⚛️ phenomenology

Electroweak Transitions of Hyperons and Charmed Baryons: Opportunities and Prospects

This paper presents a phenomenological overview of electroweak transitions in hyperons and charmed baryons, emphasizing the synergy between lattice QCD, experiment, and theory to extract fundamental parameters like $|V_{us}|$ , resolve tensions such as the $\Xi_c \to \Xi$ puzzle, and search for non-standard interactions.

Original authors: Patrik Adlarson, Constantia Alexandrou, Simone Bacchio, Varvara Batozskaya, Johan Bijnens, Alexandre Brea Rodriguez, Gernot Eichmann, Andreas Konstantinou Andrzej Kupsc, Stefan Leupold, Piotr Salabura

Published 2026-10-05

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Patrik Adlarson, Constantia Alexandrou, Simone Bacchio, Varvara Batozskaya, Johan Bijnens, Alexandre Brea Rodriguez, Gernot Eichmann, Andreas Konstantinou Andrzej Kupsc, Stefan Leupold, Piotr Salabura, Christoph Schwanda, Marcin Zieliński

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is built from a small set of fundamental particles, but the way they stick together to form the matter we see is governed by a force so complex that its inner workings remain hidden. This force, known as the strong interaction, binds quarks into protons and neutrons, and those into the atomic nuclei that make up our world. While scientists have a precise mathematical description of how these particles should behave, the equations are so difficult to solve that they cannot be calculated directly for most situations. Instead, researchers must rely on a combination of high-energy experiments, where particles are smashed together to reveal their secrets, and powerful computer simulations that attempt to recreate the rules of the universe from scratch. A critical part of this puzzle involves understanding how certain heavy particles, called baryons, transform into lighter ones. These transformations are not just random changes; they are governed by a specific set of rules that dictate how the universe balances its energy and matter. By studying these changes, scientists can test whether our current understanding of physics is complete or if there are hidden forces at play that we have yet to discover.

A new comprehensive review brings together the latest efforts to map these transformations, focusing specifically on a family of particles known as hyperons and charmed baryons. These are heavier cousins of the protons and neutrons found in ordinary matter, containing strange or charm quarks that make them unstable and short-lived. The paper acts as a roadmap for the next decade of research, connecting three distinct fields: the experimentalists who build massive detectors to catch these fleeting particles, the theorists who use supercomputers to simulate their behavior, and the phenomenologists who bridge the gap between the two. The authors argue that by working in close coordination, these groups can turn the study of these rare decays into a precision tool for testing the Standard Model, the prevailing theory of particle physics. The central goal is to measure how often these particles change and exactly how they do it, looking for any tiny deviation that might signal new physics beyond our current understanding.

The researchers highlight that while these particles have been studied for decades, recent technological advances have opened a new window of opportunity. Large data sets from colliders in Europe and China, combined with massive improvements in computer simulation techniques, now allow scientists to measure these processes with unprecedented accuracy. The paper identifies specific decay channels, such as the transformation of a lambda particle into a proton, as "golden modes" where the data is clean and the theoretical predictions are robust. In these channels, the team can extract fundamental constants of nature, specifically the strength of the interaction that changes one type of quark into another. This strength is a key piece of the puzzle that describes how the universe is organized, and measuring it with high precision helps verify if the mathematical framework of the Standard Model holds up under the most rigorous scrutiny.

One of the most significant findings presented is the potential to resolve a long-standing tension in physics regarding the value of a specific constant that governs these transformations. Different methods of measuring this value have previously yielded slightly different results, creating a discrepancy that suggests either an error in the measurements or a gap in the theory. By using lattice quantum chromodynamics—a method where the fabric of space-time is treated as a grid for computer calculations—the authors show that it is now possible to predict the behavior of these baryons from first principles. These simulations provide a model-independent way to interpret experimental data, meaning the results do not rely on assumptions about how the particles are structured. When the new simulation results are combined with fresh experimental data, they offer a path to a more precise and consistent value, potentially closing the gap between different measurement techniques.

