arXiv subjects

Nobuchika Okada

Publications and source records attributed to Nobuchika Okada.

At least 73 records · Page 4Linked to original sources

arXiv2023

Type III seesaw with R-parity violation in light of $m_W$ (CDF)

Motivated by the recently reported measurement of the $W$ boson mass $M_W = 80.4335 \pm 0.0094$ GeV by the CDF collaboration, we propose a type III seesaw extension of the minimal supersymmetric standard model (MSSM) which also includes an R-parity violating term. Without taking potential SUSY radiative corrections into account, we show that the CDF measurement of $M_W$ and the LEP measurement of the $ρ$ parameter can be simultaneously accommodated at the $2 σ$ level. A long-lived gravitino in a few GeV mass range is a unique viable dark matter candidate in this framework.

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arXiv2022

Radiative Plateau Inflation with Conformal Invariance: Dynamical Generation of Electroweak and Seesaw Scales

We investigate a scale-invariant $B-L$ scenario where the Standard Model (SM) is supplemented with a dark scalar $ϕ$ which has gauge \& Yukawa interactions, with the couplings $g_{BL}$ and $y$, respectively, leading to radiative plateau inflation at scale $ϕ=M$ in the ultraviolet (UV), while dynamically generating the Electroweak and Seesaw scales \textit{á lá} Coleman-Weinberg in the infrared (IR). This is particularly achieved by implementing threshold corrections at an energy scale $μ_T$ arising due to the presence of vector-like fermions. We show that implementing the inflationary observables makes the couplings solely dependent on the plateau scale $M$, leaving us with only two independent parameters $M$ and $μ_T$. Within the theoretically consistent parameter space defined by $m_{Z_{BL}} > 850~\rm GeV$, from the assumption of independent evolution of the dark sector couplings from the SM couplings and $M < 5.67~M_P$ required for the realisation of inflationary \textit{plateau-like} behaviour of the potential around $ϕ=M$, where $M_P=2.4\times10^{18}$ GeV is the reduced Planck mass, we identify the parameter space that is excluded by the current LHC results from the search for the heavy $Z_{BL}$ boson. For typical benchmark points in the viable parameter regions, we estimate the reheating temperature to be $\mathcal{O}(TeV)$ thus consistent with the standard Big Bang Nucleosynthesis (BBN) constraints. For typical benchmark points ($M=5.67,~1,~0.1~M_P$) we predict the scales of inflation to be $\mathcal{H}_{inf}=2.79\times10^{12}$ GeV, $1.53\times10^{10}$ GeV and $1.53\times10^7$ GeV, respectively.

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arXiv2022

eV Hubble Scale Inflation with Radiative Plateau: Very light Inflaton, Reheating & Dark Matter in B-L Extensions

We study radiative plateau-like inflation \& Z$_{BL}$-portal freeze-in fermionic dark matter (DM) in a minimal B-L extended model. The U(1)$_{B-L}$ Higgs, responsible for heavy neutrino masses, also drives inflation in the early universe, thanks to radiative corrections from the heavy neutrinos \& the Z$_{BL}$ gauge boson. In our benchmark choice for the U(1)$_{B-L}$ gauge coupling $g_{B-L}\sim10^{-4}$, a light Z$_{BL}$ boson can be explored by current and future lifetime frontier experiments, such as FASER and FASER 2 at the LHC, SHiP, Belle II and LHCb. For the benchmark, the Hubble scale of inflation ($\mathcal{H}_{inf}$) is very low ($\mathcal{H}_{inf} = \mathcal{O}(100)$ eV) \& the inflaton turns out to be very light with mass of $\mathcal{O}(1)$ eV, and consequently the decay width of inflaton is extremely small. We investigate a 2-field system with the inflaton/B-L Higgs and the Standard Model (SM) Higgs, and find that the reheating with a suffuciently high temperature occurs when the water-fall direction to the SM Higgs direction opens up in the trajectory of the scalar field evolution.

