Meson

In particle physics, a meson ( ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometre (10 m), which is about 0.6 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few tenths of a nanosecond. Heavier mesons decay to lighter mesons and ultimately to stable electrons, neutrinos and photons. Outside the nucleus, mesons appear in nature only as short-lived products of very high-energy collisions between particles made of quarks, such as cosmic rays (high-energy protons and neutrons) and baryonic matter. Mesons are routinely produced artificially in cyclotrons or other particle accelerators in the collisions of protons, antiprotons, or other particles. Higher-energy (more massive) mesons were created momentarily in the Big Bang, but are not thought to play a role in nature today. However, such heavy mesons are regularly created in particle accelerator experiments that explore the nature of the heavier quarks that compose the heavier mesons. Mesons are part of the hadron particle family, which are defined simply as particles composed of two or more quarks.…

In particle physics, a meson ( ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometre (10  m), which is about 0.6 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few tenths of a nanosecond. Heavier mesons decay to lighter mesons and ultimately to stable electrons, neutrinos and photons. Outside the nucleus, mesons appear in nature only as short-lived products of very high-energy collisions between particles made of quarks, such as cosmic rays (high-energy protons and neutrons) and baryonic matter. Mesons are routinely produced artificially in cyclotrons or other particle accelerators in the collisions of protons, antiprotons, or other particles. Higher-energy (more massive) mesons were created momentarily in the Big Bang, but are not thought to play a role in nature today. However, such heavy mesons are regularly created in particle accelerator experiments that explore the nature of the heavier quarks that compose the heavier mesons. Mesons are part of the hadron particle family, which are defined simply as particles composed of two or more quarks. The other members of the hadron family are the baryons: subatomic particles composed of odd numbers of valence quarks (at least three), and some experiments show evidence of exotic mesons, which do not have the conventional valence quark content of two quarks (one quark and one antiquark), but four or more. Because quarks have a spin , the difference in quark number between mesons and baryons results in conventional two-quark mesons being bosons, whereas baryons are fermions. Each type of meson has a corresponding antiparticle (antimeson) in which quarks are replaced by their corresponding antiquarks and vice versa. For example, a positive pion ( ) is made of one up quark and one down antiquark; and its corresponding antiparticle, the negative pion ( ), is made of one up antiquark and one down quark. Because mesons are composed of quarks, they participate in both the weak interaction and strong interaction. Mesons with net electric charge also participate in the electromagnetic interaction. Mesons are classified according to their quark content, total angular momentum, parity and various other properties, such as C-parity and G-parity. Although no meson is stable, those of lower mass are nonetheless more stable than the more massive, and hence are easier to observe and study in particle accelerators or in cosmic ray experiments. The lightest group of mesons is less massive than the lightest group of baryons, meaning that they are more easily produced in experiments, and thus exhibit certain higher-energy phenomena more readily than do baryons. But mesons can be quite massive: for example, the J/Psi meson ( ) containing the charm quark, first seen 1974, is about three times as massive as a proton, and the upsilon meson ( ) containing the bottom quark, first seen in 1977, is about ten times as massive as a proton. Particle physicists are most interested in mesons with no orbital angular momentum (  = 0), therefore the two groups of mesons most studied are the  = 1;  = 0 and  = 0;  = 0, which corresponds to  = 1 and  = 0, although they are not the only ones. It is also possible to obtain  = 1 particles from  = 0 and  = 1. How to distinguish between the  = 1,  = 0 and  = 0,  = 1 mesons is an active area of research in meson spectroscopy. Based on this, one might think that, if the wavefunction for each particle (more precisely, the quantum field for each particle type) were simultaneously mirror-reversed, then the new set of wavefunctions would perfectly satisfy the laws of physics (apart from the weak interaction). It turns out that this is not quite true: In order for the equations to be satisfied, the wavefunctions of certain types of particles have to be multiplied by −1, in addition to being mirror-reversed. Such particle types are