arXiv subjects

Svjetlana Fajfer

Publications and source records attributed to Svjetlana Fajfer.

At least 37 records · Page 2Linked to original sources

arXiv2021

Implications of $b\to s\ell^+\ell^-$ constraints on $b\to sν\barν$ and $s\to dν\barν$

We investigate the consequences of deviations from the Standard Model observed in $b\to sμμ$ transitions for flavour-changing neutral-current processes involving down-type quarks and neutrinos, under generic assumptions concerning the structure of New Physics. We derive the relevant Wilson coefficients within an effective field theory approach respecting the SM gauge symmetry, including right-handed currents and assuming a flavour structure based on approximate $U(2)$ symmetry, and only SM-like light neutrinos. We discuss correlations among $B \to K^{(*)} ν\bar ν$ and $K\to πν\bar ν$ branching ratios in the case of linear Minimal Flavour Violation and in a more general framework, highlighting in each case the role played by various New Physics scenarios proposed to explain $b\to sμμ$ deviations. This talk is based on arXiv:2005.03734.

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arXiv2021

Testing Lepton Flavour Universality in $Υ(4S)$ Decays

We propose a novel method to probe the persistent hints of Lepton Flavour Universality violation observed in semileptonic $B$ decays. Relying on the specific properties of the Belle II experiment, it consists in comparing the inclusive rates of $Υ(4S) \to e^\pm μ^\mp X$, $Υ(4S) \to μ^\pm τ_{\rm had}^\mp X$ and $Υ(4S) \to e^\pm τ_{\rm had}^\mp X$. We show that such a measurement can be directly related to the ratio $R(X)_{τ\ell} \equiv Γ(b\to X τν) / Γ(b \to X \ell ν)$ ($\ell=e$ or $μ$), once appropriate experimental cuts are applied to suppress the effects of neutral $B$ mixing and leptons emitted through charm or tau decays. Such a measurement would thus constitute an additional and potentially competitive probe of Lepton Flavour Universality in $b\to c\ellν$ transitions, complementary to existing exclusive measurements, accessible in the Belle II environment.

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arXiv2021

Novel Leptoquark Pair Production at LHC

We introduce a novel mechanism for the leptoquark pair production at LHC that is of a $t$-channel topology and is quark-quark initiated. This mechanism operates under fairly general conditions. One of them is that the two leptoquarks in question couple to the same lepton and the other one is that the fermion numbers of these two leptoquarks differ by two. The strength of the proposed mechanism provides an alternative way to the conventional processes to efficiently constrain the parameter space of the two leptoquark scenarios at LHC whenever the aforementioned conditions are met. We accordingly present one case study to outline the physics potential of this novel production mechanism.

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arXiv2021

Interplay of New Physics Effects in $(g-2)_\ell$ and $h\to\ell^+\ell^-$ -- Lessons from SMEFT

We explore the interplay of New Physics (NP) effects in $(g-2)_\ell$ and $h \to \ell^+ \ell^-$ within the Standard Model Effective Field Theory (SMEFT) framework, including one-loop Renormalization Group (RG) evolution of the Wilson coefficients as well as matching to the observables below the electroweak symmetry breaking scale. We include both the leading dimension six chirality flipping operators including a Higgs and $SU(2)_L$ gauge bosons as well as four-fermion scalar and tensor operators, forming a closed operator set under the SMEFT RG equations. We compare present and future experimental sensitivity to different representative benchmark scenarios. We also consider two simple UV completions, a Two Higgs Doublet Model and a single scalar LeptoQuark extension of the SM, and show how tree level matching to SMEFT followed by the one-loop RG evolution down to the electroweak scale can reproduce with high accuracy the $(g-2)_\ell$ and $h \to \ell^+ \ell^-$ contributions obtained by the complete one- and even two-loop calculations in the full models.

