Evidence of Mirror Symmetry Breaking in the $^{71}$Kr and $^{71}$Br Atomic Nuclei

Core Concepts

This research provides the first experimental evidence of mirror symmetry breaking in the ground states of the atomic nuclei $^{71}$Kr and $^{71}$Br, a phenomenon previously observed only in heavier mirror nuclei.

Abstract

• Bibliographic Information: Algora, A., Vitéz-Sveiczer, A., Poves, A., Kiss, G. G., ... & Kruppa, A. (2024). Isospin breaking in the $^{71}$Kr and $^{71}$Br mirror system. arXiv preprint arXiv:2411.00509v1.
• Research Objective: Investigate the beta decay of $^{71}$Kr and compare experimental data with theoretical shell model calculations to determine the spin-parity assignment of the ground state of $^{71}$Kr and assess the presence of mirror symmetry breaking in the $^{71}$Kr/$^{71}$Br mirror system.
• Methodology: The research employed a high-statistics beta decay study of $^{71}$Kr produced at the RIKEN Nishina Center. The team used the BigRIPS separator and the WAS3ABi and EURICA detectors to measure gamma rays and beta-delayed protons emitted during the decay. The experimental data was then compared to state-of-the-art shell model calculations, including Coulomb and other charge symmetry breaking terms.
• Key Findings: The experimental data, particularly the accumulated beta strength distribution, showed a significant discrepancy with theoretical predictions assuming a 5/2− ground state for $^{71}$Kr, as found in its mirror nucleus $^{71}$Br. The data instead strongly favored a 1/2− ground state for $^{71}$Kr, indicating a breakdown of mirror symmetry in this system. This conclusion is further supported by the absence of feeding to low-lying 7/2− states in $^{71}$Br, expected for a 5/2− parent state.
• Main Conclusions: The study provides compelling evidence for mirror symmetry breaking in the A=71 mirror system, specifically an inversion of the two lowest energy states in $^{71}$Kr compared to $^{71}$Br. This is the first observation of such breaking in a T=1/2 mirror pair, where a single proton-neutron exchange differentiates the nuclei. The study also identified a new state in $^{70}$Se, populated through beta-delayed proton emission, which is interpreted as a shape-coexisting 0+2 state.
• Significance: This research significantly contributes to nuclear physics by providing the first confirmed case of mirror symmetry breaking in a light mirror system. This finding challenges the conventional understanding of isospin symmetry, particularly in simpler systems, and highlights the importance of considering charge symmetry breaking effects in nuclear structure models.
• Limitations and Future Research: While the study presents strong evidence, further experimental confirmation of the spin-parity assignments of the states involved would strengthen the conclusions. Additionally, exploring other light mirror systems could reveal if this phenomenon is more widespread than previously thought. Investigating the impact of this finding on related areas like tests of the Conserved Vector Current Hypothesis and the determination of the |Vud| matrix element would be beneficial.

Stats

The beta decay of 71Kr to the isobaric analog state in 71Br has a branching ratio of 80.7(7)%, the lowest experimentally confirmed feeding among all studied mirror decays.
The beta-delayed proton emission branch for 71Kr is 2.77(24)%.
The half-life of 71Kr was determined to be 94.50(24) ms.
The proton separation energy in 71Br is 1861 keV.

Quotes

"This represents the first example of established mirror symmetry breaking in pairs with T=1/2."
"Our work shows that the A=71 system presents the lowest confirmed ground state to ground state branching ratio in mirror T=1/2 nuclei."

Key Insights Distilled From

Isospin breaking in the $^{71}$Kr and $^{71}$Br mirror system

by A. A... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00509.pdf

Deeper Inquiries

What are the broader implications of mirror symmetry breaking for our understanding of fundamental symmetries in nature?

The observation of mirror symmetry breaking in the ${}^{71}$Kr and ${}^{71}$Br mirror system, as highlighted in the paper, has potentially significant implications for our understanding of fundamental symmetries in nuclear physics and beyond:

Testing the Limits of Isospin Symmetry: Isospin symmetry, which posits that protons and neutrons are interchangeable within the nucleus, is a cornerstone of nuclear physics. While we know it's an approximate symmetry, broken by electromagnetic interactions and differences in quark masses, observing where and how it breaks provides valuable insights into the strengths and limitations of this concept. This specific case, involving a simple T=1/2 system, challenges our assumptions about the robustness of isospin symmetry even in seemingly straightforward scenarios.

Probing Nuclear Structure: Mirror symmetry breaking often arises from subtle interplay between the strong force, governing nuclear structure, and the electromagnetic force. The fact that ${}^{71}$Kr and ${}^{71}$Br exhibit different ground state spins despite their mirror relationship suggests unexpected nuclear structure effects are at play. This could be related to shape coexistence, where a nucleus can exist in different shapes with similar energies, or other complex configurations within the nucleus.

Refining Nuclear Models: Theoretical models of the nucleus rely on symmetries and approximations to remain tractable. The discovery of mirror symmetry breaking in this system necessitates a re-evaluation and refinement of these models. It highlights the need for more sophisticated calculations that can accurately account for the observed deviations from symmetry, potentially leading to a more complete and accurate description of nuclear forces.

