Scalar Singlet Research Articles

Scalar dark matter with a μτ flavored mediator

We study a renormalizable scalar singlet dark matter model based on $Z_4$ lepton flavor symmetry. A $\mu\tau$-philic scalar doublet is introduced to explain the discrepancy between the experimental value and the theoretical prediction of the muon anomalous magnetic moment. At the same time, these flavored scalars play the role of a mediator which connects dark matter and standard model particles. The observed relic abundance of the dark matter is easily maintained while satisfying the current severe constraints on the dark matter from various experiments and observations thanks to the flavor off-diagonal interactions of scalar mediators. We also explore the possibility of dark matter direct detection through the one-loop process.

Freeze-in and freeze-out of sterile neutrino dark matter

A sterile neutrino with a keV-scale mass is a compelling dark matter candidate. We propose a new production mechanism involving the decay and annihilation of a complex scalar singlet with a Higgs portal coupling which develops a vacuum expectation value. The interactions of the resulting pseudo Nambu-Goldstone boson may thermalise the dark sector.We determine the region of parameter space where dark sector thermalisation is reached and discuss the most relevant cosmological observables.The scenario can be considered as the combination of a freeze-in of the dark sector followed by relativistic freeze-out.

Leptoquark-assisted singlet-mediated di-Higgs production at the LHC

At the LHC, the gluon-initiated processes are considered to be the primary source of di-Higgs production. However, in the presence of a new resonance, the light-quark initiated processes can also contribute significantly. In this letter, we look at the di-Higgs production mediated by a new singlet scalar. The singlet is produced in both quark-antiquark and gluon fusion processes through loops involving a scalar leptoquark and right-handed neutrinos. With benchmark parameters inspired from the recent resonant di-Higgs searches by the ATLAS collaboration, we examine the prospects of such a resonance in the TeV-range at the High-Luminosity LHC (HL-LHC) in the bb¯τ+τ− mode with a multivariate analysis. We obtain the 5σ and 2σ contours and find that a significant part of the parameter space is within the reach of the HL-LHC.

Probing high scale Dirac leptogenesis via gravitational waves from domain walls

We propose a novel way of probing high-scale Dirac leptogenesis, a viable alternative to the canonical leptogenesis scenario where the total lepton number is conserved, keeping light standard model neutrinos purely Dirac. The simplest possible seesaw mechanism for generating light Dirac neutrinos involves heavy singlet Dirac fermions and a singlet scalar. In addition to unbroken global lepton number, a discrete ${Z}_{2}$ symmetry is imposed to forbid direct coupling between right and left chiral parts of light Dirac neutrinos. Generating light Dirac neutrino mass requires the singlet scalar to acquire a vacuum expectation value (VEV) that also breaks the ${Z}_{2}$ symmetry, leading to the formation of domain walls in the early Universe. These walls, if made unstable by introducing a soft ${Z}_{2}$-breaking term, generate gravitational waves (GWs) with a spectrum characterized by the wall tension or the singlet VEV, and the soft symmetry breaking scale. The scale of leptogenesis depends on the ${Z}_{2}$-breaking singlet VEV, which is also responsible for the tension of the domain wall, affecting the amplitude of GWs produced from the collapsing walls. We find that most of the near-future GW observatories will be able to probe Dirac leptogenesis scales all the way up to ${10}^{11}\text{ }\text{ }\mathrm{GeV}$.

Electroweak symmetry breaking and WIMP-FIMP dark matter

Electroweak Symmetry Breaking (EWSB) is known to produce a massive universe that we live in. However, it may also provide an important boundary for freeze-in or freeze-out of dark matter (DM) connected to Standard Model via Higgs portal as processes contributing to DM relic differ across the boundary. We explore such possibilities in a two-component DM framework, where a massive U(1)X gauge boson DM freezes-in and a scalar singlet DM freezes-out, that inherits the effect of EWSB for both the cases in a correlated way. Amongst different possibilities, we study two sample cases; first when one DM component freezes in and the other freezes out from thermal bath both necessarily before EWSB and the second, when both freeze-in and freeze-out occur after EWSB. We find some prominent distinctive features in the available parameter space of the model for these two cases, after addressing relic density and the recent most direct search constraints from XENON1T, some of which can be borrowed in a model independent way.

