arXiv subjects

Nejc Košnik

Publications and source records attributed to Nejc Košnik.

At least 19 recordsLinked to original sources

arXiv2026

Current and future constraints on heavy New Physics from $τ$ weak dipole moments

We study the weak magnetic and electric dipole moments of the $τ$ lepton as precision tests of the Standard Model (SM) and probes of heavy New Physics (NP). We present an updated SM prediction for the $τ$ weak magnetic dipole moment at one loop, including a careful assessment of theoretical uncertainties from electroweak scheme dependence. Working within the SM Effective Field Theory, we derive comprehensive current constraints on the $τ$ dipole operators from a combination of observables: the $τ$ weak and electromagnetic dipole moments, high-mass Drell-Yan tails at the LHC, $Z$ partial decay widths, and the electron electric dipole moment. Finally, we assess the prospects of measuring the SM value of the $τ$ weak magnetic moment at the FCC-$ee$ Tera-$Z$ run, and project the sensitivities of the leading observables to heavy NP at FCC-$ee$ and HL-LHC, incorporating informed assumptions about improvements of systematic uncertainties. We find that the $τ$ weak dipole moments are already among the leading probes of the $τ$ dipole operators, and will become increasingly dominant at future colliders.

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arXiv2026

Probing the neutrino mass through semileptonic meson decays

We argue that a detailed analysis of semileptonic decays can test the possibility of a massive neutrino. The key observable, related to the forward-backward asymmetry, is exactly zero for a massless neutrino but becomes non-zero if the neutral lepton is heavy and interacts with Standard Model fields via left-handed operators. For right-handed interactions, this quantity differs significantly from zero even for a massless right-handed neutrino. We demonstrate this explicitly using the example of a pseudoscalar meson decaying into another pseudoscalar meson. A similar discussion applies to decays into a vector meson, with an additional subtlety addressed in this work.

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arXiv2025

UV origins of CP-violating leptonic Yukawa couplings

Probing the CP nature of the Higgs Yukawa couplings is a promising avenue for unraveling new physics effects. In this work, we investigate the possible single- and two-field UV origins of CP-violating leptonic Yukawa couplings, using the Standard Model Effective Field Theory as a stepping stone. We demonstrate a rich set of constraints on the UV model parameters, including direct Higgs measurements, electric and magnetic dipole moments of leptons, charged lepton flavor violating observables, and electroweak precision tests. Studying representative flavor assumptions we find that the precision constraints are often, but not always, more constraining than the dedicated LHC analyses of modified leptonic Yukawa couplings.

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arXiv2025

Right-handed interactions in puzzling $B$-decays

We explain the difference between the measured decay widths of $b \to c τν$ processes and of $B\to Kν\barν$ and their values predicted in the Standard Model by introducing the right-handed interactions both to quarks and to leptons. At low energy scales, in addition to the Standard Model particles, we assume the presence of an additional neutral lepton (right-handed neutrino). We then show a specific realization of such a scenario in a model with a single scalar leptoquark ($S_1$), and discuss the corresponding phenomenology.

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arXiv2024

$R_{D^{(*)}}$ and survival of the fittest scalar leptoquark

Motivated by the long-standing discrepancy in lepton flavor universality ratios $R_D$ and $R_{D^{\ast}}$ we assess the status of scalar leptoquark states $R_2$, $\widetilde R_2$ and $S_1$ which can in principle provide a desired enhancement of $\mathcal{B}(B\to D^{(\ast )}τν)$ in a minimal setup with two Yukawa couplings only. We consider unavoidable low-energy constraints, $Z$-pole measurements as well as high-$p_T$ constraints. After setting mass of each leptoquark to $1.5$ TeV we find that of all considered states only $S_1$ leptoquark, coupled to both chiralities of leptons and quarks, is still a completely viable solution while the scenario with $R_2$ is in growing tension with $Γ(Z \to ττ)$ and with the LHC constraints on the di-tau tails at high-$p_T$. We comment on the future experimental tests of $S_1$ scenario.

