Search for dark sector and rare decays at BESIII

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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

Imagine the universe as a giant, bustling city. For decades, physicists have been the city planners, drawing up the "Standard Model" blueprint. This blueprint explains almost everything we see: the buildings (atoms), the traffic (forces), and the people (particles). It works beautifully.

But there's a problem. The blueprint is missing huge sections. It doesn't explain the "Dark Sector"—the invisible stuff that makes up most of the city's mass (Dark Matter), or why there's more matter than antimatter. It's like having a map of a city that shows all the houses but ignores the massive underground tunnels and secret societies that might be running the place.

This paper is a report from the BESIII experiment, a high-tech "security camera" system in Beijing (the Beijing Electron Positron Collider). The team, led by researchers from Sun Yat-sen University, has been watching billions of particle collisions, looking for cracks in the blueprint or signs of these hidden tunnels.

Here is what they found, explained simply:

1. The "Invisible Ghost" Hunt (Dark Matter)

The Analogy: Imagine you are watching a magician pull a rabbit out of a hat. You see the hat, you see the rabbit, but you know there's a secret compartment. If the magician pulls out a rabbit and nothing else, you might suspect a ghost is hiding in the hat, invisible to your eyes.

The Science:
The team looked at a specific particle decay called $\eta \to \pi^0 + \text{invisible}$ .

They started with a heavy particle ( $\eta$ ).
It broke apart into a lighter particle ( $\pi^0$ ) and... nothing visible.
If the "nothing" was actually a Dark Matter particle (a "ghost"), it would carry away energy without being seen.

The Result: They didn't see any ghosts. However, by not seeing them, they set a very strict rule: "If ghosts exist here, they must be weaker than this."

Why it matters: Previous experiments tried to catch dark matter by waiting for it to bump into a heavy rock (nucleus) in a deep mine. But light, fast dark matter (sub-GeV) is too weak to move the rock. BESIII acts like a high-speed camera that can spot the energy the ghost steals, even if it's too light to knock over a rock. Their limits are 100,000 times stricter than the mine experiments for this specific type of light dark matter.

2. The "Missing Person" Cases (Other Dark Searches)

The team didn't just look for one type of ghost; they looked for other strange, invisible things in different scenarios:

The $J/\psi$ Mystery: They watched a particle called $J/\psi$ decay into a $\phi$ meson and "nothing." Again, no invisible particles were found, but they set new rules on how often this could happen.
The "Dark Baryon" (The Shadow Twin): There is a theory that for every normal particle (like a proton), there might be a "dark twin" that carries a "dark charge."
- They looked at a particle called the $\Xi^-$ (a heavy cousin of the proton) decaying into a pion and a "dark baryon."
- The Result: No dark twins found. But this was the first time anyone looked for this specific decay, so they set the first-ever rules for it.

3. The "Secret Messenger" (Light Vector Bosons)

The Analogy: Imagine two people talking. Usually, they use a standard phone (the known forces). But what if they had a secret, invisible walkie-talkie (a "dark photon" or light vector boson) that only they could use?

The Science:
They looked for a particle called $V$ (the secret messenger) appearing when a heavy particle ( $\chi_{cJ}$ ) decayed. They checked if this messenger turned into an electron-positron pair ( $e^+e^-$ ).

The Result: No secret walkie-talkies were found. They ruled out the existence of these messengers for a wide range of masses.

4. The "Rule Breakers" (Rare Decays)

The Standard Model has strict laws, like "You can't turn a proton into a neutron without a specific reason" or "You can't turn an electron into a muon." But new physics might allow these "rule-breaking" events.

Lepton Flavor Violation: They looked for a particle ( $\psi(2S)$ ) turning into an electron and a muon at the same time. It's like a coin suddenly turning into a dollar bill. Result: No coin-to-dollar magic found.
Baryon/Lepton Number Violation: They looked for processes where the total number of "matter particles" changes, which would explain why the universe is made of matter and not antimatter. Result: No rule breakers found yet, but they set the tightest limits ever for these specific rare events.
Weak Decays: They checked if heavy "charmed" particles could decay in very slow, rare ways that the Standard Model predicts but are hard to see. They found no evidence of these rare events yet, but they improved the limits significantly.

The Big Picture

Think of the BESIII experiment as a super-sensitive metal detector sweeping a beach.

