In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, antiquarks, and antileptons, which generally are matter particles and antimatter particles), as well as the fundamental bosons (gauge bosons and the Higgs boson), which generally are force particles that mediate interactions among fermions. A particle containing two or more elementary particles is a composite particle.
In the Standard Model, elementary particles are represented for predictive utility as point particles. Though extremely successful, the Standard Model is limited by its omission of gravitation and has some parameters arbitrarily added but unexplained.
According to the current models of big bang nucleosynthesis, the primordial composition of visible matter of the universe should be about 75% hydrogen and 25% helium-4 (in mass). Neutrons are made up of one up and two down quarks, while protons are made of two up and one down quark. Since the other common elementary particles (such as electrons, neutrinos, or weak bosons) are so light or so rare when compared to atomic nuclei, we can neglect their mass contribution to the observable universe's total mass. Therefore, one can conclude that most of the visible mass of the universe consists of protons and neutrons, which, like all baryons, in turn consist of up quarks and down quarks.
In terms of number of particles, some estimates imply that nearly all the matter, excluding dark matter, occurs in neutrinos, which constitute the majority of the roughly 1086 elementary particles of matter that exist in the visible universe. Other estimates imply that roughly 1097 elementary particles exist in the visible universe (not including dark matter), mostly photons and other massless force carriers.
The Standard Model of particle physics contains 12 flavors of elementary fermions, plus their corresponding antiparticles, as well as elementary bosons that mediate the forces and the Higgs boson, which was reported on July 4, 2012, as having been likely detected by the two main experiments at the Large Hadron Collider (ATLAS and CMS). The Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, however, since it is not known if it is compatible with Einstein's general relativity. There may be hypothetical elementary particles not described by the Standard Model, such as the graviton, the particle that would carry the gravitational force, and sparticles, supersymmetric partners of the ordinary particles.
In the Standard Model, vector (spin-1) bosons (gluons, photons, and the W and Z bosons) mediate forces, whereas the Higgs boson (spin-0) is responsible for the intrinsic mass of particles. Bosons differ from fermions in the fact that multiple bosons can occupy the same quantum state (Pauli exclusion principle). Also, bosons can be either elementary, like photons, or a combination, like mesons. The spin of bosons are integers instead of half integers.
Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a single electroweak force at high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at the HERA collider at DESY. The differences at low energies is a consequence of the high masses of the W and Z bosons, which in turn are a consequence of the Higgs mechanism. Through the process of spontaneous symmetry breaking, the Higgs selects a special direction in electroweak space that causes three electroweak particles to become very heavy (the weak bosons) and one to remain with an undefined rest mass as it is always in motion (the photon). On 4 July 2012, after many years of experimentally searching for evidence of its existence, the Higgs boson was announced to have been observed at CERN's Large Hadron Collider. Peter Higgs who first posited the existence of the Higgs boson was present at the announcement. The Higgs boson is believed to have a mass of approximately 125 GeV. The statistical significance of this discovery was reported as 5 sigma, which implies a certainty of roughly 99.99994%. In particle physics, this is the level of significance required to officially label experimental observations as a discovery. Research into the properties of the newly discovered particle continues.
String theory is a model of physics whereby all \"particles\" that make up matter are composed of strings (measuring at the Planck length) that exist in an 11-dimensional (according to M-theory, the leading version) or 12-dimensional (according to F-theory) universe. These strings vibrate at different frequencies that determine mass, electric charge, color charge, and spin. A \"string\" can be open (a line) or closed in a loop (a one-dimensional sphere, that is, a circle). As a string moves through space it sweeps out something called a world sheet. String theory predicts 1- to 10-branes (a 1-brane being a string and a 10-brane being a 10-dimensional object) that prevent tears in the \"fabric\" of space using the uncertainty principle (e.g., the electron orbiting a hydrogen atom has the probability, albeit small, that it could be anywhere else in the universe at any given moment).
Technicolor theories try to modify the Standard Model in a minimal way by introducing a new QCD-like interaction. This means one adds a new theory of so-called Techniquarks, interacting via so called Technigluons. The main idea is that the Higgs boson is not an elementary particle but a bound state of these objects.
The most important address about the current experimental and theoretical knowledge about elementary particle physics is the Particle Data Group, where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding.
A elementary particle is one that has no internal structure; that is, both theoretically and experimentally there are no constituent particles that combine to make it up. It has proven true historically that particles considered \"elementary\" in one epoch, the atom for example, are later found to be composite. Another example is the set of mesons, once thought to be the elementary quanta whose exchange between neutrons and protons embodied the nuclear force, but now considered to be made up of quarks.
In the Standard Model elementary particles fall into different groups: the group of \"particles\" themselves, which fall under the classifications of leptons and quarks, and the particles that mediate the interactions between them, the force carriers or field quanta, that fall under the categories of photons, weak bosons, and gluons.
Many years later, when I was trying to get Not Even Wrong published, I contacted Veltman and he was quite helpful. At the time he had recently published his own popular book about particle physics, Facts and Mysteries in Elementary Particles, which contained his own version of the Not Even Wrong critique:
The reader may ask why in this book string theory and supersymmetry have not been discussed. . . The fact is that this book is about physics and this implies that theoretical ideas must be supported by experimental facts. Neither supersymmetry nor string theory satisfy this criterion. They are figments of the theoretical mind. To quote Pauli, they are not even wrong. They have no place here.
Coughlan, G. D., James Dodd, and Ben Gripaios. The ideas of particle physics: an introduction for scientists. 3rd ed. Cambridge, Cambridge University Press, c2006. 254 p. Bibliography: p. 246-249. QC793.2.D6 2006
The research was conducted by more than 400 scientist in 14 countries within the Chinese particle physics experiment BESIII, where beams of electrons and positrons with very high energy collide with each other. At a specific collision energy the probability for the resonance particle J/ψ to form is very high. On February 11, 2019 the magic limit was reached where 10 milliard J/ψ-particles had been formed and detected. The J/ψ-particle is a hadron consisting of a charm quark and an anti-charm quark. The particle has a very short life span and decays to more stable particles which are measured in the BESIII-detector. The amount of data this corresponds to is the largest which has ever been collected in an electron-positron beam experiment.
From the record sized collection of the J/ψ-particle one may study a number of different phenomena within physics concerning the very shortest length scales (10-15m) in the universe. Maybe one may even discover deviations which may have an explanation beyond the theories we know about today.
Facts about HadronsHadrons are subatomic particles, made of quarks (elementary particles) and the strong force. The most known examples of hadrons are the proton and the neutron which both consist of three quarks. Hadrons may also consist of a quark and an antiquark, such in the case of the J/ψ-particle which is made of a charm quark and an anti-charm quark. The J/ψ-particle was first detected in 1974 and for this discovery the physicists Samuel Ting and Burton Richter received the Nobel Prize in physics in 1976.
Facts about the Standard ModelThe theory which describes physics on the very shortest length scales is known as the Standard Model of elementary particle physics and it is well tested. At the same time most physicists expect that the Standard Model is not the final description and with the help of precision experiments one may look for cracks in the model. This is done by comparing experimental results with theoretical calculations based on the Standard Model.
To make progress in the study of elementary particles, one needs sophisticated experimental and theoretical tools. We use accelerators of monumental size to produce particle collisions at energies that are equal to those 10-12 s after the big bang. We routinely collide matter with antimatter, destroying the initial particles and creating new ones. The detectors that we use to study these collisions are nearly as impressive. Here at UCSB, the high-energy physics group is very active in constructing such detectors and in analyzing the results of experiments that we perform at various accelerator laboratories. 59ce067264