arXiv subjects

Tobias Felkl

Publications and source records attributed to Tobias Felkl.

6 recordsLinked to original sources

arXiv2023

When Energy Goes Missing: New Physics in $b\to sνν$ with Sterile Neutrinos

Belle II recently reported the first measurement of $B^+\to K^++\mathrm{inv}$, which is $2.8σ$ above the Standard Model prediction. We explore the available parameter space of new physics within Standard Model effective field theory extended by sterile neutrinos ($ν$SMEFT) and provide predictions for the other $B\to K^{(\star)}+\mathrm{inv}$ decay modes and invisible $B_s$ decays. We also briefly comment on charged current decays $B\to D^{(\star)}\ell\barν$ and possible ultraviolet completions of the relevant $ν$SMEFT operators.

hep-ph↗

arXiv2023

Flavour anomalies meet flavour symmetry

We construct an extension of the Standard Model with a scalar leptoquark $ϕ\sim (3,1,-\tfrac13)$ and the discrete flavour symmetry $G_f=D_{17}\times Z_{17}$ to explain anomalies observed in charged-current semi-leptonic $B$ meson decays and in the muon anomalous magnetic moment, together with the charged fermion masses and quark mixing. The symmetry $Z_{17}^{\rm diag}$, contained in $G_f$, remains preserved by the leptoquark couplings, at leading order, and efficiently suppresses couplings of the leptoquark to the first generation of quarks and/or electrons, thus avoiding many stringent experimental bounds. The strongest constraints on the parameter space are imposed by the radiative charged lepton flavour violating decays $τ\toμγ$ and $μ\to eγ$. A detailed analytical and numerical study demonstrates the feasibility to simultaneously explain the data on the lepton flavour universality ratios $R(D)$ and $R(D^\star)$ and the muon anomalous magnetic moment, while passing the experimental bounds from all other considered flavour observables.

hep-ph↗

arXiv2023

Probing general $U(1)'$ models with non-universal lepton charges at FASER/FASER2, COHERENT and long-baseline oscillation experiments

The general anomaly-free $U(1)'$ models allow non-universal lepton charges. We explore the sensitivities of FASER/FASER2, COHERENT and DUNE/T2HK precision experiments to the new gauge boson $Z'$ and the new CP-even scalar $ϕ$. With non-universal lepton charges, distinctive reaches at FASER/FASER2 emerge in the regime of low $m_{Z'}$ and small gauge coupling $g_{BL}$ for different $U(1)'$ charge setups. The COHERENT experiment and the future long-baseline experiments DUNE/T2HK also provide complementary probes to the available parameter space. For $m_ϕ< 2m_{Z'}$, the search for the scalar $ϕ$ at FASER/FASER2 is sensitive to the mixing angle between the scalar singlet and the SM Higgs. In the case of $m_ϕ> 2m_{Z'}$, the kinematically allowed decay $ϕ\to Z' Z'$ changes the lifetime and decay rates of the scalar $ϕ$. The sensitivity reach highly depends on the $Z'$ mass and the gauge coupling $g_{BL}$.

hep-ph↗

arXiv2023

Riding the Seesaw: What Higgsstrahlung May Reveal about Massive Neutrinos

We investigate if the projected high-precision measurements of the cross section of the Higgsstrahlung process $e^+ e^- \to Zh$ at a future electron-positron collider can be utilised to indirectly probe the fermionic Seesaw models. We consider the two centre-of-mass energies $\sqrt{s}=240$ GeV and $365$ GeV, and compare the collider reaches to constraints from electroweak observables, probes of lepton flavour universality and the existing and prospective bounds from searches for lepton flavour violation. For the analysis we assume the limit of an exactly conserved lepton-number symmetry. We find that while any appreciable correction to the Higgsstrahlung cross section is already strictly constrained in the Type-I Seesaw model, effects of up to $\mathcal{O}(10\%)$ are possible within Type-III Seesaw.

hep-ph↗

arXiv2021

A Tale of Invisibility: Constraints on New Physics in $b\to sνν$

The Belle II experiment will measure the rare decays $B\to Kνν$ and $B\to K^* νν$ with increased sensitivity which can hence be expected to serve as a very efficient probe of new physics. We calculate the relevant branching ratios in low-energy effective field theory (LEFT) including an arbitrary number of massive sterile neutrinos and discuss the expected sensitivity to the different operators. We also take into account the longitudinal polarisation fraction $F_L$ and the inclusive decay rate $B\to X_sνν$. In our investigation we consider new physics dominantly contributing to one and two operators both for massless and massive (sterile) neutrinos. Our results show a powerful interplay of the exclusive decay rates $B\to Kνν$ and $B\to K^*νν$, and a surprisingly large sensitivity of the inclusive decay mode to vector operators even under conservative assumptions about its uncertainty. Furthermore, the sensitivity of $F_L$ is competitive with the branching ratio of $B\to K^* νν$ in the search for new physics contributing to scalar operators and thus also complementary to $B\to Kνν$ and $B\to X_sνν$.

hep-ph↗

arXiv2021

The Singly-Charged Scalar Singlet as the Origin of Neutrino Masses

We consider the generation of neutrino masses via a singly-charged scalar singlet. Under general assumptions we identify two distinct structures for the neutrino mass matrix. This yields a constraint for the antisymmetric Yukawa coupling of the singly-charged scalar singlet to two left-handed lepton doublets, irrespective of how the breaking of lepton-number conservation is achieved. The constraint disfavours large hierarchies among the Yukawa couplings. We study the implications for the phenomenology of lepton-flavour universality, measurements of the $W$-boson mass, flavour violation in the charged-lepton sector and decays of the singly-charged scalar singlet. We also discuss the parameter space that can address the Cabibbo Angle Anomaly.

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