They didn't find any gold (new particles) this time.
However, by sweeping the beach and finding nothing, they proved that if gold is there, it's buried deeper or is much smaller than we thought.
They have effectively "fenced off" huge areas of the map where new physics cannot exist. This forces scientists to look in new, more creative places.

Conclusion:
The BESIII team has used their massive collection of data (billions of collisions) to cast a very wide, very net. While they didn't catch the "fish" (Dark Matter or new particles) yet, they have proven that the water is much clearer than before. This gives us a much better map of where to look next in the search for the secrets of the universe.

1. Problem Statement

The Standard Model (SM) of particle physics, while successful, fails to explain several fundamental phenomena, including the nature of Dark Matter (DM), the strong CP problem, the muon $g-2$ anomaly, and the matter-antimatter asymmetry. These gaps suggest the existence of a "dark sector" containing new particles and interactions.

Sub-GeV DM Challenge: Direct detection experiments struggle to detect sub-GeV DM because the recoil energy from DM-nucleon scattering is often below detection thresholds.
Rare Decays: The SM predicts extremely low branching fractions (BFs) for certain processes, such as Charged Lepton Flavor Violation (CLFV), Baryon/Lepton Number Violation (BNV/LNV), and charmonium weak decays. Observing these at rates higher than SM predictions would be a clear signature of New Physics (NP).
The Opportunity: The BESIII experiment, operating at the $\tau$ -charm energy region ($1.84 $–$ 4.95$ GeV) with massive datasets (e.g., $10^{10}$ $J/\psi$ events), offers a unique high-intensity environment to probe these sub-GeV dark sector particles and rare decays that are inaccessible to high-energy colliders or direct detection experiments.

2. Methodology

The authors utilize the extensive datasets collected by the BESIII detector at the BEPCII collider. The analysis strategies vary by channel but generally follow a "tag-and-recoil" or "inclusive search" approach:

Data Samples:
- $\sim 10.1 \times 10^9$ $J/\psi$ events.
- $\sim 2.7 \times 10^9$ $\psi(2S)$ events.
- $\sim 20$ fb $^{-1}$ at $\sqrt{s} = 3.773$ GeV (continuum).
- $> 20$ fb $^{-1}$ above $4.0$ GeV.
Specific Search Strategies:
- $\eta \to \pi^0 + \text{invisible}$ : $\eta$ mesons are tagged via $J/\psi \to K^+K^-\eta$ . The $\pi^0$ is reconstructed, and the "invisible" system is identified by the recoil of the $K^+K^-\pi^0$ system. This searches for a scalar dark boson ( $S$ ) decaying to DM ( $\chi\bar{\chi}$ ).
- $J/\psi \to \phi + \text{invisible}$ : $\phi$ mesons are reconstructed from $K^+K^-$ pairs. The invisible mass is constrained to be $< 2m_{K_L^0}$ to reduce backgrounds.
- $\Xi^- \to \pi^- + \text{invisible}$ : $\Xi^-$ candidates are tagged via the recoil of $\bar{\Xi}^+ \to \pi^+\bar{\Lambda} (\to \bar{p}\pi^+)$ . The analysis checks for missing energy in the Electromagnetic Calorimeter (EMC) to identify dark baryons.
- $\chi_{cJ} \to J/\psi V$ ( $V \to e^+e^-$ ): Searches for a light vector boson ( $V$ ) in the dilepton invariant mass spectrum ( $M_{e^+e^-}$ ) produced from $\psi(2S) \to \gamma \chi_{cJ}$ .
- Rare Decays: Searches for CLFV ( $\psi(2S) \to e\mu$ ), BNV/LNV (e.g., $J/\psi \to pe^-$ ), and weak charmonium decays (e.g., $J/\psi \to D_s^- \ell^+ \nu$ ) involve selecting specific final states and comparing observed yields against estimated SM backgrounds.

3. Key Contributions

This paper presents the first or most stringent constraints on several dark sector and rare decay channels:

Sub-GeV DM Constraints: Established the first collider-based constraints on sub-GeV DM via $\eta \to \pi^0 + \text{invisible}$ and $J/\psi \to \phi + \text{invisible}$ .
Dark Baryon Search: Conducted the first search for dark baryons in hyperon decays ( $\Xi^- \to \pi^- + \text{invisible}$ ).
Light Vector Boson: Set new limits on light vector bosons coupling to charm quarks in $\chi_{cJ}$ decays.
Rare Decay Limits: Provided the first experimental limits for several BNV/LNV processes and CLFV decays, significantly improving upon previous upper limits for charmonium weak decays.

