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Kaoru Hagiwara

Publications and source records attributed to Kaoru Hagiwara.

At least 73 records · Page 4Linked to original sources

Tau polarization effects in the CNGS tau-neutrino appearance experiments

We studied tau polarization effects on the decay distributions of tau produced in the CNGS tau-neutrino appearance experiments. We show that energy and angular distributions for the decay products in the laboratory frame are significantly affected by the tau polarization. Rather strong azimuthal asymmetry about the tau momentum axis is predicted, which may have observable consequences in experiments even with small statistics.

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Solving the degeneracy of the lepton-flavor mixing angle theta_atm by the T2KK two detector neutrino oscillation experiment

If the atmospheric neutrino oscillation amplitude, sin^2 2theta_atm is not maximal, there is a two fold ambiguity in the neutrino parameter space: sin^2 theta_atm>0.5 or sin^2 theta_atm<0.5. In this article, we study the impact of this degeneracy, the so-called octant degeneracy, on the T2KK experiment, which is a proposed extension of the T2K (Tokai-to-Kaimoka) neutrino oscillation experiment with an additional water cherenkov detector placed in Korea. We find that the degeneracy between sin^2 theta_atm= 0.40 and 0.60 can be resolved at the 3sigma level for sin^2 2theta_rct>0.12 (0.08) for the optimal combination of a 3.0^circ off-axis beam (OAB) at SK (L=295km) and a 0.5^circ OAB at L=1000km with a far detector of 100kton volume, after 5 years of exposure with 1.0(5.0) time 10^21 POT/year, if the hierarchy is normal. We also study the influence of the octant degeneracy on the capability of T2KK experiment to determine the mass hierarchy and the leptonic CP phase. The capability of rejecting the wrong mass hierarchy grows with increasing sin^2 theta_atm when the hierarchy is normal, whereas it is rather insensitive to sin^2 theta_atm for the inverted hierarchy. We also find that the 1sigma allowed region of the CP phase is not affected significantly even when the octant degeneracy is not resolved. All our results are obtained for the 22.5 kton Super-Kamiokande as a near detector and without an anti-neutrino beam.

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Electroweak symmetry breaking and precision data

We study the impact of LEP2 constraints on the dimensionless coefficients of the electroweak chiral Lagrangian on the precision observables using the improved renormalization group equations. We find that the current uncertainty in the triple and quartic gauge boson couplings can accommodate electroweak symmetry breaking models with $S(Λ=1 \textrm{TeV}) > 0$.

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Solving the neutrino parameter degeneracy by measuring the T2K off-axis beam in Korea

The T2K neutrino oscillation experiment will start in 2009. In this experiment the center of the neutrino beam from J-PARC at Tokai village will go through underground beneath Super-Kamiokande, reach the sea level east of Korean shore, and an off-axis beam at $0.5^{\circ}$ to $1.0^{\circ}$ can be observed in Korea. We study physics impacts of putting a 100 kt-level Water \cerenkov detector in Korea during the T2K experimental period. For a combination of the $3^{\circ}$ off-axis beam at SK with baseline length L = 295 km and the $0.5^{\circ}$ off-axis beam in the east coast of Korea at L = 1000 km, we find that the neutrino mass hierarchy (the sign of $m^2_{3} - m^2_1$) can be resolved and the CP phase of the MNS unitary matrix can be constrained uniquely at 3-$σ$ level when \sin^2 2θ_{\rm rct} $ \ge 0.06 $.

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The Decay of Tau Leptons Produced in Neutrino-Nucleon Scatterings

Energy and angular distributions of the tau decay products in the CERN-to-Gran Sasso $ν_τ$ appearance experiments are studied for the decay modes $τ\toπν$ and $τ\to l\barνν$ (l=e or mu). We find that the decay particle distributions in the laboratory frame are significantly affected by the tau polarization. Rather strong azimuthal asymmetry of $pi^-$ and $l^-$ about the tau momentum axis is predicted, which may have observable consequences even at small statistics experiments.

