Searches for new physics beyond the Standard Model in hyperon sector

This review summarizes recent BESIII experiment results on hyperon physics, highlighting world-leading constraints on new physics beyond the Standard Model through studies of dark and invisible particles, baryon- and lepton-number violation, and a three-orders-of-magnitude improvement in the sensitivity to the $\Lambda$ electric dipole moment using quantum-entangled pairs.

The Gist

Imagine the universe as a giant, incredibly complex puzzle. For decades, physicists have been using a rulebook called the Standard Model to solve it. This rulebook explains how almost everything works: why magnets stick, how stars burn, and what holds atoms together. But there's a problem. The rulebook is missing a few crucial pieces. It can't explain Dark Matter (the invisible stuff holding galaxies together) or why the universe is made of matter instead of being an equal mix of matter and antimatter (which would have annihilated everything instantly).

This paper is like a detective report from the BESIII experiment in China. The detectives are using a special microscope to look at a specific group of particles called Hyperons.

Who are Hyperons?

Think of protons and neutrons (the building blocks of your body) as the "popular kids" in the particle world. They are stable and well-behaved. Hyperons are like their mysterious, short-lived cousins. They contain a "strange" ingredient (a strange quark) that makes them unstable; they live for only a tiny fraction of a second before decaying.

Because they are so short-lived and "strange," they are perfect spies for looking for new physics. If the Standard Model is a perfect map, Hyperons are the places where the map might have a blank spot or a wrong turn.

The Three Main Investigations

The paper details three major "missions" the BESIII team went on to find cracks in the Standard Model.

1. The "Ghostly Spin" (Searching for Electric Dipole Moments)

The Analogy: Imagine a spinning top. If it's perfectly balanced, it spins straight up. But if it has a tiny, hidden weight on one side (an "electric dipole"), it will wobble in a specific way.
The Physics: Physicists are looking for a "wobble" in the Hyperons' spin that shouldn't exist. This wobble, called an Electric Dipole Moment (EDM), would be a smoking gun for a violation of time-reversal symmetry. In plain English: it would prove that time doesn't just move forward, but that the laws of physics treat the past and future differently. This is crucial for explaining why we exist at all!
The Result: The team used a clever trick. They created pairs of Hyperons and anti-Hyperons that were "quantum entangled" (like a pair of magic dice that always show opposite numbers). By watching how they decayed, they looked for that wobble.
The Verdict: They didn't find the wobble. But, they looked with a microscope 1,000 times more powerful than any previous attempt. They set a new, incredibly strict rule: "If this wobble exists, it must be smaller than we can currently see." This forces new theories to be much more precise.

2. The "Missing Pieces" (Searching for Invisible Decays)

The Analogy: Imagine you drop a billiard ball into a pool. You expect to see ripples. But sometimes, the ball seems to vanish without a splash. Where did it go?
The Physics: Some theories suggest that particles might decay into Dark Matter. Since Dark Matter doesn't interact with light or our detectors, it would look like the particle just disappeared, taking its energy with it. This is linked to the "Neutron Lifetime Puzzle," where scientists get different answers depending on how they count neutrons. Maybe some neutrons are secretly turning into Dark Matter!
The Result: The team watched billions of Hyperons decay, looking for the "missing energy" signature. They checked if a Hyperon could turn into a proton and nothing else (invisible).
The Verdict: No missing pieces found. They set the strictest limits yet on how often this can happen. It's like saying, "If a billiard ball ever vanishes into thin air, it happens less than once in a million tries." This rules out many popular theories about what Dark Matter could be.

3. The "Forbidden Switch" (Searching for Baryon and Lepton Number Violation)

The Analogy: Imagine a bank vault where the rules say you can never create or destroy money, only move it around. But what if you found a machine that could turn a $10 bill into a $5 bill and a $5 coin, or worse, turn a $10 bill into a $20 bill out of thin air?
The Physics: The Standard Model has strict rules: the total number of "baryons" (matter particles) and "leptons" (like electrons) must stay the same. But if these rules are broken, it could explain why the universe has more matter than antimatter.
The Result: The team looked for "impossible" events, like a particle turning into two electrons and a proton, or a Hyperon turning into an anti-Hyperon (a matter-antimatter switch).
The Verdict: No forbidden switches were found. The vault is secure. But again, they checked with such high precision that they have forced any "money-printing machine" to be incredibly rare.