4. Key Results

A. Dark Sector Searches

$\eta \to \pi^0 + \text{invisible}$ :
- Result: No significant signal observed for scalar boson mass $m_S \in [0, 400]$ MeV.
- Limits: Upper limits (ULs) on the branching fraction (BF) are $(1.8 \sim 5.5) \times 10^{-5}$ .
- Coupling Constraints: Limits on scalar-quark couplings ( $g_u, g_d$ ) are $(1.3 \sim 3.2) \times 10^{-5}$ .
- Significance: These constraints improve upon the PandaX-4T direct detection limits by a factor of 3.3 to 18.3 for sub-GeV DM. They also probe the parameter space for thermal relic DM (assuming $m_\chi = 0.45m_S$ ).
- Scattering Cross-section: The derived constraint on the DM-nucleon scattering cross-section ( $\bar{\sigma}_n$ ) is approximately 5 orders of magnitude stronger than previous direct detection experiments in the sub-GeV region.
$J/\psi \to \phi + \text{invisible}$ :
- Result: No signal observed.
- Limits: UL on BF is $7.0 \times 10^{-8}$ .
- Secondary Result: Derived an UL on $\eta \to \text{invisible}$ of $2.4 \times 10^{-5}$ , improving previous results by $>4\times$ .
$\Xi^- \to \pi^- + \text{invisible}$ (Dark Baryons):
- Result: No signal observed for dark baryon masses ranging from $1.07$ to $1.16$ GeV.
- Limits: Set new ULs on the BF, improving upon LHC constraints recast for the dark baryon model.
$\chi_{cJ} \to J/\psi V$ (Light Vector Boson):
- Result: No signal observed for $V$ mass $5 \sim 300$ MeV.
- Limits: UL on coupling strength $\epsilon$ is $(2.5 \sim 17.5) \times 10^{-3}$ . This uniquely probes couplings to charm quarks in non-universal models.

B. Rare Decay Searches

CLFV ( $\psi(2S) \to e\mu$ ):
- Result: 8 candidates observed vs. $6.2 \pm 1.4$ expected background.
- Limit: UL on BF is $1.4 \times 10^{-8}$ (90% CL).
BNV/LNV Processes:
- First searches performed for:
  - $J/\psi \to pe^-$ : UL $3.1 \times 10^{-8}$ .
  - $J/\psi \to K^+K^+e^-e^-$ : UL $2.1 \times 10^{-9}$ .
  - $\eta \to \pi^+\pi^+e^-e^-$ : UL $4.6 \times 10^{-6}$ .
  - $\omega \to \pi^+\pi^+e^-e^-$ : UL $2.8 \times 10^{-6}$ .
Charmonium Weak Decays:
- Improved ULs on Cabibbo-favored weak decays (e.g., $J/\psi \to D_s^- e^+ \nu_e$ ) by roughly one order of magnitude compared to previous limits, reaching the $10^{-7}$ to $10^{-6}$ level.

5. Significance

Complementarity to Direct Detection: The BESIII results demonstrate that collider experiments are uniquely powerful for probing sub-GeV Dark Matter, a mass region where traditional nuclear recoil experiments lose sensitivity. The constraints on DM-nucleon scattering cross-sections are superior to direct detection limits by orders of magnitude in this mass range.
Probing High-Energy Scales: The $\eta \to \pi^0 S$ analysis shows that low-energy precision experiments can probe New Physics energy scales ( $\Lambda_{NP}$ ) up to 10 TeV, bridging the gap between low-energy flavor physics and high-energy collider physics.
Thermal Relic DM: The results successfully exclude parts of the parameter space for thermal relic DM, providing critical input for cosmological models.
New Physics Frontiers: By setting the first limits on BNV/LNV and CLFV in charmonium and hyperon decays, BESIII opens new avenues for testing the Majorana nature of neutrinos and supersymmetric models (e.g., R-parity violation).
Future Potential: With the massive accumulated datasets in the $\tau$ -charm region, BESIII is poised to provide even more refined constraints, potentially discovering deviations from the Standard Model in the near future.