arXiv subjects

Kåre Fridell

Publications and source records attributed to Kåre Fridell.

16 recordsLinked to original sources

arXiv2026

Feasibility Study of Lepton Number Violation in Rare $B$ and $K$ Meson Decays

We study lepton-number-violating interactions at dimension seven in the Standard Model effective field theory that contribute to the meson decays $B \to K νν$ and $K \to πνν$. Such interactions could washout the baryon asymmetry of the Universe and also contribute to the neutrinoless double beta decay, even though the interactions involve a change in down-type quark flavors. We clarify conditions under which excesses in meson decay rates over the Standard Model predictions can be successfully observed. We also show that, although these interactions contribute to neutrino masses at the two-loop level, the Weinberg operator can be introduced consistently without spoiling the scenario.

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arXiv2025

Radiative neutrino masses from dim-7 SMEFT: a simplified multi-scale approach

Lepton-number-violating interactions occur in the Standard Model Effective Field Theory (SMEFT) at odd dimensions starting from the dimension-5 Weinberg operator. Although the operators at dimension-7 and higher are more suppressed by the heavy new scale, they can be crucial when traditional seesaw mechanisms leading to tree-level dimension-5 contributions are absent. We identify all minimal tree-level UV-completions for dimension-7 $ΔL=2$ SMEFT operators without covariant derivatives and propose a new simplified approach for estimating the radiative neutrino masses arising from such operators. This dimensional-regularisation-based approach provides a more accurate estimate for the loop neutrino masses when the new physics fields are hierarchical in mass, as compared to the cut-off-regularisation-based approach often employed in the literature. This allows us to identify viable regions of parameter space in the full list of relevant simplified models close to the current limits set by neutrinoless double beta decay and the LHC that would previously have been thought to be excluded by neutrino-mass constraints.

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arXiv2025

Lepton flavor physics at $μ^+ μ^+$ colliders

We discuss sensitivities to lepton flavor violating (and conserving) interactions at future muon colliders, especially at $μ^+μ^+$ colliders. Compared with the searches for rare decays of $μ$ and $τ$, we find that the TeV-scale future colliders have better sensitivities depending on the pattern of hierarchy in the flavor mixings. As an example, we study the case with the type-II seesaw model, where the flavor mixing parameters have direct relation to the neutrino mass matrix. At a $μ^+ μ^+$ collider, the number of events of the $μ^+ μ^+ \to μ^+ τ^+$ process can be larger than $\mathcal{O}(100)$ with the center of mass energy $\sqrt s = 2$ TeV, and with an integrated luminosity ${\cal L} = 1$ ab$^{-1}$, while satisfying bounds from rare decays of $μ$ and $τ$. We discuss impacts of the overall mass scale of neutrinos as well as CP violating phases to the number of expected events.

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arXiv2025

Leptogenesis and neutrino mass with scalar leptoquarks

Leptoquarks are known to generate a wide range of potentially observable phenomena, and have been searched for in different experiments. We show that the observed baryon asymmetry and neutrino mass scale can both be simultaneously produced in a model featuring scalar leptoquarks while avoiding existing experimental constraints and potentially leading to future observable signatures.

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arXiv2024

Impact of Bound State Formation on Baryogenesis

The mechanism behind the generation of the baryon asymmetry of the Universe (BAU) is one of the biggest open questions of (astro-)particle physics. Popular mechanisms to generate the observed baryon asymmetry include CP-violating out-of-equilibrium decays and scatterings of heavy particles. If these heavy non-relativistic particles feature long-range interactions, the formation of bound states can impact the generation of the baryon asymmetry. We outline the general conditions for when bound states are important for decay and scattering dominated baryogenesis and present the necessary Boltzmann equations for the first time. We demonstrate that bound states can impact baryogenesis in three different ways: They (i) strongly impact abundances of particles sourcing the BAU, (ii) act as a source term of the asymmetry, and (iii) mediate additional washout channels.

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arXiv2024

Non-Canonical Nucleon Decays as Window into Light New Physics

Nucleon decays are generic predictions of motivated theories, including those based on the unification of forces and supersymmetry. We demonstrate that non-canonical nucleon decays offer a unique opportunity to broadly probe light new particles beyond the Standard Model with masses below $\sim$few GeV over decades in mass range, including axion-like particles, dark photons, sterile neutrinos, and scalar dark matter. Conventional searches can misinterpret and even completely miss such new physics. We propose a general strategy based on momenta of visible decay final states to probe these processes, offering a rich physics program for existing and upcoming experiments such as Super-Kamiokande, Hyper-Kamiokande, DUNE, and JUNO.