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arXiv2024

CP-odd window into long distance dynamics in rare semileptonic $B$ decays

We consider the combined measurements of CP-averaged decay rates and direct CP asymmetries of $B^\pm\to K^\pm \ell^+ \ell^-$ and $B^\pm\to π^\pm \ell^+ \ell^-$ to probe (non-local) four-quark operator matrix element contributions to rare semileptonic B meson decays. We also explore how their effects could be in principle disentangled from possible local new physics effects using $U$-spin relations. To this end, we construct a ratio of CP-odd decay rate differences which are exactly predicted within the standard model in the $U$-spin limit, while the leading $U$-spin breaking effects can also be systematically calculated. Our results motivate binned measurements of the direct CP asymmetry in $B^\pm\to π^\pm \ell^+ \ell^-$ as well as dedicated theoretical estimates of $U$-spin breaking both in local form factors as well as in four-quark matrix elements.

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arXiv2023

New Physics in CP Violating and Flavour Changing Quark Dipole Transitions

We explore CP-violating (CPV) effects of heavy New Physics in flavour-changing quark dipole transitions, within the framework of Standard Model Effective Field Theory (SMEFT). First, we establish the relevant dimension six operators and consider the Renormalisation Group (RG) evolution of the appropriate Wilson coefficients. We investigate RG-induced correlations between different flavour-violating processes and electric dipole moments (EDMs) within the Minimal Flavour Violating and $U(2)^3$ quark flavour models. At low energies, we set bounds on the Wilson coefficients of the dipole operators using CPV induced contributions to observables in non-leptonic and radiative $B$, $D$ and $K$ decays as well as the neutron and electron EDMs. This enables us to connect observable CPV effects at low energies and general NP appearing at high scales. We present bounds on the Wilson coefficients of the relevant SMEFT operators at the high scale $Λ= 5~{\rm TeV}$, and discuss most sensitive CPV observables for future experimental searches.

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arXiv2023

Correlating New Physics Effects in Semileptonic $ΔC = 1$ and $ΔS = 1$ Processes

We present constraints on the left-handed dimension-6 interactions that contribute to semileptonic and leptonic decays of $K$, $D$, pions and to nuclear beta decay. We employ the flavour covariant description of the effective couplings, identify universal CP phases of New Physics and derive constraints from decay rates and CP-odd quantities. As a result, we can predict the maximal effects of such flavoured NP in $D$ decays from stringent $K$ decay constraints and vice-versa.

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arXiv2022

On a model with two scalar leptoquarks -- $R_2$ and $S_3$

We discuss a model that can accommodate the $B$-physics anomalies, based on combining two scalar leptoquarks, $R_2$ and $S_3$, of mass $\mathcal{O}(1\,\mathrm{TeV})$, and that we proposed in our previous paper. We update the analysis of its parameter space and show that a model remains viable and consistent with a number of low energy and high energy flavor physics constraints. Since the model predicts a non-zero New Physics phase, we discuss the possibility to test its contribution to the neutron electric dipole moment and to the angular distributions of the exclusive $b\to cτ\bar ν$ decays. We find that the model can provide a significant enhancement to $\mathcal{B}(B\to K^{(\ast)}νν)$ and provides both the upper and lower bounds to $\mathcal{B}(B\to K^{(\ast)}μτ)$.

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arXiv2021

LFU and CP violation with $S_3$

We introduce the CP violating scalar leptoquark $S_3$ to explain the measured values of the lepton universality ratios $R_{K^{(*)}}$. We derive constraints on the CP-even and CP-odd components of the leptoquark Yukawa couplings stemming from effects in $b \to s μμ$ and $B_s$ mixing. For the $b \to s μμ$ processes we impose $R_{K^{(*)}}$, $\mathcal{B}(B_s \to μ^+ μ^-)$, as well as CP-sensitive angular asymmetries $A_{7,8,9}$, whereas in the $B_s$ mixing sector $ΔM_s$ and $S_{ψϕ}$ are considered. Combining the constraints within the $S_3$ model reveals that a large CP phase with a definite sign is perfectly viable for a leptoquark of mass below a few TeV. For larger mass of the $S_3$ leptoquark the CP phase is suppressed due to the observables pertaining to the $B_s$ system. We provide predictions of direct and mixing-induced CP asymmetries in $B \to K μμ$ that could reveal the presence of the novel CP phase.

