by Seng-Pei Liew

Recently we have witnessed the birth of gravitational-wave astronomy, as gravitational waves were directly observed for the first time. Subsequently, gravitational waves due to binary neutron star merger were detected along with associated electromagnetic events, opening a new era of multi-messenger astronomy.


The field of high energy physics has arguably seen little progress in comparison. Indeed, the final piece of the Standard Model (SM), the Higgs boson, has been discovered in 2012. There is, however, no other trace of new particles at the LHC, which is promised by many theories arguing that the Higgs mass is “natural”. This drives beyond-the-SM (BSM) enthusiasts like me to near depression.

That no new particle has been discovered (so far) may imply that we are looking in the wrong direction for probing new physics. Instead of searching for new particles directly, alternate approaches are cried out for. Indeed, when life gives us the Higgs, we should figure out ways to search for BSM physics using the Higgs. One way of finding traces of new particles is by making measurements of the Higgs couplings. This is because the Higgs couplings could be significantly affected by the existence of new particles/physics. In fact, it is almost guaranteed that the “naturalness” problem mentioned above could be illuminated by measuring the Higgs couplings.

Since there is no sign of new physics, it is likely that there is a gap in energy scale between the SM and new physics. The presence of BSM, however, can then be parametrized with a set of higher dimensional operators, even without knowing the details of the BSM model. Of particular interest are those related to gluon fusion, the main mechanism of Higgs production at the LHC (taken from here):

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One way (which is of particular interest to this network) of studying these operators is via Higgs pair production at the LHC. The figure below (taken from here) shows Feynman diagrams of double Higgs production in the SM, as well as those induced by the effective operators. Work Packages 1 (focusing on Higgs pair production) and 2 (focusing on new physics searches) of this network are particularly relevant to this research direction. Last but not least, LHC Higgs measurement emerges also as an unprecedented opportunity for applying novel multivariate techniques to analyzing data.

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The results from the LHC are not very encouraging so far. The current situation mirrors the pre-LHC era: there was no direct evidence of “new” physics, i.e. the Higgs, at the Large Electron Positron Collider (LEP) or Tevatron (the top quark was discovered nevertheless) during the 90s However, electroweak measurements had been able to constrain the mass of the Higgs boson indirectly to less than 161 GeV. Perhaps we will encounter such a situation again, where no new particles can be found and Higgs measurements will play an important role, but let us wait and see what future runs of the LHC tell us.