arXiv subjects

Ken Sasaki

Publications and source records attributed to Ken Sasaki.

At least 37 records · Page 2Linked to original sources

arXiv2002

Twist-3 effects for polarized virtual photon structure function $g_2^γ$

We investigate twist-3 effects in the polarized virtual photon. The structure function $g_2^γ$, which exists only for the virtual photon target and can be measured in future polarized $e^+ e^-$ collider experiments, receives both twist-2 and twist-3 contributions. The twist-3 part is analyzed in pure QED interaction as well as in LO QCD. We find the twist-3 contribution is appreciable for the photon in contrast to the nucleon case.

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arXiv2002

Virtual photon structure functions and positivity constraints

We study the three positivity constraints among the eight virtual photon structure functions, derived from the Cauchy-Schwarz inequality and which are hence model-independent. The photon structure functions obtained from the simple parton model show quite different behaviors in a massive quark or a massless quark case, but they satisfy, in both cases, the three positivity constraints. We then discuss an inequality which holds among the unpolarized and polarized photon structure functions $F_1^γ$, $g_1^γ$ and $W_{TT}^τ$, 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, and we examine whether this inequality is satisfied by the perturbative QCD results.

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arXiv2002

Twist-3 Effects in Polarized Photon Structure

The polarized photon structure is described by two spin structure functions $g_1^γ$ and $g_2^γ$ which can be studied in the future polarized ep or e$^+$e$^-$ colliders. Here we investigate the QCD twist-3 effects in $g_2^γ$ to the leading order in QCD.

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arXiv2002

Polarized Photon Structure: $g_1^γ$ and $g_2^γ$

We investigate the polarized photon structure functions $g_1^γ$ and $g_2^γ$ which can be studied in the future polarized version of ep or e$^+$e$^-$ colliders. The NLO QCD calculations of $g_1^γ$ and the possible twist-3 effects in $g_2^γ$ are discussed.

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arXiv2002

Polarized Virtual Photon Structure Function $g_2^γ$ and Twist-3 Effects in QCD

We investigate the twist-3 effects in the polarized virtual photon structure. The structure functions $g_1^γ$ and $g_2^γ$ of polarized photon could be experimentally studied in the future polarized $ep$ or $e^+e^-$ colliders. The leading contributions to $g_1^γ$ are the twist-2 effects, while another structure function $g_2^γ$, which only exists for the virtual photon target, receives not only the twist-2 but also twist-3 contributions. We first show that the twist-3 effects actually exist in the box-diagram contributions and we extract the twist-3 part, which can also be reproduced by the pure QED operator product expansion. We then calculate the non-trivial lowest moment ($n=3$) of the twist-3 contribution to $g_2^γ$ in QCD. For large $N_c$ (the number of colors), the QCD analysis of the twist-3 effects in the flavor nonsinglet part of $g_2^γ$ becomes tractable and we can obtain its moments in a compact form for all $n$.

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arXiv2001

Zee model and phenomenology of lepton sector

The virtual effects of the Zee charged scalar boson on the lepton-family-number (LFN) violating processes are studied. We obtain the constraints on the individual Yukawa coupling constants of the Zee boson to leptons. Using these constraints, we predict the upper bounds on the muonium-antimuonium conversion probability, the branching fractions of the LFN violating decays such as $τ\to eγ$, $τ\to μγ$, $τ^- \to μ^+e^-e^-$ and $τ\to e^+μ^-μ^-$. The contribution of the Zee boson to the muon anomalous magnetic moment is also considered

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arXiv2000

Factorization Scheme and Parton Distributions in the Polarized Virtual Photon Target

We investigate spin-dependent parton distributions in the polarized virtual photon target in perturbative QCD up to the next-to-leading order (NLO). 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 NLO, but they are factorization-scheme-dependent.We analyze parton distributions in six different factorization schemes and discuss their scheme dependence. We study, in particular, the QCD and QED axial anomaly effects on the first moments of parton distributions to see the interplay between the axial anomalies and factorization schemes. We also show that the factorization-scheme dependence is characterized by the large-$x$ behaviors of quark distributions in the virtual photon. Gluon distribution is predicted to be the same up to NLO among the six factorization schemes examined. In particular, the first moment of gluon distribution is found to be factorization-scheme independent up to NLO.

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arXiv2000

Polarized and Unpolarized Structures of the Virtual Photon

We discuss the structure functions and the parton distributions in the virtual photon target, both polarized and unpolarized, beyond the leading order in QCD. We study the factorization-scheme dependence of the parton distributions.

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arXiv2000

Parton Distributions in the Virtual Photon Target and Factorization Scheme Dependence

We investigate parton distributions in the virtual photon target, both polarized and unpolarized, up to the next-leading order (NLO) in QCD. Parton distributions can be predicted completely up to NLO, but they are factorization-scheme-dependent. We analyze parton distributions in several factorization schemes and discuss their scheme dependence. Particular attentions are paid to the axial anomaly effect on the first moments of the polarized quark parton distributions, and also to the large-$x$ behaviors of polarized and unpolarized parton distributions.