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Lifting degeneracies in the oscillation parameters by a neutrino factory

We study the potential of a very long baseline neutrino oscillation experiment with a neutrino factory and a large segmented water-Cerenkov calorimeter detector in resolving the degeneracies in the neutrino oscillation parameters; the sign of the larger mass-squared difference $δm^2_{13}$, the sign of $|U_{μ3}|^2 (\equiv \sin^2θ_{ATM)-1/2$, and a possible two-fold ambiguity in the determination of the CP phase $δ_{MNS}$. We find that the above problems can be resolved even if the particle charges are not measured. The following results are obtained in our exploratory study for a neutrino factory which delivers $10^{21}$ decaying $μ^+$ and $μ^-$ at 10 GeV and a 100 kton detector which is placed 2,100 km away and is capable of measuring the event energy and distinguishing $e^\pm$ from $μ^\pm$, but not their charges. The sign of $δm^2_{13}$ can be determined for $4|U_{e3}|^2(1-|U_{e3}|^2)\equiv \sin^22θ_{RCT}\gsim 0.008$. That of $\sin^2θ_{ATM}-1/2$ can be resolved for $\sin^22θ_{ATM}=0.96$ when $\sin^22θ_{RCT}\gsim 0.06$. The CP-violating phase $δ_{MNS}$ can be uniquely constrained for $\sin^22θ_{RCT}\gsim 0.02$ if its true value is around $90^\circ$ or $270^\circ$, while it can be constrained for $\sin^22θ_{RCT}\gsim 0.03$ if its true value is around $0^\circ$ or $180^\circ$.

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Physics prospects of future neutrino oscillation experiments in Asia

The three neutrino model has 9 physical parameters, 3 neutrino masses, 3 mixing angles and 3 CP violating phases. Among them, neutrino oscillation experiments can probe 6 parameters: 2 mass squared differences, 3 mixing angles, and 1 CP phase. The experiments performed so far determined the magnitudes of the two mass squared differences, the sign of the smaller mass squared difference, the magnitudes of two of the three mixing angles, and the upper bound on the third mixing angle. The sign of the larger mass squared difference (the neutrino mass hierarchy pattern), the magnitude of the third mixing angle and the CP violating phase, and a two-fold ambiguity in the mixing angle that dictates the atmospheric neutrino oscillation should be determined by future oscillation experiments. In this talk, I introduce a few ideas of future long baseline neutrino oscillation experiments which make use of the super neutrino beams from J-PARC (Japan Proton Accelerator Research Complex) in Tokai village. We examine the potential of HyperKamiokande (HK), the proposed 1 Mega-ton water Cerenkov detector, and then study the fate and possible detection of the off-axis beam from J-PARC in Korea. We also show that very long baseline experiments with higher energy beams from J-PARC and a water Cerenkov calorimeter detector (BAND) proposed in Beijing can resolve the neutrino mass hierarchy, and lift all the degeneracies in the three neutrino model parameters.

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One-loop anomalous couplings and matching conditions in the nonlinear realization of the SU(2) Higgs Model

We study the renormalization of the nonlinear realization of the SU(2) Higgs model in the modified minimal subtraction renormalization scheme. We propose that the effective field method with truncated operator series is trustworthy even when the Higgs boson is relatively light. Using the technique of background field method in the coordinate space, we derive the matching conditions at tree and one loop order, both in the regions where the Higgs boson is heavy and where it is light. We obtain the complete one-loop anomalous couplings up to ${\Large O} (p^4)$ in the SU(2) Higgs model. We observe that the contribution of the gauge bosons and the Goldstone bosons to the anomalous couplings at the one-loop level is significant. By establishing the correspondence between our coordinate space calculation and the momentum space calculation that exists in the literature, we find agreement in all the matching conditions in the heavy Higgs boson limit.

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SUSY Loop Contributions to the $W$-Pair Production in $e^+e^-$ Collisions

We discuss one-loop contributions of supersymmetry (SUSY) particles to the process $e^+e^- \to W^+W^-$ in the minimal supersymmetric standard model. The calculation is tested by using the sum rule among the form factors, which is based on the BRS invariance. The overall normalization factor of the amplitudes is tested by using the decoupling property of the SUSY particles in the large mass limit. The correction due to the one-loop effect of the sfermions can be of a few times $\pm 0.1$ % level, whereas that of the charginos and neutralinos can be order 1%. We also study the CP-violating effects in the chargino and neutralino sector.

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One-loop contributions of charginos and neutralinos to W+ W- production in e+ e- collisions

We study one-loop effects of charginos and neutralinos on the helicity amplitudes for the process $e^+e^-\to W^+W^-$ in the MSSM. The calculation is tested by using two methods. First, the sum rules among form factors which result from BRS invariance. Second, the decoupling property in the large mass limit is used to test the overall normalization of the amplitudes. The one-loop contribution to the helicity amplitudes is shown as a function of $μ$ Higgs parameter and $\sqrt{s}$. We also study the effects of the CP-violating phase.

