arXiv subjects

Natsumi Nagata

Publications and source records attributed to Natsumi Nagata.

At least 37 records · Page 2Linked to original sources

arXiv2023

Quantifying Limits on CP Violating Phases from EDMs in Supersymmetry

We revisit the calculation of the electron, neutron, and proton electric dipole moments (EDMs) in the constrained minimal supersymmetric standard model (CMSSM). The relatively large mass of the Higgs boson, $m_H \simeq 125$ GeV coupled with the (as yet) lack of discovery of any supersymmetric particle at the LHC, has pushed the supersymmetry breaking scale to several TeV or higher. Though one might expect this decoupling to have relaxed completely any bounds on the two CP violating phases in the CMSSM ($θ_μ$ and $θ_A$), the impressive experimental improvements in the limits on the EDMs (particularly the electron EDM) still allow us to set constraints of order $(0.01 - 0.1)π$ on $θ_A$ and $(0.001 - 0.1)π$ on $θ_μ$. We also discuss the impact of future improvements in the experimental limits on supersymmetric models.

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arXiv2023

Probing New Physics in the Vector-like Lepton Model by Lepton Electric Dipole Moments

We examine the lepton dipole moments in an extension of the Standard Model (SM), which contains vector-like leptons that couple only to the second-generation SM leptons. The model naturally leads to sizable contributions to the muon $g-2$ and the muon electric dipole moment (EDM). One feature of this model is that a sizable electron EDM is also induced at the two-loop level due to the existence of new vector-like leptons in the loops. We find parameter regions that can explain the muon $g-2$ anomaly and are also consistent with the experimental constraints coming from the electron EDM and the Higgs decay $h\rightarrow μ^{+}μ^{-}$. The generated EDMs can be as large as $O(10^{-22})~e \cdot \mathrm{cm}$ for the muon and $O(10^{-30})~e \cdot \mathrm{cm}$ for the electron, respectively, which can be probed in future experiments for the EDM measurements.

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arXiv2022

Minimal Nambu-Goldstone Higgs Model in Supersymmetric SU(5) Revisited

We revisit the minimal Nambu-Goldstone (NG) Higgs supersymmetric (SUSY) SU(5) grand unified model and study its phenomenological implications. The Higgs sector of the model possesses a global SU(6) symmetry, which is spontaneously broken and results in the Higgs doublets of the minimal SUSY Standard Model (MSSM) as NG chiral superfields. Therefore, the model naturally leads to light Higgs doublets and solves the doublet-triplet splitting problem. Because of the SU(6) symmetry, the couplings of the Higgs sector are tightly restricted, and thus the model is more predictive than the minimal SUSY SU(5). We determine all the grand-unified-theory parameters via the matching conditions of the gauge coupling constants at the unification scale and calculate proton lifetime, confronting this with current experimental bounds. We discuss that this model is incompatible with the constrained MSSM, whilst it has a large viable parameter space in the high-scale SUSY scenario. The perturbativity condition on the trilinear coupling of the adjoint Higgs field imposes an upper (lower) limit on the wino (gluino) mass, implying a hierarchical mass pattern for these gauginos. Future proton-decay searches can probe a large part of the parameter space, especially if the SUSY-breaking scale is $\lesssim 100$~TeV.

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arXiv2022

Capture of Electroweak Multiplet Dark Matter in Neutron Stars

If dark matter has a sizable scattering cross section with nucleons, it can efficiently be captured by a neutron star. Its energy is then transferred to the neutron star as heat through the scattering and annihilation inside the star. This heating effect may be detectable via dedicated temperature observations of nearby old pulsars, providing an alternative method for dark matter searches. In this paper, we show that for electroweak multiplet dark matter this search strategy can probe the parameter region which is out of reach of future dark matter direct detection experiments. To see this systematically, we classify such dark matter candidates in terms of their electroweak charges and investigate the effect of ultraviolet physics by means of higher-dimensional effective operators. We then show that if the effect of ultraviolet physics is sizable, the dark matter-nucleon elastic scattering cross section becomes sufficiently large, whilst if it is suppressed, then the mass splittings among the components of the DM multiplet get small enough so that the inelastic scattering processes are operative. In any case, the electroweak multiplet dark matter particles are efficiently captured in neutron stars, making the search strategy with the temperature observation of old neutron stars promising.

