insikt - Particle physics - # B meson decays

Measurement of Branching Fractions and CP Asymmetries in B to K*(892) Gamma Decays at Belle II

Q: If this deviation in isospin asymmetry persists and is confirmed as a sign of new physics, what are the broader implications for our understanding of the fundamental building blocks and forces of nature?

Confirmation of a significant deviation in the isospin asymmetry of B → K*(892)γ decays would be a groundbreaking discovery with profound implications for our understanding of fundamental physics: Evidence for new particles and interactions: The deviation would directly point to the existence of new particles and interactions beyond the Standard Model. This would revolutionize our understanding of the building blocks of matter and the forces that govern their interactions. Insights into the flavor puzzle: The Standard Model provides no explanation for the observed hierarchy of fermion masses and mixing angles, known as the flavor puzzle. New physics contributing to flavor-changing neutral currents, as suggested by the isospin asymmetry anomaly, could offer crucial insights into the origin of flavor structure. Constraints on new physics models: The specific pattern of deviation in the isospin asymmetry, along with other flavor observables, would provide stringent constraints on the possible new physics models. This would guide theoretical developments and motivate further experimental searches for new particles and interactions. Connection to other open questions: The discovery could have implications for other open questions in particle physics and cosmology, such as the origin of dark matter, the matter-antimatter asymmetry in the universe, and the hierarchy problem. In conclusion, a confirmed deviation in the isospin asymmetry of B → K*(892)γ decays would be a major breakthrough, potentially opening a new window into physics beyond the Standard Model and deepening our understanding of the fundamental constituents and forces of nature.

Centrala begrepp

The Belle II experiment measured key parameters of B meson decays to K*(892) and a photon, finding results consistent with the Standard Model but with intriguing hints of potential new physics in the isospin asymmetry.

Sammanfattning

Bibliographic Information: The Belle II Collaboration, "Measurement of B →K∗(892)γ decays at Belle~II," arXiv:2411.10127v1 [hep-ex] (2024).
Research Objective: This research paper presents measurements of B meson decays to a K*(892) meson and a photon (B →K∗(892)γ) using data collected by the Belle II experiment at the SuperKEKB collider. The study aims to precisely determine key parameters of these decays, including branching fractions (B) and CP asymmetries (ACP), to test predictions of the Standard Model of particle physics and search for hints of new physics beyond it.
Methodology: The researchers analyzed a large dataset of electron-positron collisions at the Υ(4S) resonance, corresponding to an integrated luminosity of 365 fb−1. They reconstructed B meson candidates from their decay products, applying stringent selection criteria to isolate signal events from background processes. The signal yields were extracted from fits to the invariant mass and energy distributions of the B meson candidates, considering contributions from signal, continuum background, and background from other B meson decays.
Key Findings: The analysis yielded precise measurements of the branching fractions and CP asymmetries for both neutral (B0 →K∗0γ) and charged (B+ →K∗+γ) decay modes. The results for the branching fractions are B(B0 →K∗0γ) = (4.14 ± 0.10 ± 0.11) × 10−5 and B(B+ →K∗+γ) = (4.02 ± 0.13 ± 0.13) × 10−5. The CP asymmetries were measured to be ACP(B0 →K∗0γ) = (−3.3 ± 2.3 ± 0.4)% and ACP(B+ →K∗+γ) = (−0.7±2.9±0.6)%. The difference in CP asymmetries between the neutral and charged modes (∆ACP) was found to be (+2.6 ± 3.8 ± 0.7)%, and the isospin asymmetry (∆0+) was measured to be (+5.0 ± 2.0 ± 1.5)%.
Main Conclusions: The measured values for the branching fractions and CP asymmetries are consistent with previous measurements and theoretical predictions within the Standard Model. However, the measured isospin asymmetry, while still consistent with the Standard Model, deviates from zero by 2.0 standard deviations, hinting at the possibility of contributions from new particles or interactions not accounted for in the Standard Model.
Significance: Precise measurements of B meson decays, particularly rare processes like B →K∗(892)γ, provide stringent tests of the Standard Model and offer a window into potential new physics. The observed deviation in the isospin asymmetry, if confirmed with larger datasets, could point towards new particles or interactions that couple differently to up-type and down-type quarks, leading to an enhanced isospin violation.
Limitations and Future Research: The current measurement of the isospin asymmetry is statistically limited. Future analyses with larger datasets collected by Belle II will be crucial to improve the precision of this measurement and either confirm or refute the observed deviation from the Standard Model prediction. Further theoretical studies are also needed to refine the Standard Model predictions for the isospin asymmetry and explore possible new physics scenarios that could explain a larger-than-expected value.

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Statistik

The integrated luminosities for the on- and off-resonance datasets are (365.3 ± 1.7) fb−1 and (42.7 ± 0.2) fb−1, respectively.
The data sample contains (387 ± 6) × 10^6 BB events.
B(B0 →K∗0γ) = (4.14 ± 0.10 ± 0.11) × 10−5.
B(B+ →K∗+γ) = (4.02 ± 0.13 ± 0.13) × 10−5.
ACP (B0 →K∗0γ) = (−3.3 ± 2.3 ± 0.4)%.
ACP (B+ →K∗+γ) = (−0.7±2.9±0.6)%.
∆ACP = (+2.6 ± 3.8 ± 0.7)%.
∆0+ = (+5.0 ± 2.0 ± 1.5)%.