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arXiv2021

Leptoquarks and Real Singlets: A Richer Scalar Sector Behind the Origin of Dark Matter

We investigate scenarios with $\mathcal{O}(1\mathrm{\,TeV})$ scalar leptoquarks that act as portals between the Standard Model and Dark Matter. We assume that Dark Matter is a scalar singlet $S$ which couples to a scalar leptoquark $Δ$ and the Higgs boson via the terms in the scalar potential. In addition, the leptoquark is endowed with Yukawa couplings to quarks and leptons that may address the anomalies in $B$ meson decays. We consider the $SS$ annihilation cross sections to estimate the Dark Matter relic abundance and explore the interplay between astrophysical, collider and flavour physics bounds on such models. In the heavy Dark Matter window, $m_S > m_Δ$, the leptoquark portal becomes the dominant mechanism to explain the Dark Matter abundance. We find that the leptoquark Yukawa couplings, relevant for quark and lepton flavour physics, are decoupled from the dark matter phenomenology. By focussing on a scenario with a single leptoquark state, we find that relic density can only be explained when both $Δ$ and $S$ masses are lighter than $\mathcal{O}(10\mathrm{\,TeV})$.

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arXiv2021

Prospects of discovering new physics in rare charm decays

The LHCb bounds on the branching ratio of rare decay $D^0 \to μ^+ μ^-$ and the constraints on the branching ratio of $D^+ \to π^+ μ^+ μ^-$ in the nonresonant regions enable us to improve constraints on new physics contributions. Using the effective Lagrangian approach we determine sizes of the Wilson coefficients allowed by the existing LHCb bounds on rare charm decays. Then we discuss contributions to rare charm meson decay observables in several models of new physics: a model with an additional spin-1 weak triplet, leptoquark models, Two Higgs doublets model of type III, and a $Z'$ model. Here we complement the discussion by $D^0 - \bar D^0$ oscillations data. Among considered models, only leptoquarks can significantly modify Wilson coefficients. Assuming that the differential decay width for $D^+ \to π^+ μ^+ μ^-$ receives NP contribution, while the differential decay width for $D^+ \to π^+ e^+ e^-$ is Standard Model-like, we find that lepton flavor universality can be violated and might be observed at high dilepton invariant mass.

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arXiv2021

Optimized probes of $CP$-odd effects in the $t \bar{t} h$ process at hadron colliders

We use machine learning (ML) and non-ML techniques to study optimized $CP$-odd observables, directly and maximally sensitive to the $CP$-odd $i \tilde κ\bar t γ^5 t h$ interaction at the LHC and prospective future hadron colliders using the final state with a Higgs boson and a top quark pair, $pp\to t\bar t h$, followed by semileptonic $t$ decays. We perform phase-space optimization of manifestly $CP$-odd observables ($\boldsymbol ω$), sensitive to the sign of $\tilde κ$, and constructed from experimentally accessible final state momenta. We identify a simple optimized linear combination $\boldsymbol α\cdot \boldsymbolω$ that gives similar sensitivity as the studied fully fledged ML models. Using $\boldsymbolα\cdot \boldsymbolω$ we project the expected sensitivities to $\tilde κ$ at HL-LHC, HE-LHC, and FCC-hh.

