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Kazumasa Ohkuma

Publications and source records attributed to Kazumasa Ohkuma.

At least 19 recordsLinked to original sources
arXiv2020

Correlations in flavor-changing tqZ couplings constrained via experimental data

Possible non-standard tqZ couplings, where q=c or u, originated from general flavor-changing-neutral-current interactions are studied model-independently using the effective Lagrangian consisting of several SU(3)xSU(2)xU(1) invariant dimension-6 operators. After the electroweak symmetry breaking, these operators are recombined to form four kinds of independent terms whose coefficients are complex in general. The Lagrangian could therefore include up to eight independent coupling parameters. Through searches for experimentally allowed regions of these parameters, it is found that some correlations exist among the signs and sizes of those couplings.

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arXiv2019

Effective Lagrangian Approach to Top Decay via Flavor Changing Neutral Current

We study possible non-standard $tuZ$ and $tcZ$ couplings, which induce flavor-changing neutral-current decays of the top quark, in the effective-Lagrangian framework. The corresponding interaction Lagrangian comes from several $SU(3) \times SU(2) \times U(1)$ invariant dimension-6 effective operators, and it includes four independent complex-number coupling constants. Constraints on those non-standard coupling constants in each interaction are derived by using the present experimental limits of the branching fractions for $t \to u Z$ and $t \to c Z$ processes. Expected improvements of the constraints at future facilities are discussed as well. It is also pointed out that some correlations hold among those constrained coupling constants.

hep-ph↗
arXiv2019

Studying flavor-changing neutral tqZ couplings: Current constraints and future prospects

Possible non-standard tuZ and tcZ interactions, which induce flavor-changing neutral-current decays of the top quark, are studied in the effective-Lagrangian framework. The corresponding Lagrangian consists of four kinds of non-standard couplings coming from SU(3)xSU(2)xU(1) invariant dimension-6 effective operators. The four coupling constants in each interaction are treated as complex numbers independent of each other, and constraints on them are derived by using the present experimental limits of the branching fractions for t -> u Z and t -> c Z processes. Future improvements of those constraints are also discussed as well as possibilities of measurements of these couplings at the High-Luminosity Large Hadron Collider.

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arXiv2016

Constraints on generalized non-standard $tbW$ couplings

General non-standard $tbW$ couplings are studied as model independently as possible based on the effective Lagrangian consisting of the dimension-6 operators, which is an extension of the standard-model Lagrangian. The $tbW$-interaction Lagrangian in this framework includes four kinds of couplings, which could be complex. Constraints on those non-standard $tbW$ couplings are obtained by comparing the experimental data related to the $t\to b W$ process with the corresponding theoretical formulas derived from the effective Lagrangian. The constraints on some sets of the non-standard couplings are shown not to be so strong because those couplings balance out each other as we treat all the non-standard couplings as complex numbers at the same time.

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arXiv2016

Refined analysis and updated constraints on general non-standard tbW couplings

We recently studied possible non-standard tbW couplings based on the effective-Lagrangian which consists of four kinds of SU(3) x SU(2) x U(1) invariant dimension-6 effective operators and gave an experimentally allowed region for each non-standard coupling. We here re-perform that analysis much more precisely based on the same experimental data but on a new computational procedure using the Graphics-Processing-Unit (GPU) calculation system. Comparing these two analyses with each other, the previous one is found to have given quite reliable results despite of its limited computation capability. We then apply this new procedure to the latest data and present updated results.

hep-ph↗
arXiv2015

Full analysis of general non-standard tbW couplings

Possible non-standard couplings which could contribute to the t -> b W process are studied based on the effective-Lagrangian approach. The corresponding effective Lagrangian consists of four kinds of dimension-6 effective operators, each of which has an independent coupling constant. In this analysis, all those couplings are treated as complex numbers and constraints on them are estimated by using recent experimental data from the LHC. We point out that the resultant constraints on those couplings are still not that strong because contributions from some couplings can work oppositely with each other.

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arXiv2014

Final charged-lepton angular distribution and possible anomalous top-quark couplings in p p -> t tbar X -> l^+ X'

Possible anomalous (or nonstandard) top-quark interactions with the gluon and those with the W boson induced by SU(3) x SU(2) x U(1) gauge-invariant dimension-6 effective operators are studied in p p -> t tbar X -> l^+ X' (l=e or mu) at the Large Hadron Collider (LHC). The final charged-lepton (l^+) angular distribution is first computed for nonvanishing nonstandard top-gluon and top-W couplings with a cut on its transverse momentum. The optimal-observable procedure is then applied to this distribution in order to estimate the expected statistical uncertainties in measurements of those couplings that contribute to this process in the leading order.

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arXiv2013

Latest constraint on non-standard top-gluon couplings at hadron colliders and its future prospect

Constraints on the non-standard top-gluon couplings composed of the chromomagnetic- and chromoelectric-dipole moments of the top quark are updated by combining the latest data of top-pair productions from the Tevatron, 7-TeV LHC, and 8-TeV LHC. We find that adding the recent 8-TeV data to the analysis is effective to get a stronger constraint on the chromoelectric-dipole moment than the one from the Tevatron and 7-TeV LHC alone. We also discuss how those constraints on the non-standard couplings could be further improved when 14-TeV LHC results become available in the near future.