Citat

Viktiga insikter från

\boldmath Measurement of {\boldmath $B \to K{}^{*}(892)\gamma$} decays at Belle~II

by Belle II Col... på arxiv.org 11-18-2024

https://arxiv.org/pdf/2411.10127.pdf

$\boldmath Measurement of {\boldmath $B \to K{}^{*}(892)\gamma$} decays at Belle~II$

Djupare frågor

What specific types of new physics models could potentially lead to an enhanced isospin asymmetry in B →K*(892)γ decays, and how can these models be further tested experimentally?

Several new physics models could enhance the isospin asymmetry (∆0+) in B →K*(892)γ decays, often pushing it towards negative values compared to the small positive Standard Model (SM) prediction. Here are a few examples:

Supersymmetric models with non-minimal flavor violation: In these models, new particles like charginos and squarks can contribute to the loop-level b → sγ transition. If the couplings of these particles violate flavor symmetry, they can lead to significant deviations in ∆0+ from the SM prediction. These models can be further tested by searching for direct production of supersymmetric particles at high-energy colliders like the LHC. Additionally, precise measurements of other flavor observables, such as B → Xsγ and B → Xsℓ+ℓ− decays, can provide complementary constraints on these models.

Two-Higgs-doublet models with flavor-changing neutral currents: These models introduce an additional Higgs doublet, which can mediate flavor-changing neutral currents at tree level. These currents can contribute to the b → sγ transition and modify ∆0+.  Direct searches for the new Higgs bosons at colliders and precise measurements of other flavor-changing neutral current processes, such as B → K(*)µ+µ− and B → ℓ+ℓ−, can provide further tests of these models.

Models with leptoquarks: Leptoquarks are hypothetical particles that couple to both quarks and leptons. They can contribute to the b → sγ transition at one-loop level and potentially enhance ∆0+. Direct searches for leptoquarks at colliders and studies of lepton flavor violating decays, such as µ → eγ and τ → µγ, can provide additional information about these models.
It is important to note that these are just a few examples, and other new physics scenarios could also lead to an enhanced isospin asymmetry.  A comprehensive approach involving both direct searches for new particles and precise measurements of various flavor observables is crucial to disentangle the underlying new physics model.

Could there be subtle, unaccounted-for experimental systematic effects that might explain the observed deviation in the isospin asymmetry, and how can these be further investigated and mitigated in future analyses?

While the Belle II measurement shows a 2.0 standard deviation difference from zero in the isospin asymmetry, it's crucial to consider potential experimental systematic effects that might contribute to this deviation. Some subtle effects that warrant further investigation include:

Detector-induced charge asymmetries:  Differences in the reconstruction and identification efficiencies of K+ and K−, or π+ and π−, can lead to a spurious isospin asymmetry. These differences can arise from subtle variations in the detector response to positively and negatively charged particles.  These effects can be further investigated by studying charge asymmetries in control samples of data, such as high-statistics decays like B → J/ψ K() and D → D0π, where the underlying physics is well understood.

Background modeling:  Imperfect modeling of the background contributions, particularly from continuum events, can impact the extracted signal yields and consequently the isospin asymmetry.  This can be addressed by using more sophisticated background suppression techniques, such as multivariate analysis methods, and by performing detailed studies of the background composition in different regions of the phase space.

Photon detection:  The isospin asymmetry measurement relies heavily on the accurate reconstruction and identification of the high-energy photon.  Any subtle differences in the photon detection efficiency between data and simulation, particularly in the energy region relevant for B → K*(892)γ decays, can introduce a systematic bias.  These effects can be mitigated by improving the photon energy calibration and resolution, and by performing dedicated studies of the photon reconstruction efficiency using control samples like e+e− → µ+µ−γ.
Future analyses can mitigate these systematic uncertainties by:

Increasing the size of the control samples: Larger control samples will improve the statistical precision of the systematic uncertainty estimates.
Developing data-driven methods: Utilizing data-driven techniques to model the background and detector response can reduce reliance on simulation and minimize potential biases.
Improving detector understanding: Continuous efforts to understand and calibrate the detector response will be crucial for minimizing systematic uncertainties in future measurements.

If this deviation in isospin asymmetry persists and is confirmed as a sign of new physics, what are the broader implications for our understanding of the fundamental building blocks and forces of nature?

Confirmation of a significant deviation in the isospin asymmetry of B → K*(892)γ decays would be a groundbreaking discovery with profound implications for our understanding of fundamental physics:

Evidence for new particles and interactions:  The deviation would directly point to the existence of new particles and interactions beyond the Standard Model. This would revolutionize our understanding of the building blocks of matter and the forces that govern their interactions.

Insights into the flavor puzzle: The Standard Model provides no explanation for the observed hierarchy of fermion masses and mixing angles, known as the flavor puzzle. New physics contributing to flavor-changing neutral currents, as suggested by the isospin asymmetry anomaly, could offer crucial insights into the origin of flavor structure.

Constraints on new physics models: The specific pattern of deviation in the isospin asymmetry, along with other flavor observables, would provide stringent constraints on the possible new physics models. This would guide theoretical developments and motivate further experimental searches for new particles and interactions.

Connection to other open questions:  The discovery could have implications for other open questions in particle physics and cosmology, such as the origin of dark matter, the matter-antimatter asymmetry in the universe, and the hierarchy problem.
In conclusion, a confirmed deviation in the isospin asymmetry of B → K*(892)γ decays would be a major breakthrough, potentially opening a new window into physics beyond the Standard Model and deepening our understanding of the fundamental constituents and forces of nature.