Our understanding of fundamental interactions among the elementary matter constituents, the quarks and leptons, is based on the Standard Model (SM) gauge symmetries [SU(3) x SU(2) x U(1) x General Relativity]. In the last century, it has received an overwhelming empirical confirmation. In particular in the last 20 years several experimental breakthroughs (1998 – discovery of non-zero neutrino masses, 2000 – acceleration of the expansion of the Universe, 2012 – the discovery of the Higgs boson, 2016 – observation of gravitational waves) have reinforced our confidence in its basic principles while exacerbating some of the unanswered questions which have occupied the minds of high-energy physicists for decades.
One leading example is the discovery of the Higgs boson at the LHC, crowned in 2013 with a Nobel Prize to François Englert and Peter W. Higgs for their seminal work on the symmetry breaking mechanism that underlies the SM. Such a discovery on the one hand and the absence so far of any convincing sign of new physics at the TeV energy scale on the other, have shaken the expectations of a whole generation of physicists. Together with other puzzling observations, it has spurred a large-scale and broad experimental program worldwide to explore the fundamental interactions in many complementary directions. Of the confounding and far-reaching questions that the SM leaves unanswered, those which are of direct relevance to this project are:
- What is the origin of electroweak symmetry breaking (EWSB)? This is achieved in the SM through the vacuum expectation of the Higgs field (v=246 GeV). The huge gap between this energy scale and the Planck one (1E19 GeV, where quantum gravity effects are expected to be no longer negligible), arguably the largest scale in Nature, poses a very severe hierarchy problem to the SM, for which we do not have a solution supported by experiment. This is related to the next question:
- Is the discovered Higgs boson the SM scalar? This experimental question could have wide-ranging consequences: finding deviations from the expected properties for the SM scalar or discovering new scalar resonances would signify the existence of new symmetries (supersymmetry?) or dynamics (new strong interactions?) beyond the SM.
- What is the origin of neutrino masses? Are neutrinos Dirac or Majorana fermions? The mechanisms behind the properties of the neutrinos (such as their masses and mixing pattern) originate from physics beyond the SM.
- Why is there matter and (almost) no antimatter in the Universe? This issue is related, through baryogenesis and leptogenesis (which involves right-handed or sterile neutrinos), to time reversal violation at a fundamental level. However, a detailed mechanism for producing this asymmetry is far from being established.
- What is dark matter? We have hints that Dark Matter (DM) may be made of new elementary particles, cold remnants of the Early Universe, but the true nature of dark matter still eludes us. Is it part of a new, hidden sector of particles and forces?
All of the above points to the fact that the SM cannot offer a complete description of the fundamental interactions of our Universe. In addition, it is reasonable to expect that the next breakthrough will come from new experimental results and/or observations. In particular, there is hope that the advent of many new experiments on the high energy, high intensity and cosmological frontiers will shed light on these outstanding problems.