Durieux, G., Irles, A., Miralles, V., Peñuelas, A., Perello, M., Poschl, R., et al. (2019). The electro-weak couplings of the top and bottom quarks – Global fit and future prospects. J. High Energy Phys., 12(12), 098–44pp.
Abstract: We evaluate the implications of LHC and LEP/SLC measurements for the electro-weak couplings of the top and bottom quarks. We derive global bounds on the Wilson coefficients of ten two-fermion operators in an effective field theory description. The combination of hadron collider data with Z -pole measurements is found to yield tight limits on the operator coefficients that modify the left-handed couplings of the bottom and top quark to the Z boson. We also present projections for the high-luminosity phase of the LHC and for future electron-positron colliders. The bounds on the operator coefficients are expected to improve substantially during the remaining LHC programme, by factors of 1 to 5 if systematic uncertainties are scaled as statistical ones. The operation of an e(+)e(-) collider at a center-of-mass energy above the top-quark pair production threshold is expected to further improve the bounds by one to two orders of magnitude. The combination of measurements in pp and e(+)e(-) collisions allows for a percent-level determination of the top-quark Yukawa coupling, that is robust in a global fit.
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Bonnet, F., Hirsch, M., Ota, T., & Winter, W. (2013). Systematic decomposition of the neutrinoless double beta decay operator. J. High Energy Phys., 03(3), 055–34pp.
Abstract: We discuss the systematic decomposition of the dimension nine neutrinoless double beta decay operator, focusing on mechanisms with potentially small contributions to neutrino mass, while being accessible at the LHC. We first provide a (d = 9 tree-level) complete list of diagrams for neutrinoless double beta decay. From this list one can easily recover all previously discussed contributions to the neutrinoless double beta decay process, such as the celebrated mass mechanism or “exotics”, such as contributions from left-right symmetric models, R-parity violating supersymmetry and leptoquarks. More interestingly, however, we identify a number of new possibilities which have not been discussed in the literature previously. Contact to earlier works based on a general Lorentz-invariant parametrisation of the neutrinoless double beta decay rate is made, which allows, in principle, to derive limits on all possible contributions. We furthermore discuss possible signals at the LHC for mediators leading to the short-range part of the amplitude with one specific example. The study of such contributions would gain particular importance if there were a tension between different measurements of neutrino mass such as coming from neutrinoless double beta decay and cosmology or single beta decay.
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Helo, J. C., Hirsch, M., & Ota, T. (2016). Long-range contributions to double beta decay revisited. J. High Energy Phys., 06(6), 006–32pp.
Abstract: We discuss the systematic decomposition of all dimension-7 (d = 7) lepton number violating operators. These d = 7 operators produce momentum enhanced contributions to the long-range part of the 0 nu beta beta decay amplitude and thus are severely constrained by existing half-live limits. In our list of possible models one can find contributions to the long-range amplitude discussed previously in the literature, such as the left-right symmetric model or scalar leptoquarks, as well as some new models not considered before. The d = 7 operators generate Majorana neutrino mass terms either at tree-level, 1-loop or 2-loop level. We systematically compare constraints derived from the mass mechanism to those derived from the long-range 0 nu beta beta decay amplitude and classify our list of models accordingly. We also study one particular example decomposition, which produces neutrino masses at 2-loop level, can fit oscillation data and yields a large contribution to the long-range 0 nu beta beta decay amplitude, in some detail.
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Fileviez Perez, P., Murgui, C., & Plascencia, A. D. (2020). Axion dark matter, proton decay and unification. J. High Energy Phys., 01(1), 091–18pp.
Abstract: We discuss the possibility to predict the QCD axion mass in the context of grand unified theories. We investigate the implementation of the DFSZ mechanism in the context of renormalizable SU(5) theories. In the simplest theory, the axion mass can be predicted with good precision in the range m(a) = (2-16) neV, and there is a strong correlation between the predictions for the axion mass and proton decay rates. In this context, we predict an upper bound for the proton decay channels with antineutrinos, tau(p -> K+(nu) over bar) less than or similar to 4 x 10(37) yr and tau(p -> pi(+)(nu) over bar) less than or similar to 2 x 10(36) yr. This theory can be considered as the minimal realistic grand unified theory with the DFSZ mechanism and it can be fully tested by proton decay and axion experiments.
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Cepedello, R., Hirsch, M., & Helo, J. C. (2017). Loop neutrino masses from d=7 operator. J. High Energy Phys., 07(7), 079–21pp.
Abstract: We discuss the generation of small neutrino masses from d = 71 -loop diagrams. We first systematically analyze all possible d = 7 1 -loop topologies. There is a total of 48 topologies, but only 8 of these can lead to “genuine” d = 7 neutrino masses. Here, we define genuine models to be models in which neither d = 5 nor d = 7 tree -level masses nor a d = 5 1 -loop mass appear, such that the d = 7 1 -loop is the leading order contribution to the neutrino masses. All genuine models can then be organized w.r.t. their particle content. We find there is only one diagram with no representation larger than triplet, while there are 22 diagrams with quadruplets. We briefly discuss three minimal example models of this kind.
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