Protons are spin-1/2 fermions and are composed of three valence quarks, making them baryons

If you've been following this trail carefully you'll recall that Wikipedia through us a bit of a curve ball earlier by implying in at least one spot inconsistently that all baryons contain only three quarks:

Wikipedia: 'Since baryons are made of three quarks [DISPUTED], their spin vectors can add to make a vector of length S = 3/2, which has four spin projections (Sz = +3/2, Sz = +1/2, Sz = −1/2, and Sz = −3/2), or a vector of length S = 1/2 with two spin projections ...'

This led in a previous slide to a questionable Generalization from SpinHalfBaryon to TriQuarkBaryon, which in fact mixed two different aspects, one involving the number of valence quarks, and another involving spin. This has now been cleaned up so that instead SpinHalfBaryon extends Baryon directly and also extends SpinHalfParticle, so if, for example, there is a concrete pentaquark with spin-1/2 it can now attach to SpinHalfBaryon.

This fix also makes it possible to now cleanly include Nucleon as an intermediate abstraction shared by Proton and Neutron, where Nucleon extends TriQuarkBaryon and SpinHalfBaryon.

We are told that:

Wikipedia: 'A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks.'

It was correctly identified earlier that not all quarks "contained" in a hadron are valence quarks. These snippets clarify this further:

Wikipedia: 'Hadrons contain, along with the valence quarks that contribute to their quantum numbers, virtual quark–antiquark pairs known as sea quarks.'

Wikipedia: 'The quarks that determine the quantum numbers of hadrons are called valence quarks; apart from these, any hadron may contain an indefinite number of virtual "sea" quarks, antiquarks, and gluons, which do not influence its quantum numbers.'

That can be included in the existing SysML model easily using subsetted properties. The block Hadron now declares /quark:Quark[2..*] to be a derived union, where seaQuark:Quark[0..*] and valenceQuark:Quark[2..*] are {subsets} of /quark. And Hadron also gets a part property gluon:Gluon[0..*].

Subsetted properties can also be used in AtomicNucleus:

Wikipedia: 'One or more protons are present in the nucleus of every atom; they are a necessary part of the nucleus.'

The block AtomicNucleus has a derived union /nucleon:Nucleon[1..*], where proton:Proton[1..*] and neutron:Neutron[0..*] are {subsets}.

We learn now also:

Wikipedia: 'The free proton is stable.'

Does this mean protons that are not "free" are sometimes not stable? We can make the interpretation to date somewhat more robust by documenting the isStable:Boolean in Particle as now applying only to free particles, where Proton redefines it to be 'true'.

And once again, a "short-cut" quickie version of Parsing Analysis uses the SysML Trace together with compartment listing for 'tracedFrom' to indicate where values for mass and restEnergy were sourced.

Up next

Improved baryon Package Diagram including Nucleon, Proton, Neutron

Notes

[POLICY, STYLE]{STRICT} Webel Parsing Analysis «pa» diagrams are NOT intended as final presentation diagrams! They serve merely to traceably elicit model elements, which may then be shown in other (typically much tidier) presentation diagrams elsewhere.

[DISPLAY, STYLE, TIP]{SUGGESTED} Webel Parsing Analysis: If the dashed-line "anchors" from your «snippet» ElementGroup symbols to elicited elements make a «pa» Parsing Analysis Diagram (PAD) too hard to read, try making them light grey using symbol properties.

Snippets (quotes/extracts)

Protons are spin-1/2 fermions and are composed of three valence quarks, making them baryons (a sub-type of hadrons).

Hadrons contain, along with the valence quarks that contribute to their quantum numbers, virtual quark–antiquark pairs known as sea quarks.

The quarks that determine the quantum numbers of hadrons are called valence quarks; apart from these, any hadron may contain an indefinite number of virtual "sea" quarks, antiquarks, and gluons, which do not influence its quantum numbers.

The two up quarks and one down quark of a proton are held together by the strong force, mediated by gluons.

A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks.

A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and a mass slightly less than that of a neutron.