Why Does This Matter?

You might ask, "If they found nothing new, is this a failure?" Absolutely not.

In science, finding out what isn't there is just as important as finding what is.

• The "No" is a "Yes": By proving that these strange effects don't happen (or happen extremely rarely), the team has eliminated hundreds of wrong theories. They have narrowed down the search for the "New Physics" that will eventually solve the universe's biggest mysteries.
• The Future: The paper ends with a look ahead. They are planning to build an even bigger, brighter machine (the Super Tau-Charm Factory). This new machine will be like upgrading from a magnifying glass to a telescope. It will allow them to look for these tiny effects with even greater sensitivity, potentially finally catching a glimpse of the "ghosts" hiding in the dark sector.

The Bottom Line

This paper is a testament to the power of precision. The BESIII team didn't just look for new particles; they looked for the absence of expected behavior with such extreme accuracy that they have rewritten the rules of the game for future physicists. They are the guardians of the Standard Model, testing its limits to see if it can hold up against the weight of the universe's deepest secrets.

Technical Summary

1. Problem Statement

The Standard Model (SM) of particle physics, while successful, fails to explain fundamental cosmological observations, specifically the nature of dark matter (DM) and the origin of the cosmic matter–antimatter asymmetry (baryogenesis). These gaps suggest the existence of physics Beyond the Standard Model (BSM).

• CP Violation: The SM's CP violation (via the CKM mechanism) is insufficient to explain the observed baryon asymmetry. New sources of CP violation are required.
• Baryon/Lepton Number Violation: While conserved in the SM at the perturbative level, many Grand Unified Theories (GUTs) and mechanisms for neutrino mass generation (e.g., seesaw mechanism) predict violations of baryon number ($B$) and lepton number ($L$).
• Dark Sector: The "neutron lifetime puzzle" (a discrepancy between beam and bottle measurement methods) and the similarity in mass densities of baryonic matter and dark matter suggest the possibility of baryons decaying into invisible dark sector particles.

Hyperons (baryons containing strange quarks) offer a unique laboratory for these searches. Unlike nucleons, hyperons are "self-analyzing" due to parity-violating weak decays, allowing direct access to polarization observables. Furthermore, the BESIII experiment at the BEPCII collider produces massive samples of quantum-entangled hyperon–antihyperon pairs ($J/\psi \to B\bar{B}$), providing a clean environment for precision tests.

2. Methodology

The paper reviews recent results from the BESIII experiment, utilizing a dataset of approximately $10^{10}$ $J/\psi$ events. The core methodologies include:

• Quantum Entanglement & Spin Correlations: Utilizing the coherent production of $\Lambda\bar{\Lambda}$ pairs from $J/\psi$ decays. The entangled nature of the pair allows for the extraction of CP-violating form factors and Electric Dipole Moments (EDMs) through angular distributions of decay products, bypassing the need for long-lived spin precession (which is impossible for short-lived hyperons).
• Double-Tagging Technique: A model-independent strategy where one hyperon in the pair is fully reconstructed (the "tag") via its dominant hadronic decay modes. This tags the presence and kinematics of the recoiling hyperon, allowing for a precise search for "invisible" or rare decays on the signal side without relying on missing energy reconstruction from the initial state.
• Kinematic Fits & Background Suppression:
  • Invisible Decays: Using the sum of energies in the Electromagnetic Calorimeter ($E_{EMC}$) not associated with the tagged tracks as the primary discriminator. A value near zero indicates an invisible decay.
  • Data-Driven Corrections: To address uncertainties in simulating neutron interactions (a major background for invisible searches), the authors use control samples (e.g., $J/\psi \to \bar{p}\pi^+ n \pi^0$) to correct Monte Carlo simulations.
  • Blind Analysis: Applied in searches for rare decays (e.g., $\Xi^- \to \Sigma^+ e^- e^-$) to avoid human bias.

