Extending the sensitivity of heavy sterile neutrino searches with solar neutrino experiments

This paper presents a sensitivity study demonstrating that solar neutrino experiments can detect heavy sterile neutrinos in the MeV mass range by combining complementary detection methods for decay products ($e^+e^-$ pairs or $\nu_e$), thereby extending the observable parameter space for mixing angles and masses.

The Gist

Imagine the Sun as a giant, glowing factory that constantly spits out tiny, ghostly particles called neutrinos. For decades, physicists have been watching these ghosts, but they suspect there's a secret family member hiding in plain sight: a "heavy sterile neutrino."

Think of the standard neutrinos as invisible ninjas that barely interact with anything. The "heavy sterile" version is like a ninja wearing a heavy, bulky suit. It's so heavy and shy that it doesn't play by the usual rules of physics (the Standard Model), and it's incredibly hard to catch.

This paper is a proposal from a team at Tsinghua University on how to catch these heavy ghosts using our current solar neutrino detectors, specifically looking for ones that weigh between 2 and 15 "MeV" (a unit of mass for tiny particles).

Here is the simple breakdown of their plan:

The Setup: The Sun's Secret Exit

The Sun produces these heavy ghosts when a specific type of radioactive decay happens inside it (called $^8$B decay). It's like the Sun has a secret back door. Occasionally, instead of sending out a normal ghost, it sends out a heavy one.

The problem is, these heavy ghosts are tricky. They have a "mixing parameter" (let's call it the Shyness Factor).

• If the Shyness Factor is high, they are produced often but don't last long.
• If the Shyness Factor is low, they are produced rarely but might live a long time.

The Two Detection Strategies

The team realized that trying to catch these ghosts with just one method is like trying to catch a fish with only a net or only a hook. You need both. They propose two complementary methods based on where the heavy ghost decides to "die" (decay).

Method 1: The "Explosion" Inside the Tank

• The Scenario: Imagine the heavy ghost flies all the way from the Sun to Earth and enters our giant underground water tank (the detector). If it decays inside the tank, it bursts into a pair of particles: an electron and a positron (an anti-electron).
• The Clue: Normal solar neutrinos usually hit the water and create just one electron. But this heavy ghost creates a pair (a duo).
• The Analogy: It's like walking into a room and seeing a single person (background noise) versus seeing two people holding hands (the signal). The team calculates that if the heavy ghost has a "medium" lifespan, it will likely explode inside the tank, leaving this tell-tale duo behind.
• The Tool: They look at the energy of this duo and the angle between them. If the angle is wide enough, it's a strong sign it's the heavy ghost and not just a normal neutrino.

Method 2: The "Messenger" from Outside

• The Scenario: What if the heavy ghost is too short-lived? It might explode before it even reaches Earth, perhaps while still in space near the Sun.
• The Clue: When it explodes in space, it releases a normal neutrino ($\nu_e$) that flies the rest of the way to Earth.
• The Problem: This is hard to spot because it looks exactly like a normal solar neutrino.
• The Solution: The team found a way to tell them apart using direction.
  • Normal solar neutrinos always come straight from the Sun (like a laser beam).
  • The neutrinos from the heavy ghost's explosion in space can come from slightly different angles because the explosion happened at a random spot in space, not right at the Sun's core.
• The Analogy: Imagine looking at a lighthouse. All the light beams come from the lighthouse. But if a firework explodes in the sky near the lighthouse, the light from that explosion comes from a slightly different angle. By measuring the angle very precisely, the team hopes to spot these "off-center" messengers.

The Results: A Map of Possibility

The authors ran the numbers for a hypothetical 500-ton detector running for one year.

• The Sweet Spot: They found that by combining both methods, they could potentially see a few signal events across almost the entire range of masses and "shyness" they are interested in ($2$ to $15$ MeV mass, and a specific range of mixing parameters).
• Complementary Strengths:
  • Method 1 (the explosion inside) is best for ghosts that live long enough to reach the tank.
  • Method 2 (the off-center messenger) is best for ghosts that die too quickly to reach Earth.
• The Goal: They aren't claiming to have found the particle yet. Instead, they are drawing a map that shows exactly where to look to either find it or rule it out. They believe their combined approach is much more sensitive than what has been done before (like the Borexino experiment).

In a Nutshell

The paper says: "We have a new, two-pronged strategy to hunt for heavy, shy neutrinos using the Sun as a factory. One strategy catches them if they explode inside our detector; the other catches the messengers they send if they explode in space. Together, these methods cover a huge area of the 'unknown' that other experiments have missed."

Technical Summary

Technical Summary: Extending the Sensitivity of Heavy Sterile Neutrino Searches with Solar Neutrino Experiments

Problem Statement
The observation of neutrino flavor oscillations confirms non-zero neutrino masses, a phenomenon unexplained by the Standard Model (SM). While the Type-I seesaw mechanism offers a theoretical framework for these masses by introducing heavy sterile neutrinos ($\nu_H$), the mixing parameters ($|U_{\alpha H}|^2$) required for heavy $\nu_H$ (MeV to TeV scale) are typically extremely small, rendering their production rates in standard sources (solar, reactor, accelerator) negligible. Although current experiments have excluded significant portions of the $|U_{\alpha H}|^2$ versus $m_{\nu_H}$ parameter space, a substantial region remains unexplored, particularly in the MeV mass range where mixing parameters could be larger than the strict seesaw limit. This paper addresses the challenge of detecting heavy sterile neutrinos with masses between 2 MeV and 15 MeV produced via $^8$B solar neutrino decay, specifically targeting mixing parameters as low as $10^{-6}$.

