arXiv subjects

John N. Ng

Publications and source records attributed to John N. Ng.

At least 19 recordsLinked to original sources

arXiv2024

Probing Lepton Number Violation at Same-Sign Lepton Colliders

Same-sign lepton colliders offer a promising environment to probe lepton number violation. We study processes that change lepton number by two units in the context of Majorana heavy neutral leptons and neutrinophilic scalars at $μ$TRISTAN, a proposed same-sign muon collider. Our work shows that such colliders, with modest energy and luminosity requirements, can either reveal direct evidence of lepton number violation or significantly constrain unexplored regions of parameter space, especially in the case of a neutrinophilic scalar.

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arXiv2023

Higgs Portal Interpretation of the Belle II $B^+ \to K^+ νν$ Measurement

The Belle II experiment recently observed the decay $B^+ \to K^+ νν$ for the first time, with a measured value for the branching ratio of $ (2.3 \pm 0.7) \times 10^{-5}$. This result exhibits a $\sim 3σ$ deviation from the Standard Model (SM) prediction. The observed enhancement with respect to the Standard Model could indicate the presence of invisible light new physics. In this paper, we investigate whether this result can be accommodated in a minimal Higgs portal model, where the SM is extended by a singlet Higgs scalar that decays invisibly to dark sector states. We find that current and future bounds on invisible decays of the 125 GeV Higgs boson completely exclude a new scalar with a mass $\gtrsim 10$ GeV. On the other hand, the Belle II results can be successfully accommodated if the new scalar is lighter than $B$ mesons but heavier than kaons. We also investigate the cosmological implications of the new states and explore the possibility that they are part of an abelian Higgs extension of the SM. Future Higgs factories are expected to place stringent bounds on the invisible branching ratio of the 125 GeV Higgs boson, and will be able to definitively test the region of parameter space favored by the Belle II results.

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arXiv2020

KeV scale new fermion from a hidden sector

We studied a simple model of hidden sector consists of a Dirac fermion $χ$ and a spontaneously broken $U(1)_s$ symmetry. The dark sector is connected to the Standard Model(SM) via three righthanded SM singlet neutrinos, $N_R$'s, and the kinetic mixing between $U(1)_s$ and $U(1)_Y$. A mixing between the scalar $ϕ$ that breaks $U(1)_s$ and the SM Higgs boson, $H$, is implemented via the term $ϕ^\dagger ϕH^\dagger H$ and this provides a third connection to the SM. Integrating out the $N_R$ at a high scale not only gives the active neutrinos, $ν$, masses but generates effective Dirac type of couplings between $ν$ and $χ$. This changes the usual Type-I seesaw results for active neutrino masses and makes $χ$ behave like a sterile neutrino even though its origin is in the hidden sector. $χ$ is also split into a pair of Majorana fermions. The amount of splitting depends on the parameters. If the lighter of the pair has a mass around keV, its lifetime is longer than the age of the universe and it can be a warm dark matter candidate. Signatures of $χ$ in high precision Kurie plots of nuclei $β$ decays and low energy neutrino nuclei coherent scatterings are discussed. The model also induces new invisible $Z$ decay modes that can be searched for in future Z factories.

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arXiv2019

Alternative Perspective on Gauged Lepton Number and Implications for Collider Physics

A new anomaly-free gauged $U(1)_\ell$ lepton-number model is studied. Two standard model lepton generations acquire the same but oppositive sign $U(1)_\ell$ charges, while four exotic chiral leptons cancel the anomalies of the remaining lepton family. We discuss a simplified case which has the universal Yukawa couplings. It agrees with all the experimental constraints and predicts $m_e, m_μ\ll m_τ$, and the latter is of the electroweak scale. Due to the interference between the SM and $U(1)_\ell$ gauge interactions, this model robustly predicts that $e,μ,τ$ have distinctive forward-backward asymmetries at the $e^+e^-$ colliders. It can be searched for at the $e^+e^-$ machine with $\sim$ TeV center-of-mass energy and an integrated luminosity $\sim ab^{-1}$.

