Leptoquarks: flavour anomalies, LHC and model building
Final Report Abstract
The Standard Model (SM) of particle physics continues to be a very successful description of the microscopic world which has also been confirmed to a large extent by the experiments at the LHC in the last years. However, also some anomalies have been reported for certain decays of B-mesons hinting for physics beyond the SM. In particular certain ratios show sizeable deviations from the SM predictions hinting that lepton flavour universality could be violated. This can be caused by so-called leptoquarks which are bosons coupling simultaneously to leptons and quarks. A particular interesting one is the U1-leptoquark, which has spin one. It is fairly easy to add such degrees of freedom to the SM by hand. However, eventually this should be embedded in a larger framework to ensure the consistency of the complete picture. One possibility to get such an U1-leptoquark is to postulate at higher energy scales a SU (4) gauge group which gets spontaneously broken to SU (3)C × U (1)B−L where SU (3)C is the gauge group of the strong forces and U (1)B−L gives rise to an additional Z vector boson. Within this project we have taken as gauge group SU (4) × SU (2)L × U (1)R which can be embedded at a hypothetical scale of grand unification into SO(10). In this class of models one has in addition scalar leptoquarks. We have studied their impact on the anomalies in the B-sector and showed that they cannot be neglected. We have also shown that there are stringent constraints on the underlying model parameters due to the wealth of data, in particular in the lepton and K-meson sectors, which agree with the SM predictions. We have found that there is a rather specific part of the parameter space which can explain some of the observed anomalies, namely RK and R∗K, while being consistent with all flavour observables. It also predicts that RD and RD∗ should be close to one. Moreover, in this part of the parameter space, the branching ratios of the scalar leptoquarks are predicted leading to an unique test of this model at either the LHC or in the early phase of a prospective 100 TeV pp-collider. As a by-product of this investigations we have extended the capabilities of the public program SARAH and contributed to the WCxf-interface which considerably facilitates the inter-operability of public programs for the flavour sector and effective field theories. The attempt to build a supersymmetric version of this model consistent with existing data failed. However, it led to a second research line, namely the question if there is a way to embed the Yukawa and gauge sectors in a common framework. We have worked out the foundations of such a possibility. Here we use the fact that the exceptional group E8 contains SU (3) × E6 as a subgroup where the first factor is considered as flavour group which immediately gets broken to SU (2)F × U (1)F whereas the E6 gauge group gets broken to SU (3)C × SU (3)L × SU (3)R . The latter contains the SM gauge group as subgroup. Among the most appealing emergent properties of this theory is the Higgs-matter unification with a highly-constrained massless chiral sector featuring two universal Yukawa couplings at the scale of grand unifcation. At the electroweak scale, the minimal SM-like effective field theory limit is a specific flavored three-Higgs doublet model consistent with the observed large hierarchies in the quark mass spectra and mixing. Moreover, it predicts that new heavy leptons and quarks coming in vector-like representations of the SM gauge group should be observed in the upcoming LHC runs, latest at a prospective 100 TeV pp collider. While we have shown here the principal feasibility of this approach, the technical details still need to be worked out in the future.
Publications
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Improved predictions for intermediate and heavy Supersymmetry in the MSSM and beyond, Eur. Phys. J. C 77 (2017) no.5, 338
F. Staub and W. Porod
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A unified leptoquark model confronted with lepton non-universality in B-meson decays, Phys. Lett. B 787 (2018) 159
T. Faber, M. Hudec, M. Malinský, P. Meinzinger, W. Porod and F. Staub
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WCxf: an exchange format for Wilson coefficients beyond the Standard Model, Comput. Phys. Commun. 232 (2018) 71
J. Aebischer, I. Brivio, A. Celis, J.A. Evans, Y. Jiang, J. Kumar, X. Pan, W. Porod, J. Rosiek, D. Shih, F. Staub, D.M. Straub, D. van Dyk, and A. Vicente
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Collider phenomenology of a unified leptoquark model
T. Faber, Y. Liu, W. Porod, M. Hudec, M. Malinský, F. Staub and H. Kolešová
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Prospects for New Physics from gauge Left-Right-Colour-Family Grand Unification Hypothesis
A. P. Morais, R. Pasechnik and W. Porod
