Hiroshi Yokoya

University of Toyama, Тояма, Toyama, Japan

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Publications (31)83.28 Total impact

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    Shinya Kanemura, Koji Tsumura, Hiroshi Yokoya
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    ABSTRACT: In the two Higgs doublet model, $\tan\beta$ is an important parameter, which is defined as the ratio of the vacuum expectation values of the doublets. We study how accurately $\tan\beta$ can be determined at linear colliders via the precision measurement of the decay branching fraction of the standard model (SM) like Higgs boson. Since the effective coupling constants of the Higgs boson with the weak gauge bosons are expected to be measured accurately, the branching ratios can be precisely determined. Consequently, $\tan\beta$ can be determined with a certain amount of accuracy. Comparing the method to those using direct production of the additional Higgs bosons, we find that, depending on the type of Yukawa interactions, the precision measurement of the decay of the SM-like Higgs boson can be the best way to determine $\tan\beta$, when the deviations in the coupling constants with the gauge boson from the SM prediction are observed at linear colliders.
    Physical review D: Particles and fields 05/2013; 88(5).
  • Shinya Kanemura, Kei Yagyu, Hiroshi Yokoya
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    ABSTRACT: When the doubly-charged Higgs bosons $H^{\pm\pm}$ mainly decay into the same-sign dilepton, a lower bound on the mass is around 400 GeV by the current LHC data. On the other hand, no such bound has been reported by using the data at LEP and at the LHC for the case where the same-sign diboson decay $H^{\pm\pm}\to W^{\pm(*)} W^{\pm(*)}$ is dominant. We study limits on the mass for such a case by using the current experimental data. From the precise measurement of the total width of the Z boson at the LEP experiment, the mass below 43 GeV is excluded with the 95% C.L. It turns out that the results from four charged lepton searches at LEP do not provide any significant constraint. We show that a new lower bound is obtained in the diboson decay scenario at the LHC with the collision energy to be 7 TeV and the integrated luminosity to be 4.7 fb$^{-1}$. By using the data of the same-sign dilepton events, the lower limit is found to be 60 GeV at the 95% C.L. By the extrapolation of the data to 20 fb$^{-1}$ with the same collision energy, the lower limit is evaluated to be 85 GeV.
    Physics Letters B 05/2013; · 4.57 Impact Factor
  • Mayumi Aoki, Shinya Kanemura, Hiroshi Yokoya
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    ABSTRACT: We study collider signatures for extra scalar bosons in the inert doublet model at the international linear collider (ILC). The inert doublet model is a simple extension of the standard model by introducing an additional isospin-doublet scalar field which is odd under an unbroken Z_2 symmetry. The model predicts four kinds of Z_2-odd scalar bosons, and the lightest of them becomes stable and a candidate of the dark matter as long as it is electrically neutral. Taking into account the constraints from various theoretical and phenomenological conditions, we perform a simulation study for the distinctive signatures of the extra scalars over the standard-model background contributions at the ILC with the center-of-mass energy of sqrt{s} = 250 GeV and 500 GeV. We further discuss observables for determination of the mass of the scalars. We find that the parameter regions which cannot be detected at the large hadron collider can be probed at the ILC.
    Physics Letters B 03/2013; 725(4-5). · 4.57 Impact Factor
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    ABSTRACT: We present search prospects and phenomenology of doubly resonant signals that come from the decay of a neutral weak-singlet color-octet vector state \omega_8 into a lighter weak-triplet color-octet scalar \pi_8, which can arise in several theories beyond the Standard Model. Taking m_{\omega_8}-m_{\pi_8}>m_W, we demonstrate an analysis of the signals pp \to \omega_8 \to \pi^\pm_8 W^\mp (\pi^0_8 Z) \to g W^\pm W^\mp (g Z Z). The present 8 TeV LHC run is found to have the potential to exclude or discover the signal for a range of masses and parameters. The preferred search channel has a boosted W-tagged jet forming a resonance with a second hard jet, in association with a lepton and missing energy.
    Physical review D: Particles and fields 08/2012; 86(7).
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    Hiroaki Sugiyama, Koji Tsumura, Hiroshi Yokoya
