Information: - A neutrino (or ) (denoted by the Greek letter ) is a fermion (an elementary particle with half-integer spin) that interacts only via the weak subatomic force and gravity. The mass of the neutrino is much smaller than that of the other known elementary particles. - Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward (or "gravitate" toward) one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, also cause gravitation and are under the influence of it. On Earth, gravity gives weight to physical objects and causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing, forming stars and the stars to group together into galaxies so gravity is responsible for many of the large scale structures in the Universe. Gravity has an infinite range, although its effects become increasingly weaker on farther objects. - In particle physics, the strong interaction is the mechanism responsible for the strong nuclear force (also called the strong force or nuclear strong force), and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction and gravitation. At the range of 10 m (femtometer), the strong force is approximately 137 times stronger than electromagnetism, a million times stronger than the weak interaction and 10 times stronger than gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as proton and neutron. In addition, the strong force binds neutrons and protons to create atomic nuclei. Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton. - In particle physics , a pion ( or a pi meson , denoted with the Greek letter pi : ) is any of three subatomic particles : 0 , + , and . Each pion consists of a quark and an antiquark and is therefore a meson . Pions are the lightest mesons ( and , more generally , the lightest hadrons ) , because they are composed of the lightest quarks ( the u and d quarks ) . They are unstable , with the charged pions + and decaying with a mean lifetime of 26 nanoseconds ( 2.6 10 8 seconds ) , and the neutral pion 0 decaying with a much shorter lifetime of 8.4 10 17 seconds . Charged pions most often decay into muons and muon neutrinos , and neutral pions into gamma rays . The exchange of virtual pions , along with the vector , rho and omega mesons , provides an explanation for the residual strong force between nucleons . Pions are not produced in radioactive decay , but are produced commonly in high energy accelerators in collisions between hadrons . All types of pions are also produced in natural processes when high energy cosmic ray protons and other hadronic cosmic ray components interact with matter in the Earth 's atmosphere . Recently , detection of characteristic gamma rays originating from decay of neutral pions in two supernova remnant stars has shown that pions are produced copiously in supernovas , most probably in conjunction with production of high energy protons that are detected on Earth as cosmic rays . The concept of mesons as the carrier particles of the nuclear force was first proposed in 1935 by Hideki Yukawa . While the muon was first proposed to be this particle after its discovery in 1936 , later work found that it did not participate in the strong nuclear interaction . The pions , which turned out to be examples of Yukawa 's proposed mesons , were discovered later : the charged pions in 1947 , and the neutral pion in 1950 . - In quantum mechanics, a boson is a particle that follows BoseEinstein statistics. Bosons make up one of the two classes of particles, the other being fermions. The name boson was coined by Paul Dirac to commemorate the contribution of the Indian physicist Satyendra Nath Bose in developing, with Einstein, BoseEinstein statisticswhich theorizes the characteristics of elementary particles. Examples of bosons include fundamental particles such as photons, gluons, and W and Z bosons (the four force-carrying gauge bosons of the Standard Model), the recently discovered Higgs boson, and the hypothetical graviton of quantum gravity; composite particles (e.g. mesons and stable nuclei of even mass number such as deuterium (with one proton and one neutron, mass number = 2), helium-4, or lead-208); and some quasiparticles (e.g. Cooper pairs, plasmons, and phonons). - In particle physics, a hadron ("hadros", "stout, thick") is a composite particle made of quarks held together by the strong force in a similar way as molecules are held together by the electromagnetic force. - Particle physics (also high energy physics) is the branch of physics that studies the nature of the particles that constitute "matter" (particles with mass) and "radiation" (massless particles). Although the word "particle" can refer to various types of very small objects (e.g. protons, gas particles, or even household dust), "particle physics" usually investigates the irreducibly smallest detectable particles and the irreducibly fundamental force fields necessary to explain them. By our current understanding, these elementary particles are excitations of the quantum fields that also govern their interactions. The currently dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model. Thus, modern particle physics generally investigates the Standard Model and its various possible extensions, e.g. to the newest "known" particle, the Higgs boson, or even to the oldest known force field, gravity. - In particle physics, mesons (or ) are hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of quark sub-particles, they have a physical size, with a diameter of roughly one fermi, which is about the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Charged mesons decay (sometimes through mediating particles) to form electrons and neutrinos. Uncharged mesons may decay to photons. Both of these decays imply that color is no longer a property of the byproducts. - A quark (or ) is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as "color confinement", quarks are never directly observed or found in isolation; they can be found only within hadrons, such as baryons (of which protons and neutrons are examples) and mesons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves. After reading the paragraphs above, we are interested in knowing the entity with which 'pion' exhibits the relationship of 'interaction'. Find the answer from the choices below. Choices: - electromagnetic force - gravity - strong interaction
A:
strong interaction