arXiv subjects

Kentarou Mawatari

Publications and source records attributed to Kentarou Mawatari.

At least 19 recordsLinked to original sources

arXiv2026

Disentangling new physics with quantum entanglement in $t\bar{t}$ production at future lepton colliders

We investigate quantum entanglement and Bell-inequality violation in top-antitop pair production at future lepton colliders such as the International Linear Collider (ILC) and multi-TeV muon colliders. Within the Standard Model (SM), the process proceeds through $s$-channel $\gamma$ and $Z$ exchange and exhibits characteristic spin-correlation patterns that encode a non-trivial amount of entanglement. We then examine how these features are modified in several well-motivated extensions of the SM:(i) a neutral scalar mediator that couples to charged leptons and top quarks via Yukawa interactions and contributes as an additional $s$-channel exchange; (ii) the minimal gauged $U(1)_{B-L}$ model, which introduces a new neutral gauge boson $Z'$ coupling vectorially to SM fermions; and (iii) a Randall-Sundrum scenario, in which the exchange of massive Kaluza-Klein gravitons arising from a warped extra dimension induces additional spin-dependent interactions. For all cases, we evaluate quantum-information observables, including the entanglement marker, the concurrence, and the maximal Clauser-Horne-Shimony-Holt parameter, and study their dependence on the center-of-mass energy, scattering angle, and model parameters. We find that, relative to the SM expectation, the entanglement is typically reduced in the scalar-mediator scenario, while sizable deviations can arise in the $U(1)_{B-L}$ and Randall-Sundrum cases for phenomenologically relevant regions of parameter space. These results demonstrate the potential of quantum-information observables as sensitive probes of new particles and their interaction patterns in future lepton colliders.

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arXiv2026

Higgs production in association with a Z boson at TeV-scale lepton colliders

We study the $l^-l^+\to \nu\bar{\nu}Zh$ process for future lepton colliders, whose cross section becomes larger than that for $l^-l^+\to Zh$ in the energy region above a few TeV. We classify the amplitudes into three main groups based on the topology of each Feynman diagram; vector boson scattering, $l^-W^+$ scattering, and $W^-l^+$ scattering, and study the interference patterns among the amplitudes. We show that subtle gauge cancellation among the amplitudes at high energies in the unitary gauge is absent in the recently proposed Feynman-diagram gauge, and the physical distributions can be interpreted by the contributions from each subgroup. We also find that the interference patterns in kinematical distributions of the Z boson can be understood by those in the $l^-l^+\to \nu\bar{\nu}Z$ process.

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arXiv2026

Multi-channel phase space with Feynman-diagram-gauge amplitudes

Multi-channel phase space with a single Feynman diagram enhancement is a powerful tool for high-energy physics event generation if a diagram with a singular propagator dominates the total scattering amplitude at the corresponding singular kinematical region, and when the interference among amplitudes is not larger than the square of each amplitude. These conditions are satisfied in the Feynman-diagram-gauge amplitudes for both unbroken (QED and QCD) and broken (EW) gauge theories. We illustrate the usefulness of this method in lepton collider processes that are challenging to accurately simulate at very high energies, i.e., $ l\bar{l} \to \nu_l \bar{\nu}_l t\bar{t} H $, $ l \bar{\nu}_l t\bar{b} H $, and $ l \bar{l} t\bar{t} H $, in the SMEFT with a complex top-Yukawa coupling. The total cross sections of the latter two processes contain lepton-mass singularities arising from $t$-channel photon-exchange diagrams. To address this issue, we develop a phase-space parametrization that accurately generates the distributions of forward-emitted charged leptons. We modify the \HELAS\ library to evaluate helicity amplitudes in the singular region so that vertices at very small invariant momentum square of order $m_e^2$ can be accurately evaluated even at multi-TeV energies.

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arXiv2024

One-loop QCD amplitudes in the Feynman-Diagram gauge

Scattering amplitudes for the massless QCD process, $q\bar{q}\to q^\prime\bar{q}^\prime$, are calculated in the one-loop order in the Feynman-Diagram (FD) gauge, where gluons are quantized on the light cone with opposite direction of the three-momenta. We find non-decoupling of the Faddeev-Popov ghosts and non-conventional UV singularities in dimensional regularization. The known QCD amplitudes with asymptotic freedom are reproduced only after summing propagator and vertex corrections. By quantizing gluons in the Feynman gauge on the FD gauge background, we obtain the one-loop improved FD gauge amplitudes.

