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21 pages, 638 KiB

Open AccessArticle

Neutron Star Constraints on Neutron Dark Decays

by Dake Zhou

Universe 2023, 9(11), 484; https://doi.org/10.3390/universe9110484 - 17 Nov 2023

Cited by 2 | Viewed by 1337

Abstract

Motivated by the neutron lifetime puzzle, it is proposed that neutrons may decay into new states yet to be observed. We review the neutron star constraints on dark fermions carrying unit baryon number with masses around 939 MeV, and discuss the interaction strengths [...] Read more.

Motivated by the neutron lifetime puzzle, it is proposed that neutrons may decay into new states yet to be observed. We review the neutron star constraints on dark fermions carrying unit baryon number with masses around 939 MeV, and discuss the interaction strengths required for the new particle. The possibility of neutrons decaying into three dark fermions is investigated. While up to six flavors of dark quarks with masses around 313 MeV can be compatible with massive pulsars, any such exotic states lighter than about 270 MeV are excluded by the existence of low-mass neutron stars around ∼

1.2 M_{⊙}

. Light dark quarks in the allowed mass range may form a halo surrounding normal neutron stars. We discuss the potential observable signatures of the halo during binary neutron star mergers. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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10 pages, 453 KiB

Open AccessArticle

How Long Does the Hydrogen Atom Live?

by David McKeen and Maxim Pospelov

Universe 2023, 9(11), 473; https://doi.org/10.3390/universe9110473 - 4 Nov 2023

Cited by 2 | Viewed by 1242

Abstract

It is possible that the proton is stable while atomic hydrogen is not. This is the case in models with new particles carrying baryon number which are light enough to be stable themselves, but heavy enough so that proton decay is kinematically blocked. [...] Read more.

It is possible that the proton is stable while atomic hydrogen is not. This is the case in models with new particles carrying baryon number which are light enough to be stable themselves, but heavy enough so that proton decay is kinematically blocked. Models of new physics that explain the neutron lifetime anomaly generically have this feature, allowing for atomic hydrogen to decay through electron capture on a proton. We calculate the radiative hydrogen decay rate involving the emission of a few hundred keV photon, which makes this process experimentally detectable. In particular, we show that the low energy part of the Borexino spectrum is sensitive to radiative hydrogen decay, and turn this into a limit on the hydrogen lifetime of order

10^{30} s

or stronger. For models where the neutron mixes with a dark baryon,

χ

, this limits the mixing angle to roughly

10^{- 11}

, restricting the

n \to χ γ

branching to

10^{- 4}

, over a wide range of parameter space. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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12 pages, 355 KiB

Open AccessArticle

Constraining Dark Boson Decay Using Neutron Stars

by Wasif Husain, Dipan Sengupta and A. W. Thomas

Universe 2023, 9(7), 307; https://doi.org/10.3390/universe9070307 - 26 Jun 2023

Cited by 3 | Viewed by 837

Abstract

Inspired by the well-known anomaly in the lifetime of the neutron, we investigated its consequences inside neutron stars. We first assessed the viability of the neutron decay hypothesis suggested by Fornal and Grinstein within neutron stars, in terms of the equation of state [...] Read more.

Inspired by the well-known anomaly in the lifetime of the neutron, we investigated its consequences inside neutron stars. We first assessed the viability of the neutron decay hypothesis suggested by Fornal and Grinstein within neutron stars, in terms of the equation of state and compatibility with observed properties. This was followed by an investigation of the constraint information on neutron star cooling that can be placed on the decay rate of the dark boson into standard model particles, in the context of various BSM ideas. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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18 pages, 1413 KiB

Open AccessArticle

The Neutron Mean Life and Big Bang Nucleosynthesis

by Tsung-Han Yeh, Keith A. Olive and Brian D. Fields

Universe 2023, 9(4), 183; https://doi.org/10.3390/universe9040183 - 12 Apr 2023

Cited by 4 | Viewed by 1257

Abstract

We explore the effect of neutron lifetime and its uncertainty on standard big bang nucleosynthesis (BBN). BBN describes the cosmic production of the light nuclides,

^{1} H

, D,

^{3} H

+

^{3} He

,

^{4} He

, and

^{7} Li

+ [...] Read more.