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arXiv2018

Neutrino masses and gauged $U(1)_\ell$ lepton number

We investigate the tree-level neutrino mass generation in the gauged $U(1)_\ell$ lepton model recently proposed by us [arXiv:1805.10382]. With the addition of one Standard Model(SM) singlet, $ϕ_1(Y=0, \ell=1)$, and one SM triplet scalar, $T(Y=-1,\ell=0)$, realistic lepton masses can be accommodated. The resulting magnitude of neutrino mass is given by $\sim v_t^3/v_L^2$, where $v_t$ and $v_L$ are the vacuum expectation values of $T$ and $ϕ_1$, respectively, and it is automatically of the inverse see-saw type. Since $v_L$ is the lepton number violation scale we take it to be high, i.e. ${\cal O} \gtrsim (\mbox{TeV})$. Moreover, the induced lepton flavor violating processes and the phenomenology of the peculiar triplet are studied. An interesting bound, $0.1\lesssim v_t\lesssim24.1$ GeV, is obtained when taking into account the neutrino mass generation, $Br(μ\rightarrow e γ)$, and the limits from oblique parameters, $ΔS$ and $ΔT$. Collider phenomenology of the SM triplets is also discussed.

hep-ph↗

arXiv2018

Study of Gauged Lepton Symmetry Signatures at Colliders

We construct a new gauged $U(1)_\ell$ lepton number model which is anomaly-free for each SM generation. The active neutrino masses are radiatively generated with a minimal scalar sector. The phenomenology and collider signals are studied. The interference effects among the new gauge boson, $Z_\ell$, photon, and $Z$-boson can be probed at the future $e^+e^-$ colliders even if the center-of-mass energy is below the mass of $Z_\ell$. Moreover, the electroweak precision sets a stringent bound on the mass splitting of the new lepton doublets.

hep-ph↗

arXiv2018

Prospects for Detecting light bosons at the FCC-ee and CEPC

We look at the prospects for detecting light bosons, $X$, at proposed Z factories assuming a production of $10^{12}$ Z bosons. Such a large yield is within the design goals of future FCC-ee and CEPC colliders. Specifically we look at the cases where $X$ is either a singlet scalar which mixes with the standard model Higgs or a vector boson with mass $1\lesssim M_X \lesssim 80$ GeV. We find that several channels are particularly promising for discovery prospects. In particular $Z\rightarrow f \bar{f} X$ and $Z \rightarrow V_Q X$ gives a promising signal above a very clean standard model background. We also discuss several channels that have too large a background to be useful.

hep-ph↗

arXiv2018

Signal for a light singlet scalar at the LHC

In the general Higgs portal like models, the extra neutral scalar, $S$, can mix with the Standard Model (SM) Higgs boson, $H$. We perform an exploratory study focusing on the direct search for such a light singlet $S$ at the Large Hadron Collider (LHC). After careful study of the SM background, we find the process $pp\rightarrow t\bar{t}S$ followed by $S\rightarrow b\bar{b}$ can be used to investigate $S$ with mass in the $20<M_S<100$ GeV range, which has not been well explored at the LHC. The signal significance becomes meaningful with a luminosity around a few $\mbox{ab}^{-1}$. Also, we study the prospects of finding the light scalar at the future 100 TeV $pp$ collider, the $Z$ and Higgs factories. With similar luminosity, the current LEP limits on the mixing between $S$ and $H$ can be improved by at least one or two orders of magnitude.

