Dark Matter-Induced Nuclear De-Excitation at SBND with Ab Initio Nuclear Theory

This paper demonstrates that the Short-Baseline Near Detector (SBND) can probe previously unexplored light dark matter parameter spaces by detecting MeV-scale photon "blips" from nuclear de-excitation, utilizing state-of-the-art ab initio nuclear theory to predict signals from argon excited states up to 18 MeV.

The Gist

The Big Idea: Catching Invisible Ghosts with a "Flashlight"

Imagine you are trying to find a ghost in a dark room. You can't see the ghost directly, but you know that if the ghost bumps into a specific object (like a vase), the vase might wobble and drop a small, glowing marble. If you see that glowing marble, you know the ghost was there.

This paper is about a team of physicists looking for Dark Matter—the invisible stuff that makes up most of the universe's mass. They are using a giant detector called SBND (Short-Baseline Near Detector) located at Fermilab. Instead of looking for the dark matter particle itself, they are looking for the "glowing marbles" it leaves behind when it bumps into atoms inside the detector.

The Setup: The Factory and the Detector

1. The Factory (The Proton Beam): Scientists shoot a high-speed beam of protons (tiny particles) at a target. This is like a high-speed train crashing into a wall.
2. The Byproduct (The Mediator): When the protons hit the target, they create a burst of other particles. The theory suggests this crash also creates a "messenger" particle (called a dark photon or $A'$). This messenger is invisible to us but can decay into two dark matter particles.
3. The Target (The Detector): These dark matter particles fly 110 meters down the track and hit the SBND detector. The detector is a giant tank filled with Liquid Argon (a super-cold, liquid version of the gas in your lightbulbs).

The "Blip": How They Spot the Invisible

Usually, dark matter is thought to bounce off atoms like a billiard ball (elastic scattering). But this paper focuses on a different, trickier scenario: Inelastic Scattering.

• The Analogy: Imagine the dark matter particle hits an Argon atom not just to bounce it, but to kick it.
• The Excitation: This kick excites the Argon atom, putting it into a "stressed" or "excited" state. Think of it like ringing a bell. The bell is now vibrating with energy.
• The De-excitation (The Blip): The bell (the Argon atom) can't stay excited forever. It quickly settles down by releasing that extra energy as a flash of light (a photon).
• The Signature: In the liquid argon detector, this flash of light creates a tiny, isolated spark of energy. The scientists call this a "blip." It's a very specific, localized spark of light that looks like a tiny firework inside the tank.

The Challenge: Doing the Math Right

To know if they are seeing a real dark matter "blip" or just random noise, they need to predict exactly how often these blips should happen.

• The Old Way: Previously, scientists used "shell models" (like a simplified map of the atom) to guess how the Argon atom would react. But these maps often needed "tweaks" or adjustments to match real-world data, which made them less reliable for new physics.
• The New Way (Ab Initio): This paper uses Ab Initio calculations. Think of this as building the atom from scratch using only the fundamental laws of physics, without any "tweaks" or shortcuts.
  • They calculated the behavior of every possible excited state of the Argon atom up to 18 MeV (a specific energy level).
  • They found that the most important "kicks" happen when the atom jumps to specific states (called $1^+$ and $2^+$ states).
  • This "from-scratch" math gives them a much more trustworthy prediction of what a real dark matter signal looks like.

The Two Ways to Look

The paper looks at two different ways to run the experiment:

1. Target Mode (The Busy Factory): The proton beam hits the main target first. This creates a lot of dark matter, but it also creates a lot of "noise" (neutrinos) that can fake a signal. It's like trying to hear a whisper in a crowded stadium.
2. Dump Mode (The Quiet Room): The proton beam is aimed directly at a heavy iron wall (a "dump"), skipping the main target. This creates fewer dark matter particles, but it reduces the "noise" (neutrinos) by 50 times. It's like moving the experiment to a quiet library. The signal is cleaner, making it easier to spot the "blip."

The Results: Finding New Territory

After doing all the complex math and accounting for background noise (like random sparks from natural radiation or stray neutrons), the team found:

• SBND is sensitive: Even with the noise, the detector is powerful enough to spot these "blips."
• New Territory: They can look for dark matter in areas of "parameter space" (a map of possible masses and interaction strengths) that no one has been able to check before.
• The Promise: If they see these specific "blips" in the liquid argon, it could be the first solid evidence of light dark matter interacting with nuclei in this specific way.

Summary

In short, this paper says: "We have built a super-accurate mathematical model of how Argon atoms react when hit by dark matter. Using this model, we show that the SBND detector can spot tiny, isolated flashes of light ('blips') caused by dark matter. By running the experiment in a 'quiet mode' (Dump Mode), we can ignore most of the background noise and potentially discover a new type of dark matter that has never been seen before."

