We consider the properties of the gauge-invariant two-point correlation functions of the gauge-field strengths for QCD in the presence of a magnetic background field at zero temperature. We discuss the general structure of the correlators in this case and provide the results of an exploratory lattice study for $N_f=2$ QCD discretized with unimproved staggered fermions. Our analysis provides evidence for the emergence of anisotropies in the nonperturbative part of the correlators and for an increase of the gluon condensate as a function of the external magnetic field.

We investigate the medium modification of the generalized vector form factors of the nucleon, which include the electromagnetic and energy-momentum tensor form factors, based on an in-medium modified $\pi \text{−}\rho \text{−}\omega$ soliton model. We find that the vector form factors of the nucleon in nuclear matter fall off faster than those in free space, which implies that the charge radii of the nucleon become larger in nuclear medium than in free space. We also compute the corresponding transverse charge densities of the nucleon in nuclear matter, which clearly reveal the increasing of the nucleon size in nuclear medium.

A dedicated study of the $\pi {\mathrm{\Sigma }}_c$ scattering around its threshold is carried out in this work to probe the nature of ${\mathrm{\Lambda }}_c(2595{)}^{+}$. We first demonstrate that the effective range expansion approach fails to work near the ${\mathrm{\Lambda }}_c(2595{)}^{+}$ pole position, due to the presence of a nearby Castillejo-Dalitz-Dyson (CDD) pole around the $\pi {\mathrm{\Sigma }}_c$ thresholds. We then develop a general framework to properly handle the situation with a CDD pole accompanied by nearby thresholds, which is first elaborated for the single-channel case and then generalized to the coupled-channel study. The isospin-breaking effects of the three $\pi {\mathrm{\Sigma }}_c$ channels with different thresholds are specially taken into account in our study. The finite-width effects from the ${\mathrm{\Sigma }}_c$ baryons are considered and found to be relevant to give the ${\mathrm{\Lambda }}_c(2595{)}^{+}$ width fully compatible with its experimental value. Through the compositeness analysis, our robust conclusion is that the ${\pi }^0{\mathrm{\Sigma }}_c^{+}$ component is subdominant inside the ${\mathrm{\Lambda }}_c(2595{)}^{+}$.

The current short baseline reactor experiments, Daya Bay and RENO (Double Chooz) have measured (or are capable of measuring) an effective $\mathrm{\Delta }{m}^2$ associated with the atmospheric oscillation scale of $0.5\mathrm{km}/\mathrm{MeV}$ in electron antineutrino disappearance. In this paper, I compare and contrast the different definitions of such an effective $\mathrm{\Delta }{m}^2$ and argue that the simple, L/E independent definition given by $\mathrm{\Delta }{m}_{ee}^2\equiv {\cos }^2{\theta }_{12}\mathrm{\Delta }{m}_{31}^2+{\sin }^2{\theta }_{12}\mathrm{\Delta }{m}_{32}^2$, i.e. “the ${\nu }_e$ weighted average of $\mathrm{\Delta }{m}_{31}^2$ and $\mathrm{\Delta }{m}_{32}^2$,” is superior to all other definitions and is useful for both short baseline experiments mentioned above and for the future medium baseline experiments JUNO and RENO-50.

General neutrino-scalar couplings appear in many extensions of the Standard Model. We can probe these neutrino-scalar couplings by a leptonic decay of mesons and from a heavy neutrino search. Our analysis improves the present limits to $|g_e{|}^2<1.9\times {10}^{-6}$ and $|g_{\mu }{|}^2<1.9\times {10}^{-7}$ at 90% C.L. for massless scalars. For massive scalars, we found for the first time the constraints for $g_{\alpha }^2$ couplings to be $10^{-6}-10^{-1}$, respectively, for scalar masses between up 100 MeV, and we have no limits for masses above 300 MeV.

Measurements of the top quark-antiquark ($t\overline{t}$) spin correlations and the top quark polarization are presented for $t\overline{t}$ pairs produced in $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$. The data correspond to an integrated luminosity of $19.5{\mathrm{fb}}^{-1}$ collected with the CMS detector at the LHC. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, both inclusively and differentially, with respect to the invariant mass, rapidity, and transverse momentum of the $t\overline{t}$ system. The measurements are unfolded to the parton level and found to be in agreement with predictions of the standard model. A search for new physics in the form of anomalous top quark chromo moments is performed. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment and the imaginary part of the chromo-electric dipole moment are evaluated.

