Search

The proposed muon collider presents a promising avenue to explore various classes of beyond the Standard Model (BSM) particles. In this paper, we investigate the discovery prospects of the scalar leptoquark (LQ) $S_1$ at a muon collider. We consider two benchmark center-of-mass (C.O.M.) energy scenarios: 5 TeV and 10 TeV. We assume that the LQ decays into a top quark and a muon. The collider analysis for an LQ decaying into a top quark is distinct from that of lighter quarks. A TeV-scale LQ decaying into a top quark can produce an exotic, boosted fat-jet signature. In addition to the usual searches based on pair production of LQs, we also examine the single production mode, which depends on the $S_1 t \mu$ coupling. We demonstrate that systematically combining the pair and single production modes significantly enhances the discovery potential of the LQ at the muon collider. Our signal topology includes at least one hadronically decaying top fat-jet and two oppositely charged muons, thereby enabling the incorporation of the single production mode. We show that even with single production alone, it is possible to probe LQs as heavy as 4.5 TeV (9.0 TeV) in the 5 TeV (10 TeV) C.O.M. scenario for $\mathcal{O}(1)$ couplings., Comment: 8 pages, 4 figures, 1 table

Neutrino oscillation experiments have provided direct evidence for the existence of neutrino masses. The seesaw mechanism explains the smallness of these masses through the introduction of heavy right-handed neutrino (RHN) states. The RHN states can aslo generate Dirac neutrino masses at tree or loop level. These heavy states can exist at the electroweak scale, approximately in the $\mathcal{O}(\mathrm{GeV})$ range, and can be investigated through current and future collider experiments. This scenario, where other new physics interactions occur at scales much higher than the RHN scale, can be described using an effective field theory (EFT) framework known as $N_R$-EFT. This study focuses on constraining the Wilson coefficients of $N_R$-EFT operators, which primarily contribute to tri-lepton production and missing energy signals at the LHC. We examine both the scenarios where the RHN mass $M_N$ is less than and greater than the $W$ boson mass $M_W$, and provide predictions for the High-Luminosity run of the LHC (HL-LHC)., Comment: 21 pages, 4 tables, 10 figures , 1 appendix, v2: corrected typos, modified the appendix and added references. This version is to be submitted to Phys. Rev. D

We study the connection between collider and dark matter phenomenology in the singlet extension of the Georgi-Machacek model. In this framework, the singlet scalar serves as a suitable thermal dark matter (DM) candidate. Our focus lies on the region $v_{\chi}<1$ GeV, where $v_{\chi}$ is the common vacuum expectation value of the neutral components of the scalar triplets of the model. Setting bounds on the model parameters from theoretical, electroweak precision and LHC experimental constraints, we find that the BSM Higgs sector is highly constrained. Allowed values for the masses of the custodial fiveplets, triplets and singlet are restricted to the range $140~ {\rm GeV }< M_{H_5} < 350~ {\rm GeV }$, $150~ {\rm GeV }< M_{H_3} < 270 ~{\rm GeV }$ and $145~ {\rm GeV }< M_{H} < 300~ {\rm GeV }$. The extended scalar sector provides new channels for DM annihilation into BSM scalars that allow to satisfy the observed relic density constraint while being consistent with direct DM detection limits. The allowed region of the parameter space of the model can be explored in the upcoming DM detection experiments, both direct and indirect. In particular, the possible high values of BR$(H^0_5\to\gamma\gamma)$ can lead to an indirect DM signal within the reach of CTA. The same feature also provides the possibility of exploring the model at the High-Luminosity run of the LHC. In a simple cut-based analysis, we find that a signal of about $4\sigma$ significance can be achieved in final states with at least two photons for one of our benchmark points., Comment: 44 pages, 20 figures, 7 tables, Published as JHEP10(2024)058

Doubly-charged Higgs bosons have extensively been searched at the LHC. In this work, we study the sensitivity reach of the doubly-charged scalar ($H^{\pm\pm}$) in muon collider for the well-known Type-II seesaw scenario. First, we perform a cut-based analysis to predict the discovery prospect in the muon collider operating with 3 TeV center of mass energy. In addition to this, we have also performed a multivariate analysis and compare the cut-based result with the result obtained from the multivariate analysis. We find that the cut-based analysis is more significant as compared to the multivariate analysis in the large doubly-charged scalar mass region. We predict that a doubly-charged scalar mass, $M_{H^{\pm\pm}}$, upto 1450 GeV can be probed with $5\sigma$ significance for center of mass $\sqrt{s}= 3$ TeV and integrated luminosity $\mathcal{L} = 1000\,\textrm{fb}^{-1}$.

In the Dynamical Scotogenic Model, the global $B-L$ symmetry is supposed to be broken spontaneously resulting in a massless Goldstone boson called majoron, and massive right handed neutrinos which participate in the generation of light neutrino massses at one-loop. One of them being the lightest stable particle can be a thermal dark matter candidate. We discuss how the dark matter phenomenology differs from the original Scotogenic model, taking into account all the constraints coming from the observed neutrino masses and mixing, lepton flavor violations such as $\mu \to e\gamma, \mu \to e J$, astrophysical and cosmological observations of stellar cooling and $N_{eff}$, as well as collider signatures such as Higgs invisible decays. We find that the dark matter annihilation to majorons plays an important role to produce the right relic abundance., Comment: 20 pages, 12 figures, 3 tables, Published as JHEP08(2023)130

We study a different variant of Left-Right Symmetric Model, incorporating Dirac type neutrinos. In the absence of the bi-doublet scalars, the possibility of a universal seesaw type of mass generation mechanism for all the Standard Model charged fermions have been discussed. The model has been constructed by extending the Standard Model particle spectrum with heavy vector-like fermions as well as different scalar multiplets. We have shown that this model can generate non zero neutrino mass through loop mediated processes. The parameters which are involved in neutrino mass generation mechanism can satisfy the neutrino oscillation data for both normal and inverted hierarchy. The lightest charged Higgs plays a crucial role in neutrino mass generation mechanism and can have mass of $\mathcal{O}[\text{GeV}]$. We have systematically studied different constraints which are relevant for the charged Higgs phenomenology. In addition to that we also briefly discuss discovery prospects of the charged Higgs at different colliders.