3. Key Contributions and Results

A. Precision Tests of Symmetries (CP Violation)

• $\Lambda$ Electric Dipole Moment (EDM):
  • Result: The collaboration measured the real and imaginary parts of the $\Lambda$ EDM using the entangled system.
    • $\text{Re}(d_\Lambda) = (-3.1 \pm 3.2 \pm 0.5) \times 10^{-19} \, e\cdot\text{cm}$
    • $\text{Im}(d_\Lambda) = (2.9 \pm 2.6 \pm 0.6) \times 10^{-19} \, e\cdot\text{cm}$
  • Limit: $|d_\Lambda| < 6.5 \times 10^{-19} \, e\cdot\text{cm}$ (95% CL).
  • Significance: This improves upon the previous world limit (from Fermilab in the 1980s) by three orders of magnitude. It provides the first stringent constraint on the strange quark's chromo-electric dipole moment ($\tilde{d}_s$), a parameter largely unconstrained by neutron EDM measurements.

B. Baryon and Lepton Number Violation

• $\Lambda-\bar{\Lambda}$ Oscillations ($|\Delta B| = 2$):
  • Result: No "wrong-sign" events were observed.
  • Limit: Oscillation probability $P(\Lambda) < 1.4 \times 10^{-6}$; Transition mass $\delta m_{\Lambda\bar{\Lambda}} < 2.1 \times 10^{-18} \, \text{GeV}$.
  • Significance: Provides a critical benchmark for $|\Delta B|=2$ interactions involving second-generation quarks.
• Rare Decays ($|\Delta L| = 2$ and $|\Delta B| = 1$):
  • $\Sigma^- \to p e^- e^-$: Limit $\mathcal{B} < 6.7 \times 10^{-5}$.
  • $\Xi^0 \to K^\mp e^\pm$: Limits $\mathcal{B}(\Xi^0 \to K^- e^+) < 3.6 \times 10^{-6}$ and $\mathcal{B}(\Xi^0 \to K^+ e^-) < 1.9 \times 10^{-6}$.
  • $\Xi^- \to \Sigma^+ e^- e^-$: Limit $\mathcal{B} < 2.0 \times 10^{-5}$.
  • Significance: These are the first experimental constraints on these specific channels, testing Majorana neutrino exchange mechanisms and $B-L$ violating operators.

C. Dark Sector Searches

• Invisible $\Lambda$ Decay:
  • Result: No signal observed.
  • Limit: $\mathcal{B}(\Lambda \to \text{invisible}) < 7.4 \times 10^{-5}$.
  • Significance: First direct constraint on invisible baryon decays, constraining mirror matter models.
• Massless Dark Particles ($\Sigma^+ \to p + \text{invisible}$):
  • Result: No signal observed.
  • Limit: $\mathcal{B} < 3.2 \times 10^{-5}$.
  • Significance: Constrains the axion-fermion effective decay constant ($F^A_{sd} > 2.8 \times 10^7 \, \text{GeV}$), surpassing limits from $K-\bar{K}$ mixing.
• Dark Baryons ($\Xi^- \to \pi^- + \text{invisible}$):
  • Result: Searched for massive dark baryons ($\chi$) with masses near the GeV scale.
  • Limit: $\mathcal{B} < 6.5 \times 10^{-4}$ (for $m_\chi = m_\Lambda$).
  • Significance: First direct constraints on dark baryons from hyperon decays, providing tighter bounds on Wilson coefficients for quark-to-dark-baryon transitions than previous LHC searches.

4. Significance and Future Outlook

• Global Constraints: The results establish world-leading constraints on BSM scenarios, particularly those involving flavor-diagonal CP violation in the strange sector and baryonic dark matter. The combination of $\Lambda$ EDM and neutron EDM data allows for the isolation of the strange quark's chromo-EDM contribution.
• Neutron Lifetime Puzzle: The invisible decay searches directly test the hypothesis that neutrons (and hyperons) decay into dark sector particles, significantly restricting the viable parameter space for this explanation.
• Future Prospects: The paper highlights the Super Tau-Charm Factory (STCF) as the next frontier. With a luminosity two orders of magnitude higher than BEPCII, the STCF is projected to:
  • Reach EDM sensitivities of $10^{-20} - 10^{-21} \, e\cdot\text{cm}$.
  • Probe branching fractions for invisible/rare decays down to $10^{-6}$.
  • Extend studies to higher-mass hyperons (e.g., $\Omega^-$) and double-strange baryons ($\Xi^-, \Xi^0$).

In conclusion, this review demonstrates that hyperon physics at the intensity frontier, leveraging quantum entanglement and high-statistics datasets, is a powerful and complementary tool to high-energy colliders for uncovering new physics and fundamental symmetries.