arXiv subjects

Ken Sasaki

Publications and source records attributed to Ken Sasaki.

At least 19 recordsLinked to original sources

arXiv2021

Timelike Transition Form Factor for CP-odd Higgs Boson Production

We investigate the production of CP-odd Higgs boson $A^0$ associated with a real $γ$ in $e^+e^-$ collisions. Because of the properties of $A^0$ coupling to the other fields, the main contribution comes from the top-quark triangle loop diagrams. We obtain the timelike transition form factor which describes the production amplitude of $A^0$ in the $e^+e^-$ collisions. This timelike transition form factor is related to the spacelike transition form factor relevant for the $A^0$ production in $e^-γ$ collisions. It turns out that the possible extra contributions from chargino-sneutrino and neutralino-selectron box diagrams do not give sizable effects.

hep-ph↗

arXiv2020

Electroweak fermion triangle loop contributions to the muon anomalous magnetic moment revisited

The contribution to the muon anomalous magnetic moment from the fermion triangle loop diagrams connected to the muon line by a photon and a $Z$ boson is reanalyzed in the unitary gauge. With use of the anomalous axial-vector Ward identity, it is shown that the calculation in the unitary gauge exactly coincides with the one in the 't Hooft-Feynman gauge. The part which arises from the ordinary axial-vector Ward identity corresponds to the contribution of the neutral Goldstone boson. For the top-quark contribution, the one-parameter integral form is obtained up to the order of $m_μ^2/m_Z^2$. The results are compared with those obtained by the asymptotic expansion method.

hep-ph↗

arXiv2018

PHOTON-2017 conference proceedings

This document collects the proceedings of the PHOTON 2017 conference ("International Conference on the Structure and the Interactions of the Photon", including the 22th "International Workshop on Photon-Photon Collisions", and the "International Workshop on High Energy Photon Colliders") held at CERN (Geneva) in May 2017. The latest experimental and theoretical developments on the topics of the PHOTON conference series are covered: (i) $γ\,γ$ processes in e$^+$e$^-$, proton-proton (pp) and nucleus-nucleus (AA) collisions at current and future colliders, (ii) $γ$-hadron interactions in e$^\pm$p, pp, and AA collisions, (iii) final-state photon production (including Standard Model studies and searches beyond it) in pp and AA collisions, and (iv) high-energy $γ$-ray astrophysics. These proceedings are dedicated to the memory of Maria Krawczyk.

hep-ph↗

arXiv2018

CP-odd Higgs Boson Production in $eγ$ Collisions

We investigate the CP-odd Higgs boson production via two-photon processes in $eγ$ collisions. The CP-odd Higgs boson, which we denote as $A^0$, is expected to appear in the Two-Higgs Doublet Models (2HDM) as a minimal extension of Higgs sector for which the Minimal Supersymmetric Standard Model (MSSM) is a special case. The scattering amplitude for $eγ\rightarrow eA^0$ is evaluated at the electroweak one-loop level. The dominant contribution comes only from top-quark loops when $A^0$ boson is rather light and $\tanβ$ is not large. There are no contributions from the $W$-boson and $Z$-boson loops nor from the scalar top-quark (stop) loops. The differential cross section for the $A^0$ production is analysed.

hep-ph↗

arXiv2014

Higgs production in $e$ and real $γ$ collision

We investigate the Standard Model Higgs boson production in $e^-γ$ collision. The electroweak one-loop contributions to the scattering amplitude for $e^-γ\rightarrow e^-H$ are calculated and expressed in analytical form. We analyze the cross section for the Higgs production in $e^-γ$ collision for each combination of polarizations of the initial electron and photon beams. The feasibility of observing the Higgs boson in $e^-+γ\rightarrow e^-+b+{\overline b}$ channel is examined.

hep-ph↗

arXiv2013

Higgs Production in Two-Photon Process and Transition Form Factor

The Higgs production in the two-photon fusion process is investigated where one of the photons is off-shell while the other one is on-shell. This process is realized in either electron-positron collision or electron-photon collision where the scattered electron or positron is detected (single tagging) and described by the transition form factor. We calculate the contributions to the transition form factor of the Higgs boson coming from top-quark loops and W-boson loops. We then study the $Q^2$ dependence of each contribution to the total transition form factor and also of the differential cross section for the Higgs production.

hep-ph↗

arXiv2013

The real photon structure functions in massive parton model in NLO

We investigate the one-gluon-exchange ($αα_s$) corrections to the real photon structure functions $W_{TT} $, $W_{LT}$, $W_{TT}^{a} $ and $W_{TT}^τ$ in the massive parton model. We employ a technique based on the Cutkosky rules and the reduction of Feynman integrals to master integrals. We show that a positivity constraint, which is derived from the Cauchy-Schwarz inequality, is satisfied among the unpolarized and polarized structure functions $W_{TT}$, $W_{TT}^a$ and $W_{TT}^τ$ calculated up to the next-to-leading order in QCD.

