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Standard Model of Physics

IN SHORT - The 'Standard Model' or 'Standard Model of Physics', which had been born in the 1970's, is the general explanation of how matter works at the scale of the particles. It was then a synthesis of what existed, as it became the basis for any ulterior science. It's mainly quantum physics. The Standard Model of Physics shows how matter is sorted into

elementary particles, forces, and force carriers. Elementary particles are the basis upon which more elaborated particles, like the neutrons or protons, build. Forces are mediating the interactions at the particle and atomic level. Gravity, albeit weak at such a scale, is part of these forces. Force carriers, amazingly, are particles which are transporting the forces!

Evolution of knowledge about matter and Universe led in the 1970s to what is termed the Standard Model, or Standard Model of physics, which is the general explanation of how matter works, on Earth, and in the Universe, at atomic scale. The Standard Model is a 1970s synthesis of what was known then and since has been the basis for following experiments. It is successfull to describe the physical world. Technically speaking, the Standard Model is the quantum theory including the theory of strong interactions (QCD, quantum chromodynamics) and the unified theory of weak and electromagnetic forces (electroweak)

Basics

Matter is made of base constituents, of forces by which these constituents interact. These forces are carried by force carriers. A general principle rules matter: the Pauli Exclusion Principle. It states that two particles in the same state (color charge, intrinsic angular momentum -or spin, etc) cannot exist in the same place at the same time. Particles subject to this principle are said fermions. Fermions are matter fundamental constituents (like quarks, or electrons). Particles not subject to this principle are said bosons. Bosons are force carriers. Constituents of matter are fermions, as force carriers are bosons. Moreover any atom with an odd number of electrons, protons, and neutrons is a fermion; any atom with an even number of electrons, protons, and neutrons is a boson. For each any particle there is an antiparticle having the same characteristics but with opposite charge

Constituents

Matter Fundamental Constituents, or Fermions. They are subject to the Pauli Exclusion Principle. Fermions in turn are divided into quarks or leptons

Quarks: quarks come in three "flavors": up/down (u/d), charm/strange (c/s), top/bottom (t/b). Each quark carries what is called a "color charge", which is the basis of one of the fundamental forces, the strong force. For quarks, there are three varieties of it. Color charge is an attribute which cannot be seen and which obeys to maths (called "(SU)3"), totally different from those obeyed by electric charges. Linking together quarks form particles called hadrons. Hadrons are either of the fermionic kind: they are made of three quarks or three antiquarks and mostly are further step of matter building blocks (neutron, proton e.g.); or hadrons are of the bosonic form: they are made of one quark and one antiquark. They are also termed "mesons" and mostly act like residual force carriers binding protons and neutrons between them
Leptons: leptons come in three "flavors": electron/electron neutrino (ne/e), muon/muon neutrino (nm/m), tau/tau neutrino (nt/t). Leptons neutrinos are those which are electrically neutral (and are the famous neutrinos in three varieties); others are positively charged. All these leptons, charged or neutral, are unstable

All these elementary particles have a spin (angular momentum), an electric charge -in units of the proton's charge, energy (in electronvolt (eV)) and mass (in GeV/c²)

Forces

Four types of forces mediate matter interactions: the strong interaction force, the electromagnetic force, the weak force, the gravity. All these forces, except the gravity, work on the basis of quantum field theories

the strong force is experienced by quarks, and gluons (gluons are force carriers -see below). The strong force belongs to the quantum chromodynamics (QCD). The strong force is not of an electromagnetic nature. It acts on the "color charge" of the quarks and of the gluons. Contradictorily, the nearer the quarks, the weaker the strong force, as when the quarks get distant, the force get stronger
the electromagnetic force is experienced by any particle being electrically charged. It acts on the electrical charge. Electromagnetic force belongs to the quantum theory of electrodynamics (QED). Electromagnetic force is the synthesis of electrical charges (positive, negative) and magnetism (North, South). Hence the name. Particles (or objects) with an opposite charge attract each other, particles with the same charge repel each other. An interesting idea is that "visible" forces are based on electromagnetism: it is atoms' electromagnetic force which resists in daily objects to their displacement from their equilibrium state. As atoms are electrically neutral (they have an equal number of positively charged protons and negatively charged electrons), molecules build from atoms due to a residual force called the "residual electromagnetic force": charged parts of the atoms electromagnetically interact. Beyond the atomic and molecular interactions, it is this same force which rules over inter-molecular interactions hence for life and most forms of interactions in the Universe

