Antiparticle of the proton, discovered by the Italian physicist Emilio Segrč in 1955
at the California University (Berkeley), by bombarding a metallic target with 6,2 Gev
protons produced by a synchrotron.
It has a negative electric charge equal, in modulus, to the one of the proton and a mass
equal to the one of the proton.
Elementary particle discovered in 1897 by the English physicist Joseph John Thomson (
Nobel prize winner in 1906 ), by experiments on the electric discharges in the rarefied
gases.
Thomson, while studying the effects of combined electric and magnetic fields on the
cathode rays, that is on the electrons, succeeded to determine the e/m relationship
between the electric charge and the mass of the electron.
The measure of the negative electric charge of electron
(e = 1,6*10-19 C ) was effected in 1909 by the American physicist Millikan in a
series of very careful experiments effected on oil microscopic drops electrified by
X-rays.
The electron is sensitive to the weak and electromagnetic forces.
The quantum of electromagnetic energy E=hf, where h is the Planck universal constant and f the frequency of the
electromagnetic wave, was introduced in the first quantum theory by Albert Einstein to
explain the photoelectric effect, that consists in the electron emission by a metal struck
by light radiation.
In the theory of quantum electrodynamics, that derives from the application of the special
relativity theory to the quantum mechanics, to describe the behaviour of the charged
particles, attractive or repulsive forces between two electric charges are produced by the
exchange of a photon, that is of a packet (quantum) of electromagnetic energy produced
violating the energy conservation principle, because a system with a total energy equal to
rest-mass energy associated to total mass of two particles, trasforms into a system with a
greater total energy, just by taking account of the generation of a photon with energy E = hf.
Therefore the photon must be a virtual photon, so that the violation of the energy
conservation law isn't evidenced.
The photons associated with the radiation, that can be light or X or g
radiation , have a rest-mass equal to zero and are movingto with the velocity of the
light, transporting energy and momentum.
The photons belong to the family of the vectors of force and are bosons, since their spin
is equale to 1.
Particles with mass greater of that of the electron, belonging to the family of the
hadrons, because they are sensitive to both the strong interactions and the weak ones.
The hypothesis of the existence of a particle with a mass equal to about 200 times the
electronic one, was advanced in 1935 by the Japanese physicist Yukawa, to explain the
strong interaction between two nucleons ( proton and neutron ) in the atomic nuclei.
By analogy with the interpretation of the electromagnetic interaction between two charged
particles, provided by the theory of quantum electrodynamics and founded on the exchange
of a photon, Yukawa hypothesized the exchange of a p meson
between two neutrons
( neutral meson ) or between a neutron and a proton ( charged meson ), admitting a
violation of the mass-energy conservation principle , provided the relative flight time
(with velocity c) is compatible with the uncertainty principle of Heisemberg.
One of the formulations of this principle is equivalent in fact to affirm that, the
greater is the experimental error DE
associated with the measure of the energy of a particle, the smaller is the duration Dt of the measure, with law of inverse
proportionality: DE.Dt
~= h/(2p). Therefore, since DE = mc2 is the relativistic energy associated with
the mass m of the virtual meson generated and R = cDt is the esteemed range of the nuclear forces (~ 2*10-13cm
), we have:
mc2 (R/c) ~= h/ (2p) ; m ~= h/
(2pRc) ~= 193 electronic masses.
A particle with mass equal to about 200 times the mass of the electron was discovered in
1938 from Anderson and Neddermeyer studying the showers of the secondary particles
produced by the cosmic rays.
Such a particle was identified with the Yukawa particle till the 1947, when Powell and
Occhialini discovered the apparent p meson, that does part of the cosmic radiation and
decaies producing the " meson " of Anderson and Neddermeyer and a neutrino.
Since 1947 is known that the p meson, that is sensitive to the
strong interaction and determines the interactions among nucleons, has a rest-mass equal
to about 270 electronic masses, and is quite distinct, being a hadron, from the "
meson " discovered in 1938, which instead belongs to the family of the leptons, that
are sensitive to the electro-weak interaction.
Therefore the "pseudo-meson" of Anderson is named muon.
After 1947 were discovered in the cosmic radiation other mesons, either charged or
neutral, the K mesons, the so-called kaons, with masses equal to about 960 electronic
masses.
The kaons, since evidence some anomalies connected to the various decay modes into less
heavy particles, are named strange kaons, that is particles with a new typical category of
the subnuclear particles world: the strangeness.
Fundamental particle whose existence was hypothesized in 1933 from the Austrian
physicist Wolfgang Pauli to explain the variability of the kinetic energy of the electrons
emitted by the nuclei subjected to the b radioactive decay .
Pauli postulated the existence of a mass-less neutral particle by which it were possible
to explain the fact that the emitted electrons are characterized by a kinetic energy
variable from zero and a maximum value.
By admitting the existence of the neutrino, it is possible justifie for the b decay the
validity of the energy and momentum conservation principles.
In fact, if it is postulated that the velocities of the emitted neutrinos have opposite
directions to that of the electrons and kinetic energies such that added to that of the
electrons give a constant sum, are justified immediately the characteristics observed
experimentally for the b decay .
Since neutrinos have no mass, consequently it is deduced that they are moving with the
velocity of the light, as the photons.
Exist three neutrino types with the respective antiparticles, associated respectively to
the electron, the muon and to the lepton t .
The neutrinoes belong to the family of the leptons and are sensitive only to the weak
subnuclear force.
Particle discovered by the English physicist Chadwick in 1932, which furnished a new
interpretation of the experiments effected by Bothe and Becker ( 1930 ) and by the French
Mr. Joliot and Mrs. Joliot-Curie ( 1932 ) bombarding boron targets and of beryllium with
the a particles ( helium nuclei ) emitted by radioactive
elements.
The interpretation proposed by Chadwick evidenced the presence of the neutron in the
atomic nuclei, that before 1932 were thought to consist of protons and electrons.
The neutron is electrically neutral and its mass is nearly equal to the
one of proton.
It belongs to the family of the hadrons and is subjected therefore to the strong,
electromagnetic and weak forces.
The neutron and the proton are both named nucleons, as components of the atomic nuclei.
Antiparticle of the electron (positive electron ), discovered in 1932 by the American
physicist Anderson and subsequently ( 1933 ) by the physicists Blackett ( English ) and
Occhialini.
The discovery of the positron, the first antiparticle identified experimentally by using
the forsights of the relativistic quantum mechanics formulated by the English physicist
Dirac ( 1928 ), was effected by studying with fog-chambers (Wilson chambers ), in presence
of a magnetic field, the electron-positron
( particle-antiparticle ) pairs produced by making lead targets to be bombarded by long
range g photons furnished by the cosmic rays. Were observed for the first time some pairs
of circular trajectories with a mirror-like symmetry (trajectories with opposite bending
radii ), described in a magnetic field by particle pairs of equal mass and opposite
electric charge.
The positron is sensitive to the electromagnetic and weak (electro-weak) interactions and
belongs to the first generation of leptons.
Particle considered elementary till thirty years ago, when were obtained in the high
energy physics laboratories, in USA and in Europe , the first experimental confirmations
of the validity of the theoretical quark model proposed quark in 1964 by the American
physicist Murray Gell Mann.
The proton, that constitutes the nucleus of the atom of hydrogen, has a positive electric
charge equal, in modulus, to that of the electron, and a mass equal about to 1840 times
the electronics one.
The proton belongs to the family of the hadrons and is sensitive therefore to the strong,
electromagnetics and weak interactions.