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arXiv2022

Light $Z^\prime$ and Dirac fermion dark matter in the $B-L$ model

We consider a $U(1)_{B-L}$ model with a $Z^\prime$ portal Dirac fermion dark matter (DM) $χ$ of low mass which couples very weakly to the $B - L$ gauge boson $Z^\prime$. An arbitrary $B-L$ charge $Q\neq \pm1, \pm 3$ of the DM $χ$ ensures its stability. Motivated by the sensitivity reach of forthcoming "Lifetime Frontier" experiments, we focus on the $Z^\prime$ mass, $m_{Z^\prime}$, in the sub-GeV to few GeV range. To evaluate the DM relic abundance, we examine both the freeze-out and freeze-in DM scenarios. For the freeze-out scenario, we show that the observed DM abundance is reproduced near the $Z^\prime$ resonance, $m_χ\simeq m_{Z^\prime}/2$, where $m_χ$ is the DM mass. For the freeze-in scenario, we focus on $m_χ\ll m_{Z^\prime}$. We show that for a fixed value of $m_{Z^\prime}$, $g_{BL}$ values roughly scale as $1/Q$ to reproduce the observed DM abundance. For various $Q$ values in the range between $10^{-6}$ and $10^2$, we show that the gauge coupling values $g_{BL}$ needed to reproduce the observed DM abundance lie in the search reach of future planned and/or proposed experiments such as FASER, Belle-II, LDMX, and SHiP. In the freeze-in case, the $Q$ values to realize observable $g_{BL}$ values are found to be much smaller than that in the freeze-out case.

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arXiv2022

Affleck-Dine Leptogenesis with One Loop Neutrino Mass and strong CP

We present a unified model that solves four major problems of the standard model i.e. neutrino masses, origin of matter, strong CP problem and dark matter using the framework of Affleck-Dine (AD) mechanism. The AD-field is responsible for inflation, origin of matter and neutrino masses which arise at the one loop level. Neutrino masses are therefore intimately connected to the baryon to photon ratio of the universe. The dark matter in the model is the axion field used to solve the strong CP problem. The model has a near massless Majorana fermion which contributes to $ΔN_{\rm eff}\sim 0.1$ in the early universe, that can be tested in the upcoming CMB-S4 experiment

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arXiv2022

Axions, WIMPs, proton decay and observable $r$ in $SO(10)$

We explore some experimentally testable predictions of an $SO(10)$ axion model which includes two 10-plets of fermions in order to resolve the axion domain wall problem. The axion symmetry can be safely broken after inflation, so that the isocurvature perturbations associated with the axion field are negligibly small. An unbroken gauge $Z_2$ symmetry in $SO(10)$ ensures the presence of a stable WIMP-like dark matter, a linear combination of the electroweak doublets in the fermion 10-plets and an $SO(10)$ singlet fermion with mass $\sim 62.5 \; {\rm GeV}\; (1 \; {\rm TeV}) $ when it is mostly the singlet (doublet) fermion, that co-exists with axion dark matter. We also discuss gauge coupling unification, proton decay, inflation with non-minimal coupling to gravity and leptogenesis. With the identification of the SM singlet Higgs field in the $126$ representation of $SO(10)$ as inflaton, the magnetic monopoles are inflated away, and we find $0.963 \lesssim n_s \lesssim 0.965$ and $0.003 \lesssim r \lesssim 0.036$, where $n_s$ and $r$ denote the scalar spectral index and tensor-to-scalar ratio, respectively. These predictions can be tested in future experiments such as CMB-S4.

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arXiv2022

Dirac dark matter, dark radiation, and the type-II seesaw mechanism in alternative $U(1)_X$ standard model

We propose an extra $U(1)_X$ model with an alternative charge assignment for right-handed right-handed neutrinos. The type-II seesaw mechanism by a triplet Higgs field is promising for neutrino mass generation because of the alternative charge assignment. The small vacuum expectation value (VEV) of an additional Higgs doublet naturally leads to a very small VEV of the triplet Higgs field, and as a result, the smallness of neutrino mass can be understood. With the minimal Higgs field for $U(1)_X$ with the charge $1$, right-handed neutrinos are candidates for Dirac dark matter (DM) and dark radiation (DR). We have derived and imposed the LHC bound, the DR constraint and the bound from DM direct searches in the wide range of parameter space. Among various $U(1)_X$ choices, the DM direct search bound is found to be weakest for $U(1)_R$ where the constraints from thermal DM and non-negligible DR can be compatible. Such a number of the effective neutrino species would be interesting from the viewpoint of the so-called Hubble tension.