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arXiv2024

Decoding the $B \to K νν$ excess at Belle II: kinematics, operators, and masses

An excess in the branching fraction for $B^+ \to K^+ νν$ recently measured at Belle II may be a hint of new physics. We perform thorough likelihood analyses for different new physics scenarios such as $B \to KX$ with a new invisible particle $X$, or $B\to Kχχ$ through a scalar, vector, or tensor current with $χ$ being a new invisible particle or a neutrino. We find that vector-current 3-body decay with $m_X \simeq 0.6$ GeV - which may be dark matter - is most favored, while 2-body decay with $m_X \simeq 2$ GeV is also competitive. The best-fit branching fractions for the scalar and tensor cases are a few times larger than for the 2-body and vector cases. Past BaBar measurements provide further discrimination, although the best-fit parameters stay similar.

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arXiv2024

Heavy photophobic ALP at the LHC

We study the photophobic ALP model in high-mass regions under LHC Run-II. Since the ALP is predominantly coupled with electroweak gauge bosons such as $ZZ$, $WW$, and $Zγ$, and less with di-photon, the model may be probed via multi-boson final-state processes. We find that on-shell ALP productions with $Zγ$ final states currently provide the best sensitivities for $m_{a} > 40~{\rm GeV}$.

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arXiv2024

Probing Lepton Number Violation: A Comprehensive Survey of Dimension-7 SMEFT

Observation of lepton number violation would represent a groundbreaking discovery with profound consequences for fundamental physics and as such, it has motivated an extensive experimental program searching for neutrinoless double beta decay. However, the violation of lepton number can be also tested by a variety of other observables. We focus on the possibilities of probing this fundamental symmetry within the framework of the Standard Model Effective Field Theory (SMEFT) beyond the minimal dimension-5. Specifically, we study the bounds on $ΔL = 2$ dimension-7 effective operators beyond the electron flavor imposed by all relevant low-energy observables and confront them with derived high-energy collider limits. We also discuss how the synergy of the analyzed multi-frontier observables can play a crucial role in distinguishing among different dimension-7 SMEFT operators.

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arXiv2022

Probing Active-Sterile Neutrino Transition Magnetic Moments with Photon Emission from CE$ν$NS

In the presence of transition magnetic moments between active and sterile neutrinos, the search for a Primakoff upscattering process at coherent elastic neutrino-nucleus scattering (CE$ν$NS) experiments can provide stringent constraints on the neutrino magnetic moment. We show that a radiative upscattering process with an emitted photon in the final state can induce a novel coincidence signal at CE$ν$NS experiments that can also probe neutrino transition magnetic moments beyond existing limits. Furthermore, the differential distributions for such a radiative mode can also potentially be sensitive to the Dirac vs. Majorana nature of the sterile state mediating the process. This can provide valuable insights into the nature and mass generation mechanism of the light active neutrinos.

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arXiv2022

New Ideas in Baryogenesis: A Snowmass White Paper

The Standard Model of Particle Physics cannot explain the observed baryon asymmetry of the Universe. This observation is a clear sign of new physics beyond the Standard Model. There have been many recent theoretical developments to address this question. Critically, many new physics models that generate the baryon asymmetry have a wide range of repercussions for many areas of theoretical and experimental particle physics. This white paper provides an overview of such recent theoretical developments with an emphasis on experimental testability.

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arXiv2021

Probing baryogenesis with neutron-antineutron oscillations

In the near future, the Deep Underground Neutrino Experiment and the European Spallation Source aim to reach unprecedented sensitivity in the search for neutron-antineutron ($n\text{-}\bar{n}$) oscillations, whose observation would directly imply $|ΔB| = 2$ violation and hence might hint towards a close link to the mechanism behind the observed baryon asymmetry of the Universe. In this work, we explore the consequences of such a discovery for baryogenesis first within a model-independent effective field theory approach. We then refine our analysis by including a source of CP violation and different hierarchies between the scales of new physics using a simplified model. We analyse the implication for baryogenesis in different scenarios and confront our results with complementary experimental constraints from dinucleon decay, LHC, and meson oscillations. We find that for a small mass hierarchy between the new degrees of freedom, an observable rate for $n\text{-}\bar{n}$ oscillation would imply that the washout processes are too strong to generate any sizeable baryon asymmetry, even if the CP violation is maximal. On the other hand, for a large hierarchy between the new degrees of freedom, our analysis shows that successful baryogenesis can occur over a large part of the parameter space, opening the window to be probed by current and future colliders and upcoming $n\text{-}\bar{n}$ oscillation searches.

