arXiv subjects

Svjetlana Fajfer

Publications and source records attributed to Svjetlana Fajfer.

At least 19 recordsLinked to original sources

arXiv2026

New Avenues for $|ΔB|$ = 2 Processes Beyond Neutron-Antineutron Oscillations

We explore baryon-number-violating ($|ΔB| = 2$) processes beyond the well-known neutron-antineutron ($n - \bar{n}$) oscillations, focusing on the $Λ- \bar Λ$ system. The presence of a strange quark in the $Λ$ baryon introduces a new set of six-quark operators roughly of the form $(uds)^2$, which are different from the $(udd)^2$ operators responsible for $n - \bar{n}$ oscillations. Using the Standard Model Effective Field Theory (SMEFT), we classify all dimension-9 operators that cause $|ΔB|=2$ transitions and study their UV completions mediated by exotic scalar fields with trilinear interactions. We demonstrate that in these models, $Λ- \bar Λ$ oscillations can occur at tree level, with $n - \bar{n}$ mixing potentially appearing at higher loop levels. We employ a chiral effective theory to constrain the effective mass mixing $δm_Λ$, deriving bounds from current experimental limits on $n - \bar{n}$ oscillations and dinucleon decays such as $p \,p \to K^+ K^+$. These bounds indicate that $Λ- \barΛ$ oscillations probe a complementary parameter space, sensitive to baryon-number violation at scales up to $10^2-10^3$ TeV. We show that the existing indirect bounds make it challenging to provide a competitive bound on $δm_Λ$ at BESIII.

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arXiv2026

Neutrino Mass Induced $n$-$\overline{n}$ Oscillation

The Georgi-Glashow model is the simplest possible attempt at grand unification. However, due to its particle content, the model preserves a global $U(1)_{B-L}$ symmetry, where $B$ and $L$ are baryon and lepton numbers, respectively. It thus leaves neutrinos massless just as the Standard Model does. The extensions of the Georgi-Glashow model that break lepton number by two units, i.e., scenarios with $|ΔL|=2$ operator(s), naturally generate potentially viable Majorana neutrino masses. Since the dynamics that yields $|ΔL| = 2$ interactions unavoidably induces $|ΔB| = 2$ transitions via $B\!-\!L$ breaking, these extensions consequentially lead to intriguing processes such as neutron--antineutron ($n$--$\overline{n}$) oscillation. We investigate this intrinsic connection between neutrino mass generation and $n$--$\overline{n}$ oscillation within a number of representative extensions of the Georgi-Glashow model that can yield realistic neutrino masses and mixing parameters. These extensions include the tree-level seesaw mechanism realizations of the Type I, II, and III varieties, as well as the one-loop and two-loop radiative neutrino mass models.

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arXiv2026

Weak Triplet Models of Neutrino Magnetic Moments

Experimental limits on neutrino magnetic moments remain several orders of magnitude above the predictions of the Standard Model; therefore, any future detection would provide unambiguous evidence for new physics. In models with Dirac neutrinos, however, mechanisms that enhance the magnetic moment typically generate excessively large neutrino masses. Recently, it has been argued that in frameworks where neutrinos mix with weak-triplet Dirac fermions, the magnetic moment can be decoupled from the neutrino mass. In this work, we revisit this possibility and show that sizable enhancements remain highly nontrivial to realize naturally. We demonstrate that, although the minimal realization allows the magnetic moment to be decoupled from the neutrino mass, obtaining an observable enhancement requires a delicate adjustment of the model parameters. Moreover, in extended scenarios, the decoupling no longer persists: the magnetic moment and neutrino mass become intrinsically linked, such that attempts to enhance the former inevitably induce large contributions to the latter.

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arXiv2025

Impact of new invisible particles on $B\to K^{(*)} E_{\rm miss}$ observables

Motivated by a recent Belle~II measurement that suggests an excess in the rare decay $B \to K\, E_{\rm miss}$, and building upon our recent differential decay rate likelihood analysis of the existing experimental information, we investigate possible new physics (NP) scenarios in which light invisible states participate in flavour-changing $b \to s$ transitions. In particular, we consider the total and differential $B\to K^* E_{\rm miss}$ decay rates and $K^*$ polarisation effects in each NP scenario preferred by the $B\to K E_{\rm miss}$ measurement. We show that future measurements of these $B \to K^* E_{\rm miss}$ observables will offer decisive discrimination among the different NP explanations. Our results highlight the strong complementarity of the rare semi-invisible $b$-hadron decay observables, and underline the importance of analysing their momentum transfer spectra when probing extensions of the Standard Model that feature new light degrees of freedom.

