CERN Accelerating science

Charting the Unknown: interpreting LHC data from the energy frontier

by Panos Charitos

The LHC has started its Run2, exploring for the first time the high-energy frontier at 13 TeV. Collected data will give decisive information on the dynamics behind electroweak symmetry breaking and allow testing the weak-scale naturalness. A TH Institute “Charting the Unknown: Interpreting LHC data from the energy frontier” focused on the interpretation of the new data in the context of theories beyond the Standard Model. From 25 July to 12 August, world-leading experts came together for an in-depth discussion towards revealing the final clues of the underlying theory of the electroweak scale. The organizers aimed to have the latest data presented during the workshop in order to trigger interesting discussions. The TH institute partially coincided with ICHEP16, the world’s largest event in particle physics, where the experimental collaborations presented their latest results. 

In the past months, there was a lot of excitement about the 750 GeV diphoton excess that could have signified a new fundamental particle. This exciting hint generated many theory papers attempting to explain the origin of the 750 GeV signal. However, with more data included in the analysis, the bump disappeared, suggesting that it was a statistical fluctuation; thus refuting theories for new physics. The story of the 750 GeV bump gives an interesting example of how new data can completely shift the focus of the scientific community. Theorists are ready to embrace new ideas. The discovery of the Higgs boson has been a crucial chapter in our understanding of the electroweak breaking sector. However our understanding remains incomplete leaving space to many open questions.

For most theorists it is clear that we enter a moment of transition and most of the existing models relt on the argument of naturalness to predict new physics at the LHC. Many of these theories have been excluded with more data from the LHC while it seems that we are reaching a crossroad regarding the open questions about our nature.  Given the latest results from the 13TeV run of the LHC, the TH workshop focused on some of the open questions about the electroweak symmetry breaking; namely the existence of dark matter and the potential candidate particles, the role of naturalness and finally plans for future colliders that will push back further the energy frontier. The programme also included review talks by the LHC experimental collaborations to report on their latest results (including searches for symmetry and exotic particles as well as Higgs studies) and highlight future projections.

It is worth mentioning a novel idea that Savas Dimopoulos presented during the TH workshop on very light particles that can accumulate around a black hole altering the dynamics of the space time. The characteristic radiation (superradiance) from these particles could allow probing axions between 10-20 and 10-10 eV independent of the abundance of dark matter. Dimopoulos suggested that if this is the case then they could be directly observed by analysing the gravitational waves emitted from the collision of two black holes. His talk generated an intense discussion as it is clearly linked to the recent discovery of gravitational waves.

Another inspiring talk pointing to possible new directions in our research was given by David Kaplan. In his talk “The Lifetime Frontier” he suggested that we should search for particles with very long lifetimes (LLPs) that can be one way to go beyond the naturalness paradigm. To detect these particles one should consider building detectors on top of the LHC – as well as outside the cavern – since these very weakly interacting particles could live long and travel up to the surface. In other words, this calls to enlarge the parameter space of the present experiments by several order of magnitude.

The last talk was dedicated about physics in future colliders. LianTao Wan discussed the different options for lepton-lepton, hadron-hadron and lepton-hadron colliders and reviewed the physics potential of these machines. It is evident that future colliders can cover significant ground beyond the LHC and by extending the energy and luminosity frontiers give answers to many of the questions that remain open. The importance of lepton machines was also emphasized during the talk as they can provide a powerful and complementary probe about the energy scale where new physics might lie. In addition it will also shed more light on the nature and the properties of the Higgs boson as it seems to be a very special particle of the Standard Model.

Finally, during the TH institute three colloquia were given on ”Fundamental Physics beyond Colliders” by Mina Arvanitaki, on “New ideas in dark matter direct detection” by Kathryn Zurek and finally on “Spacetime, Quantum Mechanics and the LHC” by Nima Arkani Hamed. Arvanitaki discussed how existing and well-established techniques can improve by several orders of magnitude searches for scalar Dark Matter and the QCD axions. Zurek offered a detailed overview of past indirect and direct searches. In his presentation he shown the interplay between high-energy physics and condensed matter while he also discussed new ideas for the detection of dark matter that could pave the way forward. Finally, Arkani Hamed, in an inspirational talk, discussed the effort to go beyond the quantum field theory in order to get a breakthrough in our present understanding of quantum mechanics and general relativity. This may sound like a simple statement but the maths in doing this step are very complicated. In fact, an interesting suggestion is that we may be missing a more fundamental part of our theory – having models that offer a more complicated description of reality. For Arkani Hamed, complexity stems from the fact that we try to maintain locality in our theories (while conservation of energy & momentum is manifest in all steps).


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