arXiv subjects

Yu Hamada

Publications and source records attributed to Yu Hamada.

34 records · Page 2Linked to original sources

arXiv2022

Precision $μ^+μ^+$ and $μ^+e^-$ elastic scatterings

Expected precisions of measurements of the elastic scattering cross sections are estimated for $μ^+ μ^+$ and $μ^+ e^-$ colliders, which are recently proposed as future realistic possibilities ($μ$TRISTAN). Comparing with contributions from possible new physics represented by higher dimensional operators, we find that the measurements at a TeV energy $μ^+μ^+$ collider can probe the scale of new physics up to $O(100)$~TeV. A $μ^+ e^-$ collider for the Higgs boson factory can also improve the electroweak precision test.

hep-ph↗

arXiv2022

Understanding anomalous particle production in massless QED via time-varying $θ$ angle

The Maxwell equations imply that, under the background of non-zero $\boldsymbol{B}$, varying $θ$ term produces $\boldsymbol{E} \cdot \boldsymbol{B}$. An interesting example is the Witten effect where a magnetic monopole becomes a dyon which, however, should disappear in the exact massless limit of the fermion. Underlying mechanism of this phenomenon has been understood by Callan by the presence of an effective axion-like degree of freedom around the monopole, which is roughly the phase of the fermions. The configuration of this axion cancels the effect of the $θ$ term. Now, the chiral anomaly implies that non-vanishing $\boldsymbol{E} \cdot \boldsymbol{B}$ induces the chiral charge in the system. The question is whether the chiral charge is generated in the massless limit when we take into account the axion-like degree of freedom in the discussion. The discussion is relevant for the mechanism of baryogenesis under the background of time-dependent $θ$. We solve the system of the massless QED with time dependent $θ$ by reducing it to the two-dimensional QED. We demonstrate the occurrence of chiral charge generation in the background of static magnetic field for two cases: a magnetic monopole and a uniform magnetic flux. For the monopole case, the chiral charge comes out from the monopole while canceling the Witten effect. For the case of the uniform flux, on the other hand, the effect of the backreaction cannot be ignored, giving a more non-trivial time dependence. We also discuss their implications on baryogenesis.

hep-th↗

arXiv2022

$μ$TRISTAN

The ultra-cold muon technology developed for the muon $g-2$ experiment at J-PARC provides a low emittance $μ^+$ beam which can be accelerated and used for realistic collider experiments. We consider the possibility of new collider experiments by accelerating the $μ^+$ beam up to 1 TeV. Allowing the $μ^+$ beam to collide with a high intensity $e^-$ beam at the TRISTAN energy, $E_{e^-}= 30$ GeV, in the storage ring with the same size as TRISTAN (the circumference of 3 km), one can realize a collider experiment with the center-of-mass energy $\sqrt s = 346$ GeV, which allows productions of the Higgs bosons through the vector boson fusion processes. We estimate the deliverable luminosity with existing accelerator technologies to be at the level of $5 \times 10^{33}$ cm$^{-2}$ s$^{-1}$, with which the collider can be a good Higgs boson factory. The $μ^+ μ^+$ colliders up to $\sqrt s = 2$ TeV are also possible by using the same storage ring. They have a capability of producing the superpartner of the muon up to TeV masses.

hep-ph↗

arXiv2022

Stable $Z$-strings with topological polarization in two Higgs doublet model

We find that a $Z$-string is stable in a wide range of parameter space of the two Higgs doublet model due to a split into a pair of two topological $Z$-strings with fractional $Z$ fluxes. This configuration, a bound state of the two strings connected by a domain wall, is called a vortex molecule. Although the vortex molecule has no net topological charge, the locally induced topological charge density is polarized, namely distributed positively around one constituent string and negatively around the other constituent string, leading to the stability of the molecule. We numerically show that the vortex molecule is indeed a stable solution of the equation of motions in a much wider parameter space of the model than the usual axially symmetric $Z$-string in the Standard Model and the two Higgs doublet model, although it is not the case for experimental values of the parameters.

hep-ph↗

arXiv2022

Electroweak-Skyrmion as Asymmetric Dark Matter

We propose a scenario that the Electroweak-Skyrmion, a solitonic object made of the Higgs field and the electroweak gauge fields, is identified as an asymmetric dark matter. In this scenario, the relic abundance of the dark matter is related to the baryon asymmetry of the Universe through a sphaleron-like process. We show that the observed ratio of dark matter abundance to the baryon asymmetry can be explained by this scenario with an appropriate choice of model parameters that is allowed by currently available experimental constraints.

hep-ph↗

arXiv2022

Fixed Point Structure of Gradient Flow Exact Renormalization Group for Scalar Field Theories

