Table of contents

We continue our previous determination of the masses of the sequential fourth generation quarks in an extension of the Standard Model, and predict the mass m₄ (m_L) of the fourth generation neutrino ν₄ (charged lepton L) by solving the dispersion relations associated with heavy fermion decays. The results m₄ ≈ 170 GeV and m_L ≈ 270 GeV are extracted from the dispersive analyses of the t → de⁺ν₄ and ${L}^{-}\to {\nu }_{1}\bar{t}d$ decay widths, respectively, where t (d, e⁺, ν₁) denotes a top quark (down quark, positron, light neutrino). The predictions are cross-checked by examining the ${L}^{-}\to {\nu }_{4}\bar{u}d$ decay, $\bar{u}$ being an anti-up quark. It is shown that the fourth generation leptons with the above masses survive the current experimental bounds from Higgs boson decays into photon pairs and from the oblique parameters. We also revisit how the existence of the fourth generation leptons impacts the dispersive constraints on the neutrino masses and the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix elements. It is found that the unitarity of the 3 × 3 PMNS matrix holds well up to corrections of $O({m}_{\nu }^{2}/{m}_{W}^{2})$, m_ν (m_W) being a light neutrino (the W boson) mass, whose mixing angles and CP phase prefer the values θ₁₂ ≈ 34^∘, θ₂₃ ≈ 47^∘, θ₁₃ ≈ 5^∘ and δ ≈ 200^∘ in the normal-ordering scenario for neutrino masses.

The impact of the CPT-Odd electroweak gauge sector of the Standard Model Extension on the electromagnetic properties of charged leptons is studied. This gauge sector is characterized by the ${\left({k}_{1}\right)}_{\mu }$ and ${\left({k}_{2}\right)}_{\mu }$ Lorentz violation (LV) coefficients, which have positive mass dimension because they are associated with a U_Y(1)-invariant and with an SU_L(2)-invariant dimension-three operators, respectively. They belong to the category of relevant interactions, which can have strong effects on low-energy observables. We present a comprehensive study on the impact of this sector on the magnetic dipole moment (MDM) and the electric dipole moment (EDM) of charged leptons, up to second order in these LV coefficients, both at the tree and one-loop levels. We find that the O(k_i) contributions at the tree and one-loop levels depend on energy, while $O({k}_{i}^{2})$ ones at the tree-level do not. As for $O({k}_{i}^{2})$ one-loop effects, there are both energy-dependent and energy-independent contributions, but we have focused only on those of the latter type. We find that the EDM only is generated at O(k_i) up to one-loop level, whereas the MDM receives contributions from both O(k_i) and $O({k}_{i}^{2})$ at both tree and one-loop levels. The contributions of O(k_i) to the MDM are found to be suppressed relative to the corresponding contributions to the EDM by approximately three orders of magnitude. Using a recent experimental limit on the electron EDM the $| {\left({k}_{2}\right)}_{0}-| {{\bf{k}}}_{{\bf{2}}}| \cos {\theta }_{\gamma }| \lt 0.86\,{m}_{e}$ bound was obtained. As far as the contributions of $O({k}_{i}^{2})$ are concerned, we find that the tree-level contributions are suppressed with respect to the one-loop ones by at least a factor of $\left({m}_{l}^{2}/{m}_{Z}^{2}\right)$. We find that the contribution to the electron MDM is by far the dominant one, as it can be up to four and seven orders of magnitude greater than those of the muon and tau, respectively. The Lorentz coefficient ${\left({k}_{{AF}}\right)}_{\mu }$ of the Carroll–Field–Jackiw's QED is given by a linear combination of the ${\left({k}_{1}\right)}_{\mu }$ and ${\left({k}_{2}\right)}_{\mu }$ vectors. Assuming that $| {k}_{1}^{2}| ,| {k}_{2}^{2}| \gg | {k}_{{AF}}^{2}|$ and taking ${\left({k}_{{AF}}\right)}_{\mu }=0$, which implies that ${\left({k}_{1}\right)}_{\mu }$ and ${\left({k}_{2}\right)}_{\mu }$ are collinear, we obtain an upper bound of $\left|\tfrac{{k}_{2}^{2}}{{m}_{e}^{2}}\right|\lt 4.36\times {10}^{-10}$. The fact that ${k}_{2}^{2}$ is an observer Lorentz invariant allows us to introduce a new-physics scale through $\sqrt{{k}_{2}^{2}}={{\rm{\Lambda }}}_{\mathrm{CPT}}$, for which we obtain the upper limit Λ_CPT < 2.08 × 10⁻⁵m_e. The physical implications derived from the fact that the LV coefficients have positive mass units are discussed.

