Elementary Particles
< back to articles

Elementary Particles

By Staff Writer

A particle that is not known to be made of any other particle can be called an elementary particle. Quarks and leptons are the fundamental constituents of matter since they are not known to be made of any other particles. These particles along with the particles known as exchange particles form the family of elementary particles.

All matter, living and inanimate, is made of smaller quantities of matter and are divisible into their constituents. In ancient Greek, philosophers considered all matter to be consisting of water, earth, air and fire, the four fundamental building blocks. In the 18th and 19th centuries there were many studies to find out the constituents of these fundamental building blocks, if any. Molecules were first thought to be the tiniest, indivisible constituents of all matter. Then came the atom, followed by the discovery of electrons, protons and neutrons, all collectively called fundamental particles that goes into the making of matter. The discovery of radio activity suggested the existence of another kind of particles called neutrinos. Then came the revolution that further populated the world of particles by the discovery that protons and neutrons that constituted the atom were not fundamental any more but consisted of tinier particles known as quarks. So, what are the elementary particles?

A particle that is not known to be made of any other particle can be called an elementary particle. That is, the very basic building blocks of matter. All matter consist of quarks and lepton, two broad category of elementary particles. Of these quarks combine to form protons and neutrons that constitute the atomic nucleii. There are six distinct types of quarks, up (u), down (d), strange (s), charm (c), bottom (b), and top (t). The up, charm and top quarks carry a positive electric charge equal to 2/3 of the electric charge carried by an electron while the down, strange and the bottom quarks carry a negative charge equal to 1/3 of that of an electron. All quarks have their respective antiquarks, represented by u̅, d̅, s̅, c̅, b̅, and t̅. Leptons include the electrons (e−), muons (&mu−) and tau leptons (&tau−), all carrying a negative electric charge of unity, along with their respective neutrinos, the electron neutrino (&nue), muon neutrino (&nu&mu) and tau neutrino (&nu&tau), that are electrically neutral. Leptons, just as in the case of quarks, have their antiparticles as well. These six quarks and six leptons constitute all the matter in the world.

A proton, one of the constituents of atomic nucleus, consists of two u quarks and one d quark. Symbolically,
p &rarr uud
The fractional electric charges on the two up quarks and one down quark totals to one positive charge equal to that of a proton. Similarly, a neutron consists of one up quark and two down quarks,
n &rarr udd
the charges of constituent quarks adding up to zero making the neutron electrically neutral.

The electron has a mass of 9.11 × 10-31 kg. In particle physics it is customary to use a different unit of mass known as electron volt or eV. The mass of an electron is 0.51 MeV (0.51 × 106 eV), the muon has a mass of 105.7 MeV and the tau lepton has a mass of 1.7 GeV (1.7 × 109 eV). The neutrinos are very light particles and were considered massless until very recently. The electron neutrino has a mass of 2.2 eV, the muon neutrino has a mass less than 0.17 MeV and the tau neutrino mass is close to 15.5 MeV. Neutrinos are electrically neutral. The masses of the quarks are 2.4 MeV, 1.27 GeV and 171. 2 GeV for the up, charm and top, respectively and 4.8 MeV, 104 MeV and 4.2 GeV for the down, strange and bottom quarks, respectively. There is an uncertainty in the deduction of quark masses mainly because they are in a state bound by the strong force. The up and down quarks have comparable masses, consistent with the fact that protons and neutrons have almost the same mass, with neutron slightly heavier than proton. Neutrons are unstable and decay into protons. The up and down quarks were discovered as early as 1968, in scattering experiments at SLAC, but were identified as quarks much later. The charm and bottom quarks were discovered in 1974 and 1977 respectively. The heaviest of all quarks, the top, was discovered as recently as 1995. Very high energy experiments were required to produce particles that contained the top quarks.

All the particles have an intrinsic property called spin. It is similar to the angular momentum and is the direction of rotation about its own axis, to state it in the simplest way. In quantum mechanics, spin quantum number is one of the degrees of freedom associated with the quantum state of an elementary particle. It takes integral or half integral values n/2 of reduced Planck’s constant h/2π n = 0, 1, 2, …. By definition it is a dimensionless quantity. In statistical mechanics, the distribution of particles in a given state is described by Fermi-Dirac statistics and Bose-Einstein statistics. The former is applicable to a set of particles having half integral spin, known as fermions, while the latter applies to particles with integral spins, or bosons. Quarks and leptons are fermions having half integral spins 1/2. Neutrinos of all “flavors” (the type of neutrino is given the common term flavor; neutrinos come in three flavors, electron, muon and tau) have left handed helicity (one of the two spin states, up or down) while all antineutrinos have right handed helicity.