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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

Probing the $CP$ nature of the top quark Yukawa at hadron colliders

We analyze the prospects of probing the $CP$-odd $i \tilde κ\bar t γ^5 t h$ interaction at the LHC and its projected upgrades, the high-luminosity and high-energy LHC, directly using associated on-shell Higgs boson and top quark or top quark pair production. To this end we first construct a $CP$-odd observable based on top quark polarization in $W b \to t h$ scattering with optimal linear sensitivity to $\tilde κ$. For the corresponding hadronic process $pp \to t h j$ we present a method of extracting the phase-space dependent weight function that allows to retain close to optimal sensitivity to $\tildeκ$. We project future sensitivity to the signal in $pp \to t(\to \ell νb )h(\to b\bar b) j$. Based on these insights we propose novel $CP$-odd observables for top quark pair production in association with the Higgs, $pp \to t \bar t h$, with semileptonically decaying tops and $h\to \bar b b$, that rely solely on measuring the momenta of leptons and $b$-jets from the decaying tops without having to distinguish the charge of the $b$-jets. Among the many possibilities we single out an observable that can potentially probe $\tilde κ\sim 0.5$ at the high-luminosity LHC and $\tilde κ\sim 0.1$ at high-energy LHC with $2σ$ confidence.

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arXiv2020

Beyond the Standard Model, guided by Lepton Universality

We discuss the current status of lepton flavour universality observables in semileptonic charged and neutral current $B$ meson decays. We recapitulate the current experimental situation. Assuming the current tensions with the SM are of beyond the Standard Model origin we present their implications for the effective theory couplings. Then we discuss general leptoquark models before finally focussing on the model with two scalar leptoquarks ($R_2$ and $S_3$) which have couplings connected by the underlying Grand Unified Theory framework. We present the viability of the model with regard to lepton flavour universality and present opportunities to search for this model in future high-intensity and high-energy experiments

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arXiv2019

GUT Physics in the era of the LHC

Grand Unified Theories (GUTs) are one of the most interesting high-energy completions of the Standard Model, because they provide a rich, powerful and elegant group-theoretical framework able to resolve a variety of problems remaining in our current understanding of particle physics. They usually act as motivators for many low energy BSM theories, such as left-right symmetric or supersymmetric models, and they serve to fill the gap between the experimentally reachable low energies and the physics in the ultraviolet. In recent years, however, they have fallen slightly from the spotlight, in favour of `simplified' models with more specific phenomenological predictions. The aim of this review is to summarize the state of the art on GUTs and argue for their importance in modern physics. Recent advances in experiments permit to test the predictions of GUTs at different energy scales. First, as GUTs can play a role in the inflationary dynamics of the early Universe, their imprints could be found in the CMB observations by the Planck satellite. Remarkably enough, GUTs could manifest themselves also in terrestrial tests; several planned experiments aim to probe the proton stability and to establish order of magnitude higher bounds on its lifetime. Moreover, the predictions of specific GUT models could be tested even at the LHC thanks to its high energy reach, via searches for exotic states or additional contributions to flavour anomalies.

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arXiv2019

Palatable Leptoquark Scenarios for Lepton Flavor Violation in Exclusive $b\to s\ell_1\ell_2$ modes

We examine various scenarios that involve a light ${\cal O}(1 TeV)$ leptoquark state and select those which are compatible with the current experimental values for $\mathcal{B}(B_s\to μμ)$, $\mathcal{B}(B\to Kμμ)_{\mathrm{large}-q^2}$, $R_K=\mathcal{B^\prime}(B\to Kμμ)/\mathcal{B^\prime}(B\to Kee)$, and which lead to predictions consistent with other experimental data. We show that two such scenarios are phenomenologically plausible, namely the one with a doublet of scalar leptoquarks of hypercharge $1/6$, and the one with a triplet of vector leptoquarks of hypercharge $2/3$. We also argue that a model with a singlet scalar leptoquark of hypercharge $1/3$ is not viable. Using the present experimental data as constraints, it is shown that the exclusive lepton flavor violating decays, $\mathcal{B}(B_s\to μτ)$, $\mathcal{B}(B\to K μτ)$ and $\mathcal{B}(B\to K^\ast μτ)$, can be as large as $\mathcal{O}(10^{-5})$.

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