arXiv subjects

Olcyr Sumensari

Publications and source records attributed to Olcyr Sumensari.

46 records · Page 3Linked to original sources

arXiv2017

Lepton flavor (universality) violation in $B$-meson decays

Even though the LHC searches did not unveil the new physics particles so far, LHCb hints towards deviations from lepton flavor universality in exclusive decays based on the transition $b\to s\ell\ell$. In this proceeding, we present a new leptoquark model that can explain both $R_{K}^{\mathrm{exp}}<R_{K}^{\mathrm{SM}}$ and $R_{K^\ast}^{\mathrm{exp}}<R_{K^\ast}^{\mathrm{SM}}$ via loop effects, consistent with observations made by LHCb. We discuss the main predictions of this scenario that can be tested experimentally, which include the bounds on lepton flavor violating decays $\mathcal{B}(Z\toμτ) \lesssim \mathcal{O}(10^{-7})$ and $\mathcal{B}(B\to Kμτ)\lesssim \mathcal{O}(10^{-9})$.

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arXiv2017

Leptoquark models for the $B$-physics anomalies

The $B$-physics experiments at LHCb, BaBar and Belle hint towards deviations from Lepton Flavor Universality in both the tree-level and loop-induced $B$ meson semileptonic decays. We propose a leptoquark model with light right-handed neutrinos which can accommodate both $R_K^\mathrm{exp} R_{D^{(\ast)}}^\mathrm{SM}$. We discuss several of its predictions which can be tested in modern day experiments. We also comment on the recent finding at LHCb, namely $R_{K^\ast}^{\mathrm{exp}}<R_{K^\ast}^\mathrm{SM}$.

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arXiv2017

Seeking the CP-odd Higgs via $h\rightarrow η_{c,b}\ell^+\ell^-$

We show that the decay rates of the Higgs boson to a pseudoscalar quarkonium and a pair of leptons, $h\to P\ell^+\ell^-$ ($P\in\{η_c,η_b\}$), can be substantially enhanced in a scenario with two Higgs doublets with a softly broken $\mathbb{Z}_2$ symmetry (2HDM) when the CP-odd Higgs A is light, i.e. $m_A\lesssim m_h$. Depending on the type of 2HDM the enhancement of $\mathcal{B}(h\to η_{c,b}τ^+τ^-)$ with respect to its Standard Model value can be an order of magnitude larger, i.e. $\mathcal{O}(10^{-6}\div10^{-5})$. The decays $h\to P\ell^+\ell^-$ could therefore provide an efficient channel to investigate the presence of a light CP-odd Higgs $A$ and help to disentangle among various 2HDM scenarios.

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arXiv2017

Sterile neutrinos facing kaon physics experiments

We discuss weak kaon decays in a scenario in which the Standard Model is extended by massive sterile fermions. After revisiting the analytical expressions for leptonic and semileptonic decays we derive the expressions for decay rates with two neutrinos in the final state. By using a simple effective model with only one sterile neutrino, compatible with all current experimental bounds and general theoretical constraints, we conduct a thorough numerical analysis which reveals that the impact of the presence of massive sterile neutrinos on kaon weak decays is very small, less than $1\%$ on decay rates. The only exception is $\mathcal{B} (K_L\to νν)$, which can go up to $\mathcal{O}( 10^{-10})$, thus possibly within the reach of the KOTO experiment. In other words, if all the future measurements of weak kaon decays turn out to be compatible with the Standard Model predictions, this would not rule out the existence of massive light sterile neutrinos with non-negligible active-sterile mixing. Instead, for a sterile neutrino of mass below $m_K$, one might obtain a huge enhancement of $\mathcal{B} (K_L\to νν)$, otherwise negligibly small in the Standard Model.

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arXiv2017

SU(5) Unification with TeV-scale Leptoquarks

It was previously noted that SU(5) unification can be achieved via the simple addition of light scalar leptoquarks from two split $\bf10$ multiplets. We explore the parameter space of this model in detail and find that unification requires at least one leptoquark to have mass below $\approx16\,$TeV. We point out that introducing splitting of the $\bf24$ allows the unification scale to be raised beyond $10^{16}$ GeV, while a U(1)$_{PQ}$ symmetry can be imposed to forbid dangerous proton decay mediated by the light leptoquarks. The latest bounds from LHC searches are combined and we find that a leptoquark as light as 400 GeV is still permitted. Finally, we discuss the interesting possibility that the leptoquarks required for unification could also be responsible for the $2.6σ$ deviation observed in the ratio $R_K$ at LHCb.

