The type II seesaw mechanism is an attractive way to generate the observed light neutrino masses. It postulates a SU(2)$_\mathrm{L}$-triplet scalar field, which develops an induced vacuum expectation value after electroweak symmetry breaking, giving masses to the neutrinos via its couplings to the lepton SU(2)$_\mathrm{L}$-doublets. When the components of the triplet field have masses around the electroweak scale, the model features a rich phenomenology. We discuss the current allowed parameter space of the minimal low scale type II seesaw model, taking into account all relevant constraints, including charged lepton flavour violation as well as collider searches. We point out that the symmetry protected low scale type II seesaw scenario, where an approximate "lepton number"-like symmetry suppresses the Yukawa couplings of the triplet to the lepton doublets, is still largely untested by the current LHC results. In part of this parameter space the triplet components can be long-lived, potentially leading to a characteristic displaced vertex signature where the doubly-charged component decays into same-sign charged leptons. By performing a detailed analysis at the reconstructed level we find that already at the current run of the LHC a discovery would be possible for the considered parameter point, via dedicated searches for displaced vertex signatures. The discovery prospects are further improved at the HL-LHC and the FCC-hh/SppC.
The observation of Higgs decays into heavy neutrinos would be strong evidence for new physics associated to neutrino masses. In this work we propose a search for such decays within the Type I seesaw model in the few-GeV mass range via displaced vertices. Using 300 fb$^{-1}$ of integrated luminosity, at 13 TeV, we explore the region of parameter space where such decays are measurable. We show that, after imposing pseudorapidity cuts, there still exists a region where the number of events is larger than $\mathcal{O}(10)$. We also find that conventional triggers can greatly limit the sensitivity …
We investigate the pair production of right-handed neutrinos from the decay of a light $B-L$ scalar in the $U(1)_{B-L}$ model. The $B-L$ scalar mixes to the SM Higgs, and the physical scalar is required to be lighter than the observed Higgs. The produced right-handed neutrinos are predicted to be long-lived according to the type-I seesaw mechanism, and yield potentially distinct signatures such as displaced vertex and time-delayed leptons at the CMS/ATLAS/LHCb, as well as signatures at the far detectors including the CODEX-b, FACET, FASER, MoEDAL-MAPP and MATHUSLA. We analyze the sensitivity r…
In this article, we explore the possibility of displaced Higgs production from the decays of the heavy fermions in the Type-III seesaw extension of the Standard Model at the LHC/FCC and the muon collider. The displaced heavy fermions and the Higgs boson can be traced back by measuring the displaced charged tracks of the charged leptons along with the $b$-jets. A very small Yukawa coupling can lead to two successive displaced decays which makes the phenomenology even more interesting. The prospects of the transverse and longitudinal displaced decay lengths are extensively studied in the context…
We point out that the type III seesaw mechanism introducing fermion triplets predicts peculiar Higgs boson signatures of displaced vertices with two b jets and one or two charged particles which can be cleanly identified. In a supersymmetric theory, the scalar partner of the fermion triplet contains a neutral dark matter candidate which is almost degenerate with its charged components. A Higgs boson can be produced together with such a dark matter triplet in the cascade decay chain of a strongly produced squark or gluino. When the next lightest supersymmetric particle (NLSP) is bino/wino-like,…
In a class of Type-II seesaw dominated $SO(10)$ models proposed recently with heavy neutrinos, extra $Z'$ boson, and resonant leptogenesis, at first we show that the lightest first generation sterile neutrino that mediates dominant contributions to neutrinoless double beta decay also generates the displaced vertex leading to verifiable like-sign di-electron as well as di-muon production events outside the LHC detectors having suppressed standard model back-ground and missing energy. Resonant leptogenesis in this case is implemented by a pair of quasi-degenerate sterile neutrinos of the second …
We investigate a prospect of probing the type-III seesaw neutrino mass generation mechanism at various collider experiments by searching for a disappearing track and a displaced vertex signature originating from the decay of $SU(2)_L$ triplet fermion ($Σ$). Since $Σ$ is primarily produced at colliders through the electroweak gauge interactions, its production rate is uniquely determined by its mass. We find that a $Σ$ particle with a mass of a few hundred GeV produces a disappearing track signature from the decay of its charged component, which can be searched at the HL-LHC. Furthermore, we sh…
