Hunting for Missing Pieces: A Search for New Physics in Λc+ Decays

Author: Denis Avetisyan

Researchers are leveraging the Super Tau-Charm Facility to probe potential violations of fundamental symmetries in the decay of Λc+ baryons, seeking evidence of sterile neutrinos or other exotic particles.

The distributions of <span class="katex-eq" data-katex-display="false">m_{BC}</span> and <span class="katex-eq" data-katex-display="false">\Delta E</span>-generated from a simulation of twenty thousand events at a center-of-mass energy of 4.682 GeV and a missing mass of 1.110 GeV-define event-selection windows, with the decay of the <span class="katex-eq" data-katex-display="false">\Lambda^{+}_{c}b</span> baryon to <span class="katex-eq" data-katex-display="false">K^{+}K^{+}missing</span> exhibiting a distribution shape nearly identical to that of a pion decay. — The distributions of $m_{BC}$ and $\Delta E$ -generated from a simulation of twenty thousand events at a center-of-mass energy of 4.682 GeV and a missing mass of 1.110 GeV-define event-selection windows, with the decay of the $\Lambda^{+}_{c}b$ baryon to $K^{+}K^{+}missing$ exhibiting a distribution shape nearly identical to that of a pion decay.

This study details a search for baryon number violating decays at the STCF, potentially revealing new physics scales up to tens of TeV via missing energy signatures.

The Standard Model of particle physics, while remarkably successful, offers no explanation for the observed matter-antimatter asymmetry in the universe, hinting at physics beyond its framework. This motivates the search for baryon number violation (BNV), and here, in ‘Searching for apparent baryon number violation in $Λ_c^+$ decays at the Super Tau-Charm Facility’, we propose a novel search for apparent BNV in decays of $Λ_c^+$ baryons via missing energy signatures. Monte Carlo simulations of the proposed Super Tau-Charm Facility (STCF) demonstrate the potential to probe new-physics scales up to several TeV, interpreting sensitivities within sterile-neutrino effective field theory and R-parity violating supersymmetry. Could the STCF, therefore, unlock new insights into the fundamental symmetries governing matter and the universe?

The Enduring Matter-Antimatter Asymmetry

The universe, as it exists today, is overwhelmingly composed of matter, with a near-complete absence of antimatter – a perplexing imbalance that challenges the foundations of modern physics. Current theories, encapsulated within the Standard Model of particle physics, predict that matter and antimatter should have been created in equal amounts during the Big Bang. However, if this were truly the case, these substances would have largely annihilated each other, leaving behind only energy. The persistence of matter, and thus everything we observe – stars, galaxies, and life itself – necessitates an explanation that extends beyond the Standard Model. This asymmetry isn’t merely a quantitative discrepancy; it represents a fundamental gap in understanding the universe’s origins and evolution, prompting intensive research into potential new particles, interactions, and physical laws that could account for this enduring mystery. The observed dominance of matter isn’t simply different from what’s predicted; it actively demands a revision of established physics.

The genesis of matter’s prevalence over antimatter hinges on a set of criteria known as the Sakharov conditions, established to explain baryogenesis – the process that created this imbalance. These conditions demand, among other things, a violation of baryon number conservation (BNV), meaning processes must exist that don’t conserve the difference between matter and antimatter particles. However, the Standard Model of particle physics, despite its successes, strictly adheres to baryon number conservation; all known interactions preserve this quantity. This presents a significant impasse, indicating that any explanation for the observed matter-antimatter asymmetry requires physics beyond the Standard Model, prompting intensive searches for novel particles and interactions capable of enacting the necessary baryon number violation and ultimately, accounting for the universe’s material existence.

The persistent imbalance between matter and antimatter in the universe necessitates a deeper investigation into processes that violate baryon number conservation. While the Standard Model of particle physics elegantly describes known forces and particles, it fails to adequately account for the observed asymmetry, offering no viable mechanism for sufficient baryon number violation (BNV). Consequently, physicists are actively pursuing extensions to the Standard Model, exploring theoretical frameworks like leptogenesis, grand unified theories, and supersymmetry, all of which predict novel sources of BNV. These searches involve both direct experimental efforts – such as looking for proton decay or neutron-antineutron oscillations – and indirect probes through precision measurements of neutrino properties and searches for new particles at high-energy colliders, aiming to unveil the fundamental physics responsible for the universe’s matter dominance.

Current LHC bounds on Λ from MET+jets searches, as extracted and reinterpreted from Aadet al.(2024) and Hilleret al.(2026), define the 95% confidence level sensitivity limits for scenarios with vanishing backgrounds.

