We may finally know how heavy "ghost particles" truly are thanks to this new study

Scientists in Finland have found a rare type of nuclear decay that could help answer one of physics’ biggest open questions: the mass of the electron‑antineutrino. Neutrinos are tiny, almost massless particles that are everywhere in the universe. They are created in huge numbers by processes like nuclear reactions in the Sun, and trillions of them pass through our bodies every second.

“Their mass determination would be of utmost importance,” said Professor Anu Kankainen from the University of Jyväskylä. “Understanding them can give us a better picture of the evolution of the universe.”

Neutrinos and antineutrinos are called "ghost particles" because they have no electric charge, an almost negligible mass, and interact only via the weak nuclear force, allowing trillions to pass through matter—including your body—every second without effect. Their near invisibility makes them extremely difficult to detect, earning them a reputation as elusive messengers from the universe’s most extreme events.

One way to measure the electron‑antineutrino mass is to study the shape of the beta decay spectrum near its endpoint. Beta decay is when a nucleus changes into another nucleus, releasing an electron and an electron antineutrino. The total energy released is called the Q value. If the Q value is very small, it makes the measurement more sensitive.

“Since the electron antineutrino mass is estimated to be at least five orders of magnitude smaller than the electron mass, it is very challenging to observe its contribution to the beta decay,” said doctoral researcher Jouni Ruotsalainen. “To make it more accountable, beta decays which release very little energy, the so‑called low‑Q‑value beta decays, are of particular interest.”

The team focused on a special case: the 6⁺ isomer of silver‑110 (¹¹⁰ᵐAg) decaying to the 5₂⁺ state in cadmium‑110 (¹¹⁰Cd). Earlier data suggested the Q value for this decay, Q₍β, m₎, might be −0.12 (131) keV, which could mean the decay was impossible or barely possible.

To get a clearer answer, the researchers used the JYFLTRAP double Penning trap and a technique called phase‑imaging ion‑cyclotron resonance to measure the mass of stable silver‑109 (¹⁰⁹Ag) with cadmium‑110 as a reference. Combining this with known spectroscopic data, they found Q₍β, m₎ = 405 (135) eV. This is the lowest Q value ever recorded for an allowed beta decay.

“It was quite easy to produce the stable silver and cadmium ions with our existing electric discharge ion sources and measure their mass difference,” Ruotsalainen said. “I was thrilled to see that the resulting Q value is positive and actually the lowest for any allowed beta decay transition discovered so far.”

Shell‑model calculations using the jj45pnb Hamiltonian and atomic data gave a partial half‑life of t₁/₂ = 2.2₋₁.₂₈⁺⁵.²⁴ × 10⁷ years and a branching ratio of Br = 3.0₋₂.¹⁵⁺⁴.¹⁶ × 10⁻⁸. That means about three in every million decays from the isomer take this ultra‑low‑energy path.

“With a half‑life of around 250 days for the isomer itself, and the possibility of producing it via thermal‑neutron capture on stable silver‑109, it becomes a viable candidate for future antineutrino mass measurements,” said researcher Marlom Ramalho.

Kankainen said the combination of an allowed decay, a record‑low Q value, and easy production makes silver‑110 worth further study. “It is nice to see that measurements of stable or near stable isotopes can still be very impactful,” she said.

Source: University of Jyväskylä, APS

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