Technical Summary

Technical Summary: Dark Matter–Induced Nuclear De-Excitation at SBND with Ab Initio Nuclear Theory

Problem Statement
Current light dark matter (DM) searches, particularly those utilizing proton-beam dump experiments, often rely on elastic scattering channels or disappearance signatures. While inelastic scattering cross-sections are typically smaller than elastic ones, they offer a unique advantage: the emission of MeV-scale de-excitation photons. These photons produce distinct, localized low-energy energy depositions ("blips") in Liquid Argon Time Projection Chambers (LArTPCs). However, predicting the rates for these signals has been hindered by the limitations of phenomenological nuclear models. Traditional Large-Scale Shell Model (LSSM) calculations often require "quenching" (adjusting) operators to match experimental data, introducing uncertainties that prevent robust predictions for the full set of operators involved in DM scattering. Furthermore, the relevant parameter space for light DM (MeV scale) has not been thoroughly explored using inelastic channels in the context of the Short-Baseline Near Detector (SBND) at Fermilab.

Methodology
This work investigates the sensitivity of SBND to light fermionic dark matter ($\chi$) that interacts via a $U(1)$ gauge boson ($A'$). The analysis covers two operational modes: the ongoing "Target Mode" (protons striking a Beryllium target) and a proposed "Dump Mode" (protons striking an iron dump directly), the latter of which significantly reduces neutrino backgrounds.

The core methodological innovation lies in the calculation of the inelastic DM-nucleus scattering cross-section. Instead of relying on phenomenological models, the authors employ state-of-the-art ab initio nuclear theory.

• Nuclear Framework: The study utilizes the Valence-Space In-Medium Similarity Renormalization Group (VS-IMSRG) method. This approach starts from realistic chiral two- and three-nucleon interactions (specifically 1.8/2.0(EM), $\Delta$NNLOGO(394), and N3LOTexas) and fundamental electroweak currents, solving the nuclear many-body problem without introducing phenomenological parameters.
• Scope of States: The calculations systematically include all relevant argon excited states with energies up to 18 MeV. This includes states with spins and parities ranging from $1^+$ to $4^+$ and $0^-$ to $4^-$.
• Signal Generation: The analysis assumes a dark photon or leptophobic model where $A'$ is produced via neutral meson decays ($\pi^0, \eta \to \gamma A'$) or proton bremsstrahlung, subsequently decaying into a $\chi\bar{\chi}$ pair. These DM particles then scatter inelastically off argon nuclei, exciting them. The excited nuclei de-excite via the emission of MeV-scale gamma rays, which manifest as "blips" in the LArTPC.
• Background Estimation: The study accounts for irreducible backgrounds, primarily neutral-current neutrino-argon inelastic scattering and beam-related neutrons. For the target mode, an estimated $\sim$235 neutrino-induced inelastic events are considered. For the dump mode, the neutrino flux is suppressed by a factor of 50, allowing for a background-free search assumption in the sensitivity projections.

Key Contributions

1. First Ab Initio Application to Inelastic DM: This work extends ab initio nuclear theory, previously applied to elastic scattering, to the more computationally demanding inelastic scattering regime. It calculates wave functions for multiple excited states and high-rank transition matrix elements, providing more physically robust and predictive nuclear form factors than LSSM.
2. Comprehensive State Inclusion: The authors are the first to systematically include all relevant argon excited states up to 18 MeV in this context. They identify that the dominant contributions to the signal arise from $1^+$ states (located between 4.5 and 10 MeV) and the first $2^+$ state (at $\sim$1.5 MeV).
3. SBND Sensitivity Projections: The paper establishes SBND as a powerful probe for inelastic dark matter interactions, analyzing both its current target-mode operation and a proposed beam-dump configuration.

Results
Using the ab initio calculated cross-sections, the authors derive exclusion bounds for two scenarios: the vector-portal dark photon model and the leptophobic dark matter model.

• Signal Rates: The analysis projects sensitivity based on expected signal event counts. For the Target Mode, contours are drawn for 10 and 30 events (the latter corresponding to approximately $3\sigma$ sensitivity with estimated neutrino backgrounds). For the Dump Mode, a 2.3-event contour is presented under the assumption of a background-free search.
• Parameter Space Coverage: The results demonstrate that SBND can probe previously unexplored regions of the parameter space for light dark matter (masses $m_\chi$ from $\sim$1 MeV to 100 MeV). The exclusion limits extend beyond current constraints from elastic scattering searches (e.g., COHERENT, CCM, LSND, MiniBooNE) in specific regions, particularly where the inelastic channel offers reduced neutrino-induced backgrounds.
• Model Independence: The use of ab initio calculations ensures that the predicted signal rates are not dependent on the ad-hoc operator adjustments required by shell models, thereby increasing the reliability of the exclusion limits.

Significance
The paper claims that this work highlights the potential of MeV-scale nuclear de-excitation signatures in liquid-argon detectors as a powerful avenue for future accelerator-based dark matter searches. By combining the unique capabilities of SBND (specifically its ability to reconstruct isolated MeV-scale electromagnetic activity) with rigorous ab initio nuclear theory, the study establishes a robust framework for detecting light dark matter via inelastic scattering. The authors emphasize that this approach allows for the exploration of parameter space that was previously inaccessible or unconstrained by existing experiments, offering a complementary channel to traditional elastic scattering and disappearance searches. The work serves as a proof-of-concept that ab initio methods can successfully address the computational challenges of inelastic scattering, enabling more precise predictions for next-generation DM experiments.