Since its discovery in 2012, the Higgs boson has been in the spotlight for both experimentalists and theorists. In addition to its confirmed role in the mass mechanism, recent papers have discussed its possible role in the inflation of the universe and in the matter-antimatter imbalance. Can a single particle be responsible for everything?
“Since 2012 we have known that the Higgs boson exists, but its inner properties are yet to be completely uncovered,” says Gian Giudice, a member of the CERN Theory Unit. “Precise measurements of its decay modes are still ongoing and the LHC Run 2 will be essential to understand the nature of this particle at a deeper level.”
What we know is that this boson is not “yet another particle” among the hundreds that we deal with every day in physics labs. In agreement with the Standard Model theory, the recent experimental data confirms that the particle discovered by the CERN experiments is the key particle of the Brout-Englert-Higgs mechanism, which explains the origin of the mass of subatomic particles. Is this unique feature enough to make it “the” particle that shaped our entire universe? “The discovery of the Higgs boson has indeed opened new avenues to both cosmology and particle physics,” says Gian Giudice. “Many studies have been published about its possible role in shaping the early history of our universe, but the theoretical situation is far from clear.”
According to some theoretical models, the Higgs boson could be the ‘inflaton’, the particle responsible for the rapid expansion that the universe went through in its first moments. “The identity of the inflaton is still a mystery and it was an intriguing clue to find that the Higgs boson and the inflaton share some basic features,” explains Giudice. “However, the Standard Model interactions are not sufficient to generate inflation unless we introduce an anomalously strong coupling between the Higgs boson and gravity. Such strong coupling makes calculations unreliable, and the question of whether we can identify the Higgs boson with the inflaton is still much debated among theoreticians. In my opinion, the Higgs boson needs the assistance of other new particles to generate inflation.”
The situation is similarly fuzzy when we look at the matter-antimatter imbalance. “In some theoretical models, this imbalance is created during the primordial phase transition that led to the formation of the special state of Higgs field that we observe in the universe today,” explains Giudice. “In such models, the Higgs boson has a leading role in generating the asymmetry between matter and antimatter, but new particles and new interactions beyond the Standard Model are certainly needed to make the idea work,” confirms Giudice.
The overall situation remains unclear: while several theoretical studies are trying to explore deeply the different possible facets of the Higgs field, others are focusing on alternative scenarios. “A simpler explanation of the matter-antimatter asymmetry is offered by neutrinos if, as generally believed, neutrinos also have components in which spin and velocity point in the same direction,” says Giudice. He goes on to explain: “Originally in the universe all neutrino components were in thermal equilibrium. As the universe cooled, the neutrino components with more feeble interaction went out of equilibrium and eventually decayed, leaving behind a small excess of matter over antimatter.”
The new high-energy data from Run 2 will be like a more powerful magnifying glass for physicists to look closer at this and many other fundamental processes that nature has so far kept out of our reach.