Probing Beyond the Standard Model with Effective Theories

Effective field theories (EFTs) offer a model-independent method for analyzing potential physics beyond the Standard Model. Rather than requiring specification of the complete high-energy theory, EFTs focus on low-energy observables, expressing them as expansions in powers of $p/Λ$ , where $p$ represents the momentum scale of the process and Λ signifies the energy scale of new physics. This approach allows physicists to parameterize the effects of unknown high-energy phenomena through a series of effective operators constructed from Standard Model fields, ordered by their dimensionality. Higher-dimensional operators are suppressed by powers of $1/Λ$ , enabling calculations of low-energy effects without detailed knowledge of the ultraviolet completion. Consequently, EFTs provide a systematic way to search for and constrain new physics, even in the absence of a specific theoretical model.

Baryon number violating (BNV) processes are not predicted within the Standard Model, but can arise in the Effective Field Theory (EFT) framework through the inclusion of dimension-6 operators. These operators, representing deviations from the Standard Model at higher energy scales, introduce terms into the Lagrangian that are not subject to the accidental symmetries protecting baryon number conservation. Specifically, operators involving four fermions and a derivative, or those with multiple covariant derivatives, can mediate transitions that change baryon number by one or more units. The effects of these operators are calculable, allowing for predictions of BNV decay rates and cross-sections which can be compared with experimental searches, such as proton decay or neutron-antineutron oscillation, providing a pathway to constrain new physics beyond the Standard Model.

The Neutrino-Extended Standard Model Effective Field Theory (SMEFT) incorporates sterile neutrinos as additional degrees of freedom, allowing for the parameterization of new physics effects beyond the minimal SMEFT. This extension is particularly relevant for Baryon Number Violation (BNV) searches because sterile neutrinos can mediate BNV processes through dimension-6 operators not present in the standard SMEFT. Specifically, the inclusion of sterile neutrino fields introduces new Wilson coefficients associated with operators that directly contribute to proton decay and other BNV signatures. These new coefficients provide additional targets for experimental searches at facilities like proton decay experiments and neutrinoless double beta decay experiments, potentially offering increased sensitivity to new physics beyond the Standard Model.

Effective operators induce <span class="katex-eq" data-katex-display="false">\Lambda_c^+ \to \pi^+/K^+ + \nu_s</span> decays, as illustrated by the parton-level Feynman diagrams featuring an effective vertex represented by the black blob. — Effective operators induce $\Lambda_c^+ \to \pi^+/K^+ + \nu_s$ decays, as illustrated by the parton-level Feynman diagrams featuring an effective vertex represented by the black blob.

Hunting for Baryon Number Violation in Particle Decays

The $\Lambda_c^+$ baryon presents a favorable decay channel for searches of baryon number violation (BNV) due to its relatively long lifetime and well-understood production and decay modes within the LHCb experiment. Baryon number violation is a key prediction of certain beyond-the-Standard Model theories, and observation of such processes would constitute definitive evidence for new physics. The $\Lambda_c^+$ baryon, being a weakly decaying particle containing a $c$ quark, offers a distinct production mechanism and decay kinematics that allow for enhanced sensitivity to rare or forbidden decay processes, specifically those violating baryon number conservation. Analyses focus on searching for decay modes where baryons are not produced in the final state, implying a baryon number violating process, and reconstructing the decay products to identify potential signal events against background noise.

The detection of missing energy in $\Lambda_c^+$ decays provides a sensitive probe for physics beyond the Standard Model. Since missing energy implies the presence of undetected, and therefore invisible, decay products, its observation is not directly attributable to known particles and interactions. Specifically, a significant imbalance in the measured decay products’ momentum necessitates the existence of particles that do not interact with the detector, such as weakly interacting particles or those escaping detection due to limited tracking capabilities. Analyzing events with substantial missing transverse energy allows researchers to constrain potential new physics models and search for deviations from Standard Model predictions in $\Lambda_c^+$ decay channels.

Sterile neutrinos are hypothetical particles that do not interact via the weak force, differing from the three known active neutrinos. Their existence is proposed to explain observed anomalies in neutrino oscillation experiments and could also account for dark matter. In the context of $Λ_c^+$ decays, sterile neutrinos would manifest as missing energy and momentum, as they would be produced in the decay but escape detection. The decay channel $Λ_c^+ \rightarrow π^+/K^+ + ν_s$ , where $ν_s$ represents a sterile neutrino, provides a direct link between theoretical models incorporating sterile neutrinos and experimental searches for missing energy signatures. Precise measurements of decay rates and kinematic distributions in $Λ_c^+$ decays can therefore constrain the mass and mixing parameters of sterile neutrinos, offering a potential pathway to verify or refute their existence.

