We revisit the calculation of the fermion self-energy in QED in the presence of a magnetic field. We show that, after carrying out the renormalization procedure and identifying the most general perturbative tensor structure for the modified fermion mass operator in the large field limit, the mass develops an imaginary part. This happens when account is made of the subleading contributions associated to Landau levels other than the lowest one. The imaginary part is associated to a spectral density describing the spread of the mass function in momentum. The center of the distribution corresponds to the magnetic-field modified mass. The width becomes small as the field intensity increases in such a way that for asymptotically large values of the field, when the separation between Landau levels becomes also large, the mass function describes a stable particle occupying only the lowest Landau level. For large but finite values of the magnetic field, the spectral density represents a finite probability for the fermion to occupy Landau levels other than the lowest Landau level.

We calculate axion potential in antiferromagnetic insulators from path integral. It is derived from the effective potential for the order parameter of the antiferromagnetic phase in insulators. Static and dynamical axions are defined consistently from the potential. Consequently antiferromagnetic/paramagnetic and topological/normal orders are classified. The dynamical axion is predicted in all phases and its mass turns out to be various values up to eV scale.

Light scalar fields that couple to matter through the Higgs portal mediate long-range fifth forces. We show how the mixing of a light scalar with the Higgs field can lead to this fifth force being screened around macroscopic objects. This behavior can only be seen by considering both scalar fields as dynamical, and is missed if the mixing between the Higgs field and the scalar field is not taken into account. We explain under which conditions the naive “integrating-out" procedure fails, i.e., when the mass matrix of the Higgs-scalars system has a nearly vanishing mass eigenvalue. The resulting flat direction in field space can be lifted at the quadratic order in the presence of matter and the resulting fifth force mediated by the Higgs portal can be screened either when the gravitating objects are large enough or their surface Newton potential exceeds a threshold. Finally we discuss the implications of these results for nearly massless relaxion models.

The vector $U$ bosons, or so-called “dark photons,” are one of the possible candidates for the dark matter mediators. They are supposed to interact with the standard matter via a “vector portal” due to the $U(1)-U(1{)}^{\prime }$ symmetry group mixing which might make them visible in particle and heavy-ion experiments. While there is no confirmed observation of dark photons, the detailed analysis of different experimental data allows to estimate the upper limit for the kinetic mixing parameter ${\epsilon }^2$ depending on the mass $M_U$ of $U$ bosons which is also unknown. In this study we present theoretical constraints on the upper limit of ${\epsilon }^2(M_U)$ in the mass range $M_U\le 0.6\mathrm{GeV}$ from the comparison of the calculated dilepton spectra with the experimental data from the HADES collaboration at SIS18 energies where the dark photons are not observed. Our analysis is based on the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which reproduces well the measured dilepton spectra in $p+p$, $p+A$ and $A+A$ collisions. Additionally to the different dilepton channels originating from interactions and decays of ordinary matter particles (mesons and baryons), we incorporate the decay of hypothetical $U$ bosons to dileptons, $U\to e^{+}{e}^{-}$, where the $U$ bosons themselves are produced by the Dalitz decay of pions ${\pi }^0\to \gamma U$, $\eta$ mesons $\eta \to \gamma U$ and Delta resonances $\mathrm{\Delta }\to NU$. Our analysis can help to estimate the requested accuracy for future experimental searches of “light” dark photons by dilepton experiments.

We entertain the option of scalar leptoquarks to explain the anomalies in the semileptonic decays of $B$ mesons and the discrepancies in the lepton $(g-2{)}_l$’s including the recent results at Fermilab E989. The $R_{K^{(*)}}$ and $R_{D^{(*)}}$ anomalies can be accommodated by the specific couplings for triplet and singlet leptoquarks, respectively, subject to the bounds from $B\to K\nu \overline{\nu }$. We discuss the correlation between the leptonic signatures from leptoquarks such as $\mu \to e\gamma$ and the electric dipole moment of the electron and show that desirable neutrino masses can be generated dominantly by top-quark loops in the extension of the model with several doublet leptoquarks.

