A fifth fundamental force? (Geraint Lewis)

The scientific media was abuzz recently with hints that a fifth force of nature had been seen. If confirmed, this would be big news, a revolution of our understanding of the fundamental workings of the universe. But should we get getting excited just yet?

The story begins with a team of Hungarian physicists bombarding a target of lithium-7 with protons, resulting in an excited state of beryllium-8. This then decays to the ground-state with the emission of a positron-electron pair. By examining the properties of these ejected particles, the Hungarian team was searching for the potential signature of an additional particle playing a role in the decay, an unseen intermediate that exists for only a moment before itself decaying.

Earlier this year, the team, led by Attila Krasznahorkay, announced an anomaly in their results: an unexpected peak indicating existence of an unknown particle at an energy of about 17 MeV. Faced with this anomaly, the team offered several possible candidates, such a dark matter candidate, or dark photon, or a previously unknown boson.

That’s where things stood until the anomaly was re-examined by American researchers led by Jonathan Feng. They claim their theoretical analysis supports the idea that the beryllium decay anomaly is due to the existence of a massive ‘X-boson’, a mysterious name for a mysterious particle.

Of course, fundamental bosons have an important role in modern physics, and the X-boson would join the photon, gluon, W±, Z and the theoretical graviton in carrying fundamental forces. With its mass, this X-force would operate on very short ranges, on the scale of ~10 femtometres, and would couple to quarks and electrons. If real, it would open an exciting new avenue of research into fundamental physics.

But here we need caution! Why hasn’t this relatively light mass boson been seen in other experiments? Physics has a history of finding experimental anomalies that eventually melt away with additional data. While the Hungarian detection is claimed to be significant at 6.5-sigma, many physicists will require more measurements before being convinced.

And even if the detection is confirmed, the theoretical speculation has only just begun, and we can expect a growing flurry of papers to appear on the arxiv.

So have we discovered a fifth force fundamental force in the universe? At the moment, the answer is a simple ‘we just don’t know’.

Geraint Lewis
Sydney Institute for Astronomy