Searcharxiv⌕ Search

arXiv subjects

Junichi Kanzaki

Publications and source records attributed to Junichi Kanzaki.

10 recordsLinked to original sources
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 ν_l \barν_l t\bar{t} H $, $ l \barν_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.

hep-ph↗
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 $μ^-μ^+\toν_μ\barν_μt\bar{t}H$ 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.

hep-ph↗
arXiv2023

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 $γγ\to W^+W^-$ 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.

hep-ph↗
arXiv2023

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.

hep-ph↗
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.

hep-ph↗
arXiv2012

On the W+4 jets background to the top quark asymmetry at the Tevatron

We investigate the possibility that the W+4 jets background is a source of the anomalously large top-antitop (tt) charge asymmetry observed at the Tevatron. We simulate the tt reconstruction of the signal and background events at the matrix-element level and find that the reconstructed tt candidates from the W +4 jets background could give large forward-backward asymmetry. We suggest serious re-evaluation of the W+4 jets background for the tt candidate events by studying the distributions of the reconstructed tt systems in their rapidity difference, the transverse momentum of the top quark, and that of the tt system, which can differ significantly from those of the QCD tt production events especially at high tt invariant mass.

hep-ph↗
arXiv2008

HELAS and MadGraph/MadEvent with spin-2 particles

Fortran subroutines to calculate helicity amplitudes with massive spin-2 particles (massive gravitons), which couple to the standard model particles via the energy momentum tensor, are added to the {\tt HELAS} ({\tt HEL}icity {\tt A}mplitude {\tt S}ubroutines) library. They are coded in such a way that arbitrary scattering amplitudes with one graviton production and its decays can be generated automatically by {\tt MadGraph} and {\tt MadEvent}, after slight modifications. All the codes have been tested carefully by making use of the invariance of the helicity amplitudes under the gauge and general coordinate transformations.

hep-ph↗
arXiv2007

Potential for measuring the H^\pm W^\mp Z^0 vertex from WZ fusion at the Large Hadron Collider

We investigate the possibility of measuring the H^\pm W^\mp Z^0 vertex from the single $H^\pm$ production process via WZ fusion at the CERN Large Hadron Collider (LHC). This vertex strongly depends on the structure of the Higgs sector in various new physics scenarios, so that its measurement can be useful to distinguish the models. A signal and background simulation under the expected detector performance at the LHC is done for the processes of pp \to W^\pm Z^0 X \to H^\pm X \to tbX and pp \to W^\pm Z^0 X \to H^\pm X \to W^\pm Z^0 X, and the required magnitudes of the H^\pm W^\mp Z^0 vertex for observation are evaluated. It is found that although the loop induced H^\pm W^\pm Z^0 vertex in multi-Higgs doublet models cannot be measurable, the latter process can be useful to test the model with a real and a complex triplets.

hep-ph↗
arXiv2004

Study of Black Holes with the ATLAS detector at the LHC

We evaluate the potential of the ATLAS detector for discovering black holes produced at the LHC, as predicted in models with large extra dimensions where quantum gravity is at the TeV scale. We assume that black holes decay by Hawking evaporation to all Standard Model particles democratically. We comment on the possibility to estimate the Planck scale.

hep-ph↗