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Mihoko M. Nojiri

Publications and source records attributed to Mihoko M. Nojiri.

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arXiv2001

Implications of Muon anomalous magnetic moment for Direct detection of Neutralino Dark Matter

We investigate the implications of the recent measurement of muon anomalous magnetic moment for the direct detection of neutralino dark matter in the three different SUSY models: mSUGRA, a model with non-universal Higgs mass, and an SO(10) GUT model. We consider two extreme scenario for $Δa_μ$ bound, i.e. $27 \times 10^{-10} < Δa_μ< 59 \times 10^{-10}$ (1$σ$ bound) and $0 < Δa_μ< 11 \times 10^{-10}$ ($2σ$ below). In mSUGRA model, the counting ratio may be above the sensitivity of the future experiments when parameters are within $1σ$ bound of $Δa_μ$. However, the $Ω_χ$ tends to be high compared to the currently favored value $Ω=0.3$. For models with the non-universal scalar masses, the possibility to have the consistent $Ω_χ$ and the high counting ratio is open up in the region of parameter space where Higgsino mass $μ$ is smaller than mSUGRA prediction. In particular, in the SO(10) model, the LSP dark matter detection rate may be enhanced by almost one order of magnitude compared to mSUGRA and the model with non-universal Higgs mass, for cosmologically acceptable $Ω_χ h^2$. The highest detection rate of LSP dark matter occurs in the region where the LSP constitutes a subdominant part of local halo DM. Implication of SUSY mass parameter measurement under the cosmological constraint is also discussed.

hep-ph↗
arXiv2001

The fate of the B ball

The gauge-mediated SUSY-breaking (GMSB) model needs entropy production at a relatively low temperature in the thermal history of the Universe for the unwanted relics to be diluted. This requires a mechanism for the baryogenesis after the entropy production, and the Affleck and Dine (AD) mechanism is a promising candidate for it. The AD baryogenesis in the GMSB model predicts the existence of the baryonic Q ball, that is the B ball, and this may work as the dark matter in the Universe. In this article, we discuss the stability of the B ball in th presence of baryon-number violating interactions. We find that the evaporation rate increases monotonically with the B-ball charge because the large field value inside the B ball enhances the effect of the baryon-number-violating operators. While there are some difficulties to evaluate the evaporation rate of the B ball, we derive the evaporation time (lifetime) of the B ball for the mass-to-charge ratio $ω_0\gsim 100 \MEV$. The lifetime of the B ball and the distortion of the cosmic ray positron flux and the cosmic background radiation from the B ball evaporation give constraints on the baryon number of the B ball and the interaction, if the B ball is the dark matter. We also discuss some unresolved properties of the B ball.

hep-ph↗
arXiv2001

Direct Detection of Neutralino Dark Matter and the Anomalous Dipole Moment of the Muon

We compare predictions for the spin-independent contribution to the neutralino-proton scattering cross section $\sig$ and for the anomalous magnetic dipole moment of the muon, $a_μ= (g_μ-2)/2$, in models with gravity-mediated supersymmetry breaking. We nearly always find a positive correlation between these two measurables, i.e. scenarios with larger $a_μ$ also tend to have larger $\sig$, but the detailed prediction differs greatly between models. In particular, we find that for the popular mSUGRA scenario with universal soft breaking masses at the scale of Grand Unification, measurements of $a_μ$ currently seem more promising. On the other hand, if scalar soft breaking masses at the GUT scale receive sizable contributions from SO(10) D-terms, one often finds scenarios with large $\sig$ but $a_μ$ below the currently foreseen sensitivity. A string-inspired model with non-universal scalar spectrum at the GUT scale falls between these two cases.

hep-ph↗
arXiv2000

Scrutinizing LSP Dark Matter at the LHC

We show that LHC experiments might well be able to determine all the parameters required for a prediction of the present density of thermal LSP relics from the Big Bang era. If the LSP is an almost pure bino we usually only need to determine its mass and the mass of the SU(2) singlet sleptons. This information can be obtained by reconstructing the cascade $\tilde{q}_L \to \tildeχ_2^0 q \to \tilde{\ell}_R \ell q \to \tildeχ_1^0 \ell^+ \ell^- q$. The only requirement is that $m_{\tilde{\ell}_R} < m_{\tildeχ_2^0}$, which is true for most of the cosmologically interesting parameter space. If the LSP has a significant higgsino component, its predicted thermal relic density is smaller than for an equal--mass bino. We show that in this case squark decays also produce significant numbers of $\tildeχ_4^0$ and $\tildeχ_2^\pm$. Reconstructing the corresponding decay cascades then allows to determine the higgsino component of the LSP.