A certain class of new physics models includes long-lived particles which are singlet under the Standard Model (SM) gauge group.A displaced vertex is a spectacular signature to probe such particles productions at the high energy colliders, with a negligible SM background. In the context of the minimal gauged $B-L$ extended SM, we consider a pair creation of Majorana right-handed neutrinos (RHNs) at the high energy colliders through the production of the SM and the $B-L$ Higgs bosons and their subsequent decays into RHNs. With parameters reproducing the neutrino oscillation data, we show that t…
The type-II seesaw mechanism with an isospin-triplet scalar $Δ_L$ 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-ener…
Extending the Standard Model with right-handed neutrinos (RHNs) is well motivated by the observation of neutrino oscillations. In the type-I seesaw model, the RHNs interact with the SM particles via tiny mixings with the active neutrinos, which makes their discovery in the laboratory, and in particular at collider experiments in general challenging. In this work we instead consider an extension of the type-I seesaw model with the addition of a leptoquark (LQ), and employ a non-minimal production mechanism of the RHN via LQ decay, which is unsuppressed by neutrino mixing. We focus on relatively…
We consider a $U(1)'$ extended supersymmetric model with a right-handed neutrino superfield which can generate light neutrino mass by Type I seesaw mechanism. The lighter superpartner of the right-handed neutrino could be the scalar dark matter. These right-handed sneutrinos can come from the decay of $\tilde{Z'}$, superpartner of the extra gauge boson $Z'$. Left-right handed sneutrino mixings affect their decays further, giving rise to displaced "lepton flavour violating" signatures. A wino-like chargino NLSP (next to lightest supersymmetric particle) creates even more interesting decay topol…
We discuss the implementation of the "minimal" type III seesaw model, i.e. with one fermionic triplet, in FeynRules/MadGraph. This is the first step in order to realize a real study of LHC data recorded in the LHC detectors. With this goal in mind, we comment on the possibility of discovering this kind of new physics at the LHC running at 7 TeV with a luminosity of few fb^-1.
Electron proton (ep) colliders could provide particle collisions at TeV energies with large data rates while maintaining the clean and pile~up-free environment of lepton colliders, which makes them very attractive for heavy neutrino searches. Heavy (mainly sterile) neutrinos with masses around the electroweak scale are proposed in low scale seesaw models for neutrino mass generation. In this paper, we analyse two of the most promising signatures of heavy neutrinos at ep colliders, the lepton-flavour violating (LFV) lepton-trijet signature and the displaced vertex signature. In the considered b…
Displaced vertices are relatively unusual signatures for dark matter searches at the LHC. We revisit the model of pseudo-Dirac dark matter (pDDM), which can accommodate the correct relic density, evade direct detection constraints, and generically provide observable collider signatures in the form of displaced vertices. We use this model as a benchmark to illustrate the general techniques involved in the analysis, the complementarity between monojet and displaced vertex searches, and provide a comprehensive study of the current bounds and prospective reach.
We investigate the pair-production of Right-Handed Neutrinos (RHNs) via a $B-L$ $Z'$ boson and the detection prospects at the High-Luminosity run of the LHC (HL-LHC) and a future $pp$ collider (FCC-hh). We focus on RHN states with a mass of $10-70$ GeV which naturally results in displaced vertices for small active-sterile mixing strengths. Being produced through a mass resonance with $m_{Z'} \ge 1$ TeV, the RHNs are heavily boosted, leading to collimated decay products that give rise to fat-jets. We investigate the detection prospect of dedicated signatures in the inner detector and the muon s…
The presence of small neutrino masses and flavour mixings can be accounted for naturally in various models about extensions of the standard model, particularly in the seesaw mechanism models. In this work, we present a minimally extended seesaw framework with two right-handed neutrinos, where the active neutrino masses are derived in the radiative regime. Using the framework it can be shown that within certain mass limits, the light neutrino mass term can approach a form that is similar to its form under the type-I seesaw mechanism. Apart from this, we show that the decay width of right-handed…