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    ABSTRACT: The doubly charged scalar boson (H^{\pm\pm}) is introduced in several models of the new physics beyond the standard model. The H^{\pm\pm} has Yukawa interactions with two left-handed charged leptons or two right-handed charged leptons depending on the models. We study kinematical properties of H^{\pm\pm} decay products through tau leptons in order to discriminate the chiral structures of the new Yukawa interaction. The chirality of tau leptons can be measured by the energy distributions of the tau decay products, and thus the chiral structure of the new Yukawa interaction can be traced in the invariant-mass distributions of the H^{\pm\pm} decay products. We perform simulation studies for the typical decay patterns of the H^{\pm\pm} with simple event selections and tau-tagging procedures, and show that the chiral structure of the Yukawa interactions of H^{\pm\pm} can be distinguished by measuring the invariant-mass distributions.
    Physics Letters B 07/2012; 717(s 1–3). · 4.57 Impact Factor
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    Shinya Kanemura, Koji Tsumura, Hiroshi Yokoya
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    ABSTRACT: We study the feasibility of the Type-X two Higgs doublet model (THDM-X) at collider experiments. In the THDM-X, new Higgs bosons mostly decay into tau leptons in the wide range of the parameter space. Such scalar bosons are less constrained by current experimental data, because of the suppressed quark Yukawa interactions. We discuss a search strategy of the THDM-X with multi-tau lepton final states at International linear collider and Large Hadron Collider. By using the collinear approximation, we show that a four tau lepton signature (e^+e^- -> HA -> 4\tau) can be a clean signal.
    01/2012;
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    Shinya Kanemura, Koji Tsumura, Hiroshi Yokoya
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    ABSTRACT: A detailed simulation study is performed for multi-tau-lepton signatures at the Large Hadron Collider, which can be used to probe additional Higgs bosons with lepton-specific Yukawa interactions. Such an extended Higgs sector is introduced in some of new physics models at the TeV scale. We here consider the two Higgs doublet model with the Type-X Yukawa interaction, where nonstandard Higgs bosons predominantly decay into tau leptons. These extra Higgs bosons can be pair produced via s-channel gauge boson mediation at hadron colliders; q \bar q \to Z \to HA and q \bar q' \to W^\pm \to HH^\pm (AH^\pm), where H, A and H^\pm are CP-even, odd and charged Higgs bosons, respectively. Consequently, multi-tau-lepton originated signals appear in the final state as a promising signature of such a model. We find that the main background can be considerably reduced by requiring the high multiplicity of leptons and tau-jets with appropriate kinematical cuts in the final state. Thus, assuming the integrated luminosity of a hundred of inverse fb, the excess can be seen in various three- and four-lepton channels. With the integrated luminosity of thousands of inverse fb, the determination of the mass as well as ratios of leptonic decay branching ratios of these Higgs bosons would also be possible.
    Physical review D: Particles and fields 11/2011; 85(9).
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    ABSTRACT: Current limits from the LHC on fourth generation quarks are already at the unitarity bound of 500 GeV or so. If they exist, the strong Yukawa couplings are turning nonperturbative, and may form bound states. We study the domain of mb′ and mt′ in the range of 500 to 700 GeV, where we expect binding energies are mainly of Yukawa origin, with QCD subdominant. To be consistent with electroweak precision tests, the t′ and b′ quarks have to be nearly degenerate, exhibiting a new “isospin.” Comparing relativistic expansion with a relativistic bound state approach, we find the most interesting is the production of a color octet, isosinglet vector meson (a “gluon-prime”) via qq̅ →ω8. Leading decay modes are π8±W∓, π80Z0, and constituent quark decay, with qq̅ and tt̅ ′ and bb̅ ′ subdominant. The color octet, isovector pseudoscalar π8 meson decays via constituent quark decay, or to Wg. These decay rates are parameterized by the decay constant, the binding energy and mass differences, and Vtb′. For small Vt′b, one could have a spectacular signal of WWg, where a soft W accompanies a very massive Wg pair. In general, however, one has high multiplicity signals with b, W, and t jet substructures that are not so different from the t′t̅ ′ and b′b̅ ′ search.
    Physical review D: Particles and fields 11/2011; 84(9).
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    Xiao-Gang He, German Valencia, Hiroshi Yokoya
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    ABSTRACT: We consider the phenomenology of CP violation in a color-octet extended scalar sector for t\bar{t} production and decay at the LHC. In particular we study the effect of the two neutral color-octet scalars S_I and S_R that occur in the model. There are two new sources of CP violation: a phase in the couplings of S_{I,R} to top-quarks; and two phases in the quartic couplings of the scalar potential. In resonant production of a single S_{I,R} followed by its decay into t\bar{t} pairs through the parton level process gg to S_{I,R} to t\bar{t}, we find large raw CP asymmetries which can reach 12%. These raw asymmetries are, of course, diluted by standard model (SM) t\bar{t} pairs making observation of CP violation contingent on whether the resonance itself can be extracted from the SM background.