Vector-like leptons (VLLs), as one kind of interesting new particle, can produce a rich phenomenology in low- and high-energy experiments. In the framework of the singlet vector-like lepton with scalar (VLS) model, we investigate the discovery potential of the VLL via its single production at the International Linear Collider (ILC) with a center of mass energy $\sqrt{s}=$ 1 TeV and an integrated luminosity ${ \mathcal L }$ = 1 ab⁻¹, taking into account the appropriate polarization. For the signal and standard model (SM) background analysis, we have considered two kinds of decay channels for the W boson, i.e. pure leptonic and fully hadronic decay channels. Our analytic results show that the parameter space M_F ∈  [300, 675] GeV and κ ∈  [0.0294, 0.1] might be detected by the proposed ILC for a pure leptonic decay channel. For a fully hadronic decay channel, a larger detection region of the parameter space isderived as M_F ∈  [300, 700] GeV and κ ∈  [0.0264, 0.0941].

We have examined a production of a muon pair through vector boson fusion and direct μ⁺μ⁻ annihilation at future high-energy muon colliders in the framework of the Randall–Sundrum-like model with a small curvature of spacetime. The collision energies of 3 TeV, 14 TeV and 100 TeV are addressed. Differential and total cross sections are both calculated, and exclusion bounds on a five-dimensional gravity scale are obtained depending on collision energy and the integrated luminosity of the muon colliders. For comparison, the effects of extra dimensions in the Arkani-Hamed–Dimopoulos–Dvali model at the muon collider are studied.

Amid the uncertainty regarding the fundamental nature of neutrinos, we adhere to the Dirac description, and construct a model in the framework of Δ(27) symmetry. The model successfully accounts for the hierarchical patterns of both charged lepton and neutrino masses. The neutrino mass matrix exhibits four texture zeroes, and the associated mixing scheme aligns with the experimental data, notably controlled by a single parameter.

Lepton flavor violation (LFV) represents a clear new physics (NP) signal beyond the Standard Model (SM). In this paper, we study LFV decay $Z\to {l}_{i}^{\pm }{l}_{j}^{\mp }$ with ${l}_{i}^{-}$ representing the lepton and ${l}_{i}^{+}$ representing the antilepton in the B-L Supersymmetric Standard Model (B-LSSM). We calculate these processes separately in the mass eigenstate basis and the electroweak interaction basis, and for the latter adopt the mass insertion approximation (MIA) method. The effect of the LFV decays $Z\to {l}_{i}^{\pm }{l}_{j}^{\mp }$ changing with parameters can be shown clearly at the analytic level by the MIA, which provides a new way for us to analyze the LFV processes. At the same time, the corresponding constraints from the LFV decays ${l}_{j}^{-}\to {l}_{i}^{-}\gamma$ and (g − 2)_μ are considered to analyze the numerical results.

An in-beam gamma-ray spectroscopy study of the even–even nucleus ⁹²Mo has been carried out using the ³⁰Si + ⁶⁵Cu, ¹⁸O + ⁸⁰Se reactions at beam energies of 120 and 99 MeV, respectively. Angular distribution from the oriented state ratio (R_ADO) and linear polarization (Δ_asym) measurements have fixed most of the tentatively assigned spin-parity of the high-energy levels. A large-scale shell-model calculation using the GWBXG interaction has been carried out to understand the configuration and structure of both positive and negative parity states up to the highest observed spin. The high-spin states primarily originate from the coupling of excited proton- and neutron-core structures in an almost stretched manner. The systematics of the energy required to form a neutron particle-hole pair excitation, νg_9/2 → νd_5/2, is discussed. The lifetimes of a few high-spin states have been measured using the Doppler shift attenuation method. Additionally, a qualitative argument is proposed to explain the comparatively strong E1 transition feeding the 7310.9 keV level.

