Hadrons and Leptons
Hadrons and Leptons
Particles can also be revealingly grouped according to what forces they feel between them. All particles (even those that are massless) are affected by gravity, since gravity affects the space and time in which particles exist. All charged particles are affected by the electromagnetic force, as are neutral particles that have an internal distribution of charge (such as the neutron with its magnetic moment). Special names are given to particles that feel the strong and weak nuclear forces. Hadrons are particles that feel the strong nuclear force, whereas leptons are particles that do not. The proton, neutron, and the pions are examples of hadrons. The electron, positron, muons, and neutrinos are examples of leptons, the name meaning low mass. Leptons feel the weak nuclear force. In fact, all particles feel the weak nuclear force. This means that hadrons are distinguished by being able to feel both the strong and weak nuclear forces.
This table lists the characteristics of some of the most important subatomic particles, including the directly observed carrier particles for the electromagnetic and weak nuclear forces, all leptons, and some hadrons. Several hints related to an underlying substructure emerge from an examination of these particle characteristics. Note that the carrier particles are called gauge bosons. First mentioned in Patterns in Spectra Reveal More Quantization, a boson is a particle with zero or an integer value of intrinsic spin (such as \(s=0, 1, 2, ...\)), whereas a fermion is a particle with a half-integer value of intrinsic spin (\(s=1/2,\phantom{\rule{0.25em}{0ex}}3/2,\phantom{\rule{0.25em}{0ex}}\text{.}\text{.}\text{.}\)). Fermions obey the Pauli exclusion principle whereas bosons do not. All the known and conjectured carrier particles are bosons.
Selected Particle Characteristics
| Category | Particle name | Symbol | Antiparticle | Rest mass \((\text{MeV}/{c}^{2})\) | \(B\) | \({L}_{e}\) | \({L}_{\mu }\) | \({L}_{\tau }\) | \(S\) | Lifetime (s) |
|---|---|---|---|---|---|---|---|---|---|---|
| Gauge | Photon | \(\gamma \) | Self | 0 | 0 | 0 | 0 | 0 | 0 | Stable |
| Bosons | \(W\) | \({W}^{+}\) | \({W}^{-}\) | \(\text{80}\text{.}\text{39}×{\text{10}}^{3}\) | 0 | 0 | 0 | 0 | 0 | \(1.6×{\text{10}}^{-\text{25}}\) |
| \(Z\) | \({Z}^{0}\) | Self | \(\text{91}\text{.}\text{19}×{\text{10}}^{3}\) | 0 | 0 | 0 | 0 | 0 | \(1.32×{\text{10}}^{-\text{25}}\) | |
| Leptons | Electron | \({e}^{-}\) | \({e}^{+}\) | 0.511 | 0 | \(±1\) | 0 | 0 | 0 | Stable |
| Neutrino (e) | \({\nu }_{e}\) | \({\overline{v}}_{e}\) | \(0(7\text{.}0\text{eV})\) | 0 | \(±1\) | 0 | 0 | 0 | Stable | |
| Muon | \({\mu }^{-}\) | \({\mu }^{+}\) | 105.7 | 0 | 0 | \(±1\) | 0 | 0 | \(2\text{.}\text{20}×{\text{10}}^{-6}\) | |
| Neutrino \((\mu )\) | \({v}_{\mu }\) | \({\stackrel{-}{v}}_{\mu }\) | \(0(<0.27)\) | 0 | 0 | \(±1\) | 0 | 0 | Stable | |
| Tau | \({\tau }^{-}\) | \({\tau }^{+}\) | 1777 | 0 | 0 | 0 | \(±1\) | 0 | \(2\text{.}\text{91}×{\text{10}}^{-\text{13}}\) | |
| Neutrino \((\tau )\) | \({v}_{\tau }\) | \({\stackrel{-}{v}}_{\tau }\) | \(0(<31)\) | 0 | 0 | 0 | \(±1\) | 0 | Stable | |
| Hadrons (selected) | ||||||||||
| Mesons | Pion | \({\pi }^{+}\) | \({\pi }^{-}\) | 139.6 | 0 | 0 | 0 | 0 | 0 | 2.60 × 10 −8 |