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arXiv2022

The Forward Physics Facility: Sites, Experiments, and Physics Potential

The Forward Physics Facility (FPF) is a proposal to create a cavern with the space and infrastructure to support a suite of far-forward experiments at the Large Hadron Collider during the High Luminosity era. Located along the beam collision axis and shielded from the interaction point by at least 100 m of concrete and rock, the FPF will house experiments that will detect particles outside the acceptance of the existing large LHC experiments and will observe rare and exotic processes in an extremely low-background environment. In this work, we summarize the current status of plans for the FPF, including recent progress in civil engineering in identifying promising sites for the FPF and the experiments currently envisioned to realize the FPF's physics potential. We then review the many Standard Model and new physics topics that will be advanced by the FPF, including searches for long-lived particles, probes of dark matter and dark sectors, high-statistics studies of TeV neutrinos of all three flavors, aspects of perturbative and non-perturbative QCD, and high-energy astroparticle physics.

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arXiv2022

Probing heavy Majorana neutrino pair production at ILC in a $U(1)_{\rm B-L}$ extension of the Standard Model

We consider a gauged B$-$L (Baryon number minus Lepton number) extension of the Standard Model (SM), which is anomaly free in the presence of three SM singlet Right Handed Neutrinos (RHNs). Associated with the $U(1)_{\rm B-L}$ gauge symmetry breaking, the RHNs acquire Majorana masses and then with the electroweak symmetry breaking, tiny Majorana masses for the SM(-like) neutrinos are naturally generated by the seesaw mechanism. As a result of the seesaw mechanism, the heavy mass eigenstates which are mainly composed of the SM-singlet RHNs obtain suppressed electroweak interactions through small mixings with the SM neutrinos. To investigate the seesaw mechanism, we study the pair production of heavy Majorana neutrinos through the $U(1)_{\rm B-L}$ gauge boson $Z^\prime$ at the 250 GeV and 500 GeV International Linear Collider (ILC). Considering the current and prospective future bounds on the B$-$L model parameters from the search for a resonant $Z^\prime$ boson production at the Large Hadron Collider (LHC), we focus on a "smoking-gun" signature of the Majorana nature of the heavy neutrinos: a final state with a pair of same-sign, same-flavor leptons, small missing momentum, and four hadronic jets. We estimate the projected significance of the signature at the ILC.

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arXiv2022

Neutrino Mass from Affleck-Dine Leptogenesis and WIMP Dark Matter

Affleck-Dine (AD) mechanism for leptogenesis involves the cosmological evolution of a complex scalar field (AD field) that carries non-zero lepton number. We show how explicit lepton number breaking terms, which involve the AD field needed to implement this scenario combined with fermionic WIMP dark matter, can generate neutrino mass at the one loop level, thus providing a unified framework for solving four major puzzles of the standard model i.e. inflation, baryogenesis, dark matter and neutrino mass. We discuss some phenomenological implications of this model.

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arXiv2022

Are Low-energy Data already Hinting at Five Dimensions?

Low-energy data, combined with renormalization group (RG) equations, can predict new physics at far higher energy scales. In this paper, we consider the possibility that the measured Higgs boson mass and top quark mass hint at a five-dimensional gauge-Higgs unification (5D GHU) model at a scale above TeV. We note that the vanishing of the Higgs quartic coupling and the proximity of the top quark Yukawa coupling and weak gauge coupling at high scales, inferred from the experimental data, are in harmony with 5D GHU, because in 5D GHU models the Higgs quartic coupling is forbidden by the 5D gauge symmetry and the Yukawa couplings and the weak gauge coupling originate from a common 5D gauge coupling. Based on the above insight, we propose a 5D GHU model where the Standard Model fermions are embedded in 5D fermions in a way to tightly relate the top Yukawa coupling with the weak gauge coupling. Also, the model predicts the presence of vector-like fermions (other than the Kaluza-Klein modes), which can affect the RG evolutions of the 4D theory and reconcile the scale of vanishing Higgs quartic coupling and that of equality of the top Yukawa and weak gauge couplings, thereby achieving a successful matching of the 4D theory with 5D GHU. We predict the vector-like fermion mass and the compactification scale of 5D GHU from the conditions for the successful matching.

