For the sake of some of our readers, we’ll bring here, in alphabetical order, the definitions of some physical concepts used in the other articles.
Comment: electric charges are brought here in units of the proton charge.
Antiparticle: every elementary particle fulfilling the Dirac equation, has an anti-particle. The anti-particle has an opposite charge to that of the particle.
Antiquark: there are 6 of them-each one of the 6 quarks has a corresponding antiquark.
Baryon: A particle family which includes the proton, the neutron and many other particles. Except for the proton, all free particles of this family disintegrate, and the final state includes a proton and particles whose Baryonic number adds up to zero. Every Baryon is characterized by a 3-quark combination of a specific quantum state.
Boson: a denomination for an integral (non-negative) spin particle. See also Fermion below.
Dirac Equation: Quantum Mechanics’ fundamental equation for massive, spin 1/2 elementary particles. The equation successfully predicted major phenomena and finds a natural place in Field Theory. At its non- relativistic limit, and if spin effects are ignored, the Dirac Equation becomes the Schrodinger Equation.
Electromagnetic Force: the force applied by electric and magnetic fields on electric charges. Electric and magnetic fields are related to the physical laws described by the Maxwell Equations.
Electron: an elementary particle present in the atomic shell. The electron has an electric charge of -1.
Elementary particle: a pointlike particle.
Fermion: particles are classified into two categories based on their spin: Particles with integral spin are called Bosons, and particles with spin equals to a non-negative integer plus one-half are called Fermions.
Gluons: Physical QCD objects playing a role analogous to that of the electromagnetic field in the conventional atomic theory. However, the electromagnetic field also contains a radiation field (photons) emitted and absorbed by electric charge systems. On the other hand, there is no gluon radiation and that’s why gluons have never been directly observed in any experimental device. This means that their existence is QCD dependent.
h bar: see- Planck’s constant.
Hadrons: a general term designating Baryons and Mesons.
Impact of Special Relativity: The influence of this Theory on nucleon component’s masses: the Strong Interactions are indeed very strong, and it takes a huge amount of energy to separate between bound particles. According to Special Relativity energy is equivalent to mass, and therefore the mass of bound quarks is significantly smaller than the (unknown) total of their self masses.
Klein-Gordon Equation: a relativistic quantum mechanics equation supposedly describing a spin-0, massive particle.
Liquid drop model: a model describing a nuclear state supposing all nuclei, except for the lighter ones, has the same nucleon density. This is a common model describing properties of ground state of nuclei.
Maxwell Equations: Electromagnetic Theory’s fundamental equations. These equations predicted the existence of electromagnetic waves and are perfectly coherent with Special Relativity.
Meson: a particle made of a quark-antiquark pair.
Muon: this is a particle similar to the electron whose mass is about 200 times larger than that of the electron.
Neutrino: a particle carrying no electric charge, of spin 1/2, of a very low mass. There are 3 types of neutrinos, corresponding to the 3 members of the electron family: the electron, the muon and the tauon (called neutrino, muon –neutrino and tau-neutrino). Neutrinos were believed to be massless, but recent experiments conducted during the last decade suggest that they do have a mass. However, the possibility of their being massless is not totally discarded yet. The neutrino does not participate in Strong and Electromagnetic Interactions.
Neutron: a particle similar to the proton, present within the atomic nucleus. Its total electric charge is zero.
Nucleon: a global name for protons and neutrons.
Pauli principle: states that two identical fermions are not allowed to be at the same quantum state. Mathematically speaking, the Pauli principle requires that the wave function of any two identical fermions is an anti-symmetric function of 2 orthogonal single-particle wave functions. The incompressibility of fluids and solids is the consequence of Pauli’s principle as it is manifested in dense electron systems like that of liquids (and solids): a slight compression increases the electrons’ probability to jump up to a higher energy level and the liquid’s self energy increases. Since every system “wants” to minimize its energy, the liquid responds with a strong resistence to compression.
PDG: Particle Data Group: an international organization centralizing and processing particle related experimental results. This organization is considered as the oracle of the validity of particle existence and their physical properties.
Photon: a name describing the particle aspect of an electromagnetic wave.
Pi-meson (Pion): a meson composed of a quark and an antiquark from the u,d group of a specific quantum state (there are 3 of them). Its shortened name is pion. Pions are the lightest mesons.
Planck’s constant: a very fundamental constant in Quantum Theory. It serves as a natural measure unit for the angular momentum and the spin of particles and quantum states. The original constant is marked with h. A more useful constant, called h-bar, is h/(2*pi).
Positron: anti-electron. It has a positive charge.
Positronium: a bound quantum state of an electron and a positron.
