We numerically investigate the Araki-Uhlmann relative entropy in Quantum Field Theory, focusing on a free massive scalar field in <math altimg='si1.svg'><mn>1</mn><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mn>1</mn></math>-dimensional Minkowski spacetime. Using Tomita-Takesaki modular theory, we analyze the relative entropy between a coherent state and the vacuum state, with several types of test functions localized in the right Rindler wedge. Our results confirm that relative entropy decreases with increasing mass and grows with the size of the spacetime region, aligning with theoretical expectations.

Brazil

Marcelo S. Guimaraes

Itzhak Roditi

Silvio P. Sorella

Arthur F. Vieira

A numerical analysis of Araki-Uhlmann relative entropy in Quantum Field Theory

We consider a dark photon model in which the dark U(1) gauge symmetry is spontaneously broken by a vacuum expectation value of a new scalar boson. We focus on the ForwArd Search ExpeRiment (FASER) and calculate its sensitivity to the dark photon produced from the off-shell decay of the new scalar boson. It is found that the off-shell production extends the sensitivity region beyond the kinematical threshold of the on-shell decay of the scalar boson, and that the sensitivity region can be spanned to unexplored region. We also show the parameter space in which perturbative calculation is valid for the unitarity of an S matrix.

Japan

Takeshi Araki

Kento Asai

Yohei Nakashima

Takashi Shimomura

Dark photon pair production via off-shell dark Higgs at FASER

The T2K experiment is one of the most powerful long-baseline experiments to investigate neutrino oscillations. The off-axis near detector called ND280 is installed 280 m downstream from the neutrino production target to measure the neutrino energy spectrum. In this paper, we study the capability of the ND280 detector to search for the dark photon produced through the meson rare decay and proton bremsstrahlung processes at the proton beam dump. We find that the ten-year operation of T2K with the ND280 detector excludes the unexplored parameter region for the dark photon mass and kinetic mixing. We also show that a broader parameter region can be searched by the ND280 in the future T2K operation for dark photon as well as U(1) B−L  gauge boson.

Tomoya Iizawa

Hidetoshi Otono

Yosuke Takubo

New constraint on dark photon at T2K off-axis near detector

FASER is one of the promising experiments which search for long-lived particles beyond the Standard Model. In this paper, we consider charged lepton flavor violation (CLFV) via a light and weakly interacting boson and discuss the detectability by FASER. We focus on four types of CLFV interactions, i.e., the scalar-, pseudoscalar-, vector-, and dipole-type interaction, and calculate the sensitivity of FASER to each CLFV interaction. We show that, with the setup of FASER2, a wide region of the parameter space can be explored. Particularly, it is found that FASER2 has a sensitivity to very small coupling regions in which the rare muon decays, such as μ → eγ, cannot place bounds, and that there is a possibility to detect CLFV decays of the new light bosons.

Search for lepton flavor violating decay at FASER

We study constraints on light and feebly interacting bosons having charged lepton flavor violating couplings from electron beam dump experiments. Scalar, pseudoscalar, vector, and dipole type interactions of the bosons are analyzed, and excluded regions from the searches for decays into electron-positron pairs are derived. It is found that parameter regions unconstrained by flavor violating decays of muon can be excluded using the results of the E137 experiment. We also discuss the impact of the search for flavor violating decays of the light bosons in electron beam dump experiments.

Electron beam dump constraints on light bosons with lepton flavor violating couplings

Abstract We explore the possibility of resolving the Hubble tension and  anomaly simultaneously in a U(1) model with Majoron. We only focus on the case where the Majoron  does not exist at the beginning of the universe and is created by neutrino inverse decay  after electron–positron annihilation. In this case, the contributions of the new gauge boson  and the Majoron  to the effective number of neutrino species  can be calculated in separate periods. These contributions are labelled  for the U(1) gauge boson and  for the Majoron. The effective number  is evaluated by the evolution equations of the temperatures and the chemical potentials of light particles in each period. As a result, we find that the heavier  mass  results in a smaller  and requires a larger  to resolve the Hubble tension. Therefore, compared to previous studies, the parameter region where the Hubble tension can be resolved is slightly shifted toward the larger value of .

