Theory and phenomenology of two-Higgs-doublet models

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, $\tau \to 3\mu $ and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals—scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

/pdf/a-facility-to-search-for-hidden-particles-at-the-cern-sps-3pxaggnjt6.pdf

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

The final results of the search for the lepton flavour violating decay $$\mathrm {\mu }^+ \rightarrow \mathrm {e}^+ \mathrm {\gamma }$$
 based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling $$7.5\times 10^{14}$$
 stopped muons on target are presented. No significant excess of events is observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of $$ \mathcal{B} (\mu ^+ \rightarrow \mathrm{e}^+ \gamma ) < 4.2 \times 10^{-13}$$
 (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date.

/pdf/search-for-the-lepton-flavour-violating-decay-m-e-g-with-the-2lwbokwsjx.pdf

Search for the lepton flavour violating decay μ + → e + γ with the full dataset of the MEG experiment

This article reviews the current theoretical and experimental status of the field of muon decay and its potential to search for new physics beyond the standard model. The importance of rare muon processes with lepton flavor violation is highly stressed, together with precision measurements of normal muon decay. Recent up-to-date motivations of lepton flavor violation based on supersymmetric models, in particular supersymmetric grand unified theories, are described along with other theoretical models. Future prospects of experiments and muon sources of high intensity for further progress in this field are also discussed.

Muon decay and physics beyond the standard model

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (Search for Hidden Particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, $\tau\to 3\mu$ and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the Standard Model and describe interactions between new particles and four different portals - scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the Standard Model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation

/pdf/a-facility-to-search-for-hidden-particles-at-the-cern-sps-brmu5s54o3.pdf

A facility to Search for Hidden Particles at the CERN SPS: the SHiP physics case

Mass bounds of the lightest CP-even Higgs boson in the two-Higgs-doublet model

$\mu^{+} \to e^{+} \gamma$ and $\mu^{+} \to e^{+}e^{+}e^{-}$ processes are analyzed in detail with polarized muons in supersymmetric grand unified theories. We first present Dalitz plot distribution for $\mu^{+} \to e^{+}e^{+}e^{-}$ decay based on effective Lagrangian with general lepton-flavor-violating couplings and define various P- and T-odd asymmetries. We calculate branching ratios and asymmetries in supersymmetric SU(5) and SO(10) models taking into account complex soft supersymmetry breaking terms. Imposing constraints from experimental bounds on the electron, neutron and atomic electric dipole moments, we find that the T-odd asymmetry for $\mu^{+} \to e^{+}e^{+}e^{-}$ can be 15% in the SU(5) case. P-odd asymmetry with respect to muon polarization for $\mu^{+} \to e^{+} \gamma$ varies from -20% to -100% for the SO(10) model while it is $+100%$ in the SU(5) case. We also show that the P-odd asymmetries in $\mu^{+} \to e^{+}e^{+}e^{-}$ and the ratio of $\mu^{+} \to e^{+}e^{+}e^{-}$ and $\mu^{+} \to e^{+} \gamma$ branching fractions are useful to distinguish different models.

μ-->eγ and μ-->3e processes with polarized muons and supersymmetric grand unified theories

Flavor changing neutral current processes such as $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\gamma},$ $\stackrel{\ensuremath{\rightarrow}}{b}{\mathrm{sl}}^{+}{l}^{\ensuremath{-}},$ $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\nu}\overline{\ensuremath{\nu}},$ ${\ensuremath{\epsilon}}_{K},$ $\ensuremath{\Delta}{m}_{B},$ ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}},$ and ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ are calculated in the supersymmetric standard model based on supergravity. We consider two assumptions for the soft supersymmetry breaking terms. In the minimal case soft breaking terms for all scalar fields are taken to be universal at the GUT scale, whereas those terms are different for the squark-slepton sector and the Higgs sector in the nonminimal case. In the calculation we have taken into account the next-to-leading order QCD correction to the $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\gamma}$ branching ratio, the results from the CERN LEP II superparticles search, and the condition of the radiative electroweak symmetry breaking. We show that $\ensuremath{\Delta}{m}_{B}$ and ${\ensuremath{\epsilon}}_{K}$ can be enhanced up to 40% compared to the standard model values in the nonminimal case. In the same parameter region the $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\nu}\overline{\ensuremath{\nu}},$ ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}},$ and ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ branching ratios are reduced up to 10%. The corresponding deviation in the minimal case is 20% for $\ensuremath{\Delta}{m}_{B}$ and ${\ensuremath{\epsilon}}_{K}$ and within 3% for $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\nu}\overline{\ensuremath{\nu}},$ ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}},$ and ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}.$ For the $\stackrel{\ensuremath{\rightarrow}}{b}{\mathrm{sl}}^{+}{l}^{\ensuremath{-}}$ process a significant deviation from the standard model is realized only when the $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\gamma}$ amplitude has an opposite sign to the standard model prediction. The significance of these results from possible future improvements of the $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\gamma}$ branching ratio measurement and top squark search is discussed.

Flavor changing neutral current processes in B and K decays in the supergravity model

A triple vector correlation in the ${\ensuremath{\mu}}^{+}\ensuremath{\rightarrow}{e}^{+}{e}^{+}{e}^{\ensuremath{-}}$ decay with polarized muons is investigated as a probe to $\mathrm{CP}$ violating coupling constants in supersymmetric models. A sizable triple correlation can be induced due to a complex phase in the supersymmetric soft-breaking terms in the SU(5) grand unified theory. Correlation with the electric dipole moments of electron and neutron are investigated and it is shown that these quantities give independent information on possible $\mathrm{CP}$ violating sources.

Yasuhiro Okada

Papers

Muon decay and physics beyond the standard model

Mass bounds of the lightest CP-even Higgs boson in the two-Higgs-doublet model

μ-->eγ and μ-->3e processes with polarized muons and supersymmetric grand unified theories

Flavor changing neutral current processes in B and K decays in the supergravity model

CP violation in the μ→3e process and supersymmetric grand unified theory