arXiv subjects

Kaoru Hagiwara

Publications and source records attributed to Kaoru Hagiwara.

89 records · Page 5Linked to original sources

arXiv2000

Higgs, SUSY and the Standard Model at $γγ$ Colliders

In this report I surveyed physics potential of the gamma-gamma option of a Linear e+e- Collider with the following questions in mind: What new discovery can be expected at a gamma-gamma collider in addition to what will be learned at its `parent' e+e- Linear Collider? By taking account of the hard energy spectrum and polarization of colliding photons, produced by Compton back-scattering of laser light off incoming e- beams, we find that a gamma-gamma collider is most powerful when new physics appears in the neutral spin-zero channel at an invariant mass below about 80% of the c.m. energy of the colliding e-e- system. If a light Higgs boson exists, its properties can be studied in detail, and if its heavier partners or a heavy Higgs boson exists in the above mass range, they may be discovered at a gamma-gamma collider. CP property of the scalar sector can be explored in detail by making use of linear polarization of the colliding photons, decay angular correlations of final state particles, and the pattern of interference with the Standard model amplitudes. A few comments are given for SUSY particle studies at a gamma-gamma collider, where a pair of charged spinless particles is produced in the s-wave near the threshold. Squark-onium may be discovered. An e^\pm gamma collision mode may measure the Higgs-Z-gamma coupling accurately and probe flavor oscillations in the slepton sector. As a general remark, all the Standard Model background simulation tools should be prepared in the helicity amplitude level, so that simulation can be performed for an arbitrary set of Stokes parameters of the incoming photon beams.

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arXiv2000

One-loop contributions of the super-partner particles to $e^-e^+ \to W^-W^+$ in the MSSM

One-loop contributions of super-partner particles to $W$-pair production at $e^+e^-$ collision are discussed in the MSSM. To obtain trustworthy results we test our calculation using three methods: (1) sum rules among form factors which result from the BRS invariance, (2) the decoupling theorem, (3) the high-energy stability. We examine the corrections taking into account constraints from the direct search experiments and the precision data. The results for the sfermion contributions are presented.

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arXiv2000

One-loop sfermion corrections to $e^-e^+ \to W^-W^+$ in the MSSM

We study one-loop effects of sfermions on helicity amplitudes for $\eeww$ in the Minimal Supersymmetric Standard Model. The one-loop contributions are calculated in the $\bar{\rm MS}$ renormalization scheme. In order to verify the validity of the analytic calculation and the numerical program, the following tests are performed. (i) The BRS sum rules hold exactly among the analytic expressions of the form factors of the $\eeww$ amplitude and those of the amplitudes where the external $W^\pm$ bosons are replaced by the corresponding Goldstone bosons $χ^\pm$, hence they hold within the expected accuracy of the numerical program. (ii) The one-loop sfermion contribution to the amplitudes decouple in the heavy mass limit. This property is used to test the overall normalization of the amplitudes. In order to observe the analytically exact decoupling, the amplitudes are expanded by the $\bar{\rm MS}$ couplings of the Standard Model. (iii) The high-energy analytic formulas of the helicity amplitudes, which are verified by using the equivalence theorem analytically, are used for the numerical test of the high energy behavior of the amplitudes. We then investigate the magnitude of the one-loop effects on each helicity amplitude which may be measured by experiments at future linear colliders. Under the constraint from available precision data, the one-loop corrections to a few helicity amplitudes (for example, the longitudinal-$W$-pair production) can be at most from -0.8% to $+0.6$% in magnitude in the observables. The corrections in the helicity-summed cross sections are smaller, typically a few times $\pm 0.1$% at large scattering angles.

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arXiv2000

Measuring the Higgs CP Property through Top Quark Pair Production at Photon Linear Colliders

We present a model-independent study of the effects of a neutral Higgs boson without definite CP-parity in the process $γγ\to t\bar{t}$ around the mass pole of the Higgs boson. Near the resonance pole, the interference between the Higgs-exchange and the continuum amplitudes can be sizable if the photon beams are polarized and helicities of the top and anti-top quarks are measured. Study of these interference effects enables one to determine the CP property of the Higgs boson completely. An example of the complete determination is demonstrated in the context of the minimal supersymmetric standard model.

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arXiv2000

Muon g-2 and precision electroweak physics in the MSSM

The minimal supersymmetric extension of standard model (MSSM) is examined by analyzing its quantum effects on the precision electroweak measurements and the muon g-2. We examine carefully the effects of light charginos and neutralinos that are found to improve the fit to the electroweak data. We identify two distinct regions on the $(μ, M_2)$-plane that fit well to the electroweak data and give significant contribution to muon g-2.

