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Tit_El modelo Standard de Partículas Elementales

The model Standard for Elementary Particles

seminar room of group Science, Reason and Faith.
Luis Joaquín Boya Balet. Professor Emeritus, department of Theoretical Physics, University of Zaragoza. partner of group CRYF, University of Navarra.
Tuesday, 24 January 2012.

Diapositiva_El modelo Standard de Partículas Elementales


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Txt_El modelo Standard de Partículas Elementales


The model Particle Standard was established in 1975. There are no experiments to contradict it, but it seems very arbitrary, and we are looking for new avenues.

The particles are divided into two groups, "bricks", constituents of the subject, and "tails", which describe the forces between the bricks. The bricks are fermions, (they have half-integer spin) and the "tails" are bosons, (they have integer spin). The fermions obey the exclusion principle (manager of the valence 


Chemistry), while bosons can act coherently, in the same state (e.g. laser).

The universe is described by four types of forces, which in historical order were: Gravity (discovered by Newton), which binds the Earth to the Sun, etc.; the well-known electromagnetic force. There are two forces (or interactions) in the domain of the atomic nucleus, with very short range: they are the "strong" interaction (manager of the stability of nuclei) and the "weak" interaction (manager of beta decay). In quantum mechanics, each force has a carrier, which is the "tail" particle(s) of that force. The graviton would be the carrier of the gravitational force; the photon, with zero mass and spin one, that of the electric force. The strong nuclear force has as carriers the gluons (there are eight) and the weak force has three carriers, the Z particle, neutral, with a mass of 90 GeV, and the W± pair, charged, of somewhat lower mass.

The "bricks" are separated into two large groups, the leptons and the quarks. There are 2x3 leptons: charged, like the electron, or neutral, like neutrinos; and there are 2x3x3= 18 quarks. Leptons and quarks are grouped into three families or generations: the u ("up") and d ("down") quarks, the electron and the first neutrino constitute the first family, and there are three of them. The quarks have an extra Degree of freedom, the "colour": there are three colours per quark, and the gluons are also "coloured". Colour is shielded (only occurs in confinement): the observed uud protons and udd neutrons are "colourless".

Atoms and molecules form the subject. Atoms have nuclei, with protons and neutrons, and electrons dance around them (hence the neutrality of the normal atom).

A further particle is the "Higgs boson", H, a (still) hypothetical particle, predicted by model, with zero spin, neutral and very heavy. Its function is to make the weak force, of very short range, compatible with the others, of unlimited range: the weak carriers are massive. In a sense, the Higgs is manager to endow all particles with mass.

Unsatisfactory aspects of the model standard are: the large mass variation, ranging from massless photons to the Z particle with almost a trillion times the mass of the neutrino. Nor do we understand why there are three almost identical families: the electron has two "siblings", the more massive muon and tauon, which are not found in the normal subject , but are generated in cosmic rays. Nor do we understand the four "forces" that exist, either as symmetries of nature or in relation to each other. In particular, gravity is not quantizable.

Let us now comment on two recent experiments: At CERN (Geneva), they believe they have identified the Higgs particle, with a very large mass, 125 GeV (if confirmed, it would be the heaviest "elementary particle"). Another somewhat earlier experiment, originating at CERN, but detected in Italy (at Gran Sasso) claims that neutrinos, which are massive, could go faster than the speed of light (which is not possible from agreement with the Theory of Relativity). The experiment must be repeated (which will take more than a year), because, if it were correct, a large part of 20th century physics would collapse. A (partial) consensus among experts is that the result of the experiment is very doubtful and, therefore, the final, experimental verdict must be awaited before speculating.