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4.8.4 Subatomic particles
Subatomic particles are classified according to whether they do or do not respond to the strong nuclear force. Those that do are named ‘hadrons’, of which the protons and neutron are particular examples, while those that do not respond to the strong force are called ‘leptons’, and the electron and neutrino are examples.
The leptons appear to be pointlike even when probed to the highest resolution currently available ( 10−18 m). The hadrons are known to be extended objects ( 10−15 m diameter) which are built from pointlike particles known as ‘quarks’. Five varieties of quark have been identified which are distinguished by their electrical charges and further intrinsic properties named ‘strangeness’, ‘charm’, ‘bottom’ (or ‘beauty’) which are additive quantum numbers. Thus a hadron containing two charmed quarks will have net 2 units of charm and is referred to as a charmed hadron; the more strange quarks there are contained in a hadron, the more will be its net strangeness. The total electrical charge of a hadron is the sum of the electrical charges of its constituent quarks.
The quarks respond to the strong nuclear force but otherwise behave very much like leptons. This has led to conjectures that leptons and quarks may be fundamental and related to one another, though the precise nature of this relationship is still unclear. Recent evidence for a sixth variety of quark (variously called the ‘top’ quark or sometimes ‘truth’) completes a parallelism with the six leptons.
For each variety of particle listed nature also has an antiparticle with the same mass and spin but opposite charge, strangeness and other additive quantum numbers. Their symbols are the same as those for their particle equivalents but for a bar above them, e.g. p is a proton and is the antiproton. The anti-electron is denoted −, or most commonly e+, and named the ‘positron’.
The data on antiparticles are much poorer than for particles. General theorems require that antiparticles have the same mass and lifetimes as their particle equivalents: the poor data are all consistent with this and are not listed here.
In addition, there are particles classified by the family name ‘gauge bosons’. These are carriers of the fundamental forces and have spin. The photon is the carrier of the electromagnetic force; the W and Z bosons are the carriers of the weak forces.
Fundamental particles with spin
The observed mean life of a particle in flight is longer than its mean life at rest by a factor
No individual quarks have been isolated so the concept of mass is not well defined. The listing is merely a qualitative guide extracted from the masses of the lightest hadrons built from the respective quarks. For more details see F. E. Close (1979) An Introduction to Quarks and Partons, Academic Press.
Stable and metastable hadrons
These data refer only to particles immune to decay via the strong interaction; they are derived by the Particle Data Group (Phys. Rev., D50, July 1994) which contains a considerable expansion of the data of this table together with data on unstable mesons and baryons. At present these data are updated biennially by the Particle Data Group.
The quark content is listed by symbols, e.g. the proton built of two up quarks and one down quark is denoted uud. Several metastable heavy particles have been found built from a charmed quark (c) and charmed antiquark ( ) or from a bottom quark (b) and a bottom antiquark ( ). The resulting bound states of these quarks appear to be effectively non-relativistic systems with spin 0 or 1 and orbital angular momentum similar to positronium. By analogy they are known as ‘charmonium’ and ‘bottomonium’ respectively. The spectrum is listed under Heavy quark spectroscopy, below. The charmonium and bottomonium states all have zero baryon number, strangeness, charm and bottom quantum numbers.
The charm (C), strangeness (S) and baryon number (B) of the hadrons which appear in the subsequent tables are as follows:
Mesons consist of a quark and an antiquark; baryons (B = 1) consist of
three quarks (each with B =
). The superscripts denote charges in units of the proton
† The difference (mp − mn) between the proton and neutron masses is known very accurately to be − 1.293 318 ± 0.000 009 MeV.
The difference (MKL − MKS)
between the K 0L and K
0S masses is known very accurately to be
* This is averaged over all B mesons.
Heavy quark spectroscopy
The massive charmed and bottom quarks form non-relativistic bound states with their corresponding antiquarks. The resulting spectroscopy is similar to that of positronium and is known as ‘charmonium’ (charmed quark and charmed antiquark) or ‘bottomonium’. The low-lying energy levels are metastable. These energy levels yield important information on the nature of the interquark forces. The quark and antiquark couple their spins to a total of spin 0 or 1 and are in a state of relative orbital angular momentum L. The spectrum is listed in standard 2S + 1LJ notation.
Heavier charmonium states decay rapidly to charmed hadrons.
Heavier bottomonium states decay rapidly to bottom hadrons.
Gauge bosons with spin 1
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