We search for the rare radiative decay $D^0\to \gamma \gamma$ using a data sample with an integrated luminosity of $832{\mathrm{fb}}^{-1}$ recorded by the Belle detector at the KEKB $e^{+}{e}^{-}$ asymmetric-energy collider. We find no statistically significant signal and set an upper limit on the branching fraction of $\mathcal{B}(D^0\to \gamma \gamma )<8.5\times 10^{-7}$ at 90% confidence level. This is the most restrictive limit on the decay channel to date.

Chirality of neutrinos modifies the conventional kinetic theory and hydrodynamics, leading to unusual chiral transport related to quantum anomalies in field theory. We argue that these corrections have new phenomenological consequences for hot and dense neutrino gases, especially in core-collapse supernovae. We find that the neutrino density can be converted to the fluid helicity through the chiral vortical effect. This fluid helicity effectively acts as a chiral chemical potential for electrons via the momentum exchange with neutrinos and induces a “helical plasma instability” that generates a strong helical magnetic field. This provides a new mechanism for converting the gravitational energy released by the core collapse to the electromagnetic energy and potentially explains the origin of magnetars. The other possible applications of the neutrino chiral transport theory are also discussed.

We consider a static spherically symmetric thin shell wormhole that collides with another thin shell consisting of ordinary matter. By employing the geometrical constraint, which leads to the conservation of energy and momentum, we show that the state after the collision can be solved from the initial data. In the low speed approximation, the solutions are rather simple. The shell may either bounce back or pass through the wormhole. In either case, the wormhole shrinks right after the collision. In the “bouncing” case, a surprising result is that the radial speeds before and after the collision satisfy an addition law, which is independent of other parameters of the wormhole and the shell. Once the shell passes through the wormhole, we find that the shell always expands. However, the expansion rate is the same as its collapsing rate right before the collision. Finally, we find the solution for the shell moving together with the wormhole. This work sheds light on the interaction between wormholes and matter.

We prove perturbative renormalizability of projectable Hořava gravity. The key element of the argument is the choice of a gauge which ensures the correct anisotropic scaling of the propagators and their uniform falloff at large frequencies and momenta. This guarantees that the counterterms required to absorb the loop divergences are local and marginal or relevant with respect to the anisotropic scaling. Gauge invariance of the counterterms is achieved by making use of the background-covariant formalism. We also comment on the difficulties of this approach when addressing the renormalizability of the nonprojectable model.

We critically discuss the measure of very short time intervals. By means of a Gedankenexperiment, we describe an ideal clock based on the occurrence of completely random events. Many previous thought experiments have suggested fundamental Planck-scale limits on measurements of distance and time. Here we present a new type of thought experiment, based on a different type of clock, that provide further support for the existence of such limits. We show that the minimum time interval $\mathrm{\Delta }t$ that this clock can measure scales as the inverse of its size $\mathrm{\Delta }r$. This implies an uncertainty relation between space and time: $\mathrm{\Delta }r\mathrm{\Delta }t>G\hslash /c^4,$ where $G$, $\hslash$, and $c$ are the gravitational constant, the reduced Planck constant, and the speed of light, respectively. We outline and briefly discuss the implications of this uncertainty conjecture.

In dense stellar environments, nuclei may become unstable against electron captures and/or neutron emissions. These processes are of particular importance for determining the internal constitution of white-dwarf cores and neutron-star crusts. In this paper, the role of electron exchange and polarization effects is studied. In particular, the instability condition for the onset of electron captures and neutron emissions is extended so as to account for electron exchange and polarization. Moreover, general analytical expressions for the corresponding density and pressure are derived. The corrections to the electron-capture threshold in white-dwarf cores are found to be very small. Likewise, the neutron-drip density and pressure in the crusts of accreting and nonaccreting neutron stars are only slightly shifted. Depending on the nuclear mass model employed, electron polarization may change the composition of the crust of nonaccreting neutron stars. On the other hand, the current uncertainties in the masses of neutron-rich Kr and Sr isotopes are found to be more important than electron exchange and polarization effects.