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arXiv2011

Addendum to: Search for anomalous top-gluon couplings at LHC revisited

In our latest paper "Search for anomalous top-gluon couplings at LHC revisited" in Eur. Phys. J. C65 (2010), 127-135 (arXiv:0910.3049 [hep-ph]), we studied possible effects of nonstandard top-gluon couplings through the chromoelectric and chromomagnetic moments of the top quark using the total cross section of ppbar/pp --> ttbar X at Tevatron/LHC. There we pointed out that LHC data could give a stronger constraint on those two parameters, which would be hard to obtain from Tevatron data alone. We show here the first CMS measurement of this cross section actually makes it possible.

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arXiv2009

Search for anomalous top-gluon couplings at LHC revisited

Through top-quark pair productions at LHC, we study possible effects of nonstandard top-gluon couplings yielded by SU(3)xSU(2)xU(1) invariant dimension-6 effective operators. We calculate the total cross section and also some distributions for p p -> t tbar X as functions of two anomalous-coupling parameters, i.e., the chromoelectric and chromomagnetic moments of the top, which are constrained by the total cross section sigma(p pbar -> t tbar X) measured at Tevatron. We find that LHC might give us some chances to observe sizable effects induced by those new couplings.

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arXiv2007

Studying possible CP-violating Higgs couplings through top-quark pair productions at muon colliders

We study possible anomalous CP-violating Higgs couplings to mu mu-bar and t t-bar fully model-independent way through top-quark pair productions at muon colliders. Assuming additional non-standard neutral Higgs bosons, whose couplings with top-quark and muon are expressed in the most general covariant form, we carry out analyses of effects which they are expected to produce via CP-violating asymmetries and also the optimal-observable (OO) procedure under longitudinal and transverse muon polarizations. We find the measurement of the asymmetry for longitudinal beam polarization could be useful to catch some signal of CP violation, and an OO analysis might also be useful if we could reduce the number of unknown parameters with a help of other experiments and if the size of the parameters is at least O(1) ~ O(10).

hep-ph↗
arXiv2006

New-Physics Search through gamma gamma -> t tbar -> lX/bX

We probe optimal beam polarizations for new-physics search in top-quark and Higgs-boson sectors at Photon Linear Colliders (PLC). Expressing possible non-standard effects generated by SU(2)xU(1) gauge-invariant dimension-6 effective operators as anomalous top and Higgs couplings, we estimate expected statistical sensitivities of these couplings in gamma gamma -> t tbar -> lX/bX, using the optimal-observable method.

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arXiv2005

Optimal Beam Polarizations for New-Physics Search through gamma gamma -> t tbar -> lX/bX

We perform an optimal-observable analysis of the final charged-lepton/b-quark momentum distributions in gamma+gamma -> t+tbar -> lX/bX for various beam polarizations in order to study possible anomalous t-tbar-gamma, tbW and gamma-gamma-H couplings, which could be generated by SU(2)xU(1) gauge-invariant dimension-6 effective operators. We find optimal beam polarizations that will minimize the uncertainty in determination of those non-standard couplings. We also compare e+ebar and gamma+gamma colliders from the viewpoint of the anomalous-top-quark-coupling determination.

hep-ph↗
arXiv2005

Studying Possible Anomalous Top-Quark Couplings at Photon Colliders

Search for new-physics through possible anomalous t tbar gamma, tbW and gamma gamma H couplings which are generated by SU(2) X U(1) gauge-invariant dimension-6 effective operators is discussed, using energy and angular distributions of final charged-lepton/ b-quark in gamma gamma -> t tbar -> l X / bX for various beam polarizations. Optimal beam polarizations that minimize uncertainty in determination of those non-standard couplings are found performing an optimal-observable analysis.

hep-ph↗
arXiv2005

Probing Polarized Gluon Distributions through Charmed Hadron Production in Polarized pp Collisions at BNL--RHIC Energy

In order to extract information about behavior of polarized gluons in the nucleon, charmed hadron productions, i.e. D* meson and Lambda_c baryon productions, are studied in polarized pp reactions at BNL-RHIC energy. For these processes, the spin correlation asymmetry D_{LL} between the target proton and the produced charmed hadron, and its statistical sensitivity delta D_{LL} are calculated. From analyses on these processes, we found that the pseudo-rapidity distribution of D_{LL} in the limited transverse momentum region is quite effective for distinguishing the model of polarized gluons as well as the model of spin-dependent fragmentation functions.

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arXiv2004

Optimal-Observable Analysis of Possible New Physics Using the b-quark in gamma gamma -> tt -> bX

We study possible anomalous top-quark couplings generated by SU(2) x U(1) gauge invariant dimension-6 effective operators, using the final b-quark momentum distribution in gamma gamma -> tt -> bX. Taking into account non-standard tt gamma, tbW and gamma gamma H couplings, we perform an optimal-observable analysis in order to estimate the precision for the determination of all relevant non-standard couplings.

hep-ph↗
arXiv2004

Probing Anomalous Top-Quark Couplings Induced by Dim.6 Operators at Photon Colliders

Possible anomalous top-quark couplings induced by SU(2) x U(1) gauge-invariant dimension-6 effective operators were studied in the process of t t-bar productions and decays at polarized gamma gamma colliders. Two CP-violating asymmetries, a linear-polarization asymmetry and a circular-polarization asymmetry, were computed including both non-standard [t t-bar gamma] and [gamma gamma H] couplings. An optimal-observable analysis for the process gamma gamma --> t t-bar --> l^{+-} ... was performed in order to estimate the precision for determination of all relevant non-standard couplings, including the anomalous tbW coupling.

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