Protons

Main Properties > s.a. hadrons; neutrons.
* General properties: Mass m_p = 1.6726 × 10⁻²⁷ kg = 938.27 MeV/c² = 1.0073 u; Magnetic dipole moment μ_p = 2.792847356(23) μ_N, where μ_N is the nuclear magneton; It is made of uud valence quarks + sea quarks, which give the right charge, but valence quarks account only 9% of the proton's mass! Its momentum distribution is 30% valence quarks, 50% gluons, 20% virtual particles.
* Size: 2010, There is a discrepancy between electron-based measurements (and CODATA value) of 0.8768 × 10⁻¹⁵ m and recent muon-based measurements of the proton's charge radius; 2013, More precise results from a different transition in muonic hydrogen reaffirm the puzzle, giving 0.84087(39) fm for the electric charge radius, a 7σ variance with respect to the 2010-CODATA value, 0.87(6) fm for the magnetic radius (and 1.082(37) fm for the Zemach radius); 2019, New 0.83 fm measured value and likely solution of puzzle; 2020, Hydrogen spectroscopy favors the smaller value.
* Mass: 2017, Precision mass measurements of the proton are typically done with Penning traps, combinations of magnetic and electric fields in which a proton oscillates back and forth within the electric-field potential well while following a helical path due to the magnetic field; The new measurement disagrees with the previous CODATA value by about 3 standard deviations.
@ General references: news sn(17)apr [main open questions]; Heiße et al PRL(17) [new precision mass measurement]; Walker-Loud Phy(18), news sn(18)nov [mass, contributions]; Lincoln TPT(19)aug [discovery centennial].
@ Antiproton: DiSciacca et al PRL(13) + Hudson & Saltzberg Phy(13) + news CERN(13)mar [precise measurement of magnetic moment].
@ Size: Pohl et al Nat(10)jul + news at(10)jul, SA(10)sep [0.841 fm, from muonic hydrogen Lamb shift]; De Rújula PLB(10) [and QCD], PLB(11) [and QED]; Bernauer et al PRL(10), news sn(10)dec [electric and magnetic form factors]; Block & Halzen PRL(11)-a1109 + news physorg(11)nov [inelastic p-p cross sections and the proton as a black disk]; Kelkar et al NPB(12) [r = 0.813 fm from Lamb shift in muonic hydrogen]; Coleman-Smith & Müller PRD(14) [fluctuating size and shape]; Higinbotham et al PRC(16)-a1510 [from electron scattering data]; Miller PRC(19) [treatment based on photon-proton interactions]; news sn(19)sep.
@ Proton radius puzzle: Miller et al PRA(11); news psi(13)jan, sn(13)jan, mnn(13)apr [the puzzle continues]; Dahia & Lemos EPJC(16)-a1509 [evidence for extra dimensions?]; news pt(16)aug [muonic deuterium nucleus adds to the puzzle]; news sn(17)oct [the puzzle persists]; news pt(19)nov [puzzle apparently solved]; news pw(20)nov [hydrogen spectroscopy measurements]; Angeli a2103 [hist].

Internal Structure > s.a. Bag Model.
* Spin: 2017, We cannot account for the sources of the proton's spin.
@ General references: Rith & Schäfer SA(99)jul; Thomas & Weise 01 [r PT(02)jun]; Ellis & Karliner MPLA(06) [gluon polarization]; news pn(06)may [HAPPEx results]; Bass & Aidala IJMPA(06) [rev]; Eichmann et al AP(08); Devenish NPPS(08) [structure functions, overview]; news sn(18)may [pressure].
@ Spin problem: Jaffe PT(95)sep [p "spin crisis"]; Bass RMP(05), 07 [spin structure in QCD]; Fritzsch NPPS(07) [spin structure]; Myhrer & Thomas PLB(08) [possible solution]; Bass MPLA(09); Thomas et al IJMPA(10), Aidala et al RMP(13) [rev]; de Florian et al PRL(14) [gluons may provide some of the proton's missing spin]; Tiwari a1503 [Lorentz covariance and gauge invariance]; Tiwari IJMPA(16)-a1612 [spin as a topological invariant]; Bass Phy(17) [50% of the proton spin comes from the gluons]; Adam et STAR PRD(19) + news sn(19)mar [up and down antiquark contributions].
@ Magnetic moment: Belinfante PR(53) [and neutron]; Kumar pw(05)sep; Ulmer et al PRL(11) + Myers Phy(11) [single-p spin flip]; Ulmer et al JPCS(14)-a1402.
@ Models: Kim a1408; Savvidy JPA(14) [tensor-gluons as additional partons].

Phenomenology > s.a. QCD phenomenology.
* Lifetime: 2017, Despite decades of searches, no proton decay has been detected; From Superkamiokande, in 1998, τ ≥ 1.6 × 10³³ yr [@ Shiozawa et al PRL(98)]; in 2014, τ ≥ 5.9 × 10³³ yr [@ Abe et al PRD(14)].
@ Decay, in general: Horwitz & Katznelson PRL(83) [inhibition inside a nucleus]; Nath & Fileviez PRP(07) [rev, in GUTs, strings and branes]; Abe et al PRD(17) [Super-Kamiokande water Cherenkov detector search]; > s.a. GUTs.
@ Decay, and quantum gravity: Harnik et al NPB(04)hp; Danielsson gq/05; Bambi & Freese CQG(08)-a0803, comment Frampton CQG(09)-a0808, reply a0902 [from minimal length]; Al-Modlej et al CJP(19)-a1903 [non-commutative spacetime].
@ Related topics: Androic et Q_weak PRL(13) [weak charge, first measurement]; > s.a. black-hole formation.

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