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arXiv2000

Supersymmetry versus precision experiments revisited

We study constraints on the supersymmetric standard model from the updated electroweak precision measurements --- the Z-pole experiments and the $W$-boson mass measurements. The supersymmetric-particle contributions to the universal gauge-boson-propagator corrections are parametrized by the three oblique parameters Sz, Tz and mw. The oblique corrections, the Zqq and Zll vertex corrections, and the vertex and box corrections to the μ-decay width are separately studied in detail. We first study individual contribution from the four sectors of the model, the squarks, the sleptons, the supersymmetric fermions (charginos and neutralinos), and the supersymmetric Higgs bosons, to the universal oblique parameters, where the sum of individual contributions gives the total correction. We find that the light squarks or sleptons, whose masses just above the present direct search limits, always make the fit worse than that of the Standard Model (SM), whereas the light charginos and neutralinos generally make the fit slightly better. The contribution from the supersymmetric Higgs sector is found small. We then study the vertex/box corrections carefully when both the supersymmetric fermions (-inos) and the supersymmetric scalars (squarks and sleptons) are light, and find that no significant improvement over the SM fit is achieved. The best overall fit to the precision measurements are found when charginos of mass $\sim 100\gev$ with a dominant wino-component are present and the doublet squarks and sleptons are all much heavier. The improvement over the SM is marginal, however, where the total χ^2 of the fit to the 22 data points decreases by about one unit, due mainly to a slightly better fit to the $Z$-boson total width.

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arXiv1998

Constraints on leptophobic Z' models from electroweak experiments

We study the constraints from updated electroweak data on the three leptophobic Z' models, the $η$ model with an appropriate $U(1)'-U(1)_Y$ kinetic mixing, a Z' model motivated by the flipped SU(5) \times U(1) unification, and the phenomenological Z' model of Agashe, Graesser, and Hinchliffe. The $Z-Z'$ mixing effects are parametrized in terms of a positive contribution to the $T$ parameter, $T_{new}$, and the effective mass mixing parameter, $\barξ$. All the theoretical predictions for the $Z$ boson parameters, the $W$ boson mass and the observables in low-energy neutral current experiments are presented together with the standard model radiative corrections. The allowed region in the ($\barξ,T_{new}$) plane is shown for the three models. The 95% CL lower limit on the heavier mass eigenstate $Z_2$ is given as a function of the effective $Z-Z'$ mixing parameter $ζ$.

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arXiv1998

Sum rules for $e^+e^- \to W^+W^-$ helicity amplitudes from BRS invariance

The BRS invariance of the electroweak gauge theory leads to relationships between amplitudes with external massive gauge bosons and amplitudes where some of these gauge bosons are replaced with their corresponding Nambu-Goldstone bosons. Unlike the equivalence theorem, these identities are exact at all energies. In this paper we discuss such identities which relate the process $e^+e^- \to W^+W^-$ to $W^\pmχ^\mp$ and $χ^+χ^-$ production. By using a general form-factor decomposition for $e^+e^- \to W^+W^-$, $e^+e^- \to W^\pm χ^\mp$ and $e^+e^- \to χ^+χ^-$ amplitudes, these identities are expressed as sum rules among scalar form factors. Because these sum rules may be applied order by order in perturbation theory, they provide a powerful test of higher order calculations. By using additional Ward-Takahashi identities we find that the various contributions are divided into separately gauge-invariant subsets, the sum rules applying independently to each subset. After a general discussion of the application of the sum rules we consider the one-loop contributions of scalar-fermions in the Minimal Supersymmetric Standard Model as an illustration.

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arXiv1998

$Z'$ bosons in supersymmetric $E_6$ models confront electroweak data

We study constraints on additional $Z'$ bosons predicted in the supersymmetric (SUSY) $E_6$ models by using the updated results of electroweak experiments -- $Z$-pole experiments, $m_W$ measurements and low-energy neutral current (LENC) experiments. We find that the effects of $Z$-$Z'$ mixing are parametrized by (i) a tree-level contribution to the $T$-parameter, (ii) the effective $Z$-$Z'$ mass mixing angle $\xibar$. In addition, the effect of the direct exchange of the heavier mass eigenstate $Z_2$ in the LENC processes is parametrized by (iii) a contact term $\contact$. We give the theoretical predictions for the observables in the electroweak experiments together with the standard model radiative corrections. Constraints on $T_{\rm new}$ and $\xibar$ from the $Z$-pole and $m_W$ experiments and those on $\contact$ from the LENC experiments are separately shown. Impacts of the kinetic mixing between the ${\rm U(1)}_Y$ and ${\rm U(1)'}$ gauge bosons on the $χ^2$-analysis are studied. We show the 95% CL lower mass limit of $Z_2$ as a function of the effective $Z$-$Z'$ mixing parameter $ζ$, a combination of the mass and kinetic mixings. Theoretical prediction on $ζ$ and $g_E$ is found for the $χ, ψ, η$ and $ν$ models by assuming the minimal particle content of the SUSY $E_6$ models. In a certain region of the parameter space, the $Z_2$ boson mass in the detectable range of LHC is still allowed.

