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Looking for physics beyond the standard model in unexplored regions at the LHC and flavour experiments

Final Report Summary - BSMEXPLORE (Looking for physics beyond the standard model in unexplored regions at the LHC and flavour experiments)

The goals of this project were very ambitious, as it aimed at studying both physics at the LHC and flavour physics, in relation to physics beyond the Standard Model. Obviously, the direction of this study depended a lot on the eventual discovery of new physics, fact that did not happened so far. So, on the LHC side, the work has been more related to the possibility of placing new bounds on some new physics model. On the other hand, on the flavour side, even if there has not been a discovery, several anomalies have emerged, together with a huge literature trying to explain them. As it will be explained in detail below, the beneficiary, Prof. Carla Biggio, has worked more on the systematic study of the consistency of some of these models, namely the ones involving massive vectors. In what follows the obtained results will be discussed more in detail.

First of all, regarding LHC physics, Prof. Biggio carried on a collaboration with the Padova group of the CMS experiment in order to study the possibility of discovering the responsible for neutrino masses at the LHC. In particular, they considered the type-III seesaw model, where the Standard Model Lagrangian is extended with some fermionic SU(2) triplet and, in absence of a signal, they put new bounds on the value of their masses using data at 13 TeV.

The study of this model, which has very interesting phenomenological consequences in flavour physics, i.e. the second line of this research project, has not finished yet. Indeed, in the last year, together with her Ph.D student and other collaborators at SISSA (Trieste, Italy) and IFT (Madrid, Spain), Prof. Biggio is performing a new analysis of this model in order to set indirect bounds (coming precisely from flavour physics) on its parameters, once it is assumed that the new particles introduced are relatively light. Again, the peculiar characteristics of the variation of the model they are analyzing have interesting consequences on LHC physics, suggesting a follow-up of the CMS analysis. A paper containing the new bounds on this model is going to appear soon.

As already explained in reporting period one, instead of performing a model-independent classification of all the one-particle extensions of the Standard Model that could give rise to a significant mu into e-gamma decay, as written in the project, Prof. Biggio decided to analyse all the one-particle extensions of the Standard Model that could explain the existing discrepancy between the experimental result and the theoretical prediction of the muon (g-2). This study has been done in collaboration with her former Master student, and the main result was that among the particles that could explain that discrepancy there are leptoquarks, and, in particular, one predicted in a supersymmetric model that she proposed few years ago. Therefore this result enforced the need of deepening the study of that model on the one side, and of looking for those particular leptoquarks in LHC data on the other.

The first of these objective has been reached with a paper that Prof. Biggio wrote in collaboration with people at Cornell University (USA), where they considered the mentioned SUSY model and study its implications in neutrino physics and its discovery potential at future collider. On the other hand, the search for these leptoquarks at the LHC, is the subject of a collaboration which she is still carry on with the Genova CMS group and Dr. L. Panizzi, one of the two Research fellows payed with this Marie Curie fellowship.

The expertise gained on leptoquarks while working on the previuos projects led Prof. Biggio to try to explain the anomalies measured in B-meson decays still with leptoquarks, or, more ambitiously, to try to explain at once both the muon (g-2) discrepancy and B-physics anomalies. However, studying the literature, me and Prof. Biggio, together Dr. L. Di Luzio, the other Research fellow payed with this MC-CIG, and her former master student M. Bordone, we realised that many models already proposed were involving massive vector leptoquarks and people were performing loop calculations without considering the consistency of it. Therefore, instead of proposing another model, we decided to study the consistency of those models and we showed that, in order to avoid ambiguities in loop calculations in models with massive vectors, these vectors must be embedded into a spontaneously broken gauge theories. This fact has dramatic implications on the bounds on the masses of these particles, excluding, in most of the cases, the possibility to adopt them as the only new particle able to explain either the the muon (g-2) anomaly or the ones in B-physics.

All the work briefly described above has been published in papers and also has been presented at conferences. Moreover, on top of it, the two research fellows payed with this MC grant have published other papers, all in line with this project.