The gauge singlet right-handed neutrinos (RHNs) are essential fields in several neutrino mass models that explain the observed eV scale neutrino mass. We assume RHN field to be present in the vicinity of the electroweak scale and all the other possible beyond the standard model (BSM) fields arise at high energy scale $\ge\Lambda$. In this scenario, the BSM physics can be described using effective field theory (EFT) where the set of canonical degrees of freedoms consists of both RHN and SM fields. EFT of this kind is usually dubbed as $N_{R}$-EFT. We systematically construct relevant operators that can arise at dimension five and six while respecting underlying symmetry. To quantify the phenomenological implication of these EFT operators we calculate different couplings that involve RHN fields. We discuss the constraints on these EFT operators coming from different energy and precision frontier experiments. For $pp$, $e^{-}p$ and $e^{+}e^{-}$ colliders, we identify various channels which crucially depends on these operators. We analytically evaluate the decay widths of RHN considering all relevant operators and highlight the differences that arise because of the EFT framework. Based upon the signal cross-section we propose different multi-lepton channels to search for the RHN at 14 TeV LHC as well as \emph{future} particle colliders.

This is the Snowmass2021 Energy Frontier (EF) Beyond the Standard Model (BSM) report. It combines the EF topical group reports of EF08 (Model-specific explorations), EF09 (More general explorations), and EF10 (Dark Matter at Colliders). The report includes a general introduction to BSM motivations and the comparative prospects for proposed future experiments for a broad range of potential BSM models and signatures, including compositeness, SUSY, leptoquarks, more general new bosons and fermions, long-lived particles, dark matter, charged-lepton flavor violation, and anomaly detection., Comment: 108 pages + 38 pages references and appendix, 37 figures, Report of the Topical Group on Beyond the Standard Model Physics at Energy Frontier for Snowmass 2021. The first nine authors are the Conveners, with Contributions from the other authors

We present an extension of the SM involving three triplet fermions, one triplet scalar and one singlet fermion, which can explain both neutrino masses and dark matter. One triplet of fermions and the singlet are odd under a $Z_2$ symmetry, thus the model features two possible dark matter candidates. The two remaining $Z_2$-even triplet fermions can reproduce the neutrino masses and oscillation parameters consistent with observations. We consider the case where the singlet has feeble couplings while the triplet is weakly interacting and investigate the different possibilities for reproducing the observed dark matter relic density. This includes production of the triplet WIMP from freeze-out and from decay of the singlet as well as freeze-in production of the singlet from decay of particles that belong to the thermal bath or are thermally decoupled. While freeze-in production is usually dominated by decay processes, we also show cases where the annihilation of bath particles give substantial contribution to the final relic density. This occurs when the new scalars are below the TeV scale, thus in the reach of the LHC. The next-to-lightest odd particle can be long-lived and can alter the successful BBN predictions for the abundance of light elements, these constraints are relevant in both the scenarios where the singlet or the triplet are the long-lived particle. In the case where the triplet is the DM, the model is subject to constraints from ongoing direct, indirect and collider experiments. When the singlet is the DM, the triplet which is the next-to-lightest odd particle can be long-lived and can be probed at the proposed MATHUSLA detector. Finally we also address the detection prospects of triplet fermions and scalars at the LHC., Comment: 52 pages, 26 figures, Published as JHEP11(2022)133

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 spectrometer, namely a pair of displaced fat-jets and the associated tracks, respectively. We find that both the HL-LHC and FCC-hh can be sensitive to small active-sterile mixing $V_{\mu N} > 10^{-6}$ and $V_{\mu N} > 10^{-7}$ with the number of events reaching $\mathcal{O}(10)$ and $\mathcal{O}(10^3)$, respectively. This allows probing the generation of light neutrino masses through the Seesaw mechanism in this scenario., Comment: 29 pages, 17 figures, Contribution to Snowmass 2021

We consider the gauged $B-L$ model which is extended with a secluded dark sector, comprising of two dark sector particles. In this framework the lightest $\mathcal{Z}_2$-odd particle is the dark matter candidate, having a feeble interaction with all other SM and BSM states. The next-to-lightest $\mathcal{Z}_2$-odd particle in the dark sector is a super-wimp, with large interaction strength with the SM and BSM states. We analyse all the relevant production processes that contribute to the dark matter relic abundance, and broadly classify them in two different scenarios, a) dark matter is primarily produced via the non-thermal production process, b) dark matter is produced mostly from the late decay of the next-to-lightest $\mathcal{Z}_2$-odd particle. We discuss the dependency of the relic abundance of the dark matter on various model parameters. Furthermore, we also analyse the discovery prospect of the BSM Higgs via invisible Higgs decay searches., Comment: 37 pages, 17 figures, 5 tables

The existence of light sterile neutrinos, as predicted in several models, can help to explain a number of observations starting from dark mater to recent anomalies in short baseline experiments. In this paper we consider two models - Left-Right Symmetric Zee model and Extended Seesaw model, that can naturally accommodate the presence of light sterile neutrinos in the eV to MeV mass scale. We perform a detailed study on the neutrinoless double beta decay process which receives major contributions from diagrams involving these light sterile neutrinos. Considering a number of theoretical and experimental constraints, including light neutrino masses and mixings, unitarity of the mixing matrix etc., we compare our predicted values of the half-life of neutrinoless double beta decay with the experimental limits. This can put significant constraints on the neutrino mass, active-sterile neutrino mixing and several other important parameters in these models., Comment: 33 pages, 16 figures. Comments are welcome

