Neutrons |

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

* __History__: 1932, Discovered by
James Chadwick; 1934, Chadwick and Goldhaber measured 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, about 8 seconds (or 2.6 σ), and the discrepancy is still unresolved (2018);
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; 2018, The "bottle method" gives 877.7 s;
2020, Proposed space-based measurement.

* __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, The best current upper bound
is 2.9 × 10^{−26} *e*·cm
(90% c.l.), but soon other experiments will do better; 2020, New experimental
value of (0.0 ± 1.1_{stat} ±
0.2_{sys}) × 10^{−26}
*e*·cm; > 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 GRG(17)-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];
Tureanu PRD(18)-a1804 [neutron-antineutron oscillations];
Sponar et al a2012
[tests of fundamental concepts of quantum mechanics].

@ __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 AHEP(16)-a1601 [rev].

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