arXiv subjects

Nejc Košnik

Publications and source records attributed to Nejc Košnik.

26 records · Page 2Linked to original sources

arXiv2018

Scalar leptoquarks from GUT to accommodate the $B$-physics anomalies

We address the $B$-physics anomalies within a two scalar leptoquark model. The low-energy flavor structure of our set-up originates from two $SU(5)$ operators that relate Yukawa couplings of the two leptoquarks. The proposed scenario has a UV completion, can accommodate all measured lepton flavor universality ratios in $B$-meson decays, is consistent with related flavor observables, and is compatible with direct searches at the LHC. We provide prospects for future discoveries of the two light leptoquarks at the LHC and predict several yet-to-be-measured flavor observables.

hep-ph↗

arXiv2017

The role of the $S_3$ GUT leptoquark in flavor universality and collider searches

We investigate the ability of the $S_3$ scalar leptoquark to address the recent hints of lepton universality violation in $B$ meson decays. The $S_3$ leptoquark with quantum numbers $(\overline{\mathbf{3}},\mathbf{3},1/3)$ naturally emerges in the context of an $SU(5)$ GUT model without any conflict with the stringent limits from observed nucleon stability. Scalar leptoquark $S_3$ with left-handed couplings to 2nd and 3rd generations of charged leptons and down-type quarks seems well-suited to address both $R_{K^{(*)}}$ and $R_{D^{(*)}}$. We quantify this suitability with numerical fits to a plethora of relevant flavor observables. The proposed $SU(5)$ model calls for a second leptoquark state, i.e., $\tilde{R}_2$ with quantum numbers $(\mathbf{3},\mathbf{2},1/6)$, if one is to generate gauge coupling unification and neutrino mass. We accordingly include it in our study to investigate $\tilde{R}_2$'s ability to offset adverse effects of $S_3$ and thus improve a quality of numerical fits. A global fit of the leptoquark Yukawa couplings shows that large couplings of light $S_3$ to $τ$ leptons are preferred. We furthermore identify $B \to K^{(*)} \barνν$ as the most sensitive channel to probe the preferred region of parameter space. Large couplings of $S_3$ to $τ$ leptons are finally confronted with the experimental searches for $τ$ final states at the Large Hadron Collider. These searches comprise a study of decay products of the leptoquark pair production, as well as, and more importantly, an analysis of the high-mass $ττ$ final states.

hep-ph↗

arXiv2017

Leptoquark mechanism of neutrino masses within the grand unification framework

We demonstrate viability of the one-loop neutrino mass mechanism within the framework of grand unification when the loop particles comprise scalar leptoquarks (LQs) and quarks of the matching electric charge. This mechanism can be implemented in both supersymmetric and non-supersymmetric models and requires the presence of at least one LQ pair. The appropriate pairs for the neutrino mass generation via the up-type and down-type quark loops are $S_3$-$R_2$ and $S_{1,\,3}$-$\tilde{R}_2$, respectively. We consider two phenomenologically distinct regimes for the LQ masses in our analysis. First regime calls for very heavy LQs in the loop. It can be naturally realised with the $S_{1,\,3}$-$\tilde{R}_2$ scenarios when the LQ masses are roughly between $10^{12}$ GeV and $5 \times 10^{13}$ GeV. These lower and upper bounds originate from experimental limits on partial proton decay lifetimes and perturbativity constraints, respectively. Second regime corresponds to the collider accessible LQs in the neutrino mass loop. That option is viable for the $S_3$-$\tilde{R}_2$ scenario in the models of unification that we discuss. If one furthermore assumes the presence of the type II see-saw mechanism there is an additional contribution from the $S_3$-$R_2$ scenario that needs to be taken into account beside the type II see-saw contribution itself. We provide a complete list of renormalizable operators that yield necessary mixing of all aforementioned LQ pairs using the language of $SU(5)$. We furthermore discuss several possible embeddings of this mechanism in $SU(5)$ and $SO(10)$ gauge groups.

hep-ph↗

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.