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arXiv2025

Beyond Neutrino Mass: Observable $n$-$\overline{n}$ Oscillations in UV Complete Seesaw Models

Next-generation experiments, such as the Deep Underground Neutrino Experiment and the European Spallation Source, are set to improve sensitivity to neutron-antineutron oscillation, a direct probe of $ΔB = 2$ baryon number violation, with particularly significant gains expected at the latter. The discovery of such a rare $ΔB = 2$ process would indicate physics beyond the Standard Model and could point to specific unified theories that allow observable $n-\overline{n}$ transitions. We accordingly examine $n-\overline{n}$ oscillations within a unified framework that accounts for charged fermion masses and generates viable neutrino masses via the seesaw mechanism. More specifically, we show that $n-\overline{n}$ oscillations can arise from two specific topologies within two distinct $SU(5)$ scenarios. One topology requires a presence of two color-sextet scalars in the Type II seesaw framework, whereas the other involves a scalar sextet and a color-octet fermion in the Type III seesaw framework. While the former topology can be realized in the $SO(10)$/Pati-Salam frameworks, the latter finds a natural embedding in $SU(5)$, which constitutes one of the key novelties of our work. Remarkably enough, the same dynamics responsible for fermion masses also induces baryon number violation, thus linking $n-\overline{n}$ oscillations to the flavor structure of the theory. We show that, given a TeV-scale mass for one of the colored states, upcoming searches for such $ΔB = 2$ processes can probe for a presence of the other colored states with masses up to $10^{11}\,\mathrm{GeV}$, well beyond the reach of colliders. This positions $n-\overline{n}$ oscillations as a rare low-energy portal to grand unification and ultra-heavy new physics.

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arXiv2025

Baryon Number Violation: From Nuclear Matrix Elements to BSM Physics

Processes that violate baryon number, most notably proton decay and $n\bar n$ transitions, are promising probes of physics beyond the Standard Model (BSM) needed to understand the lack of antimatter in the Universe. To interpret current and forthcoming experimental limits, theory input from nuclear matrix elements to UV complete models enters. Thus, an interplay of experiment, effective field theory, lattice QCD, and BSM model building is required to develop strategies to accurately extract information from current and future data and maximize the impact and sensitivity of next-generation experiments. Here, we briefly summarize the main results and discussions from the workshop "INT-25-91W: Baryon Number Violation: From Nuclear Matrix Elements to BSM Physics," held at the Institute for Nuclear Theory, University of Washington, Seattle, WA, January 13-17, 2025.

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arXiv2025

Signatures of Light New Particles in $B\to K^{(*)} E_{\rm miss}$

The recent Belle II observation of $B \to K E_{\rm miss}$ challenges theoretical interpretations in terms of Standard Model neutrino final states. Instead, we consider new physics scenarios where up to two new light-invisible particles of spin 0 up to 3/2 are present in the final state. We identify viable scenarios by reconstructing the (binned) likelihoods of the relevant $B \to K^{(*)} E_{\rm miss}$ and also $B_s \to E_{\rm miss}$ experimental analyses and present preferred regions of couplings and masses. In particular, we find that the current data prefers two-body decay kinematics involving the emission of a single massive scalar or a vector particle, or alternatively, three-body decays involving pairs of massive scalars or spin 1/2 fermions. When applicable, we compare our findings with existing literature and briefly discuss some model-building implications.

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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

Gauge and Scalar Boson Mediated Proton Decay in a Predictive SU(5) GUT Model

We assess proton decay signatures in the simplest viable $SU(5)$ model with regard to constraints on parameters governing the Standard Model fermion mass spectrum. Experimental signals for all eight two-body proton decay processes result from exchange of two gauge bosons, a single scalar leptoquark, or their combination. Consequently, it enables us to delve into an in-depth anatomy of proton decay modes and anticipate future signatures. Our findings dictate that observing a proton decay into $p\toπ^0e^+$ indicates gauge boson mediation, with the potential for observation of $p\toη^0e^+$ mode. Alternatively, if decay is through $p\to K^+\overlineν$ process, it is mediated by a scalar leptoquark, possibly allowing the observation of $p\toπ^0μ^+$. Detection of both $p\toπ^0 e^+$ and $p\to K^+\overlineν$ could enhance $p\toπ^0μ^+$ through constructive interference. The model predicts inaccessibility of $p\toπ^+\overlineν$, $p\toη^0μ^+$, $p\to K^0e^+$, and $p\to K^0μ^+$, regardless of the dominant mediation type, in the coming decades. In summary, through a comprehensive analysis of proton decay signals, gauge coupling unification, and fermion masses and mixing, we precisely constrain the parameter space of the $SU(5)$ model in question.

