The original three-quark model (up,down,strange) proposed by Gell-Mann and Zweig, working at the Brookhaven National Laboratory, allowed to explain the properties (mass, electric charge, spin and strangeness ) of all the discovered particles, and was considered valid until in 1974 was discovered
a new particle with a 3,1 GeV mass, whose existence, differently from the W-baryon, wasn't foreseen within the model.
The new particle was discovered indipendently by the research group working at the Brookhaven National Laboratory, who assigned to it the J symbol ,and by the one working at the SLAC (Stanford Linear Accelerator Center- California), who assigned to it the Y symbol.
The new particle, that resulted be a neutral meson, with an unitary spin, was produced by making collide high energy electrons against positrons in a storage ring, that is a special circular accelerator in which circulate in opposite directions both an electron and a positron beam, that are accelerated and are made to collide, by means of suitable deflection magnetic fields, in two or more cross points of the circular trajectory, in correspondence of which take place the annihilation of electron-positron pairs and the production of an high energy photon .
Subsequently the produced photon generates the Y particle across its decay products.
To include the new particle in the quark model, it was needful to introduce a new type of quark, the charm quark, with spin 1/2 and an electric charge equal to 2/3, so denominated because it allowed to explain, as happened for the strange quark, the slow decay of the new particle, which, being an hadron, has to decay in a time of 10-23 s, that is typical of the particles subjected to the strong subnuclear interaction, whereas it is observed to decay in three pions ( positive, negative and neutral ) in about 10-20 s.
The Y particle is formed by a charm-quark (c) and a charm antiquark (/c), with a total electric charge equal to zero (2/3 -2/3) , and whose spin are added to give the observed unitary spin.
It must be considered that quarks have never been observed as free particles, because the strong subnuclear forces that tie them to form hadrons ( mesons and baryons ), are characterized by an intensity that increases with increasing distance between quarks, and that implicates a very great bond energy, much greater of the energy available in today particle accelerators ( 2000 Gev).
The existence of quarks is therefore deduced indirectly, by observing the particles derived from their combinations.
The modification of quark model, that was needful after discovery of Y particle and the implicit discovery of the new charm "flavour", in addition to the up, down and strange "flavours", allowed also to explain, by a symmetry law, the absence of experimental observations of decaying particles containing the strange quark into particles containing the down quark.
This decays, denominated neutral decays with flavour changing, because both quarks have the same electric charge and different flavours, are explained with the fact that the charm quark (c) stretch preferentially to decay into a strange quark, by the exchange of an intermediate W+ boson, whose positive electric charge allows to respect the conservation law of the electric charge ( 2/3 = -1/3 + 1) : c --> s + W+.
Instead are observed normally decays of particles containing the down quark into particles containing particles the up quark (-1/3 = 2/3-1 ): d --> u + W-, whereas have never been observed decays particles containing the charm quark into particle containing the up quark.
So the second quark generation (charm quark and strange quark) has been introduced, together with the second lepton generation (muon and muonic neutrino), in addition to the quark (up and down ) and to the leptons ( electron and electron neutrino ) of the first generation, that constitute ordinary matter.
The elementary particles (with the respective antiparticles ) formed by the second quark generation, were much more abundant in the initial phase of the universe evolution,when energy were so high to allow to generate them, and the structure of the matter was based exclusively on charm and strange quarks, muons and muonic neutrinos, with the respective antiparticles.
In 1977, at the Fermilab of Chicago, while concluding a long period of researches started in 1967 at Brookhaven, by the study of the muon-antimuon pairs produced by bombarding uranium nuclei with 30 GeV protons, was discovered a new heavy particle, the Y meson, with a mass of about 10 Gev, the most heavy particle never till that time produced by high energy collisions.
To insert the new particle in the standard model, with considerations analogous to that expressed for the introduction of the charm quark, it was needful, by considering the slow decay of the Y meson, to hypothesize the existence of a new quark-antiquark pair.
Therefore was introduced the fifth quark, to which was given the name of bottom ,or beauty, with spin 1/2 and an electric negative charge equal to -1/3, with the respective antiquark.
The discovery of the sixth quark, with a 174 GeV mass and an electric positive charge equal to 2/3, to which was given the name of top or truth, took place at the Fermilab in 1995, after about 15 years of fruitless researches, because of the big mass of the top quark, about 187 times more heavy than the proton and 35000 times more heavy than the up quark .