hep-ph↗
arXiv2000

Lepton Energy Asymmetry and Precision SUSY study at Hadron Colliders

We study the distribution of lepton pairs from the second lightest neutralino decay \tildeχ^0_2-->\tilde{l}l followed by \tilde{l}\to \tildeχ^0_1 l. The distribution of the ratio of lepton transverse momenta A_T shows peak structure if m_{ll}< m^{max}_{ll}/2 is required. The peak position A_T^{peak} is described by a simple function of the ino and slepton masses in the m_{ll}\sim 0 limit. When a moderate m_{ll} cut is applied, A_T^{peak} depends on the \tildeχ^0_2 velocity distribution, but the dependence would be corrected by studying the lepton P_T distribution. A_T^{peak} and the edge of m_{ll} distributions are used to determine the mass parameters involved in the decay for parameters of interest to LHC experiments. For some cases the ino and slepton masses may be determined within 10% by the lepton distribution only independent of model assumptions. Correct combinations of A_{T}^{peak} and m_{ll}^{edge} would be identified even if different \tildeχ^0_2 decay chains are co-existing. The analysis could be extended to the Tevatron energy scale or other cascade decays.

hep-ph↗
arXiv1999

Neutralino Decays at the CERN LHC

We study the distribution of lepton pairs from the second lightest neutralino decay \tchi^0_2\to\tchi^0_1 l^+l^-. This decay mode is important to measure the mass difference between \tchi^0_2 and the lightest neutralino \tchi^0_1, which helps to determine the parameters of the minimal supersymmetric standard model at the CERN LHC. We found that the decay distribution strongly depends on the values of underlying MSSM parameters. For some extreme cases, the amplitude near the end point of the lepton invariant mass distribution can be suppressed so strongly that one needs the information of the whole m_{ll} distribution to extract m_{\tchi^0_2}-m_{\tchi^0_1}. On the other hand, if systematic errors on the acceptance can be controlled, this distribution can be used to constrain slepton masses and the Z\tchi^0_2\tchi^0_1 coupling. Measurements of the velocity distribution of \tchi^0_2 from samples near the end point of the m_{ll} distribution, and of the asymmetry of the p_T of leptons, would be useful to reduce the systematic errors.

hep-ph↗
arXiv1998

Lepton-Flavor Violation in the Left-handed Slepton Production at Future Lepton Colliders

The Super-Kamiokande atmospheric neutrino data suggest existence of the large lepton-flavor violating (LFV) interaction in the higher energy scale. If the minimal supersymmetric standard model is extended to have right-handed neutrinos, the left-handed sleptons in the second and third generations are expected to have the LFV masses in the minimal supergravity scenario. In this article we study the LFV signals in the left-handed slepton production at $μ^+μ^-$ colliders and $\e^+\e^-$ linear colliders (LC's), $μ^+μ^-(\e^+\e^-)\toτμ+4jets + \E$ and $μ^+μ^-(\e^+\e^-)\toτμl+ 2jets+ \E$. The main background comes from decay of a tau lepton into a muon in the lepton-flavor conserving slepton pair production. They are significantly reduced by the energy and the impact parameter cuts for the muon. At $μ^+μ^-$ colliders (LC's) it may be possible to reach the mixing angle $\sin 2θ_{\tildeν} \gsim 0.1(0.5)$ and the mass difference $Δm_{\tildeν}\gsim 0.1(0.4)$ GeV for the sneutrinos in the second and third generations at the statistical significance of $3σ$. Such small mass difference and a mixing angle may be induced radiatively even if the Yukawa coupling constant for the tau neutrino is of the order of 0.1.

hep-ph↗
arXiv1998

Radiative Corrections to a Supersymmetric Relation: A New Approach

Recently it has been realized that the production and decay processes of charginos, neutralinos, and sleptons receive corrections which grow like log m_squark for large m_squark. In this paper we calculate the chargino pair production cross section at e+e- colliders with quark/squark loop corrections. We introduce a novel formulation, where the one-loop amplitude is reorganized into two parts. One part is expressed in terms of the ``effective'' chargino coupling gbar and mixing matrices U^P, V^P, and includes all O(log m_squark) corrections, while the other decouples for large m_squark. The form of the one-loop cross section then becomes physically transparent. Our formulation can be easily extended to other loops and processes. Numerically, we find significant corrections due to the effective t-channel coupling gbar, for gaugino-like charginos. In the mixed region, where the chargino has large gaugino and Higgsino components, the corrections due to (U^P,V^P) are also significant. Our numerical results disagree with a previous calculation. We revisit previous studies of the determination of gbar through the measurement of the chargino production cross section. We point out that a previous study, which claimed that the measurement suffers large systematic errors, was performed at a ``pessimistic'' point in MSSM parameter space. We provide reasons why the systematic errors are not a limiting factor for generic parameter choices.

hep-ph↗
arXiv1997

Toward loop level study of supersymmetric models at future e^+e^- linear colliders

In the case with a large splitting between the squark and slepton masses, the supersymmetric identities which enforce the equality of the gauge and gaugino couplings are violated in the effective theory below the squark mass threshold. We compute the full one-loop (s)quark corrected slepton production cross-sections. We find that the one-loop corrected slepton production cross-sections can depend on the squark mass strongly, up to 9% *log_{10}(M_{\tilde{Q}}/m_{\tilde{l}}). We investigate the squark mass sensitivity of the slepton cross-section measurements at a future linear collider. For sneutrino production accessible at \sqrt{s}=500 GeV there can be sensitivity to squark masses at or larger than 1 TeV.

