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Beltran Jimenez, J., de Andres, D., & Delhom, A. (2020). Anisotropic deformations in a class of projectively-invariant metric-affine theories of gravity. Class. Quantum Gravity, 37(22), 225013–25pp.
Abstract: Among the general class of metric-affine theories of gravity, there is a special class conformed by those endowed with a projective symmetry. Perhaps the simplest manner to realise this symmetry is by constructing the action in terms of the symmetric part of the Ricci tensor. In these theories, the connection can be solved algebraically in terms of a metric that relates to the spacetime metric by means of the so-called deformation matrix that is given in terms of the matter fields. In most phenomenological applications, this deformation matrix is assumed to inherit the symmetries of the matter sector so that in the presence of an isotropic energy-momentum tensor, it respects isotropy. In this work we discuss this condition and, in particular, we show how the deformation matrix can be anisotropic even in the presence of isotropic sources due to the non-linear nature of the equations. Remarkably, we find that Eddington-inspired-Born-Infeld (EiBI) theories do not admit anisotropic deformations, but more general theories do. However, we find that the anisotropic branches of solutions are generally prone to a pathological physical behaviour.
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Vicente, A. (2018). Anomalies in b -> s transitions and dark matter. Adv. High. Energy Phys., 2018, 3905848–11pp.
Abstract: Since 2013, the LHCb collaboration has reported on the measurement of several observables associated with b -> s transitions, finding various deviations from their predicted values in the Standard Model. These include a set of deviations in branching ratios and angular observables, as well as in the observables R-k and R-k*, specially built to test the possible violation of Lepton Flavor Universality. Even though these tantalizing hints are not conclusive yet, the b -> s* anomalies have gained considerable attention in the flavor community. Here we review new physics models that address these anomalies and explore their possible connection to the dark matter of the Universe. After discussing some of the ideas introduced in these works and classifying the proposed models, two selected examples are presented in detail in order to illustrate the potential interplay between these two areas of current particle physics.
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Dai, L. R., Pavao, R., Sakai, S., & Oset, E. (2018). Anomalous enhancement of the isospin-violating Lambda(1405) production by a triangle singularity in Lambda(c) ->pi(+)pi(0)pi(0)Sigma(0). Phys. Rev. D, 97(11), 116004–10pp.
Abstract: The decay of Lambda(+)(c) into pi(+)pi(0) Lambda(1405) with the Lambda(1405) decay into pi(0)Sigma(0) through a triangle diagram is studied. This process is initiated by Lambda(+)(c) -> pi(+) (K) over bar N-*, and then the (K) over bar (*) decays into (K) over bar (pi) and (K) over bar N produce the Lambda(1405) through a triangle loop containing (K) over bar N-* (K) over bar which develops a singularity around 1890 MeV. This process is prohibited by the isospin symmetry, but the decay into this channel is enhanced by the contribution of the triangle diagram, which is sensitive to the mass of the internal particles. We find a narrow peak in the pi(0)Sigma(0) invariant mass distribution, which originates from the (K) over bar amplitude, but is tied to the mass differences between the charged and neutral (K) over bar or N states. The observation of the unavoidable peak of the triangle singularity in the isospin- violating Lambda(1405) production would provide further support for the hadronic molecular picture of the Lambda(1405) and further information on the (K) over bar N interaction.
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Khosa, C. K., & Sanz, V. (2023). Anomaly Awareness. SciPost Phys., 15(2), 053–24pp.
Abstract: We present a new algorithm for anomaly detection called Anomaly Awareness. The algorithm learns about normal events while being made aware of the anomalies through a modification of the cost function. We show how this method works in different Particle Physics situations and in standard Computer Vision tasks. For example, we apply the method to images from a Fat Jet topology generated by Standard Model Top and QCD events, and test it against an array of new physics scenarios, including Higgs production with EFT effects and resonances decaying into two, three or four subjets. We find that the algorithm is effective identifying anomalies not seen before, and becomes robust as we make it aware of a varied-enough set of anomalies.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Anomaly detection search for new resonances decaying into a Higgs boson and a generic new particle X in hadronic final states using √s=13 TeV pp collisions with the ATLAS detector. Phys. Rev. D, 108(5), 052009–33pp.
Abstract: A search is presented for a heavy resonance Y decaying into a Standard Model Higgs boson H and a new particle X in a fully hadronic final state. The full Large Hadron Collider run 2 dataset of proton-proton collisions at root s =13 TeV collected by the ATLAS detector from 2015 to 2018 is used and corresponds to an integrated luminosity of 139 fb(-1). The search targets the high Y-mass region, where the H and X have a significant Lorentz boost in the laboratory frame. A novel application of anomaly detection is used to define a general signal region, where events are selected solely because of their incompatibility with a learned background-only model. It is constructed using a jet-level tagger for signal-model-independent selection of the boosted X particle, representing the first application of fully unsupervised machine learning to an ATLAS analysis. Two additional signal regions are implemented to target a benchmark X decay into two quarks, covering topologies where the X is reconstructed as either a single large-radius jet or two small-radius jets. The analysis selects Higgs boson decays into bb, and a dedicated neural-network-based tagger provides sensitivity to the boosted heavy-flavor topology. No significant excess of data over the expected background is observed, and the results are presented as upper limits on the production cross section sigma(pp -> Y -> XH -> qqbb) for signals with m(Y) between 1.5 and 6 TeV and m(X) between 65 and 3000 GeV.