We describe basics of a new approach to transverse momentum dependence in hard exclusive processes. We develop it in application to the transition process ${\gamma }^{*}\gamma \to {\pi }^0$ at the handbag level. Our starting point is coordinate representation for matrix elements of operators [in the simplest case, bilocal $\mathcal{O}(0,z)$] describing a hadron with momentum $p$. Treated as functions of $(pz)$ and $z^2$, they are parametrized through virtuality distribution amplitudes (VDA) $\mathrm{\Phi }(x,\sigma )$, with $x$ being Fourier conjugate to $(pz)$ and $\sigma$ Laplace conjugate to $z^2$. For intervals with $z^{+}=0$, we introduce the transverse momentum distribution amplitude (TMDA) $\mathrm{\Psi }(x,k_{\perp })$, and write it in terms of VDA $\mathrm{\Phi }(x,\sigma )$. The results of covariant calculations, written in terms of $\mathrm{\Phi }(x,\sigma )$, are converted into expressions involving $\mathrm{\Psi }(x,k_{\perp })$. Starting with scalar toy models, we extend the analysis onto the case of spin-$1/2$ quarks and QCD. We propose simple models for soft VDAs/TMDAs, and use them for comparison of handbag results with experimental (BABAR and BELLE) data on the pion transition form factor. We also discuss how one can generate high-$k_{\perp }$ tails from primordial soft distributions.

We calculate the branching ratios of ${\chi }_{b0}\to {\tau }^{+}{\tau }^{-}$ via an $s$-channel Higgs boson and estimate the sensitivity to this process from $\mathrm{\Upsilon }\to \gamma {\chi }_{b0}\to \gamma {\tau }^{+}{\tau }^{-}$. We show that future running at the $\mathrm{\Upsilon }(3S)$ at a very high luminosity super $B$ factory can put significant constraints on the type-II two-Higgs-doublet model when the discovered 125 GeV Higgs boson is the heavier of the two charge parity ($CP$)-even scalars.

We consider a simple renormalizable dark matter model consisting of two real scalars with a mass splitting $\delta$, interacting with the SM particles through the Higgs portal. We find a viable parameter space respecting all the bounds imposed by invisible Higgs decay experiments at the LHC, the direct detection experiments by XENON100 and LUX, and the dark matter relic abundance provided by WMAP and Planck. Despite the singlet scalar dark matter model that is fragile against the future direct detection experiments, the scalar split model introduced here survives such forthcoming bounds. We emphasize the role of the coannihilation processes and the mixing effects in this feature. For $m_{\mathrm{DM}}\sim 63\mathrm{GeV}$ in this model we can explain as well the observed gamma-ray excess in the analyses of the Fermi-LAT data at Galactic latitudes $2°\le |b|\le 20°$ and Galactic longitudes $|l|<20°$.

We propose two simple quirk models to explain the recently reported 750 GeV diphoton excesses at ATLAS and CMS. It is already well known that a real singlet scalar $\varphi$ with Yukawa couplings $\varphi \overline{X}X$ to vectorlike fermions $X$ with mass $m_X>m_{\varphi }/2$ can easily explain the observed signal, provided $X$ carries both SM color and electric charge. We instead consider first the possibility that the pair production of a fermion, charged under both SM gauge groups and a confining $SU(3{)}_v$ gauge group, is responsible. If pair produced it forms a quirky bound state, which promptly annihilates into gluons, photons, v-gluons and possibly SM fermions. This is an extremely minimal model to explain the excess, but is already in some tension with existing displaced searches, as well as dilepton and dijet resonance bounds. We therefore propose a hybrid quirk-scalar model, in which the fermion of the simple $\varphi \overline{X}X$ toy model is charged under the additional $SU(3{)}_v$ confining gauge group. Constraints on the new heavy fermion $X$ are then significantly relaxed. The main additional signals of this model are possible dilepton, dijet and diphoton resonances at $\sim 2\mathrm{TeV}$ or more from quirk annihilation, and the production of v-glueballs through quirk annihilation and $\varphi$ decay. The glueballs can give rise to spectacular signatures, including displaced vertices and events with leptons, photons and $Z$-bosons. If the quirk-scalar model is responsible for the 750 GeV excess it should be discovered in one of these channels with 20 or $300{\mathrm{fb}}^{-1}$ of LHC Run 2 data.