We explore the effect of neutron lifetime and its uncertainty on standard big bang nucleosynthesis (BBN). BBN describes the cosmic production of the light nuclides,

^{1} H

, D,

^{3} H

+

^{3} He

,

^{4} He

, and

^{7} Li

+

^{7} Be

, in the first minutes of cosmic time. The neutron mean life

τ_{n}

has two roles in modern BBN calculations: (1) it normalizes the matrix element for weak

n \leftrightarrow p

interconversions, and (2) it sets the rate of free neutron decay after the weak interactions freeze-out. We review the history of the interplay between

τ_{n}

measurements and BBN, and present a study of the sensitivity of the light element abundances to the modern neutron lifetime measurements. We find that

τ_{n}

uncertainties dominate the predicted

^{4} He

error budget, but these theory errors remain smaller than the uncertainties in

^{4} He

observations, even with the dispersion in recent neutron lifetime measurements. For the other light element predictions,

τ_{n}

contributes negligibly to their error budget. Turning the problem around, we combine present BBN and cosmic microwave background (CMB) determinations of the cosmic baryon density to predict a “cosmologically preferred” mean life of

τ_{n} (BBN + CMB) = 870 \pm 16 s

, which is consistent with experimental mean life determinations. We show that if future astronomical and cosmological helium observations can reach an uncertainty of

σ_{obs} (Y_{p}) = 0.001

in the

^{4} He

mass fraction

Y_{p}

, this could begin to discriminate between the mean life determinations. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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Review

Jump to: Research

15 pages, 1058 KiB

Open AccessReview

Isovector Axial Charge and Form Factors of Nucleons from Lattice QCD

by Rajan Gupta

Universe 2024, 10(3), 135; https://doi.org/10.3390/universe10030135 - 12 Mar 2024

Viewed by 809

Abstract

A survey of the calculations of the isovector axial vector form factor of the nucleon using lattice QCD is presented. Attention is paid to statistical and systematic uncertainties, in particular those due to excited state contributions. Based on a comparison of results from [...] Read more.

A survey of the calculations of the isovector axial vector form factor of the nucleon using lattice QCD is presented. Attention is paid to statistical and systematic uncertainties, in particular those due to excited state contributions. Based on a comparison of results from various collaborations, a case is made that lattice results are consistent within 10%. A similar level of uncertainty is in the axial charge

g_{A}^{u - d}

, the mean squared axial charge radius

⟨ r_{A}^{2} ⟩

, the induced pseudoscalar charge

g_{P}^{*}

, and the pion–nucleon coupling

g_{π N N}

. Even with the current methodology, a significant reduction in errors is expected over the next few years with higher statistics data on more ensembles closer to the physical point. Lattice QCD results for the form factor

G_{A} (Q^{2})

are compatible with those obtained from the recent MINER

ν

A experiment but lie 2–3

σ

higher than the phenomenological extraction from the old

ν

–deuterium bubble chamber scattering data for

Q^{2} > 0.3

GeV². Current data show that the dipole ansatz does not have enough parameters to fit the form factor over the range

0 \leq Q^{2} \leq 1

GeV², whereas even a

z^{2}

truncation of the z expansion or a low order Padé are sufficient. Looking ahead, lattice QCD calculations will provide increasingly precise results over the range

0 \leq Q^{2} \leq 1

GeV², and MINER

ν

A-like experiments will extend the range to

Q^{2} \sim 2

GeV² or higher. Nevertheless, improvements in lattice methods to (i) further control excited state contributions and (ii) extend the range of

Q^{2}

are needed. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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31 pages, 8221 KiB

Open AccessReview

Probing Dark Sectors with Neutron Stars

by Susan Gardner and Mohammadreza Zakeri

Universe 2024, 10(2), 67; https://doi.org/10.3390/universe10020067 - 1 Feb 2024

Cited by 2 | Viewed by 943

Abstract

Tensions in the measurements of neutron and kaon weak decays, such as of the neutron lifetime, may speak to the existence of new particles and dynamics not present in the Standard Model (SM). In scenarios with dark sectors, particles that couple feebly to [...] Read more.

Tensions in the measurements of neutron and kaon weak decays, such as of the neutron lifetime, may speak to the existence of new particles and dynamics not present in the Standard Model (SM). In scenarios with dark sectors, particles that couple feebly to those of the SM appear. We offer a focused overview of such possibilities and describe how the observations of neutron stars, which probe either their structure or dynamics, limit them. In realizing these constraints, we highlight how the assessment of particle processes within dense baryonic matter impacts the emerging picture—and we emphasize both the flavor structure of the constraints and their broader connections to cogenesis models of dark matter and baryogenesis. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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20 pages, 873 KiB

Open AccessReview

Neutron Dark Decay

by Bartosz Fornal

Universe 2023, 9(10), 449; https://doi.org/10.3390/universe9100449 - 16 Oct 2023

Cited by 5 | Viewed by 1411

Abstract

There exists a puzzling disagreement between the results for the neutron lifetime obtained in experiments using the beam technique versus those relying on the bottle method. A possible explanation of this discrepancy postulates the existence of a beyond-Standard-Model decay channel of the neutron [...] Read more.