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arXiv2016

Renormalization Group Study of the Minimal Majoronic Dark Radiation and Dark Matter Model

We study the 1-loop renormalization group equation running in the simplest singlet Majoron model constructed by us earlier to accommodate the dark radiation and dark matter content in the universe. A comprehensive numerical study was performed to explore the whole model parameter space. A smaller effective number of neutrinos $\triangle N_{eff}\sim 0.05$, or a Majoron decoupling temperature higher than the charm quark mass, is preferred. We found that a heavy scalar dark matter, $ρ$, of mass $1.5-4$ TeV is required by the stability of the scalar potential and an operational type-I see-saw mechanism for neutrino masses. A neutral scalar, $S$, of mass in the $10-100$ GeV range and its mixing with the standard model Higgs as large as $0.1$ is also predicted. The dominant decay modes are $S$ into $b\bar{b}$ and/or $ωω$. A sensitive search will come from rare $Z$ decays via the chain $Z\rightarrow S+ f\bar{f}$, where $f$ is a Standard Model fermion, followed by $S$ into a pair of Majoron and/or b-quarks. The interesting consequences of dark matter bound state due to the sizable $Sρρ$-coupling are discussed as well. In particular, shower-like events with an apparent neutrino energy at $M_ρ$ could contribute to the observed effective neutrino flux in underground neutrino detectors such as IceCube.

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arXiv2015

Search for Heavy Right-Handed Neutrinos at the LHC and Beyond in the Same-Sign Same-Flavor Leptons Final State

In this study we explore the LHC's Run II potential to the discovery of heavy Majorana neutrinos, with luminosities between $30$ and $3000$ fb$^{-1}$ in the $l^{\pm}l^{\pm}j~j$ final state. Given that there exist many models for neutrino mass generation, even within the Type I seesaw framework, we use a simplified model approach and study two simple extensions to the Standard Model, one with a single heavy Majorana neutrino, singlet under the Standard Model gauge group, and a limiting case of the left-right symmetric model. We then extend the analysis to a future hadron collider running at $100$ TeV center of mass energies. This extrapolation in energy allows us to study the relative importance of the resonant production versus gauge boson fusion processes in the study of Majorana neutrinos at hadron colliders. We analyze and propose different search strategies designed to maximize the discovery potential in either the resonant production or the gauge boson fusion modes.

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arXiv2014

Probing Radiative Neutrino Mass Generation through Monotop Production

We present a generalization of a model where the right-handed up-type quarks serve as messengers for neutrino mass generation and as a portal for dark matter. Within this framework the Standard Model is extended with a single Majorana neutrino, a coloured electroweak-singlet scalar and a coloured electroweak-triplet scalar. We calculate the relic abundance of dark matter and show that we can match the latest experimental results. Furthermore, the implications from the scattering between nuclei and the dark matter candidate are studied and we implement the latest experimental constraints arising from flavour changing interactions, Higgs production and decay and LHC collider searches for a single jet and jets plus missing energy. In addition, we implement constraints arising from scalar top quark pair-production. We also study the production of a single top in association with missing energy and calculate the sensitivity of the LHC to the top quark semileptonic decay mode with the current $20$ fb$^{-1}$ data set at a centre of mass energy of $\sqrt{s}=8$ TeV. Furthermore, we carry out the analysis to centre of mass energies of $\sqrt{s}=14$ TeV with $30$ and $300$ fb$^{-1}$ of data.

hep-ph↗

arXiv2014

A Minimal Model of Majoronic Dark Radiation and Dark Matter

We extend the singlet Majoron model of dark radiation by adding another singlet scalar of unit lepton charge. The spontaneous breaking of global $U(1)_L$ connects dark radiation with neutrino mass generation via the type-I seesaw mechanism. The model naturally has a stable scalar dark matter field. It also predicts the existence of a light scalar of mass less than 1 GeV that mixes with the Standard Model Higgs boson. We perform a numerical analysis of the parameters of the model by imposing constraints from giving correct relic abundance and satisfying bounds from direct dark matter detection, rare decays of B-meson, and invisible width of the Higgs boson. The viability of the model in accommodating the gamma rays from the Galactic center is discussed as well. The model gives rise to new rare Higgs boson decays such as four-muon final states with displaced vertices. Another unique signal is two muons and missing energy recoil against the muon pair. Our result also shows that such a bridge between dark radiation and the seesaw mechanism will put the seesaw scale in the range of 1-100 TeV.