Shedding light on the electroweak phase transition from exotic Higgs boson decays at the lifetime frontiers

We study the scenarios where a strongly first-order electroweak phase transition (EWPT) is triggered by a light singlet scalar, which has feeble interactions to the Higgs. Since the singlet scalar is light and has weak couplings, it can decay at a macroscopic distance away from the collision point. Therefore, it can be regarded as a long-lived particles (LLP) in such scenarios. We perform the searches of the LLPs from the exotic Higgs decays, at the FASER, MAPP and CMS-Timing detectors of the 14 TeV HL-LHC, to probe the strongly first-order EWPT. In certain scenarios, we show that the LLP searches can help to reach the parameter space of the strongly first-order EWPT remarkably, where the searches for promptly exotic Higgs decays are not valid.

Constraints on heavy neutral leptons interacting with a singlet scalar

Heavy neutral leptons (HNLs) are an attractive minimal extension of the Standard Model, as is a singlet scalar $s$ mixing with the Higgs boson. If both are present, it is natural for HNLs to interact with $s$. For a light singlet,the decay $N\to s\nu$ can dominate over weak HNL decays. We reinterpret existing constraints on HNL mixing from the DELPHI, CHARM and Belle experiments for 0.5-100 GeV mass HNLs, taking into account the new decay channel. Although the constraints are typically weakened, in some cases they can become stronger, due to observable $s\to\ell^+\ell^-$ decays in the detectors. The method presented here could be used to recast constraints from other (older) experiments without resorting to computationally expensive Monte Carlo simulations. In addition, we update and correct some errors in the analysis of the original constraints, in the absence of the singlet.

Primordial black holes from an electroweak phase transition

We propose a mechanism that forms primordial black holes (PBHs) via a first-order electroweak phase transition (FOEWPT). The FOEWPT is realized by extending the Standard Model with a real singlet scalar, while the PBH formation is achieved by the collapse of non-topological solitons called Fermi-balls. Such solitons form via trapping fermions in the false vacuum during the FOEWPT, and they eventually collapse into PBHs due to the internal Yukawa attractive force. We demonstrate that a scenario with PBH dark matter candidate can exist, and the typical experimental signals include FOEWPT gravitational waves and the multi-lepton/jet or displaced vertex final states at the LHC.

Positivity bounds in the standard model effective field theory beyond tree level

Focusing on four-Higgs interactions, we analyse the robustness of tree-level-derived positivity bounds on Standard Model effective field theory (SMEFT) operators under quantum corrections. Among other results, we demonstrate that: (i) Even in the simplest extensions of the Standard Model, e.g. with one new scalar singlet or with a neutral triplet, some positivity bounds are strictly violated; (ii) the mixing of the dimension-eight operators under renormalisation, which we compute here for the first time, can drive them out of their positivity region; (iii) the running of the dimension-eight interactions triggered by solely dimension-six terms respects the positivity bounds. Our results suggest, on one hand, that departures from positivity within the SMEFT, if ever found in the data, do not necessarily imply the breaking of unitarity or causality, nor the presence of new light degrees of freedom. On the other hand, they lead to strong constraints on the form of certain anomalous dimensions.

