Cottin, G., Helo, J. C., & Hirsch, M. (2018). Displaced vertices as probes of sterile neutrino mixing at the LHC. Phys. Rev. D, 98(3), 035012–6pp.
Abstract: We investigate the reach at the LHC to probe light sterile neutrinos with displaced vertices. We focus on sterile neutrinos N with masses m(N) similar to (5-30) GeV that are produced in rare decays of the standard model gauge bosons and decay inside the inner trackers of the LHC detectors. With a strategy that triggers on the prompt lepton accompanying the N displaced vertex and considers charged tracks associated with it, we show that the 13 TeV LHC with 3000/fb is able to probe active-sterile neutrino mixings down to vertical bar V-lN vertical bar(2) approximate to 10(-9), with l = e, mu, which is an improvement of up to 4 orders of magnitude when comparing with current experimental limits from trileptons and proposed lepton-jets searches. In the case when tau mixing is present, mixing angles as low as vertical bar V-tau N vertical bar(2) approximate to 10(-8) can be accessed.
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Helo, J. C., Hirsch, M., Ota, T., & Pereira dos Santos, F. A. (2015). Double beta decay and neutrino mass models. J. High Energy Phys., 05(5), 092–40pp.
Abstract: Neutrinoless double beta decay allows to constrain lepton number violating extensions of the standard model. If neutrinos are Majorana particles, the mass mechanism will always contribute to the decay rate, however, it is not a priori guaranteed to be the dominant contribution in all models. Here, we discuss whether the mass mechanism dominates or not from the theory point of view. We classify all possible (scalar-mediated) short-range contributions to the decay rate according to the loop level, at which the corresponding models will generate Majorana neutrino masses, and discuss the expected relative size of the different contributions to the decay rate in each class. Our discussion is general for models based on the SM group but does not cover models with an extended gauge. We also work out the phenomenology of one concrete 2-loop model in which both, mass mechanism and short-range diagram, might lead to competitive contributions, in some detail.
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Hirsch, M., Staub, F., & Vicente, A. (2012). Enhancing l(i) -> 3l(j) with the Z(0)-penguin. Phys. Rev. D, 85(11), 113013–5pp.
Abstract: Lepton flavor violation has been observed in neutrino oscillations. For charged lepton flavor violation decays only upper limits are known, but sizable branching ratios are expected in many neutrino mass models. High-scale models, such as the classical supersymmetric seesaw, usually predict that decays l(i) -> 3l(j) are roughly a factor alpha smaller than the corresponding decays l(i) -> l(j)gamma. Here we demonstrate that the Z(0)-penguin diagram can give an enhancement for decays l(i) -> 3l(j) in many extensions of the minimal supersymmetric standard model (MSSM). We first discuss why the Z(0)-penguin is not dominant in the MSSM with seesaw and show that much larger contributions from the Z(0)-penguin are expected in general. We then demonstrate the effect numerically in two example models, namely, the supersymmetric inverse seesaw and R-parity violating supersymmetry.
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Carquin, E., Neill, N. A., Helo, J. C., & Hirsch, M. (2019). Exotic colored fermions and lepton number violation at the LHC. Phys. Rev. D, 99(11), 115028–9pp.
Abstract: Majorana neutrino mass models with a scale of lepton number violation of order tem-electron-volts potentially lead to signals at the LHC. Here, we consider an extension of the standard model with a colored octet fermion and a scalar leptoquark. This model generates neutrino masses at two-loop order. We make a detailed Monte Carlo study of the lepton number violating signal at the LHC in this model, including a simulation of standard model backgrounds. Our forecast predicts that the LHC with 300/fb should be able to probe this model up to color-octet fermion masses in the range of (2.6-2.7) TeV, depending on the lepton flavor of the final state.
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Deppisch, F. F., Harz, J., Huang, W. C., Hirsch, M., & Pas, H. (2015). Falsifying high-scale baryogenesis with neutrinoless double beta decay and lepton flavor violation. Phys. Rev. D, 92(3), 036005–6pp.
Abstract: Interactions that manifest themselves as lepton number violating processes at low energies in combination with sphaleron transitions typically erase any preexisting baryon asymmetry of the Universe. In this article, we discuss the constraints obtained from an observation of neutrinoless double beta decay in this context. If a new physics mechanism of neutrinoless double beta decay other than the standard light neutrino exchange is observed, typical scenarios of high-scale baryogenesis will be excluded unless the baryon asymmetry is stabilized via some new mechanism. We also sketch how this conclusion can be extended beyond the first lepton generation by incorporating lepton flavor violating processes.
