Belle Experiment Sets Toughest Limits Yet on Invisible Particles in Rare B Decays

Three of the five decay channels—specifically B⁺ → π⁺ X, B⁺ → Dₛ⁺ X, and B⁺ → p X—had never been directly searched for before this study . To identify the invisible particle, the team employed a clever 'B-tagging' technique. Since B mesons at KEKB are produced in pairs, they fully reconstructed one B meson to precisely infer the properties of its partner. The partner they were interested in was the one that decayed into a known track plus the tell-tale missing energy .

After all the analysis, no significant excess of missing-energy events above the expected background was observed. Consequently, the collaboration set 90% confidence level (CL) upper limits on the branching fractions that range between 10⁻⁴ and 10⁻⁶. In simpler terms, the probability of these invisible-harbouring decays is, at most, between one in ten thousand and one in a million, depending on the mass of the hypothetical particle .

Immediate Squeeze on Axion-like Particles and Dark Scalars

This absence of a signal delivers an immediate reality check for a broad class of theoretical models. Invisible FIPs, including axion-like particles (ALPs) and dark scalars, are common predictions in theories trying to explain dark matter. The strength with which they interact with Standard Model particles directly dictates how frequently they would appear in B decays. By finding no signal, the Belle analysis directly translates into tighter limits on the possible interaction strengths, or coupling constants, of these particles .

These new limits don't rule out ALPs or dark scalars entirely—they could still exist with interactions too weak for Belle to detect. However, they substantially narrow the allowed parameter space, essentially herding future experimental efforts toward the most promising theoretical hiding spots .

A First Direct Blow to B-mesogenesis

Perhaps the most impactful result is from the channel involving a proton: B⁺ → p X. This provides the first direct experimental constraint on the 'B-mesogenesis' mechanism. This bold theoretical scenario suggests that in the very early universe, the decays of B mesons generated an excess of antimatter that was then funnelled into a dark sector, leaving behind the matter-dominated universe we see today .

The Belle collaboration's upper limits on this decay are now strong enough to rule out the mechanism for a range of dark-sector particle masses, placing significant pressure on the model. However, the squeeze isn't fatal yet. A recent theoretical paper notes that to definitively test B-mesogenesis, experimental limits on the branching fraction of B⁺ → p + missing energy would need to be pushed down to the level of 10⁻⁷ or 10⁻⁸—a sensitivity that's still a hundred to a thousand times tougher than Belle's new limits .

The Path Forward with Belle II

These groundbreaking constraints are, in many ways, a starting gun. They are largely statistically limited, meaning the Belle data sample is simply not big enough to sniff out extremly rare decays. The upgraded Belle II experiment, now operating at the SuperKEKB collider, has already collected a data sample many times larger than Belle’s and is on track to eventually gather 50 times more data .

With this much larger fire hose of data, Belle II will be able to improve the sensitivity on these invisible decay channels by orders of magnitude, probing far deeper into the theoretical wilderness for all the models covered by this search. The Belle results therefore serve as both a critical benchmark and a launchpad for the next generation of discovery, pointing Belle II directly at the mass ranges and theoretical models where a breakthrough could be hiding .