    Journal of High Energy Physics 10/2011; · 5.62 Impact Factor
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    ABSTRACT: We propose a class of observables constructed from lepton energy distribution, which are independent of the velocity of the parent particle if it is scalar or unpolarized. These observables may be used to measure properties of various particles in the LHC experiments. We demonstrate their usage in a determination of the Higgs boson mass.
    Physics Letters B 07/2011; · 4.57 Impact Factor
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    ABSTRACT: The recent data on $W+$dijet excess reported by CDF may be interpreted as the associated production of a $W$ and a new particle of mass about 150 GeV which subsequently decays into two hadron jets. We study the possibility of explaining the $W+$dijet excess by colored scalar bosons. There are several colored scalars which can have tree level renormalizable Yukawa couplings with two quarks, $({\bf 8}, {\bf 2},1/2)$, $(\bar{\bf 6}({\bf 3}), {\bf 3}({\bf 1}), -1/3)$, $(\bar {\bf 6}({\bf 3}), {\bf 1}, -4/3(2/3))$. If one of these scalars has a mass about 150 GeV, being colored it can naturally explain why the excess only shows up in the form of two hadron jets. Although the required production cross section and mass put constraints on model parameters and rule out some possible scenarios when confronted with other existing data, in particular FCNC data, we find that there are strong constraints on the Yukawa couplings of these scalars. Without forcing the couplings to be some special texture forms most of the scalars, except the $({\bf 3}, {\bf 3}, -1/3)$, are in trouble with FCNC data. We also study some features for search of these new particles at the RHIC and the LHC and find that related information can help further to distinguish different models.
    Journal of High Energy Physics 05/2011; · 5.62 Impact Factor
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    ABSTRACT: We report on the feasibility of the direct measurement of the top Yukawa coupling g_t at the International Linear Collider (ILC) during its first phase of operation with a center-of-mass energy of 500 GeV. The signal and background models incorporate the non-relativistic QCD corrections which enhance the production cross section near the t-tbar threshold. The e+e- -> t tbar H signal is reconstructed in the 6-jet + lepton and the 8-jet modes. The results from the two channels are combined. The background processes considered are e+e- -> t bbar W- / tbar b W+ (which includes e+e- -> t tbar), e+e- -> t tbar Z, and e+e- -> t tbar g* -> t tbar b bar. We use a realistic fast Monte-Carlo detector simulation. Signal events are selected using event shape variables, through jet clustering, and by identifying heavy flavor jets. Assuming a Higgs mass of 120 GeV, polarized electron and positron beams with (Pe-,Pe+) = (-0.8,+0.3), and an integrated luminosity of 1 ab-1, we estimate that the e+e- -> t tbar H events can be seen with a statistical significance of 5.2 sigma, corresponding to the relative top Yukawa coupling measurement accuracy of |Delta g_t / g_t| = 10%.
    Physical review D: Particles and fields 04/2011; 84(1).
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    Yukinari Sumino, Hiroshi Yokoya
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    ABSTRACT: First we present a theoretical framework to compute the fully differential cross sections for the top-quark productions and their subsequent decays at hadron colliders, incorporating the bound-state effects which are important in the t\bar{t} threshold region. We include the bound-state effects such that the cross sections are correct in the LO approximation both in the threshold and high-energy regions. Then, based on this framework we compute various kinematical distributions of top quarks as well as of their decay products at the LHC, by means of Monte-Carlo event-generation. These are compared with the corresponding predictions based on conventional perturbative QCD. In particular, we find a characteristic bound-state effect on the (bW^+)-(\bar{b}W^-) double-invariant-mass distribution, which is deformed to the lower invariant-mass side in a correlated manner.
    Journal of High Energy Physics 07/2010; 2010(9). · 5.62 Impact Factor
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    Kaoru Hagiwara, Hiroshi Yokoya
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    ABSTRACT: We study bound-state effects on the pair production of gluinos at hadron colliders, in a context of the minimal supersymmetric extension of the standard model. Due to the expected large mass and the octet color-charge of gluinos, the bound-state effects can be substantial at the LHC. We find significant deformation of the invariant-mass distributions of a gluino-pair near the mass threshold, as well as an additional correction to the total cross-section. Both the invariant-mass distribution and the correction to the total cross section depend crucially on the decay width of the gluino. Comment: 17 pages, 6 figures; references added, typos corrected