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arXiv2016

Lepton Flavor Violation in Exclusive $b\to s$ Decays

Starting from the general effective hamiltonian relevant to the $b\to s$ transitions, we derive the expressions for the full angular distributions of the $B \to K^{(\ast )} \ell_1 \ell_2$ decay modes, as well as for ${\cal B}(B_s \to \ell_1\ell_2 )$ ($\ell_1\neq \ell_2$). We point out the differences in the treatment of the lepton flavor violating modes with respect to the lepton flavor conserving ones. Concerning the relevant Wilson coefficients we evaluate them in two different scenarios: (i) The (pseudo-)scalar coefficients are obtained using the (pseudo-)scalar coupling extracted from the experimental non-zero value of ${\cal B}(h\to μτ)$, (ii) Revisiting a $Z^\prime$-model in which the flavor changing neutral couplings are allowed. We provide the numerical estimates of the branching fractions of the above-mentioned modes in both scenarios.

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arXiv2016

Leptoquark model to explain the $B$-physics anomalies, $R_K$ and $R_D$

We show that a model with a scalar leptoquark of hypercharge $Y=1/6$ which includes the light right-handed neutrinos, can successfully describe both of the $B$-physics anomalies, $R_K^{\rm exp} < R_K^{\rm SM}$ and $R_D^{\rm exp} > R_D^{\rm SM}$. We discuss the corresponding low energy effective theory and, after using the known experimental data as constraints, we show that the model is viable and that it offers several predictions which can be tested experimentally.

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arXiv2016

Can the new resonance at LHC be a CP-Odd Higgs boson?

A plausible explanation of the recent experimental indication of a resonance in the two-photon spectrum at LHC is that it corresponds to the CP-odd Higgs boson. We explore such a possibility in a generic framework of the two Higgs doublet models (2HDM), and combine $m_A \approx 750$GeV with the known $m_h =125.7(4)$~GeV to show that the charged Higgs boson and the other CP-even scalar masses become bounded from bellow and from above. We show that this possibility is also consistent with the electroweak precision data and the low energy observables, which we test in a few leptonic and semileptonic decay modes.

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arXiv2016

Limits on Neutrinophilic Two-Higgs-Doublet Models from Flavor Physics

We derive stringent limits on neutrinophilic two-Higgs-doublet models from low-energy observables after the discovery of the Higgs boson and of the mixing angle $θ_{13}$. These decays can constrain the plane spanned by $m_{H^\pm}$, the mass of the new charged Higgs, and $v_2$, the vacuum expectation value of the new neutrinophilic scalar doublet. Lepton flavor conserving decays are not able to set meaningful bounds, since they depend strongly on the unknown neutrino absolute mass scale. On the other hand, loop induced lepton flavor violating decays, such as $μ\to e γ$, $μ\to 3 e $ or $μ\to e$ in nuclei are currently responsable for the best limits today. If $v_2 \lesssim 1 \, (0.1)$ eV we get $m_{H^\pm} \gtrsim 250 \, (2500)$ GeV at 90% CL. In the foreseen future these limits can improve by at least a factor of 100.

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arXiv2015

Lepton flavor violating decays of vector quarkonia and of the $Z$ boson

We address the impact of sterile fermions on the lepton flavor violating decays of quarkonia as well as of the $Z$ boson. We compute the relevant Wilson coefficients and show that the ${\rm B}(V\to\ell_α\ell_β)$, where $V=ϕ,ψ^{(n)}$, $Υ^{(n)},Z$ can be significantly enhanced in the case of large sterile fermion masses and a non-negligible active-sterile mixing. We illustrate that feature in a specific minimal realization of the inverse seesaw mechanism, known as $(2,3)$-ISS, and in an effective model in which the presence of non-standard sterile fermions is parameterized by means of one heavy sterile (Majorana) neutrino.

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