arXiv subjects

K. Müürsepp

Publications and source records attributed to K. Müürsepp.

5 recordsLinked to original sources

arXiv2026

Sensitivity targets for dark photon searches in charged kaon decays

We investigate the prospects for detecting invisible dark photons in charged kaon decays through the two-pion channel $K^+\toπ^+π^0\,γ_D$ and the one-pion radiative decay $K^+\toπ^+γ\,γ_D$, where $γ$ and $γ_D$ denote the ordinary photon and dark photon, respectively. The former decay was previously proposed as a golden channel for massless dark photon searches at NA62, while here we introduce the radiative process as a complementary probe and extend both analyses to massive dark photons. The three-body modes are particularly relevant at low dark photon masses, where the two-body decay $K^+\toπ^+\, γ_D$ becomes strongly suppressed and vanishes in the massless limit. For massless dark photons, renormalizable interactions induced by kinetic mixing are absent, and the dominant contribution arises from the effective non-renormalizable flavor-changing quark interaction $s\to d\, γ_D$ mediated by a dimension-five magnetic-dipole operator. This is also true for massive dark photons. Their kinetic mixing contributes negligibly to these channels, after considering the existing bounds on the mixing parameter. Only the same flavor changing magnetic-dipole operator as in the massless case is relevant. Using the chiral quark model, we estimate the relevant hadronic matrix elements of the dipole operator and account for constraints from $K^+\toπ^+\,X$, where $X$ denotes resonant missing energy, $K^+\toπ^+ν\barν$, and $K^0\!\!-\!\bar{K}^0$ mixing. Based on these calculations, we determine the allowed branching ratios for these decay modes and the sensitivities required at NA62 and future charged-kaon facilities.

hep-ph↗

arXiv2026

Testing the unitarity of the light neutrino mixing matrix

We propose a novel test of the unitarity of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix at collider experiments. Our approach exploits the incomplete cancellation between $t$-channel neutrino exchange and $s$-channel gauge-boson contributions that arises in the presence of violation of the flavor-diagonal PMNS unitarity conditions in weak boson pair production, leading to an anomalous growth of the cross section with energy. Such effects are generic in extensions of the Standard Model in which light neutrinos mix with heavier states, and can manifest at colliders as long as the characteristic energy of the process remains below the mass threshold of the new degrees of freedom. After briefly reviewing these scenarios, we employ our strategy to derive model-independent bounds on flavor diagonal unitarity-violating effects using LEP~II data. We then present sensitivity projections for future lepton and hadron colliders, demonstrating that they are well suited to probe the unitarity of the neutrino mixing matrix with this method.

hep-ph↗

arXiv2026

Testing the CKM unitarity at high energy via the $W^+W^-$ production at the LHC and future colliders

We propose a novel test to assess the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, $V_{\rm CKM}$, at present and future collider experiments. Our strategy makes use of the $W^+W^-$ production cross section to directly probe the $V_{\rm CKM}^\dagger V_{\rm CKM}$ product, which regulates the high-energy behavior of the observable. The violation of unitarity is signalled by an anomalous behavior of the cross section that grows quadratically with the $W^+W^-$ invariant mass with respect to the Standard Model prediction. By using the recent ATLAS measurements of the $W^+W^-$ cross section we are able to constrain the maximal unitarity violation allowed by current data, producing a bound complementary to the results of flavor physics experiments. Forecasts for the high luminosity phase of the LHC and for the future 100 TeV hadron collider are also discussed.

hep-ph↗

arXiv2024

Explaining the $B^+\to K^+ ν\barν$ excess via a massless dark photon

The Belle II collaboration has recently observed the rare decay $B^+\to K^+ ν\barν$, finding an excess with respect to the Standard Model prediction. We explore the possibility that the data entails long-distance interactions induced by a massless dark photon, $γ_{\scriptscriptstyle{D}}$. This couples at the tree-level to an invisible, dark sector and to the Standard Model via higher-dimensional operators, such as the chromomagnetic-dipole coupling that we use to explain the excess. As the process $B^+\to K^+ γ_{\scriptscriptstyle{D}}$ is forbidden by angular momentum conservation, the transition mediated by the off-shell dark photon yields a three-body final state comprising a pair of dark fermions that show as a missing energy continuum in the detector, faking the neutrino signature. We show that the Belle II data is explained for perturbative values of the parameters of the model. This scenario predicts new contributions to the neutral $B$ meson decays $B^0\to K^* γ_{\scriptscriptstyle{D}}$, in which the emission of a on-shell dark photon is allowed, yielding a monochromatic missing energy signature. Analogously, an excess due to the emission of a dark photon is predicted for the $B^0_s\to ϕ+ E_{\rm miss}$ decay that could be scrutinized next at the LHCb experiments.

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arXiv2021

New fermions in the light of the $(g-2)_μ$

The very precise measurement of the anomalous magnetic moment of the muon, recently released by the Muon g-2 experiment at Fermilab, can serve to set stringent constraints on new particles. If the observed 4$σ$ discrepancy from the Standard Model value is indeed real, it will set a tight margin on the scale of the masses and couplings of these particles. Instead, if the discrepancy is simply a result of additional theoretical and experimental uncertainties to be included, strong constraints can be put on their parameters. In this mini-review, we summarize the impact of the latest muon g-2 measurement on new fermions that are predicted by a wide range of new physics models and with exotic quantum numbers and interactions. We will particularly discuss the case of vector-like leptons, excited leptons, and supersymmetric fermions, as well as spin-3/2 isosinglet fermions, which have been advocated recently.

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