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One-loop contributions of charginos and neutralinos to W-pair production in E+ E- collisions

We study the one-loop effects of charginos and neutralinos on the helicity amplitudes for $\eeww$ in the minimal supersymmetric standard model. The calculation is tested by using two methods. First, the sum rule for the form factors between $\eeww$ and the process where the external $W^\pm$ bosons are replaced by the corresponding Goldstone bosons $ω^\pm$ is employed to test the analytic expression and the accuracy of the numerical program. Second, the decoupling property in the large mass limit is used to test the overall normalization of the amplitudes. These two tests are most effectively carried out when the amplitudes are expanded in terms of the modified minimal subtraction ($\bar{\rm MS}$) couplings of the standard model. The resulting perturbation expansion is valid at collider energies below and around the threshold of the light supersymmetric particles. We find that the corrections to the cross section of the longitudinally polarized $W$-pair production can be as large as -1.4% at the threshold of the light chargino-pair production for large scattering angles. We also study the effects of the CP-violating phase in the chargino and neutralino sectors on the helicity amplitudes. We find that the resulting CP-violating asymmetries can be at most 0.1%.

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Probing the CP nature of the Higgs bosons by top-pair production at photon linear colliders

We study effects of heavy Higgs bosons on the top-pair production process at photon linear colliders. The interference patterns between the resonant Higgs-production amplitudes and the continuum QED amplitudes are examined. The patterns tell us not only the CP nature of the Higgs bosons but also the phase of the $γγ$--Higgs vertex which gives new information about the Higgs couplings to new charged particles. We point out that it is necessary to use circularly polarized photon beams to produce efficiently heavy Higgs bosons whose masses exceed the electron beam energy, and show that the above interference patterns of the production amplitudes can be studied by observing top decay angular distributions. Analytic expressions for the helicity amplitudes for the sequential process $γγ\to t \bar{t} \to (bW^+) (\bar{b}W^-) \to (b f_1 \bar{f}_2) (\bar{b} f_3 \bar{f}_4)$ are presented in terms of the generic $γγ\to t \bar{t}$ production amplitudes.

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Measuring the CP-violating phase by a long base-line neutrino experiment with Hyper-Kamiokande

We study the sensitivity of a long-base-line (LBL) experiment with neutrino beams from the High Intensity Proton Accelerator (HIPA), that delivers 10^{21} POT per year, and a proposed 1Mt water-Cherenkov detector, Hyper-Kamiokande (HK) 295km away from the HIPA, to the CP phase (delta_{M N S}) of the three-flavor lepton mixing matrix. We examine a combination of the nu_mu narrow-band beam (NBB) at two different energies, vev{p_pi}=2, 3GeV, and the bar{nu}_mu NBB at vev{p_pi}=2GeV. By allocating one year each for the two nu_mu beams and four years for the bar{nu}_mu beam, we can efficiently measure the nu_mu to nu_e and bar{nu}_mu to bar{nu}_e transition probabilities, as well as the nu_mu and bar{nu}_mu survival probabilities. CP violation in the lepton sector can be established at 4sigma (3sigma) level if the MSW large-mixing-angle scenario of the solar-neutrino deficit is realized, |\dmns| or |delta{M N S}-180^{circ}| > 30^{circ}, and if 4|U_{e3}|^2 (1-|U_{e3}|^2) equiv sin^2 2 theta_{CHOOZ} > 0.03 (0.01). The phase delta_{M N S} is more difficult to constrain by this experiment if there is little CP violation, delta_{M N S} sim 0^{circ} or 180^{circ}, which can be distinguished at 1sigma level if sin^2 2 theta_{CHOOZ} >~ 0.01.