A Standard Model extension with two Majorana neutrinos can explain the measured neutrino masses and mixings, and also account for the matter-antimatter asymmetry in a region of parameter space that could be testable in future experiments. The testability of the model relies to some extent on its minimality. In this paper we address the possibility that the model might be extended by extra generic new physics which we parametrize in terms of a low-energy effective theory. We consider the effects of the operators of the lowest dimensionality, $d=5$, and evaluate the upper bounds on the coefficie…
The DAMPE $e^+ e^-$ excess at around 1.4 TeV could be explained in the type-II seesaw model with a scalar dark mater $D$ which is stabilized by a discrete $Z_2$ symmetry. The simplest scenario is the annihilation $DD \to H^{++} H^{--}$ followed by the subsequent decay $H^{\pm\pm} \to e^\pm e^\pm$, with both the DM and triplet scalars roughly 3 TeV with a small mass splitting. In addition to the Drell-Yan process at future 100 TeV hadron colliders, the doubly-charged components could also be produced at lepton colliders like ILC and CLIC in the off-shell mode, and mediate lepton flavor violatin…
We recapitulate the latest results on the class of the simplest SO(10) grand unified models in which the GUT-scale symmetry breaking is triggered by an adjoint Higgs representation. We argue that the minimal survival approximation traditionally used in the GUT- and seesaw-scale estimates tends to be blind to very interesting parts of the parameter space in which some of the intermediate-scale states necessary for non-supersymmetric unification of the SM gauge couplings can be as light as to leave their imprints in the TeV domain. The stringent minimal-survival-based estimates of the B-L scale …
We point out that in generic TeV scale seesaw models for neutrino masses with local $B-L$ symmetry breaking, there is a phenomenologically allowed range of parameters where the Higgs field responsible for $B-L$ symmetry breaking leaves a physical real scalar field with mass around GeV scale. This particle (denoted here by $H_3$) is weakly mixed with the Standard Model Higgs field ($h$) with mixing $θ_1\lesssim m_{H_3}/m_h$, barring fine-tuned cancellation. In the specific case when the $B-L$ symmetry is embedded into the TeV scale left-right seesaw scenario, we show that the bounds on the $h-H…
Observation of non-zero neutrino masses at a scale $\sim 10^{-1} - 10^{-2}$ eV is a major problem in the otherwise highly successful Standard Model. The most elegant mechanism to explain such tiny neutrino masses is the seesaw mechanism with right handed neutrinos. However, the required seesaw scale is so high, $\sim 10^{14}$ GeV, it will not have any collider implications. Recently, an explicit model has been constructed to realize the seesaw mechanism with the right handed neutrinos at the electroweak scale. The model has a mirror symmetry having both the left and right lepton and quark doub…
We investigate a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. For this the Minimal Supersymmetric Standard Model is extended by adding light right-handed neutrinos and their superpartners, the R-sneutrinos. Moreover, we consider the lightest electroweakinos to be higgsino-like. We demonstrate that a prospective international $e^+ e^-$ linear collider with a center of mass energy of 1 TeV will be able to discover sleptons in scenarios which can be difficult for current colliders. Moreover, we also show that a measurement of the…
Upcoming Lepton Flavour Violation experiments searching for $μ\rightarrow 3e$ and $μ$ to $ e $ conversion in nuclei processes will provide new opportunities to test the fundamental properties of the neutrino sector, and possibly the origin of matter. In recent work, it was shown that the Type II Seesaw mechanism alone can simultaneously explain the neutrino masses, Leptogenesis, and inflation. A key prediction of this model was the possibility of signals being produced in Lepton Flavour Violation decays. Searches at future experiments such as Mu3e and COMET will be integral to determining the …
In this paper, we study the prospect of ECAL barrel timing to develop triggers dedicated to long-lived particles decaying to jets, at the level-1 of HL-LHC. We construct over 20 timing based variables, and identify three of them which have better performances and are robust against increasing PU. We estimate the QCD prompt jet background rates accurately using the "stitching" procedure for varying thresholds defining our triggers, and compute the signal efficiencies for different LLP scenarios for a permissible background rate. The trigger efficiencies can go up to $\mathcal{O}(80\%)$ for the …
Sterile neutrinos with masses in the range (1-100) GeV, have been searched for in a variety of experiments. Here, we discuss the prospects to search for sterile neutrinos at the LHC using displaced vertices. Two different cases are discussed: (i) the standard model extended with sterile neutrinos and (ii) right-handed neutrinos in a left-right symmetric extension of the standard model. A dedicated displaced vertex search will allow to probe parts of the parameter space not accessible to other searches, but will require large luminosity in both cases.