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arXiv2024

W-boson pair production at lepton colliders in the Feynman-diagram gauge

We calculate helicity amplitudes for $e^-e^+\to W^-W^+$ analytically in the Feynman-diagram (FD) gauge. We show that, unlike in the unitary gauge, there is no energy growth of the individual Feynman amplitudes for the longitudinally polarized W bosons, and the contributions from the associated Goldstone bosons are manifest even without taking the high-energy limit. We also find that the physical distributions can be interpreted by the individual amplitudes in the FD gauge.

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arXiv2024

Automatic generation of helicity amplitudes in Feynman-Diagram gauge

We develop a method to calculate helicity amplitudes of an arbitrary tree-level process in Feynman-Diagram (FD) gauge for an arbitrary gauge model with MadGraph5_aMC@NLO. We start from the 't Hooft-Feynman gauge Lagrangian in FeynRules and generate scattering amplitudes by identifying the Goldstone boson as the $5^{\rm th}$ component of each weak boson. All the vertices of the 5-component weak bosons are then created automatically by assembling the relevant weak boson and Goldstone boson vertices in the Feynman gauge. The 5-component weak boson vertices are then connected by the $5\times5$ matrix propagator in the FD gauge. As a demonstration of the method we calculate the cross section for the process $\mu^-\mu^+\to\nu_\mu\bar{\nu}_\mu t\bar{t}H$ with complex top Yukawa coupling, which can be obtained by adding a gauge invariant dimension-6 operator to the Standard Model (SM) Lagrangian. The FD gauge and the unitary (U) gauge amplitudes give exactly the same cross section, and subtle gauge theory cancellation among diagrams in the U gauge at high energies is absent in the FD gauge, as has been observed for various SM processes. In addition, we find that the total cross sections at high energies are dominated by a single, or a set of non-vanishing Feynman amplitudes with the higher dimensional vertices in the FD gauge.

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arXiv2023

Long-lived Higgsino pairs decaying within the LHC detectors

Pair-produced long-lived lightest Higgsinos decaying within a few meters in the LHC detectors are considered. The relatively light nearly-pure Higgsino states object of this study are those of a supersymmetric minimal model with gauge-mediated supersymmetry breaking and heavy sfermions and gauginos. The heavier Higgsinos decay instantly into the lightest one, with very little activities. Hence, all pair-produced Higgsinos end up as a pair of the lightest neutral Higgsinos, possibly long lived and decaying into the gravitino and either the Higgs or the Z boson. If they decay inside the detectors, these bosons deposit huge energies in the calorimeters. The cross section $\sigma_2$ ($\sigma_1$) for the events with two (at least one) decaying-in-flight lightest Higgsinos is examined. The combination $(\sigma_1)^2/\sigma_2$ is found to be almost insensitive to the lightest Higgsino lifetime, thereby providing a good variable to measure its mass. The ratio $\sigma_1/\sigma_2$, instead, mildly dependent on its mass, can give good information on its lifetime. It is argued that the lightest Higgsinos in the events are rather slow, as the very relativistic ones escape away. Thus, the initial state radiations tend to be soft and a trigger system appropriate for such events is necessary to detect them.

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arXiv2022

Helicity amplitudes in light-cone and Feynman-diagram gauges

Recently proposed Feynman-diagram (FD) gauge propagator for massless and massive gauge bosons is obtained from a light-cone (LC) gauge propagator, by choosing the gauge vector along the opposite direction of the gauge boson three-momentum. We implement a general LC gauge propagator for all the gauge bosons of the Standard Model (SM) in the HELicity Amplitude Subroutines (HELAS) codes, such that all the SM helicity amplitudes can be evaluated at the tree level in the LC gauge by using MadGraph. We confirm that our numerical codes produce physical helicity amplitudes which are consistent among all gauge choices. We then study interference patterns among Feynman amplitudes, for a few $2\to3$ scattering processes in QED and QCD, and the process $\gamma\gamma\to W^+W^-$ followed by the $W^\pm$ decays. We find that in a generic LC gauge, where all the gauge boson propagators share a common gauge vector, we cannot remove the off-shell current components which grow with their energy systematically from all the Feynman amplitudes in $2\to3$ processes. On the other hand, the $5\times5$ LC gauge propagator for the weak bosons removes components which grow with energy due to the longitudinal polarization mode of the external bi-fermion currents, and hence can give $2\to2$ weak boson scattering amplitudes which are free from subtle cancellation at high energies. The particular choice of the FD gauge vector has advantages over generic LC gauge, not only because all the terms which grow with energy of off-shell and on-shell currents are removed systematically from all the diagrams, but also because no artificial gauge vector direction dependence of individual amplitudes appears.

