We consider quantum corrections to coannihilation processes of weakly interacting massive particles (WIMPs) due to the exchange of light bosons in the initial state (“Sommerfeld corrections”). We work at the one-loop level, i.e. we assume that these corrections can be treated perturbatively. Coannihilation is important if there is at least one additional new particle with mass close to the lightest WIMP, which is a dark matter candidate. In this case the exchange of a (relatively light) boson in the initial state can change the identity of the annihilating particles. The corrections we are interested in factorize, as in the case of WIMP self-annihilation treated previously, but they can mix different tree-level amplitudes. Moreover, even small mass splittings between the external particles and those in the loop can change the relevant loop functions significantly. We find exact analytical expressions for these functions and illustrate the effects by considering the cases of wino- or Higgsino-like neutralinos as examples.

We suggest a new perspective on the cosmological constant problem by scrutinizing its standard formulation. In classical and quantum mechanics without gravity, there is no definition of the zero point of energy. Furthermore, the Casimir effect only measures how the vacuum energy changes as one varies a geometric modulus. This leads us to propose that the physical vacuum energy in a Friedmann-Lemaître-Robertson-Walker expanding universe only depends on the time variation of the scale factor $a(t)$. Equivalently, requiring that empty Minkowski space is gravitationally stable is a principle that fixes the ambiguity in the zero-point energy. On the other hand, if there is a meaningful bare cosmological constant, this prescription should be viewed as a fine-tuning. We describe two different choices of vacuum, one of which is consistent with the current universe consisting only of matter and vacuum energy. The resulting vacuum energy density ${\rho }_{\mathrm{vac}}$ is constant in time and approximately $k_c^2{H}_0^2$, where $k_c$ is a momentum cutoff and $H_0$ is the current Hubble constant; for a cutoff close to the Planck scale, values of ${\rho }_{\mathrm{vac}}$ in agreement with astrophysical measurements are obtained. Another choice of vacuum is more relevant to the early universe consisting of only radiation and vacuum energy, and we suggest it as a possible model of inflation.

We compute the light-cone momentum distributions of pions in the nucleon in chiral effective theory using both pseudovector and pseudoscalar pion-nucleon couplings. For the pseudovector coupling we identify $\delta$-function contributions associated with end-point singularities arising from the pion-nucleon rainbow diagrams, as well as from pion bubble and tadpole diagrams which are not present in the pseudoscalar model. Gauge invariance is demonstrated, to all orders in the pion mass, with the inclusion of Kroll-Ruderman couplings involving operator insertions at the $\pi NN$ vertex. The results pave the way for phenomenological applications of pion cloud models that are manifestly consistent with the chiral symmetry properties of QCD.

We present an antenna shower formalism that includes helicity dependence for massless partons. The formalism applies to both traditional (global) showers and sector-based variants. We combine the shower with vincia’s multiplicative approach to matrix-element matching, generalized to operate on each helicity configuration separately. The result is a substantial gain in computational speed for high parton multiplicities. We present an implementation of both sector and global showers, with min/max variations, and helicity-dependent tree-level matching applied through $n\le 4$ for $V/H\to q\overline{q}+n$ partons and through $n\le 5$ for $H\to n$ gluons.

We analyze the low-energy spin structure of the nucleon in a covariant effective field theory with explicit spin-$3/2$ degrees of freedom to third order in the small scale expansion. Using the available data on the strong and electromagnetic width of the $\Delta$ resonance, we give parameter-free predictions for various spin polarizabilities and moments of spin structure functions. We find an improved description of the nucleon spin structure at finite photon virtualities for some observables and point out the necessity of a fourth-order calculation.

Models based on flavor symmetries are the most often studied approaches to explain the unexpected structure of lepton mixing. In many flavor symmetry groups a product of two triplet representations contains a symmetric and an antisymmetric contraction to a triplet. If this product of two triplets corresponds to a Majorana mass term, then the antisymmetric part vanishes, and in economic models tribimaximal mixing is achieved. If neutrinos are Dirac particles, the antisymmetric part is, however, present and leads to deviations from tribimaximal mixing, in particular, nonzero $U_{e3}$. Thus, the nonvanishing value of $U_{e3}$ and the nature of the neutrino are connected. We illustrate this with a model based on $A_4$ within the framework of a neutrinophilic two Higgs doublet scenario.

