Ken Sasaki

QCD higher order effects in spin-dependent deep inelastic electroproduction

QCD effects beyond the leading logarithmic approximation are considered for polarized deep inelastic electroproduction. The corrections of the order g2 to the coefficient functions in the Wilson expansion are calculated by evaluating the Green function for the current product as well as those for the composite operators. We shall clarify how to normalize the composite operators and discuss the renormalization-scheme dependence. It is pointed out that the Q2 dependence is different between the massless and massive quark models.

Dynamical symmetry breaking and the top quark mass in the minimal supersymmetric standard model

The minimal supersymmetric extension of the standard model is studied, when the electroweak symmetry breakdown relies on the formation of condensates involving the third generation of quarks and their supersymmetric partners. The top quark mass is obtained as a function of the compositeness scale Λ and the soft supersymmetry breaking scale, ΔS. The dependence of the top quark mass on the ration of the Higgs vacuum expectation values, as well as a function of the lightest Higgs mass, is analyzed. We show that, when Λ ≅ 1016 GeV, the characteristics top quark mass in this model is 140 GeV ⩽ mt ⩽ 195 Gev, a prediction that is only slightly dependent on the value of ΔS.

Renormalization-group analysis of the Higgs sector in the minimal supersymmetric standard model

We examine the particle spectra of the Higgs sector in the minimal supersymmetric extension of the standard model using the one-loop renormalization group evolution of the scalar self-couplings. We treat the general case in which there are two light Higgs doublets below the soft supersymmetry breaking scale MSUSY. The model with only one light CP-even neutral scalar particle appears as a particular limiting case in our analysis. Using theoretical and experimental results, we obtain upper and lower bounds on the neutral CP-even and charged Higgs mass spectrum. The triviality bounds on the top and Higgs masses are also analyzed, yielding constraints on the range of mass parameters consistent with possible grand unification scenarios. We obtain that for an effective cutoff scale Λ ⋍ 1016 GeV (1010 Gev), a soft supersymmetry breaking scale MSUSY ⋍ 1 TeV and a CP-odd state mass mA ⋍ 40 GeV, the lightest CP-even state mass is bounded to be mh ⩽ 52 GeV (60 GeV). In addition, for the same values of Λ and MSUSY, and for mA ⩾ MZ, the tree-level relation mh ⩽ mA is fulfilled. Our analysis indicates that, in the simplest grand unification scenarios with MGUT ⋍ 1016 GeV and MSUSY ⋍ 1 TeV, the decay mode h → AA is only allowed if mA ⩽ 26 GeV.

Background field method: Alternative way of deriving the pinch technique's results.

We show that the background field method (BFM) is a simple way of deriving the same gauge-invariant results which are obtained by the pinch technique (PT). For illustration we construct gauge-invariant selfenergy and three-point vertices for gluons at one-loop level by BFM and demonstrate that we get the same results which were derived via PT. We also calculate the four-gluon vertex in BFM and show that this vertex obeys the same Ward identity that was found with PT.

Mass of the lightest supersymmetric Higgs boson beyond the leading logarithm approximation.

We examine the radiative corrections to the mass of the lightest Higgs boson in the minimal supersymmetric extension of the standard model. We use the renormalization-group-improved effective potential which includes the next-to-leading-order contributions. We find that the higher-order corrections to the lightest Higgs boson mass are non-negligible, adding 3--11 GeV (3--9 GeV) to the result in the leading logarithm approximation for the range of top quark mass 100 GeV [lt][ital m][sub [ital t]][lt]200 GeV and for the supersymmetry-breaking scale [ital M][sub SUSY]=1 TeV ([ital M][sub SUSY]=10 TeV). Also we find that our result is stable under the change of the renormalization parameter [ital t].

Renormalization Group Equations for the {Kobayashi-Maskawa} Matrix

The renormalization group equations for the parametrization-convention independent quadratic parameters |Vij|2 of the KM matrix are derived. Numerical analysis of these equations shows that the heavy quark family (t, b) tends to mix with the lighter families (c, s) and (u, d) with increasing energy, although the variation is very much slow. The CP-nonconservation effects are shown to get larger with energy.

Spin Structure Function of the Virtual Photon Beyond the Leading Order in QCD

Polarized photon structure can be studied in the future polarized e + e − colliding-beam experiments. We investigate the spin-dependent structure function of the virtual photon g 1(x, Q 2 , P 2 ), in perturbative QCD for � 2 ≪ P 2 ≪ Q 2 , where −Q 2 (−P 2 ) is the mass squared of the probe (target) photon. The analysis is performed to next-to-leading order in QCD. We particularly emphasize the renormalization scheme independence of the result.The non-leading corrections significantly modify the leading log result, in particular, at large x as well as at small x. We also discuss the non-vanishing first moment sum rule of g 1 , where O(αs) corrections are computed.

Polarized parton distributions in the photon and factorization scheme dependence

Abstract Spin-dependent parton distributions in the polarized virtual photon are investigated in QCD up to the next-to-leading order (NLO). In the case Λ 2 ≪ P 2 ≪ Q 2 , where − Q 2 (− P 2 ) is the mass squared of the probe (target) photon, parton distributions can be predicted completely up to the NLO, but they are factorization-scheme-dependent. Parton distributions are analyzed in four different factorization schemes and their scheme dependence are discussed. Particular attentions are paid to the axial anomaly effect on the first moments of quark parton distributions, and also to the large- x behaviors of the parton distributions. Gluon distribution in the virtual photon is found to be factorization-scheme independent up to the NLO.

Motion of the Tippe Top: Gyroscopic Balance Condition and Stability ∗

We reexamine avery classical problem, the spinning behavior of the tippe top on a horizontal table. The analysis is made for an eccentric sphere version of the tippe top, assuming a modified Coulomb law for the sliding friction, which is a continuous function of the slip velocity

Muonium-antimuonium conversion in models with dilepton gauge bosons.

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