The quarks and leptons are classified into generations. There are three types, generation I, II and III. In the first category, there are the up and down quarks, and the the electron and the electron neutrino. That is, two quarks and two leptons. The charm and strange quarks muon and the muon neutrino belong to generation II. The top and bottom quarks and tau lepton and tau neutrinos belong to the third generation. It is noted that the elementary particles become more massive as we move from generation I to generation II and then to generation III. All particles in one generation have the same properties, such as charge and spin.

Electron Volt (eV)

A particle carrying an electric charge q passing through an electric potential of V volts acquires an energy E = qV. An electron volt (eV) is the kinetic energy acquired by a free electron when it is accelerated through an electric potential difference of one volt. The change on the electron is 1.602 × 10-19 Coulomb. The potential difference of one volt is equal to one Joule of energy per one Coloumb of charge. Therefore, one electron volt is 1.602 × 10-19 Joules. The electron volt is a unit of energy. However, according to the theory of relativity, mass and energy are interchangeable through
E = Mc2
where M is the mass of the particle and c is the speed of light. Substituting,
eV = Mc2
or
M = eV/c2
The mass of an electron is 9.1 × 10-31 kg or 0.51 MeV (M stands for mega or 106).

Quarks, though a fundamental particle, never exist as a free particle. They are bound together by strong force acting on a very short range of the order of femtometer, 10-15 m, which is smaller than typical diameter of nucleii. Applying more and more force to overcome the strong force between them helps create new quarks which soon combines with other particles. Leptons, on the other hand, are seen as free particles, electrons and electron neutrinos, for example. Muons as well as muon neutrinos are found in the upper atmosphere, produced by cosmic rays. The tau lepton and its neutrinos are found only in laboratories. The electron neutrinos are produced in &beta decays, atomic reactors and inside the solar core. Electrons, the first ever detected elementary particle, can be found in atoms, produced in &beta decays and also in electric currents. And, all leptons can be produced in laboratories. Muons are more massive than electrons, so they decay into electrons and neutrinos.

An interesting property of quarks is that they can appear as group of three quarks (qqq) or three antiquarks (q̅q̅q̅) or as quark-antiquark pair (qq̅). This behavior is ensured by the strong force that is responsible for all interactions involving quarks. Other types called tetraquarks (qqq̅q̅) and pentaquarks (qqqqq̅) have been postulated, as early as 1987, but there are not enough evidence to prove their existence. Such states are called exotic hadrons.

Hadron is the collective name given to components of matter consisting of quarks and the interactions being mediated by strong force. Quarks carry a color charge, one of red, blue, green and antiquarks carry one of anti-red, anti-blue and anti-green. Quarks combine in such a way that the resulting product is colorless. That is, either a combination of red, blue and green quarks or a colored quark with an anti-colored one. In either case the resulting one is colorless. Hadrons consisting of three quarks with three different colors are called baryons where as those with quark-antiquark pair, carrying one color charge and its anti-color, are known as mesons. The bound state of three antiquarks form antibaryons.

In addition to the physical properties such as mass and electric charge, quarks have internal properties. One such property is the baryon number. All quarks have baryon number equal to 1/3 and antiquarks have baryon number -1/3. Since baryons are made up of three quarks, the baryon number for all baryons become 1. In the case of mesons, which consist of quark-antiquark pair, the baryon number is zero. In all interactions baryon number should be conserved.

Most familiar baryons are protons and neutrons. All baryons except protons are highly unstable. Proton is the lightest baryon, and all heavier baryons decay into lighter baryons, according to the conservation baryon number, which undergoes decay producing still lighter baryons. This process goes on until the final product is proton which is stable and doesn’t decay. Mesons are not common in the sense that we do not encounter them in everyday life. They are produced by the interaction of cosmic rays with matter and were discovered in the cosmic ray experiments.

Quarks also have flavor number. As the different types of quarks are termed as flavors, the flavor number for each quark of the same flavor is 1 and for all other flavors it is zero. That is, there are 6 flavor numbers bearing the name of each flavors, upnes, downness, charm, strange, topness and bottomness. For the up quark, the upness is one and all five other flavors are zero. Similar is the case with all other flavor numbers. Not to mention that the antiquarks have same flavor number as quarks but negative.

Continued in the next issue …