The hypothetical inflaton is almost certainly not the particle behind the universe’s rapid expansion soon after the Big Bang. This is according to an international collaboration of physicists working at the LHCb experiment on the Large Hadron Collider (LHC) at CERN, who have been looking for traces of the inflaton in the decay of B+ mesons. Back in 1981, Alan Guth proposed a new model of the early universe to explain why it looks the same in all directions today. He theorized that after the Big Bang the universe initially expanded slowly, allowing time for matter to interact and the temperature to level out. Then, there was a very short, extremely fast expansion of space–time, which happened so rapidly that the universe now appears uniform throughout. For such an expansion to take place, however, there must have been a force field behind it. “A new [force] field always means the existence of a particle that is the carrier of the effect,” explains team member Marcin Chrzaszcz from the Institute of Nuclear Physics of the Polish Acadamy of Sciences (IFJ PAN). For a while, it was thought that this particle was the Higgs boson – however, when it was observed in 2012, the boson was too heavy to be the correct candidate. So theoreticians proposed a new particle called the inflaton, which had the properties of the Higgs boson but a smaller mass. To prove its existence, physicists looked at the decay of B+ mesons, which sometimes decay into K+ mesons and Higgs bosons. According to quantum mechanics, the near-identical nature of the “brother” particles means that they transform and oscillate between each other, so the Higgs boson should then convert into the inflaton. Rather than directly measuring the inflaton or Higgs, the LHCb detects their decay into a muon and antimuon. “Depending on the parameter describing the frequency of the inflaton–Higgs oscillation, the course of B+ meson decay should be slightly different,” Chrzaszcz explains. “We found nothing. We can therefore say with great certainty that the light inflaton simply does not exist.” The work is presented in Physics Review D.