hep-ph↗

arXiv2012

The polarized photon structure function $g_1^γ(x,Q^2)$ in massive parton model in NLO

We investigate the one-gluon-exchange ($αα_s$) corrections to the polarized real photon structure function $g_1^γ(x,Q^2)$ in the massive parton model. We employ a technique based on the Cutkosky rules and the reduction of Feynman integrals to master integrals. The NLO contribution is noticeable at large $x$ and does not vanish at the threshold of the massive quark pair production due to the Coulomb singularity. It is found that the first moment sum rule of $g_1^γ$ is satisfied up to the NLO.

hep-ph↗

arXiv2010

Heavy quark effects on parton distribution functions in the unpolarized virtual photon up to the next-to-leading order in QCD

We investigate the heavy quark mass effects on the parton distribution functions in the unpolarized virtual photon up to the next-to-leading order in QCD. Our formalism is based on the QCD-improved parton model described by the DGLAP evolution equation as well as on the operator product expansion supplemented by the mass-independent renormalization group method. We evaluate the various components of the parton distributions inside the virtual photon with the massive quark effects, which are included through the initial condition for the heavy quark distributions, or equivalently from the matrix element of the heavy quark operators. We discuss some features of our results for the heavy quark effects and their factorization-scheme dependence.

hep-ph↗

arXiv2009

Parton distributions in the virtual photon target up to NNLO in QCD

Parton distributions in the virtual photon target are investigated in perturbative QCD up to the next-to-next-to-leading order (NNLO). In the case $Λ^2 \ll P^2 \ll Q^2$, where $-Q^2$ ($-P^2$) is the mass squared of the probe (target) photon, parton distributions can be predicted completely up to the NNLO, but they are factorisation-scheme-dependent. We analyse parton distributions in two different factorisation schemes, namely $\bar{\rm MS}$ and ${\rm DIS}_γ$ schemes, and discuss their scheme dependence. We show that the factorisation-scheme dependence is characterised by the large-$x$ behaviours of quark distributions. Gluon distribution is predicted to be very small in absolute value except in the small-$x$ region.

hep-ph↗

arXiv2008

Heavy Quark Effects in the Virtual Photon Structure Functions

We investigate the heavy quark mass effects in the virtual photon structure functions $F_{2}^γ(x, Q^2, P^2)$ and $F_{L}^γ(x, Q^2, P^2)$ in the framework of the mass-independent renormalization group equation (RGE). We study a formalism in which the heavy quark mass effects are treated based on parton picture as well as on the operator product expansion (OPE), and perform the numerical evaluation of $F_{\rm eff}^γ(x, Q^2, P^2)$ to the next-leading order (NLO) in QCD.

hep-ph↗

arXiv2008

NNLO QCD analysis of the virtual photon structure functions

The next-to-next-to-leading order (NNLO) QCD analysis is performed for the virtual photon structure functions which can be measured in the double-tag events in two-photon processes in $e^+e^-$ collisions. We investigate the perturbative QCD evaluation of $F_2^γ(x,Q^2,P^2)$ to NNLO and $F_L^γ(x,Q^2,P^2)$ to NLO with and without taking into account the target mass effects, which are relevant for the large $x$ region. We also carry out the phenomenological analysis for the experimentally accessible effective structure function $F_{\rm eff}^γ=F_2^γ+(3/2)F_L^γ$.

hep-ph↗

arXiv2008

Virtual Photon Structure Functions to NNLO in QCD

The unpolarized virtual photon structure functions $F_2^γ(x,Q^2,P^2)$ and $F_L^γ(x,Q^2,P^2)$ are investigated in perturbative QCD for the kinematical region $Λ^2 \ll P^2 \ll Q^2$, where $-Q^2(-P^2)$ is the mass squared of the probe (target) photon and $Λ$ is the QCD scale parameter. In the framework of operator product expansion supplemented by the renormalization group method, the definite predictions are derived for the moments of $F_2^γ(x,Q^2,P^2)$ up to the next-to-next-to-leading order (the order $αα_s$) and for the moments of $F_L^γ(x,Q^2,P^2)$ up to the next-to-leading order (the order $αα_s$)

hep-ph↗

arXiv2008

Target Mass Corrections for the Virtual Photon Structure Functions to the Next-to-next-to-leading Order in QCD