In a simple extension of the standard model (SM), a pair of vector like lepton doublets ($L_1$ and $L_2$) and a $SU(2)_L$ scalar doublet ($\eta$) have been introduced to help in accommodating the discrepancy in determination of the anomalous magnetic moments of the light leptons, namely, $e$ and $\mu$. Moreover, to make our scenario friendly to a Dirac like neutrino and also for a consistent dark matter phenomenology, we specifically add a singlet scalar ($S$) and a singlet fermion ($\psi$) in the set-up. However, the singlet states also induce a meaningful contribution in other charged lepton processes. A discrete symmetry $\mathcal {Z}_2 \times \mathcal {Z}_2^\prime$ has been imposed under which all the SM particles are even while the new particles may be assumed to have odd charges. In a bottom-up approach, with a minimal particle content, we systematically explore the available parameter space in terms of couplings and masses of the new particles. Here a number of observables associated with the SM leptons have been considered, e.g., masses and mixings of neutrinos, $(g-2)$ anomalies of $e$, $\mu$, charged lepton flavor violating (cLFV) observables and the dark matter (DM) phenomenology of a singlet-doublet dark matter. Neutrinos, promoted as the Dirac type states, acquire mass at one loop level after the discrete $\mathcal{Z}_2^\prime$ symmetry gets softly broken, while the unbroken $\mathcal{Z}_2$ keeps the dark matter stable. The mixing between the singlet $\psi$ and the doublet vector lepton can be constrained to satisfy the electroweak precision observables and the spin independent (SI) direct detection (DD) cross section of the dark matter. In this analysis, potentially important LHC bounds have also been discussed., Comment: 61 pages, 18 figures, 7 tables (published version)

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 light RHN with mass $\mathcal{O}(10)$ GeV and LQ with mass 1.0 TeV, and explore the discovery prospect of the RHN at the proposed Large Hadron electron Collider. In the considered mass range and with the given interaction strength, the RHN is long lived and, due to it stemming from the LQ decay, it is also heavily boosted, resulting in collimated decay products. The unique signature under investigation is thus a displaced fat jet. We use kinematic variables to separate signal from background, and demonstrate that the ratio variables with respect to energy/number of displaced and prompt tracks are useful handles in the identification of displaced decays of the RHN. We also show that employing a positron beam provides order of magnitude enhancement in the detection prospect of this signature., Comment: 6 pages, 6 figures, 2 tables

We consider an effective field theory framework with three standard model (SM) gauge singlet right handed neutrinos, and an additional SM gauge singlet scalar field. The framework successfully generates eV masses of the light neutrinos via seesaw mechanism, and accommodates a feebly interacting massive particle (FIMP) as dark matter candidate. Two of the gauge singlet neutrinos participate in neutrino mass generation, while the third gauge singlet neutrino is a FIMP dark matter. We explore the correlation between the $\textit{vev}$ of the gauge singlet scalar field which translates as mass of the BSM Higgs, and the mass of dark matter, which arises due to relic density constraint. We furthermore explore the constraints from the light neutrino masses in this set-up. We chose the gauge singlet BSM Higgs in this framework in the TeV scale. We perform a detailed collider analysis to analyse the discovery prospect of the TeV scale BSM Higgs through its di-fatjet signature, at a future $pp$ collider which can operate with $\sqrt{s}=100$ TeV c.m.energy., Comment: 39 pages, 15 figures

The present anomaly in muon anaomalous magnetic moment can be explained by the presence of a muon-philic $X$ boson which could be a scalar particle or a vector particle with mass less than twice the mass of muon. The muon-philic $X$ boson could interestingly not be a parity eigenstate as well. If there exists such a boson, irrespective of its parity, it can be directly observed in the decay $J/\psi \to \mu^- \mu^+ X$ where $X$ remains invisible. We show that by using the angular distribution or the distribution of events in the square Dalitz plot, along with two well defined dimensionless ratios, one can clearly distinguish among the various spin-parity possibilities. This would constitute an important probe of both the existence and the nature of this new physics possibility., Comment: 14 pages, 11 figures

The various global analyses of available neutrino oscillation data indicate the presence of the standard $3+0$ neutrino oscillation picture. However, there are a few short baseline anomalies that point to the possible existence of a fourth neutrino (with mass in the eV-scale), essentially sterile in nature. Should sterile neutrino exist in nature and its presence is not taken into consideration properly in the analyses of neutrino data, the interference terms arising due to the additional CP phases in presence of a sterile neutrino can severely impact the physics searches in long baseline (LBL) neutrino oscillation experiments. In the current work we consider one light (eV-scale) sterile neutrino and probe all the three CP phases ($\delta_{13}$, $\delta_{24}$, $\delta_{34}$) in the context of the upcoming Deep Underground Neutrino Experiment (DUNE) and also estimate how the results improve when data from NOvA, T2K and T2HK are added in the analysis. We illustrate the $\Delta \chi^2$ correlations of the CP phases among each other, and also with the three active-sterile mixing angles. Finally, we briefly illustrate how the relevant parameter spaces in the context of neutrinoless double beta decay get modified in light of the bounds in presence of a light sterile neutrino., Comment: 25 pages; 10 figures; added more details about simulation, prior and event spectra; corrected NDBD analysis; accepted for publication in JHEP

We explore relativistic freeze-in production of scalar dark matter in gauged $B-L$ model, where we focus on the production of dark matter from the decay and annihilation of Standard Model (SM) and $B-L$ Higgs bosons. We consider the Bose-Einstein (BE) and Fermi-Dirac (FD) statistics, along with the thermal mass correction of the SM Higgs boson in our analysis. We show that in addition to the SM Higgs boson, the annihilation and decay of the $B-L$ scalar can also contribute substantially to the dark matter relic density. Potential effects of electroweak symmetry breaking (EWSB) and thermal mass correction in BE framework enhance the dark matter relic substantially as it freezes-in near EWSB temperature via scalar annihilation. However, such effects are not so prominent when the dark matter freezes-in at a later epoch than EWSB, dominantly by decay of scalars. The results of this analysis are rather generic, and applicable to other similar scenarios., Comment: 32 pages, 21 figures, 6 tables, Published as JHEP05(2021)150

We consider the extension of the Standard Model (SM) with an inert Higgs doublet that also contains two or three sets of $SU(2)_L$ triplet fermions with hypercharge zero and analyze the stability of electroweak vacuum for the scenarios. The model represents a Type-III inverse seesaw mechanism for neutrino mass generation with a Dark matter candidate.An effective potential approach calculation with two-loop beta function have been carried out in deciding the fate of the electroweak vacuum. Weak gauge coupling $g_2$ shows a different behaviour as compared to the Standard Model. The modified running of $g_2$, along with the Higgs quartic coupling and Type-III Yukawa couplings become crucial in determining the stability of electroweak vacuum. The interplay between two and three generations of such triplet fermions reveals that extensions with two generations is favoured if we aspire for Planck scale stability. Bounds on the Higgs quartic couplings, Type-III Yukawa and number of triplet fermion generations are drawn for different mass scale of Type-III fermions. The phenomenologies of inert doublet and Type-III fermions at the LHC and other experiments are commented upon., Comment: 38 pages, 32 figures, 1 table, Published version