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arXiv2021

SO(10) Grand Unification with Minimal Dark Matter and Color Octet Scalars

The minimal dark matter (MDM) scenario is a very simple framework of physics beyond the Standard Model (SM) to supplement the SM with a DM candidate. In this paper, we consider an ultraviolet completion of the scenario to an SO(10) grand unified theory, which is a well-motivated framework in light of the neutrino oscillation data. Considering various phenomenological constraints, such as the successful SM gauge coupling unification, the proton stability, and the direct/indirect DM detection constraints as well as the absolute electroweak vacuum stability, we have first singled out the minimal particle content of the MDM scenario at low energies. In addition to the SM particle content, our MDM scenario includes an SU(2)$_L$ quintet scalar DM with a 9.4 TeV mass and three degenerate color-octet scalars with mass of 2 TeV. We then have found a way to embed the minimal particle content into SO(10) representations, in which a remnant $Z_2$ symmetry after the SO(10) symmetry breaking ensures the stability of the DM particle. The production cross section of the color-octet scalars at the Large Hadron Collider is found to be a few orders of magnitude below the current experimental bound.

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arXiv2021

Inflation and type III seesaw mechanism in $ν$-gauge mediated supersymmetry breaking

We discuss realization of cosmic inflation in the $ν$-gauge mediated supersymmetry breaking scenario, in which a set of 24-dimensional chiral superfields responsible for the type III seesaw mechanism play the role of the messenger fields in gauge mediation. Using the data from neutrino oscillations, we show that the model satisfies constraints from the lepton flavor violation, perturbativity of the unified gauge couplings, the observed abundance of dark matter as well as the Higgs mass of 125.1 GeV. The predicted spectrum of the cosmic microwave background radiation fits well with the observation. We also comment on the falsifiability of this scenario by future experiments.

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arXiv2021

A Unified Model for Inflation, pseudo-Goldstone Dark Matter, Neutrino Mass and Baryogenesis

We present a unified theory of inflation, neutrino mass, baryogenesis and dark matter where global lepton number symmetry and its breaking play a crucial role. The basic idea is to use a lepton number carrying complex scalar field as the inflaton as well as the field that implements Affleck-Dine (AD) leptogenesis. Dark matter is the massive majoron which is a pseudo-Goldstone boson, resulting from the spontaneous breaking of lepton number symmetry supplemented by explicit lepton number violation needed to implement AD leptogenesis. The magnitude of the resulting $n_B/s$ in the model is related to the mass of the pseudo-Goldstone dark matter, connecting two apparently disconnected cosmological observations. Inverse seesaw mechanism with lepton number breaking at low scale is crucial to prevent washout of the lepton asymmetry during the universe's evolution. The model seems to provide an economical solution to several puzzles of the standard model of particle physics and cosmology in one stroke.

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arXiv2021

Pseudo-Goldstone Dark Matter in $SO(10)$

We propose a pseudo-Goldstone boson dark matter (pGDM) particle in $SO(10)$ grand unified theory (GUT). Due to its Goldstone nature, this pGDM evades the direct DM detection experiments which, otherwise, severely constrain the parameter space of DM models. In $SO(10)$, the pGDM is embedded as a linear combination of the Standard Model (SM) singlet scalars in ${\bf 16_H}$ and ${\bf 126_H}$ representations. We consider two scenarios for the intermediate route of $SO(10)$ symmetry breaking (SB) to the SM: $SU(5) \times U(1)_X$ and Pati-Salam the $SU(4)_c \times SU(2)_L \times SU(2)_R$ (4-2-2) gauge groups. The vacuum expectation value of ${\bf 126_H}$, which triggers the breaking of $U(1)_X$ and 4-2-2 symmetry in the two scenarios, respectively, determines the pGDM lifetime whose astrophysical lower bound provides one of the most stringent constraints. For the 4-2-2 route to $SO(10)$, the successful SM gauge coupling unification requires the 4-2-2 breaking scale to be ${\cal O} (10^{11})$ GeV, and most of the parameter space is excluded. For the $SU(5) \times U(1)_X$ route, on the other hand, the $U(1)_X$ breaking scale can be significantly higher, and a wide range of the parameter space is allowed. Furthermore, the proton lifetime in the $SU(5)$ case is predicted to be $4.53 \times 10^{34}$ years, which lies well within the sensitivity reach of the Hyper-Kamiokande experiment. We also examine the constraints on the model parameter space from the Large Hadron Collider and the indirect DM search by Fermi-LAT and MAGIC experiments.

