Displaced vertex signatures of doubly charged scalars in the type-II seesaw and its left-right extensions

The type-II seesaw mechanism with an isospin-triplet scalar $\Delta_L$ provides one of the most compelling explanations for the observed smallness of neutrino masses. The triplet contains a doubly-charged component $H_L^{\pm\pm}$, which dominantly decays to either same-sign dileptons or to a pair of $W$ bosons, depending on the size of the triplet vacuum expectation value. However, there exists a range of Yukawa couplings $f_L$ of the triplet to the charged leptons, wherein a relatively light $H_L^{\pm\pm}$ tends to be long-lived, giving rise to distinct displaced-vertex signatures at the high-energy colliders. We find that the displaced vertex signals from the leptonic decays $H_L^{\pm\pm} \to \ell_\alpha^\pm \ell_\beta^\pm$ could probe a broad parameter space with $10^{-10} \lesssim |f_L| \lesssim 10^{-6}$ and 45.6 GeV $< M_{H_L^{\pm\pm}} \lesssim 200$ GeV at the high-luminosity LHC. Similar sensitivity can also be achieved at a future 1 TeV $e^+e^-$ collider. The mass reach can be extended to about 500 GeV at a future 100 TeV proton-proton collider. Similar conclusions apply for the right-handed triplet $H_R^{\pm\pm}$ in the TeV-scale left-right symmetric models, which provide a natural embedding of the type-II seesaw. We show that the displaced vertex signals are largely complementary to the prompt same-sign dilepton pair searches at the LHC and the low-energy, high-intensity/precision measurements, such as neutrinoless double beta decay, charged lepton flavor violation, electron and muon anomalous magnetic moments, muonium oscillation and M{\o}ller scattering.Comment: 49 pages, 25 figures and 2 tables, minor changes, version to appear in JHE

Asymmetric Dark Matter in the Sun and the Diphoton Excess at the LHC

It has been recently pointed out that a momentum-dependent coupling of the asymmetric Dark Matter (ADM) with nucleons can explain the broad disagreement between helioseismological observables and the predictions of standard solar models. In this paper, we propose a minimal simplified ADM model consisting of a scalar and a pseudoscalar mediator, in addition to a Dirac fermionic DM, for generating such momentum-dependent interactions. Remarkably, the pseudoscalar with mass around 750 GeV can simultaneously explain the solar anomaly and the recent diphoton excess observed by both ATLAS and CMS experiments in the early $\sqrt s=13$ TeV LHC data. In this framework, the total width of the resonance is naturally large, as suggested by the ATLAS experiment, since the resonance mostly decays to the ADM pair. The model predicts the existence of a new light scalar in the GeV range, interacting with quarks, and observable dijet, monojet and $t\bar{t}$ signatures for the 750 GeV resonance at the LHC.Comment: 7 pages, 4 figures. Version to appear in PR

Looking for New Naturally Aligned Higgs Doublets at the LHC

Since the current LHC Higgs data suggest the couplings of the observed 125 GeV Higgs boson to be close to the Standard Model (SM) expectations, any extended Higgs sector must lead to the so-called SM alignment limit, where one of the Higgs bosons behaves exactly like that of the SM. In the context of the Two Higgs Doublet Model (2HDM), this alignment is often associated with either decoupling of the heavy Higgs sector or accidental cancellations in the 2HDM potential. We present a novel symmetry justification for 'natural' alignment without necessarily decoupling or fine-tuning. We show that there exist only three different symmetry realizations of the natural alignment scenario in 2HDM. We analyze new collider signals for the heavy Higgs sector in the natural alignment limit, which dominantly lead to third-generation quarks in the final state and can serve as a useful observational tool during the Run-II phase of the LHC.Comment: 20 pages, 7 figures, 1 table; added references. Based on arXiv:1408.3405 and arXiv:1503.09140. To appear in the proceedings of Planck 2015, Ioannina, Greec

Natural Alignment in the Two Higgs Doublet Model

As the LHC Higgs data persistently suggest the couplings of the observed 125 GeV Higgs boson to be consistent with the Standard Model (SM) expectations, any extended Higgs sector must lead to the so-called SM alignment limit, where one of the Higgs bosons behaves exactly like that of the SM. In the context of the Two Higgs Doublet Model (2HDM), this alignment is often associated with either decoupling of the heavy Higgs sector or accidental cancellations in the 2HDM potential. We present a novel symmetry justification for `natural' alignment without necessarily decoupling or fine-tuning. We show that there exist only three different symmetry realizations of the natural alignment scenario in 2HDM. We identify the 2HDM parameter space satisfying the natural alignment condition up to the Planck scale. We also analyze new collider signals for the heavy Higgs sector in the natural alignment limit, which dominantly lead to third-generation quarks in the final state and can serve as a useful observational tool during the Run-II phase of the LHC.Comment: 15 pages, based on a plenary presentation given by A. Pilaftsis at the Fifth Symposium on Prospects in the Physics of Discrete Symmetries (2016, Warsaw, Poland), (significant text overlap with arXiv:1408.3405, arXiv:1503.09140 and arXiv:1510.08790

Natural Standard Model Alignment in the Two Higgs Doublet Model

The current LHC Higgs data provide strong constraints on possible deviations of the couplings of the observed 125 GeV Higgs boson from the Standard Model (SM) expectations. Therefore, it now becomes compelling that any extended Higgs sector must comply with the so-called SM alignment limit. In the context of the Two Higgs Doublet Model (2HDM), this alignment is often associated with either decoupling of the heavy Higgs sector or accidental cancellations in the 2HDM potential. Here we present a new solution realizing natural alignment based on symmetries, without decoupling or fine-tuning. In particular, we show that in 2HDMs where both Higgs doublets acquire vacuum expectation values, there exist only three different symmetry realizations leading to natural alignment. We discuss some phenomenological implications of the Maximally-Symmetric 2HDM based on SO(5) symmetry group and analyze new collider signals for the heavy Higgs sector, involving third-generation quarks, which can be a useful observational tool during the Run-II phase of the LHC.Comment: 15 pages, 1 table, 8 figures; Contribution to the Proceedings of DISCRETE 2014, Londo