We study the effect of interference on the lepton number violating~(LNV) and lepton number conserving~(LNC) three-body meson decays $M_1^{+}\to l_i^{+} l_j^{\pm}\pi^{\mp}$, that arise in a TeV scale Left Right Symmetric model~(LRSM) with degenerate or nearly degenerate right handed~(RH) neutrinos. LRSM contains three RH neutrinos and a RH gauge boson. The RH neutrinos with masses in the range of $M_N \sim$ (MeV - few GeV) can give resonant enhancement in the semi-leptonic LNV and LNC meson decays. In the case, where only one RH neutrino contributes to these decays, the predicted new physics branching ratio of semi-leptonic LNV and LNC meson decays $M_1^{+}\to l_i^{+} l_j^{+}\pi^{-}$ and $M_1^{+}\to l_i^{+} l_j^{-}\pi^{+}$ are equal. We find that with at least two RH neutrinos contributing to the process, the LNV and LNC decay rates can differ. Depending on the neutrino mixing angles and $CP$ violating phases, the branching ratios of LNV and LNC decay channels mediated by the heavy neutrinos can be either enhanced or suppressed, and the ratio of these two rates can differ from unity., Comment: 40 pages, 12 figures

We explore the signatures of the $\tilde{R}_2$ class of leptoquark (LQ) models at the proposed $e^- p$ and $e^+p$ colliders. We carry out an analysis for the proposed colliders LHeC and FCC-eh with center of mass (c.m.) energy 1.3 TeV and 3.46 TeV, respectively. For $\tilde{R}_2$ class of LQ models, there are a number of final states that can arise from LQ production and its subsequent decay. In this report we do a detailed cut-based analysis for the $l^{\pm}j$ final state. We also discuss the effect of polarized electron and positron beams on LQ production and in turn on $l^{\pm}j$ production. At LHeC, the final state $l^+j$ has very good discovery prospect. We find that, only 100 $\text{fb}^{-1}$ of data can probe LQ mass upto 1.2 TeV with $5\sigma$ significance, even with a generic set of cuts. On the contrary, at FCC-eh, one can probe LQ masses upto 2.2 TeV (for $e^-$ beam) and 3 TeV (for $e^+$ beam), at more than $5\sigma$ significance with luminosity $1000\,\text{fb}^{-1}$ and $500\,\text{fb}^{-1}$, respectively., Comment: 10 pages, 11 figures

We analyze a novel signature of the type II seesaw model - same-sign tetra-lepton signal arising from the mixing of neutral Higgs bosons and their subsequent decays to singly and doubly charged Higgs bosons. For this, we consider wide ranges of the triplet vacuum expectation value (vev) and Yukawa couplings, that are consistent with the observed neutrino masses and mixing as well as the LHC search limits. We find that a doubly charged Higgs boson with mass around 250 GeV and triplet vev around $10^{-4}-10^{-2}$ GeV can give significantly large number of events through it decay to same-sign $W$ gauge bosons at High-Luminosity LHC with $3000 \text{fb}^{-1}$ of data. We also pursue the analysis for a future hadron collider with the c.m. energy of 100 TeV. Considering a heavy Higgs boson around 900 GeV and an intermediate region of the triplet vev, where both same-sign dilepton and gauge boson decays can occur, we identify a limited range of the parameters where the number of same-sign tetra-lepton events are as large as 1000., Comment: 20 pages, 9 figures

We analyse the signal sensitivity of multi-lepton final states at collider that can arise from doubly and singly charged Higgs decay in a type-II seesaw framework. We assume triplet vev to be very small and degenerate masses for both the charged Higgs states. The leptonic branching ratio of doubly and singly charged Higgs states have a large dependency on the neutrino oscillation parameters, lightest neutrino mass scale, as well as neutrino mass hierarchy. We explore this as well as the relation between the leptonic branching ratios of the singly and doubly charged Higgs states in detail. We evaluate the effect of these uncertainties on the production cross-section. Finally, we present a detailed analysis of multi-lepton final states for a future hadron collider HE-LHC, that can operate with center of mass energy $\sqrt{s}=27$ TeV., Comment: 39 pages, 9 figures

We study the discovery prospect of different three body lepton number violating~(LNV) meson decays $M_{1}^{-}\to\ell_{1}^{-}\ell_{2}^{-}M_{2}^{+}$ in the framework of right handed~(RH) neutrino extended Standard Model~(SM). We consider a number of ongoing experiments, such as, NA62 and LHCb at CERN, Belle II at SuperKEK, as well as at the proposed future experiments, SHiP, MATHUSLA and FCC-ee. The RH Majorana neutrino $N$ mediating these meson decays provides a resonant enhancement of the rates, if the mass of $N$ lies in the range $(100\, \text{MeV}-6\, \text{GeV})$. We consider the effect of parent mesons velocity, as well as, the effect of finite detector size. Using the expected upper limits on the number of events for the LNV decay modes, $M_{1}^{-} \to\ell_1^{-}\ell_2^{-}\pi^{+}$~($M_{1}=B, B_c,D, D_{s}\,\text{and}\,K$), we analyze the sensitivity reach of the mixing angles $|V_{e N}|^{2}$, $|V_{\mu N}|^{2}$, $|V_{\tau N}|^{2}$, $|V_{e N}V_{\mu N}|$, $|V_{e N}V_{\tau N}|$ and $|V_{\mu N}V_{\tau N}|$ as a function of heavy neutrino mass $M_{N}$. We show that, inclusion of parent meson velocity can account to a large difference for active-sterile mixing, specially for $D$, $D_s$ meson decay at SHiP and $K$ meson decay at NA62. Taking into account the velocity of the $D_s$ meson, the future beam dump experiment SHiP can probe $|V_{eN}|^2 \sim 10^{-9}$. For RH neutrino mass in between 2 - 5 GeV, MATHUSLA can provide best sensitivity reach of active-sterile mixings., Comment: 40 pages, 46 figures