We consider seesaw type-I models including at least one (mostly-)sterile neutrino with mass at the eV scale. Three distinct situations are found, where the presence of light extra neutrinos is naturally justified by an approximately conserved lepton number symmetry. To analyse these scenarios consistently, it is crucial to employ an exact parametrisation of the full mixing matrix. We provide additional exact results, including generalised versions of the seesaw relation and of the Casas-Ibarra parametrisation, valid for every scale of seesaw. We find that the existence of a light sterile neutr…
Majoron-driven spontaneous leptogenesis in type-II seesaw model
Korea Institute for Advanced Study（韩国高等科学研究院）；Institute for Basic Science (IBS)（基础科学研究院）；Tsung-Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University（上海交大李政道研究所与物理与天文学院）
We propose a $B-L$ gauged extension of the Standard Model where light neutrino masses arise from type III seesaw mechanism. Unlike the minimal $B-L$ model with three right handed neutrinos having unit lepton number each, the model with three fermion triplets is however not anomaly free. We show that the leftover triangle anomalies can be cancelled by two neutral Dirac fermions having fractional $B-L$ charges, both of which are naturally stable by virtue of a remnant $\mathbb{Z}_2 \times \mathbb{Z}'_2$ symmetry, naturally leading to a two component dark matter scenario without any ad-hoc symmet…
We implement a minimal linear seesaw model (LSM) for addressing the Quasi-Dirac (QD) behaviour of heavy neutrinos, focusing on the mass regime of $M_{N} \lesssim M_{W}$. Here we show that for relatively low neutrino masses, covering the few GeV range, the same-sign to opposite-sign dilepton ratio, $R_{\ell \ell}$, can be anywhere between 0 and 1, thus signaling a Quasi-Dirac regime. Particular values of $R_{\ell \ell}$ are controlled by the width of the QD neutrino and its mass splitting, the latter being equal to the light-neutrino mass $m_ν$ in the LSM scenario. The current upper bound on $m…
The type-III seesaw seems to explain the very minuteness of neutrino masses readily and naturally. The high-energy see-saw theories usually involve a larger number of effective parametres than the physical and measurable parametres appearing in the low-energy neutrino phenomenology. Casas-Ibarra parametrisation facilitates to encode the information lost in integrating the heavy fermions out in an arbitrary complex orthogonal matrix. The CMS collaboration has already searched for triplet fermions in the type-III seesaw model with only one generation of triplet fermion flavour democratically dec…
Dark matter as the source of neutrino mass: theory overview and experimental prospects
Universidad Adolfo Ibáñez（阿道夫·伊巴涅斯大学）；Universitat de Valencia（瓦伦西亚大学）；CSIC（西班牙国家研究委员会）；INFN Cosenza（科森扎意大利国家核物理研究所）；Università della Calabria（卡拉布里亚大学）；Korea Institute for Advanced Study（韩国高等研究院）；Ramakrishna Mission Residential College (Autonomous)（罗摩克里希纳传教士寄宿学院（自治））
We analyze a displaced dilepton signal expected at the LHC for a tau left sneutrino as the lightest supersymmetric particle with a mass in the range $45$-$100$ GeV. The sneutrinos are pair produced via a virtual $W$, $Z$ or $γ$ in the $s$ channel and, given the large value of the tau Yukawa coupling, their decays into two dileptons or a dilepton plus missing transverse energy from neutrinos can be significant. The discussion is carried out in the $μν$SSM, where the presence of $R$-parity violating couplings involving right-handed neutrinos solves the $μ$ problem and can reproduce the neutrino …
The inverse seesaw mechanism allows the neutrino masses to be generated by new physics at an experimentally accessible scale, even with O(1) Yukawa couplings. In the inverse seesaw scenario, the smallness of neutrino masses is linked to the smallness of a lepton number violating parameter. This parameter may arise radiatively. In this paper, we study the cosmological implications of two contrasting radiative inverse seesaw models, one due to Ma and the other to Law and McDonald. The former features spontaneous, the latter explicit lepton number violation. First, we examine the effect of the le…
The charged current production of long-lived heavy neutrinos at the LHC can use a prompt charged lepton for triggering the measurement of the process. However, in order to fully characterize the heavy neutrino interactions, it is necessary to also probe Higgs or $Z$ mediated neutral current production. In this case the charged lepton is not available, so other means of triggering are required. In this work, we explore the possibility of using a vector boson fusion trigger in the context of a GeV-scale Type I Seesaw model. We consider a minimal model, where both Higgs and Z-mediated contributio…
We explore the possibility of a new dark matter candidate in the supersymmetric type III seesaw mechanism where a neutral scalar component of the Y=0 triplet can be the lightest supersymmetric particle. Its thermal abundance can be in the right range if non-standard cosmology such as kination domination is assumed. The enhanced cross-section of the dark matter annihilation to W+W- can leave detectable astrophysical and cosmological signals whose current observational data puts a lower bound on the dark matter mass. The model predicts the existence of a charged scalar almost degenerate with the…