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arXiv1998

Constraints on Non-Standard Contributions to the Charged-Current Interactions

The success of the quantum level predictions of the Standard Model on the $Z$ boson properties, on $\mw$ and on $\mt$, which makes use of the muon lifetime as an input, implies a stringent constraint on new physics contributions to the $V-A$ charged-current interactions among leptons. Observed unitarity of the CKM matrix elements then implies constraints on non-standard contributions to the lepton-quark charged-current interactions. By using the recent electroweak data as inputs, we find the 95%~CL limits for the corresponding contact interactions: $Λ_{CC,+}^{\ell\ell}>7.5$ TeV and $Λ_{CC,-}^{\ell\ell}>10.2$ TeV for the purely leptonic terms, and $Λ_{CC,+}^{\ell q}>5.8$ TeV and $Λ_{CC,-}^{\ell q}>10.1$~TeV for the lepton-quark contact interactions.

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arXiv1997

Constraints on four-Fermi contact interactions from low-energy electroweak experiments

We investigate the constraints on four-Fermi contact interactions from low-energy lepton-quark and lepton-lepton scattering experiments --- polarization asymmetries in electron (muon)-nucleon scattering experiments, cesium and thallium atom parity violation measurements, neutrino-nuclei and neutrino-electron scattering experiments. These constraints are then combined by assuming the lepton and quark universalities and SU(2)*U(1) gauge invariance of the contact interaction, which leave independent six lepton-quark and three pure-leptonic interactions.Impacts of these constraints on models with an additional Z-boson are briefly discussed. We also present updates of the low-energy constraints on the S and T parameters.

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arXiv1997

String unification scale and the hyper-charge Kac-Moody level in the non-supersymmetric standard model

The string theory predicts the unification of the gauge couplings and gravity. The minimal supersymmetric Standard Model, however, gives the unification scale $\sim 2\times 10^{16}$ GeV which is significantly smaller than the string scale $\sim 5\times 10^{17}$ GeV of the weak coupling heterotic string theory. We study the unification scale of the non-supersymmetric minimal Standard Model quantitatively at the two-loop level. We find that the unification scale should be at most $\sim 4\times 10^{16}$ GeV and the desired Kac-Moody level of the hyper-charge coupling should be $1.33 \lsim \ky \lsim 1.35$ in the absence of significant string threshold effects.

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arXiv1997

Analysis of Electroweak Precision Data and Prospects for Future Improvements

We update our previous work on an analysis of the electroweak data by including new and partly preliminary data available up to the 1996 summer conferences. The new results on the Z partial decay widths into b and c hadrons now offer a consistent interpretation of all data in the minimal standard model. The value extracted for the strong interaction coupling constant α_s(mZ) agrees well with determinations in other areas. New constraints on the universal parameters S, T and U are obtained from the updated measurements. No signal of new physics is found in the S,T,U analysis once the SM contributions with mt \sim 175GeV and those of not a too heavy Higgs boson are accounted for. The naive QCD-like technicolor model is now ruled out at the 99%CL even for the minimal model with SU(2)_{TC}. In the absence of a significant new physics effect in the electroweak observables, constraints on masses of the top quark, mt, and Higgs boson, mH, are derived as a function of α_s and the QED effective coupling $\barα(m_Z^2)$. The preferred range of mH depends rather strongly on the actual value of mt: mH<360 GeV for mt=170GeV, while mH>130GeV for mt=180GeV at 95%CL. Prospects due to forthcoming improved measurements of asymmetries, the mass of the weak boson W mW, mt and $\barα(m_Z^2)$ are discussed. Anticipating uncertainties of 0.00020 for $\sbar^2(m_Z^2)$, 20MeV for mW, and 2GeV for mt, the new physics contributions to the S,T,U parameters will be constrained more severely, and, within the SM, the logarithm of the Higgs mass can be constrained to about +-0.35. The better constraints on S,T,U and on mH within the minimal SM should be accompanied with matching precision in $\barα(m_Z^2)$.