Hadron-level Feynman diagrams illustrate <span class="katex-eq" data-katex-display="false">\Lambda_c^+ \to \pi^+/K^+ + \nu_s</span> decays within the <span class="katex-eq" data-katex-display="false">\nu\nu</span>LEFT framework, distinguishing between pole and non-pole production mechanisms. — Hadron-level Feynman diagrams illustrate $\Lambda_c^+ \to \pi^+/K^+ + \nu_s$ decays within the $\nu\nu$ LEFT framework, distinguishing between pole and non-pole production mechanisms.

The Super Tau-Charm Facility: A Precision Frontier

The Super Tau-Charm Facility is poised to become a leading center for the investigation of $\Lambda_c^+$ decays due to a confluence of factors. Unlike previous experiments, the STCF is specifically designed to maximize the production rate of these charmed baryons, offering an unprecedented statistical advantage. This increased data sample will allow physicists to meticulously measure the properties of $\Lambda_c^+$ decays with far greater precision than currently possible, enabling sensitive tests of the Standard Model and the search for subtle deviations that might indicate new physics. The facility’s design incorporates innovative techniques for particle identification and momentum reconstruction, critical for isolating the signals of interest from the complex background environment. These capabilities will facilitate detailed studies of both the well-understood decay modes and the rarer, potentially groundbreaking ones, offering a unique window into the fundamental forces governing particle interactions.

The Super Tau-Charm Facility’s experimental program relies heavily on the OSCAR detector simulation framework, a sophisticated tool designed to meticulously model the complex interactions within the detector. OSCAR doesn’t simply predict what will be observed, but rather details the full chain of events – from the initial particle collisions to the final signals registered by the detector components. This granular approach is critical for distinguishing genuine decay signals from the overwhelming background noise inherent in high-energy physics experiments. By simulating billions of events, researchers can optimize detector design, refine data analysis techniques, and ultimately enhance the sensitivity to rare decay processes, such as those involving $\Lambda_c^+$ baryons.

The Super Tau-Charm Facility is poised to investigate baryon number violation, a phenomenon hinting at physics beyond the Standard Model. This pursuit leverages a synergistic approach, uniting sophisticated theoretical predictions with unprecedented experimental precision. Current theoretical frameworks, like the Standard Model, largely forbid processes that don’t conserve baryon number; however, extensions to this model suggest subtle violations are possible, potentially explaining the matter-antimatter asymmetry observed in the universe. The STCF aims to detect these rare decays – such as those involving Lambda_c⁺ baryons – with enough statistical power to rigorously test these theoretical predictions and, crucially, to search for deviations that would signal the existence of new particles or interactions. By meticulously reconstructing decay events and precisely measuring relevant parameters, the facility seeks to either confirm the Standard Model’s limitations or uncover compelling evidence for new physics governing the behavior of matter itself.

Decay branching ratios for <span class="katex-eq" data-katex-display="false">\Lambda_c^+ \to M^+ + \nu_s</span> (where <span class="katex-eq" data-katex-display="false">M^+= \pi^+</span> or <span class="katex-eq" data-katex-display="false">K^+</span>) are shown as a function of <span class="katex-eq" data-katex-display="false">m_{\nu_s}</span>, with bands representing variations in the form factor <span class="katex-eq" data-katex-display="false">\beta_{\Lambda_c^+} </span> and a fixed value of <span class="katex-eq" data-katex-display="false">c_{211}/\Lambda^2 = c_{212}/\Lambda^2 = c_{221}/\Lambda^2 = 1/(1000 \text{ GeV})^2</span>. — Decay branching ratios for $\Lambda_c^+ \to M^+ + \nu_s$ (where $M^+= \pi^+$ or $K^+$ ) are shown as a function of $m_{\nu_s}$ , with bands representing variations in the form factor $\beta_{\Lambda_c^+}$ and a fixed value of $c_{211}/\Lambda^2 = c_{212}/\Lambda^2 = c_{221}/\Lambda^2 = 1/(1000 \text{ GeV})^2$ .

Towards a Complete Picture of Baryogenesis

A more complete picture of how baryons – protons and neutrons – interact at low energies is emerging from the synergy between two powerful theoretical frameworks. Neutrino-Extended Low-Energy Effective Field Theories (EFTs) provide a robust method for incorporating the subtle influence of neutrinos on these interactions, while Baryon Chiral Perturbation Theory offers a systematic way to describe the strong force dynamics governing baryons themselves. By combining these approaches, physicists can move beyond simplified models and explore the complex interplay between neutrino physics and baryon interactions with greater precision. This refined understanding is crucial because these low-energy processes may hold vital clues to the origin of matter asymmetry in the universe – the puzzle of why there is so much more matter than antimatter – and could reveal new physics beyond the Standard Model.

