One or many?

How many Higgs bosons are out there? Several theoretical models foresee the existence of more than one such boson but the current data cannot confirm any of these scenarios. What should we expect from the future data and the results of the current analysis?


Image: Xavier Cortada (with the participation of physicist Pete Markowitz), "In search of the Higgs boson: H -> ZZ", digital art, 2013. Please note that this image is a detail from Xavier Cortada's art work installed at LHC P5 CMS.

According to some theories, the mechanism that requires the existence of the Higgs boson (known by physicists as “electroweak symmetry breaking”) could also imply the involvement of other spin-zero – “boson” – particles. How would these relate to the newly-discovered particle? “The properties that the other Higgs bosons would have vary a lot depending on the theoretical model you consider,” explains Gian Giudice, member of the Theoretical Physics Unit at CERN. “The theory of supersymmetry predicts the existence of replicas of the Higgs field and these replicas would be associated with new spin-zero particles that could be neutral or charged. Instead, if the Higgs boson were a composite particle, we would expect to find new heavy short-lived states of matter with various units of spin.”

So far, the data analysis seems to show a very standard Higgs boson. However, the information we have is not enough to rule out most of the theoretical models. “There is still room for speculation in this field,” confirms Gian Giudice. “For us it is hard to swallow that the minimal Higgs structure, which is so inapt to address many fundamental issues, could be the full story. There are good reasons to believe that the Higgs boson is only the first native we have encountered in an as yet unexplored, but vastly populated, new territory.”

The discovery of the Higgs boson was a great achievement that required a huge effort from all the scientists involved in the data analysis. Searching for additional Higgs bosons is an even greater challenge. “The lack of clear hints in favour of a specific theory forces researchers to follow many different avenues,” says Gian Giudice. “Some of these avenues do not lead to easy discoveries. Take the example of the Higgs replicas in supersymmetry. Present experimental data constrain the new Higgs fields to have only a marginal role in the breaking of electroweak symmetry. As a result, the production of new Higgs bosons is a relatively rare event at the LHC and their discovery requires a data-set larger than the one required for the by-now familiar Higgs boson.”

In the sea of speculations that will take several years of data collection to explore, there is one regarding the matter-antimatter asymmetry in the Universe – one of the most intriguing mysteries that physicists are trying to unveil. “Some theorists have hypothesised that the Higgs field could have triggered the matter-antimatter asymmetry a tenth of a billionth of a second after the Big Bang,” explains Gian Giudice. “The information that we have so far does not confirm this scenario. But, if we postulate the existence of new particles involved in the Higgs mechanism, this scenario could be reconsidered.”

by Antonella Del Rosso