Scalar dark matter and radiative Dirac neutrino mass in an extended U(1)B−L model

We explore a gauged U(1)B−L extension of the standard model with inclusion of three right-handed neutrinos of exotic B−L charges to cancel the gauge anomaly. Non-trivial transformation of new particles under B−L symmetry forbids the generation of neutrino mass at tree level and hence a small Dirac mass can be generated radiatively at one loop with scalar, fermion doublets and singlet scalars. We also discuss the phenomenology of a scalar dark matter, which can be obtained from the mixing of neutral CP even component of a doublet scalar and a real singlet scalar. An adhoc Z2 symmetry is required in the current framework to stabilize the dark matter candidate. Presence of new particles with Z2 odd charges and small mass splitting, makes the phenomenology more interesting by governing the relic density with co-annihilation processes. We explore the spin-independent direct detection constraints on dark matter via the scalar mediation. Constraints from lepton flavor violating decay processes like μ→eγ are also discussed.

Pseudo-Goldstone dark matter model with CP violation

We consider an explicitly CP-violating model with two Higgs doublets and one complex singlet scalar. The singlet S is charged under a global U(1) symmetry which is softly broken by a mass term μ2S2 + H.c.. Imaginary part of S is a stable dark matter candidate which at the tree level, in the limit of zero momentum transfer, decouples from nucleons naturally satisfying all existing direct detection limits on dark matter scattering cross-section. It is explicitly shown that within this framework in the alignment limit CP violation is still present in contrast to a popular version of a 2-Higgs doublet model with softly broken Z2 symmetry. In this context, we investigate dark matter implications of the model both with and without CP violation in the scalar sector. In particular, dark matter relic abundance is calculated and the possibility for its indirect detection is discussed.

Singlet-doublet fermion origin of dark matter, neutrino mass and W-mass anomaly

Motivated by the recently reported anomaly in W boson mass by the CDF collaboration with 7σ statistical significance, we consider a singlet-doublet (SD) Majorana fermion dark matter (DM) model where the required correction to W boson mass arises from radiative corrections induced by SD fermions. While a single generation of SD fermions, odd under an unbroken Z2 symmetry, can not explain the W boson mass anomaly while being consistent with DM phenomenology, two generations of SD fermions can do so with the heavier generation playing the dominant role in W-mass correction and lighter generation playing the role in DM phenomenology. Additionally, such multiple generations of SD fermions can also generate light neutrino masses radiatively if a Z2-odd singlet scalar is included.

Robustness of ARS leptogenesis in scalar extensions

Extensions of the Standard Model (SM) with sterile neutrinos are well motivated from the observed oscillations of the light neutrinos and they have shown to successfully explain the Baryon Asymmetry of the Universe (BAU) through, for instance, the so-called ARS leptogenesis. Sterile neutrinos can be added in minimal ways to the SM, but many theories exist where sterile neutrinos are not the only new fields. Such theories often include scalar bosons, which brings about the possibility of further interactions between the sterile neutrinos and the SM. In this paper we consider an extension of the SM with two sterile neutrinos and one scalar singlet particle and investigate the effect that an additional, thermalised, scalar has on the ARS leptogenesis mechanism. We show that in general the created asymmetry is reduced due to additional sterile neutrino production from scalar decays. When sterile neutrinos and scalars are discovered in the laboratory, our results will provide information on the applicability of the ARS leptogenesis mechanism.

Muon g − 2, B → K(∗)μ+μ− anomalies, and leptophilic dark matter in U(1)μ−τ gauge symmetry

We propose a new class of U(1)μ−τ gauged model that can explain recent flavor anomalies such as muon g − 2, b → sμ+μ−, as well as include a scalar dark matter candidate while satisfying all the phenomenological constraints. For this purpose, we add new vectorlike quarks and leptons as well as two inert singlet scalars, one leptophilic and the other leptophobic. In our model b → sμ+μ− anomalies can be explained by loop induced interactions among the SM, exotic quarks, and Z′ gauge boson. In our numerical analysis, we show allowed region of our model to be narrow, and it would be tested soon, for example by searching for 4 muons and dimuon + missing transverse momentum from pp → μ+μ−Z′ and ppto {nu}_{mu, tau }{overline{nu}}_{mu, tau }Z^{prime } followed by Z^{prime}to {mu}^{+}{mu}^{-},{nu}_{mu, tau }{overline{nu}}_{mu, tau } at the LHC.