Kei Honda

Ryuta Kasuya

Joe Sato

Masaki J S Yang

Resolving the Hubble tension in a U(1) model with the Majoron

We propose models for neutrino masses and mixing in the framework of low scale <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><msub><mi>L</mi><mi>μ</mi></msub><mo>−</mo><msub><mi>L</mi><mi>τ</mi></msub></mrow></msub></math> gauge extension of the standard model. The models are designed to spontaneously break <math><mrow><mi>U</mi><mo>(</mo><mn>1</mn><msub><mrow><mo>)</mo></mrow><mrow><msub><mrow><mi>L</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo>−</mo><msub><mrow><mi>L</mi></mrow><mrow><mi>τ</mi></mrow></msub></mrow></msub></mrow></math> so that the <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><msub><mi>L</mi><mi>μ</mi></msub><mo>−</mo><msub><mi>L</mi><mi>τ</mi></msub></mrow></msub></math> gauge boson acquires an MeV scale mass, which is required to solve the longstanding problem of muon anomalous magnetic moment. Tiny neutrino masses are obtained by simultaneously invoking the linear and the inverse seesaw mechanism, and we succeed in realizing two types of one-zero textures in the active neutrino mass matrix. Both of the obtained textures favor inverted neutrino mass ordering and are testable in next generation experiments of neutrinoless double beta decay. We also show that some of extra scalar bosons can have MeV scale masses and would have significant impacts on observations of high energy cosmic neutrinos.

Low scale seesaw models for low scale <math><mrow><mi>U</mi><mo>(</mo><mn>1</mn><msub><mrow><mo>)</mo></mrow><mrow><msub><mrow><mi>L</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo>−</mo><msub><mrow><mi>L</mi></mrow><mrow><mi>τ</mi></mrow></msub></mrow></msub></mrow></math> symmetry

We make a rigorous computation of the relative entropy between the vacuum state and a coherent state for a free scalar in the framework of algebraic description of quantum field theory (AQFT). We study the case of the Rindler wedge. Previous calculations including path integral methods and computations from the lattice give a result for such relative entropy which involves integrals of expectation values of the energy-momentum stress tensor along the considered region. However, the stress tensor is in general nonunique. That means that if we start with some stress tensor, then we can “improve” it adding a conserved term without modifying the Poincaré charges. On the other hand, the presence of such an improving term affects the naive expectation for the relative entropy by a nonvanishing boundary contribution along the entangling surface. In other words, this means that there is an ambiguity in the usual formula for the relative entropy coming from the nonuniqueness of the stress tensor. The main motivation of this work is to solve this puzzle. We first show that all choices of stress tensor except the canonical one are not allowed by positivity and monotonicity of the relative entropy. Then we fully compute the relative entropy between the vacuum and a coherent state in the framework of AQFT using the Araki formula and the techniques of modular theory. After all, both results coincide and give the usual expression for the relative entropy calculated with the canonical stress tensor.

Argentina

Horacio Casini

Sergio Grillo

Diego Pontello

Relative entropy for coherent states from Araki formula

We consider a situation where the leading-order neutrino mass matrix is derived by a theoretical ansatz and reproduces the experimental data well, but not completely. Then, the next stage is to try to fully reproduce the data by adding small perturbation terms. In this paper, we obtain the analytical method to diagonalize the perturbed mass matrix and find a consistency condition that parameters should satisfy not to change $\sin \theta _{12}$ much. This condition could cause parameter tuning and plays a crucial role in relating the added perturbation terms with the prediction analytically, in particular, for the case of the partially quasi-degenerated neutrino masses $(m_2 \simeq m_1)$ where neutrinoless double beta decays would be observed in the phase-II experiments.

Eiichi Takasugi

Effects on sin 12 from perturbation of the neutrino mixing matrix with the partially degenerated neutrino masses

Inspired by the small mass-squared difference measured in solar neutrino oscillation experiments and by the testability, we suggest that a limit of the partial mass degeneracy, in which the masses of the first two-generation fermions are degenerate, may be a good starting point for understanding the observed fermion mass spectra and mixing patterns. The limit indicates the existence of a 2D rotation symmetry, such as , , and so on, in the flavor space of the first two generations. We propose simple models for the lepton sector based on and show that the models can successfully reproduce the experimental data without imposing unnatural hierarchies among dimensionless couplings, although at least tuning is necessary in order to explain the large atmospheric mixing. In particular, it is found that the subgroup of the symmetry plays an important role in understanding the smallness of the electron mass and . We also discuss the testability of the models by the future neutrinoless-double-beta-decay experiments and cosmological observations.