Nonlinear electrodynamic models are reassessed in this paper to pursue an investigation of the kinematics of the Compton effect in a magnetic background. Before considering specific models, we start off by presenting a general non-linear Lagrangian built up in terms of the most general Lorentz- and gauge-invariant combinations of the electric and magnetic fields. The extended Maxwell-like equations and the energy-momentum tensor conservation are presented and discussed in their generality. We next expand the fields around a uniform and time-independent electric and magnetic backgrounds up to second order in the propagating wave, and compute dispersion relations which account for the effect of the external fields. We obtain thereby the refraction index and the group velocity for the propagating radiation in different situations. In particular, we focus on the kinematics of the Compton effect in presence of external magnetic fields. This yields constraints that relate the derivatives of the general Lagrangian with respect to the field invariants and the magnetic background under consideration. We carry out our inspection by focusing on some specific nonlinear electrodynamic effective models: Hoffmann-Infeld, Euler-Heisenberg, generalized Born-Infeld and logarithmic.

Inspired by the neutrino oscillations data, we consider the exact $\mu -\tau$ symmetry, implemented at the level of the neutrino mass matrix, as a good initial framework around which to study and describe neutrino phenomenology. Working in the diagonal basis for the charged leptons, we deviate from $\mu -\tau$ symmetry by just modifying the phases of the neutrino mass matrix elements. This deviation is enough to allow for a nonvanishing neutrino mixing entry $|V_{e3}|$ (i.e., ${\theta }_{13}$) but it also gives a very stringent (and eventually falsifiable) prediction for the atmospheric neutrino mixing element $|V_{\mu 3}|$ as a function of $|V_{e3}|$. The breaking by phases is characterized by a single phase and is shown to lead to interesting lower bounds on the allowed mass of the lightest neutrino depending on the ordering of neutrino masses (normal or inverted) and on the value of the Dirac $\mathcal{C}P$ violating phase ${\delta }_{CP}$. The allowed parameter space for the effective Majorana neutrino mass $m_{ee}$ is also shown to be nontrivially constrained.

We describe a proposal for constructing a lattice theory that we argue may be capable of yielding free Weyl fermions in the continuum limit. The model employs reduced staggered fermions and uses site parity dependent Yukawa interactions of Fidkowski-Kitaev type to gap a subset of the lattice fermions without breaking symmetries. The possibility for such symmetric mass generation is tied to the cancellation of certain discrete anomalies arising in the continuum limit. The latter place strong constraints on the number of lattice fermions—constraints that are satisfied by this model. We present numerical results for the model in two dimensions which support this scenario.

We introduce a reweighting technique which allows for a continuous sampling of temperatures in a single simulation and employ it to compute the temperature dependence of the QCD topological susceptibility ${\chi }_{\mathrm{top}}$ at high temperatures. The method determines the ratio of ${\chi }_{\mathrm{top}}$ between any two temperatures within the explored temperature range. We find that the results from the method agree with our previous determination and that it is competitive with but not better than existing methods of determining $\mathrm{d}{\chi }_{\mathrm{top}}/\mathrm{d}T$. The method may also be useful in exploring the temperature dependence of other thermodynamical observables in QCD in a continuous way.

We perform a fit of the finite-volume QCD spectrum of three pions at maximal isospin to constrain the three-body force. We use the unitarity-based relativistic three-particle quantization condition, with the GWUQCD spectrum obtained at 315 MeV and 220 MeV pion mass in two-flavor QCD. For the heavier pion mass we find that the data is consistent with a constant contact term close to zero, whereas for the lighter mass we see a statistically significant energy dependence in tension with the prediction of leading order ChPT. Our results also suggest that with enough three-body energy levels, the two-body amplitude could be constrained.