Gradient Flow Exact Renormalization Group (GFERG) is a framework to define the Wilson action via a gradient flow equation. We study the fixed point structure of the GFERG equation associated with a general gradient flow equation for scalar field theories and show that it is the same as that of the conventional Wilson-Polchinski (WP) equation in general. Furthermore, we discuss that the GFERG equation has a similar RG flow structure around a fixed point to the WP equation. We illustrate these results with the $O(N)$ non-linear sigma model in $4-ε$ dimensions and the Wilson-Fisher fixed point.

hep-th↗

arXiv2021

Electroweak axion string and superconductivity

We study the axion strings with the electroweak gauge flux in the DFSZ axion model and show that these strings, called the electroweak axion strings, can exhibit superconductivity without fermionic zero modes. We construct three types of electroweak axion string solutions. Among them, the string with $W$-flux can be lightest in some parameter space, which leads to a stable superconducting cosmic string. We also show that a large electric current can flow along the string due to the Peccei-Quinn scale much higher than the electroweak scale. This large current induces a net attractive force between the axion strings with the same topological charge, which opens a novel possibility that the axion strings form Y-junctions in the early universe.

hep-ph↗

arXiv2020

Gravitational instantons and anomalous chiral symmetry breaking

We study anomalous chiral symmetry breaking in two-flavour QCD induced by gravitational and QCD-instantons within asymptotically safe gravity within the functional renormalisation group approach. Similarly to QCD-instantons, gravitational ones, associated to a K3-surface connected by a wormhole-like throat in flat spacetime, generate contributions to the 't~Hooft coupling proportional to $\exp(-1/g_N)$ with the dimensionless Newton coupling $g_N$. Hence, in the asymptotically safe gravity scenario with a non-vanishing fixed point coupling $g_N^*$, the induced 't Hooft coupling is finite at the Planck scale, and its size depends on the chosen UV-completion. Within this scenario the gravitational effects on anomalous $U(1)_A$-breaking at the Planck scale may survive at low energy scales. In turn, fermion masses of the order of the Planck scale cannot be present. This constrains the allowed asymptotically safe UV-completion of the Gravity-QCD system. We map-out the parameter regime that is compatible with the existence of light fermions in the low-energy regime.

hep-th↗

arXiv2020

Topological structure of Nambu monopole in Two Higgs doublet models -- Fiber bundle, Dirac's quantization and dyon --

We find a topologically non-trivial structure of the Nambu monopole in two Higgs doublet model (2HDM), which is a magnetic monopole attached by two topologically stable $Z$ strings ($Z$ flux tubes) from two opposite sides. The structure is in sharp contrast to the topological triviality of the Nambu monopole in the standard model (SM), which is attached by a single non-topological $Z$ string. It is found that the Nambu monopole in 2HDM possesses the same fiber bundle structure with those of the `t Hooft-Polyakov monopole and the Wu-Yang description of the Dirac monopole, as a result of the fact that the electromagnetic gauge field is well-defined even inside the strings and is non-trivially fibered around the monopole, while the Nambu monopole in the SM is topologically trivial because electroweak gauge symmetry is restored at the core of the string. Consequently, the Nambu monopole in 2HDM can be regarded as an embedding of the 't Hooft-Polyakov monopole into the $SU(2)_W$ gauge symmetry, and the Dirac's quantization condition always holds, which is absent for the Nambu monopole in the SM. Furthermore, we construct a dyon configuration attached with the two strings.

hep-th↗

arXiv2020

Dynamics of Nambu monopole in two Higgs doublet models -- Cosmological Monopole Collider --

We study the dynamics of the Nambu monopole in two Higgs doublet models, which is a magnetic monopole attached by two topological $Z$ strings ($Z$ flux tubes) from two opposite sides. The monopole is a topologically stable solution of the equation of motions when the Higgs potential has global $U(1)$ and $\mathbb{Z}_2$ symmetries. In this paper, we consider more general cases without the $\mathbb{Z}_2$ symmetry, and find that it is no longer a static solution but moves along the $Z$ string being pulled by the heavier string. After analytically constructing an asymptotic form of the monopole, we confirm such a motion using the numerical relaxation method. In addition, we analyze the real time dynamics of the monopole based on a point-like approximation. Consequently, if there were long string networks with the monopoles in the early universe, the monopole accelerates nearly to the speed of light emitting electromagnetic radiations as a synchrotron accelerator, and collides to an anti-monopole on the string. This collision event, which we call the cosmological monopole collider, can produce much heavier particles than those we can see today, e.g., at the Large Hadron Collider.