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arXiv2021

Coloured Scalar Mediated Nucleon Decays to Invisible Fermion

We investigate nucleon decays to light invisible fermion mediated by the coloured scalar $\bar S_1= (\bar 3, 1, -2/3)$ and compare them with the results coming from the mediation of $S_1 = (\bar 3,1,1/3)$. In the case of $\bar S_1= (\bar 3, 1, -2/3)$ up-like quarks couple to the invisible fermion, while in the case of $S_1 = (\bar 3,1,1/3)$ the down-like quarks couple to the invisible fermion. For the mass of invisible fermion smaller than the mass $m_p - m_K$, proton (neutron) can decay to $K$ and invisible fermion and the masses of $\bar S_1$ and $S_1$ are in the region $\sim 10^{15}$ GeV. The decays of nucleons to pions and invisible fermion can occur at the tree-level, but in the case of $\bar S_1$ they come from dimension-9 operator and are therefore suppressed by several orders of magnitude compared to the decays into kaons. For the invisible fermion mass in the range $(937.8 \, {\rm MeV},\, 938.8 \, {\rm MeV})$, decay of neutron $n \toχγ$ induced by $\bar S_1$ is possible at the loop level, while the proton remains stable. The branching ratio of such decay is $\le 10^{-6}$, which does not explain neutron decay anomaly, but is in agreement with the Borexino experiment bound. We comment on low-energy processes with the nucleon-like mass of $χ$ in the final state as $Λ\to χγ$ and heavy hadron decays to invisibles.

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arXiv2021

Coloured Scalars Mediated Rare Charm Meson Decays to Invisible Fermions

We consider effects of coloured scalar mediators in decays $c\to u \, {\it invisibles}$. In particular, in these processes, as invisibles, we consider massive right-handed fermions. The coloured scalar $\bar S_1\equiv (\bar 3, 1, -2/3)$, due to its coupling to weak singlets up-quarks and invisible right-handed fermions ($χ$), is particularly interesting. Then, we consider $\tilde R_2 \equiv (\bar 3, 2, 1/6)$, which as a weak doublet is a subject of severe low-energy constraints. The $χ$ mass is considered in the range $(m_K - m_π)/2\leq m_χ\leq (m_D - m_π)/2$. We determine branching ratios for $D\to χ\bar χ$, $D\to χ\bar χγ$ and $D\to πχχ$ for several $χ$ masses, using most constraining bounds. For $\bar S_1$, the most constraining is $D^0 -\bar D^0$ mixing, while in the case of $\tilde R_2$ the strongest constraint comes from $B\to K {\it missing\, energy}$ . We find in decays mediated by $\bar S_1$ that branching ratios can be $\mathcal B(D\to χ\bar χ)< 10^{-8}$ for $m_χ=0.8$ GeV, $\mathcal B(D\to χ\bar χγ) \sim 10^{-8}$ for $m_χ=0.18$ GeV, while $\mathcal B(D^+ \to π^+ χ\bar χ)$ can reach $ \sim 10^{-8}$ for $m_χ=0.18$ GeV. In the case of $\tilde R_2$ these decay rates are very suppressed. We find that future tau-charm factories and Belle II experiments offer good opportunities to search for such processes. Both $\bar S_1$ and $\tilde R_2$ might have masses within LHC reach.

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arXiv2020

$μ\to e γ$ selecting scalar leptoquark solutions for the $(g-2)_{e,μ}$ puzzles

We investigate all potentially viable scenarios that can produce the chiral enhancement required to simultaneously explain the $(g-2)_{e}$ and $(g-2)_μ$ data with either a single scalar leptoquark or a pair of scalar leptoquarks. We provide classification of these scenarios in terms of their ability to satisfy the existing limits on the branching ratio for the $μ\to e γ$ process. The simultaneous explanation of the $(g-2)_{e,μ}$ discrepancies, coupled with the current experimental data, implies that the $(g-2)_e$ loops are exclusively due to the charm quark propagation whereas the $(g-2)_μ$ loops are due to the top quark propagation. The scenarios where the $(g-2)_e$ loops are due to the top (bottom) quark propagation are, at best, approximately nine (three) orders of magnitude away from the experimental limit on the $μ\to e γ$ branching ratio. All in all, there are only three particular scenarios that can pass the $μ\to e γ$ test and simultaneously create large enough impact on the $(g-2)_{e,μ}$ discrepancies when the new physics is based on the Standard Model fermion content. These are the $S_1$, $R_2$, and $S_1 \& S_3$ scenarios, where the first two are already known to be phenomenologically viable candidates with respect to all other flavor and collider data constraints. We show that the third scenario, where the right-chiral couplings to charged leptons are due to $S_1$, the left-chiral couplings to charged leptons are due to $S_3$, and the two leptoquarks mix through the Standard Model Higgs field, cannot address the $(g-2)_{e}$ and $(g-2)_μ$ discrepancies at the $1σ$ level due to an interplay between $K_L^0 \to e^\pm μ^\mp$, $Z \to e^+ e^-$, and $Z \to μ^+ μ^-$ data despite the ability of that scenario to avoid the $μ\to e γ$ limit.