hep-ph↗
arXiv1997

Slepton Production as a Probe of the Squark Mass Scale

We investigate an important radiative correction to the slepton and chargino production processes. We consider a supersymmetric spectrum with a large splitting between the squark and slepton masses. In this case, in the effective theory below the squark mass threshold, the supersymmetric Slavnov-Taylor identities which enforce the equality of the gauge and gaugino couplings are violated. The gaugino propagators receive potentially large corrections due to virtual quark/squark loop effects. We compute the full one-loop (s)quark corrected slepton production cross-sections. The t-channel one-loop scattering amplitudes are factorized into an effective chargino/neutralino mass matrix and an effective fermion-sfermion-gaugino coupling. The difference between the effective gaugino coupling and the gauge coupling is proportional to log(Msq/msl) in the large squark mass limit. We find that the one-loop corrected slepton production cross-sections can depend on the squark mass strongly, up to 9% times log(Msq/msl). We investigate the squark mass sensitivity of the slepton cross-section measurements at a future linear collider. For sneutrino production accessible at sqrt{s}= 500 GeV there can be sensitivity to squark masses at or larger than 1 TeV.

hep-ph↗
arXiv1996

Confronting the Minimal Supersymmetric Standard Model with the Study of Scalar Leptons at Future Linear e+e- Colliders

Sleptons can easily be found at future linear e+e- colliders if kinematically accessible. Measurements of their masses and decay distributions would then determine MSSM parameters. This paper presents a detailed MC study of the production and decay of the lighter scalar tau lepton, stau1. We found that mstau1 and the left-right mixing angle of stau would be measured within an error of a few percent. tanbeta is determinable in some region of the parameter space through simultaneous studies of stau1-and selectron-pair production: the polarization measurement of the tau leptons from stau1 decays and the M1, mchi1 determination using selectron pair production and decay. We also point out the possibility to determine bino-selectron-e coupling through the measurement of the angular distribution of the selectron-pair production. The error on the coupling is expected to be comparable to its typical SUSY radiative correction, which is proportional to log(msquark/mslepton). The radiative correction affects M1 and tanbeta determination, necessitating the full 1-loop radiative correction to the selectron production processes. The implication of these measurements of the MSSM parameters on selecting models of the origin of supersymmetry breaking is also discussed.

hep-ph↗
arXiv1995

Precision Study of MSSM at future $e^+e^-$ linear colliders

The lighter scalar tau lepton $\sti$ may be the lightest scalar lepton and therefore would be found earlier in future collider experiments. We point out the impact of the measurement of the mass and the mixing angle of $\st$ to discriminate the models of SUSY breaking. Furthermore, the measurement of the polarization of $τ$ lepton($P_τ$) from the decaying $\sti$ helps to determine the Yukawa sector of minimal supersymmetric standard model. We present our MC study of the production and the decay of $\sti$ lepton at a future linear collider at $\sqrt{s}=500$ GeV. The mass, mixing angle of $\sti$ and $P_τ(\sti\rightarrowτχ_1^0)$ would be measured precisely at the future LC. ( talks given at Yukawa International Seminar(YIKS) '95 on some very hot and humid day in August, and also at Workshop on {\it Physics and Experiments with Linear $e^+e^-$ Colliders} Appi, Iwate Japan Sep.8-12 1995.)

hep-ph↗
arXiv1994

Polarization of $τ$ lepton from scalar tau decay as a probe of neutralino mixing

The $τ$ lepton arising from the scalar tau ($\st$) decay is naturally polarized. $\ptau$ depends on the left--right mixing of the $\st$ and the gaugino--higgsino mixing of the neutralino. The polarization $\ptau$ could be measured from the energy distribution of the decay products of $τ$ at future $\epem$ colliders. A measurement of $\ptau$and of the $\st$ production cross section allows to determine both these mixing angles.

hep-ph↗
arXiv1994

Implications for Supersymmetric Dark Matter Detection from Radiative $b$ Decays

We point out that combinations of parameters that predict large counting rates in experiments searching for supersymmetric dark matter often tend to predict a very large branching ratio for the inclusive decay \bsg. The recent measurement of this branching ratio, therefore, indicates that searches for supersymmetric dark matter might be even more difficult than previously anticipated.

hep-ph↗
arXiv1992

New Contribution to Coherent neutralino-Nucleus Scattering

We discuss coherent scattering between supersymmetric neutralinos and nuclei via an effective neutralino-gluon interaction. We identify two new classes of diagrams which are not treated in the existing literature. These occur at the same order of perturbation theory as the previously considered diagrams. The new contributions can be numerically important, and can even dominate the total amplitude. This affects the clculation fo event rates in experiments searching for direct or indirect evidence of cosmic relic neutralinos.

hep-ph↗