We investigate a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. For this the Minimal Supersymmetric Standard Model is extended by adding light right-handed neutrinos and their superpartners, the R-sneutrinos. Moreover, we consider the lightest neutralinos to be Higgsino-like. We first update a previous analysis and assess to which extent does existing LHC data constrain the allowed slepton masses. Here we find scenarios where sleptons with masses as low as 175 GeV are consistent with existing data. However, we also show that the…
The explanation of the small neutrino mass can be depicted using some handsome models like type-I and inverse seesaw where the Standard Model gauge singlet heavy right handed neutrinos are deployed. The common thing in these two models is a lepton number violating parameter, however, its order of magnitude creates a striking difference between them making the nature of the right handed heavy neutrinos a major play factor. In the type-I seesaw a large lepton number violating parameter involves the heavy right handed neutrinos in the form of Majorana fermions while a small lepton number violatin…
We discuss the prospects of studying lepton number violating processes in order to identify Majorana neutrinos from low scale seesaw mechanisms at lepton-proton colliders. In particular, we consider the scenarios of colliding electrons with LHC energy protons and, motivated by the efforts towards the construction of a muon collider, the prospects of muon-proton collisions. We find that present constraints on the mixing of the Majorana neutrinos still allow for a detectable signal at these kind of facilities given the smallness of the Standard Model background. We discuss possible cuts in order…
We consider an inverse seesaw mechanism of neutrino mass generation in which the Standard Model is extended by $3+3$ (heavy) sterile states, and endowed with a flavour symmetry $G_f$, $G_f=Δ(3 \, n^2)$ or $G_f=Δ(6 \, n^2)$, and a CP symmetry. These symmetries are broken in a peculiar way, so that in the charged lepton sector a residual symmetry $G_\ell$ is preserved, while the neutral fermion sector remains invariant under the residual symmetry $G_ν=Z_2 \times CP$. We study the concrete setup, where the Majorana mass term for three of the sterile states conserves $G_ν$, while the remaining mas…
Seesaw Models under the Lens of Angular Distributions at $μ^+μ^-,\, μ^+ μ^+, \, μ^+ γ$ and $μ^+ e^-$ Colliders
Indian Institute of Technology Hyderabad（印度理工学院海德拉巴分校）；INFN, Gruppo Collegato di Cosenz & Dipartimento di Fisica, Università della Calabria（意大利国家核物理研究所科森扎合作组与卡塔尼亚大学物理系）；Theoretical Physics Division, Physical Research Laboratory（物理研究实验室理论物理部）
Non-Abelian Vector Boson Dark Matter (VBDM), arising from an $SU(2)_N$ extension of the Standard Model (SM) has been studied. The Dark Matter (DM) is stabilized by imposing an additional discrete global symmetry $S^{'}$ with charges, such that it satisfies $S=S^{'}+T_{3N}$ even when $SU(2)_N$ is completely broken. This model, apart from a single-component DM also offers a two-component DM augmented by a scalar and a vector boson, which lives over a large parameter space evading strong direct search bounds. Apart from potential DM candidates, this model also explains the smallness of neutrino m…
We revisit discovery prospects for a long-lived sterile neutrino $N$ at the Large Hadron Collider (LHC) in the context of left-right symmetric theories. We focus on a displaced vertex search strategy sensitive to $\mathcal{O}$(GeV) neutrino masses produced via a right-handed $W_{R}$ boson. Both on-shell and off-shell Drell-Yan production of $W_{R}$ are considered. We estimate the reach as a function of $m_{N}$ and $m_{W_{R}}$. With $\sqrt{s}=13$ TeV and 300/fb of integrated luminosity, the LHC can probe neutrino masses as high as $\sim 30$ GeV and $m_{W_{R}}$ around 6 TeV. The reach goes up to…
We study discovery prospects for long-lived sterile neutrinos at the LHC with multitrack displaced vertices, with masses below the electroweak scale. We reinterpret current displaced vertex searches making use of publicly available, parametrized selection efficiencies for modeling the detector response to displaced vertices. We focus on production of right-handed $W_{R}$ bosons and neutrinos $N$ in a left-right symmetric model, and find poor sensitivity. After proposing a different trigger strategy (considering the prompt lepton accompanying the neutrino displaced vertex) and optimized cuts in…
We revisit the potential of a Linear Collider concerning the study of lepton flavour violation, in view of new LHC bounds and of the (very) recent developments in lepton physics. Working in the framework of a type I supersymmetric seesaw, we evaluate the prospects of observing seesaw-induced lepton flavour violating final states of the type e μ+ missing energy, arising from e+ e- and e- e- collisions. In both cases we address the potential background from standard model and supersymmetric charged currents. We also explore the possibility of electron and positron beam polarisation. The statisti…