| \({\pi }^{0}\) | Self | 135.0 | 0 | 0 | 0 | 0 | 0 | 8.4 × 10 −17 | ||
| Kaon | \({K}^{+}\) | \({K}^{-}\) | 493.7 | 0 | 0 | 0 | 0 | \(±1\) | 1.24 × 10 −8 | |
| \({K}^{0}\) | \({\stackrel{-}{K}}^{0}\) | 497.6 | 0 | 0 | 0 | 0 | \(±1\) | 0.90 × 10 −10 | ||
| Eta | \({\eta }^{0}\) | Self | 547.9 | 0 | 0 | 0 | 0 | 0 | 2.53 × 10 −19 | |
| (many other mesons known) | ||||||||||
| Baryons | Proton | \(p\) | \(\stackrel{-}{p}\) | 938.3 | ± 1 | 0 | 0 | 0 | 0 | Stable |
| Neutron | \(n\) | \(\stackrel{-}{n}\) | 939.6 | ± 1 | 0 | 0 | 0 | 0 | 882 | |
| Lambda | \({\text{Λ}}^{0}\) | \({\stackrel{-}{\text{Λ}}}^{0}\) | 1115.7 | ± 1 | 0 | 0 | 0 | \(\mp 1\) | 2.63 × 10 −10 | |
| Sigma | \({\text{Σ}}^{+}\) | \({\stackrel{-}{\text{Σ}}}^{-}\) | 1189.4 | ± 1 | 0 | 0 | 0 | \(\mp 1\) | 0.80 × 10 −10 | |
| \({\text{Σ}}^{0}\) | \({\stackrel{-}{\text{Σ}}}^{0}\) | 1192.6 | ± 1 | 0 | 0 | 0 | \(\mp 1\) | 7.4 × 10 −20 | ||
| \({\text{Σ}}^{-}\) | \({\stackrel{-}{\text{Σ}}}^{+}\) | 1197.4 | ± 1 | 0 | 0 | 0 | \(\mp 1\) | 1.48 × 10 −10 | ||
| Xi | \({\text{Ξ}}^{0}\) | \({\stackrel{-}{\text{Ξ}}}^{0}\) | 1314.9 | ± 1 | 0 | 0 | 0 | \(\mp 2\) | 2.90 × 10 −10 | |
| \({\text{Ξ}}^{-}\) | \({\text{Ξ}}^{+}\) | 1321.7 | ± 1 | 0 | 0 | 0 | \(\mp 2\) | 1.64 × 10 −10 | ||
| Omega | \({\Omega }^{-}\) | \({\Omega }^{+}\) | 1672.5 | ± 1 | 0 | 0 | 0 | \(\mp 3\) | 0.82 × 10 −10 | |
| (many other baryons known) | ||||||||||
All known leptons are listed in the table given above. There are only six leptons (and their antiparticles), and they seem to be fundamental in that they have no apparent underlying structure. Leptons have no discernible size other than their wavelength, so that we know they are pointlike down to about \({\text{10}}^{-\text{18}}\phantom{\rule{0.25em}{0ex}}\text{m}\). The leptons fall into three families, implying three conservation laws for three quantum numbers. One of these was known from \(\beta \) decay, where the existence of the electron’s neutrino implied that a new quantum number, called the electron family number \({L}_{e}\) is conserved. Thus, in \(\beta \) decay, an antielectron’s neutrino \({\stackrel{-}{v}}_{e}\) must be created with \({L}_{e}=-1\) when an electron with \({L}_{e}\text{=+}1\) is created, so that the total remains 0 as it was before decay.
Once the muon was discovered in cosmic rays, its decay mode was found to be
\({\mu }^{-}\to {e}^{-}+{\stackrel{-}{v}}_{e}+{v}_{\mu }\text{,}\)
which implied another “family” and associated conservation principle. The particle \({v}_{\mu }\) is a muon’s neutrino, and it is created to conserve muon family number \({L}_{\mu }\). So muons are leptons with a family of their own, and conservation of total \({L}_{\mu }\) also seems to be obeyed in many experiments.
More recently, a third lepton family was discovered when \(\tau \) particles were created and observed to decay in a manner similar to muons. One principal decay mode is
\({\tau }^{-}\to {\mu }^{-}+{\stackrel{-}{v}}_{\mu }+{v}_{\tau }\text{.}\)
Conservation of total \({L}_{\tau }\) seems to be another law obeyed in many experiments. In fact, particle experiments have found that lepton family number is not universally conserved, due to neutrino “oscillations,” or transformations of neutrinos from one family type to another.
This lesson is part of:
Particle Physics