Methodology
The authors propose a dual-method approach to detect heavy sterile neutrinos produced in the Sun via the decay chain $^8\text{B} \to {}^8\text{Be}^* + e^+ + \nu_H$. The detection strategy relies on the decay products of $\nu_H$, which depend on the particle's lifetime relative to its flight time from the Sun to the Earth. The study assumes a 500-ton solar neutrino detector operating for one year.

1. Production and Decay Kinematics:
   The flux of $\nu_H$ is derived from the $^8$B solar neutrino spectrum, suppressed by the mixing parameter $|U_{eH}|^2$ and a phase-space factor. The primary decay channels considered are $\nu_H \to \nu_e e^+ e^-$ and $\nu_H \to 3\nu_e$. The paper calculates the proper lifetime of $\nu_H$ as a function of mass and mixing, establishing that for small masses and small mixing, $\nu_H$ is long-lived, while for large masses and large mixing, it decays rapidly.

2. Method 1: In-Detector Decay ($e^+e^-$ Signal):
   For $\nu_H$ with intermediate lifetimes (decaying within the detector volume), the search focuses on the $e^+e^-$ pair produced in the decay.

   • Signal Characteristic: The total energy deposition of the $e^+e^-$ pair forms a distinct spectral peak at the tail of the background spectrum.
   • Background: The primary background is single electrons from elastic scattering of solar $^8$B neutrinos.
   • Discrimination: The paper utilizes the total energy spectrum and, for detectors with directional sensitivity (e.g., Cherenkov or advanced scintillators like the Jinping Neutrino Experiment), the opening angle between the $e^+$ and $e^-$. Since $\nu_H$ decay produces a pair while background events produce single electrons, a large opening angle serves as a key discriminator.

3. Method 2: Out-of-Detector Decay ($\nu_e$ Signal):
   For $\nu_H$ with very short lifetimes (decaying before reaching Earth), the only detectable product is the active neutrino $\nu_e$.

   • Signal Characteristic: The $\nu_e$ from $\nu_H$ decay has a significantly softer energy spectrum (peaking below 5 MeV) compared to the original solar neutrino.
   • Discrimination: The paper introduces the "solar angle" ($\theta_{Sun}$), defined as the angle between the detected $\nu_e$ direction and the Sun-Earth line. While standard solar neutrinos originate strictly from the Sun ($\theta_{Sun} \approx 0$), $\nu_e$ from $\nu_H$ decays occurring in space can arrive from various angles, creating a tail in the angular distribution that extends beyond the angular resolution of standard detectors (approx. $25^\circ$).

Key Contributions and Results

• Complementary Sensitivity: The study demonstrates that Method 1 and Method 2 are complementary. Method 1 is most sensitive to the "crest" of the parameter space where the $\nu_H$ lifetime allows decay within the detector (roughly $10^{-6} < |U_{eH}|^2 < 1$ and $2 < m_{\nu_H} < 15$ MeV). Method 2 covers the region of very short lifetimes (large mixing/large mass) where $\nu_H$ decays before reaching Earth.
• Event Yield Estimates: For a 500-ton detector operating for one year, the authors estimate that at least a handful of signal events can be observed across most of the parameter space defined by $10^{-6} < |U_{eH}|^2 < 1$ and $2 \text{ MeV} < m_{\nu_H} < 15 \text{ MeV}$ when combining both methods.
• Exclusion Contours: Using a profile likelihood method with Asimov datasets, the paper presents expected 90% Confidence Level (C.L.) exclusion contours.
  • Method 1 using energy spectra alone improves upon existing Borexino limits.
  • Method 1 utilizing the $e^+e^-$ opening angle (assuming $\cos \theta_{e^+e^-} < 0.9$) further extends sensitivity.
  • Method 2, utilizing the solar angle distribution of scattered electrons, provides sensitivity in the high-mixing/short-lifetime region inaccessible to the $e^+e^-$ search.
• Background Rejection: The paper details how spectral fitting and angular cuts (solar angle and opening angle) can distinguish the signal from the dominant solar neutrino background, particularly for detectors capable of directional reconstruction.

Significance
The paper claims that solar neutrino experiments, particularly those with new designs offering precise energy and directional measurements (such as the Jinping Neutrino Experiment), are promising venues for searching for heavy sterile neutrinos in the MeV mass range. By combining the detection of in-detector $e^+e^-$ pairs and out-of-detector $\nu_e$ events, these experiments can significantly extend the sensitivity limits in the $|U_{eH}|^2$ vs. $m_{\nu_H}$ plane, probing regions closer to the seesaw limit that are currently uncovered by collider experiments or previous solar neutrino analyses. The work provides a concrete framework for utilizing existing and future solar neutrino data to test Beyond Standard Model physics in the sterile neutrino sector.