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arXiv2020

Enhanced CP asymmetries in $B \to K μ^+ μ^-$

We show that the current values of $R_K^\mathrm{exp}$ and $R_{K^\ast}^\mathrm{exp}$ can be accommodated by allowing a nonzero New Physics coupling $δC_9^{μμ}$ to be complex, both in the scenario in which only $δC_9^{μμ}$ is affected, and in the scenario with complex $δC_{9,10}^{μμ}$ satisfying $δC_{9}^{μμ}=-δC_{10}^{μμ}$. A presence of the weak CP-violating phase can then be tested by measuring the CP-asymmetry, $\mathcal{A}_\mathrm{CP}$. We show that this asymmetry is enhanced around the peak of each $c\bar c$-resonance, and in fact more pronounced in the close vicinity of $J/ψ$ and $ψ(2S)$. Therefore, measuring $\mathcal{A}_\mathrm{CP}$ before and after the resonances' peak could be revelatory of the CP-violation that originates from beyond the Standard Model, or to be a significant constrain when building a realistic scenario of New Physics.

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arXiv2020

Muon $g-2$ and scalar leptoquark mixing

The observed muon anomalous magnetic moment deviates from the Standard Model predictions. There are two scalar leptoquarks with simultaneous couplings to the quark-muon pairs of both chiralities that can singly explain this discrepancy. We discuss an alternative mechanism that calls for the mixing of two scalar leptoquarks of the same electric charge through the interaction with the Higgs field, where the two leptoquarks separately couple to the quark-muon pairs of opposite chirality structures. Three scenarios that satisfy this requirement are $S_1\,\&\, {S}_3$, $\widetilde{S}_1\,\&\, S_3$, and $\widetilde{R}_2\,\&\, R_2$, where the first scenario is realised with the up-type quarks running in the loops while the other two scenarios proceed through the down-type quark loops. We introduce only two non-zero Yukawa couplings to the relevant quarks and a muon, at the time, to study ability of these three scenarios to explain $(g-2)_μ$ and be in accord with available experimental constraints. We find that the $S_1\,\&\, {S}_3$ scenario with the top-quark loops is consistent with all existing measurements. The $\widetilde{S}_1\,\&\, S_3$ and $\widetilde{R}_2\,\&\, R_2$ scenarios can accommodate the observed discrepancy through the bottom-quark loops but exhibit significant tension with the existing data on the high-$p_T$ dilepton-tails at LHC for the required values of Yukawa couplings and leptoquark masses.

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arXiv2020

Implications of $b\to sμμ$ Anomalies for Future Measurements of $B \to K^{(*)} ν\bar ν$ and $K\to πν\bar ν$

We investigate the consequences of deviations from the Standard Model observed in $b\to sμμ$ transitions for flavour-changing neutral-current processes involving down-type quarks and neutrinos. We derive the relevant Wilson coefficients within an effective field theory approach respecting the SM gauge symmetry, including right-handed currents, a flavour structure based on approximate $U(2)$ symmetry, and assuming only SM-like light neutrinos. We discuss correlations among $B \to K^{(*)} ν\bar ν$ and $K\to πν\bar ν$ branching ratios in the case of linear Minimal Flavour Violation and in a more general framework, highlighting in each case the role played by various New Physics scenarios proposed to explain $b\to sμμ$ deviations.

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arXiv2019

A comparative study of the $S_1$ and $U_1$ leptoquark effects in the light quark regime

We study the phenomenology of two leptoquarks, the Standard Model $SU(2)$ singlets $S_1$ and $U_1$, with regard to the latest experimental data from the low-energy flavor physics measurements, LHC, and the IceCube neutrino experiment. We consider a scenario when scalar (vector) leptoquark $S_1$ ($U_1$) couples exclusively to the down quark and the neutrinos (charged leptons) of all flavors, where the leptoquark in question couples to the SM lepton doublets. The couplings of $S_1$ ($U_1$) to the up-type quarks and the charged leptons (neutrinos) are in turn uniquely determined via $SU(2)$ symmetry. We find that the most important constraints on the leptoquark parameter space originate from flavor physics measurements, followed by the LHC search limits that take over the flavor physics ones in the large LQ mass regime. We furthermore show that $S_1$ ($U_1$) marginally improves (spoils) the fit of the current IceCube data with respect to the SM case within the region of parameter space that is otherwise consistent with various low-energy flavor physics measurements and the latest LHC input. Our study offers an up-to-date analysis for these two leptoquarks in view of the latest experimental data.

