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Arbelaez, C., Helo, J. C., Hirsch, M., & Ota, T. (2026). n → Kℓ and the baryon asymmetry of the universe. J. High Energy Phys., 09(9), 099–21pp.
Abstract: The observed baryon asymmetry (BAU) of the universe puts strong constraints on any (B – L)-violating interaction. An observation of a (B – L)-violating nucleon decay channel would therefore have profound implications for our understanding of the BAU. Here we point out that the observation of the final state with a kaon and a charged lepton in a future nucleon decay experiment would hint at (B – L) violation even if the charge of the lepton is not determined experimentally. In SMEFT, this follows from the fact that n -> K + & ell; – arises already at dimension seven, while the (B – L)-conserving decay n -> K – & ell; + requires dimension-ten operators that, in addition, would be accompanied by lower-dimensional (B + L)-violating decay modes. An observation of n -> K & ell; in the absence of other modes such as p -> pi 0 & ell; +, would then strongly suggest that (B – L) is violated.
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Bas i Beneito, A., Fajfer, S., & Petrov, A. A. (2026). New avenues for |△B|=2 processes beyond neutron-antineutron oscillations. J. High Energy Phys., 03(3), 124–35pp.
Abstract: We explore baryon-number-violating (|∆B| = 2) processes beyond the well-known neutron-antineutron (n−n‾) oscillations, focusing on the Λ−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−n‾ oscillations. Using the Standard Model Effective Field Theory (SMEFT), we classify all dimension-9 operators that cause |∆B| = 2 transitions and study their UV completions mediated by exotic scalar fields with trilinear interactions. We demonstrate that in these models, Λ−Λ‾ oscillations can occur at tree level, with n−n‾ mixing potentially appearing at higher loop levels. We employ a chiral effective theory to constrain the effective mass mixing δmΛΛ, deriving bounds from current experimental limits on n−n‾ oscillations and dinucleon decays such as pp → K+K+. These bounds indicate that Λ−Λ‾ 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 δmΛΛ at BESIII.
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Bas i Beneito, A., Gargalionis, J., Herrero-Garcia, J., Santamaria, A., & Schmidt, M. A. (2024). An EFT approach to baryon number violation: lower limits on the new physics scale and correlations between nucleon decay modes. J. High Energy Phys., 07(7), 004–37pp.
Abstract: Baryon number is an accidental symmetry of the Standard Model at the Lagrangian level. Its violation is arguably one of the most compelling phenomena predicted by physics beyond the Standard Model. Furthermore, there is a large experimental effort to search for it including the Hyper-K, DUNE, JUNO, and THEIA experiments. Therefore, an agnostic, model-independent, analysis of baryon number violation using the power of Effective Field Theory is very timely. In particular, in this work we study the contribution of dimension six and seven effective operators to |triangle(B – L)| = 0, 2 nucleon decays taking into account the effects of Renormalisation Group Evolution. We obtain lower limits on the energy scale of each operator and study the correlations between different decay modes. We find that for some operators the effect of running is very significant.
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Bas i Beneito, A., Gargalionis, J., Herrero-Garcia, J., & Schmidt, M. A. (2025). Squeezing proton decay and neutrino masses: upper bounds on standard model extensions. J. High Energy Phys., 10(10), 083–40pp.
Abstract: Baryon and lepton number are excellent low-energy symmetries of the Standard Model (SM) that tightly constrain the form of its extensions. In this paper we investigate the possibility that these accidental symmetries are violated in the deep UV, in such a way that one multiplet necessary for their violation lives at an intermediate energy scale M above the electroweak scale. We write down the simplest effective operators containing each multiplet that may couple linearly to the SM at the renormalisable level and estimate the dominant contribution of the underlying UV model to the pertinent operators in the SMEFT: the dimension-5 Weinberg operator and the baryon-number-violating operators up to dimension 7. Our results are upper bounds on the scale M for each multiplet-operator pair, derived from neutrino-oscillation data as well as prospective nucleon-decay searches. We also analyse the possibility that both processes are simultaneously explained within a natural UV model. In addition, we advocate that our framework provides a convenient and digestible way of organising the space of UV models that violate these symmetries.
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Bas i Beneito, A., Palavric, A., & Sainaghi, A. (2026). EFT pathways to |△B|=2: chiral constructions and phenomenology. J. High Energy Phys., 09(9), 091–64pp.
Abstract: We develop a systematic effective field theory framework for studying |triangle B| = 2 interactions across energy scales. Using chiral symmetry, we construct the complete and non-redundant set of operators governing these interactions at low energies and establish their connection to the corresponding operators in the Standard Model effective field theory, as well as to their realizations in baryon chiral perturbation theory. The framework is then applied to the phenomenology of baryon-antibaryon oscillations and dinucleon decay. While oscillations probe only a limited subset of operator structures, dinucleon decay is sensitive to a significantly broader class, including transitions that are otherwise inaccessible. In addition, we identify previously unexplored dinucleon decay channels, which can probe these unconstrained regions of parameter space. More generally, this formalism makes explicit the complementarity of different probes and provides a consistent way to trace baryon-number-violating effects from their ultraviolet origin to low-energy hadronic observables, thereby providing a basis for systematic studies of ultraviolet models generating such interactions.