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arXiv2022

Probing the $L_μ$-$L_τ$ Gauge Boson at the MUonE Experiment

We discuss the prospects of probing the $L_μ- L_τ$ gauge boson at the MUonE experiment. The $L_μ- L_τ$ gauge boson $Z^\prime$ with a mass of $\lesssim 200$ MeV, which can explain the discrepancy between the measured value of the muon $g-2$ and the value calculated in the Standard Model, can be produced at the MUonE experiment through the process $μe \to μe Z^\prime$. The $Z^\prime$ in the final state decays into a pair of neutrinos, and therefore we cannot observe the decay of $Z^\prime$ directly. It is, however, still possible to probe this signature by searching for events with a large scattering angle of muon and a less energetic final-state electron. The background events coming from the elastic scattering $μe \to μe$ as well as radiative process $μe \to μe γ$ can be removed by the kinematical cuts on the muon scattering angle and the electron energy, in addition to a photon veto. The background events from the electroweak process $μe \to μe ν\barν$ are negligible. With our selection criteria, the number of signal events $μe \to μe Z^\prime$ is found to be as large as $\sim 10^3$ in the parameter region motivated by the muon $g-2$ discrepancy. It is, therefore, quite feasible to probe the $L_μ- L_τ$ gauge boson at the MUonE experiment -- without introducing additional devices -- and we strongly recommend recording the events relevant to this $Z^\prime$ production process.

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arXiv2022

Neutron Star Heating in Dark Matter Models for the Muon $g-2$ Discrepancy

The observed value of the muon magnetic dipole moment, which deviates from the Standard Model prediction by $4.2σ$, can be explained in models with weakly-interacting massive particles (WIMPs) coupled to muons. However, a considerable range of parameter space of such models will remain unexplored in the future LHC experiments and dark matter (DM) direct searches. In this work we discuss the temperature observation of neutron stars (NSs) as a promising way to probe such models given that WIMPs are efficiently captured by NSs through DM-muon or spin-dependent DM-nucleon scattering. The captured WIMPs eventually annihilate in the star core and heat the NS. This effect can be observed in old NSs as it keeps the NS surface temperature at a few thousand K at most, which is much higher than the predicted values of the standard NS cooling theory for NSs older than $\sim 10^7$ years. We consider two classes of representative models, where the DM couples or does not couple to the Higgs field at tree level, and show that the maximal DM heating is realized in both scenarios.

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arXiv2022

Axion Quality Problem Alleviated by Non-Minimal Coupling to Gravity

Global symmetries are expected to be violated by gravity, which may cause a serious problem to models based on these symmetries. A famous example is the Peccei-Quinn solution to the strong CP problem in QCD axion models; it requires a global U(1) symmetry to be respected with high quality, and even the Planck-scale suppressed operators that violate the U(1) symmetry would spoil this solution. Indeed, it is known that gravitational instantons do break the U(1) Peccei-Quinn symmetry and induce the U(1)-violating operators, bringing back the strong CP problem to the axion models. This conclusion is, however, dependent highly on the structure of the theory around the Planck scale and, therefore, may be evaded if we go beyond the minimal setup. In this paper, we study the effect of a non-minimal coupling of the Peccei-Quinn field to gravity, $ξ$, on the gravitational instanton effect. This setup is frequently considered in cosmology as it can realize a successful inflation if $ξ\lesssim 10^5$. We find that the U(1)-breaking effect of the gravitational instantons can sufficiently be suppressed for $ξ\gtrsim 2 \times 10^3$, which suggests that the U(1) symmetry may be maintained with high quality even in the presence of gravity. Our result thus points to a new way to avoid the quality problem of global symmetries.

