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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., & Reichert, L. (2012). Supersymmetric mass spectra and the seesaw type-I scale. J. High Energy Phys., 02(2), 112.
Abstract: We calculate supersymmetric mass spectra with cMSSM boundary conditions and a type-I seesaw mechanism added to explain current neutrino data. Using published, estimated errors on SUSY mass observables for a combined LHC+ILC analysis, we perform a theoretical chi(2) analysis to identify parameter regions where pure cMSSM and cMSSM plus seesaw type-I might be distinguishable with LHC+ILC data. The most important observables are determined to be the (left) smuon and selectron masses and the splitting between them, respectively. Splitting in the (left) smuon and selectrons is tiny in most of cMSSM parameter space, but can be quite sizeable for large values of the seesaw scale, m S S. Thus, for very roughly m(SS) >= 10(14) GeV hints for type-I seesaw might appear in SUSY mass measurements. Since our numerical results depend sensitively on forecasted error bars, we discuss in some detail the accuracies, which need to be achieved, before a realistic analysis searching for signs of type-I seesaw in SUSY spectra can be carried out.
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Hirsch, M., Malinsky, M., Porod, W., Reichert, L., & Staub, F. (2012). Hefty MSSM-like light Higgs in extended gauge models. J. High Energy Phys., 02(2), 084.
Abstract: It is well known that in the MSSM the lightest neutral Higgs h(0) must be, at the tree level, lighter than the Z boson and that the loop corrections shift this stringent upper bound up to about 130GeV. Extending the MSSM gauge group in a suitable way, the new Higgs sector dynamics can push the tree-level mass of h(0) well above the tree-level MSSM limit if it couples to the new gauge sector. This effect is further pronounced at the loop level and h(0) masses in the 140GeV ballpark can be reached easily. We exemplify this for a sample setting with a low-scale U(1)(R) x U(1)(B-L) gauge symmetry in which neutrino masses can be implemented via the inverse seesaw mechanism.
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Cottin, G., Helo, J. C., Hirsch, M., Pena, C., Wang, C. S. A., & Xie, S. (2023). Long-lived heavy neutral leptons with a displaced shower signature at CMS. J. High Energy Phys., 02(2), 011–16pp.
Abstract: We study the LHC discovery potential in the search for heavy neutral leptons (HNL) with a new signature: a displaced shower in the CMS muon detector, giving rise to a large cluster of hits forming a displaced shower. A new Delphes module is used to model the CMS detector response for such displaced decays. We reinterpret a dedicated CMS search for neutral long-lived particles decaying in the CMS muon endcap detectors for the minimal HNL scenario. We demonstrate that this new strategy is particularly sensitive to active-sterile mixings with tau leptons, due to hadronic tau decays. HNL masses between similar to 1-6 GeV can be accessed for mixings as low as vertical bar V-tau N vertical bar(2) similar to 10(-7), probing unique regions of parameter space in the tau sector.
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Cepedello, R., Deppisch, F. F., Gonzalez, L., Hati, C., & Hirsch, M. (2019). Neutrinoless Double-Beta Decay with Nonstandard Majoron Emission. Phys. Rev. Lett., 122(18), 181801–6pp.
Abstract: We present a novel mode of neutrinoless double-beta decay with emission of a light Majoron-like scalar particle phi. We assume it couples via an effective seven-dimensional operator with a (V + A) lepton current and (V +/- A) quark currents leading to a long-range contribution that is unsuppressed by the light neutrino mass. We calculate the total double-beta decay rate and determine the fully differential shape for this mode. We find that future double-beta decay searches are sensitive to scales of the order Lambda(NP) approximate to 1 TeV for the effective operator and a light scalar m(phi) < 0.2 MeV, based on ordinary double-beta decay Majoron searches. The angular and energy distributions can deviate considerably from that of two-neutrino double-beta decay, which is the main background. We point out possible ultraviolet completions where such an effective operator can emerge.
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