Muon Bremsstrahlung as a New Probe of Dark Sector at Neutrino Experiments

Technical Summary: Muon Bremsstrahlung as a New Probe of Dark Sector at Neutrino Experiments

Problem Statement
Fixed-target accelerator neutrino facilities (e.g., MiniBooNE, SBND, DUNE) are traditionally optimized for neutrino production via focused meson decays. While these facilities generate intense neutrino beams, they also produce significant fluxes of secondary charged leptons, predominantly muons, which are typically absorbed in the beam dump. Previous searches for Beyond the Standard Model (BSM) physics at these sites have focused on meson decays, neutrino scattering, and proton bremsstrahlung. However, the potential of the secondary muon beam itself as a source for new physics production has remained largely unexplored. Specifically, there is a need to investigate production mechanisms that can access heavier mass regimes for Heavy Neutral Leptons (HNLs) where conventional meson decay channels become kinematically suppressed.

Methodology
The authors propose a novel production mechanism: muon bremsstrahlung occurring when the focused secondary muon beam impinges on the beam dump at the end of the decay pipe.

• Benchmark Model: The study utilizes a model featuring a muonphilic real scalar mediator ($\phi$) that couples to muons ($y_\mu$) and Heavy Neutral Leptons ($N$, with coupling $y_N$). The HNLs are assumed to mix exclusively with tau neutrinos ($U_{\tau N} \neq 0$, $U_{eN} = U_{\mu N} = 0$) to minimize existing constraints.
• Simulation Framework:
  • Flux Calculation: The authors simulate the focused charged-meson flux ($\pi^\pm, K^\pm$) using the RKHorn package for 8 GeV beams (SBND, MiniBooNE, ICARUS) and modeled fluxes for the 120 GeV LBNF beam (DUNE ND).
  • Muon Propagation: They calculate the flux of muons reaching the absorber, accounting for decay pipe lengths (~50 m for BNB, ~220 m for LBNF) and geometric acceptance.
  • Production Cross-Section: Scalar production is simulated via muon bremsstrahlung ($\mu + \text{nucleus} \to \mu + \phi + \text{nucleus}$) using the formalism from Ref. [62]. The differential cross-section depends on incident muon energy ($E_\mu$), scalar mass ($m_\phi$), and coupling $y_\mu$.
  • Decay Chain: The produced scalars decay promptly into HNLs ($\phi \to NN$). The HNLs then propagate to the detector and decay into visible final states ($\nu \pi^0$, $\nu \mu^+\mu^-$, $\nu e^+e^-$).
  • Event Rate: The number of signal events is calculated by convolving the HNL flux with the decay probability within the detector volume and the branching ratios for specific final states.

Key Contributions

1. Identification of a New Production Channel: The paper establishes muon bremsstrahlung in the beam dump as a viable and powerful mechanism for producing light new physics at neutrino facilities.
2. Kinematic Advantage: The authors demonstrate that muons from two-body meson decays carry a larger fraction of the parent meson energy compared to the accompanying neutrinos. Consequently, muon bremsstrahlung can produce scalars (and subsequently HNLs) with masses significantly higher than those accessible via direct meson decays, which are limited by the meson mass scale ($m_K - m_\mu$).
3. Comparison with Conventional Experiments: Unlike dedicated beam-dump experiments (e.g., SHiP, DarkQuest) where mesons are absorbed immediately, neutrino facilities utilize magnetic horns to focus mesons. This focusing results in a high-energy, collimated muon flux at the dump, making these facilities uniquely suited for this production mechanism.
4. Background Discrimination: The study highlights that signals from this mechanism exhibit distinctive kinematics (energy and angular distributions) that allow for efficient discrimination against Standard Model neutrino-induced backgrounds.

Results

• Mass Reach: The muon bremsstrahlung mechanism extends the discovery potential for HNLs to masses up to $\mathcal{O}(1)$ GeV. This is particularly relevant for DUNE Near Detector (ND) and SBND, where conventional meson-decay searches are kinematically limited.
• Sensitivity Projections:
  • DUNE ND: Can probe previously unexplored parameter space for HNL masses up to $\sim 1$ GeV through decays into $\nu \pi^0$, $\nu \mu^+\mu^-$, and $\nu e^+e^-$.
  • SBND/MiniBooNE/ICARUS: These experiments can also probe new parameter space, particularly for single $\pi^0$ final states.
  • Event Rates: The study provides sensitivity contours for 2.3, 10, and 100 signal events. For DUNE ND, the 100-event contour reaches $m_N \approx 1$ GeV.
• Model Constraints: The analysis assumes $m_\phi = 2.1 m_N$ and saturates existing bounds on $y_\mu$ from invisible scalar searches (NA64$\mu$, BaBar, CMS). The resulting sensitivity is proportional to $y_\mu^2 |U_{\tau N}|^2$.
• Vector/Axial-Vector Mediators: The authors note that similar or larger fractions of HNLs would be produced if the mediator were a vector or axial-vector boson, as the bremsstrahlung cross-section for these is approximately an order of magnitude larger.

Significance and Claims
The paper claims that muon bremsstrahlung substantially extends the discovery potential of existing and upcoming accelerator neutrino experiments.

• Novelty: It is the first demonstration that the intense, energetic secondary muon flux at neutrino facilities serves as a powerful probe for new physics via bremsstrahlung interactions in the beam absorber.
• Complementarity: This mechanism complements proton bremsstrahlung and meson decay searches. While proton bremsstrahlung produces highly forward-directed, high-energy particles, muon bremsstrahlung yields a softer, less forward-peaked spectrum, allowing access to different regions of parameter space.
• Future Implications: The study establishes a conceptual link between neutrino facility searches and future muon collider experiments, suggesting that high-energy muon beams offer unprecedented sensitivity to HNLs and dark sector particles.
• Modesty: The authors explicitly state that their results for the MiniBooNE anomaly are not helpful in explaining the observed low-energy excess, as the muon bremsstrahlung signal is significantly higher in energy. They also note that BBN constraints may be modified by the presence of the muonphilic scalar, requiring dedicated future studies to recalculate those bounds.