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Long-Distance Contributions to Exclusive c → uγ Transitions of the Bc Meson


Core Concepts
While the Bc → B*γ decay is sensitive to new physics in the rare c → uγ transition, the Bc → B1'γ channel, despite being less affected by long-distance contributions, suffers from hadronic suppression, making it less suitable for new physics searches.
Abstract

This research paper investigates the potential of the rare decay Bc → B1'γ to reveal new physics beyond the Standard Model (SM) in the context of flavor-changing neutral current (FCNC) transitions.

Bibliographic Information: Losacco, N. (2024). Exclusive c → uγ transitions of Bc meson. arXiv preprint arXiv:2410.17616.

Research Objective: The study aims to determine if the Bc → B1'γ decay channel offers a cleaner environment to study the rare c → uγ transition and potential new physics contributions compared to the previously studied Bc → B*γ decay.

Methodology: The authors employ a model-independent approach based on heavy quark spin symmetry to analyze the short-distance and long-distance contributions to the decay amplitudes of both Bc → B1'γ and Bc → B*γ. They utilize existing form factor calculations from lattice QCD and light-cone QCD sum rules to estimate the hadronic matrix elements involved.

Key Findings: The study reveals a significant hadronic suppression in the short-distance amplitude of the Bc → B1'γ decay due to a cancellation between contributing terms. This suppression, absent in the Bc → B*γ mode, reduces the sensitivity of the Bc → B1'γ channel to potential new physics effects.

Main Conclusions: While the Bc → B1'γ decay might experience fewer contaminations from long-distance effects compared to Bc → B*γ, the hadronic suppression in its short-distance amplitude makes it less ideal for probing new physics in the c → uγ transition. The Bc → B*γ channel, despite being more affected by long-distance contributions, remains a more promising avenue for new physics searches.

Significance: This research contributes to the field of particle physics phenomenology by providing a detailed analysis of the Bc → B1'γ decay and its potential for new physics searches. It highlights the importance of considering hadronic effects when studying rare decays and emphasizes the need for precise theoretical calculations to disentangle new physics signals from SM background.

Limitations and Future Research: The study relies on certain assumptions and approximations in calculating the decay amplitudes. Future research could explore these aspects further by employing more refined theoretical tools and incorporating experimental data to constrain the model parameters. Additionally, investigating other decay channels involving the c → uγ transition could provide complementary information and further our understanding of this rare process.

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Stats
C7 ~ 10-3 in the Standard Model. |C7|, |C7'| ≤ 0.5 in a beyond the Standard Model scenario. ΓB*c → Bcγ = 33 eV.
Quotes

Key Insights Distilled From

by Nicola Losac... at arxiv.org 10-24-2024

https://arxiv.org/pdf/2410.17616.pdf
Exclusive $c \to u \gamma$ transitions of $B_c$ meson

Deeper Inquiries

How could future experimental measurements of the Bc → B1'γ and Bc → B*γ branching fractions further constrain new physics models?

Answer: Future experimental measurements of the Bc → B1'γ and Bc → B*γ branching fractions at LHCb and Belle II experiments hold the potential to offer significant constraints on new physics models. Here's how: Enhanced Sensitivity to Wilson Coefficients: The presence of new physics particles can modify the values of the Wilson coefficients C7 and C'7, which govern the strength of the electromagnetic dipole operators responsible for these decays. Precise measurements of the branching fractions would allow us to determine these coefficients with higher accuracy. Any significant deviation from the Standard Model predictions would provide a strong hint of new physics. Disentangling Long-Distance and Short-Distance Contributions: As discussed in the paper, the Bc → B*γ decay is dominated by long-distance contributions, making it challenging to isolate the short-distance effects sensitive to new physics. On the other hand, while the Bc → B1'γ decay experiences a hadronic suppression in the short-distance amplitude within the Standard Model, this suppression might be absent in certain new physics scenarios. Therefore, comparing the branching fractions of both decays can help disentangle the long-distance and short-distance contributions, providing a clearer picture of the underlying physics. Model Discrimination: Different new physics models predict distinct patterns of deviations in the Wilson coefficients. For instance, some models might enhance C7 while others might suppress it. By measuring the branching fractions with high precision, we can differentiate between these models and narrow down the possibilities for new physics. In summary, precise measurements of the Bc → B1'γ and Bc → B*γ branching fractions can serve as powerful probes of new physics by providing valuable information about the Wilson coefficients, helping to disentangle long-distance and short-distance effects, and allowing for discrimination between different new physics models.