It was necessary to reach the energy of 1800 Gev to record an enough number of top quarks in proton-antiproton collisions, by using a sophisticated system to detect the decay products of the particle.
The proton-antiproton pairs produced a top quark (t) and a top antiquark (/t), that decaied respectively into a bottom quark (b), produced together with an intermediate W+ boson, and into a bottom antiquark (/b), produced together with an intermediate W- boson:
p + /p ---> t + /t ;
t ---> b + W+ ;
/t ---> /b + W- .
As concerns leptons, in 1977 at the SLAC was discovered the t lepton,
with a mass of about 1,8 Gev, by observing the decay products of it, that is either muon, muon antineutrinos and tauon antineutrinos, or ,alternatively, electrons, electron antineutrinos and tauon neutrinos.
The masses of the three types of neutrino, considered equal to zero in a first approximation, have values included in the interval between 2,5 eV to 17 Mev, and are today being studied by sophisticate researches with the purpose of observing the transformation of a type of neutrino in the other two (japanes experiments made by the "SuperKamiokande" on the mass oscillation of neutrinos ).
Therefore, after the discovery of the sixth quark, the standard model had to up-to-dated including the third generation quarks ( top and bottom) and the third generation leptons ( tau and tauon neutrino ), with the corresponding antiparticles.
The particles of the third generation were much abundant in the initial phases of the universe evolution, with energies much higher of the ones compatible with the particle of the second generation.
In other words, the symmetry of the standard model evidences the existence of other two possible structures of matter, analogous to the ones of ordinary matter we know, that consists of leptons and quarks of the first generation.
The fundamental forces of the universe are the following four: the gravitational
forces, the subnuclear weak forces, the electromagnetic forces and the strong subnuclear
The gravitational forces are described in classical physics by Newton's law of universal gravitation ( 1686 ), and in relativistic physics by Einstein's theory of the general relativity (1916 ).
Newton's theory, that is still valid as a theory of a first approximation, allows to effect calculations of celestial mechanics with a sufficient accuracy in the most cases.
The theory of universal gravitation describes attractive forces acting between two point-like masses M1 and M2, or between two masses assimilable to point-like masses ( material points ), keeped at a distance R between the two:F = G M1M2 /R2 , where G is an universal constant.
The gravitational attractive force is directly proportional to the product of the masses, and inversely proportional to the square of their distance and is trending to zero with increasing R toward the infinite.
Einstein's theory of the general relativity is based instead on the geometric interpretation of gravitation, which derives from the following consideration: in a gravitational field, in absence of other forces, as for example the fluidodynamic ones that act on the bodies moving in fluids, all the bodies, independently from their mass, are moving following the same law, according to the ones enunciated by Galileo .
Einstein affirms that gravitation is a property of space-time, whose geometry depends on the distribution and the motion of the gravitational masses, since the geometry of the space, that is of Euclidean type in absence of gravitational masses, becomes non-Euclidean when they are present.
The mass has the property to bend the space-time.
Therefore the rectilinear trajectory that a body would follow in the Euclidean space in absence of other masses, is transformed into a non-rectilinear trajectory in a non-Euclidean space, because of the presence of other masses that change the bending of the space-time.
Einstein's theory allows to effect calculations of celestial mechanics much more accurate than the ones founded on Newton's theory, and it is fundamental for the development of the theories on the evolution of the universe ( cosmological theories ).
The gravitational forces are the most weak forces acting in the universe, 1038 times less intense than the strong subnuclear forces acting between quarks, because of the very small value of the universal Newton's constant G.
Gravitational effects are important only in the presence of great mass distributions ( stars,planets, galaxies ).
The attractive or repulsive electromagnetic forces acting between two point-like charges Q1 and Q2, or between two charge assimilable to point-like charges, keeped at a distance Rfrom each other, is given from the formula
F = k Q1Q2/ R2, where k is an universal Coulomb's constant.
It is mathematically analogous to the law of Newton's universal gravitation and is directly proportional to the product of the charges, ed inversely proportional to the square of their distance, and is trending to zero if R becomes infinite.
The intensity of the electromagnetic force is 1/1000 of the one of the strong subnuclear force.
In classical physics the synthesis of the description of the electric and magnetic phenomena was effected by James Clerk Maxwell in the second half of the XIX century.