The enduring puzzle of why the universe contains so much more matter than antimatter necessitates investigation beyond the Standard Model, and alternative scenarios for baryon number violation represent a crucial avenue for exploration. R-parity violating supersymmetry (RPV-SUSY) offers one such possibility, predicting interactions that directly induce baryon-decaying processes without requiring the extremely high energy scales associated with conventional Grand Unified Theories. Unlike scenarios relying on sphaleron processes at high temperatures, RPV-SUSY allows for baryon number violation at lower, potentially experimentally accessible, energies. This framework introduces new particles and interactions – specifically, couplings that permit quarks and leptons to transform into each other – thereby opening up possibilities for both direct detection of baryon-decay signatures and indirect observation through precision measurements of particle properties. Consequently, dedicated searches for supersymmetric particles and investigations into their decay modes are not only tests of supersymmetry itself, but also powerful probes of potential mechanisms driving the observed matter-antimatter asymmetry.

The enduring puzzle of baryogenesis – the observed asymmetry between matter and antimatter in the universe – demands a sustained commitment to both theoretical innovation and experimental precision. Progress hinges on refining models that venture beyond the Standard Model, seeking new sources of charge-parity (CP) violation and baryon number violation. Crucially, advancements in collider physics, neutrino astronomy, and precision measurements of fundamental constants are essential to either confirm or refute these theoretical predictions. Moreover, sophisticated numerical simulations, coupled with insights from cosmology and astrophysics, will be vital in interpreting experimental results and narrowing the range of viable baryogenesis scenarios. Ultimately, a comprehensive understanding of why matter prevails requires a synergistic approach, where theoretical frameworks are rigorously tested against an ever-growing body of experimental evidence.

The <span class="katex-eq" data-katex-display="false">\Lambda_c^+</span> decay into a <span class="katex-eq" data-katex-display="false">K^+</span> meson and a neutralino <span class="katex-eq" data-katex-display="false">\tilde{\chi}^0_1</span> is mediated by the R-parity violating coupling <span class="katex-eq" data-katex-display="false">\lambda^{\prime\prime}_{212}</span>, as illustrated by these Feynman diagrams. — The $\Lambda_c^+$ decay into a $K^+$ meson and a neutralino $\tilde{\chi}^0_1$ is mediated by the R-parity violating coupling $\lambda^{\prime\prime}_{212}$ , as illustrated by these Feynman diagrams.

The search for baryon number violation, as detailed in this study, mirrors the principles of systemic evolution. Just as infrastructure should evolve without rebuilding the entire block, so too must theoretical frameworks adapt to accommodate new experimental evidence. The STCF’s investigation into Λc+ decays, probing scales up to tens of TeV, represents a focused adjustment to the existing Standard Model-a refinement rather than a wholesale reconstruction. This methodical approach, seeking subtle deviations from predicted behavior, acknowledges that even profound changes often emerge from incremental adjustments to a fundamentally sound structure. As Richard Feynman once stated, “The first principle is that you must not fool yourself – and you are the easiest person to fool.” This relentless self-assessment is crucial when examining the foundations of particle physics and interpreting the delicate signatures of new physics.

Where Do We Go From Here?

The search for baryon number violation, as detailed within this work, ultimately reveals more about the limitations of current frameworks than any immediate discovery. The pursuit of sterile neutrinos, or any long-lived particle mediating such decay, feels less like a direct hunt and more like a probing of the seams where the Standard Model begins to fray. If a signal emerges from the Super Tau-Charm Facility, it will not be a triumph of prediction, but an acknowledgement that something fundamental remains hidden.

A clever experimental design, however, cannot compensate for a lack of theoretical clarity. The effective field theory approach, while pragmatic, merely parameterizes ignorance. A truly elegant solution will require a deeper understanding of the underlying symmetries-or lack thereof-governing baryon number. It is likely that future progress will depend not on increasing luminosity, but on developing more constrained and predictive models, even if those models ultimately prove incorrect.

The true value of this research, then, may not lie in finding new particles, but in refining the questions. Simplicity always wins. A fragile, over-parameterized theory, capable of accommodating any result, is ultimately useless. The search continues, not for confirmation, but for a more honest description of the universe-a description that acknowledges what it does not know.

Original article: https://arxiv.org/pdf/2604.11329.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/