Everyday matter is composed of atoms , once presumed to be matter's elementary particles— atom meaning "unable to cut" in Greek—although the atom's existence remained controversial until about , as some leading physicists regarded molecules as mathematical illusions, and matter as ultimately composed of energy. As the s opened, the electron and the proton had been discovered, along with the photon , the particle of electromagnetic radiation.
Via quantum theory, protons and neutrons were found to contain quarks — up quarks and down quarks —now considered elementary particles. Around , an elementary particle's status as indeed elementary—an ultimate constituent of substance—was mostly discarded for a more practical outlook,  embodied in particle physics' Standard Model , what's known as science's most experimentally successful theory. All elementary particles are either bosons or fermions. These classes are distinguished by their quantum statistics : fermions obey Fermi—Dirac statistics and bosons obey Bose—Einstein statistics.
Notes: 1. In the Standard Model , elementary particles are represented for predictive utility as point particles. Though extremely successful, the Standard Model is limited to the microcosm by its omission of gravitation and has some parameters arbitrarily added but unexplained.
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. Some estimates imply that there are roughly 10 80 baryons almost entirely protons and neutrons in the observable universe. The number of protons in the observable universe is called the Eddington number. 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 10 86 elementary particles of matter that exist in the visible universe.
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, , as having been likely detected by the two main experiments at the Large Hadron Collider ATLAS and CMS. However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, 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. The remaining six particles are quarks discussed below. For example, the most accurately known quark mass is of the top quark t at Estimates of the values of quark masses depend on the version of quantum chromodynamics used to describe quark interactions.
Quarks are always confined in an envelope of gluons which confer vastly greater mass to the mesons and baryons where quarks occur, so values for quark masses cannot be measured directly. Since their masses are so small compared to the effective mass of the surrounding gluons, slight differences in the calculation make large differences in the masses.
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Isolated quarks and antiquarks have never been detected, a fact explained by confinement. Every quark carries one of three color charges of the strong interaction ; antiquarks similarly carry anticolor. Color-charged particles interact via gluon exchange in the same way that charged particles interact via photon exchange. However, gluons are themselves color-charged, resulting in an amplification of the strong force as color-charged particles are separated.
Unlike the electromagnetic force , which diminishes as charged particles separate, color-charged particles feel increasing force. However, color-charged particles may combine to form color neutral composite particles called hadrons. A quark may pair up with an antiquark: the quark has a color and the antiquark has the corresponding anticolor. The color and anticolor cancel out, forming a color neutral meson. Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green".
These three colored quarks together form a color-neutral baryon. Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutral antibaryon. Quarks also carry fractional electric charges , but, since they are confined within hadrons whose charges are all integral, fractional charges have never been isolated. Evidence for the existence of quarks comes from deep inelastic scattering : firing electrons at nuclei to determine the distribution of charge within nucleons which are baryons.
If the charge is uniform, the electric field around the proton should be uniform and the electron should scatter elastically. Low-energy electrons do scatter in this way, but, above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has much less energy and a jet of particles is emitted. This inelastic scattering suggests that the charge in the proton is not uniform but split among smaller charged particles: quarks.
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.
Gluons mediate the strong interaction , which join quarks and thereby form hadrons , which are either baryons three quarks or mesons one quark and one antiquark.
Protons and neutrons are baryons, joined by gluons to form the atomic nucleus. Like quarks, gluons exhibit color and anticolor—unrelated to the concept of visual color—sometimes in combinations, altogether eight variations of gluons. The W bosons are known for their mediation in nuclear decay.
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The Z 0 does not convert charge but rather changes momentum and is the only mechanism for elastically scattering neutrinos. The weak gauge bosons were discovered due to momentum change in electrons from neutrino-Z exchange. The massless photon mediates the electromagnetic interaction. These four gauge bosons form the electroweak interaction among elementary particles.
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. 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.
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On 4 July , 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. 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.
The graviton is a hypothetical elementary spin-2 particle proposed to mediate gravitation. While it remains undiscovered due to the difficulty inherent in its detection , it is sometimes included in tables of elementary particles. Although experimental evidence overwhelmingly confirms the predictions derived from the Standard Model , some of its parameters were added arbitrarily, not determined by a particular explanation, which remain mysterious, for instance the hierarchy problem.
Theories beyond the Standard Model attempt to resolve these shortcomings. One extension of the Standard Model attempts to combine the electroweak interaction with the strong interaction into a single 'grand unified theory' GUT. Such a force would be spontaneously broken into the three forces by a Higgs-like mechanism. The most dramatic prediction of grand unification is the existence of X and Y bosons , which cause proton decay. Supersymmetry extends the Standard Model by adding another class of symmetries to the Lagrangian.
These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts the existence of supersymmetric particles , abbreviated as sparticles , which include the sleptons , squarks , neutralinos , and charginos.
Due to the breaking of supersymmetry , the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing particle colliders would not be powerful enough to produce them. String theory is a model of physics where all "particles" that make up matter are composed of strings measuring at the Planck length that exist in an dimensional according to M-theory , the leading version or 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, like a circle.
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As a string moves through space it sweeps out something called a world sheet. String theory predicts 1- to branes a 1- brane being a string and a brane being a dimensional object that prevent tears in the "fabric" of space using the uncertainty principle e. String theory proposes that our universe is merely a 4-brane, inside which exist the 3 space dimensions and the 1 time dimension that we observe. Some predictions of the string theory include existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string and existence of a massless spin-2 particle behaving like the graviton.
Technicolor theories try to modify the Standard Model in a minimal way by introducing a new QCD-like interaction. What rules govern energy, matter, space, and time at the most elementary levels?