Heavy neutrino searches via same-sign lepton pairs at a Higgs boson factory

This paper investigates the $e^-e^+\to Zh_1$ sensitivity for Higgs boson's rare decay into heavy neutrinos $h_1 \to NN$ at the proposed electron-positron collider, with the focus on multi-lepton final states that contain same-sign lepton pairs. $h_1 \to NN$ decay can derive from Higgs boson's mixing with new physics scalar(s) that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar which gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic and mixed $NN$ decay scenarios, and categorize the signal on the number of leptons in the final state: $ \ell^\pm \ell^\pm$ with at least 3 jets, $\ell^\pm \ell^\pm \ell$ with at least 2 jets, and $e^\pm e^\pm \mu^\mp \mu^\mp$ plus with at least 1 jet, each containing one or two same-sign dilepton system(s). Selection cuts are optimized for the presence of the associated $Z$-boson, which leads to additional backgrounds at the $e^-e^+$ collider. The Standard Model background channels are systematically analyzed. Sensitivity limits on $h_1 \rightarrow NN$ branching fractions are derived for signals with 2-4 final leptons assuming the heavy neutrino masses are between 10 and 60 GeV. With 240 GeV center-of-mass energy and 5.6 ab$^{-1}$ design luminosity, $h_1 \rightarrow NN$ branching fraction can be probed to $2\times 10^{-4}$ in $2\ell$ and $3\ell$ channels, and $6\times 10^{-4}$ in the $4\ell$ channel at $95\%$ confidence level. $3\ell, 4\ell$ channels expect one or fewer background event, and their sensitivities saturate the statistic limit at 5.6 ab$^{-1}$ luminosity. A same-sign trilepton ($\ell^\pm \ell^\pm \ell^\pm$) signal in the $3\ell$ channel is also discussed. Our search strategy provides an approach to discovering the singlet scalar and exploring the origin of neutrino masses at future $e^-e^+$ colliders.

Minimal SU(6) gauge-Higgs grand unification

We present a minimal viable Gauge-Higgs Grand Unification scenario in warped space based on a $SU(6)$ bulk symmetry - unifying the gauge symmetries of the SM and their breaking sector. We show how the issue of light exotic new states is eliminated by appropriately breaking the gauge symmetry on the UV and IR boundaries by either brane scalars or gauge boundary conditions. The SM fermion spectrum is naturally reproduced including Dirac neutrinos and we compute the Higgs potential at one-loop, finding easily solutions with a realistic $m_h \sim 125$ GeV. The problem of proton decay is addressed by showing that baryon number is a hidden symmetry of the model. Among the phenomenological consequences, we highlight the presence of a scalar leptoquark and a scalar singlet. The usual $X,Y$ gauge bosons from $SU(5)$ GUTs are found at collider accessible masses.

Two dark matter candidates: The case of inert doublet and singlet scalars

We consider a multi-component dark matter model where the dark sector contains a scalar doublet and a complex scalar singlet. We impose a discrete $Z_4$ symmetry to ensure such that the lightest component of the doublet, $\tilde{A}$, and the singlet, $\tilde{S}$, are both stable. Interactions between the dark sectors impact significantly dark matter observables, they allow in particular to significantly relax the direct detection constraints on the model. To determine the parameter space that satisfies relic density, theoretical and collider constraints as well as direct and indirect detection limits, we perform two separate scans, the first includes the full parameter space of the model while the second is dedicated to scenarios with a compressed inert doublet spectrum. In the first case we find that the singlet is generally the dominant dark matter component while in the compressed case the doublet is more likely to be the dominant dark matter component. In both cases we find that the two dark matter particles can have masses that ranges from around $m_h/2$ to over the TeV scale. We emphasize the interplay between cosmological astrophysical and collider constraints and show that a large fraction of the parameter space that escapes current constraints is within the sensitivity reach of future detectors such as XENON-nT, Darwin or CTA. Important collider signatures are mostly found in the compressed spectrum case with the possibility of probing the model with searches for heavy stable charged particles and disappearing tracks. We also show that semi-annihilation processes such as $\tilde{S}\tilde{S}\to \tilde{A}Z$ could give the dominant signature in indirect detection searches.

