the neutron decay. Radioactive decay and the chemical properties of radioac- tive substances were studied around 1900 by Marie Curie (Nobel prizes in physics and chemistry in 1903 and 1911, respectively), Pierre Curie (Nobel prize in physics in 1903), and Rutherford (Nobel prize in physics in 1908).
These three scientists found out that radioactive decay generates three types of rays:
• α-rays (helium nuclei42He),
• β-rays (fast electrons),
• γ-rays (high-energy photons).
Visible light has a wave length between 4ã10−7m and 8ã10−7m. In contrast to this,X-rays andγ-rays have a short wave length of 10−10m and 10−13m, respectively. The energy of X-rays and γ-rays is much stronger than the energy of light rays. Note that the energy of photons increases if the wave length decreases. Cosmic rays were discovered by Hess in 1911 (Nobel prize in physics in 1936). In 1928 Gamow explained the production of α-rays.
If α-particles would be classical particles, they could not leave the nucleus because of the existence of a strong potential barrier. Therefore, it is crucial thatα-particles are quantum particles. They possess stochastic properties. In particular, they can leave the nucleus by “tunnelling” the potential barrier. In 1934 Fermi used slow neutrons in order to produce new radioactive elements containing a large number of nucleons. Fermi was awarded the Nobel prize in physics in 1938.
The neutrino was experimentally discovered outside a nuclear reactor in 1956. At the Savannah River reactor (Georgia, U.S.A.), the number of neu- trinos emerging per second was extremely high, and physicists waited with their detector until they eventually detected some. Other neutrino sources are our sun and outbursts of supernovae. Note that the energy production of the sun is based on a series of nuclear reactions which converts hydrogen into helium. These reactions start by the process
p+p→ 21D +e++νe.
Here, the fusion of two protons yields one deuteron21D plus one positrone+ and one electron neutrinoνe.This process is caused by the weak force.27For his theory of the energy production in stars, Bethe was awarded the Nobel prize in physics in 1967. Neutrinos coming from the sun were detected by Davies in 1970. In 1987, a supernova explosion took place about 160 000
27The nucleus of an atom consists of protons and neutrons called nucleons. The symbolNZA stands for the nucleus of an atom that consists ofN nucleons andZ protons. Hence the number of neutrons is equal toN−Z.
The symbole− (resp.e+) tells us that the electron (resp. positron) has the negative (resp. positive) elementary electric charge −e (resp. e). Similarly, Z0 tells us that theZ-boson has no electric charge.
light years away in the Magellanic Cloud next to our Milky Way galaxy. This star had 8 sun masses. The released energy was enormous,
E= 1046J.
More then 99% of energy came out in invisible form – as neutrinos – based on the reaction
e−+p→n+νe.
Two experimental groups in the United States and Japan reported detect- ing neutrinos at the time of the supernova. The experimental detection of neutrinos is a highly nontrivial task, since their interaction with matter is extremely weak.
Phase transitions of the fundamental forces in the early uni- verse. Many physicists assume that there was only one fundamental force at the time of the Big Bang. The cooling of the universe was responsible for phase transitions of this fundamental force which led to a splitting into the four fundamental forces observed in nature today.