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arXiv2022

Searching for stop LSP at the LHC

We analyze relevant signals expected at the LHC for a stop as the lightest supersymmetric particle (LSP). The discussion is carried out in the framework of the $μν$SSM, where the presence of $R$-parity violating couplings involving right-handed neutrinos solves the $μ$-problem and reproduces neutrino data. The stops are pair produced at the LHC, decaying with displaced vertices to a lepton and a bottom quark or a neutrino and a top quark. We compare the predictions of this scenario with ATLAS and CMS searches for long-lived particles. To analyze the parameter space we sample the $μν$SSM for a stop LSP using a likelihood data-driven method, and paying special attention to reproduce the current experimental data on neutrino and Higgs physics, as well as flavor observables. Our results translate into a lower limit on the mass of the left (right) stop LSP of 1068 GeV (1341 GeV), corresponding to a decay length of 1.86 mm (6.61 mm).

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arXiv2021

Flipped SU(5) GUT Phenomenology: Proton Decay and $g_μ-2$

We consider proton decay and $g_μ- 2$ in flipped SU(5) GUT models. We first study scenarios in which the soft supersymmetry-breaking parameters are constrained to be universal at some high scale $M_{in}$ above the standard GUT scale where the QCD and electroweak SU(2) couplings unify. In this case the proton lifetime is typically $\gtrsim 10^{36}$~yrs, too long to be detected in the foreseeable future, and the supersymmetric contribution to $g_μ- 2$ is too small to contribute significantly to resolving the discrepancy between the experimental measurement and data-driven calculations within the Standard Model. However, we identify a region of the constrained flipped SU(5) parameter space with large couplings between the 10- and 5-dimensional GUT Higgs representations where $p \to e^+ π^0$ decay may be detectable in the Hyper-Kamiokande experiment now under construction, though the contribution to $g_μ-2$ is still small. A substantial contribution to $g_μ- 2$ is possible, however, if the universality constraints on the soft supersymmetry-breaking masses are relaxed. We find a `quadrifecta' region where observable proton decay co-exists with a (partial) supersymmetric resolution of the $g_μ- 2$ discrepancy and acceptable values of $m_h$ and the relic LSP density.

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arXiv2021

The new $(g-2)_μ$ result and the $μν$SSM

The $μν$SSM is a highly predictive alternative model to the MSSM. In particular, the electroweak sector of the model can explain the longstanding discrepancy between the experimental result for the anomalous magnetic moment of the muon, $(g-2)_μ$, and its Standard Model prediction, while being in agreement with all other theoretical and experimental constraints. The recently published MUON G-2 result is within $0.8\,σ$ in agreement with the older BNL result on $(g-2)_μ$. The combined result was announced as $a_μ^{\rm exp} = (11659206.1 \pm 4.1) \times 10^{-10}$, yielding a new deviation from the Standard Model prediction of $Δa_μ= (25.1 \pm 5.9) \times 10^{-10}$, corresponding to $4.2\,σ$. Using this improved bound we update the analysis in the $μν$SSM as presented in Ref. [1] and set new limits on the allowed parameters space of the electroweak sector of the model. We conclude that significant regions of the model can explain the new $(g-2)_μ$ data.

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arXiv2021

Flipped $g_μ-2$

We analyze the possible magnitude of the supersymmetric contribution to $g_μ- 2$ in a flipped SU(5) GUT model. Unlike other GUT models which are severely constrained by universality relations, in flipped SU(5) the U(1) gaugino mass and the soft supersymmetry-breaking masses of right-handed sleptons are unrelated to the other gaugino, slepton and squark masses. Consequently, the lightest neutralino and the right-handed smuon may be light enough to mitigate the discrepancy between the experimental measurement of $g_μ- 2$ and the Standard Model calculation, in which case they may be detectable at the LHC and/or a 250 GeV $e^+ e^-$ collider, whereas the other gauginos and sfermions are heavy enough to escape detection at the LHC.