We explore the discovery prospect of the doubly-charged component of an $SU(2)_L$-triplet scalar at the future $e^- p$ collider FCC-eh, proposed to operate with an electron beam energy of 60 GeV and a proton beam energy of 50 TeV. We consider the associated production of the doubly-charged Higgs boson along with leptons and jet(s), and its subsequent prompt decay to same-sign lepton pair. This occurs for ${\cal O}(1)$ Yukawa coupling of the scalar triplet with charged leptons, which is expected for reasonably small vacuum expectation values of the neutral component of the triplet field that governs the neutrino mass generation in the type-II seesaw. We present our analysis for two different final states, $3l+\ge1j$ and an inclusive $\ge2l+\ge1j$ channel. Considering its decay to electrons only, we find that the doubly-charged Higgs with mass around a TeV could be observed at $3\sigma$ confidence level with $\mathcal{O}(200)\, \rm{fb}^{-1}$ of integrated luminosity, while masses up to 2 TeV could be probed within a few years of data accumulation. The signal proposed here becomes essentially background-free, if it is triggered in the $\mu\mu$ mode and a $5\sigma$ discovery is achievable in this channel for a TeV-scale doubly-charged Higgs with an integrated luminosity as low as $\mathcal{O} (50)\, \rm{fb}^{-1}$. We also highlight the sensitivity of FCC-eh to the Yukawa coupling responsible for production of the doubly-charged Higgs boson as a function of its mass in both the $ee$ and $\mu\mu$ channels., Comment: 28 pages, 6 figures, 8 tables; minor changes, published version

We explore the discovery prospect of a very heavy neutrino at the proposed $e^+e^-$ collider for two different c.m.energies $\sqrt{s}=1.4$ TeV and 3 TeV. We consider production of heavy neutrino via $s$ and $t$-channel processes, and its subsequent prompt decays leading to semi-leptonic final states, along with significant missing energy. For our choice of masses, the gauge boson produced from heavy neutrino decay is highly boosted, leading to a fat-jet. We carry out a detail signal and background analysis for $e^{\pm}+j_{\rm{fat}}+\cancel{E}_{T}$ final state using both cut based and multivariate techniques. We show that a heavy neutrino of mass $600-2700$ GeV and active-sterile mixing $|V_{eN}|^2 \sim 10^{-5}$ can be probed with $5\sigma$ significance at $e^{+}e^{-}$ collider after collecting $\mathcal{L}=500$ $\rm{fb}^{-1}$ of data. We find the sensitivity reach at $e^{+}e^{-}$ collider is order of magnitude enhanced as compared to LHC., Comment: 25 pages, 13 figures and 6 tables

We have explored a minimal supersymmetric standard model scenario extended by one pair of gauge singlets per generation. In the model light neutrino masses and their mixings are generated via inverse seesaw mechanism. In such a scenario, a right-handed sneutrino can be the lightest supersymmetric particle and a cold Dark Matter (DM) candidate. If Casas-Ibarra parametrisation is imposed on the Dirac neutrino Yukawa coupling matrix ($Y_{\nu}$) to fit the neutrino oscillation data, the resulting $Y_{\nu}$ is highly constrained from the lepton flavor violating (LFV) decay constraints. The smallness of $Y_{\nu}$ requires the sneutrino DM to co-annihilate with other sparticle(s) in order to satisfy DM relic density constraint. We have studied sneutrino co-annihilation with wino and observed that this sneutrino-wino compressed parameter space gives rise to a novel same-sign trilepton signal for the stop quark, which is more effective than the conventional stop search channels in the present framework. We have shown that the choice of neutrino mass hierarchy strongly affects the signal event rate, making it easier to probe the scenario with inverted mass hierarchy., Comment: Text and references added, accepted in Phys.Rev.D

We explore leptoquark production and decay for the $\tilde{R}_{2}$ class of models at the proposed $e^{-} p$ collider LHeC, planned to operate with 150 GeV electron and 7 TeV proton beams. In addition to the coupling of leptoquark with the lepton and jet, the model also has right handed neutrinos, coupled to the leptoquark. We analyse the collider signatures of a number of final states, that can originate from leptoquark decay into the standard model particles, as well as, the final states that originate from further decay of the heavy neutrinos, produced from leptoquark. We find that the final state $\ell^{-}+\text{n-jets}\,(1\leq\text{n} \leq 2)$ has the largest discovery prospect, more than $5\sigma$ with only few fb of data to probe a leptoquark of mass 1.1 TeV, even with a generic set of cuts. The significance falls sharply with increasing leptoquark mass. However, with 100\,$\rm{fb}^{-1}$ of data, a 5$\sigma$ discovery for leptoquarks of mass upto 1.4 TeV is still achieveable. Also for the same luminosity, final state $\bar{b}\ell^{+}\tau^{-}+\text{n-jets}\,(\text{n}\geq 2)+\text{MET}$, resulting from the cascade decay of the leptoquark to an $\bar{t}$ and right handed neutrino, followed by further decays of $\bar{t}$ and the neutrino, is expected to yield a rather large number of events~($\approx 180$)., Comment: 29 pages, 32 figures

We study a simple left-right symmetric (LRS) extension of the Zee model for neutrino mass generation. An extra $SU(2)_{L/R}$ singlet charged scalar helps in generating a loop-induced Majorana mass for neutrinos in this model. The right-handed neutrinos in this case are very light of the order of a few eV to a few MeV which makes this scenario quite different from other LRS models. We have analyzed the scalar potential and Higgs spectrum in detail, which also play an important role for the neutrino phenomenology. We identified the parameter regions in the model which satisfy the experimentally observed neutrino masses and mixings along with other experimental constraints. We have then studied the collider signatures of the charged scalar at $e^+e^-$ colliders with different benchmark points. It is possible to get a huge enhancement in the production cross-section of the charged scalar at lepton collider compared to the hadron colliders, resulting in a much stronger signal which can be easily observed at the upcoming ILC or CLIC experiments., Comment: 14 pages, 5 figures, 5 tables

