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Physicists hint at a mysterious new 'particle X' messing with our Universe

Exciting times.

FIONA MACDONALD
8 JUN 2016
 

For months, physicists have been quietly freaking out over tantalising evidence of a brand new particle lying outside the standard model of physics. First seen as a 'blip' in Large Hadron Collider data, the hunt is now on to confirm its existence, which experts say would be "bigger than the [discovery of the] Higgs boson".

And physicists have just made the case that another new particle could be waiting to be discovered, by showing that the existence of a mysterious new particle, which they're calling 'particle X', could explain a significant conundrum in physics: where the heck all the missing lithium in the Universe went to.

 

If you haven't heard about the case of the missing cosmic lithium, don't worry, we weren't across it either. But it turns out that scientists have calculated all the lithium that should have formed in the early Universe, and it's about three times more lithium than we observe today.

For more than a decade they've been scratching their heads and trying to figure out what might have happened, and now an international team of physicists have come up with a new hypothesis - a new type of particle, beyond our current understanding of physics, could have interacted with protons and neutrons shortly after the Big Bang to destroy most of the cosmic lithium-7. Hence why we can't find it today.

To be clear, this is just a hypothesis waiting to be tested for now, so let's not get too excited just yet. And this particle X isn't the same one that scientists have been catching glimpses of over at the Large Hadron Collider.

But if the hypothesis was proven, it would be just as important, because it would also be evidence of physics beyond the standard model - the set of rules that, according to our best understanding, govern how the Universe works. For now though, let's just be content with the fact that we have a new idea to investigate.

So how do scientists know how much lithium-7 was supposed to be around in the first place? The initial numbers are based on a hypothesis about what happened at the birth of the Universe, known as the 'Big Bang nucleosynthesis'. 

According to this idea, protons and neutrons fused to form nuclei in the first few minutes after the Big Bang. "This process generated deuterium, large amounts of helium-4 and smaller amounts of helium-3 – the latter two combined to create beryllium-7, which eventually decayed to lithium-7," explains Edwin Cartlidge over at Physics World.

 

Based on this assumption, and observations of the cosmic microwave background, researchers can make very precise predictions about the relative proportions of these nuclei.

But while the predictions for helium and deuterium are pretty spot on, the proposed value for lithium is way off, at anywhere between two to five times too high.

One explanation for this is that some astrophysical process, such as the birth and death of stars, could be gobbling up all the lithium - but no one's been able to identify any behaviour so far that fits the bill. 

Instead, researchers have looked to a particle physics solution for what's going on, and one answer that makes sense is that there's something else in play that we haven't detected yet - a hypothetical particle X.

A team of researchers led by Maxim Pospelov from the Perimeter Institute in Waterloo, Canada, has now calculated that this particle would be electrically neutral and fairly stable, and would interact "fairly strongly with both protons and neutrons, and has a mass lying somewhere between 1.6-20 megaelectronvolts (MeV)," as Cartlidge explains.

According to Pospelov, particle X could get rid of lithium in two ways: either breaking up beryllium nuclei into helium-3 and -4 before they decay into lithium-7; or breaking down deuterium, releasing neutrons that would destroy lithium before reforming, to keep the deuterium levels unchanged.

Pospelov told Cartlidge that their proposal for particle X also fits the description for a long-suspected particle that acts as the mediator between normal matter and dark matter, which is estimated to have a similar mass, between 10 and 30 MeV.

As exciting as all this is, there's still a long way to go before we have evidence on whether particle X might really exist - for now it's all just a hypothesis. Researchers will be able to investigate further by studying the decay of particles known as kaons, as well as through 'beam-dump' experiments that blast an object with protons and electrons. 

In the meantime, physicists are remaining skeptical, with Keith Olive from the University of Minnesota in the US telling Physics World that the paper "serves as more of an example of what might work, rather than as a solution".

Kenneth Nollett of San Diego State University admits that he still thinks there's an astrophysical explanation for the lithium dilemma, but thinks it's worth pursuing the particle idea.

"Forward progress in science requires work on all fronts," he said. We couldn't agree more.

The research has been published in Physical Review Letters.

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