Mean lifetime in the context of Muons

A muon (/ˈm(j)uː.ɒn/ M(Y)OO-on; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of ⁠1/2⁠ ħ, but with a much greater mass. It is classified as a lepton. As with other leptons, the muon is not thought to be composed of any simpler particles.

The muon is an unstable subatomic particle with a mean lifetime of 2.2 μs, much longer than many other subatomic particles. As with the decay of the free neutron (with a lifetime around 15 minutes), muon decay is slow (by subatomic standards) because the decay is mediated only by the weak interaction (rather than the more powerful strong interaction or electromagnetic interaction), and because the mass difference between the muon and the set of its decay products is small, providing few kinetic degrees of freedom for decay. Muon decay almost always produces at least three particles, which must include an electron of the same charge as the muon and two types of neutrinos.

In particle physics, a pion (/ˈpaɪ.ɒn/, PIE-on) or pi meson, denoted with the Greek letter pi (π), is any of three subatomic particles: π
, π
, and π
. 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. They are unstable, with the charged pions π
and π
decaying after a mean lifetime of 26.033 nanoseconds (2.6033×10 seconds), and the neutral pion π
decaying after a much shorter lifetime of 85 attoseconds (8.5×10 seconds). Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays.

The exchange of virtual pions, along with vector, rho and omega mesons, provides an explanation for the residual strong force between nucleons. Pions are not produced in radioactive decay, but commonly are in high-energy collisions between hadrons. Pions also result from some matter–antimatter annihilation events. 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 Earth's atmosphere. In 2013, the detection of characteristic gamma rays originating from the decay of neutral pions in two supernova remnants has shown that pions are produced copiously after supernovas, most probably in conjunction with production of high-energy protons that are detected on Earth as cosmic rays.

A free neutron refers to a neutron that is not part of an atomic nucleus. When embedded in a stable nuclide, neutrons have not been observed to decay. Free neutrons, however, decay with a mean lifetime of 877.75+0.50
−0.44 s or 879.6±0.8 s (about 14 min and 37.75 s or 39.6 s, depending on the specific measurement technique). This corresponds to a half-life of 611±1 s (about 10 min, 11 s).

The free neutron decays primarily by beta decay, with small probability of other channels. Considering the most common decay and only the stable resultant products the process may be described:

The sigma baryons are a family of subatomic hadron particles which have two quarks from the first flavour generation (up and / or down quarks), and a third quark from a higher flavour generation, in a combination where the wavefunction sign remains constant when any two quark flavours are swapped. They are thus baryons, with total isospin of 1, and can either be neutral or have an elementary charge of +2, +1, 0, or −1. They are closely related to the lambda baryons, which differ only in the wavefunction's behaviour upon flavour exchange.

The third quark can hence be either a strange (symbols Σ
, Σ
, Σ
), a charm (symbols Σ
_c, Σ
_c, Σ
_c), a bottom (symbols Σ
_b, Σ
_b, Σ
_b) or a top (symbols Σ
_t, Σ
_t, Σ
_t) quark. However, the top sigmas are expected to never be observed, since the Standard Model predicts the mean lifetime of top quarks to be roughly 5×10 s. This is about 20 times shorter than the timescale for strong interactions, and therefore it does not form hadrons.

In particle physics, a resonance is the peak located around a certain energy found in differential cross sections of scattering experiments. These peaks are associated with subatomic particles, which include a variety of bosons, quarks and hadrons (such as nucleons, delta baryons or upsilon mesons) and their excitations. Resonances can be explained as excited states of the reacting particles, or as virtual particles in intermediate steps of the reaction with very short lifetimes (10 seconds or less).

The width of the resonance (Γ) is related to the mean lifetime (τ) of the particle or excited state by the relation

The J/ψ (J/psi) meson /ˈdʒeɪ ˈsaɪ ˈmiːzɒn/ is a subatomic particle, a flavor-neutral meson consisting of a charm quark and a charm antiquark. Mesons formed by a bound state of a charm quark and a charm anti-quark are generally known as "charmonium" or psions. The J/ψ is the most common form of charmonium, due to its spin of 1 and its low rest mass. The J/ψ has a rest mass of 3.0969 GeV/c, just above that of the η
_c (2.9836 GeV/c), and a mean lifetime of 7.2×10 s. This lifetime was about a thousand times longer than expected.

Its discovery was made independently by two research groups, one at the Stanford Linear Accelerator Center, headed by Burton Richter, and one at the Brookhaven National Laboratory, headed by Samuel Ting of MIT. They discovered that they had found the same particle, and both announced their discoveries on 11 November 1974. The importance of this discovery is highlighted by the fact that the subsequent, rapid changes in high-energy physics at the time have become collectively known as the "November Revolution". Richter and Ting were awarded the 1976 Nobel Prize in Physics.