Contemporary heavy-ion physics research aims to explore the phase diagram of strongly interacting matter and search for signs of the possible critical endpoint on the Quantum Chromodynamics phase diagram. Femtoscopy is among the important tools used for this endeavor; there have been indications that combinations of femtoscopic radii parameters (referred to as HBT radii for identical boson pairs) can be related to the system's emission duration. An apparent non-monotonic behavior in their excitation function thus might signal the location of the critical point. In this paper, we show that conclusions drawn from the results obtained with a Gaussian approximation for the pion source shape might be altered if one utilizes a more general Lévy-stable source description. We find that the characteristic size of the pion source function is strongly connected to the shape of the source and its possible power-law behavior. Taking this into account properly changes the observed behavior of the excitation function.

Longitudinal (R_L) and transverse (R_T) responses from inclusive electron scattering from ¹²C and ⁴⁰Ca nuclei are computed within a fully relativistic model, and one- and two-body current operators leading to a final one-particle one-hole state. We find that the two-body contributions have no effect on R_L but they increase R_T by up to 30%, depending on the energy and momentum transfer. Inclusive cross sections have also been computed, obtaining an increase which depends on the degree of transversity of each kinematic. The comparison with carbon data is good for the responses and the cross sections. In the case of calcium, while the model compares well with the cross section data, the agreement with the responses is generally poor. However, the inconsistencies between different data sets for the separate responses in this nucleus indicate that uncertainties in the responses are largely underestimated in the data. Our calculation is fully relativistic, does not require a factorized model and includes a full quantum mechanical description of both the initial and final nucleon states. We also show that incorporating the distortion of the nucleons while making the initial and final states orthogonal is essential to reproduce the shape and magnitude of the cross section data and carbon responses. The good agreement with the electron scattering experimental data supports the use of this approach to describe the analogous neutrino-induced scattering reaction.

The reaction rates of ²²Ne(α, n)²⁵Mg and its competing channel ²²Ne(α, γ)²⁶Mg control the production of neutron flux for weak s-process nucleosynthesis in low mass asymptotic giant branch stars and in massive stars with M ≥ 10M_⊙. The temperature range of interest for these reactions lies between 0.2 and 0.4 GK. However, the rates of these reactions are poorly constrained at these temperatures due to uncertainties in the nuclear properties of several resonance states in the compound nucleus ²⁶Mg, lying within the Gamow window. The present work reports a full R-matrix evaluation of the ²²Ne(α, n)²⁵Mg and ²²Ne(α, γ)²⁶Mg reaction rates using updated nuclear data of ²⁶Mg states. Previous rate evaluation by Adsley et al and R-matrix calculations of Wiescher et al were limited by using narrow resonance approximations and omission of the resonances below E_r = 705 keV, respectively. In this work, the R-matrix fit to the available ²²Ne(α, n)²⁵Mg reaction data is performed by including the contributions of previously neglected resonances below E_r = 705 keV and considering the interference effects. The (α, n) reaction rate from the present R-matrix evaluations is noticeably higher than the narrow resonance approximation calculations in the temperature range 0.1−0.3 GK. In particular, the present (α, n) reaction rate is significantly higher (7.5 − 4.5 times) compared to Adsley et al at 0.2−0.3 GK and ≈2 times greater than Wiescher et al at 0.3 GK. The estimated reaction rate ratio of (α, n) to (α, γ) in the relevant temperature window 0.2−0.8 GK indicates that the production of neutrons for the s-process is more likely than the radiative alpha capture reaction, compared to the previous estimate by Adsley et al.

We consider a general model for a quasi-spherical explosion in which a part of the explosion energy is thermalized, forming an expanding photosphere around a compact object, and the second part of energy is taken by the expanding shell of material which forms a shock wave in the surrounding medium. Different types of particles (electrons, hadrons) can, in principle, be accelerated at the shock. They interact with the thermal radiation from the photosphere and also with the material at the shell. We determine the equilibrium spectra of particles in the shell as a function of time after explosion and calculate the time-dependent γ-ray spectra by taking into account the effects which are due to the anisotropy of the photo-spheric radiation field on the Inverse Compton (IC) process; we also include the absorption of IC γ-rays in the photosphere radiation. We conclude that, in principle, both leptonic and hadronic models can explain the GeV-TeV gamma-ray emission recently detected from the Nova RS Oph. However, the hadronic model, in comparison to the leptonic model, is more energetically demanding and requires much stronger magnetization of the nova shell.