QCD and strongly coupled gauge theories : challenges and perspectives

We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe

Testing the low scale seesaw and leptogenesis

Heavy neutrinos with masses below the electroweak scale can simultaneously generate the light neutrino masses via the seesaw mechanism and the baryon asymmetry of the universe via leptogenesis. The requirement to explain these phenomena imposes constraints on the mass spectrum of the heavy neutrinos, their flavour mixing pattern and their $CP$ properties. We first combine bounds from different experiments in the past to map the viable parameter regions in which the minimal low scale seesaw model can explain the observed neutrino oscillations, while being consistent with the negative results of past searches for physics beyond the Standard Model. We then study which additional predictions for the properties of the heavy neutrinos can be made based on the requirement to explain the observed baryon asymmetry of the universe. Finally, we comment on the perspectives to find traces of heavy neutrinos in future experimental searches at the LHC, NA62, BELLE II, T2K, SHiP or a future high energy collider, such as ILC, CEPC or FCC-ee. If any heavy neutral leptons are discovered in the future, our results can be used to assess whether these particles are indeed the common origin of the light neutrino masses and the baryon asymmetry of the universe. If the magnitude of their couplings to all Standard Model flavours can be measured individually, and if the Dirac phase in the lepton mixing matrix is determined in neutrino oscillation experiments, then all model parameters can in principle be determined from this data. This makes the low scale seesaw a fully testable model of neutrino masses and baryogenesis.Comment: We corrected errors in the experimental sensitivities and in the discussion of the full testability of the model. We also added and updated plots and references. 37 pages plus appendix, 12 figure

Kubo relations and radiative corrections for lepton number washout

Bödeker D, Laine M. Kubo relations and radiative corrections for lepton number washout. Journal of Cosmology and Astroparticle Physics. 2014;2014(05):041.The rates for lepton number washout in extensions of the Standard Modelcontaining right-handed neutrinos are key ingredients in scenarios forbaryogenesis through leptogenesis. We relate these rates to real-timecorrelation functions at finite temperature, without making use of any particleapproximations. The relations are valid to quadratic order in neutrino Yukawacouplings and to all orders in Standard Model couplings. They take into accountall spectator processes, and apply both in the symmetric and in the Higgs phaseof the electroweak theory. We use the relations to compute washout rates atnext-to-leading order in g, where g denotes a Standard Model gauge or Yukawacoupling, both in the non-relativistic and in the relativistic regime. Even inthe non-relativistic regime the parametrically dominant radiative correctionsare only suppressed by a single power of g. In the non-relativistic regimeradiative corrections increase the washout rate by a few percent at hightemperatures, but they are of order unity around the weak scale and in therelativistic regime