Probing Long-Lived Particle Production in Muon Decays at the SNS with a Highly Capable Hydrocarbon Detector

This paper proposes using a tons-scale hydrocarbon scintillator detector at the Spallation Neutron Source to search for sub-GeV dark sector particles, such as axion-like particles and heavy neutral leptons, produced in muon decays, demonstrating the potential for order-of-magnitude sensitivity improvements over current limits through effective cosmic ray background rejection.

The Gist

Imagine the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory as a massive, high-speed particle factory. Every second, it fires a beam of protons like a shotgun blast at a target, creating a flood of tiny particles called pions. These pions quickly slow down and turn into muons (a heavier cousin of the electron), which then sit still for a tiny fraction of a second before decaying.

Usually, scientists look at what happens during the beam blast. But this paper proposes a different strategy: waiting in the quiet after the storm.

The Mystery: "Ghost" Particles

Scientists suspect that hidden in the shadows of the Standard Model are "Long-Lived Particles" (LLPs). Think of these as cosmic ghosts. They are produced when the muons decay, but unlike normal particles, they don't vanish immediately. They travel a bit, then decay into a pair of an electron and a positron (anti-electron).

The paper focuses on two types of these "ghosts":

1. Heavy Neutral Leptons (HNLs): Heavy, invisible neutrinos that might explain why real neutrinos have mass.
2. Axion-Like Particles (ALPs): Tiny, light particles that could be related to the mysterious "dark matter" holding the universe together.

The Detective: The "HC2" Detector

To catch these ghosts, the authors propose building a new detector called HC2. Imagine a giant, 4-ton block of glowing plastic (hydrocarbon scintillator) that acts like a high-tech honeycomb.

• The Honeycomb: Instead of one big tank, the detector is sliced into many long, thin segments (like a stack of pancakes or a honeycomb). This allows scientists to see exactly where a particle hits.
• The Flash: When a particle hits the plastic, it makes a flash of light. The detector is so sensitive it can count individual photons of light.
• The Time Machine: The key feature is timing. The beam blast lasts only 0.4 microseconds. The "ghost" particles arrive a few microseconds after the blast. By waiting for this quiet window, the detector ignores the chaotic noise of the beam itself.

The Challenge: The Cosmic Noise

The biggest problem isn't the beam; it's the sky. Earth is constantly bombarded by cosmic rays (muons and neutrons) from space. These create "noise" that looks exactly like the ghost particles the scientists are hunting.

The paper uses a clever trick to solve this: The PROSPECT Detector.
The team didn't just guess how well their new detector would work. They used data from an existing detector called PROSPECT (which was built to study nuclear reactors). PROSPECT sits on the surface, exposed to the same cosmic noise the new detector would face.

By analyzing PROSPECT's "reactor-off" data (times when the reactor was silent), they could see exactly how many cosmic "false alarms" happen. They then used powerful computer simulations to predict how the new, improved HC2 detector would handle this noise.

The Results: A Clearer View

The paper claims that with this new setup, they can filter out the cosmic noise incredibly well.

• The Filter: By using the honeycomb structure and special light-discrimination techniques (telling the difference between a "heavy" hit from a neutron and a "light" hit from an electron), they can reject 99.9% of the background noise.
• The Payoff: They predict that over a three-year run, the detector would see only a few hundred background events. This is so quiet that if even a handful of "ghost" particles showed up, it would be a massive discovery.

What They Can Find

The paper shows that this setup could improve our ability to find these particles by 10 to 100 times compared to current global limits.

• For Heavy Neutrinos: They could find particles with masses between 10 and 100 MeV that other experiments have missed.
• For Axion-Like Particles: They could probe a specific type of particle that other experiments struggle to see, especially near the "edge" of what is physically possible for muon decay.

Bonus: A Side Quest

While hunting for ghosts, the detector would also be an excellent tool for studying neutrinos from the SNS. It could measure how neutrinos interact with carbon atoms in the plastic with high precision, helping scientists understand the universe's most elusive particles better.

The Bottom Line

This paper is a blueprint for a "listening post" at the SNS. Instead of shouting over the noise of the beam, the scientists propose waiting for the silence, using a highly sensitive, segmented detector to catch the faint whispers of new physics that have been hiding in plain sight. If built, it could rewrite our understanding of the dark sector of the universe.

Technical Summary

Technical Summary: Probing Long-Lived Particle Production in Muon Decays at the SNS with a Highly Capable Hydrocarbon Detector

Problem Statement
The search for dark sector particles, specifically Long-Lived Particles (LLPs) such as Heavy Neutral Leptons (HNLs) and Axion-Like Particles (ALPs), has expanded beyond cosmic frontiers to terrestrial sources like proton beams. The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) is an ideal facility for probing the sub-GeV mass regime (10–100 MeV) because it produces an intense flux of muons ($\mu^+$) that decay at rest. These decays can produce LLPs which subsequently decay into observable $e^+e^-$ pairs. However, detecting these isolated electromagnetic signatures is challenging due to the dominance of steady-state cosmic ray backgrounds (neutrons and muons) during the post-beam time window where the signal is expected. Existing detectors at the SNS, such as those in the COHERENT experiment, have reported limits, but the ultimate sensitivity is often clouded by uncertainties regarding achievable background rejection levels in surface-level detectors.