We consider central exclusive diffractive dipion production in the reactions $pp\to pp{\pi }^{+}{\pi }^{-}$ and $p\overline{p}\to p\overline{p}{\pi }^{+}{\pi }^{-}$ at high energies. We include the dipion continuum, the dominant scalar $f_0(500)$, $f_0(980)$, and tensor $f_2(1270)$ resonances decaying into the ${\pi }^{+}{\pi }^{-}$ pairs. The calculation is based on a tensor Pomeron model and the amplitudes for the processes are formulated in terms of vertices respecting the standard crossing and charge-conjugation relations of quantum field theory. The formulas for the dipion continuum and tensor meson production are given here for the first time. The theoretical results are compared with existing STAR, CDF, CMS experimental data and predictions for planned or current experiments (ALICE, ATLAS) are presented. We show the influence of the experimental cuts on the integrated cross section and on various differential distributions for outgoing particles. Distributions in rapidities and transverse momenta of outgoing protons and pions as well as correlations in azimuthal angle between them are presented. We find that the relative contribution of the resonant $f_2(1270)$ and dipion continuum strongly depends on the cut on proton transverse momenta or four-momentum transfer squared $t_{1,2}$ which may explain some controversial observations made by different ISR experiments in the past. The cuts may play then the role of a $\pi \pi$ resonance filter. We suggest some experimental analyses to fix model parameters related to the Pomeron-Pomeron–$f_2$ coupling.

We perform a systematic study on the spin decomposition of an electron in QED at one-loop order. It is found that the electron orbital angular momentum defined in Jaffe-Manohar and Ji spin sum rules agree with each other, and the so-called potential angular momentum vanishes at this order. The calculations are performed in both dimensional regularization and Pauli-Villars regularization for the ultraviolet divergences, and they lead to consistent results. We further investigate the calculations in terms of light-front wave functions and find a missing contribution from the instantaneous interaction in light-front quantization. This clarifies the confusing issues raised recently in the literature on the spin decomposition of an electron and will help consolidate the spin physics program for nucleons in QCD.

Effective magnetic $SU(N)$ gauge theory with classical $Z_N$ flux tubes of intrinsic width $\frac{1}{M}$ is an effective field theory of the long-distance quark-antiquark interaction in $SU(N)$ Yang-Mills theory. Long-wavelength fluctuations of the $Z_N$ vortices of this theory lead to an effective string theory. In this paper, we clarify the connection between effective field theory and effective string theory, and we propose a new constraint on these vortices. We first examine the impact of string fluctuations on the classical dual superconductor description of confinement. At interquark distances $R\sim \frac{1}{M}$, the classical action for a straight flux tube determines the heavy quark potentials. At distances $R\gg \frac{1}{M}$, fluctuations of the flux tube axis $\tilde{x}$ give rise to an effective string theory with an action $S_{\text{eff}}(\tilde{x})$, the classical action for a curved flux tube, evaluated in the limit $\frac{1}{M}\to 0$. This action is equal to the Nambu-Goto action. These conclusions are independent of the details of the $Z_N$ flux tube. Further, we assume the QCD flux tube satisfies the additional constraint, ${\int }_0^{\infty }rdr\frac{T_{\theta \theta }(r)}{r^2}=0,$ where $\frac{T_{\theta \theta }(r)}{r^2}$ is the value of the $\theta \theta$ component of the stress tensor at a distance $r$ from the axis of an infinite flux tube. Under this constraint, the string tension $\sigma$ equals the force on a quark in the chromoelectric field $\overrightarrow{E}$ of an infinite straight flux tube, and the Nambu-Goto action can be represented in terms of the chromodynamic fields of effective magnetic $SU(N)$ gauge theory, yielding a field theory interpretation of effective string theory.