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arXiv2023

$S$-wave contribution to rare $D^0 \to π^+ π^- \ell^+ \ell^-$ decays in the Standard Model and sensitivity to New Physics

Physics of the up-type flavour offers unique possibilities of testing the Standard Model (SM) compared to the down-type flavour sector. Here, we discuss SM and New Physics (NP) contributions to the rare charm-meson decay $ D^0 \to π^+ π^- \ell^+ \ell^- $. In particular, we discuss the effect of including the lightest scalar isoscalar resonance in the SM picture, namely, the $f_0 (500)$, which manifests in a big portion of the allowed phase space. Other than showing in the total branching ratio at an observable level of about $ 20\% $, the $f_0 (500)$ resonance manifests as interference terms with the vector resonances, such as at high invariant mass of the leptonic pair in distinct angular observables. Recent data from LHCb optimize the sensitivity to $P$-wave contributions, that we analyse in view of the inclusion of vector resonances. We propose the measurement of alternative observables which are sensitive to the $S$-wave and are straightforward to implement experimentally. This leads to a new set of null observables, that vanish in the SM due to its gauge and flavour structures. Finally, we study observables that depend on the SM interference with generic NP contributions from semi-leptonic four-fermion operators in the presence of the $S$-wave.

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arXiv2023

Decay dynamics of $N\to \ell π$ and $N\to \ell γ$

SuperKamiokande experiment tightly bounds the lifetimes of the baryon number violating proton decays. The decay widths for the nucleon to an antilepton and a photon are not so well bounded experimentally as $\ell π$ modes. Using an effective Lagrangian approach, we relate the decay widths of $N\to \ell γ$ to the decay widths of $N\to \ell π$. Our result points out factor $10^{3}$ suppression of the decay widths $Γ(p \to \ell^{+} γ)$ and $Γ(n \to \bar νγ)$ compared to the decay widths $Γ(p \to \ell^{+} π^{0})$ and $Γ(n \to \bar νπ^{0})$, respectively. This result is independent of the model of new physics. Then, we investigate the dynamics of $N\to \ell π$ and $N\to \ell γ$ amplitudes at the tree and loop level in which scalar leptoquarks are mediators of a new interaction. At the tree level, leptoquarks probe the physics beyond the Standard Model at a scale of $10^{16}$ GeV, while at the loop level, such decay can occur at a scale of $10^{7}$ GeV.

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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

Probing CP violation in exclusive $b \to s ν\bar ν$ transitions

We consider the time-dependent analysis of rare $B_d$ and $B_s$ decays mediated by $b\to s ν\bar ν$ transitions. The inclusion of time evolution allows us to construct novel observables with specific sensitivity to CP-odd phases in these processes. The sensitivity to CP violation of corresponding time-integrated measurements in presence of flavor-tagging is also explored. We provide precise predictions for these observables in the SM and explore their sensitivity to new CP-violating NP contributions at present and planned future $B$-physics experiments. As such, these observables provide unique probes of CP violation in $b \to s ν\bar ν$ transitions.

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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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arXiv2022

Triple-leptoquark interactions for tree- and loop-level proton decays

We study the impact of triple-leptoquark interactions on matter stability for two specific proton decay topologies that arise at the tree- and one-loop level if and when they coexist. We demonstrate that the one-loop level topology is much more relevant than the tree-level one when it comes to the proton decay signatures despite the usual loop-suppression factor. We subsequently present detailed analysis of the triple-leptoquark interaction effects on the proton stability within one representative scenario to support our claim, where the scenario in question simultaneously features a tree-level topology that yields three-body proton decay $p\to e^+ e^+ e^-$ and a one-loop level topology that induces two-body proton decays $p\to π^0 e^+$ and $p\to π^+ \barν$. We also provide a comprehensive list of the leading-order proton decay channels for all non-trivial cubic and quartic contractions involving three scalar leptoquark multiplets that generate triple-leptoquark interactions of our interest, where in the latter case one of the scalar multiplets is the Standard Model Higgs doublet.

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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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