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Chakraborty, S., Gupta, A., & Vanvlasselaer, M. (2023). Anomaly induced cooling of neutron stars: a Standard Model contribution. J. Cosmol. Astropart. Phys., 10(10), 030–23pp.
Abstract: Young neutron stars cool via the emission of neutrinos from their core. A precise understanding of all the different processes producing neutrinos in the hot and degenerate matter is essential for assessing the cooling rate of such stars. The main Standard Model processes contributing to this effect are nu bremsstrahlung, mURCA among others. In this paper, we investigate another Standard Model process initiated by the Wess-Zumino-Witten term, leading to the emission of neutrino pairs via N gamma -> N nu nu over bar . We find that for proto-neutron stars, such processes with degenerate neutrons can be comparable and even dominate over the typical and well-known cooling mechanisms.
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Han, C., Lopez-Ibañez, M. L., Melis, A., Vives, O., & Yang, J. M. (2022). Anomaly-free ALP from non-Abelian flavor symmetry. J. High Energy Phys., 08(8), 306–21pp.
Abstract: Motivated by the XENON1T excess in electron-recoil measurements, we investigate the prospects of probing axion-like particles (ALP) in lepton flavor violation experiments. In particular, we identify such ALP as a pseudo-Goldstone from the spontaneous breaking of the flavor symmetries that explain the mixing structure of the Standard Model leptons. We present the case of the flavor symmetries being a non-Abelian U(2) and the ALP originating from its U(1) subgroup, which is anomaly-free with the Standard Model group. We build two explicit realistic examples that reproduce leptonic masses and mixings and show that the ALP which is consistent with XENON1T anomaly could be probed by the proposed LFV experiments.
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Fileviez Perez, P., Golias, E., Li, R. H., Murgui, C., & Plascencia, A. D. (2019). Anomaly-free dark matter models. Phys. Rev. D, 100(1), 015017–15pp.
Abstract: We investigate the predictions of anomaly-free dark matter models for direct and indirect detection experiments. We focus on gauge theories where the existence of a fermionic dark matter candidate is predicted by anomaly cancellation, its mass is defined by the new symmetry breaking scale, and its stability is guaranteed by a remnant symmetry after the breaking of the gauge symmetry. We find an upper bound on the symmetry breaking scale by applying the relic density and perturbative constraints. The anomaly-free property of the theories allows us to perform a full study of the gamma lines from dark matter annihilation. We investigate the correlation between predictions for final-state radiation processes and gamma lines. Furthermore, we demonstrate that the latter can be distinguished from the continuum gamma-ray spectrum.
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Caron, S., Casas, J. A., Quilis, J., & Ruiz de Austri, R. (2018). Anomaly-free dark matter with harmless direct detection constraints. J. High Energy Phys., 12(12), 126–24pp.
Abstract: Dark matter (DM) interacting with the SM fields via a Z-boson (Z-portal') remains one of the most attractive WIMP scenarios, both from the theoretical and the phenomenological points of view. In order to avoid the strong constraints from direct detection and dilepton production, it is highly convenient that the Z has axial coupling to DM and leptophobic couplings to the SM particles, respectively. The latter implies that the associated U(1) coincides with baryon number in the SM sector. In this paper we completely classify the possible anomaly-free leptophobic Z with minimal dark sector, including the cases where the coupling to DM is axial. The resulting scenario is very predictive and perfectly viable from the present constraints from DM detection, EW observables and LHC data (di-lepton, di-jet and mono-jet production). We analyze all these constraints, obtaining the allowed areas in the parameter space, which generically prefer mZ less than or similar to 500 GeV, apart from resonant regions. The best chances to test these viable areas come from future LHC measurements.
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Han, C., Lopez-Ibañez, M. L., Melis, A., Vives, O., & Yang, J. M. (2021). Anomaly-free leptophilic axionlike particle and its flavor violating tests. Phys. Rev. D, 103(3), 035028–7pp.
Abstract: Motivated by the recent Xenon1T result, we study a leptophilic flavor-dependent anomaly-free axionlike particle (ALP) and its effects on charged-lepton flavor violation. We present two representative models. The first one considers that the ALP origins from the flavon that generates the charged-lepton masses. The second model assumes a larger flavor symmetry such that more general mixings in the charged-lepton are possible, while maintaining flavor-dependent ALP couplings. We find that a keV ALP explaining the Xenon1T result is still viable for lepton flavor violation and stellar cooling astrophysical limits. On the other hand, if the Xenon1T result is confirmed, future charged-lepton flavor violation measurements can be complementary to probe such a possibility.
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