Neutrons |

**In General** > s.a. experiments
in quantum mechanics; hadrons [structure]; protons.

* __History__: 1932, Discovered
by Chadwick; 1934, Chadwick and Goldhaber determined the mass accurately enough
to determine that a neutron was not an electron-proton bound state (as
Rutherford had predicted); 2002, Evidence for neutrinoless double-*β* decay
reported, and quantum states in gravitational field.

* __Mass__: Its value is *m*_{n} =
1.6749 × 10^{–27} kg.

* __Lifetime__: An isolated neutron survives just 15 minutes before it decays into a proton, electron, and an antineutrino; Astrophysicists rely on a precise value of the free neutron lifetime to calculate the rate of nucleosynthesis during the big bang, and particle physicists use it to constrain fundamental parameters of the standard model; Yet measured lifetimes vary by about a percent, or 8 seconds (or 2.6 σ), and the discrepancy is still unresolved (2013); 2015, Cosmological data give 905.7 ± 7.8 s, while the "bottle method" with ultracold neutrons gives 905.7 ± 7.8 s, and the "beam method" 888.0 ± 2.1 s; 2018, the discrepancy is now 3.8 σ; One possibility is that the neutron can decay into a dark matter particle.

* __Electric dipole moment__:
In the standard model, the strong interaction should violate
T-reversal symmetry, and thus CP symmetry; But such a symmetry violation
would result in a neutron electric dipole moment 10 orders of magnitude larger
than the current bound; In 1977, Roberto Peccei
and Helen Quinn discovered a simple dynamical mechanism to enforce strong
CP symmetry which, as Steven Weinberg and Frank Wilczek independently realized,
implies the existence of the axion; 2006,
Currently the best experimental upper bound is 2.9 × 10^{–26} *e*·cm
(90% c.l.), but soon other experiments will do better; > s.a. CPT theorem.

* __n____- p mass difference__: The measured difference is only 0.14% of the average of the two masses (a slightly smaller or larger value would have led to a dramatically different universe), and results from a competition between electromagnetic effects and the mass difference between the up and down quarks.

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**Neutron Interferometry** > s.a. experiments
in quantum mechanics; geometric
phase;
quantum equivalence principle.

* __And gravity__: Gravity-induced phases have already been detected, and they
show that gravity at the quantum level is not a purely geometric effect,
since the mass of the employed particles appears explicitly in the interference
expression.

@ __General references__: Greenberger RMP(83)
[and quantum mechanics, rev]; Rauch HPA(88);
Unnerstall PLA(90)
[comment]; Rauch & Vigier PLA(90);
Rauch FP(93);
Benatti & Floreanini
PLB(99)qp [semigroups
and dissipative evolution]; Felber et al FP(99)
[in space and time]; Rauch & Werner
00 [r PT(02)jun];
Wu et al IJTP(10)-a0910 [quantum
theory approach]; Klein FP(12) [history]; Klepp et al PTEP-a1407 [and fundamental quantum phenomena].

@ __Geometric and quantum phases__: Werner CQG(94);
Littrell et al PRA(97);
Allman et al PRA(97);
Bhandari qp/01/PRL;
Rauch et al Nat(02)jun
[confinement-induced]; Sponar et al JPA(10)-a1002; Werner FP(12) [observation of geometric phase].

@ __And gravity__: Wolf FP(90)
[and quantum gravity]; Werner CQG(94);
Camacho PLA(99)qp,
PLA(99)qp;
Varjú & Ryder AJP(00)may
[general relativistic treatment]; Nandi & Zhang PRD(02)gq [and
equivalence principle]; Camacho & Macías PLB(05)
[and torsion]; Abele & Leeb NJP(12); Galiautdinov & Ryder a1701 [derivation in the weak-field approximation].

@ __Related topics__: Gähler & Zeilinger AJP(91)apr
[wave phenomena, interference and diffraction]; > s.a. Goos-Hänchen
Effect; gravitomagnetism.

**Other Phenomenology** > s.a. Beta Decay; CPT
tests; neutron stars.

@ __General references__:
Snow PT(13)mar [slow neutrons and fundamental physics].

@ __Quantum states in gravitational field__:
Nesvizhevsky et al NIM(00),
Nat(02)jan;
Schwarzschild PT(02)mar;
Olevik et al qp/02;
Westphal gq/02 [theory];
Nesvizhevsky PRD(03),
comment Hansson et al PRD(03)qp,
reply PRD(03);
Jenke et al PRL(14)
[and constraints on dark energy and dark matter];
> s.a. tests of newtonian gravity.

@ __Scattering and interactions__:
news pw(06)dec [and paper dating];
Alexandrov G&C(08) [and higher-dimensional gravity];
Furrer et al 09 [in condensed-matter physics];
Chen & Kotlarchyk 07 [interaction with matter].

@ __Neutrinoless double- β decay__:
Klapdor-Kleingrothaus in(02)hp,
et al FP(02);
Klapdor-Kleingrothaus FP(03);
Dell'Oro et al a1601 [rev].

>

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