hep-ph↗

arXiv2013

Tracking Down the Top Quark Forward-Backward Asymmetry with Monotops

We revisit the possibility that the top quark forward-backward asymmetry arises from the on-shell production and decay of scalar top partners to top-antitop pairs with missing transverse energy. Although the asymmetry is produced by t-channel exchange of a light mediator, the model remains unconstrained by low energy atomic parity violation tests. An interesting connection to the active neutrino sector through a Type-I seesaw operator helps to evade stringent monojet constraints and opens up a richer collider phenomenology. After performing a global fit to top data from both the Tevatron and the LHC, we obtain a viable region of parameter space consistent with all phenomenological and collider constraints. We also discuss the discovery potential of a predicted monotop signal and related lepton charge asymmetry at the LHC.

hep-ph↗

arXiv2013

Some consequences of the Majoron being the dark radiation

We discuss some phenomenological consequences in a scenario where a singlet Majoron plays the role of dark radiation. We study the interrelations between neutrino mass generation and the scalar potential arising from this identification. We find the extra scalar has to be light with a mass at or below the GeV level. The mixing of this scalar with the Standard Model Higgs impacts low energy phenomena such as the muonic hydrogen Lamb shift and muon anomalous magnetic moment. Demanding that the light scalar solves the puzzle in the muon magnetic moment requires the scalar to be lighter still with mass at or below the 10 MeV level. The cross-sections for the production of heavy neutrinos at LHC14 are also given.

hep-ph↗

arXiv2013

Top Quark as a Dark Portal and Neutrino Mass Generation

We present a new model for radiatively generating Majorana active neutrino masses while incorporating a viable dark matter candidate. This is possible by extending the Standard Model with a single Majorana neutrino endowed with a dark parity, a colour electroweak singlet scalar, as well as a colour electroweak triplet scalar. Within this framework, the $up$-type quarks play a special role, serving as a portal for dark matter, and a messenger for neutrino mass generation. We consider three benchmark scenarios where the abundance of dark matter can match the latest experimental results, while generating neutrino masses in the milli-electronvolt range. We show how constraints from lepton flavour violation, in particular the branching fraction of $μ\to eγ$, can place lower bounds on the coupling between our dark matter candidate and top quarks. Furthermore, we show that this coupling can also be constrained using collider data from the Tevatron and the LHC.

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arXiv2012

Constraints on New Scalars from the LHC 125 GeV Higgs Signal

We study the implications the recent results from the LHC Higgs searches have on scalar new physics. We study the impact on both the Higgs production and decay from scalars with and without colour, and in cases where decoupling do and do not happen. We investigate possible constraints on scalar parameters from the production rate in the diphoton channel, and also the two vector boson channels. Measurements from both channels can help disentangle new physics due to colour from that due to charge, and thus reveal the nature of the new scalar states.

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arXiv2012

Top Quark Forward-Backward Asymmetry and Anomalous Right-Handed Charge Currents

Recent measurements of the top quark forward-backward asymmetry at the Tevatron could hint at new physics with an unexpected flavor structure. The significance of such an abnormal flavor structure in alleviating the tension between the various determinations of $|V_{ub}|$ via right-handed charge currents is studied. In particular, we elaborate on how the associated new flavor changing couplings naturally allow for the generation of anomalous loop-induced right-handed charge currents which can simultaneously remove the tension in the determinations of $|V_{ub}|$ and escape the tight indirect bounds from $B \ra X_s γ$.

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arXiv2010

Neutron-Antineutron Oscillations in a Warped Extra Dimension

We investigate neutron-antineutron oscillations in the Randall-Sundrum warped extra dimensional scenario. The four dimensional effective strengths of the relevant operators that induce the oscillations are calculated up to an arbitrary coupling along with their corresponding enhancements due to QCD 1-loop running effects. We find that the $ΔB = 2$ operators can be geometrically suppressed without fine tuning to within current experimental limits with a warped down four dimensional mass scale which can be as low as a fraction of a TeV.

hep-th↗