The Standard Model and low-energy experiments: from lepton-flavour violation to dark photons

This note is a concise theoretical summary of the first session on new physics searches at the high-intensity frontier of the IFAE2017 Conference. Recent theoretical developments related to muonic lepton-flavour violation and $g-2$ are reviewed.Comment: 5 pages, IFAE2017 Conferenc

Leptonic CP violation in the charged sector and effective field theory approach

These proceedings introduce the main techniques and ideas for a systematic effective field theory analysis of CP violation in the charged lepton sector. This study is required because the Standard Model of particle physics predicts a very high degree of CP violation suppression in the lepton sector, and this implies that possible new physics effects can be parameterised in terms of new interactions among the Standard Model fields, in the framework of the so-called Standard Model Effective Field Theory. In analogy with previous investigations of charged lepton flavour violating observables, this document illustrates how the current limits on leptonic CP violation coming from the electric-dipole moment of the leptons can be recast into constraints on the effective coefficients defined at a given decoupling scale. Furthermore, important bounds acting on previously unconstrained parameters are discussed.Comment: 10 pages, 1 figure, 5 table

Lepton-flavour violating decays in theories with dimension 6 operators

Despite a large experimental effort, so far no evidence for flavour-violating decays of charged leptons such as $l_i\to l_j\gamma$ and $l_i\to l_j l_k l_k$ has been found. The absence of a signal puts very severe constraints on many extensions of the Standard Model. Here we apply a model independent approach by studying such decays in the Standard Model effective field theory. Going beyond leading order in the Standard Model couplings and considering all dimension 6 operators that might lead to lepton-flavour violation, we are able to extract limits on a large number of Wilson coefficients of such operators. We are also able to compare the impact of particular searches and find, for example, that flavour-violating decays of the $Z$-boson $Z\to \mu e$ are much more constrained from low-energy experiments $\mu\to e \gamma$ than from the limits of current and future direct searches at high energy.Comment: 7 pages, 5 Tables; to appear in the Proceedings of the FCCP2015 Worksho

The pure B−LB-L model and future linear colliders: the Higgs sector

We summarise the phenomenology of the Higgs sector of the minimal $B-L$ extension of the Standard Model at an $e^+e^-$ Linear Collider. Within such a scenario, we show that (in comparison with the Large Hadron Collider) several novel production and decay channels involving the two physical Higgs states could experimentally be accessed at such a machine. In particular, we present the scope of the $Z'$ strahlung process for single and double Higgs production, the only suitable mechanism for accessing an almost decoupled heavy scalar state.Comment: This proceeding of the 2011 International Workshop on Future Linear Colliders (LCWS11) is published through the SLAC Electronic Conference Proceedings Archiv

Fully differential NLO predictions for the radiative decay of muons and taus

We present a general purpose Monte Carlo program for the calculation of the radiative muon decay $\mu\to e\,\nu \bar{\nu}\gamma$ and the radiative decays $\tau\to e\, \nu \bar{\nu}\gamma$ and $\tau\to\mu\,\nu \bar{\nu}\gamma$ at next-to-leading order in the Fermi theory. The full dependence on the lepton masses and polarization of the initial-sate lepton are kept. We study the branching ratios for these processes and show that fully-differential next-to-leading order corrections are important for addressing a tension between BaBar's recent measurement of the branching ratio $\mathcal{B}(\tau\to e\, \nu\bar{\nu}\gamma)$ and the Standard Model prediction. In addition, we study various distributions of the process $\mu\to e\,\nu \bar{\nu}\gamma$ and obtain precise predictions for the irreducible background to $\mu\to e \gamma$ searches, tailored to the geometry of the MEG detector.Comment: 14 pages, 5 figures, published versio

Charged-Higgs production in the Two-Higgs-Doublet Model --- the \tau\nu channel

We update the allowed parameter space of the CP-violating 2HDM with Type II Yukawa couplings, that survives the current experimental and theoretical constraints on the model. For a representative set of allowed parameter points, we study the production of charged Higgs bosons, both at the LHC at 14 TeV and at a possible future hadronic collider at 30 TeV. Two classes of production mechanisms are considered, "bosonic" (pp\to H^\pm W^\mp X) and "fermionic" (pp \to H^+ \bar t (b) X). After commenting on our previous H^\pm\to W^\pm H_1 investigation, we focus on the tauonic decay mode, H^\pm \to \tau\nu, performing a detailed signal-over-background analysis at the parton level. The increased features provided when considering CP violation, i.e., the extension of the parameter space and the mixing of the would-be CP-odd scalar boson, only marginally increase the discovery prospects, which remain very challenging both when increased luminosities and higher energies are considered.Comment: 26 pages, 13 figures. v2 Version published in JHE