After the formulation of the relativistic quantum mechanics by Dirac ( 1928), was developed a quantum theory of the electromagnetism, that is known as quantum electrodynamic (Q.E.D. Quantum Electro-Dynamics) and was subsequently improved by the physicists ( Nobel prize winner 1965 ) Sin-Itiro Tomonaga, Julian Schwinger and Richard Feynman , who transformed it into a coherent and complete theory, by which can be given much accurate results for all the electromagnetic interactions between elementary particles.
In quantum electrodynamics attractive or repulsive forces between two charged particles are described by the exchange of a virtual photon (g), whose transfer from a particle to the other doesn't violate the energy conservation principle, provided its flight time and the required mass-energy are compatible with Heisenberg's uncertainty principle.
How much greater is the energy of the exchanged photon,
the smaller has to be the flight time Dt : D E .Dt ~= h/(2p) .
The weak subnuclear forces, thate produce the radioactive decay phenomena of atomic nuclei and of unstable elementary particles, is 1013 times more weak than the strong subnuclear force, and has a short range ( 10-15 cm ), that is trending to zero for very small distances between the particles on which it acts, that is the leptons
(electrons, muons, tauons and respective neutrinoes ).
The first theory on the weak forces is owing to to Enrico Fermi ( 1934).
In fact the weak forces are called also the "Fermi forces".
Subsequently, in the context of the research of an unified theory of the four fundamental forces, in the '60 years, independently each from the other, the physicists ( Nobel prize winner in 1979 ) Sheldon she Glashow and Steven Weinberg ,working at the Harvard University, and Abdus Salam, working at the international center of theoretical physics of Trieste, developed an unified theory of the weak and electromagnetic interactions (the unified theory of electro-weak forces ).
The electro-weak theory, founded on the exchange of the intermediary virtual bosons W+, W- and Z°,with spin 1, that were subsequently detected at the CERN by the physicists ( Nobel prize winner in 1984 ) Carlo Rubbia and Simon Van der Meer, extends to the weak and electromagnetic forces the model by which in the 1935 Hideki Yukawa ( Nobel prize winner in 1949 ) developed the first theory of the nuclear forces acting among protons and neutrons, which is based on the exchange of pmesons (pions), that was subsequently discovered in 1947 by Powell ( Nobel prize winner in 1950 ),Occhialini, Muirhead and Lattes in particle showers of cosmic radiation.
The electro-weak theory by Glashow, Salam and Weinberg shows that , while energy increases towards 100 Gev, the characteristic value of the electro-weak unification , the electromagnetic and weak forces are trending to assume the same intensity, and they can be explained by the same boson exchange mechanism, which is based on the photon for the electromagnetic interactions and on the W e Z° bosons for the weak ones.
A further boson has to be detected, the boson of Higgs, whose existence was proposed in 1964 by Peter Higgs at the Edinburgh University, to explain the mass difference among the force vectors in the weak and electromagnetic interactions.
The discovery of the neutral heavy boson of Higgs, with zero spin, that the physicists hope could take place by using the new LHC ( Large Hadron Collider) at the CERN, would allow to explain the symmetry breakup of electro-weak interactions, by the acquisition of mass of the intermediate bosons W and Z°, that are the vectors of the weak force, and the separation of the photon, with zero rest mass, that is the vector of the electromagnetic force.
The strong subnuclear force is the most intense of the fundamental forces, and acts among nucleons by the exchange of pions according to the model of Yukawa, whereas inside the hadrons ( mesons and baryons ), that are made of quarks, the force vectors are eight bosons with spin 1 and zero mass, denominated gluons (from the Latin glus = glue) .
The strong force has a short range ( 10-13cm ), and it is increasing with the increasing distance between quarks, which implicates to be very huge the bond energy of quarks that compose hadrons, and then the impossibility to detect an isolate quark.
By starting from quantum electrodynamics, has been formulated a theory analogous to it, the quantum chromodynamics ( Q.C.D. - Quantum Chromo-Dynamics ) which, coherently with Pauli's principle, that imposes to quarks , that are all with spin 1/2, to haven't the same quantum state, introduces , besides the "flavour" of the quark ( up, down, strange, charm, bottom and top), also the so called "colors" ( red, green and blue ),that are some conventional parameters (color charges ),that , likewise to electric charges, are needful to distinguish the quark with the same flavour in the hadrons.