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arXiv1999

Polarized Parton Distributions in the Photon and Factorization Scheme Dependence

Spin-dependent parton distributions in the polarized virtual photon are investigated in QCD up to the next-to-leading order (NLO). In the case $Λ^2 \ll P^2 \ll Q^2$, where $-Q^2$ ($-P^2$) is the mass squared of the probe (target) photon, parton distributions can be predicted completely up to the 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.

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arXiv1999

Spin structure function of the virtual photon

We investigate the spin structure of the virtual photon beyond the leading order in QCD. The first moment of the virtual photon spin structure function $g_1^γ(x,Q^2,P^2)$ with QCD effects turns out to be non-vanishing in contrast to the real photon case. Numerical analysis for virtual as well as real photon case is presented.

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arXiv1998

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 $g_1^γ(x,Q^2,P^2)$, in perturbative QCD for $Λ^2 \ll P^2 \ll Q^2$, 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 $g_1^γ$, where ${\cal O}(α_s)$ corrections are computed.

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arXiv1998

Polarized Virtual Photon Structure Function in the Next-to-leading Order in QCD

The virtual photon structure function $g_1^γ(x,Q^2,P^2)$, which can be obtained in polarized $e^{+}e^{-}$ colliding-beam experiments, is investigated for $Λ^2 \ll P^2 \ll Q^2$, where $-Q^2$ ($-P^2$) is the mass squared of the probe (target) photon. The analysis is made to next-to-leading order in QCD, in the framework of the QCD improved parton model with the DGLAP evolution equations. The non-leading corrections significantly modify the leading log result, in particular, at large $x$ as well as at small $x$.

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arXiv1998

Spin Structure Function g_2(x,Q^2) and Twist-3 Operators in large-N_C QCD

It is shown in the framework of the operator product expansion and the renormalization group method that the twist-3 part of flavour nonsinglet spin structure function g_2(x,Q^2) obeys a simple Dokshitzer-Gribov- Lipatov-Altarelli-Parisi (DGLAP) equation in the large N_C limit even in the case of massive quarks (N_C is the number of colours). There are four different types of twist-3 operators which contribute to g_2, including quark-mass-dependent operators and the ones proportional to the equation of motion. They are not all independent but are constrained by one relation. A new choice of the independent operator bases leads to a simple form of the evolution equation for g_2 at large N_C.

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arXiv1996

Gauge-independent resummed gluon self-energy in hot QCD

The pinch technique (PT) is applied to obtain the gauge-independent resummed gluon self-energy in a hot Yang-Mills gas. Calculation is performed at one- loop level in linear gauges which preserve rotational invariance and the res ummed propagators and vertices are used. The effective gluon self-energy, w hich is obtained as the sum of the resummed gluon self-energy and the resumm ed pinch contributions, is not only gauge-independent but also satisfies the transversality relation. Using this gauge-independent effective gluon self- energy, we calculate the damping rate for transverse gluons in the leading o rder and show that the result coincides with the one obtained by Braaten and Pisarski.

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arXiv1996

Gauge-independent Thermal $β$ Function In Yang-Mills Theory via the Pinch Technique

It is proposed to use the pinch technique (PT) to obtain the gauge-independent thermal $β$ function $β_T$ in a hot Yang-Mills gas. Calculations of the thermal $β$ function are performed at one-loop level in four different gauges, (i) the background field method with an arbitrary gauge parameter, (ii) the Feynman gauge, (iii) the Coulomb gauge, and (iv) the temporal axial gauge. When the pinch contributions to the gluon self-energy are included, the same result is derived for $β_T$ in all four cases.

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arXiv1995

Gauge-independent Thermal $β$ Function In Yang-Mills Theory

It is proposed to use the pinch technique (PT) to obtain the gauge-independent thermal $β$ function in a hot Yang-Mills gas. Calculations of the thermal $β$ function are performed at one-loop level in four different gauges, (i) the background field method with an arbitrary gauge, (ii) the Feynman gauge, (iii) the Coulomb gauge, and (iv) the temporal axial gauge, and they yield the same result in all four cases.

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arXiv1995

Two-loop electroweak corrections to the $ρ$ parameter beyond the leading approximation

We show that in the framework of the pinch technique the universal part of the $ρ$ parameter can be meaningfully defined, beyond one loop. The universal part so obtained satisfies the crucial requirements of gauge-independence, finiteness, and process-independence, even when subleading contributions of the top quark are included. The mechanism which enforces the aforementioned properties is explained in detail, and several subtle field theoretical issues are discussed. Explicit calculations of the sub-leading two-loop corrections of order $O(G_μ^{2}m^{2}_{t}M_{Z}^{2})$ are carried out in the context of an $SU(2)$ model, with $M_{W}=M_{Z}$, and various intermediate and final results are reported.

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