The paper also draws attention to a specific puzzle involving a particle called the Xi-c. Experimental measurements of its decay rate appear to disagree with the latest computer simulations, creating a tension that the authors describe as an emerging problem. This discrepancy is not a failure but a vital clue; it suggests that either the experiments need to be refined to reduce uncertainties, or the theoretical models need to be adjusted to account for effects that were previously overlooked. The authors propose that this tension is exactly the kind of challenge that drives progress, pushing both experimentalists and theorists to improve their methods. They outline a strategy where future experiments will focus on measuring not just the rate of decay, but the detailed shape of the energy distribution of the particles produced, which can reveal the underlying structure of the interaction.

Beyond the standard transformations, the review explores the search for rare and forbidden processes that could reveal physics beyond the known laws. The authors examine decays where particles change their flavor in ways that are heavily suppressed or completely forbidden in the current theory, such as a particle decaying into a different type of lepton than expected. These rare events are sensitive probes for new forces or particles that might exist but have not yet been detected. The paper emphasizes that the spin and polarization of these baryons provide a unique advantage over other particles, offering a richer set of observables that can distinguish between different types of new physics. By measuring how the decay products are oriented in space, scientists can test for violations of fundamental symmetries, such as the difference between matter and antimatter, which is crucial for understanding why the universe is made of matter at all.

The roadmap laid out in the paper extends to the next generation of experiments, including the Large Hadron Collider in Europe and the Super Tau-Charm Factory in China. These facilities are expected to produce billions of these heavy particles, providing the statistical power needed to measure the rarest decays with high precision. The authors stress that the success of this program depends on a tight feedback loop between the experimental data and the theoretical calculations. As experiments gather more data, theorists will refine their simulations to match the precision, and as simulations become more accurate, they will guide the experiments on where to look for the most significant signals. This coordinated effort aims to transform the study of hyperon and charmed baryon decays from a niche area of particle physics into a cornerstone for testing the fundamental laws of nature, potentially uncovering the first cracks in the Standard Model that could lead to a deeper understanding of the universe.

Technical Summary: Electroweak Transitions of Hyperons and Charmed Baryons

Problem Statement
Electroweak transitions among low-lying strange and charmed baryons represent a critical frontier for precision tests of the Standard Model (SM), offering a regime complementary to meson decays. While mesonic processes have been extensively studied, baryon weak decays present unique challenges due to their richer spin and helicity structures, which provide enhanced sensitivity to hadron form factors and potential Beyond the Standard Model (BSM) physics. However, these processes are significantly more difficult to study than their mesonic counterparts due to exponentially poorer signal-to-noise ratios in baryon correlation functions and the larger number of independent form factors required for their description. Despite recent progress in lattice Quantum Chromodynamics (LQCD) and experimental capabilities, a coordinated roadmap linking phenomenology, experiment, and theory is needed to fully exploit the potential of hyperon and charmed-baryon semileptonic decays (SLDs) for determining CKM matrix elements (specifically $|V_{us}|$ and $|V_{cd}|$ ), testing flavor symmetries, and searching for non-standard interactions.

Methodology and Framework
The paper adopts a multi-faceted approach, integrating three primary theoretical and experimental pillars:

Theoretical Framework: The analysis is grounded in the effective Lagrangian for charged-current weak interactions, parameterized by vector and axial-vector form factors ( $F^V_i, F^A_i$ ). The paper details the formalism for $1/2^+ \to 1/2^+$ , $3/2^+ \to 1/2^+$ , and $3/2^+ \to 3/2^+$ transitions, emphasizing the role of second-class currents and radiative corrections. It incorporates Chiral Perturbation Theory (ChPT) to understand SU(3) flavor symmetry breaking and low-energy constants, while acknowledging convergence challenges due to the strange quark mass.
Lattice QCD (LQCD): The paper reviews the status of LQCD calculations, highlighting the shift from quenched approximations to dynamical simulations at or near physical quark masses. It discusses the extraction of form factors via three-point correlation functions and the emerging capability to compute inclusive decay rates via four-point functions. A key methodological focus is the control of systematic uncertainties, including excited-state contamination, discretization effects, and the incorporation of isospin-breaking and QED corrections.
Functional Methods: The paper outlines the use of Dyson-Schwinger and Bethe-Salpeter equations (DSE/BSE) and the Functional Renormalization Group (FRG) as complementary non-perturbative tools. These methods provide a unified framework for calculating elastic and transition form factors, offering insights into the "quark core" and meson-cloud effects, particularly in the timelike region.
Experimental Landscape: The review assesses current and future experimental capabilities at BESIII, STCF, LHCb, Belle II, HADES, and the future CBM experiment at FAIR. It evaluates their specific strengths, such as the double-tag technique at $e^+e^-$ colliders for absolute branching fractions and the high-statistics, forward-geometry capabilities of LHCb for strange baryons.