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B^0_{d,s} - \bar{B}^0_{d,s} mass-differences from QCD spectral sum rules

We present the first QCD spectral sum rules analysis of the SU(3) breaking parameter ξand an improved estimate of the renormalization group invariant (RGI) bag constant \hat{B}_{B_q} both entering into the B^0_{d,s} - \bar{B}^0_{d,s} mass-differences. The averages of the results from the Laplace and moment sum rules to order α_s are f_B\sqrt{\hat B_B} \simeq (247 \pm 59) MeV and ξ\equiv f_{B_s} \sqrt{\hat B_{B_s}} / f_{B} \sqrt{\hat B_{B}} \simeq (1.18\pm 0.03), in units where f_π=130.7 MeV. Combined with the experimental data on the mass-differences ΔM_{d,s}, one obtains the constraint on the CKM weak mixing angle |V_{ts}/V_{td}|^2 > 20.0 (1.1). Alternatively, using the weak mixing angle from the analysis of the unitarity triangle and the data on ΔM_d, one predicts ΔM_s=18.6 (2.2) ps^{-1} in agreement with the present experimental lower bound and within the reach of Tevatron 2.

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Observability of the Lightest MSSM Higgs Boson with Explicit CP Violation via Gluon Fusion at the LHC

We investigate the observability of the lightest Higgs boson in the gluon-fusion channel at the CERN Large Hadron Collider (LHC) in the minimal supersymmetric Standard Model with explicit CP-violating mixing among three neutral Higgs bosons. The lightest Higgs boson with its mass less than 130 GeV can be detected at the LHC via its gluon-fusion production followed by the decay into two photons. The explicit CP violation can suppress both the production cross section and the two-photon decay branching fraction so significantly that the signal cross section may be more than ten times smaller than the SM signal. This reduction factor can be as small as 1/40 if the lightest Higgs boson mass is 115 GeV and its production cross section at LEP2 is more than 90 % that of the SM case.

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Ultra High Energy Cosmic Ray, Superheavy Dark Matter and Extra Dimension

We propose a new mechanism for explaining the very long lifetime of superheavy dark matter X, which is proposed as a source of the ultra high-energy cosmic rays above the GZK cutoff (5 \times 10^{19} eV). The singlet X particle couples to the MSSM particles only through a bulk singlet field which develops the v.e.v. in the ``hidden'' brane. The distance between this hidden brane and the ``visible'' brane naturally leads to the exponential suppression of the coupling. The X particle decays predominantly into the higgsino and higgs boson of the MSSM, and its decay spectrum is completely determined once their properties are known.

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Supersymmetric contributions to muon g-2 and the electroweak precision measurements

In view of the recent measurement of the muon g-2 and finalization of the LEP electroweak data, we re-examine all the new particle contributions to those observables in the MSSM. The SM fits the latest electroweak data excellently with the observed top-quark mass and the Higgs-boson mass of around 100 GeV, and so does the MSSM in the decoupling limit. The MSSM gives a slightly better fit to the data than the SM (Δχ^2_{min} \sim -2) when relatively light left-handed sleptons of mass \sim 200 GeV and a light chargino of mixed higgsino-wino character (μ/M_2 \sim 1) with mass \sim 100 GeV co-exist. The improvement in the fit diminishes quickly for wino- or higgsino- dominant charginos, for heavier charginos, and for lighter sleptons. We find that the MSSM contributions to the muon g-2 is most efficient when the light chargino has a mixed character. If \tanβ< 10, the set of a light mixed chargino and light sleptons that is favored by the electroweak data is also favored by the g-2 data for μ>0. Models with light gauginos (μ/M_2 > 10) or light higgsinos (μ/M_2 < 0.1) give significant contributions to muon g-2 only for large \tanβ(\simgt 15).

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Higgs Boson Decays in the Minimal Supersymmetric Standard Model with Radiative Higgs Sector CP Violation

We re-evaluate the decays of the Higgs bosons in the minimal supersymmetric standard model (MSSM) where the tree-level CP invariance of the Higgs potential is explicitly broken by the loop effects of the third-generation squarks with CP-violating soft-breaking Yukawa interactions. This study is based on the mass matrix of the neutral Higgs bosons that is valid for arbitrary values of all the relevant MSSM parameters. It extends the previous work considerably by including neutral Higgs-boson decays into virtual gauge bosons and those into top-squark pairs, by implementing squark-loop contributions to the two-gluon decay channel, and by incorporating the decays of the charged Higgs boson. The constraints from the electron electric dipole moment on the CP phases are also discussed. We find that the branching fractions of both the neutral and charged Higgs-boson decays and their total decay widths depend strongly on the CP phases of the top (and bottom) squark sectors through the loop-induced neutral Higgs boson mixing as well as the direct couplings of the neutral Higgs bosons to top squark pairs.

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