hep-ph↗

arXiv2020

TeV-scale Majorogenesis

The Majoron, the Nambu-Goldstone boson of lepton number symmetry, is an interesting candidate for dark matter as it deeply connects the dark matter and neutrino physics. In this paper, we consider the Majoron dark matter as pseudo Nambu-Goldstone boson with TeV-scale mass. The heavy Majoron generally has the large decay constant and tiny Yukawa couplings to light right-handed neutrinos which are required by cosmological and astrophysical observations. That makes it difficult to realize the desired amount of the relic abundance of Majoron dark matter. We consider three improved scenarios for the generation of Majoron, dubbed as Majorogenesis, in the early universe and find in all cases the parameter space compatible with the relic abundance and cosmic-ray constraints.

hep-ph↗

arXiv2020

Obtaining the sphaleron field configurations with gradient flow

We propose a formalism to obtain the electroweak sphaleron, which is one of the static classical solutions, using the gradient flow method. By adding a modification term to the gradient flow equation, we can obtain the sphaleron configuration as a stable fixed point of the flow in the large flow time. Applying the method to the $SU(2)$-Higgs model (the Weinberg angle $θ_W$ is $0$) in four dimensions, we obtain the sphaleron configuration whose energy coincides with previous studies numerically. We can also show that the Chern-Simons number of the solution has a half-integer.

hep-th↗

arXiv2020

Scalegenesis and fermionic dark matters in the flatland scenario

We propose an extension of the standard model with Majorana-type fermionic dark matters based on the flatland scenario where all scalar coupling constants, including scalar mass terms, vanish at the Planck scale, i.e. the scalar potential is flat above the Planck scale. This scenario could be compatible with the asymptotic safety paradigm for quantum gravity. We search the parameter space so that the model reproduces the observed values such as the Higgs mass, the electroweak vacuum and the relic abundance of dark matter. We also investigate the spin-independent elastic cross section for the Majorana fermions and a nucleon. It is shown that the Majorana fermions as dark matter candidates could be tested by dark matter direct detection experiments such as XENON, LUX and PandaX-II. We demonstrate that within the minimal setup compatible with the flatland scenario at the Planck scale or asymptotically safe quantum gravity, the extended model could have a strong predictability.

hep-ph↗

arXiv2020

Topological Nambu monopole in two Higgs doublet models

We show that a topological Nambu monopole exists as a regular solution for a large range of parameters in two Higgs doublet models, contrary to the standard model admitting only non-topological Nambu monopoles. We analyze a Higgs potential with a global $U(1)$ symmetry and a discrete symmetry $\mathbb{Z}_2$. The monopole is attached by two topological $Z$ strings ($Z$ flux tubes) from both sides. Despite of a trivial second homotopy group, the discrete symmetry $\mathbb{Z}_2$ together with a non-trivial first homotopy group for $Z$ strings topologically ensures the topological stability. After analytically constructing an asymptotic form of such a configuration, we explicitly construct a solution of the equation of motion based on a 3D numerical simulation, in which magnetic fluxes spherically emanating from the monopole at large distances are deformed in the vicinity of the monopole. Since the monopoles are expected to be abundant in the present universe, they might be observed in the current monopole searches.

hep-ph↗

arXiv2018

A novel regularization of chiral gauge theory

We propose a novel gauge-invariant regularization for the perturbative chiral gauge theory.Our method consists of the two ingredients: use of the domain-wall fermion to describe a chiral fermion with Pauli-Villars regulators and application of the di- mensional regularization only to the gauge field. This regularization is implemented in the Lagrangian level, unlike other gauge-invariant regularizations (eg. the covariant regularizations). We show that the Abelian (fermion number) anomaly is reproduced correctly in this formulation. We also show that once we add the counter terms to the full theory, then the renormalization in the chiral gauge theory is automatically achieved.

hep-th↗

arXiv2017

Axial U(1) current in Grabowska and Kaplan's formulation

Recently, Grabowska and Kaplan suggested a non-perturbative formulation of a chiral gauge theory, which consists of the conventional domain-wall fermion and a gauge field that evolves by the gradient flow from one domain wall to the other. In this paper, we discuss the U(1) axial-vector current in 4 dimensions using this formulation. We introduce two sets of domain-wall fermions belonging to complex conjugate representations so that the effective theory is a 4-dimensional vector-like gauge theory. Then, as a natural definition of the axial-vector current, we consider a current that generates the simultaneous phase transformations for the massless modes in 4 dimensions. However, this current is exactly conserved and does not reproduce the correct anomaly. In order to investigate this point precisely, we consider the mechanism of the conservation. We find that this current includes not only the axial current on the domain wall but also a contribution from the bulk, which is non-local in the sense of 4-dimensional fields. Therefore, the local current is obtained by subtracting the bulk contribution from it.

hep-lat↗