Starting from the first renormalized factorization theorem for a process described at subleading power in soft-collinear effective theory, we discuss the resummation of Sudakov logarithms for such processes in renormalization-group improved perturbation theory. Endpoint divergences in convolution integrals, which arise generically beyond leading power, are regularized and removed by systematically rearranging the factorization formula. We study in detail the example of the $b$-quark induced $h\to \gamma \gamma$ decay of the Higgs boson, for which we resum large logarithms of the ratio $M_h/m_b$ at next-to-leading logarithmic order. We also briefly discuss the related $gg\to h$ amplitude.

We study the existence of fully-heavy hidden-flavor $bc\overline{b}\overline{c}$ tetraquark states with various $J^{PC}=0^{\pm +},0^{--},1^{\pm \pm },2^{++}$, by using the moment QCD sum rule method augmented by fundamental inequalities. Using the moment sum rule analyses, our calculation shows that the masses for the $S$-wave positive parity $bc\overline{b}\overline{c}$ tetraquark states are about 12.2–12.4 GeV in both $[{\overline{\mathbf{3}}}_{\mathbf{c}}{]}_{bc}\otimes [{\mathbf{3}}_{\mathbf{c}}{]}_{\overline{b}\overline{c}}$ and $[{\mathbf{6}}_{\mathbf{c}}{]}_{bc}\otimes [{\overline{\mathbf{6}}}_{\mathbf{c}}{]}_{\overline{b}\overline{c}}$ color configuration channels. Except for two $0^{++}$ states, such results are below the thresholds $T_{{\eta }_c{\eta }_b}/T_{\mathrm{\Upsilon }\psi }$ and $T_{B_c{B}_c}$, implying that these $S$-wave positive parity $bc\overline{b}\overline{c}$ tetraquark states are probably stable against the strong interaction. For the $P$-wave negative parity $bc\overline{b}\overline{c}$ tetraquarks, their masses in the $[{\overline{\mathbf{3}}}_{\mathbf{c}}{]}_{bc}\otimes [{\mathbf{3}}_{\mathbf{c}}{]}_{\overline{b}\overline{c}}$ channel are around 12.9–13.2 GeV, while a bit higher in the $[{\mathbf{6}}_{\mathbf{c}}{]}_{bc}\otimes [{\overline{\mathbf{6}}}_{\mathbf{c}}{]}_{\overline{b}\overline{c}}$ channel. They can decay into the $c\overline{c}+b\overline{b}$ and $c\overline{b}+b\overline{c}$ final states via the spontaneous dissociation mechanism, including the $J/\psi \mathrm{\Upsilon }$, ${\eta }_c\mathrm{\Upsilon }$, $J/\psi {\eta }_b$, $B_c^{+}{B}_c^{-}$ channels.

When the Standard Model Higgs sector is extended with a complex singlet that breaks global lepton number symmetry spontaneously, a massless Goldstone boson called the Majoron $J$ arises. In addition to increasing Higgs invisible decay through mixing, the Majoron can generally have flavor-changing interactions with fermions. We find that Type-III seesaw model poses such interesting properties with both charged leptons and neutrinos. This opens up new channels to search for the Majoron. We use the experimental data such as muonium-antimuonium oscillation and flavor-changing neutrino and charged lepton decays to put constraints on the couplings. As a novel way to reveal the chiral properties of these interactions, we propose an experimentally measurable polarization asymmetry of flavor-changing $\ell \to {\ell }^{\prime }J$ decays.

Combined bounds on the Majorana neutrino mass for light and heavy neutrino exchange mechanisms are derived from current neutrinoless double beta decay ($0\nu \beta \beta$) search results for a variety of nuclear matrix element (NME) models. The approach requires self-consistency of a given model to predict NMEs across different isotopes. The derived bounds are notably stronger than those from any single experiment and show less model-to-model variation, highlighting the advantages of using multiple isotopes in such searches. Projections indicate that the combination of near-term experiments should be able to probe well into the inverted neutrino mass hierarchy region. A method to visually represent $0\nu \beta \beta$ experimental results is also suggested to more transparently compare across different isotopes and explicitly track model dependencies.