As a consequence of the LSND anomaly and other hints of an eV scale sterile neutrino from particle physics and cosmology, the neutrino sector of the standard model of particle physics has to be extended and the smallest extension is the ($3+1$) model, i.e., three active neutrinos plus one sterile one. In this work we study the neutrino mass matrix $M_{\nu }$ with three texture zeros in the ($3+1$) model, assuming all the neutrino states are of Majorana type. With the mass hierarchy between the active and sterile neutrinos as well as the smallness of the reactor neutrino mixing and active-sterile mixing, the analytical expressions can be greatly simplified. We systematically examine all the 120 texture zeros, via both analytical and numerical analysis, and find that the 100 textures with zeros in the fourth column and row of $M_{\nu }$ can hardly be compatible with experiments; whereas, 19 of the other 20 are feasible. We can foresee that the active neutrino masses are required to be around 0.01 eV and the next generation neutrinoless double beta decay experiments are promising to test these textures. Many other phenomenologically meaningful predictions are also obtained. All the $CP$ conserving ($3+1$) textures with three zeros are excluded by experiments.

Hernandez and Smirnov discovered an interesting formula to parametrize each column of a neutrino mixing matrix by six integers related to the residual symmetry. We point out that these six integers are not independent and propose a way to find the allowed combinations using structure constants of finite groups.

Results are reported of a search for a deviation in the jet production cross section from the prediction of perturbative quantum chromodynamics at next-to-leading order. The search is conducted using a 7 TeV proton-proton data sample corresponding to an integrated luminosity of $5.0{\mathrm{fb}}^{-1}$, collected with the Compact Muon Solenoid detector at the Large Hadron Collider. A deviation could arise from interactions characterized by a mass scale $\Lambda$ too high to be probed directly at the LHC. Such phenomena can be modeled as contact interactions. No evidence of a deviation is found. Using the ${\mathrm{CL}}_{\mathrm{s}}$ criterion, lower limits are set on $\Lambda$ of 9.9 TeV and 14.3 TeV at 95% confidence level for models with destructive and constructive interference, respectively. Limits obtained with a Bayesian method are also reported.

In black hole thermodynamics of certain models, the thermodynamic first law may contain the pressure term. The corresponding entropy follows the area law whereas the thermodynamic energy is not the same with the black hole mass. If the pressure can be decomposed into two parts and recombined with the original thermodynamic quantities, then the thermodynamic energy becomes the black hole mass and the entropy satisfying the area law turns out to be the corrected entropy called the Wald entropy, respectively.

It is known that monopoles can be confined by vortex strings in $d=3+1$ while vortices can be confined by domain lines in $d=2+1$. Here, as a higher dimensional generalization of these, we show that Yang-Mills instantons can be confined by monopole strings in $d=4+1$. We achieve this by putting the system into the Higgs phase in which the configuration can be constructed inside a non-Abelian vortex sheet.

We consider a coupling of dilaton gravity to the classically scale-invariant B-L extended standard model, which has been recently proposed as a phenomenologically viable model realizing the Coleman-Weinberg mechanism of breakdown of the electroweak symmetry. It is shown in the present model that without recourse to the Coleman-Weinberg mechanism, the B-L gauge symmetry is broken in the process of spontaneous symmetry breakdown of scale invariance at the tree level, and as a result the B-L gauge field becomes massive via the Higgs mechanism. Since the dimensionful parameter is only the Planck mass in our model, one is forced to pick up very small coupling constants if one wishes to realize the breaking of the B-L symmetry at the TeV scale.

Motivated by recent progress in studying the duality-symmetric models of nonlinear electrodynamics, we revert to the auxiliary tensorial (bispinor) field formulation of the $O(2)$ duality proposed by us in Ivanov and Zupnik [Nucl. Phys. B618, 3 (2001); Yad. Fiz. 67, 2212 (2004)]. In this approach, the entire information about the given duality-symmetric system is encoded in the $O(2)$-invariant interaction Lagrangian which is a function of the auxiliary fields $V_{\alpha \beta }$, ${\overline{V}}_{\dot{\alpha }\dot{\beta }}$. We extend this setting to duality-symmetric systems with higher derivatives and show that the recently employed “nonlinear twisted self-duality constraints” amount to the equations of motion for the auxiliary tensorial fields in our approach. Some other related issues are briefly discussed and a few instructive examples are explicitly worked out.