Key Contributions and Results

Semileptonic Hyperon Decays ( $s \to u$ ): The paper identifies the determination of $|V_{us}|$ as a primary goal. It demonstrates that combining precise experimental measurements of branching fractions (e.g., $\Lambda \to p \ell \nu$ ) with LQCD calculations of all form factors allows for a model-independent extraction of $|V_{us}|$ , bypassing the reliance on SU(3) symmetry assumptions used in the traditional Cabibbo method. Recent LHCb measurements of $\Lambda \to p \mu \nu$ and BESIII results for $\Lambda \to p e \nu$ are highlighted as pivotal.
Charmed Baryon Decays ( $c \to s/d$ ): The paper reviews the status of $\Lambda_c$ and $\Xi_c$ decays. It highlights a significant tension between experimental measurements of form factors (specifically the $q^2$ dependence) in $\Lambda_c \to \Lambda \ell \nu$ and recent LQCD predictions. This discrepancy motivates further theoretical refinement and more precise differential measurements. The paper also notes the first baryonic determination of $|V_{cd}|$ from $\Lambda_c \to n \ell \nu$ .
Non-Standard Interactions (NSI): The document emphasizes the unique sensitivity of hyperon SLDs to scalar and tensor operators, particularly in muonic modes where helicity suppression enhances the interference with non-standard amplitudes. It presents constraints on Wilson coefficients ( $\epsilon_S, \epsilon_T$ ) derived from lepton-flavor-universality (LFU) ratios ( $R_{\mu e}$ ), noting that combining multiple hyperon channels provides complementary constraints in the parameter space.
Rare and Electromagnetic Transitions: The paper reviews weak radiative and dileptonic decays (FCNCs), such as $\Lambda \to n \gamma$ and $\Lambda_b \to \Lambda \mu^+ \mu^-$ . It underscores the role of baryon spin in providing access to angular observables not available in meson decays, offering distinct probes for BSM physics. It also discusses electromagnetic Dalitz decays ( $B^* \to B \ell^+ \ell^-$ ) as a means to probe the timelike structure of form factors and test U-spin symmetry.
Inclusive vs. Exclusive: The review distinguishes between exclusive form factor determinations and the emerging capability to calculate inclusive decay rates directly on the lattice using spectral density methods, which could provide cross-checks for total rates without reconstructing specific final states.

Significance and Outlook
The paper positions itself as a "white paper" intended to identify the most promising opportunities at the interface of experiment, phenomenology, and LQCD. Its significance lies in:

Synergy: It argues that the convergence of high-statistics datasets from LHCb, BESIII, and Belle II with high-precision LQCD calculations creates a unique window for precision SM tests that is currently underutilized compared to meson physics.
Precision Targets: It outlines realistic precision targets for the coming decade, such as sub-percent determinations of $|V_{us}|$ and $|V_{cd}|$ from baryonic decays, and percent-level measurements of branching fractions for hyperon SLDs.
BSM Sensitivity: It establishes hyperon decays, particularly muonic modes and LFU ratios, as theoretically clean laboratories for constraining scalar and tensor interactions, offering constraints complementary to those from kaon decays and collider searches.
Roadmap: The document provides a strategic roadmap for the field, prioritizing the measurement of "golden modes" ( $\Lambda \to p \ell \nu$ , $\Xi^- \to \Lambda \ell \nu$ ), the resolution of the $\Xi_c \to \Xi$ tension, and the expansion of studies to excited baryon states and beauty baryons.

The authors conclude that while baryon weak decays remain challenging, the rapid progress in LQCD and the advent of new experimental facilities make this a developing area with the potential to deliver stringent tests of the Standard Model and new insights into non-perturbative QCD dynamics over the next decade.

More like this