Beltran Jimenez, J., & Delhom, A. (2019). Ghosts in metric-affine higher order curvature gravity. Eur. Phys. J. C, 79(8), 656–7pp.
Abstract: We disprove the widespread belief that higher order curvature theories of gravity in the metric-affine formalism are generally ghost-free. This is clarified by considering a sub-class of theories constructed only with the Ricci tensor and showing that the non-projectively invariant sector propagates ghost-like degrees of freedom. We also explain how these pathologies can be avoided either by imposing a projective symmetry or additional constraints in the gravity sector. Our results put forward that higher order curvature gravity theories generally remain pathological in the metric-affine (and hybrid) formalisms and highlight the key importance of the projective symmetry and/or additional constraints for their physical viability and, by extension, of general metric-affine theories.
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Beltran Jimenez, J., Delhom, A., Olmo, G. J., & Orazi, E. (2021). Born-Infeld gravity: Constraints from light-by-light scattering and an effective field theory perspective. Phys. Lett. B, 820, 136479–6pp.
Abstract: By using a novel technique that establishes a correspondence between general relativity and metric-affine theories based on the Ricci tensor, we are able to set stringent constraints on the free parameter of Born-Infeld gravity from the ones recently obtained for Born-Infeld electrodynamics by using light-by light scattering data from ATLAS. We also discuss how these gravity theories plus matter fit within an effective field theory framework.
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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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Beltran, R., Cottin, G., Helo, J. C., Hirsch, M., Titov, A., & Wang, Z. S. (2023). Long-lived heavy neutral leptons from mesons in effective field theory. J. High Energy Phys., 01(1), 015–38pp.
Abstract: In the framework of the low-energy effective field theory of the Standard Model extended with heavy neutral leptons (HNLs), we calculate the production rates of HNLs from meson decays triggered by dimension-six operators. We consider both lepton number-conserving and lepton-number-violating four-fermion operators involving either a pair of HNLs or a single HNL. Assuming that HNLs are long-lived, we perform simulations and investigate the reach of the proposed far detectors at the high-luminosity LHC to (i) active-heavy neutrino mixing and (ii) the Wilson coefficients associated with the effective operators, for HNL masses below the mass of the B-meson. We further convert the latter to the associated new-physics scales. Our results show that scales in excess of hundreds of TeV and the active-heavy mixing squared as small as 10(-15 )can be probed by these experiments.
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Beltran, R., Cottin, G., Helo, J. C., Hirsch, M., Titov, A., & Wang, Z. S. (2022). Long-lived heavy neutral leptons at the LHC: four-fermion single-N-R operators. J. High Energy Phys., 01(1), 044–18pp.
Abstract: Interest in searches for heavy neutral leptons (HNLs) at the LHC has increased considerably in the past few years. In the minimal scenario, HNLs are produced and decay via their mixing with active neutrinos in the Standard Model (SM) spectrum. However, many SM extensions with HNLs have been discussed in the literature, which sometimes change expectations for LHC sensitivities drastically. In the N-R SMEFT, one extends the SM effective field theory with operators including SM singlet fermions, which allows to study HNL phenomenology in a “model independent” way. In this paper, we study the sensitivity of ATLAS to HNLs in the N-R SMEFT for four-fermion operators with a single HNL. These operators might dominate both production and decay of HNLs, and we find that new physics scales in excess of 20 TeV could be probed at the high-luminosity LHC.
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Beltran, R., Cottin, G., Hirsch, M., Titov, A., & Wang, Z. S. (2023). Reinterpretation of searches for long-lived particles from meson decays. J. High Energy Phys., 05(5), 031–31pp.
Abstract: Many models beyond the Standard Model predict light and feebly interacting particles that are often long-lived. These long-lived particles (LLPs) in many cases can be produced from meson decays. In this work, we propose a simple and quick reinterpretation method for models predicting LLPs produced from meson decays. With the method, we are not required to run Monte-Carlo simulation, implement detector geometries and efficiencies, or apply experimental cuts in an event analysis, as typically done in recasting and reinterpretation works. The main ingredients our method requires are only the theoretical input, allowing for computation of the production and decay rates of the LLPs. There are two conditions for the method to work: firstly, the LLPs in the models considered should be produced from a set of mesons with similar mass and lifetime (or the same meson) and second, the LLPs should, in general, have a lab-frame decay length much larger than the distance between the interaction point and the detector. As an example, we use this method to reinterpret exclusion bounds on heavy neutral leptons (HNLs) in the minimal “3+1” scenario, into those for HNLs in the general effective-field-theory framework as well as for axion-like particles. We are able to reproduce existing results, and obtain new bounds via reinterpretation of past experimental results, in particular, from CHARM and Belle.