said to have negative or odd parity (  = −1, or alternatively  = −), whereas the other particles are said to have positive or even parity (  = +1, or alternatively  = +). For mesons, parity is related to the orbital angular momentum by the relation: P = \left( -1 \right)^{L + 1} where the is a result of the parity of the corresponding spherical harmonic of the wavefunction. The "+1" comes from the fact that, according to the Dirac equation, a quark and an antiquark have opposite intrinsic parities. Therefore, the intrinsic parity of a meson is the product of the intrinsic parities of the quark (+1) and antiquark (−1). As these are different, their product is −1, and so it contributes the "+1" that appears in the exponent. As a consequence, all mesons with no orbital angular momentum (  = 0) have odd parity (  = −1). |q\bar{q}\rangle = |\bar{q}q\rangle then, the meson is " even" (  = +1). On the other hand, if |q\bar{q}\rangle = -|\bar{q}q\rangle then the meson is " odd" (  = −1). -parity rarely is studied on its own, but more commonly in combination with P-parity into CP-parity. -parity was originally thought to be conserved, but was later found to be violated on rare occasions in weak interactions. |q_1\bar{q}_2\rangle = |\bar{q}_1 q_2\rangle then, the meson is " even" (  = +1). On the other hand, if |q_1\bar{q}_2\rangle = -|\bar{q}_1 q_2\rangle then the meson is " odd" (  = −1). = a quantum superposition of ) and states but they all have similar masses ( ) as they are each composed of a same total number of up and down quarks and antiquarks. Under the isospin model, they were considered a single particle in different charged states. After the quark model was adopted, physicists noted that the isospin projections were related to the up and down quark content of particles by the relation I_3 = \frac{1}{2}\left[\left(n_\text{u} - n_\bar{\text{u}}\right) - \left(n_\text{d} - n_\bar{\text{d}}\right)\right], where the -symbols are the count of up and down quarks and antiquarks. In the "isospin picture", the three pions and three rhos were thought to be the different states of two particles. However, in the quark model, the rhos are excited states of pions. Isospin, although conveying an inaccurate picture of things, is still used to classify hadrons, leading to unnatural and often confusing nomenclature. Because mesons are hadrons, the isospin classification is also used for them all, with the quantum number calculated by adding for each positively charged up-or-down quark-or-antiquark (up quarks and down antiquarks), and for each negatively charged up-or-down quark-or-antiquark (up antiquarks and down quarks). Q = I_3 + \frac{1}{2}(B + S + C + B^\prime + T), where S, C, , and T represent the strangeness, charm, bottomness and topness flavour quantum numbers respectively. They are related to the number of strange, charm, bottom, and top quarks and antiquark according to the relations: \begin{align} S &= -(n_\text{s} - n_\bar{\text{s}}) \\ C &= +(n_\text{c} - n_\bar{\text{c}}) \\ B^\prime &= -(n_\text{b} - n_\bar{\text{b}}) \\ T &= +(n_\text{t} - n_\bar{\text{t}}), \end{align} meaning that the Gell-Mann–Nishijima formula is equivalent to the expression of charge in terms of quark content: Q=\frac{2}{3}[(n_\text{u}-n_\bar{\text{u}})+(n_\text{c}-n_\bar{\text{c}})+(n_\text{t}-n_\bar{\text{t}})]-\frac{1}{3}[(n_\text{d}-n_\bar{\text{d}})+(n_\text{s}-n_\bar{\text{s}})+(n_\text{b}-n_\bar{\text{b}})]. Flavourless mesons are mesons made of pair of quark and antiquarks of the same flavour (all their flavour quantum numbers are zero: = 0, = 0, = 0, = 0). The rules for flavourless mesons are: In addition : When the spectroscopic state of the meson is known, it is added in parentheses. When the spectroscopic state is unknown, mass (in MeV/c 2 ) is added in parentheses. When the meson is in its ground state, nothing is added in parentheses. If P is in the "normal series" (i.e., P = 0 + , 1 − , 2 + , 3 − , ...), a superscript ∗ is added. If the meson is not pseudoscalar ( P = 0 − ) or vector ( P = 1 − ), is added as a subscript. When the spectroscopic state of the meson is known, it is added in parentheses. When the spectroscopic state is unknown, mass (in MeV/c 2 ) is added in parentheses. When the meson is in its ground state, nothing is added in parentheses. [b] PDG reports the resonance width (Γ). Here the conversion τ =  is given instead. [c] Strong eigenstate. No definite lifetime (see kaon notes below) [d] The mass of the and are given as that of the . However, it is known that a difference between the masses of the and on the order of exists. [e] Weak eigenstate. Makeup is missing small CP–violating term (see notes on neutral kaons below). [f] PDG reports the resonance width (Γ). Here the conversion τ =  is given instead. [g] The exact value depends on the method used. See the given reference for detail.

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