We investigate target mass effects in the unpolarized virtual photon structure functions $F_2^γ(x,Q^2,P^2)$ and $F_L^γ(x,Q^2,P^2)$ in perturbative QCD for the kinematical region $Λ^2 \ll P^2 \ll Q^2$, where $-Q^2(-P^2)$ is the mass squared of the probe (target) photon and $Λ$ is the QCD scale parameter. We obtain the Nachtmann moments for the structure functions and then, by inverting the moments, we get the expressions in closed form for $F_2^γ(x,Q^2,P^2)$ up to the next-to-next-to-leading order and for $F_L^γ(x,Q^2,P^2)$ up to the next-to-leading order, both of which include the target mass corrections. Numerical analysis exhibits that target mass effects appear at large $x$ and become sizable near $x_{\rm max}(=1/(1+\frac{P^2}{Q^2}))$, the maximal value of $x$, as the ratio $P^2/Q^2$ increases.

hep-ph↗

arXiv2005

Motion of the Tippe Top : Gyroscopic Balance Condition and Stability

We reexamine a very 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 $\vec v_P$ at the point of contact and vanishes at $\vec v_P=0$. We study the relevance of the gyroscopic balance condition (GBC), which was discovered to hold for a rapidly spinning hard-boiled egg by Moffatt and Shimomura, to the inversion phenomenon of the tippe top. We introduce a variable $ξ$ so that $ξ=0$ corresponds to the GBC and analyze the behavior of $ξ$. Contrary to the case of the spinning egg, the GBC for the tippe top is not fulfilled initially. But we find from simulation that for those tippe tops which will turn over, the GBC will soon be satisfied approximately. It is shown that the GBC and the geometry lead to the classification of tippe tops into three groups: The tippe tops of Group I never flip over however large a spin they are given. Those of Group II show a complete inversion and the tippe tops of Group III tend to turn over up to a certain inclination angle $θ_f$ such that $θ_f<π$, when they are spun sufficiently rapidly. There exist three steady states for the spinning motion of the tippe top. Giving a new criterion for stability, we examine the stability of these states in terms of the initial spin velocity $n_0$. And we obtain a critical value $n_c$ of the initial spin which is required for the tippe top of Group II to flip over up to the completely inverted position.

physics.class-ph↗

arXiv2004

Mass effects in the polarized virtual photon structure

We discuss target mass effects in the polarized virtual photon structure functions $g_1^γ(x,Q^2,P^2)$, $g_2^γ(x,Q^2,P^2)$ for the kinematic region $Λ^2\ll P^2 \ll Q^2$, where $-Q^2 (-P^2)$ is the mass squared of the probe (target) photon. We obtain the expressions for the structure functions in closed form by inverting the Nachtmann moments for the twist-2 and twist-3 operators. Numerical analysis shows that target mass effects appear at large $x$ and become sizable near the maximal value of $x$, as the ratio $P^2/Q^2$ increases. Target mass effects for the QCD sum rules of $g_1^γ$ and $g_2^γ$ are also investigated.

hep-ph↗

arXiv2003

Spinning eggs--which end will rise?

We examine the spinning behavior of egg-shaped axisymmetric bodies whose cross sections are described by several oval curves similar to real eggs with thin and fat ends. We use the gyroscopic balance condition of Moffatt and Shimomura and analyze the slip velocity of the bodies at the point of contact as a function of $θ$, the angle between the axis of symmetry and the vertical axis, and find the existence of the critical angle $θ_c$. When the bodies are spun with an initial angle $θ_{\rm initial}>θ_c$, $θ$ will increase to $π$, implying that the body will spin at the thin end. Alternatively, if $θ_{\rm initial}<θ_c$, then $θ$ will decrease. For some oval curves, $θ$ will reduce to 0 and the corresponding bodies will spin at the fat end. For other oval curves, a fixed point at $θ_f$ is predicted, where $0 <θ_f< θ_c$. Then the bodies will spin not at the fat end, but at a new stable point with $θ_f$. The empirical fact that eggs more often spin at the fat than at the thin end is explained.

physics.class-ph↗

arXiv2003

Target Mass Effects in Polarized Virtual Photon Structure Functions

We study target mass effects in the polarized virtual photon structure functions $g_1^γ(x,Q^2,P^2)$, $g_2^γ(x,Q^2,P^2)$ in the kinematic region $Λ^2\ll P^2 \ll Q^2$, where $-Q^2 (-P^2)$ is the mass squared of the probe (target) photon. We obtain the expressions for $g_1^γ(x,Q^2,P^2)$ and $g_2^γ(x,Q^2,P^2)$ in closed form by inverting the Nachtmann moments for the twist-2 and twist-3 operators. Numerical analysis shows that target mass effects appear at large $x$ and become sizable near $x_{\rm max}(=1/(1+\frac{P^2}{Q^2}))$, the maximal value of $x$, as the ratio $P^2/Q^2$ increases. Target mass effects for the sum rules of $g_1^γ$ and $g_2^γ$ are also discussed.

hep-ph↗