Naturalness at the low and high energy fronts



Gilad Perez (CERN & Weizmann), Matthias Schlaffer (Weizmann Institute of Science)

For several decades the leading strategy to look for new physics followed from the paradigm that the Higgs fine tuning problem is solved via a symmetry principle. This principle predicts that the SM fields are part of a larger multiplet, formally invariant, under some fundamental symmetry. The other component of these multiplets are denoted as SM partner fields. These partners are required to have masses around the electroweak scale and thus we generically expected them to be found at the Large Hadron Collider (LHC) era. This argument is not-loopehole free. But, it is fair to say that it does describe the generic prediction of natural SM extensions. However, despite an intense theoretical and enormous experimental effort done by the LHC experiments at the energy frontier, so far none of these partners was found. Thus, in spite of its long track-record of success, the mere notion of naturalness gets questioned. Recently, a few proposals have emerged to account for the apparent unnatural fine-tuning, forcing the community to reconsider the need for TeV scale new physics. This is a highly speculative topic with potentially ground-breaking implications. These innovative new physics (NP) scenarios inevitably come with unusual signatures, qualitatively different than the ones that have been searched for over the past decades. Therefore, new experimental approaches need to be strategized to actively probe this emerging new physics frontier.