We investigate the pair-production of right-handed neutrinos via the Standard Model (SM) Higgs boson in a gauged $B-L$ model. The right-handed neutrinos with a mass of few tens of GeV generating viable light neutrino masses via the seesaw mechanism naturally exhibit displaced vertices and distinctive signatures at the LHC and proposed lepton colliders. The production rate of the right-handed neutrinos depends on the mixing between the SM Higgs and the exotic Higgs associated with the $B-L$ breaking, whereas their decay length depends on the active-sterile neutrino mixing. We focus on the displaced leptonic final states arising from such a process, and analyze the sensitivity reach of the LHC and proposed lepton colliders in probing the active-sterile neutrino mixing. We show that mixing to muons as small as $V_{\mu N} \approx 10^{-7}$ can be probed at the LHC with 100 fb$^{-1}$ and at proposed lepton colliders with 5000 fb$^{-1}$. The future high luminosity run at LHC and the proposed MATHUSLA detector may further improve this reach by an order of magnitude., Comment: 23 pages, 9 figures, matches published version

We investigate the production and decays of doubly-charged Higgs bosons for the Type-II seesaw mechanism at an $e^{+} e^{-}$ collider with two center of mass energies, $\sqrt{s}=380$ GeV and 3 TeV, and analyze the fully hadronic final states in detail. Lower mass ranges can be probed during the 380 GeV run of the collider, while high mass ranges, which are beyond the 13 TeV Large Hadron Collider discovery reach, can be probed with $\sqrt{s}=3$ TeV. For such a heavy Higgs boson, the final decay products are collimated, resulting in fat-jets. We perform a substructure analysis to reduce the background and find that a doubly-charged Higgs boson in the mass range 800-1120 GeV can be discovered during the 3 TeV run, with integrated luminosity $\mathcal{L} \sim 95\, \rm{fb}^{-1}$ of data. For 380 GeV center of mass energy, we find that for the doubly-charged Higgs boson in the range 160-172 GeV, a $5\sigma$ significance can be achieved with only integrated luminosity $\mathcal{L} \sim 24 \, \rm{fb}^{-1}$. Therefore, a light Higgs boson can be discovered immediately during the run of a future $e^{+} e^{-}$ collider., Comment: 20 pages, 8 figures and 5 tables

It is well known that for the pure standard model triplet fermionic WIMP-type dark matter (DM), the relic density is satisfied around 2 TeV. For such a heavy mass particle, the production cross-section at 13 TeV run of LHC will be very small. Extending the model further with a singlet fermion and a triplet scalar, DM relic density can be satisfied for even much lower masses. The lower mass DM can be copiously produced at LHC and hence the model can be tested at collider. For the present model we have studied the multi jet ($\geq 2\,j$) + missing energy ($\cancel{E}_{T}$) signal and show that this can be detected in the near future of the LHC 13 TeV run. We also predict that the present model is testable by the earth based DM direct detection experiments like Xenon-1T and in future by Darwin., Comment: 34 pages, 12 figures, 8 tables, text added, matches with the published version

In the light of recent experimental results from IceCube, LHC searches for scalar leptoquark, and the flavor anomalies $R_K$ and $R_{K^*}$, we analyze two scalar leptoquark models with hypercharge $Y=1/6$ and $Y=7/6$. We consider the 53 high-energy starting events from IceCube and perform a statistical analysis, taking into account both the Standard Model and leptoquark contribution together. The lighter leptoquark states that are in agreement with IceCube are strongly constrained from LHC di-lepton+dijet search. Heavier leptoquarks in the TeV mass range are in agreement both with IceCube and LHC. We furthermore show that leptoquark which explains the $B$-physics anomalies and does not have any coupling with the third generation of quarks and leptons, can be strongly constrained., Comment: 24 pages, 8 figures, 1 table; typos corrected, discussions and references added; matches published version

We consider an extension of the Standard Model (SM) augmented by two neutral singlet fermions per generation and a leptoquark. In order to generate the light neutrino masses and mixing, we incorporate inverse seesaw mechanism. The right handed neutrino production in this model is significantly larger than the conventional inverse seesaw scenario. We analyze the different collider signatures of this model and find that the final states associated with three or more leptons, multi jet and at least one b-tagged and (or) $\tau$-tagged jet can probe larger RH neutrino mass scale. We have also proposed a same-sign dilepton signal region associated with multiple jets and missing energy that can be used to distinguish the the present scenario from the usual inverse seesaw extended SM., Comment: Text added, accepted in PRD

We analyze the lepton number violating (LNV) meson decays that arise in a TeV scale Left Right Symmetry model. The right handed Majorana neutrino $N$ along with the right handed or Standard Model gauge bosons mediate the meson decays and provide a resonant enhancement of the rates if the mass of $N$ ($M_N$) lies in the range $\sim (100\, \rm{MeV}-5\, \rm{GeV})$. Using the expected upper limits on the number of events for the LNV decay modes $M_{1}^{+} \to\ell^{+}\ell^{+}\pi^{-}$~($M_{1}=B, D, D_{s}, K$), we derive constraints plausible on the mass of the right handed charged gauge boson by future searches at the ongoing NA62 and LHCb experiments at CERN, the upcoming Belle II at SuperKEK, as well as at the proposed future experiments, SHiP and FCC-ee. These bounds are complimentary to the limits from same-sign dilepton search at Large Hadron Collider (LHC). The very high intensity of Charmed mesons expected to be produced at SHiP will result in a far more stringent bound, $M_{W_R}>18.4$~TeV (corresponding to $M_N=1.46$~GeV), than the other existing bounds from collider and neutrinoless double beta decay searches., Comment: 14 pages, 12 figures

The discovery of lepton flavour violating interactions will be striking evidence for physics beyond the Standard Model. Focusing on the three decays $\tau^\mp \to \mu^\pm \mu^\mp \mu^\mp$, $\tau^\mp \to e^\pm \mu^\mp \mu^\mp$ and $\tau^\mp \to e^\mp \mu^\mp \mu^\pm$, we evaluate the discovery potential of current and future high-energy colliders to probe lepton flavour violation in the $\tau$ sector. Based on this potential we determine the expected constraints on parameters of new physics in the context of the Type-II Seesaw Model, the Left-Right Symmetric Model, and the Minimal Supersymmetric Standard Model. The existing and ongoing 13 TeV run of the Large Hadron Collider has the potential to produce constraints that outperform the existing $e^+ e^-$ collider limits for the $\tau^\mp \to \mu^\pm \mu^\mp \mu^\mp$ decay and achieve a branching fraction limit of $\lesssim 10^{-8}$. With a future circular $e^+ e^-$ collider, constraints on the $\tau \to l \mu \mu$ branching fractions could reach as low as a few times $10^{-12}$., Comment: 26 pages, 19 figures. V2: Typo fixes, references added, figure file sizes reduced. Discussion added on consistency with mu -> 3e limits in the Type-II Seesaw Model. Agrees with version published in JHEP