It is through such colliding events that physicists study deepest levels of matter. Accelerators make particles collide and yield others. This colliding event particularly has been produced at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, USA. This accelerator is specially dedicated to produce primeval states of matter, as they are believed to have existed in the early Universe. courtesy RHIC, Brookhaven National Laboratory

the weak force is experienced by quarks and leptons. It acts on the flavor. The weak force is responsible for the radioactive decay of atoms, quarks, and leptons. Most massive quarks and leptons decay into lighter ones through the weak force. They are said to "change flavor"
the electromagnetic and the weak force have been theoretically unified in the 1960s and are called the electroweak force -or electroweak
gravity is the well known newtonian force. It is experienced by all particles (even if it does not act strongly at this level of matter. It is tiny in high-energy physics situations). Gravity acts on particles' mass and energy. Technically speaking gravity does not belong to the Standard Model but is one of the fundamental forces however

Forces Carriers

Forces are carried between particles due to "force carriers". Force carriers are particles specially dedicated to transport of forces. Particles are interacting between themselves due to interactions. Interactions are exchange of force carrying particles: the latter are exerting a force of a somehow newtonian type on particles receiving them (action-reaction). Force carriers are not subject to the Pauli Exclusion Principle. Technically all interactions may be said mediated (carried) by integer-spin or Boson field quanta. Force carriers are bosons. Odd spin bosons mediate repulsive forces; even spin bosons mediate attractive forces

Carrier for the Strong Force is the Gluon. Gluon comes from "glue", as gluons glue quarks together. Gluons are massless. Like quarks they have a color charge. For them it comes in 8 types. Gluons generally speaking mediate strong interactions between color-charged particles; it is color-charged force carriers mediating color-charged particles interaction. Leptons, photons and bosons W and Z have no color-charge as they are not subject to strong interaction.
Carrier for the Electromagnetic Force is the Photon. Photon is massless. It is interesting to think that a large part of Universe's interactions are mediated by light. Photons mediate interaction between electrically charged particles
Carrier for the Weak Force are W and Z Particles. They are also said W and Z bosons. W are electrically charged (W⁺ or W^-); Z is neutral (Z⁰). These particles are massive
Carrier for Gravity is the Graviton. This particle has not yet been discovered. Graviton is thought to be the force carrier in Einstein's General Relativity

Synthesis

All these matter constituents, forces, and force-carriers are the building blocks of atoms. Quarks unite to form protons or neutrons. Protons and neutrons unite to form atom nucleus. Electrons are orbiting the nucleus. Atom is held together due to forces mediated by force carriers: quarks are linked together by the strong force which is carried by gluons. Protons and neutrons are linked together inside the nucleus by a residual strong force emanating from their quarks constituents. Electrons are linked to nucleus by the electromagnetic force. Further, weak force acting on quarks constituents and electrons bring natural atom decay

thumbnail to a diagram of the Standard Model of physics

click to a diagram of the Standard Model of physics

All this is of importance and show a tremendous variety of particles and forces at work in matter: electrons and quarks belong to the same level of matter building blocks. Both are fermions subject to the Pauli Exclusion Principle. Quarks interact via the strong force only. Electrons are subject to the electromagnetic force only. Interaction between quarks composites like protons and neutrons is mediated by mesons which are formed of quarks themselves, by-products of quarks and gluons. Electrons are linked to the atom nucleus through the electromagnetic force. Result is an atom where if protons and neutrons were 10 cm across, quarks and electrons would be 0.1 mm wide as the outer limit of electrons orbits would be 10 km away from the nucleus!

Most of matter in the Universe is made of up (u) and down (d) quarks which are the most massive quarks, and of electrons which are the most massive leptons. This is called the first generation of particles, most stable of them. These are light particles. They are the by-products of second and third generation of particles decay. These particles are heavier, unstable, and decay

Beyond the Standard Model?

A question to the Standard Model are how particles got their masses. It is the famous question of the Higgs boson. Higgs boson would be the carrier to the "Higgs force". Another question is the question of very weak forces which might be involved into very long duration decays like the decay of the proton (10³² years) or be responsible for the mass of the neutrinos. Two main other centers of interest for physicists are the idea of unification of the strong, weak and electromagnetic forces into one "Grand Unified Theory" (GUT) -mathematical theories for these different forces are somehow similar, and the famous attempt of the String Theory to unify the four forces -strong, weak, electromagnetic and gravity- although gravity rests on a different mathematical theory and that there is no quantum theory of gravity. If all this proves to be real and based, it is likely that, in the same way that the Standard Model included its predecessors -the atom and the newtonian models- any further model for physics will embed the Standard Model, Relativity, and quantum physics. Understanding the Standard Model of physics if of importance for astronomy: Big Bang is seen as the moment when unified forces were progressively splitted and when elementary particles were progressively created and linked together. The latest discovery about the Standard Model is a new particle, called "theta". This particle was found at Brookhaven RHIC and is strangely composed of four quarks and an antiquark as usual baryons and mesons are formed by three quarks (or antiquarks) or one quark and one antiquark only. Some other advances in the particules science should be provided by the next 'Large Hadron Collider Particle Accelerator' being built by the European Organization for Nuclear Research (CERN) at the French-Swiss border near Geneva