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arXiv2022

Quantum information and CP measurement in $H \to \tau^+ \tau^-$ at future lepton colliders

We introduce a methodology and investigate the feasibility of measuring quantum properties of tau lepton pairs in the $H \to \tau^+ \tau^-$ decay at future lepton colliders. In particular, observation of entanglement, steerability and violation of Bell inequalities are examined for the ILC and FCC-ee. We find that detecting quantum correlation crucially relies on precise reconstruction of the tau lepton rest frame and a simple kinematics reconstruction does not suffice due to the finite energy resolution of the colliding beams and detectors. To correct for energy mismeasurements, a log-likelihood method is developed that incorporates the information of impact parameters of tau lepton decays. We demonstrate that an accurate measurement of quantum properties is possible with this method. As a by-product, we show that a novel model-independent test of CP violation can be performed and the CP-phase of $H \tau \tau$ interaction can be constrained with an accuracy comparable to dedicated analyses, i.e., up to $7.9^{\circ}$ and $5.4^{\circ}$ at ILC and FCC-ee, respectively.

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arXiv2022

Helicity amplitudes without gauge cancellation for electroweak processes

In the 5-component representation of weak bosons, the first four components make a Lorentz four vector, representing the transverse and longitudinal polarizations excluding the scalar component of the weak bosons, whereas its fifth component corresponds to the Goldstone boson. We obtain the $5\times 5$ component propagators of off-shell weak bosons, proposed previously and named after the Goldstone boson equivalence theorem, by starting from the unitary-gauge representation of the tree-level scattering amplitudes, and by applying the BRST (Becchi--Rouet--Stora--Tyutin) identities to the two sub-amplitudes connected by each off-shell weak-boson line. By replacing all weak boson vertices with those among the off-shell 5-component wavefunctions, we arrive at the expression of the electroweak scattering amplitudes, where the magnitude of each Feynman amplitude has the correct on-shell limits for all internal propagators, and hence with no artificial gauge cancellation among diagrams. Although our derivation is limited to the tree-level only, it allows us to study the properties of each Feynman amplitude separately, and then learn how they interfere in the full amplitudes. We implement the 5-component weak boson propagators and their vertices in the numerical helicity amplitude calculation code HELAS (Helicity Amplitude Subroutines), so that an automatic amplitude generation program such as MadGraph can generate the scattering amplitudes without gauge cancellation. We present results for several high-energy scattering processes where subtle gauge-theory cancellation among diagrams takes place in all the other known approaches.

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arXiv2021

Next-to-leading-order corrections to the Higgs strahlung process from electron-positron collisions in extended Higgs models

We present the cross section for $e^{+}e^{-}\to hZ$ with arbitrary sets of electron and $Z$ boson polarizations at the full next-to-leading order in various extended Higgs models, such as the Higgs singlet model (HSM), the inert doublet model (IDM) and the two Higgs doublet model (2HDM). We systematically perform complete one-loop calculations to the helicity amplitudes in the on-shell renormalization scheme, and present the full analytic results as well as numerical evaluations. The deviation $\Delta R^{hZ}$ in the total cross section from its standard model (SM) prediction is comprehensively analyzed, and the differences among these models are discussed in details. We find that new physics effects appearing in the renormalized $hZZ$ vertex almost govern the behavior of $\Delta R^{hZ}$, and it takes a negative value in most cases. The possible size of $\Delta R^{hZ}$ reaches several percent under the theoretical and experimental bounds. We also analyze the deviation $\Delta R^{hZ}_{XY}$ in the total cross section times decay branching ratios of the discovered Higgs boson by utilizing the $\texttt{H-COUP}$ program. It is found that the four types of 2HDMs can be discriminated by analyzing the correlation between $\Delta R^{hZ}_{\tau\tau}$ and $\Delta R^{hZ}_{bb}$ and those between $\Delta R^{hZ}_{\tau\tau}$ and $\Delta R^{hZ}_{cc}$. Furthermore, the HSM and the IDM can be discriminated from the 2HDMs by measuring $\Delta R^{hZ}_{WW}$. These signatures can be tested by precision measurements at future Higgs factories such as the International Linear Collider.

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arXiv2021

Probing a degenerate-scalar scenario in a pseudoscalar dark-matter model

We study a pseudoscalar dark-matter model arising from a complex singlet extension of the standard model (SM), and show that the dark-matter--nucleon scattering is suppressed when two CP-even scalars are degenerate. In such a degenerate-scalar scenario we explore the model parameter space which satisfies constraints from the direct detection experiments and the relic density of dark matter. In addition, we discuss a possibility to verify such a scenario by using the recoil mass technique at the International Linear Collider. We find that a pair of states separated by 0.2 GeV can be distinguished from the single SM-like Higgs state at 5$\sigma$ with integrated luminosity of 2 ab$^{-1}$.