Methodology
This paper proposes and evaluates a "HC2" campaign: the deployment of a highly capable, tons-scale hydrocarbon scintillator detector at the SNS. The study benchmarks the sensitivity of such a detector using the PROSPECT experiment as a prototype. PROSPECT, originally designed for reactor antineutrino searches, demonstrated excellent background rejection through high light collection, optical segmentation, $^6$Li doping, and Pulse Shape Discrimination (PSD).

The methodology involves:

1. Phenomenological Modeling: Calculating the production and decay rates of HNLs (via $\mu^+ \to e^+ \nu_e N$) and Lepton Flavor Violating (LFV) ALPs (via $\mu^+ \to e^+ a$) at the SNS. The study focuses on the $e^+e^-$ decay channel of these LLPs.
2. Detector Simulation: Utilizing PROSPECT's validated Monte Carlo (SG4) simulations of cosmic neutron and muon interactions. The study models a nominal HC2 detector (approx. 4 m$^3$, 3.8 tons of scintillator) with design variations, including:
   • Case A (Nominal): $^6$Li-doped liquid scintillator (LiLS) with full segmentation and PSD.
   • Case B: Undoped liquid scintillator (LS) with PSD.
   • Variations: Testing the impact of removing PSD capabilities or optical segmentation (monolithic targets).
3. Signal Selection: Defining selection criteria to isolate $e^+e^-$ pairs, including multiplicity cuts (2–10 pulses), PSD cuts to reject hadronic backgrounds, and fiducialization cuts to reject edge effects. Time-coincident vetoes are applied to reject muon decays and spallation products.
4. Sensitivity Analysis: Performing a binned Poissonian $\chi^2$ analysis to project 90% confidence level (C.L.) exclusion limits for HNL mixing angles ($|U_{\mu N}|^2$) and ALP decay constants ($f_a$) over a 3-year data-taking period (5 $\times$ 10$^{23}$ protons on target).

Key Contributions and Results

• Background Benchmarking: By applying selection cuts to PROSPECT's reactor-off data, the authors demonstrate that the combination of segmentation and PSD can reduce cosmic background rates by a factor of $\sim 10^3$ in the 20–100 MeV range. The data-simulation agreement for cosmic backgrounds is found to be within 10%, validating the use of SG4 simulations for future SNS predictions.
• Detector Design Optimization: The study finds that for the SNS BSM context, $^6$Li doping is not strictly necessary for background rejection, as the primary backgrounds are cosmic muons and neutrons rather than reactor-induced neutrons. However, Pulse Shape Discrimination (PSD) and optical segmentation are critical. Removing PSD increases neutron-induced backgrounds by over 30$\times$, while removing segmentation increases both muon and neutron backgrounds by factors of 2–6.
• Projected Sensitivity:
  • HNLs: A nominal HC2 campaign (with segmentation and PSD) is projected to improve current global limits on HNL mixing ($|U_{\mu N}|^2$) by more than an order of magnitude in the 10–100 MeV mass range. The sensitivity is competitive with, and in some regions superior to, higher-energy beam experiments like MicroBooNE, particularly for masses below the muon mass.
  • ALPs: For LFV ALPs, the HC2 strategy offers a distinct advantage near the kinematic endpoint of the muon decay (80–100 MeV). While traditional peak-search experiments (e.g., TWIST, PIENU) lose sensitivity as the electron energy drops, the decay-in-flight strategy of HC2 remains stable, potentially improving limits by nearly two orders of magnitude in this specific region.
• Background Reduction Strategies: The authors note that if an external muon veto system is added to reduce muon-induced backgrounds to sub-dominant levels, the total background count could be reduced to the few-hundred level over three years. Furthermore, a basement deployment location (Location B in the study) with additional overburden could further suppress cosmic backgrounds.

Significance and Claims
The paper claims that the HC2 concept represents a significant advancement in the search for sub-GeV dark sector particles. By leveraging the time-isolated nature of muon decays at rest at the SNS and the advanced background rejection capabilities of modern hydrocarbon scintillators (exemplified by PROSPECT), the proposed experiment can achieve world-leading sensitivity in the 10–100 MeV mass regime.

The authors emphasize that this approach complements existing searches:

• It probes mass ranges and coupling structures (specifically the product of flavor-violating and flavor-conserving couplings for ALPs) that are inaccessible to precision muon decay experiments.
• It accesses the low-mass region where higher-energy proton beam experiments (like those at Fermilab) have reduced sensitivity due to kinematic thresholds.

Additionally, the paper notes that such a deployment would simultaneously allow for high-precision measurements of inelastic neutrino interactions on $^{12}$C, potentially matching or exceeding the precision of historical measurements from LSND and KARMEN, provided systematic uncertainties in the neutrino flux can be constrained. The study concludes that while the projected sensitivity is robust, it is contingent on the characterization of uncharacterized neutron-induced $\gamma$-ray backgrounds at the specific SNS deployment sites, suggesting that near-term radiation surveys are a necessary next step.