Spot the strange particles ESO/T.Preibisch

Ghostly particles could be haunting our universe. A new theory claims that the cosmos is full of unseen particle families that don’t interact with each other. If true, the model could explain why gravity is so puzzlingly weak.

The idea is an alternative to supersymmetry, a theory in which every known particle has a heavier partner. Such superparticles would explain why the mass of our familiar set of particles is low enough to account for gravity’s weakness. But the particle-smashing Large Hadron Collider, near Geneva, Switzerland, still hasn’t seen any superparticles, despite years of searching.

Maybe that’s because we need not just one set of partner particles, but many, says Nima Arkani-Hamed at Princeton University.


“The idea is a little wild,” admits Tim Cohen at the University of Oregon in Eugene, who teamed up with Arkani-Hamed and others to work on the new theory. “The absence of new physics at the LHC has motivated us to – instead of introducing a few new particles – introduce 1016 new particles.”

The group began by tweaking the story of what happened in the big bang. After its birth, the universe inflated until it was cold and flat. Arkani-Hamed’s team believes that at this point, all the energy was locked up in the form of a single kind of particle, which they call the reheaton.

Then, because particles always want to decay into something less energetic, the reheatons broke down into smaller particles – the ones that structure the universe today.

Next, the physicists took our familiar standard model of particles, and introduced many copies of it into the story. These families live side-by-side, but scarcely interact with each other. Each family has the same kinds of particles, but a slightly different mass for its Higgs boson – the famous “missing piece” of the standard model, which was found to have a surprisingly light mass by the LHC in 2012. Because the Higgs gives mass to other particles, varying it leads to big differences in how much the particles in a group pull on each other.

Extreme families

If the universe was full of particles from families with very heavy or zero-mass Higgs bosons, then gravity between particles would be too strong or too weak for atoms to form. Only one family would have a Higgs boson just light enough to let planets and people develop – the Goldilocks amount of gravity. So how did our universe come to have more of the right sort of particles?

The team says that for families with a Higgs mass just above zero, the reheatons would have decayed into one Higgs boson and another particle of the remaining energy. This wouldn’t have been the case for other families. Particles can only decay into things lighter than themselves, so for families with high Higgs boson masses, the reheatons would have decayed into two smaller entities – instead of the Higgs – plus the remainder particle. Particles prefer to decay into fewer entities, so most of the big bang’s energy would be funnelled into light-Higgs families, and only a bit to those with slightly heavier Higgs bosons.

As a result, our familiar standard model of particles – which has the lightest possible non-zero-mass Higgs – reigns supreme.

That’s one step towards solving the weak-gravity problem. The next step comes from the team’s observation that the equation for gravity between particles can be fine-tuned by adjusting the number of particle sectors.

The team calculated that about 1016 particle families results in a small value for gravity’s strength. If you combine the effect of large numbers of particles on the gravity equation and the reheatons favouring light Higgs bosons, you get gravity just weak enough for playing basketball instead of collapsing into a singularity, says Cohen.

Occam’s razor

Some physicists are sceptical of this first incarnation of the new theory. Assuming that there are huge numbers of particle classes isn’t an appealing solution, says Peter Woit at Columbia University in New York. “It’s a huge violation of Occam’s razor,” he says.

“This is not a bad idea – in fact, it’s a pretty clever idea in terms of how things might work,” says Matt Strassler at Harvard University. However, the mathematics used to flesh out the idea is pretty contrived, he says. Hopefully now that the idea is out there, the model can be improved, he adds.

Paddy Fox at the Fermi National Accelerator Laboratory near Batavia, Illinois, says there’s a way to test this theory within the next 10 years. A few particles from the sets with slightly heavier Higgses probably got produced. We could see these ghostly particle families haunting the universe via their effects on the cosmic microwave background – leftover radiation from the universe’s birth.

“There’s an exciting possibility that you could see such an imprint in the next round of experiments,” says Fox. “This is not something we have to wait around and one day hope to see.”

This means the search for new particles could move out of underground colliders and into astronomy, says Cohen. So don’t call the Ghostbusters until the astronomy results are in.

Journal reference: arxiv.org/abs/1607.06821

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