The NEMO experimentPhysics goals - The Standard Model of Particles Physics
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To bottom of this pageIntroductionOne aim of Physics is to understand what is matter? How can it be created, transformed and eventually destroyed? How interact its components with each other? Since the end of the 19th century, we have known that matter is made of individual components: atoms. More than one hundred different kind of atoms (atomic elements) have been discovered. Atoms are small particles of matter, their size is typically about 10-10 m. Atomic elements have different masses, the lighter being the hydrogen. They have different electromagnetic properties that enable the assembling of several atoms in more complex structures: molecules and cristals. Depending on the local conditions, matter can exist in different atomic or molecular states: solid, liquid, gaseous, plasma. As far as we know, observed (visible) matter in the universe is almost exclusively made of the hydrogen (H) and helium (He) elements while other elements (C,N,O,Si,Fe...) are present only as trace (not on the Earth of course!). Scientists have understood quite early - thanks to experimental results - that atoms where composite particles and that something else exists at a more fondamental level and smaller scales. They discovered that the atom was made of a central nucleus with a positive electric charge while electrons orbit around it. The study of the atomic nucleus gave birth to nuclear physics. Figure 1 shows the naive representation of a hydrogen atom where electric forces play a fundamental role. The atomic nucleus is made of one single proton (brown ball) with an electron (blue ball) spinning around. Protons and electrons are particles of matter, while exchanged photons (yellow light) between the proton and the electron are the messengers of the electromagnetic interaction that makes the atom a stable object (at low temperature only!).
The Standard Model of Particle PhysicsThe Standard Model of Particle Physics (SM) is a theoretical framework that scientists involved in high energy physics (HEP) have elaborated during the last 70 years thanks to a huge amount of experimental and theoretical efforts. The SM is not an exact theory. It does not allow us to understand and predict everything that may happen in the subatomic real world. Nevertheless the SM has met considerable success over years, making some aspects of the known fundamental interactions and the compositeness of matter understandable, predicting the existence of new particles and explaining new processes at the subatomic scales but also in cosmology and astrophysics. The SM is still not complete (will it ever be?). In its present minimal form, it cannot answer some fundamental questions:
The answer to these questions can only come thanks to new experimental and theoretical work. To top of this page Fundamental interactionsPhysicists have discovered four fundamental interactions:
All fundamental interactions obey some special rules, i.e. they follow (or violate) some fundamental symmetries and conservation laws. Some well known conservation laws are:
Other conservation rules exist... It is now well established that the electromagnetic and the weak interactions are two different aspects of the same fundamental interaction (the electroweak theory). More, physicists believe that all interactions come from a same fundamental process at the early beginning of the universe. HEP physics aims to find the way to unify all processes and objects observed in our universe in a unique (simple?) theroretical framework. To top of this page Elementary particlesThe SM proposes a classification of elementary particles that is based on fundamental symmetries. We know two families of particles: bosons and fermions. More, to each known particle corresponds an anti-particle with the same mass, lifetime and opposite electrical charge (matter/antimatter symmetry). Particles interact with each other thanks to fundamental interactions. Bosons are the messenger particles of the four fundamental interactions (electromagnetic/weak, strong and gravitational interactions). For example the well known massless photon (γ) is the messenger of the electromagnetic interaction. The bosons W+, W- and Z0 are the massive messengers of the weak interaction (beta decay in atomic nucleus, muon decay...). The gluon (g) has never been observed experimentally but it is believed that it is the messenger of the strong interaction. To top of this page QuarksFermions are the particles from which the matter is made of. This family consists in two groups of elementary particles: quarks (q is a general symbol) and leptons (l is a general symbol). No free quark has never been directly observed up to now. Nevertheless we believe from the analysis of many HEP experiments that six different quarks exist together with six corresponding anti-particles (anti-quarks, see table 1). These quarks are labelled up (u), down (d), strange (s), charm (c), bottom (b) and top (t).
In addition to the specific numbers associated to them (Iz,S,C,B,T), all quarks have a special quantum number named the baryonic number: its value is +1/3 for quarks and -1/3 for anti-quarks. Quarks are sensitive to all known fundamental interactions. Physicists believe that nucleons (neutron: n , proton: p ) - which are the components of the atomic nucleus - are made of quarks (figure 2). Other hundreds of observed composite particles are also made of quarks (hadrons). Baryons are hadrons made of 3 quarks and mesons are hadrons made of a quark and an anti-quark.
To top of this page LeptonsIn the Standard Model of elementary particles, leptons are spin 1/2 fundamental particles that interact only through electro-weak processes (and gravity). Up to now, physicists have discovered/observed six different leptons together with their six associated anti-particles. Table 1 shows the main properties of the six charged leptons (electrons...) and the six neutral leptons (neutrinos and anti-neutrinos).
One distinguishes three different leptonic numbers, one for each neutrino flavor:
As an example, have a look on the beta decay of the free neutron on figure 3: Within the SM framework, we can represent this weak interaction process at the elementary particles level as shown in figure 4: Figure 5 illustrates the muon decay process: To top of this page |
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Copyright © 2004 by François Mauger (NEMO Collaboration) Last update: 30-03-2004 |