We investigate multilepton signatures, arising from the decays of doubly charged and singly charged Higgs bosons in the Type-II Seesaw model. Depending on the vacuum expectation value of the triplet $v_{\Delta}$, the doubly and singly charged Higgs bosons can decay into a large variety of multi-lepton final states. We explore all possible decay modes corresponding to different regimes of $v_{\Delta}$, that generate distinguishing four and five leptonic signatures. We focus on the 13 TeV Large Hadron Collider (LHC) and further extend the study to a very high energy proton-proton collider (VLHC) with a center-of-mass energy of 100 TeV. We find that a doubly charged Higgs boson of masses around 375 GeV can be discovered at immediate LHC runs. A heavier mass of 630 GeV can instead be discovered at the high-luminosity run of the LHC or at the VLHC with 30 $\rm{fb}^{-1}$., Comment: 21 pages, 10 figures

Hadronic decays of boosted resonances, e.g., top quark jets, at hadronic super colliders are frequent predictions in TeV-scale extensions of the Standard Model of Particle Physics. In such scenarios, accurate modeling of QCD radiation is necessary for trustworthy predictions. We present the automation of fully differential, next-to-leading-order (NLO) in QCD corrections with parton shower (PS) matching for an effective Left-Right Symmetric Model (LRSM) that features $W_R^\pm, Z_R$ gauge bosons and heavy Majorana neutrinos $N$. Publicly available universal model files require remarkably fewer user inputs for predicting benchmark collider processes than leading order LRSM constructions. We present predictions for inclusive $W_R^\pm, Z_R$ production at the $\sqrt{s} = 13$ TeV Large Hadron Collider (LHC) and a hypothetical future 100 TeV Very Large Hadron Collider (VLHC), as well as inclusive $N$ production for a hypothetical Large Hadron Electron Collider (LHeC). As a case study, we investigate at NLO+PS accuracy the properties of heavy neutrino (color-singlet) jets and top quark (color-triplet) jets from decays of high-mass $W_R$ bosons at the LHC. Contrary to top jets, we find that the kinematic properties of heavy neutrinos jets, and in particular jet mass, are resilient against the effects of parton showers and hard QCD radiation. This suggests that in searches for neutrino jets, aggressive selection cuts that would otherwise be inappropriate for top jets can be imposed with minimal signal loss., Comment: 28 pages, 7 figures, and 3 small appendices. Model files available from http://feynrules.irmp.ucl.ac.be/wiki/EffLRSM Happy Halloween!

In the light of the recent result from KamLAND-Zen (KLZ) and GERDA Phase-II, we update the bounds on the effective mass and the new physics parameters, relevant for neutrinoless double beta decay ($0\nu \beta \beta$). In addition to the light Majorana neutrino exchange, we analyze beyond standard model contributions that arise in Left-Right symmetry and R-Parity violating supersymmetry. The improved limit from KLZ constrains the effective mass of light neutrino exchange down to sub-eV mass regime 0.06 eV. Using the correlation between the $^{136}\rm{Xe}$ and $^{76}\rm{Ge}$ half-lives, we show that the KLZ limit individually rules out the positive claim of observation of $0\nu\beta\beta$ for all nuclear matrix element compilation. For the Left-Right symmetry and R-parity violating supersymmetry, the KLZ bound implies a factor of 2 improvement of the effective mass and the new physics parameters. The future ton scale experiments such as, nEXO will further constrain these models, in particular, will rule out standard as well as Type-II dominating LRSM inverted hierarchy scenario., Comment: 9 pages, 7 pdf figures, 7 tables, results from GERDA Phase-II included, to appear in PRD

We re-examine the discovery potential at hadron colliders of high-mass right-handed (RH) gauge bosons $W_R$ - an inherent ingredient of Left-Right Symmetric Models (LRSM). We focus on the regime where the $W_R$ is very heavy compared to the heavy Majorana neutrino $N$, and investigate an alternative signature for $W_R \rightarrow N$ decays. The produced neutrinos are highly boosted in this mass regime. Subsequently, their decays via off-shell $W_R$ bosons to jets, i.e., $N \rightarrow \ell^\pm j j$ are highly collimated, forming a single neutrino jet $(j_N)$. The final-state collider signature is then $\ell^\pm j_N$, instead of the widely studied $\ell^\pm\ell^\pm jj$. Present search strategies are not sensitive to this hierarchical mass regime due to the breakdown of the collider signature definition. We take into account QCD corrections beyond next-to-leading order (NLO) that are important for high-mass Drell-Yan processes at the 13 TeV Large Hadron Collider (LHC). For the first time, we evaluate $W_R$ production at NLO with threshold resummation at next-to-next-to-leading logarithm (NNLL) matched to the threshold-improved parton distributions. With these improvements, we find that a ${W_R}$ of mass $M_{W_R} = 3~(4)~[5]$ TeV and mass ratio of $(m_N/M_{W_R})<0.1$ can be discovered with a $5-6 \sigma$ statistical significance at 13 TeV after ${10~(100)~[2000]~\text{fb}^{-1}}$ of data. Extending the analysis to the hypothetical 100 TeV Very Large Hadron Collider (VLHC), $5\sigma$ can be obtained for $W_R$ masses up to $M_{W_R}=15~(30)$ with approximately $100~\text{fb}^{-1}~(10~\text{ab}^{-1})$., Comment: v2, journal version. PRD (2016). Results unchanged

