New high-sensitivity search for neutron to mirror-neutron oscillations at the PSI UCN source

A high-sensitivity search for neutron-to-mirror-neutron oscillations conducted at the PSI UCN source found no evidence of anomalous neutron losses, thereby excluding 99.98% of the previously claimed parameter space and establishing new limits on the oscillation time constant.

The Gist

The Great Neutron Hide-and-Seek: A New Search for "Mirror" Particles

Imagine the universe as a giant, bustling city. We know about all the people living here: electrons, protons, and neutrons. But what if there was a parallel city right next to ours, a "Mirror City," inhabited by exact twins of everyone in our city? These "mirror people" look just like us, but they are invisible to us. They don't talk to us, they don't bump into us, and we can't see them with our eyes. They only interact with us through gravity.

Physicists call these invisible twins Mirror Neutrons.

For a long time, scientists have wondered: Could a regular neutron from our world suddenly turn into a mirror neutron and vanish into the Mirror City? If this happened, it would explain some weird mysteries in physics, like why there is more matter than antimatter in the universe, or even what "Dark Matter" is made of.

The Previous Mystery: "Ghostly" Disappearances

In the past, a few experiments suggested that neutrons were indeed disappearing at a rate that didn't make sense. It was as if neutrons were sneaking out of a room through a secret door that shouldn't exist. Some scientists thought they had found evidence of these "Mirror Neutrons."

However, other experiments couldn't find this secret door. The mystery remained: Was the door real, or was it just a glitch in the measurement?

The New Experiment: The Ultimate Search

A team of scientists at the Paul Scherrer Institute (PSI) in Switzerland decided to build the most sensitive "ghost detector" ever to solve this mystery once and for all. They used Ultra-Cold Neutrons (UCNs).

Think of these neutrons like slow-motion bowling balls. Because they are so cold (almost absolute zero), they move very slowly. The scientists put them inside a shiny, empty metal tank (the "storage vessel") and waited to see if any of them disappeared.

The Setup:

1. The Trap: They trapped thousands of these slow neutrons in a magnetic field.
2. The Test: They flipped the magnetic field back and forth (like flipping a switch from North to South).
3. The Theory: If mirror neutrons exist, the "secret door" would open and close depending on the direction of the magnetic field. If the door opened, neutrons would vanish into the mirror world and never come back.

The Challenge: The "Noisy Room"

The tricky part is that the magnetic field inside the tank isn't perfectly uniform. It's like trying to hear a whisper in a room where the wind is blowing unevenly. To fix this, the team built a 3D Magnetic Mapper.

Imagine sending a tiny, robotic spider with five eyes (sensors) crawling all over the inside of the tank to map every single tiny fluctuation of the magnetic field. They did this to create a perfect 3D map of the "wind" inside the tank. This allowed them to calculate exactly how likely a neutron was to slip through the secret door at any specific spot.

The Results: The Door is Locked

After running thousands of tests in 2021, the team counted the neutrons that came back.

• The Expectation: If the "Mirror Neutron" theory was right, they should have seen a specific pattern of missing neutrons when they flipped the magnetic field.
• The Reality: The number of neutrons that disappeared was exactly what you would expect from normal physics (like neutrons just decaying naturally). There were no extra disappearances.

The "secret door" to the Mirror City was not found.

What Does This Mean?

The scientists used their data to draw a giant net over the possible "locations" where these mirror neutrons could hide.

• They scanned a huge range of magnetic strengths.
• They checked almost every possible angle the "mirror world" could be tilted.
• The Verdict: They have now ruled out 99.98% of the area where scientists previously thought the "Mirror Neutron" signal might be hiding.

In simple terms:
If the "Mirror Neutron" was a ghost hiding in a giant house, previous experiments thought they heard a noise in the living room. This new experiment checked the living room, the kitchen, the attic, and the basement with super-sensitive ears. They found no ghosts. The "ghosts" were likely just the house settling (statistical noise).

Why Does This Matter?

Even though they didn't find the mirror neutrons, this is a huge success.

1. It clears the air: It tells us that the weird signals seen in the past were likely mistakes, not new physics.
2. It narrows the search: Scientists now know exactly where not to look. If mirror neutrons exist, they must be hiding in a very tiny, very specific corner of the universe that this experiment couldn't quite reach (less than 0.02% of the possibilities).
3. It proves our tools work: The experiment showed that we can measure the quantum world with incredible precision, using magnetic maps and slow neutrons.

The Bottom Line:
The universe is still full of mysteries, but the "Mirror Neutron" hiding in the spot everyone was looking at? It's not there. The search continues, but now we know the map is much clearer than before.

Technical Summary

1. Problem Statement and Motivation

The paper addresses the search for neutron ($n$) to mirror-neutron ($n'$) oscillations, a phenomenon predicted by theories proposing a "mirror sector" of particles that are exact duplicates of Standard Model particles but sterile to ordinary interactions (except gravity and potentially feeble interactions).