There exists a puzzling disagreement between the results for the neutron lifetime obtained in experiments using the beam technique versus those relying on the bottle method. A possible explanation of this discrepancy postulates the existence of a beyond-Standard-Model decay channel of the neutron involving new particles in the final state, some of which can be dark matter candidates. We review the current theoretical status of this proposal and discuss the particle physics models accommodating such a dark decay. We then elaborate on the efforts undertaken to test this hypothesis, summarizing the prospects for probing neutron dark decay channels in future experiments. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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26 pages, 1599 KiB

Open AccessReview

The Standard Model Theory of Neutron Beta Decay

by Mikhail Gorchtein and Chien-Yeah Seng

Universe 2023, 9(9), 422; https://doi.org/10.3390/universe9090422 - 19 Sep 2023

Cited by 5 | Viewed by 1157

Abstract

We review the status of the Standard Model theory of neutron beta decay. Particular emphasis is put on the recent developments in the electroweak radiative corrections. Given that some existing approaches give slightly different results, we thoroughly review the origin of discrepancies, and [...] Read more.

We review the status of the Standard Model theory of neutron beta decay. Particular emphasis is put on the recent developments in the electroweak radiative corrections. Given that some existing approaches give slightly different results, we thoroughly review the origin of discrepancies, and provide our recommended value for the radiative correction to the neutron and nuclear decay rates. The use of dispersion relation, lattice Quantum Chromodynamics, and an effective field theory framework allows for high-precision theory calculations at the level of

10^{- 4}

, turning neutron beta decay into a powerful tool to search for new physics, complementary to high-energy collider experiments. We offer an outlook to the future improvements. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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10 pages, 283 KiB

Open AccessReview

Strong Interaction Dynamics and Fermi β Decay in the Nucleon and the Nucleus

by Gerald A. Miller

Universe 2023, 9(5), 209; https://doi.org/10.3390/universe9050209 - 27 Apr 2023

Viewed by 879

Abstract

Nuclear super-allowed

β

decay has been used to obtain tight limits on the value of the CKM matrix element

V_{u d}

that is important for unitarity tests and, therefore, for tests of the standard model. Current requirements on precision are so intense [...] Read more.

Nuclear super-allowed

β

decay has been used to obtain tight limits on the value of the CKM matrix element

V_{u d}

that is important for unitarity tests and, therefore, for tests of the standard model. Current requirements on precision are so intense that effects formerly thought too small to matter are now considered relevant. This article is a brief review of personal efforts to include the effects of strong interactions on Fermi

β

decay. First, I examine the role of isospin violation in the decay of the neutron. The size of the necessary correction depends upon detailed strong-interaction dynamics. The isospin violating parts of the nucleon wave function, important at the low energy of

β

decay, can be constrained by data taken at much higher energies, via measurements, for example, of

e d \to e^{'} π^{\pm} + X

reactions at Jefferson Laboratory. The next point of focus is on the role of nuclear short-ranged correlations, which affect the value of the correction needed to account for isospin violation in extracting the value of

V_{u d}

. The net result is that effects previously considered as irrelevant are now considered relevant for both neutron and nuclear

β

decay. Full article

(This article belongs to the Special Issue Neutron Lifetime)

27 pages, 896 KiB

Open AccessReview

Neutron Lifetime Anomaly and Mirror Matter Theory

by Wanpeng Tan

Universe 2023, 9(4), 180; https://doi.org/10.3390/universe9040180 - 11 Apr 2023

Cited by 7 | Viewed by 1461

Abstract

This paper reviews the puzzles in modern neutron lifetime measurements and related unitarity issues in the CKM matrix. It is not a comprehensive and unbiased compilation of all historic data and studies, but rather a focus on compelling evidence leading to new physics. [...] Read more.

This paper reviews the puzzles in modern neutron lifetime measurements and related unitarity issues in the CKM matrix. It is not a comprehensive and unbiased compilation of all historic data and studies, but rather a focus on compelling evidence leading to new physics. In particular, the largely overlooked nuances of different techniques applied in material and magnetic trap experiments are clarified. Further detailed analysis shows that the “beam” approach of neutron lifetime measurements is likely to give the “true”

β

-decay lifetime, while discrepancies in “bottle” measurements indicate new physics at play. The most feasible solution to these puzzles is a newly proposed ordinary-mirror neutron (

n - n^{'}

) oscillation model under the framework of mirror matter theory. This phenomenological model is reviewed and introduced, and its explanations of the neutron lifetime anomaly and possible non-unitarity of the CKM matrix are presented. Most importantly, various new experimental proposals, especially lifetime measurements with small/narrow magnetic traps or under super-strong magnetic fields, are discussed in order to test the surprisingly large anomalous signals that are uniquely predicted by this new

n - n^{'}

oscillation model. Full article

(This article belongs to the Special Issue Neutron Lifetime)

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