For convention all the baryons have to be formed by 3 quarks with such colors that their sum gives the white ( red + green + blue = white ).
In the case of mesons, that are formed by a quark and an antiquark, the color of the quark neutralizes the anticolor of the respective antiquark.
Altogether, being 6 the "flavours" of quarks, they are considered 18 "colored" quarks and the 18 respective antiquarks.
A quark is screened from a cloud which consists of virtual gluons and virtual quark-antiquark pairs , that are continually emitted and re-absorbed.
The strong force between two quarks and between a quark and an antiquark are produced by the exchange of 8 different types of gluons, two of which don't have a colour charge (colourless gluon that are emitted and absorbed by only one quark, without changing its colour and 6 that transport each 2 "color charges " :
For example, the anti-red color, that is gotten adding the green and blue colors, corresponds to the lightblue color, that is the anti-colour ( complementary colour ) of the red colour, since adding colours lightblue ( green + blue ) and red is gotten the white colour ( red + green + blue ).
Likewise the anti-green colour, that is gotten adding red and blue colours, corresponds to the purple color, that is the anticolour of the green colour, while the antiblue, that is gotten adding red and green colurs, corresponds to the yellow colour, that is the anticolour of the blue colour.
The difference among the photons, the quanta of the electromagnetic field, and the gluons, the quanta of the gluon field, is in the fact that, whereas the photon don't transport any electric charge, 6 of the 8 gluon types, instead, ransport a color and an anticolour (a charge-anticharge pair).
As in classical and in quantum electrodynamics (Q.E.D.) is fundamental the conservation principle of electric charge , so in quantum cromodynamics ( Q.C.D.) is fundamental the conservation principle of the color charge ( chromatic charge ) of two or several quarks, antiquark and gluons, in the sense that, although the colours and the anticolours of quark and gluons are allowed to change, their sum (the resultant color ) is preserved.
For example, if a red quark emits a red-antigreen gluon that is absorbed by a green quark, the colours of the quark are exchanged, but the total color ( yellow = red + green ) is preserved:
- initial total color: quark1-red + quark2-green = yellow;
- total color after the emission of the red-antigreen gluon:
quark1-green + red-antigreen gluon + quark2-green = yellow ;
( green and antigreen colours are neutralized giving the white); - total color after the absorption of the red-antigreen gluon: quark1-verde + quark2-rosso = yellow.
If subsequently the second quark ( red ) emits a red-antigreen gluon that is absorbed from the first quark ( green ), the colours of the quarks are again exchanged, while the total color ( yellow ) is preserved:
- initial total color: quark1-green + quark2-red = yellow;
- total color after the emission of the red-antigreen gluon:
quark1-green+ red-antigreen gluon + quark2-green = yellow;
- total color after the absorption of the red-antigreen gluon:
quark1-rosso + quark2-green = yellow.
The quarks and the antiquarks are subjected to all the 4 fundamental forces: strong,
electromagnetic, weak and gravitational.
The charged leptons ( electrons, muons and tauons ) are subjected to the electromagnetic, weak and gravitational interactions.
The neutral leptons (neutrinos) are subjected to weak and gravitational interactions.
The standard model, based on the electro-weak theory and on quantum chromodynamics,
till now don't allow to include in it, in a coherent way, a quantum theory of the
gravitation,based on the exchange of quanta of gravitational energy, the gravitons, with
mass equal to zero and spin 2.
Since long time the physicists formulated some theories with the purpose to unify at very high energies, in an only supersimmetric theory, three or all the fundamental forces of nature, respectively at the 1016 Gev energy (the value corresponding to the unification of the strong and electro-weak forces) and at the 1018 Gev energy (the value corresponding to Planck' scale of the gravity quantum theory).
The supersimmetric theory would be able to describe the structure of matter in the early instants of the universe evolution, 1,4 * 10-43 seconds after the Big Bang, when the universe had dimensions of the order of Planck' scale ,
4,1*10-35 meters, with a temperature of 3,6*1032 °K.
Yet, the various proposed theories, as the so-called Grand Unification Theories (G.U.T.) (the supersymmetric theories), as the Theories Of Everything (T.O.E.), like the superstring theory, that includes gravity, don't succeed at present to give a coherent vision of all the till now discovered particles.
The difficulty consists in the unification of quantum mechanics, that is the base of the standard model,with the relativistic theory of gravitation, that is a classical theory and isn't based on the quantization of space-time.