• Context: Previous experiments using Ultra-Cold Neutrons (UCNs) established upper bounds on the oscillation time constant ($\tau_{nn'}$). However, re-analyses of data from earlier experiments (specifically [37, 40, 42]) revealed significant deviations from the null hypothesis (3$\sigma$, 5$\sigma$, and 2.5$\sigma$ effects).
• The Anomaly: These deviations were hypothesized to be caused by $n-n'$ oscillations resonantly amplified by a mirror magnetic field ($B'$) present on Earth. The oscillation probability depends on the alignment and magnitude of the applied magnetic field ($B$) relative to this unknown $B'$.
• The Gap: While a previous PSI experiment (PSI-2021) excluded a large portion of the parameter space, specific regions—particularly those involving specific magnitudes and directions of $B'$—remained untested. The goal was to investigate these remaining parameter spaces with higher sensitivity.

2. Methodology

Experimental Setup

The experiment was conducted at the Paul Scherrer Institute (PSI) UCN source using a dedicated apparatus installed in Area West.

• Apparatus: A 1.5 m³ stainless steel storage vessel (1 m high) surrounded by a system of eight rectangular coils capable of generating tunable magnetic fields in the range 5 $\mu$T < $B$ < 109 $\mu$T.
• Detection: UCNs were stored for 180 seconds and then counted using a CASCADE detector for 95 seconds.
• Field Control: The magnetic field direction was reversed between cycles (pattern ABBA BAAB) to measure directional asymmetry. The system utilized 16 three-axis fluxgates for external monitoring and a custom 3D magnetic field mapper (containing five fluxgates on a rotating/vertical probe) to characterize the internal field inhomogeneities with high precision.

Theoretical Framework and Analysis

• Oscillation Probability: The probability of $n \to n'$ transition depends on the free flight time, the ordinary field $B$, the mirror field $B'$, and the angle $\beta$ between them. In the presence of a non-zero $B'$, the transition is suppressed at $B=0$ but resonantly amplified when $B \approx B'$.
• Asymmetry Observable ($A_B$): The experiment measured the asymmetry in neutron counts between positive ($+B$) and negative ($-B$) field polarities:
  $$A_B = \frac{n(+B) - n(-B)}{n(+B) + n(-B)}$$
  Conventional losses (decay, wall collisions) cancel out in this ratio, isolating potential $n-n'$ losses.
• Handling Inhomogeneity: Recognizing that the magnetic field inside the vessel is not perfectly homogeneous, the authors employed Monte Carlo (MCUCN) simulations. They mapped the field gradients and calculated the cumulative oscillation probability for individual neutron trajectories.
• Resonance Functions: The analysis introduced "resonance functions" ($F_A^i$) derived from simulations to relate the measured asymmetry to the oscillation time $\tau_{nn'}$, accounting for the unknown direction (angles $a, b$) of the mirror field $B'$.

3. Key Contributions

1. High-Sensitivity Apparatus: Deployment of a dedicated setup with a wide magnetic field scanning range (5–109 $\mu$T) and a sophisticated 3D magnetic field mapping system to characterize field inhomogeneities.
2. Advanced Simulation: Integration of detailed magnetic field maps into Monte Carlo simulations to accurately model the cumulative oscillation probability for neutrons in an inhomogeneous field, moving beyond the assumption of a homogeneous field used in simpler analyses.
3. Comprehensive Parameter Space Scan: A systematic scan of the remaining parameter space claimed by previous anomalies, explicitly treating the mirror field direction as a variable to be marginalized over.

4. Results

• Null Result: The measured mean asymmetry $\langle A_B \rangle$ was consistent with the null hypothesis (zero asymmetry) within standard errors across all target fields. No evidence of anomalous neutron losses was found.
• Exclusion Limits:
  • New limits for the oscillation time constant $\tau_{nn'}$ were established as a function of the mirror field magnitude $B'$ and direction.
  • 99.98% Exclusion: The experiment excluded the parameter space previously claimed for potential signals (specifically the anomalies reported in [42]) to 99.98% confidence within the full $4\pi$ solid angle of directional coordinates.
  • Robustness: Even when assuming the anomaly signals were artificially increased by factors of 2 and 5, the experiment could still exclude 99.62% and 89.84% of the solid angle, respectively.
  • Remaining Region: A tiny region ($| \cos(b) | - 1 < 10^{-4}$) where the mirror field vector is nearly perpendicular to the vertical axis remains unexcluded due to lower sensitivity in that specific orientation.

5. Significance

• Resolution of Anomalies: This study effectively rules out the "mirror magnetic field" explanation for the previously observed anomalies in neutron loss data within the vast majority of the possible parameter space.
• Dark Matter Constraints: By constraining $n-n'$ mixing, the results place tighter bounds on mirror matter models, which are candidates for dark matter.
• Methodological Benchmark: The combination of high-precision magnetic field mapping with Monte Carlo trajectory simulations sets a new standard for future searches for exotic neutron transitions and sterile particle mixing.
• Future Directions: While the primary anomaly regions are excluded, the paper notes that complementary searches (e.g., neutron spin-precession frequency variations) are still needed to probe very small mirror fields ($B' < 1 \mu$T) outside the range of this experiment.

In conclusion, the PSI collaboration has performed the most sensitive search to date for neutron-mirror neutron oscillations, successfully excluding the parameter space associated with previous experimental anomalies with 99.98% confidence, thereby significantly narrowing the search for mirror matter.