Observation of a broad $p$-wave resonant state in $^{9}$He

Researchers observed a broad $p$-wave resonant state in $^{9}$He at 1.28(1) MeV using two-body invariant-mass spectroscopy following the knockout reaction of $^{11}$Li at approximately 250 MeV/nucleon.

The Gist

The Mystery of the "Ghostly" Helium Nucleus

Imagine you are trying to study a spinning top. Usually, a top spins predictably, stays upright, and eventually slows down. But imagine a top that is so unstable and "jittery" that it barely exists for a fraction of a second before it flies apart into pieces.

That is essentially what scientists are dealing with when they study $^{9}\text{He}$ (Helium-9).

The Background: A Family of Unstable Atoms

In the world of physics, atoms are like tiny solar systems. Most atoms are stable (like the Helium in a party balloon), but there are "extreme" versions that are incredibly heavy and unstable.

The researchers in this paper are looking at Helium-9. Think of Helium-9 as a very fragile house made of Lego bricks. It has a core (Helium-8) and one extra "guest" neutron. The problem is, this guest neutron doesn't want to be there. It’s like a person trying to stand on a vibrating platform; the moment they step on, they are immediately flung off. Because the guest neutron leaves so fast, the whole "house" (the nucleus) is incredibly hard to photograph or measure.

The Problem: Conflicting Clues

For years, scientists have been arguing about what Helium-9 actually looks like.

• Some scientists thought the "guest" neutron stayed for a tiny, tiny moment (a narrow resonance).
• Others thought it was just a passing shadow that never really formed a stable shape at all (a virtual state).

It was like trying to describe a ghost. One person says, "I saw a solid figure for a second!" and another says, "No, it was just a mist that passed through!" Because the previous experiments didn't have enough "light" (data), the results were blurry and contradictory.

The Breakthrough: A High-Speed Collision

The team in this paper decided to use a "high-speed camera" to get a clearer picture.

Instead of trying to build Helium-9 directly, they used a "parent" nucleus called Lithium-11 (which is like a much larger, more complex structure). They blasted this Lithium-11 with a high-energy beam, essentially "knocking out" pieces of it to see what was left behind. This is called a knockout reaction.

By using a massive, high-tech facility (the RIKEN Nishina Center in Japan), they were able to catch the fragments of this explosion with incredible precision.

The Discovery: A "Broad" Personality

Here is what they found: They observed a "broad p-wave resonant state."

In plain English: They found that the Helium-9 nucleus does exist for a brief moment, but it is much "wider" or "fuzzier" than anyone previously thought.

The Analogy:
If previous scientists thought Helium-9 was like a sharp, clear bell ringing (a narrow resonance), this new study shows it is actually more like a deep, heavy thud (a broad resonance). A bell ring is precise and distinct; a thud is messy, wide, and spreads out.

This "broadness" (the width) tells us that the neutron is escaping very, very quickly. The fact that it is a "p-wave" state tells us about the specific "dance move" or orbital path the neutron was taking right before it flew away.

Why Does This Matter?

Why spend millions of dollars to watch a "thud" that lasts for a trillionth of a second?

Because these tiny, unstable nuclei are the "extreme sports" of the universe. By understanding how Helium-9 behaves, scientists can test their mathematical models of how the universe is glued together. If our math can predict the "messy thud" of Helium-9, we know our math is correct for understanding the very building blocks of everything in existence.

Technical Summary

Technical Summary: Observation of a Broad p-wave Resonant State in $^9\text{He}$

1. Problem and Motivation

The structure of $^9\text{He}$ (consisting of an $^8\text{He}$ core and one valence neutron) has been a subject of long-standing debate in nuclear physics. Previous experimental studies have yielded conflicting results regarding its low-lying states:

• The p-wave resonance ($J^\pi = 1/2^-$): Earlier experiments reported varying energy levels and, crucially, very narrow widths ($\Gamma \approx 100\text{--}130$ keV).
• The s-wave virtual state ($J^\pi = 1/2^+$): There is significant disagreement regarding the s-wave scattering length ($a_s$), with values ranging from near zero (weak interaction) to $\le -10$ fm (strong interaction).
• The Conflict: Recent high-resolution elastic scattering data suggested that $^9\text{He}$ might not possess narrow resonances at all, but rather a broad s-wave resonance around 3 MeV.

The lack of consensus on these properties makes it difficult to understand the underlying $^8\text{He}\text{-}n$ interaction and its implications for the structure of the neighboring nucleus, $^{10}\text{He}$.

2. Methodology

The researchers employed two-body invariant-mass spectroscopy to probe the $^9\text{He}$ system.

• Reaction Mechanism: The experiment used the $^{11}\text{Li}(-1p1n)$ knockout reaction at approximately 250 MeV/nucleon. By knocking out one proton and one neutron from the two-neutron halo nucleus $^{11}\text{Li}$, the $^9\text{He}$ nucleus was populated.
• Facility and Instrumentation: The study was conducted at the RIKEN Nishina Center (Japan) using the BigRIPS fragment separator to produce a purified $^{11}\text{Li}$ secondary beam. The SAMURAI spectrometer was used to detect the $^8\text{He}$ fragments, while the NEBULA neutron detector array was used to detect the decay neutrons.
• Data Analysis and Contamination Control: A major challenge was the contamination from $^{10}\text{He}$ three-body phase-space (3PS) decay ($^{10}\text{He} \to ^8\text{He} + 2n$), where only one neutron is detected. The team used Monte Carlo simulations to model this 3PS decay and subtract it from the experimental spectrum.
• Fitting Procedure: The relative energy ($E_{\text{rel}}$) spectrum was fitted using a Breit-Wigner p-wave resonance shape, convoluted with the experimental energy resolution (determined via GEANT4 simulations to be 0.40 MeV at 1.3 MeV), placed on top of a constant background.

3. Key Results

• Observation of Resonance: After contamination subtraction and efficiency correction, a clear p-wave resonant peak was identified.
• Resonance Parameters: The study determined the resonant energy to be $E_0 = 1.28(1)$ MeV and the decay width to be $\Gamma = 0.82(4)$ MeV.
• Comparison with Previous Data:
  • The energy ($1.28$ MeV) is consistent with previous (though statistically limited) transfer and charge-exchange reaction data.
  • The width ($\Gamma \approx 0.8$ MeV) is nearly an order of magnitude larger than previously reported values ($\sim 0.1$ MeV).
• Selection Rules: The absence of a prominent s-wave state in the spectrum is attributed to the selection rules of the $^{11}\text{Li}(p, pd)$ reaction, which preferentially populates the $1/2^-$ configuration.

4. Significance and Implications

• Validation of Nuclear Models: The discovery of a much broader resonance aligns with modern theoretical predictions. The results are consistent with ab initio calculations (Variational Monte Carlo, No-Core Shell Model with Continuum) and the Shell Model in Continuum (CSM). These models suggest the large width is due to the strong single-particle character of the state.
• Resolving Discrepancies: By providing high-statistics data, this work suggests that previous reports of "narrow" resonances were likely due to insufficient statistical significance.
• Structural Insight: This observation provides a crucial benchmark for understanding the $^8\text{He}\text{-}n$ interaction, which is essential for accurately modeling the physics of neutron-rich nuclei near the drip line, specifically the structure of $^{10}\text{He}$.