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arXiv2021

A Minimal Supersymmetric SU(5) Missing-Partner Model

We explore a missing-partner model based on the minimal SU(5) gauge group with $\bf{75}$, $\bf{50}$ and $\bf{\overline{50}}$ Higgs representations, assuming a super-GUT CMSSM scenario in which soft supersymmetry-breaking parameters are universal at some high scale $M_{\rm in}$ above the GUT scale $M_{\rm GUT}$. We identify regions of parameter space that are consistent with the cosmological dark matter density, the measured Higgs mass and the experimental lower limit on $τ(p \to K^+ ν)$. These constraints can be satisfied simultaneously along stop coannihilation strips in the super-GUT CMSSM with $\tan β\sim 3.5 - 5$ where the input gaugino mass $m_{1/2} \sim 15 - 25$~TeV, corresponding after strong renormalization by the large GUT Higgs representations between $M_{\rm in}$ and $M_{\rm GUT}$ to $m_{\rm LSP}, m_{\tilde t_1} \sim 2.5 - 5$~TeV and $m_{\tilde g} \sim 13 - 20$~TeV, with the light-flavor squarks significantly heavier. We find that $τ(p \to K^+ ν) \lesssim 3 \times 10^{34}$~yrs throughout the allowed range of parameter space, within the range of the next generation of searches with the JUNO, DUNE and Hyper-Kamiokande experiments.

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arXiv2021

Atomic Form Factors and Inverse Primakoff Scattering of Axion

We reexamine the inverse Primakoff scattering of axions, whose scattering cross section depends on the distribution of electrons in target atoms. We evaluate it using a form factor computed with a relativistic Hartree-Fock wave function and compare it with the previous results obtained with those based on the screened Coulomb potential for the electrostatic field in the atom. We take xenon as an example for the target atom and show that the scattering cross section was overestimated by more than an order of magnitude for axions with $\lesssim O(10)$ keV energies, like solar axions. It is also found that inelastic scattering processes, in which the final state contains an excited or ionized atom, can be comparable or even be dominant when the size of momentum transfer is $\lesssim 1$ keV. For more energetic axions, on the other hand, the total scattering cross section is found to be well approximated by a simple expression and has little dependence on the atomic structure. As an application of this result, we consider supernova axions, whose energy is 10-100 MeV, and show that $O(1)$ inverse Primakoff events are expected for axions from a nearby supernova in the future neutrino experiments, which may warrant a more detailed study on the search strategy of this process.

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arXiv2021

Explaining muon $g-2$ data in the $μν$SSM

We analyze the anomalous magnetic moment of the muon $g-2$ in the $μν$SSM. This $R$-parity violating model solves the $μ$ problem reproducing simultaneously neutrino data, only with the addition of right-handed neutrinos. In the framework of the $μν$SSM, light left muon-sneutrino and wino masses can be naturally obtained driven by neutrino physics. This produces an increase of the dominant chargino-sneutrino loop contribution to muon $g-2$, solving the gap between the theoretical computation and the experimental data. To analyze the parameter space, we sample the $μν$SSM using a likelihood data-driven method, paying special attention to reproduce the current experimental data on neutrino and Higgs physics, as well as flavor observables such as $B$ and $μ$ decays. We then apply the constraints from LHC searches for events with multi-leptons + MET on the viable regions found. They can probe these regions through chargino-chargino, chargino-neutralino and neutralino-neutralino pair production. We conclude that significant regions of the parameter space of the $μν$SSM can explain muon $g-2$ data.

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arXiv2020

Low-Energy Probes of No-Scale SU(5) Super-GUTs

We explore the possible values of the $μ\to e γ$ branching ratio, $\text{BR}(μ\rightarrow eγ)$, and the electron dipole moment (eEDM), $d_e$, in no-scale SU(5) super-GUT models with the boundary conditions that soft supersymmetry-breaking matter scalar masses vanish at some high input scale, $M_{\rm in}$, above the GUT scale, $M_{\rm GUT}$. We take into account the constraints from the cosmological cold dark matter density, $Ω_{CDM} h^2$, the Higgs mass, $M_h$, and the experimental lower limit on the lifetime for $p \to K^+ \bar ν$, the dominant proton decay mode in these super-GUT models. Reconciling this limit with $Ω_{CDM} h^2$ and $M_h$ requires the Higgs field responsible for the charge-2/3 quark masses to be twisted, and possibly also that responsible for the charge-1/3 and charged-lepton masses, with model-dependent soft supersymmetry-breaking masses. We consider six possible models for the super-GUT initial conditions, and two possible choices for quark flavor mixing, contrasting their predictions for proton decay with versions of the models in which mixing effects are neglected. We find that $τ\left(p\rightarrow K^+ \barν\right)$ may be accessible to the upcoming Hyper-Kamiokande experiment, whereas all the models predict $\text{BR}(μ\rightarrow eγ)$ and $d_e$ below the current and prospective future experimental sensitivities or both flavor choices, when the dark matter density, Higgs mass and current proton decay constraints are taken into account. However, there are limited regions with one of the flavor choices in two of the models where $μ\to e$ conversion on a heavy nucleus may be observable in the future. Our results indicate that there is no supersymmetric flavor problem in the class of no-scale models we consider.

