Flavour-changing neutral current processes beyond the Standard Model

In dieser Arbeit untersuche ich, im Rahmen von Lösungen des Dunkle-Materie-Problems, seltene Zerfälle in der Higgs- und $B$-Physik. Ich werde alle Aspekte und Techniken diskutieren, die zur Durchführung der Rechnungen benötigt werden. In dieser Arbeit hebe ich Mechanismen zur Unterdrückung von flavour-ändernden neutralen Strömen, wie beispielsweise $b \to s(d) X$, hervor. Ich werde ebenso die Rolle eines fundamentalen Skalars im Standardmodell aufzeigen

Higgs boson decay into a lepton pair and a photon: A roadmap to the discovery of H →zγ and probes of new physics

The decay H→ℓ$^{+}$ℓ$^{–}$γ, ℓ=e, μ receives contributions from H→Z[→ℓ$^{+}$ℓ$^{–}$]γ and a nonresonant contribution, both of which are loop induced. We describe how one can separate these subprocesses in a gauge-independent way, define the decay rate Γ(H→Zγ), and extract the latter from differential H→ℓ$^{+}$ℓ$^{–}$γ branching ratios. For ℓ=μ, also the tree decay rate, which is driven by the muon Yukawa coupling, is important. We propose kinematic cuts optimized to separate the three contributions, paving the way to the milestone (i) discovery of H→Zγ, (ii) discovery of H→μ$^{+}$μ$^{–}$γ| $_{tree}$, and (iii) quantification of new physics in both the effective H−Z−γ and nonresonant H−ℓ$^{+}$ℓ$^{–}$−γ couplings

Higgs portal to dark matter and B→K(∗)B\to K^{(*)} decays

We consider a Higgs portal model in which the 125-GeV Higgs boson mixes with a light singlet mediator $h_2$ coupling to particles of a Dark Sector and study potential $b\to s h_2$ decays in the Belle II experiment. Multiplying the gauge-dependent off-shell Standard-Model $b$-$s$-Higgs vertex with the sine of the Higgs mixing angle does not give the correct $b$-$s$-$h_2$ vertex. We clarify this issue by calculating the $b$-$s$-$h_2$ vertex in an arbitrary $R_\xi$ gauge and demonstrate how the $\xi$ dependence cancels from physical decay rates involving an on-shell or off-shell $h_2$. Then we revisit the $b\to s h_2$ phenomenology and point out that a simultaneous study of $B\to K^* h_2$ and $B\to K h_2$ helps to discriminate between the Higgs portal and alternative models of the Dark Sector. We further advocate for the use of the $h_2$ lifetime information contained in displaced-vertex data with $h_2$ decaying back to Standard-Model particles to better constrain the $h_2$ mass or to reveal additional $h_2$ decay modes into long-lived particles.Comment: 9 pages, 4 figures. v.2: minor clarifications, added Table 1, added references, acknowledgements extende

Higgs portal to dark matter and B→K(∗)B\rightarrow K^{(*)} decays

We consider a Higgs portal model in which the 125-GeV Higgs boson mixes with a light singlet mediator h$_{2}$ coupling to particles of a Dark Sector and study potential b→sh$_{2}$ decays in the Belle II experiment. Multiplying the gauge-dependent off-shell Standard-Model b-s-Higgs vertex with the sine of the Higgs mixing angle does not give the correct b-s-h$_{2}$ vertex. We clarify this issue by calculating the b-s-h$_{Zahl}$ vertex in an arbitrary R$_{ξ}$ gauge and demonstrate how the ξ dependence cancels from physical decay rates involving an on-shell or off-shell h$_{2}$. Then we revisit the b→sh$_{2}$ phenomenology and point out that a simultaneous study of B→K∗h$_{2}$ and B→Kh$_{2}$ helps to discriminate between the Higgs portal and alternative models of the Dark Sector. We further advocate for the use of the h$_{2}$ lifetime information contained in displaced-vertex data with h$_{2}$ decaying back to Standard-Model particles to better constrain the h$_{2}$ mass or to reveal additional h$_{2}$ decay modes into long-lived particles

Dark SU(2) Stueckelberg portal

We study the non-abelian $SU(2)_D$ extension of the $U(1)_D$ Stueckelberg portal, which plays a role of the mediator between the Standard Model (SM) and Dark Sector (DS). This portal is specified by the Stueckelberg mechanism for generation of dark gauge boson masses. Proposed $U(1)_D\times SU(2)_D$ Stueckelberg portal has a connection with SM matter fields in analogy with Familon Model. We derive bounds on the couplings of dark portal bosons and SM particles, which govern diagonal and non-diagonal flavor transitions of quarks and leptons.Comment: 12 pages, 4 figures, journal versio

Lepton phenomenology of Stueckelberg portal to dark sector

We propose an extension of the Standard Model (SM) with a UA′(1) gauge-invariant dark sector connected to the SM via a new portal—the Stueckelberg portal, arising in the framework of dark photon A′ mass generation via the Stueckelberg mechanism. This portal opens through the effective dim=5 operators constructed from the covariant term of the auxiliary Stueckelberg scalar field σ providing flavor nondiagonal renormalizable couplings of both σ and A′ to the SM fermions ψ. The Stueckelberg scalar plays a role of Goldstone boson in the generation of mass of the dark photon. Contrary to the conventional kinetic mixing portal, in our scenario, flavor diagonal A′−ψ couplings are not proportional to the fermion charges and are, in general, flavor nondiagonal. These features drastically change the phenomenology of dark photon A′ relaxing or avoiding some previously established experimental constraints. We focus on the phenomenology of the described scenario of the Stueckelberg portal in the lepton sector and analyze the contribution of the dark sector fields A′ to the anomalous magnetic moment of muon (g−2)μ, lepton flavor–violating decays li→lkγ, and μ−e conversion in nuclei. We obtain limits on the model parameters from the existing data on the corresponding observables