nLab proton

Idea

A proton is a bound state of two up quarks and a down quark, hence a baryon.

flavors of fundamental fermions in the standard model of particle physics:
generation of fermions	1st generation	2nd generation	3d generation
quarks ($q$)
up-type	up quark ($u$)	charm quark ($c$)	top quark ($t$)
down-type	down quark ($d$)	strange quark ($s$)	bottom quark ($b$)
leptons
charged	electron	muon	tauon
neutral	electron neutrino	muon neutrino	tau neutrino
bound states:
mesons	light mesons: pion ($u d$) ρ-meson ($u d$) ω-meson ($u d$) f1-meson a1-meson	strange-mesons: ϕ-meson ($s \bar s$), kaon, K*-meson ($u s$, $d s$) eta-meson ($u u + d d + s s$) charmed heavy mesons: D-meson ($u c$, $d c$, $s c$) J/ψ-meson ($c \bar c$)	bottom heavy mesons: B-meson ($q b$) ϒ-meson ($b \bar b$)
baryons	nucleons: proton $(u u d)$ neutron $(u d d)$

The bound states of protons and neutrons (jointly: nucleons) in turn are nuclei, the cores of atoms. The proton alone is the nucleus of the hydrogen atom.

In the standard model of particle physics the proton is stable, and experiment highly constrains possible rare proton decay (see there). But some models beyond the standard model (notably many, but not all, GUT models) do imply rare proton decay.

Properties

Mass

A proton has rest mass about a GeV:

$$m_{proton} \simeq 0.938 GeV \,.$$

Spin

The contribution of the constituents (quarks, gluons) to the total spin of the proton turns out to be subtle (“proton spin crisis”) Thomas 08.

Related entries

• neutron

• Lambda baryon

• proton spin crisis

• proton radius problem

References

See also

• Wikipedia, Proton

On the spin of the proton (cf. proton spin crisis):

• Anthony Thomas, The spin of the proton, Progress in Particle and Nuclear Physics Volume 61, Issue 1, July 2008, Pages 219–228 Quarks in Hadrons and Nuclei — 29th Course International Workshop on Nuclear Physics (arXiv:0805.4437)

• Anthony Thomas, The resolution of the proton spin crisis, 2008 (pdf)

On the mass of the proton and the mass gap problem ($\sim$confinement-problem) of QCD:

• Craig Roberts, On Mass and Matter, AAPPS Bulletin 31 6 (2021) [arXiv:2101.08340, doi:10.1007/s43673-021-00005-4]

On the pressure of the proton, via lattice QCD:

• Phiala Shanahan, William Detmold, Pressure Distribution and Shear Forces inside the Proton, Physical Review Letters 122 7 (2019) [doi:10.1103/PhysRevLett.122.072003, arXiv:1810.07589]

On proton stability:

• Juven Wang, Zheyan Wan, Yi-Zhuang You, Proton Stability: From the Standard Model to Beyond Grand Unification, Phys. Rev. D 106 (2022) 025016 [arXiv:2204.08393, doi:10.1103/PhysRevD.106.025016]