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arXiv2020

Probing extended Higgs sectors by the synergy between direct searches at the LHC and precision tests at future lepton colliders

We discuss a possibility that the parameter space of the two Higgs doublet model is significantly narrowed down by considering the synergy between direct searches for additional Higgs bosons at the LHC and its luminosity upgraded operation and precision measurements of the Higgs boson properties at future electron-positron colliders such as the International Linear Collider. We show that, in the case where the coupling constants of the discovered Higgs boson are slightly different from the predicted values in the standard model, most of the parameter space is explored by the direct searches of extra Higgs bosons, in particular for the decays of the extra Higgs bosons into the discovered Higgs boson, and also by the theoretical arguments such as perturbative unitarity and vacuum stability. This can be done because there appears an upper limit on the mass of the extra Higgs bosons as long as the deviation exists in the Higgs boson coupling. We also show that in the alignment limit where all the Higgs boson couplings take the standard model like values most of the parameter space cannot be excluded because most of the Higgs to Higgs decays are suppressed and also there is no upper limit on the masses from the theoretical arguments.

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arXiv2020

QED and QCD helicity amplitudes in parton-shower gauge

We introduce photon and gluon propagators in which the scalar polarization component is subtracted systematically by making use of the BRST invariance of the off-shell vector boson created from physical on-shell states. The propagator has the light-cone gauge form, where the spacial component of the gauge vector points along the negative of the off-shell vector boson momentum. We call the gauge as parton-shower gauge, since in collinear configurations the absolute value squared of each Feynman amplitude reproduces all the singular behaviors of the corresponding parton shower in this gauge. We introduce new HELAS codes that can be used to calculate the tree-level helicity amplitudes of arbitrary QED and QCD processes by using MadGraph. The absence of subtle gauge cancellation among Feynman amplitudes allows numerical codes to evaluate singular behaviors accurately, and helps us gaining physical insights on interference patterns.

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arXiv2019

H-COUP Version 2: a program for one-loop corrected Higgs boson decays in non-minimal Higgs sectors

We present the concept of H-COUP_ver 2, which evaluates the decay rates (including higher order corrections) for the Higgs boson with a mass of 125 GeV in various extended Higgs models. In the previous version (H-COUP_1.0), only a full set of the Higgs boson vertices are evaluated at one-loop level in a gauge invariant manner in these models. H-COUP_ver 2 contains all the functions of H-COUP_1.0. After shortly introducing these extended Higgs models and discussing their theoretical and experimental constraints, we summarize formulae for the renormalized vertices and the decay rates. We then explain how to install and run H-COUP_ver 2 with some numerical examples.

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arXiv2019

Automated simulations beyond the Standard Model: supersymmetry

The MadGraph5 aMC@NLO framework aims to automate all types of leading- and next-to-leading-order-accurate simulations for any user-defined model that stems from a renormalisable Lagrangian. In this paper, we present all of the key ingredients of such models in the context of supersymmetric theories. In order to do so, we extend the FeynRules package by giving it the possibility of dealing with different renormalisation options that are relevant to supersymmetric models. We also show how to deal with the problem posed by the presence of narrow resonances, thus generalising the so-called on-shell subtraction approaches. We extensively compare our total rate results with those of both Prospino2 and Resummino, and present illustrative applications relevant to the 13 TeV LHC, both at the total-rate and differential levels. The computer programs that we have used to obtain the predictions presented here are all publicly available.

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arXiv2019

Proposal for the validation of Monte Carlo implementations of the standard model effective field theory

We propose a procedure to cross-validate Monte Carlo implementations of the standard model effective field theory. It is based on the numerical comparison of squared amplitudes computed at specific phase-space and parameter points in pairs of implementations. Interactions are fully linearised in the effective field theory expansion. The squares of linear effective field theory amplitudes and their interference with standard-model contributions are compared separately. Such pairwise comparisons are primarily performed at tree level and a possible extension to the one-loop level is also briefly considered. We list the current standard model effective field theory implementations and the comparisons performed to date.

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arXiv2019

Full next-to-leading-order calculations of Higgs boson decay rates in models with non-minimal scalar sectors

We present a complete set of decay rates of the Higgs boson with the mass of 125 GeV at the full next-to-leading order in a variety of extended Higgs models; i.e., a model with an additional real singlet scalar field, four types of two Higgs doublet models and the inert doublet model. All the one-loop contributions due to QCD and electroweak interactions as well as scalar interactions are taken into account, and the calculations are systematically performed. Branching ratios for all the decay modes are evaluated in these models, and patterns of deviations in each decay mode from the standard model predictions are comprehensively analyzed. We show how these models with extended Higgs sectors can be distinguished by using our calculation of the branching ratios and future precision measurements of the Higgs boson decays.

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