We study bound states of Abelian gauge theory in $D=1+1$ dimensions using an equal-time, Poincaré-covariant framework. The normalization of the linear confining potential is determined by a boundary condition in the solution of Gauss’ law for the instantaneous $A^0$ field. As in the case of the Dirac equation, the norm of the relativistic fermion-antifermion ($f\overline{f}$) wave functions gives inclusive particle densities. However, while the Dirac spectrum is known to be continuous we find that regular $f\overline{f}$ solutions exist only for discrete bound-state masses. The $f\overline{f}$ wave functions are consistent with the parton picture when the kinetic energy of the fermions is large compared to the binding potential. We verify that the electromagnetic form factors of the bound states are gauge invariant and calculate the parton distributions from the transition form factors in the Bjorken limit. For relativistic states we find a large sea contribution at low $x_{Bj}$. Since the potential is independent of the gauge coupling the bound states may serve as “Born terms” in a perturbative expansion, in analogy to the usual plane wave in and out states.

The Lorentzian metric structure used in any field theory allows one to implement the relativistic notion of causality and to define a notion of time dimension. This article investigates the possibility that at the microscopic level the metric is Riemannian, i.e., locally Euclidean, and that the Lorentzian structure, that we usually consider as fundamental, is in fact an effective property that emerges in some regions of a four-dimensional space with a positive definite metric. In such a model, there is no dynamics nor signature flip across some hypersurface; instead, all the fields develop a Lorentzian dynamics in these regions because they propagate in an effective metric. It is shown that one can construct a decent classical field theory for scalars, vectors, and (Dirac) spinors in flat spacetime. It is then shown that gravity can be included but that the theory for the effective Lorentzian metric is not general relativity but of the covariant Galileon type. The constraints arising from stability, the equivalence principle, and the constancy of fundamental constants are detailed and a phenomenological picture of the emergence of the Lorentzian metric is also given. The construction, while restricted to classical fields in this article, offers a new view on the notion of time.

We study the renormalization group flow of the $\mathrm{O}(N)$ nonlinear sigma model in arbitrary dimensions. The effective action of the model is truncated to fourth order in the derivative expansion, and the flow is obtained by combining the nonperturbative renormalization group and the background field method. We investigate the flow in three dimensions and analyze the phase structure for arbitrary $N$. While a nontrivial fixed point is present in a reduced truncation of the effective action and has critical properties that can be related to the well-known features of the $\mathrm{O}(N)$ universality class, one of the fourth-order operators destabilizes this fixed point and has to be discussed carefully. The results about the renormalization flow of the models will serve as a reference for upcoming simulations with the Monte Carlo renormalization group.

We present an $SO(2,4)$-covariant quantization of the free electromagnetic field in conformally flat spaces (CFSs). A CFS is realized in a six-dimensional space as an intersection of the null cone with a given surface. The smooth move of the latter is equivalent to perform a Weyl rescaling. This allows to transport the $SO(2,4)$-invariant quantum structure of the Maxwell field from Minkowski space to any CFS. Calculations are simplified and the CFS Wightman two-point functions are given in terms of their Minkowskian counterparts. The difficulty due to gauge freedom is surpassed by introducing two auxiliary fields and using the Gupta-Bleuler quantization scheme. The quantum structure is given by a vacuum state and creators/annihilators acting on some Hilbert space. In practice, only the Hilbert space changes under Weyl rescalings. Also, the quantum $SO(2,4)$-invariant free Maxwell field does not distinguish between two CFSs.

We show that the power-law decay modes found in linear perturbations of Schwarzschild black holes, generally called tails, do not produce caustics on a naturally defined family of null surfaces in the neighborhood of $i^{+}$ of a black hole horizon. This null congruence thus yields a well-behaved coordinate system both near the event horizon and at null infinity.

In this paper we deal with several issues regarding the localization properties of the Unruh-DeWitt (UdW) detector model. Since its original formulation as a pointlike detector, the UdW model has been used to study extensively the physics of quantum fields in presence of accelerations or curved backgrounds. Natural extensions of it have tried to take into account the spatial profile of such detectors, but all of them have met a series of problems in their spectral response which render them useless to study some of the most interesting physical scenarios. In this paper we provide a derivation of the smeared UdW interaction from QED first principles, then we analyze the spectral response of spatially smeared UdW detectors, and discuss the kind of spatial profiles which are useful for the study of relevant cases.