Could there be other beyond the Standard Model scenarios where the hadronic suppression in the Bc → B1'γ decay is lifted, making it a more sensitive probe of new physics?

Answer: Yes, several beyond the Standard Model scenarios could potentially lift the hadronic suppression in the Bc → B1'γ decay, enhancing its sensitivity to new physics. Some possibilities include: New heavy particles contributing to the loop: The presence of new heavy particles, such as those predicted in supersymmetric extensions of the Standard Model or in models with extra dimensions, could contribute to the loop diagrams governing the c → uγ transition. These contributions could modify the relationship between the tensor form factors, potentially alleviating the suppression observed in the Standard Model. New right-handed currents: Models with extended gauge symmetries, such as left-right symmetric models, introduce new right-handed currents that can couple to the quarks involved in the decay. These new interactions could alter the hadronic matrix elements, potentially leading to an enhancement of the Bc → B1'γ amplitude. Scalar-mediated interactions: Some new physics models propose the existence of new scalar particles that can mediate interactions between quarks. These scalar-mediated interactions could introduce new operators in the effective Hamiltonian, modifying the decay dynamics and potentially lifting the hadronic suppression. Resonant effects: If the mass of the new particles involved in the loop diagrams happens to be close to the Bc meson mass, resonant effects could significantly enhance the Bc → B1'γ decay rate, making it a more sensitive probe of new physics. It's important to note that the specific impact of these new physics scenarios on the Bc → B1'γ decay rate would depend on the details of the model, such as the masses, couplings, and quantum numbers of the new particles involved. Nevertheless, the possibility of lifting the hadronic suppression highlights the importance of studying this decay channel in detail, as it could provide valuable insights into physics beyond the Standard Model.

How does the study of rare B meson decays contribute to our understanding of the hierarchy problem and the search for a more fundamental theory of nature?

Answer: The study of rare B meson decays, like those discussed in the paper, plays a crucial role in our quest to understand the hierarchy problem and search for a more fundamental theory of nature. Here's how: Indirect Probes of New Physics: The hierarchy problem arises from the vast difference in energy scales between the electroweak scale (∼100 GeV) and the Planck scale (∼1019 GeV), where gravity becomes comparable in strength to the other fundamental forces. Rare B meson decays, being suppressed in the Standard Model, are highly sensitive to even minute contributions from new particles or interactions at higher energy scales. Observing deviations from Standard Model predictions in these decays could indirectly point towards the existence of new physics beyond the Standard Model, potentially offering clues to address the hierarchy problem. Testing New Physics Models: Many proposed solutions to the hierarchy problem, such as supersymmetry, extra dimensions, and composite Higgs models, predict the existence of new particles and interactions. These new particles and interactions can manifest themselves in rare B meson decays by modifying the decay rates, angular distributions, and other observables. Precise measurements of these decays can therefore be used to test and constrain these new physics models, guiding us towards a more fundamental theory. Flavor Physics and CP Violation: Rare B meson decays are also excellent laboratories to study flavor physics and CP violation, which deal with the different generations of quarks and the asymmetry between matter and antimatter. The Standard Model explanation for CP violation is insufficient to explain the observed matter-antimatter asymmetry in the universe. New sources of CP violation are expected in many extensions of the Standard Model, and these could potentially be observed in rare B decays. Complementarity with Direct Searches: While direct searches at high-energy colliders aim to produce new particles directly, rare B meson decay studies offer a complementary approach by indirectly probing new physics through virtual effects. This complementarity is crucial because new particles might be too heavy to be produced directly at current collider energies, but their effects could still be detectable in precision measurements of rare decays. In conclusion, the study of rare B meson decays provides a powerful and multifaceted tool for exploring physics beyond the Standard Model. By offering indirect probes of new physics, testing new physics models, investigating flavor physics and CP violation, and complementing direct searches, these studies contribute significantly to our understanding of the hierarchy problem and our search for a more fundamental theory of nature.
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