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Beltran, R., Günther, J., Hirsch, M., Titov, A., & Wang, Z. S. (2024). Heavy neutral leptons from kaons in effective field theory. Phys. Rev. D, 109(11), 115014–19pp.
Abstract: In the framework of the low -energy effective theory containing, in addition to the Standard -Model fields, heavy neutral leptons (HNLs), we compute the decay rates of neutral and charged kaons into HNLs. We consider both lepton -number -conserving and lepton -number -violating four-fermion operators, taking into account also the contribution of active -heavy neutrino mixing. Assuming that the produced HNLs are longlived, we perform simulations and calculate the sensitivities of future long -lived -particle (LLP) detectors at the high -luminosity LHC as well as the near detector of the Deep Underground Neutrino Experiment (DUNE -ND) to the considered scenario. When applicable, we also recast the existing bounds on the minimal mixing case obtained by NA62, T2K, and PS191. Our findings show that, while the future LHC LLP detectors can probe currently allowed parameter space only in certain benchmark scenarios, DUNE -ND should be sensitive to parameter space beyond the current bounds in almost all the benchmark scenarios, and, for some of the effective operators considered, it can even probe new -physics scales in excess of 3000 TeV.
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Beltran-Palau, P., del Rio, A., Nadal-Gisbert, S., & Navarro-Salas, J. (2021). Note on the pragmatic mode-sum regularization method: Translational-splitting in a cosmological background. Phys. Rev. D, 103(10), 105002–9pp.
Abstract: The point-splitting renormalization method offers a prescription to calculate finite expectation values of quadratic operators constructed from quantum fields in a general curved spacetime. It has been recently shown by Levi and Ori that when the background metric possesses an isometry, like stationary or spherically symmetric black holes, the method can be upgraded into a pragmatic procedure of renormalization that produces efficient numerical calculations. In this paper we show that when the background enjoys three-dimensional spatial symmetries, like homogeneous expanding universes, the above pragmatic regularization technique reduces to the well-established adiabatic regularization method.
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Beltran-Palau, P., del Rio, A., & Navarro-Salas, J. (2023). Quantum corrections to the Schwarzschild metric from vacuum polarization. Phys. Rev. D, 107(8), 085023–15pp.
Abstract: We explore static and spherically symmetric solutions of the 4-dimensional semiclassical Einstein's equations using the quantum vacuum polarization of a conformal field as a source. These solutions may be of interest for the study of exotic compact objects (ECOs). The full backreaction problem is addressed by solving the semiclassical Tolman-Oppenheimer-Volkoff (TOV) equations making use of effective equations of state inspired by the trace anomaly and an extra simplifying and reasonable assumption. We combine analytical and numerical techniques to solve the resulting differential equations, both perturbatively and nonperturbatively in h. In all cases the solution is similar to the Schwarzschild metric up p ffiffito the vicinity of the classical horizon r = 2M. However, at r = 2M + epsilon, with epsilon similar to O(root h), we find a coordinate singularity. In the case of matching with a static star, this leads to an upper bound in the compactness, and sets a constraint on the family of stable ECOs. We also study the corrections that the quantum-vacuum polarization induces on the propagation of waves, and discuss the implications. For the pure vacuum case, we can further extend the solution by using appropriate coordinates until we reach another singular point, where this time a null curvature singularity arises and prevents extending beyond. This picture qualitatively agrees with the results obtained in the effective two-dimensional approach, and reinforces the latter as a reasonable method.
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Beltran-Palau, P., Ferreiro, A., Navarro-Salas, J., & Pla, S. (2019). Breaking of adiabatic invariance in the creation of particles by electromagnetic backgrounds. Phys. Rev. D, 100(8), 085014–12pp.
Abstract: Particles are spontaneously created from the vacuum by time-varying gravitational or electromagnetic backgrounds. It has been proven that the particle number operator in an expanding universe is an adiabatic invariant. In this paper we show that, in some special cases, the expected adiabatic invariance of the particle number fails in presence of electromagnetic backgrounds. In order to do this, we consider as a prototype a Sauter-type electric pulse. Furthermore, we also show a close relation between the breaking of the adiabatic invariance and the emergence of the axial anomaly.
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