A renormalizable left-right symmetric model with low scale seesaw mechanisms

We propose a low scale renormalizable left-right symmetric theory that successfully explains the observed SM fermion mass hierarchy, the tiny values for the light active neutrino masses and is consistent with the lepton and baryon asymmetries of the Universe, the muon and electron anomalous magnetic moments as well as with the constraints arising from the meson oscillations. In the proposed model the top and exotic quarks obtain masses at tree level, whereas the masses of the bottom, charm and strange quarks, tau and muon leptons are generated from a tree level Universal Seesaw mechanism, thanks to their mixings with the charged exotic vector like fermions. The masses for the first generation SM charged fermions arise from a radiative seesaw mechanism at one loop level, mediated by charged vector like fermions and electrically neutral scalars. The light active neutrino masses are produced from a one-loop level inverse seesaw mechanism mediated by electrically neutral scalar singlets and right handed Majorana neutrinos. Our model is also consistent with the experimental constraints arising from the Higgs diphoton decay rate as well as with the constraints arising from charged lepton flavor violation. We also discuss the Z′ and heavy scalar production at a proton-proton collider.

Electroweak bubble wall expansion: gravitational waves and baryogenesis in Standard Model-like thermal plasma

Computing the properties of the bubble wall of a cosmological first order phase transition at electroweak scale is of paramount importance for the correct prediction of the baryon asymmetry of the universe and the spectrum of gravitational waves. By means of the semiclassical formalism we calculate the velocity and thickness of the wall using as theoretical framework the scalar singlet extension of the SM with a parity symmetry and the SM effective field theory supplemented by a dimension six operator. We use these solutions to carefully predict the baryon asymmetry and the gravitational wave signals. The singlet scenario can easily accommodate the observed asymmetry but these solutions do not lead to observable effects at future gravity wave experiments. In contrast the effective field theory fails at explaining the baryon abundance due to the strict constraints from electric dipole moment experiments, however, the strongest solutions we found fall within the sensitivity of the LISA experiment. We provide a simple analytical approximation for the wall velocity which only requires calculation of the strength and temperature of the transition and works reasonably well in all models tested. We find that generically the weak transitions where the fluid approximation can be used to calculate the wall velocity and verify baryogenesis produce signals too weak to be observed in future gravitational wave experiments. Thus, we infer that GW signals produced by simple SM extensions visible in future experiments are likely to only result from strong transitions described by detonations with highly relativistic wall velocities.

Freezing-in a hot bath: resonances, medium effects and phase transitions

Relic density calculations of dark matter freezing out from the primordial plasma have reached a high level of sophistication, with several numerical tools readily available that match the observationally required accuracy. Dark matter production via the freeze-in mechanism, on the other hand, is sensitive to much higher temperatures than in the freeze-out case, implying both technical and computational difficulties when aiming for the same level of precision. We revisit the formulation of freeze-in production in a way that facilitates the inclusion of in-medium corrections like plasma effects and the spin statistics of relativistic quantum gases, as well as the temperature dependence of dark matter production rates induced by the electroweak and strong phase transitions, and we discuss in detail the additional complications arising in the presence of s-channel resonances. We illustrate our approach in the context of Higgs portal models, and provide the most accurate calculation to date of the freeze-in abundance of Scalar Singlet dark matter. We explore in particular the case of small reheating temperatures, for which the couplings implied by the freeze-in mechanism may be testable at the LHC. Together with this article we present a major update 6.3 of DarkSUSY with the added capability of performing general freeze-in calculations, including all complications mentioned above.