“It’s not a bad day at work. Five of the biggest fundamental problems in physics seem sorted in one go. The model that can do this, formulated by Guillermo Ballesteros at the University of Paris-Saclay in France and his colleagues, may explain dark matter, neutrino oscillations, baryogenesis, inflation and the strong CP problem.
Dubbed SMASH, the model is based on the standard model of particle physics, but has a few bits tacked on. The standard model is a collection of particles and forces that describes the building blocks of the universe. Although it has passed every test thrown at it, it can’t explain some phenomena.
For example, we don’t understand dark matter, the mysterious substance that makes up 84 per cent of the universe’s mass. Nor why there is more matter than antimatter. Nor why the universe grew so rapidly in its youth during a period known as inflation. The list continues.
So something is still missing from the standard model. “Presumably we need some new particles,” says Mikhail Shaposhnikov at the Swiss Federal Institute of Technology in Lausanne. “The question is, how many new particles do we need?”
Smashing theories together
Some models, like supersymmetry, add hundreds of particles – none of which have been spotted at colliders like the LHC. But SMASH adds only six: three neutrinos, a fermion and a field that includes two particles.
That’s a reasonable approach, Shaposhnikov says. “I would start by assuming that the number of new particles is very small,” he says. “And then add new particles only if you really need them.”
SMASH is several theories smashed together, says co-author Andreas Ringwald at the German Electron Synchrotron, DESY, in Hamburg. It builds on Shaposhnikov’s model from 2005, which added three neutrinos to the three already known in order to solve four fundamental problems in physics: dark matter, inflation, some questions about the nature of neutrinos, and the origins of matter.
SMASH adds a new field to explain some of those problems a little differently. This field includes two particles: the axion, a dark horse candidate for dark matter, and the inflaton, the particle behind inflation.
As a final flourish, SMASH uses the field to introduce the solution to a fifth puzzle: the strong CP problem, which helps explain why there is more matter than antimatter in the universe.
“The best thing about the theory is that it can be tested or checked within the next 10 years or so,” Ringwald says. “You can always invent new theories, but if they can only be tested in 100 years, or never, then this is not real science but meta-science.”
SMASH predicts that the axion should be about ten billion times lighter than the electron. Particles this small could be probed by the CULTASK experiment running in South Korea, or the proposed ORPHEUS experiment in the US and the planned MADMAX experiment in Germany.
This doesn’t mean it’s game over. It’s more like game on. Physicists will continue to compete to find experimental evidence or a better model.
“The battle is open,” Ringwald says.”