Connections to Fundamental Interactions: While the observed symmetry breaking is likely rooted in nuclear structure effects, it's crucial to consider potential connections to more fundamental interactions beyond the Standard Model of particle physics. For instance, certain hypothetical particles or forces could contribute to isospin symmetry violation. While these connections are speculative at this stage, they cannot be entirely ruled out without further investigation.
In summary, the breaking of mirror symmetry in this seemingly simple system serves as a reminder that our understanding of nuclear forces and symmetries is not yet complete. It motivates further experimental and theoretical investigations to unravel the underlying mechanisms and their broader implications for nuclear physics and potentially beyond.

Could the observed mirror symmetry breaking be explained by physics beyond the Standard Model, or are nuclear structure effects sufficient?

The paper primarily attributes the observed mirror symmetry breaking in the ${}^{71}$Kr and ${}^{71}$Br system to nuclear structure effects, specifically highlighting:

Coulomb Interaction: The repulsion between protons within the nucleus, arising from the electromagnetic force, is a significant contributor to isospin symmetry breaking. The paper explicitly includes Coulomb corrections in their shell model calculations.

Charge Symmetry Breaking (CSB) Terms: Beyond the simple Coulomb repulsion, more subtle charge-dependent components of the nuclear force, collectively termed CSB terms, are also considered. These terms account for differences in the strong force between proton-proton, neutron-neutron, and proton-neutron pairs.

Shell Model Calculations: The researchers employ state-of-the-art shell model calculations, which are a well-established theoretical framework for describing nuclear structure. These calculations, incorporating Coulomb and CSB corrections, successfully reproduce the observed ground state spin inversion when the specific interaction PFSDG-U is used.
Beyond the Standard Model:
While the paper focuses on nuclear structure explanations, it's worth considering the possibility of contributions from physics beyond the Standard Model. Some hypothetical scenarios include:

New Particles or Forces: New, undiscovered particles or fundamental forces could interact differently with protons and neutrons, leading to additional isospin symmetry violation.

Modifications to Existing Interactions: Extensions to the Standard Model might subtly modify the properties of known particles or forces, potentially impacting isospin symmetry.
Current Understanding:
At present, there's no compelling evidence to suggest that physics beyond the Standard Model is required to explain the observed mirror symmetry breaking in this specific case. The detailed shell model calculations, incorporating known nuclear structure effects, provide a plausible and consistent explanation.
Further Investigation:
However, it's crucial to remain open to the possibility of new physics. Further experimental studies of mirror nuclei, particularly those involving precise measurements of decay rates, branching ratios, and energy levels, could help constrain or potentially reveal deviations from Standard Model predictions. Additionally, more refined theoretical calculations, exploring a wider range of nuclear models and interactions, would be valuable to solidify our understanding of this phenomenon.

If we consider the nucleus as a microcosm of the universe, how might this discovery of asymmetry in a seemingly simple system change our perspective on the universe's evolution and the prevalence of matter over antimatter?

The discovery of asymmetry in the seemingly simple mirror nuclei system, while primarily a nuclear physics finding, can offer intriguing parallels to the grander question of matter-antimatter asymmetry in the universe. Here's how:

Symmetry Breaking and the Big Bang: The Big Bang theory posits that the early universe was in an extremely hot, dense state with equal amounts of matter and antimatter. However, the universe we observe today is dominated by matter. This implies a violation of symmetry, known as CP violation, occurred at some point, leading to a slight excess of matter over antimatter.

Microcosm-Macrocosm Connection: While the energy scales and specific mechanisms are vastly different, the principle of symmetry breaking in a seemingly symmetric system resonates with the cosmological puzzle. Just as the mirror nuclei exhibit unexpected asymmetry due to subtle interplay of forces, perhaps similar, yet-unknown mechanisms in the early universe tipped the balance in favor of matter.

New Physics Hints: The Standard Model of particle physics, while remarkably successful, cannot fully explain the observed matter-antimatter asymmetry. This points to the existence of new physics beyond the Standard Model. Similarly, the observed mirror symmetry breaking, while currently attributed to nuclear structure effects, could also potentially harbor subtle hints of new physics if studied with higher precision and theoretical scrutiny.

Rethinking Fundamental Symmetries: This discovery encourages us to re-examine our assumptions about fundamental symmetries in nature. Perhaps what we perceive as fundamental symmetries are only approximate, holding true at certain scales or under specific conditions. The breaking of these symmetries, both at the nuclear level and on a cosmological scale, might be essential for the complexity and diversity we observe in the universe.
Caution and Future Directions:
It's crucial to avoid overstretching the analogy. The physics governing the nucleus and the early universe are vastly different. However, the shared theme of symmetry breaking in seemingly symmetric systems highlights a profound connection.
This discovery should motivate further research in both nuclear and particle physics:

Precision Measurements: More precise measurements of mirror nuclei properties could reveal subtle deviations from Standard Model predictions, potentially hinting at new physics.
Theoretical Refinement: Improved theoretical models of both nuclear structure and the early universe are crucial to understand the underlying mechanisms of symmetry breaking and their implications.
By exploring these seemingly disparate yet interconnected realms, we might gain a deeper understanding of the fundamental laws governing the universe and the reasons for its very existence.