Proton: a particle in the atomic nucleus. The proton has a positive +1 electric charge.
Quantum Field Theory: This is a theory with many consequences. With regard to the proton (and the neutron) the following phenomenon is mentioned here: in fact, the proton’s state is not defined merely by 3 quarks. According to quantum field theory, a probability exists that the proton’s quantum state will include one or several additional pairs of quark-antiquark. And indeed, antiquarks were directly measured in protons. This means that for the proton, this probability is not negligible.
Quark: an elementary particle. There are 6 kinds of quarks: u,d,s–Strange, c–Charmed, b and t. The proton’s state is characterized by three uud quarks. u’s electric charge is 2/3 and d’s electric charge is -1/3. The neutron is characterized by 3 udd quarks. There is a terminological convention here. In atoms, the nature of chemical properties is mainly defined by the electrons in the external shell. They are called valence electrons. Similarly, these 3 quarks are called in the literature “the valence quarks”.
Screening phenomenon: a phenomenon known from electricity theory. An example relevant to the present issue, is that the atom is made of an electrically charged nucleus and its geometric dimensions are very small. The nucleus is wrapped in a sort of electron cloud and in a non-ionized atom, the absolute value of the sum of the electron negative charges is equal to the nucleus’ charge. At a relatively great distance from the atom, there is no electric field related to the atom in question, because the field of the electron charges cancels that of the nucleus charge (i.e., screens it). On the other end, at the proximity of the nucleus, the prominent field is the one related to the nucleus and the field related to the charge of the electrons is negligible. In other words: at a great distance from the atom, electrons totally “screen” the nucleus’ field. Very close to the nucleus, they don’t screen it at all; at some intermediate point there is a partial screening, which increases with the distance from the nucleus.
Schrödinger Equation: The first successful equation of Quantum Mechanics. This is an approximate equation which does not deal with the spin and neglects Special Relativity Effects.
Special relativity Theory: see the Impact of Special Relativity.
Spin: A property characterizing a quantum state in general and an elementary particle in particular. The spin is like a kind of a quantum top. The spin can have values of N/2, when N is an integral, non-negative number. An important particle family is the “Dirac particles” having spin 1/2. The common measure unit of spins is actually h-bar.
Strangeness: an expression related to one of the quark types’ name.
Strong Interaction: holding quarks together (in Baryons or Mesons).
Strong Nuclear Interaction: a residual force of the Strong Interaction, binding protons and neutrons in the atomic nucleus.
Tau lepton, (sometimes called Tauon) is the third and heaviest electron-like particle. The electron family includes the electron, the muon and the tauon. These particles are all elementary, massive, have spin 1/2, carrying a negative electric charge and having each an anti-particle. The Tau lepton is some 3500 times heavier than the electron and about 17 times heavier than the muon. Its half-life is measured in units of 10-13 seconds.
Uncertainty principle: this principle defines a lower limit to the imprecision related to physical quantities, formulated by means of h-bar. This principle applies to the imprecision product of time and energy. For example, a particle whose typical lifetime is very short, allows, in principle, a good knowledge of the time interval during which it took part in the measurement. Therefore, its intrinsic energy measure (i.e. its mass) will be associated with an appropriate inaccuracy. These relations also apply to position and momentum.
Van der Waals force: a force interacting between neutral molecules.
Wave: every particle has a wave quality, and therefore has a wavelength. A particle’s wavelength depends on the particle’s velocity (or, more accurately, on the particle’s momentum). Here, the wave length is inversely proportional to the momentum. A fundamental wave property states that when a particle with a relatively large wavelength hits a target of composite particles, it does not notice the target’s small details. Modern particle accelerators’ growing capacity to accelerate particles to increasingly higher energy and momentum leads to the production of particle beams of increasingly smaller wavelengths, allowing researchers to obtain more detailed information on the target particles’ structure. This is how, 100 years ago, the atomic structure had been studied, then some time later, the nucleus’ structure was explored, and since about 40 years now, the proton’s structure has been revealed. (Note: the medical imaging instrument called “ultra-sound scanner” uses an extremely short wavelength, operating on the same principle. The same is true for an electron microscope where a short electron wavelength enables detecting tiny details that cannot be seen by optical waves.)
Weak interaction: In states whose energy is not extremely high, the weak force reveals itself only in systems that are stable against strong and electromagnetic transitions. Here the weak interaction’s time scale is much longer. The Weak Interaction can disintegrate particles and produce other particles. For example, a free neutron decays into a proton, plus an electron and an anti-neutrino as a result of the Weak interaction. At very high energies, such as those of the W particle, the weak interaction is quite significant and it is accountable of nearly one third of the disintegrations.