Free neutron decay
This article provides insufficient context for those unfamiliar with the subject.(January 2023) |
When embedded in an
−0.44 s[1] or 879.6±0.8 s[2] (about 14 min and 37.75 s or 39.6 s, respectively). Therefore, the half-life for this process (which differs from the mean lifetime by a factor of ln(2) ≈ 0.693) is 611±1 s (about 10 min, 11 s).[3][4]
The beta decay of the neutron described in this article can be notated at four slightly different levels of detail, as shown in four layers of Feynman diagrams in a section below.
-
ν
e
The hard-to-observe
W−
quickly decays into an
W−
W−
boson from one of the down quarks hidden within the neutron, thereby converting the down quark into an up quark and consequently the neutron into a proton
3 quark composite neutron ( n0 ) |
3 quark composite proton ( p+ ) |
||||||
︷ | ︷ | ||||||
( u d d ) |
→ | ( u d u ) |
+ | W− |
|||
⤷ | e− |
+ ν e | |||||
︸ | |||||||
subsequent W− |
- The quark pair not shown in bold are inert bystanders to the whole event.
For diagrams at several levels of detail, see § Decay process, below.
Energy budget
For the free neutron, the
A small fraction (about 1 in 1,000) of free neutrons decay with the same products, but add an extra particle in the form of an emitted gamma ray:
This gamma ray may be thought of as a sort of "internal bremsstrahlung" that arises as the emitted beta particle (electron) interacts with the charge of the proton in an electromagnetic way. In this process, some of the decay energy is carried away as photon energy. Gamma rays produced in this way are also a minor feature of beta decays of bound neutrons, that is, those within a nucleus.
A very small minority of neutron decays (about four per million) are so-called "two-body (neutron) decays", in which a proton, electron and antineutrino are produced as usual, but the electron fails to gain the 13.6 eV necessary energy to escape the proton (the ionization energy of hydrogen), and therefore simply remains bound to it, as a neutral hydrogen atom (one of the "two bodies"). In this type of free neutron decay, in essence all of the neutron decay energy is carried off by the antineutrino (the other "body").
The transformation of a free proton to a neutron (plus a positron and a neutrino) is energetically impossible, since a free neutron has a greater mass than a free proton. However, see proton decay.
Decay process viewed from multiple levels
Understanding of the beta decay process developed over several years, with the initial understanding of
W−
- 1 superficial BARYON level
n0
→
p+
+
e−
+
ν
eThe ). 2 deeper BOSON level
n0
→
p+
+
W−The ). ⤷
e−
+
ν
eThe ).
ν
e
(
u
d
d
)→ (
u
d
u
)+
W−One of the down quarks in the neutron emits a boson and becomes an up quark.
W−⤷
e−
+
ν
eThe ).
ν
e
4 deepest QUARK level
d
→
u
+
W−A down quark (). ⤷
e−
+
ν
eThe ).
ν
e
Neutron lifetime puzzle
While the neutron lifetime has been studied for decades, there currently exists a lack of consilience on its exact value, due to different results from two experimental methods ("bottle" versus "beam"[6][a]).
The "neutron lifetime anomaly" was discovered after the refinement of experiments with ultracold neutrons.
On 13 October 2021 the lifetime from the bottle method was updated to [13][1] increasing the difference to 10 seconds below the beam method value of [14][15] and also on the same date a novel third method using data from the past NASA's Lunar prospector mission reported a value of [14][16] but with great uncertainty.
Yet another approach similar to the beam method has been explored with the Japan Proton Accelerator Research Complex (J-PARC) but it is too imprecise at the moment to be of significance on the analysis of the discrepancy.[17][18]
See also
- Halbach array-used in the "bottle" method
Footnotes
- ^ When physicists strip neutrons from atomic nuclei, put them in a bottle, then count how many remain there after some time, they infer that neutrons radioactively decay in 14m39s, on average. But when other physicists generate beams of neutrons and tally the emerging protons — the particles that free neutrons decay into — they peg the average neutron lifetime at around 14m48s. The discrepancy between the "bottle" and "beam" measurements has persisted [ever] since both methods of gauging the neutron's longevity began yielding results in the 1990s. At first, all the measurements were so imprecise that nobody worried. Gradually, though, both methods have improved, and still they disagree. — Wolchover (2018)[6]
- ^
The scientists have already used the new nucleon axial coupling calculation to derive a purely theoretical prediction of the lifetime of the neutron. Right now, this new value is consistent with the results from both types of experimental measurement, which differ by a mere 9sec.
- "We have a number for the neutron lifetime: 14m40s, with an error bar of 14s. That is right in the middle of the values measured by the two types of experiments, with an error bar that is big, and overlaps both,"
References
- ^ a b
UCNτ Collaboration; Gonzalez, F. M.; Fries, E. M.; Cude-Woods, C.; Bailey, T.; Blatnik, M.; Broussard, L. J.; Callahan, N. B.; Choi, J. H.; Clayton, S. M.; Currie, S. A. (13 October 2021). "Improved Neutron Lifetime Measurement with UCNτ". Physical Review Letters. 127 (16): 162501. S2CID 235490073.
- ^
Lawrence Berkeley Laboratory.
- ^
Beringer, J.; et al. (S2CID 118588567.
- ^
Particle Data Group (2007). Summary Data Table on Baryons (PDF). lbl.gov (Report). Lawrence Berkeley Laboratory. Retrieved 16 August 2012.
- ^
Heyde, K. (2004). "Beta-decay: the weak interaction at work". Basic Ideas and Concepts in Nuclear Physics: An Introductory Approach (third ed.). Taylor & Francis. ISBN 978-0-7503-0980-6. Archived from the originalon 19 January 2013 – via archive.today; link is to archived ch. 5 text.
- ^ a b c Wolchover, Natalie (13 February 2018). "Neutron lifetime puzzle deepens, but no dark matter seen". Quanta Magazine. Retrieved 31 July 2018.
- ^
Serebrov, A.P.; Fomin, A.K. (2011). "New evaluation of neutron lifetime from UCN storage experiments and beam experiments". Physics Procedia. 17: 199–205. S2CID 119204009.
- ^
Paul, Stephan (2009). "The puzzle of neutron lifetime". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 611 (2–3): 157–166. S2CID 9765336.
- ^ a b
Moskowitz, Clara (2014). "Neutron death mystery has physicists stymied". Nature. S2CID 123870434.
- ^
Greene, Geoffrey L.; Geltenbort, Peter (2016). "The Neutron Enigma". PMID 27082189.
- ^
Mumm, Pieter (2018). "Resolving the neutron lifetime puzzle". Science. 360 (6389): 605–606. S2CID 206667316.
- ^ a b "Nuclear scientists calculate value of key property that drives neutron decay". Brookhaven National Laboratory (Press release). 30 May 2018. Retrieved 31 July 2018.
- ^ "How Long Does a Neutron Live?". California Institute of Technology. 13 October 2021. Retrieved 14 October 2021.
- ^ S2CID 226955795.
- .
- S2CID 226307043.
- ISSN 2050-3911.
- ^ "KEK tackles neutron-lifetime puzzle". CERN Courier. 2 July 2021. Retrieved 2 December 2021.
Bibliography
- Ерозолимский, Б.Г. (1975). "Beta decay of the neutron" Бета-распад нейтрона [Neutron beta decay]. Успехи Физических Наук Успехи физических наук. 116 (1): 145–164. .