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arXiv2021

Messenger inflation in gauge mediation and superWIMP dark matter

We discuss phenomenological viability of a novel inflationary model in the minimal gauge mediated supersymmetry breaking scenario. In this model, cosmic inflation is realized in the flat direction along the messenger supermultiplets and a natural dark matter candidate is the gravitino from the out-of-equilibrium decay of the bino-like neutralino at late times, which is called the superWIMP scenario. The produced gravitino is warmish and can have a large free-streaming length; thus the cusp anomaly in the small scale structure formation may be mitigated. We show that the requirement of the Standard Model Higgs boson mass to be $m_{h^0}=125.1$ GeV gives a relation between the spectrum of the cosmic microwave background and the messenger mass $M$. We find, for the e-folding number $N_e=60$, the Planck 2018 constraints (TT, TE, EE+lowE+lensing+BK15+BAO, 68% confidence level) give $M > 3.64\times 10^7$ GeV. The gravitino dark matter mass is $m_{3/2} < 5.8$ GeV and the supersymmetry breaking scale $Λ$ is found to be in the range $(1.28-1.33)\times 10^6$ GeV. Future CMB observation is expected to give tighter constraints on these parameters.

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arXiv2021

$SU(5) \times U(1)_X$ Axion Model with Observable Proton Decay

We propose a $SU(5) \times U(1)_X \times U(1)_{PQ}$ model, where $U(1)_X$ is the generalization of the $B-L$ (baryon minus lepton number) gauge symmetry and $U(1)_{PQ}$ is the global Peccei-Quinn (PQ) symmetry. There are four fermions families in $\bf{\overline 5} + \bf{10}$ representations of $SU(5)$, a mirror family in $\bf{5}+\bf{\overline {10}}$ representations, and three $SU(5)$ singlet Majorana fermions. The $U(1)_X$ related anomalies all cancel in the presence of the Majorana neutrinos. The $SU(5)$ symmetry is broken at $M_{GUT} \simeq (6-9)\times 10^{15}$ GeV and the proton lifetime $τ_p$ is estimated to be well within the expected sensitivity of the future Hyper-Kamiokande experiment, $τ_p \lesssim 1.3 \times 10^{35}$ years. The $SU(5)$ breaking also triggers the breaking of the PQ symmetry, resulting in axion dark matter (DM), with the axion decay constant $f_a$ of order $M_{GUT}$ or somewhat larger. The CASPEr experiment can search for such an axion DM candidate. The Hubble parameter during inflation must be low, $H_{inf} \lesssim 10^9 $ GeV, in order to successfully resolve the axion domain wall, axion DM isocurvature and $SU(5)$ monopole problems. With the identification of the $U(1)_X$ breaking Higgs field with the inflaton field, we implement inflection-point inflation, which is capable of realizing the desired value for $H_{inf}$. The vectorlike fermions in the model are essential for achieving successful unification of the SM gauge couplings as well as the phenomenological viability of both axion DM and inflation scenario.

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arXiv2021

Affleck-Dine Baryogenesis with Observable Neutron-Anti-Neutron Oscillation

We discuss the implications of Affleck-Dine (AD) baryogenesis for different classes of baryon and lepton number violating processes: specially focussing on implications for neutron-anti-neutron ($n-\bar{n}$) oscillation. The class of AD baryogenesis scenarios we work with uses the AD field also as the inflaton which is nonminimally coupled to gravity. We find that adequate baryogenesis and no washout by the baryon number ($B$) or the lepton number ($L$) violating operators implies constraints on the observability of the process or in the case of neutrino mass with compatibility with neutrino oscillation observations. In particular, for $n-\bar{n}$ oscillation, we study some of the familiar operators that connect the AD field to $n-\bar{n}$ oscillation and find that a split scalar spectrum model turns out to be most advantageous for obtaining an observable $n-\bar{n}$ while remaining consistent with AD baryogenesis. It is interesting that this spectrum is similar to a non-supersymmetyric SO(10) model for observable $n-\bar{n}$ oscillation discussed before, suggesting that this AD scenario can be embedded into a grand unified SO(10) model. We also find that for a low scale (all scales in the 100 TeV range), there is a narrow range of parameters where the observable $n-\bar{n}$ oscillation is compatible with viable AD baryogenesis. A feature of this baryogenesis scenario for $n-\bar{n}$ oscillation is that it necessarily predicts processes with $ΔB=4$ or higher, all be it with highly suppressed amplitudes.

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