    Journal of High Energy Physics 09/2009; · 5.62 Impact Factor
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    Kaoru Hagiwara, Yukinari Sumino, Hiroshi Yokoya
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    ABSTRACT: We study bound-state effects on the production cross section in the threshold region at hadron colliders. The bound-state effects are important particularly at the LHC where the gluon fusion is the dominant subprocess. Due to the formation of resonances in the J=0 color-singlet channel of and the large width of the top quark, the invariant-mass distribution peaks at a few GeV below the threshold, and it is significantly enhanced over the naive NLO prediction until several GeV above the threshold. We present predictions of the invariant-mass distribution which incorporate both the bound-state effects and initial-state radiations up to NLO. The bound-state effects would lead to a substantial deformation of top-quark kinematical distributions in the threshold region.
    Physics Letters B 01/2008; · 4.57 Impact Factor
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    ABSTRACT: We study the angular distribution of the charged lepton in the top-quark decay into a bottom quark and a W boson which subsequently decays into \ell\nu_{\ell}, when a hard gluon is radiated off. The absorptive part of the t \to bWg decay amplitudes, which gives rise to T-odd asymmetries in the distribution, is calculated at the one-loop level in perturbative QCD. The asymmetries at a few percent level are predicted, which may be observable at future colliders. Comment: 21 pages, 9 figures; discussions for the decay of polarized top-quarks added in a new chapter, references added, version to appear in JHEP
    Journal of High Energy Physics 07/2007; · 5.62 Impact Factor
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    Hiroshi Yokoya
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    ABSTRACT: We study the single longitudinal-spin asymmetries in lepton-pair production with large transverse-momentum at RHIC and J-PARC experiments. The asymmetries in the azimuthal angular distribution of a lepton can arise from an absorptive part of production amplitudes. We revisit the one-loop calculation for the absorptive part of production amplitudes in perturbative QCD, and show that the asymmetries can be sizable at RHIC and J-PARC. Measurement of the asymmetries would test the one-loop prediction for the scattering phase of this process, and provide support for a study of the single transverse-spin asymmetries in the same kinematical region. Comment: 7 pages PTPTex, 4 figures
    Progress of Theoretical Physics 05/2007; · 2.48 Impact Factor
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    Kaoru Hagiwara, Ken-Ichi Hikasa, Hiroshi Yokoya
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    ABSTRACT: Parity-odd asymmetries in the decay angular distribution of a W boson produced with a hard jet in pp[over ] collisions arise only from QCD rescattering effects. If observed, these asymmetries will provide a first demonstration that perturbative QCD calculation is valid for the absorptive part of scattering amplitudes. We propose a simple observable to measure these asymmetries and perform realistic Monte Carlo simulations at energies reached at the Fermilab Tevatron. It is shown that the Tevatron run II should provide sufficient statistics to test the prediction.
    Physical Review Letters 01/2007; 97(22):221802. · 7.94 Impact Factor
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    ABSTRACT: 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.
    12/2006;
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    Hiroshi Yokoya, Werner Vogelsang
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    ABSTRACT: We present studies of QCD corrections to dilepton production in transversely polarized pp and \bar{p}p scattering. In particular we briefly discuss the effects of NNLL threshold resummation on the rapidity distribution of the lepton pair.
    08/2006;

Publication Stats

125 Citations
78 Downloads
1k Views
83.28 Total Impact Points

Institutions

  • 2013
    • University of Toyama
      Тояма, Toyama, Japan
  • 2011–2012
    • National Taiwan University
      • Department of Physics
      T’ai-pei, Taipei, Taiwan
  • 2007–2011
    • CERN
      • Physics Department (PH)
      Genève, Geneva, Switzerland
  • 2006–2007
    • Niigata University
      • Department of Physics
      Niahi-niigata, Niigata, Japan
    • Stony Brook University
      • Institute for Theoretical Physics (C.N. Yang)
      Stony Brook, New York, United States
  • 2003–2005
    • Hiroshima University
      Hirosima, Hiroshima, Japan
    • RIKEN
      Вако, Saitama, Japan