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Beltran, R., Cepedello, R., & Hirsch, M. (2023). Tree-level UV completions for NRSMEFT d=6 and d=7 operators. J. High Energy Phys., 08(8), 31pp.
Abstract: We study ultra-violet completions for operators in standard model effective field theory extended with right-handed neutrinos (NRSMEFT). Using a diagrammatic method, we generate systematically lists of possible tree-level completions involving scalars, fermions or vectors for all operators at d = 6 and d = 7, which contain at least one right-handed neutrino. We compare our lists of possible UV models to the ones found for pure SMEFT. We also discuss how the observation of LNV processes via NRSMEFT operators at the LHC can be related to Majorana neutrino masses of the standard model neutrinos.
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Candela, P. M., Khoze, V. V., & Turner, J. (2025). Monopoles at future neutrino detectors. J. High Energy Phys., 07(7), 034–21pp.
Abstract: We investigate the potential of future neutrino experiments, DUNE and Hyper-Kamiokande, to probe magnetic monopoles via Callan-Rubakov (CR) processes. We consider both relativistic and non-relativistic monopoles and focus on two primary detection signatures: high-energy antiproton production and proton decay catalysis. For relativistic monopoles, our analysis of the CR process indicates antiproton production with energies near 900 GeV and we find that both experiments can provide limits on the fluxes an order of magnitude below the Parker bound (approximately Phi less than or similar to 10-16 cm-2 s-1 sr-1). For non-relativistic monopoles, we recast the experimental sensitivity to proton decay catalysis and obtain upper limits on the monopole flux of Phi less than or similar to 2.3 x 10-23 cm-2 s-1 sr-1 for Hyper-Kamiokande and Phi less than or similar to 1.1 x 10-22 cm-2 s-1 sr-1 for DUNE.
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Coito, L., Faubel, C., Herrero-Garcia, J., Santamaria, A., & Titov, A. (2022). Sterile neutrino portals to Majorana dark matter: effective operators and UV completions. J. High Energy Phys., 08(8), 085–36pp.
Abstract: Stringent constraints on the interactions of dark matter with the Standard Model suggest that dark matter does not take part in gauge interactions. In this regard, the possibility of communicating between the visible and dark sectors via gauge singlets seems rather natural. We consider a framework where the dark matter talks to the Standard Model through its coupling to sterile neutrinos, which generate active neutrino masses. We focus on the case of Majorana dark matter, with its relic abundance set by thermal freeze-out through annihilations into sterile neutrinos. We use an effective field theory approach to study the possible sterile neutrino portals to dark matter. We find that both lepton-number-conserving and lepton-number-violating operators are possible, yielding an interesting connection with the Dirac/Majorana character of active neutrinos. In a second step, we open the different operators and outline the possible renormalisable models. We analyse the phenomenology of the most promising ones, including a particular case in which the Majorana mass of the sterile neutrinos is generated radiatively.
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Esser, F., Graf, L., & Hati, C. (2026). Cartography of LNV dim-9 SMEFT: implications for radiative neutrino masses and 0νββ. J. High Energy Phys., 07(7), 206–46pp.
Abstract: We perform a systematic study of lepton-number-violating (LNV) dimension-9 operators in the Standard Model Effective Field Theory (SMEFT) that can mediate neutrinoless double beta decay (0 nu beta beta) at tree level, and map them to their possible tree-level ultraviolet completions. Using a diagram-based classification, we enumerate all such completions and isolate minimal two-particle models that avoid generating the dimension-5 Weinberg operator or dimension-7 LNV operators at tree level. We then chart how these minimal models populate the operator landscape and organise them by the loop order at which they radiatively induce lower-dimensional LNV operators, highlighting scenarios in which the tree-level dimension-9 contribution can compete with or dominate loop-suppressed neutrino-mass (dimension-5) effects. Representative one-loop and two-loop classes are matched onto the SMEFT, and their implications for neutrino masses, charged-lepton flavour violation, and the relative size of dimension-9 versus dimension-5 contributions to 0 nu beta beta are analysed, delineating regions of parameter space where upcoming experiments can be sensitive to genuinely short-range LNV dynamics.
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Fridell, K., Graf, L., Harz, J., & Hati, C. (2025). Radiative neutrino masses from dim-7 SMEFT: a simplified multi-scale approach. J. High Energy Phys., 09(9), 050–39pp.
Abstract: 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 triangle L = 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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