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arXiv2020

Building Models of Inflation in No-Scale Supergravity

After reviewing the motivations for cosmological inflation formulated in the formalism of supersymmetry, we argue that the appropriate framework is that of no-scale supergravity. We then show how to construct within this framework inflationary models whose predictions for the tilt in the spectrum of scalar perturbations, $n_s$, and the ratio, $r$, of tensor and scalar perturbations coincide with those of the $R + R^2$ model of inflation proposed by Starobinsky. A more detailed study of no-scale supergravity reveals a structure that is closely related to that of $R^2$ modifications of the minimal Einstein-Hilbert action for general relativity, opening avenues for constructing no-scale de Sitter and anti-de Sitter models by combining pairs of Minkowski models, as well as generalizations of the original no-scale Starobinsky models of inflation. We then discuss the phenomenology of no-scale models of inflation, including inflaton decay and reheating, and then the construction of explicit scenarios based on SU(5), SO(10) and string-motivated flipped SU(5)$\times$U(1) GUT models. The latter provides a possible model of almost everything below the Planck scale, including neutrino masses and oscillations, the cosmological baryon asymmetry and cold dark matter, as well as $n_s$ and $r$.

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arXiv2020

R-Symmetric Flipped SU(5)

We construct a supersymmetric flipped SU(5) grand unified model that possesses an $R$ symmetry. This $R$ symmetry forbids dangerous non-renormalizable operators suppressed by a cut-off scale up to sufficiently large mass dimensions so that the SU(5)-breaking Higgs field develops a vacuum expectation value of the order of the unification scale along the $F$- and $D$-flat directions, with the help of the supersymmetry-breaking effect. The mass terms of the Higgs fields are also forbidden by the $R$ symmetry, with which the doublet-triplet splitting problem is solved with the missing partner mechanism. The masses of right-handed neutrinos are generated by non-renormalizable operators, which then yield a light neutrino mass spectrum and mixing through the seesaw mechanism that are consistent with neutrino oscillation data. This model predicts one of the color-triplet Higgs multiplets to lie at an intermediate scale, and its mass is found to be constrained by proton decay experiments to be $\gtrsim 5 \times 10^{11}$ GeV. If it is $\lesssim 10^{12}$ GeV, future proton decay experiments at Hyper-Kamiokande can test our model in the $p \to π^0 μ^+$ and $p \to K^0 μ^+$ decay modes, in contrast to ordinary grand unified models where $p \to π^0 e^+ $ or $p \to K^+ \barν$ is the dominant decay mode. This characteristic prediction for the proton decay branches enables us to distinguish our model from other scenarios.

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arXiv2020

Supernova-scope for the Direct Search of Supernova Axions

If a supernova explosion occurs within a few hundred parsecs from the Earth, a huge number of axions, in addition to neutrinos, may arrive at the Earth. In this paper, we discuss in detail the prospect of detecting those supernova axions by an axion helioscope. With the help of a pre-supernova neutrino alert system, it is possible to point a helioscope at an exploding supernova in advance. The supernova axions can then be detected by a gamma-ray detector installed at the end of the helioscope. We call such a detection system an axion supernova-scope (SNscope). We propose a conceptual design for an axion SNscope, where the gamma-ray detector is installed at the opposite end to the X-ray detector for the solar axion. It still functions as an axion helioscope during the normal operation time, and once a pre-SN neutrino alert is received, the scope is temporarily turned around and targeted to a SN candidate, waiting for the supernova axions. We estimate the sensitivity of supernova axion detection and find that SNscopes based on the next-generation axion helioscopes, such as IAXO, have potential to explore the invisible axions and to test the axion interpretation of stellar cooling anomalies.

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