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Deppisch, F. F., Harz, J., & Hirsch, M. (2014). Falsifying High-Scale Leptogenesis at the LHC. Phys. Rev. Lett., 112(22), 221601–5pp.
Abstract: Measuring a nonzero value for the cross section of any lepton number violating (LNV) process would put a strong lower limit on the washout factor for the effective lepton number density in the early Universe at times close to the electroweak phase transition and thus would lead to important constraints on any high-scale model for the generation of the observed baryon asymmetry based on LNV. In particular, for leptogenesis (LG) models with masses of the right-handed neutrinos heavier than the mass scale observed at the LHC, the implied large washout factors would lead to a violation of the out-of-equilibrium condition and exponentially suppress the net lepton number produced in such LG models. We thus demonstrate that the observation of LNV processes at the LHC results in the falsification of high-scale LG models. However, no conclusions about the viability of LG models can be drawn from the nonobservation of LNV processes.
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Arbelaez, C., Hirsch, M., & Restrepo, D. (2017). Fermionic triplet dark matter in an SO(10)-inspired left-right model. Phys. Rev. D, 95(9), 095034–9pp.
Abstract: We study a left right (LR) extension of the Standard Model (SM) where the Dark Matter(DM) candidate is composed of a set of fermionic Majorana triplets. The DM is stabilized by a remnant Z(2) symmetry from the breaking of the LR group to the SM. Two simple scenarios where the DM particles plus a certain set of extra fields lead to gauge coupling unification with a low LR scale are explored. The constraints from relic density and predictions for direct detection are discussed for both scenarios. The first scenario with a SUd(2)(R) vectorlike fermion triplet contains a DM candidate which is almost unconstrained by current direct detection experiments. The second scenario, with an additional SU(2)R triplet, opens up a scalar portal leading to direct detection constraints which are similar to collider limits for right gauge bosons. The DM parameter space consistent with phenomenological requirements can also lead to successful gauge coupling unification in a SO(10) setup.
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Fonseca, R. M., & Hirsch, M. (2017). Gauge vectors and double beta decay. Phys. Rev. D, 95(3), 035033–14pp.
Abstract: We discuss contributions to neutrinoless double beta (0 nu beta beta) decay involving vector bosons. The starting point is a list of all possible vector representations that may contribute to 0 nu beta beta decay via d = 9 or d = 11 operators at tree level. We then identify gauge groups which contain these vectors in the adjoint representation. Even though the complete list of vector fields that can contribute to 0 nu beta beta up to d = 11 is large (a total of 46 vectors), only a few of them can be gauge bosons of phenomenologically realistic groups. These latter cases are discussed in some more detail, and lower (upper) limits on gauge boson masses (mixing angles) are derived from the absence of 0 nu beta beta decay.
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Cordero-Carrion, I., Hirsch, M., & Vicente, A. (2020). General parametrization of Majorana neutrino mass models. Phys. Rev. D, 101(7), 075032–25pp.
Abstract: We discuss a general formula which allows to automatically reproduce experimental data for Majorana neutrino mass models, while keeping the complete set of the remaining model parameters free for general scans, as necessary in order to provide reliable predictions for observables outside the neutrino sector. We provide a proof of this master parametrization and show how to apply it for several well-known neutrino mass models from the literature. We also discuss a list of special cases, in which the Yukawa couplings have to fulfill some particular additional conditions.
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Helo, J. C., Hirsch, M., & Wang, Z. S. (2018). Heavy neutral fermions at the high-luminosity LHC. J. High Energy Phys., 07(7), 056–23pp.
Abstract: Long-lived light particles (LLLPs) appear in many extensions of the standard model. LLLPs are usually motivated by the observed small neutrino masses, by dark matter or both. Typical examples for fermionic LLLPs (a.k.a. heavy neutral fermions, HNFs) are sterile neutrinos or the lightest neutralino in R-parity violating supersymmetry. The high luminosity LHC is expected to deliver up to 3/ab of data. Searches for LLLPs in dedicated experiments at the LHC could then probe the parameter space of LLLP models with unprecedented sensitivity. Here, we compare the prospects of several recent experimental proposals, FASER, CODEX-b and MATHUSLA, to search for HNFs and discuss their relative merits.s
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