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arXiv2019

Light Mediators in Anomaly Free $U(1)_X$ Models I - Theoretical Framework

We examine theoretical features of $U(1)_X$ extensions of the Standard Model whose quantum anomalies are canceled per generation. Similarly to other versions, the theory consists of a Two-Higgs-Doublet Model plus a scalar singlet embedded into the $SM \otimes U(1)_X$ gauge group, and introduces small modifications to the $Z$-boson interactions. These changes can be minimized by exclusively charging right-handed fermions under the new Abelian symmetry, and are compensated by the neutral $X$-boson exchange. Non-universality of fermion couplings can also be achieved by requiring one single $X$-charged family. In general, $X$ gauge bosons can be separated into $A'$ (dark photons) and $Z'$ subsets, distinguished by the presence of axial-vector currents. $A'$ physics is commonly simpler to constrain and therefore favored by experimental tests. Finally, the model can be UV completed both by stable $χ$ fermions or by right-handed neutrinos. The prior case may provide cold WIMPs in the theory.

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arXiv2019

Light Mediators in Anomaly Free $U(1)_X$ Models II - Constraints on Dark Gauge Bosons

We consider experimental constraints in the MeV region in order to determine the parameter space for the $U(1)_X$ extension of the Standard Model, presented in the first part of our work. In particular, we focus on the model UV-completed by cold WIMPs. We conclude that the electron anomalous magnetic moment, the neutrino trident production and the relic abundance $Ω_{CDM}$ provide the most stringent bounds and, in particular cases, they are sufficient to exclude dark-photon ($A'$) models. By allowing the axial-vector coupling of the dark gauge boson $Z'$, the interference effect with the SM gauge bosons may reduce the bounds coming from the trident neutrino production. At the same time, such coupling allows a region of the parameter space already favored both by the relic abundance and by the discrepancy between experimental result and theoretical prediction for the muon anomalous magnetic moment. We emphasize that light-$Z'$ interactions, non-universal for the two first lepton families, necessarily create a difference in the proton charge radius measured in the Lamb shift of the $e$-hydrogen and $μ$-hydrogen. Finally, we determine the effects of the new gauge boson on the forward-backward asymmetry in $e^+ e^- \rightarrow \bar{f} f$, $f = μ, τ$, and on the leptonic decays $M \rightarrow j ν_j l^+ l^-$, where $M = π, K, D, D_s, B$ and $j,l = e, μ$.

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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.

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arXiv2018

Testing spin-2 mediator by angular observables in $b\rightarrow s μ^+ μ^-$

We consider effects of spin-2 particle in the $b\rightarrow s μ^+ μ^-$ transition assuming that the spin-2 particle couples in a flavour non-universal way to $b$ and $s$ quarks and in the leptonic sector couples only to the muons, thereby only contributing to the process $b\rightarrow s μ^+ μ^-$. The $B_s - \bar B_s$ transition gives the strong constraint on the coupling of the spin-2 mediator and $b$ and $s$ quarks while the observed discrepancy from the SM prediction for the muon anomalous magnetic moment $ (g-2)_μ$ serves us to constrain the $μ$-coupling to spin-2 particle. We find that the spin-2 particle can modify the angular observables in the $B \to K μ^+ μ^-$ and $B \to K^* μ^+ μ^-$ decays and produce effects which do not exist in the SM. The generated forward-backward asymmetries in these processes can reach $15\%$, while other observables for these decays receive tiny effects.

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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.

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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.

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arXiv2016

Restrained Dark $U(1)_d$ at Low Energies

We investigate a spontaneously broken $U(1)_d$ gauge symmetry with a muon-specific dark Higgs. Our first goal is to verify how the presence of a new dark Higgs, $ϕ$, and a dark gauge boson, $V$, can simultaneously face the anomalies from the muon magnetic moment and the proton charge radius. Secondly, by assuming that $V$ must decay to an electron-positron pair, we explore the corresponding parameter space determined with the low energy constraints coming from $ K \to μX$, electron $(g-2)_e$, $K \to μν_μe^+ e^-$, $K \to μν_μμ^+ μ^-$, $τ\to ν_τμν_μe^+ e^-$. We focus in the scenario where the $V$ mass is below $ \sim 2 m_μ$ and the $ϕ$ mass runs from few MeV till $250$ MeV, with V-photon mixing of the order $ \sim\mathcal{O}(10^{-3})$. Among weak process at low energies, we check the influence of the new light vector on kaon decays as well as on the scattering $e^+ e^- \rightarrow μ^+ μ^- e^+ e^-$ and discuss the impact of the dark Higgs on $e^+ e^- \rightarrow μ^+ μ^- μ^+ μ^- $. Finally, we consider contributions of the V-photon mixing in the decays $π^0 \to γe^+ e^-$, $η\to γe^+ e^-$, $ρ\to πe^+ e^-$, $K^* \to K e^+ e^-$ and $ϕ(1020) \to ηe^+ e^-$.

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