Using data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies from 4.178 to 4.600 GeV, we study the process $e^{+}{e}^{-}\to {\pi }^{0}X(3872)\gamma$ and search for $Z_c(4020{)}^0\to X(3872)\gamma$. We find no significant signal and set upper limits on $\sigma (e^{+}{e}^{-}\to {\pi }^{0}X(3872)\gamma )·\mathcal{B}(X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi )$ and $\sigma (e^{+}{e}^{-}\to {\pi }^0{Z}_c(4020{)}^0)·\mathcal{B}(Z_c(4020{)}^0\to X(3872)\gamma )·\mathcal{B}(X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi )$ for each energy point at 90% confidence level, which is of the order of several tenths pb.

We examine the status of the Chern-Simons (or Kodama) state from the point of view of a formulation of gravity that uses only real connection and metric variables and a real action. We may package the real connection variables into the complex self-dual Ashtekar connection (and will do so to make contact with previous work), but that operation is essentially cosmetic and can be undone at any step or even bypassed altogether. The action will remain the (real) Einstein-Cartan action, forgoing the addition of the usual Holst (or Nieh-Yan) term with an imaginary coefficient. It is then found that the constraints are solved by a modification of the Chern-Simons state which is a pure phase (in the Lorentzian theory, we stress), the phase containing only the fully gauge-invariant imaginary part of the Chern-Simons functional. Thus, the state for the “real theory” is nonpathological with regards to the most egregious criticisms facing its “nonreal” cousin, solving the complex theory. A straightforward modification of the real Chern-Simons state is also a solution in quasitopological theories based on the Euler invariant, for which the cosmological constant, $\mathrm{\Lambda }$, is dynamical. In that case it is enough to shift the usual factor of $\mathrm{\Lambda }$ in the wave function to the inside of the spatial Chern-Simons integral. The trick only works for the quasi-Euler theory with a critical coupling previously identified in the literature. It does not apply to the quasi-Pontryagin theory.

We study holographically the zero and finite temperature behavior of the potential energy and holographic subregion complexity corresponding to a probe meson in a nonconformal model. Interestingly, in the specific regime of the model parameters, at zero and low temperatures, we find a nicely linear relation between dimensionless meson potential energy and dimensionless volume implying that the less binding meson state needs less information to be specified and vice versa, but this behavior can not be confirmed in the high temperature limit. We also observe that the nonconformal corrections increase the holographic subregion complexity in both the zero and finite temperature states. However, nonconformality has a decreasing effect on the dimensionless meson potential energy. We finally find that in the vicinity of the phase transition, the zero temperature meson state is more favorable than the finite temperature state, from the holographic subregion complexity point of view.

The decoupling of heavy fields as required by the Appelquist-Carazzone theorem plays a fundamental role in the construction of any effective field theory. However, it is not a trivial task to implement a renormalization prescription that produces the expected decoupling of massive fields, and it is even more difficult in curved spacetime. Focused on this idea, we consider the renormalization of the one-loop effective action for the Yukawa interaction with a background scalar field in curved space. We compute the beta functions within a generalized DeWitt-Schwinger subtraction procedure and discuss the decoupling in the running of the coupling constants. For the case of a quantized scalar field, all the beta function exhibit decoupling, including also the gravitational ones. For a quantized Dirac field, decoupling appears almost for all the beta functions. We obtain the anomalous result that the mass of the background scalar field does not decouple.

In a recent paper we showed that the collapse to a black hole in one-parameter families of initial data for massless, minimally coupled scalar fields in spherically symmetric semiclassical loop quantum gravity exhibited a universal mass scaling similar to the one in classical general relativity. In particular, no evidence of a mass gap appeared as had been suggested by previous studies. The lack of a mass gap indicated the possible existence of a self-similar critical solution as in general relativity. Here we provide further evidence for its existence. Using an adaptive mesh refinement code, we show that “echoes” arise as a result of the discrete self-similarity in space-time. We also show the existence of “wiggles” in the mass scaling relation, as in the classical theory. The results from the semiclassical theory agree well with those of classical general relativity unless one takes unrealistically large values for the polymerization parameter.