hep-ph↗

arXiv2016

Lepton flavor non-universality in $b \to s \ell^+ \ell^-$ processes

We explore a scenario of New Physics entering the description of $B\to K^{(\ast )} μμ$ decay through couplings to the operators ${\cal O}_{9,10}^\prime$, satisfying $C_9^\prime = - C_{10}^\prime$. From the current data on ${\cal B}(B_s\to μμ)$ and ${\cal B}(B\to K μμ)_{[15,22] \,\mathrm{GeV}^2}$, we obtain constraints on $\mathrm{Re} C_{10}^\prime$ and $\mathrm{Im} C_{10}^\prime$ which we then assume to be lepton specific, and find $R_K= {\cal B}(B\to K μμ)/ {\cal B}(B\to K ee)_{[1,6]\,\mathrm{GeV}^2}=0.88(8)$, consistent with recent value measured at LHCb. A specific realization of this scenario is the one with a scalar leptoquark state $Δ$, in which $C_{10}^\prime$ is related to the mass of $Δ$ and its Yukawa couplings. We then show that this scenario does not make any significant impact on $B_s-\overline B_s$ mixing amplitude nor to $ {\cal B}(B\to Kν\bar ν)$. Instead, it can modify $R_{K^\ast}= {\cal B}(B\to K^\ast μμ)/ {\cal B}(B\to K^\ast ee)_{[1,6]\,\mathrm{GeV}^2}$, which will soon be experimentally measured and we find it to be $R_{K^\ast}= 1.11(8)$, while $R_{K^\ast}/R_K= 1.27(19)$. A similar ratio of forward-backward asymmetries also becomes lower than in the Standard Model.

hep-ph↗

arXiv2016

Vector leptoquark resolution of $R_K$ and $R_{D^{(*)}}$ puzzles

We propose that three recent anomalies in $B$ meson decays, $R_{D^{(*)}}$, $R_K$, and $P_5'$, might be explained by a single vector leptoquark weak triplet state. The constraints on the parameter space are obtained by considering $t \to b τ^+ ν$ data, lepton flavor universality tests in the kaon sector, bounds on $B \to K^{(*)} \bar νν$, bound on the lepton flavor violating decay $B \to K μτ$, and measurements of $b \to c μ^- \barν$ decays. The presence of such vector leptoquark could be exposed in precise measurements of $t \to b τν$ and $B \to K^{(*)} \bar νν$ decays. The model also predicts approximate equality of lepton flavor universality ratios $R_{K^*}$, $R_{K}$, and suppressed branching fraction of $B_s \to μ^+ μ^-$.

hep-ph↗

arXiv2015

New Physics Models Facing Lepton Flavor Violating Higgs Decays

We speculate about the possible interpretations of the recently observed excess in the $h \to τμ$ decay. We derive a robust lower bound on the Higgs boson coupling strength to a tau and a muon, even in presence of the most general new physics affecting other Higgs properties. Then we reevaluate complementary indirect constraints coming from low energy observables as well as from theoretical considerations. In particular, the tentative signal should lead to $τ\to μγ$ at rates which could be observed at Belle II. In turn we show that, barring fine-tuned cancellations, the effect can be accommodated within models with an extended scalar sector. These general conclusions are demonstrated in explicit new physics models. Finally we show how, given the $h \to τμ$ signal, the current and future searches for $μ\to e γ$ and $μ\to e$ nuclear conversions unambiguously constrain the allowed rates for $h \to τe$.

hep-ph↗

arXiv2008

On the Dalitz plot analysis of the B --> K eta gamma decays

Recently B-factories have published new results on the $B \to K ηγ$ decays being inspired by the theoretical suggestion to search for new physics in $B \to P_1 P_2 γ$ decays. Using heavy meson chiral perturbation theory we find mechanism which governs the amplitude in parts of the Dalitz plot where either $K$ or $η$ mesons are soft. The dominant contributions in these cases are coming from the nonresonant decay modes. We discuss also $B \to K η' γ$ Dalitz plot. Our partially integrated rates are in agreement with the experimental findings.

hep-ph↗