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arXiv1996

Looking Beyond the Standard Model through Precision Electroweak Physics

The most important hint of physics beyond the Standard Model (SM) from the 1995 precision electroweak data is that the most precisely measured quantities, the total, leptonic and hadronic decay widths of the $Z$ and the effective weak mixing angle, $\sin^2θ_W$, measured at LEP and SLC, and the quark-lepton universality of the weak charged currents measured at low energies, all agree with the predictions of the SM at a few $\times 10^{-3}$ level. By taking into account the above constraints I examine implications of three possible disagreements between experiments and the SM predictions. It is difficult to interpret the 11\% (2.5-$σ$) deficit of the $Z$-partial-width ratio $R_c=Γ_c/Γ_h$, since it either implies an unacceptably large $α_s$ or a subtle cancellation among hadronic $Z$ decay widths in order to keep all the other successful predictions of the SM. The 2\% (3-$σ$) excess of the ratio $R_b=Γ_b/Γ_h$ may indicate the presence of a new rather strong interaction, such as the top-quark Yukawa coupling in the supersymmetric (SUSY) SM or a new interaction responsible for the large top-quark mass in the Technicolor scenario of dynamical electroweak symmetry breaking. Another interpretation may be additional tree-level gauge interactions that couple only to the third generation of fermions. A common consequence of these attempts is a rather small $α_s$, $α_s(\mz )_{\msbar}=0.104\pm 0.08$. The 0.17\% (1-$σ$) deficit ...

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arXiv1995

Implications of Precision Electroweak Data

There are two aspects to the 1995 summer update of the combined preliminary electroweak data from LEP and SLC. On the one hand, agreement between experiments and the Standard Model (SM) has improved for the line-shape and the asymmetry data. The $τ$ widths and asymmetries are now consistent with $e$--$μ$--$τ$ universality, and all the asymmetry data including the left-right asymmetry from SLC are consistent with the SM (16\%CL). On the other hand, a discrepancy between experiments and SM predictions is sharpened for two observables, $R_b$ and $R_c$, where $R_q$ is the partial $Z$ boson width ratio $Γ_q/Γ_h$. $R_b$ is 3\% larger (3.7$σ$) and $R_c$ is 11\% smaller (2.5$σ$) than the SM predictions. When combined, the SM is ruled out at the 99.99\%CL for $m_t>170$GeV. It is difficult to interpret the $11\pm 4$\% deficit of $R_c$, since if we allow only $Γ_b$ and $Γ_c$ to deviate from the SM then the precisely measured ratio $R_h=Γ_h/Γ_\ell$ forces the QCD coupling to be $α_s\equiv α_s(m_Z)_{\overline{\rm MS}}=0.185\pm 0.041$, which is uncomfortably large. The data can be consistent with the prefered $α_s$ ($0.10<α_s<0.13$) only if the sum $Γ_h=Σ_q Γ_q$ does not deviate significantly from the SM prediction. Possible experimental causes for the under-estimation of $R_c$ are discussed. By assuming the SM value for $R_c$, the discrepancy in $R_b$ decreases to 2\% (3$σ$). The double tagging technique used for the $R_b$ measurements is critically reviewed. A few theoretical models that can explain large $R_b$ and small $α_s$ ($= 0.104\pm 0.008$) are discussed. If the QCD coupling $α_s$ is allowed to be fitted by the data, the standard $S$, $T$, $U$ analysis for a new physics

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arXiv1995

Electroweak Precision Measurements and the Minimal Supersymmetric Standard Model

We discuss supersymmetric contributions to the electroweak precision measurements in the Minimal Supersymmetric Standard Model for two cases: the quark-lepton universality violation $δ_{q\ell}$ in charged currents and the ratio $R_b=Γ(Z\rightarrow b\bar{b})/Γ(Z\rightarrow \rm hadrons)$. The recent experimental data suggest deviations from the Standard Model for these observables, at the 1-$σ$ level for the former and at more than 3-$σ$ level for the latter. We analyze the non-oblique corrections from the SUSY particles to explain these discrepancies. The observed non-zero $δ_{q\ell}$ may be explained by relatively light sleptons, charginos and neutralinos. Although the observed excess of $R_b$ can be explained by very light scalar top and chargino for low $\tanβ$, this interpretation is severely constrained by the opening of exotic decay modes for the top quark.

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arXiv1995

Extended technicolor contribution to the Zbb vertex

We show that the flavor-diagonal gauge boson of the extended technicolor theory contributes with opposite sign to the standard model correction for the $Zbb$ vertex. This mechanism can naturally explain the deviation of the LEP result from the standard model prediction for the partial width $Γ(Z \rightarrow b{\bar b})$. A smaller value of the QCD coupling, $α_s(m_Z) \simeq 0.115$, is then preferred by the $Γ(Z \rightarrow \mbox{hadron})$ data, which is consistent with both the recent Lattice-QCD estimate and the Particle Data Group average.

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