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arXiv2021

Direct detection of fermionic and vector dark matter with polarised targets

We study the scattering of Milky Way dark matter (DM) particles by spin-polarised target nuclei within a set of simplified models for fermionic and vector DM where DM interacts with spin 1/2 point-like nuclei through the exchange of a vector or pseudo-vector mediator particle. This study is motivated by the possibility of using polarised targets to gain novel insights into the nature of DM. For fermionic DM, we provide an explicit expression for the polarised DM-nucleus scattering cross section refining previous results found in the literature. For vector DM, we calculate the polarised cross section for DM-nucleus scattering for the first time. We find that polarised targets can in principle be used to discriminate fermionic from vector DM.

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arXiv2021

Probing lepton number violating interactions in rare kaon decays

We investigate the possibility to probe lepton number violating (LNV) operators in the rare kaon decay $K\toπνν$. Performing the analysis in the Standard Model effective field theory with only light active Majorana neutrinos, we determine the current limits on the corresponding LNV physics scale from the past E949 experiment at BNL as well as the currently operating experiments NA62 at CERN and KOTO at J-PARC. We focus on the specific signature of scalar currents in $K\toπνν$ arising from the LNV nature of the operators and study the effect on the experimental sensitivity, stressing the need for dedicated searches for beyond the SM currents. We find that the rare kaon decays probe high operator scales $Λ_\text{LNV} \approx 15$ to $20$ TeV in different quark and neutrino flavours compared to neutrinoless double beta decay. Furthermore, we comment that the observation of LNV in kaon decays can put high-scale leptogenesis under tension. Finally, we discuss the connection with small radiatively generated neutrino masses and show how the severe constraints therefrom can be evaded in a minimal ultraviolet-complete scenario featuring leptoquarks.

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arXiv2020

Implications of Rare Kaon Decays on Lepton Number Violating Interactions

We explore the possibility of lepton number violation (LNV) manifesting in the rare kaon decay $K\toπνν$, and its consequences for radiative neutrino mass generation and the washout of Lepton asymmetry in high-scale leptogenesis scenarios. We perform the analysis in a model-independent framework, the Standard Model effective field theory (SMEFT), and discuss the possible LNV nature of the rare kaon decay in the context of the currently operating NA62 experiment at CERN. We find that, in case of a LNV interaction, its detection would put high-scale leptogenesis under tension and would hint to small radiatively generated neutrino masses.

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arXiv2019

Non-relativistic Effective Interactions of Spin 1 Dark Matter

We investigate the non-relativistic reduction of simplified models for spin 1 dark matter (DM) with the aim of identifying features in the phenomenology of DM-quark interactions which are specific to vector DM. In the case of DM-quark interactions mediated by a spin 1 particle, we find two DM-nucleon interaction operators arising from the non-relativistic reduction of simplified models for spin 1 DM that are specific to spin 1 DM, and which were not considered in previous studies. They are quadratic in the momentum transfer, linear in a symmetric combination of polarisation vectors for the DM particle, and arise from simplified models which do not generate momentum transfer independent operators as leading interactions in the non-relativistic expansion of DM-nucleon scattering amplitudes. Within these simplified models, the new operators cannot be neglected when computing DM signals at direct detection experiments. For example, we find that nuclear recoil energy spectra computed by including or neglecting the new operators can differ by up to one order of magnitude for nuclear recoil energies larger than about 20 keV and DM masses below $50$ GeV. Furthermore, the shape of the expected nuclear recoil spectra depends significantly on whether the new operators are taken into account or not. Finally, neglecting the contribution to DM direct detection signals from the new operators leads to inaccurate conclusions when assessing the compatibility of a future direct detection signal with CMB constraints on the DM relic density, especially when the number of signal events is small, e.g. $\mathcal{O}(1)$.

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