Run-I results from the CMS collaboration show an excess of events in the decay $h \to \mu \tau_e$ with a local significances of 2.4$\sigma$. This could be the first hint of flavour violation in the Higgs sector. We summarise the bounds on the flavour violating Yukawa couplings from direct searches, low energy measurements and projected future experiments. We discuss the sensitivity of upcoming HL-LHC runs and future lepton colliders in measuring lepton-flavour violating couplings using an effective field theory framework. For the HL-LHC we find limits on BR($h\to \mu \tau$) and BR$(h\to e \tau) \lesssim \mathcal{O}(0.5)\%$ and on BR($h \to e \mu) \lesssim \mathcal{O}(0.02)\%$. For an ILC with center-of-mass energy of 1 TeV we expect BR($h \to e \tau$) and BR($h\to \mu \tau$) to be measurable down to $\mathcal{O}(0.2)\%$., Comment: 27 pages, 1 figure and 13 tables; v2 : Minor corrections, conclusions unchanged. To appear in JHEP

The recent results of 13 TeV ATLAS and CMS di-photon searches show an excess at di-photon invariant mass of 750 GeV. We look for possible explanation of this within minimal left right symmetric model (MLRSM). The possible candidate is a neutral Higgs of mass 750 GeV that can decay to di-photon via charged Higgs and right handed gauge boson loop. However, the cross-section is not consistent with the ATLAS and CMS results. We then discuss one possible variation of this model with universal seesaw for fermion masses that can explain this excess., Comment: 19 pages, 5 figures

We analyse in detail the scalar triplet contribution to the low-energy lepton flavour violating (LFV) and lepton number violating (LNV) processes within a TeV-scale left-right symmetric framework. We show that in both type-I and type-II seesaw dominance for the light neutrino masses, the triplet of mass comparable to or smaller than the largest right-handed neutrino mass scale can give sizeable contribution to the LFV processes, except in the quasi-degenerate limit of light neutrino masses, where a suppression can occur due to cancellations. In particular, a moderate value of the heaviest neutrino to scalar triplet mass ratio $r\lesssim {\cal O}(1)$ is still experimentally allowed and can be explored in the future LFV experiments. Similarly, the contribution of a relatively light triplet to the LNV process of neutrinoless double beta decay could be significant, disfavouring a part of the model parameter space otherwise allowed by LFV constraints. Nevertheless, we find regions of parameter space consistent with both LFV and LNV searches, for which the values of the total effective neutrino mass can be accessible to the next generation ton-scale experiments. Such light triplets can also be directly searched for at the LHC, thus providing a complementary probe of this scenario. Finally, we also study the implications of the triplet contribution for the left-right symmetric model interpretation of the recent diboson anomaly at the LHC., Comment: 28 pages, 17 figures; minor changes, version to appear in JHEP

Inspired by the recent diboson excess observed at the LHC and possible interpretation within a TeV-scale Left-Right symmetric framework, we explore its implications for low-energy experiments searching for lepton number and flavor violation. Assuming a simple Type-II seesaw mechanism for neutrino masses, we show that for the right-handed (RH) gauge boson mass and coupling values required to explain the LHC anomalies, the RH contribution to the lepton number violating process of neutrinoless double beta decay ($0\nu\beta\beta$) is already constrained by current experiments for relatively low-mass (MeV-GeV) RH neutrinos. The future ton-scale $0\nu\beta\beta$ experiments could probe most of the remaining parameter space, irrespective of the neutrino mass hierarchy and uncertainties in the oscillation parameters and nuclear matrix elements. On the other hand, the RH contribution to the lepton flavor violating process of $\mu\to e\gamma$ is constrained for relatively heavier (TeV) RH neutrinos, thus providing a complementary probe of the model. Finally, a measurement of the absolute light neutrino mass scale from future precision cosmology could make this scenario completely testable., Comment: 5 pages, 4 figures

We study the discovery prospects of a heavy neutral scalar arising from a $U(1)_{B-L}$ extension of the Standard Model (SM) during the Large Hadron Collider's high luminosity runs (HL-LHC). This heavy neutral scalar mixes with the SM Higgs boson through a Higgs portal and interacts with the SM particles with an interaction strength proportional to the sine of the mixing angle. The mixing between the two Higgs bosons is constrained by direct and indirect measurements. We choose an experimentally viable mixing angle and explore in detail the $ZZ$ and $WW$ decay modes of the heavy Higgs boson. For the $ZZ$ case, we focus on the cleanest $4\ell$ and $2\ell 2j$ final states and find that a heavy Higgs boson of mass smaller than 500 GeV can be discovered at the HL-LHC. For the $WW$ decay mode, we analyze the $\ell jj \slashed{E}_T$ signature. We implement novel background reduction techniques in order to tackle the huge background by performing both cut-based and multivariate analyses. However, large backgrounds render this channel challenging. We briefly discuss the discovery prospects of the heavy $Z'$-boson arising in this model., Comment: 44 pages, 14 figures and 10 tables; v3 : Some typos fixed

We discuss the future prospects of heavy neutrino searches at next generation lepton colliders. In particular, we focus on the planned electron-positron colliders, operating in two different beam modes, namely, $e^+e^-$ and $e^-e^-$. In the $e^+e^-$ beam mode, we consider various production and decay modes of the heavy neutrino ($N$), and find that the final state with $e+2j+{E\!\!\!/}_T$, arising from the $e^+e^-\to N\nu$ production mode, is the most promising channel. However, since this mode is insensitive to the Majorana nature of the heavy neutrinos, we also study a new production channel $e^+e^-\to Ne^\pm W^\mp$, which leads to a same-sign dilepton plus four jet final state, thus directly probing the lepton number violation in $e^+e^-$ colliders. In the $e^-e^-$ beam mode, we study the prospects of the lepton number violating process of $e^-e^-\to W^-W^-$, mediated by a heavy Majorana neutrino. We use both cut-based and multivariate analysis techniques to make a realistic calculation of the relevant signal and background events, including detector effects for a generic linear collider detector. We find that with the cut-based analysis, the light-heavy neutrino mixing parameter $|V_{eN}|^2$ can be probed down to $\sim 10^{-4}$ at 95\% C.L. for the heavy neutrino mass up to $400$ GeV or so at $\sqrt s=500$ GeV with $100 \rm{fb}^{-1}$ of integrated luminosity. For smaller mixing values, we show that a multivariate analysis can improve the signal significance by up to an order of magnitude. These limits will be at least an order of magnitude better than the current best limits from electroweak precision data, as well as the projected limits from $\sqrt s=14$ TeV LHC., Comment: 21 pages, 19 figures, 9 tables. Minor changes. Accepted in PRD. Matches with published version