We have recently proposed a special class of scalar-tensor theories known as the Fab Four. These arose from attempts to analyze the cosmological constant problem within the context of Horndeski’s most general scalar-tensor theory. The Fab Four together give rise to a model of self-tuning, with the relevant solutions evading Weinberg’s no-go theorem by relaxing the condition of Poincaré invariance in the scalar sector. The Fab Four are made up of four geometric terms in the action with each term containing a free potential function of the scalar field. In this paper we rigorously derive this model from the general model of Horndeski, proving that the Fab Four represents the only classical scalar-tensor theory of this type that has any hope of tackling the cosmological constant problem. We present the full equations of motion for this theory, and give an heuristic argument to suggest that one might be able to keep radiative corrections under control. We also give the Fab Four in terms of the potentials presented in Deffayet et al’s version of Horndeski.

Coupling dark energy to dark matter provides one of the simplest way to effectively modify gravity at large scales without strong constraints from local (i.e. Solar System) observations. Models of coupled dark energy have been studied several times in the past and are already significantly constrained by cosmic microwave background experiments. In this paper we estimate the constraints that future large-scale observations will be able to put on the coupling and in general on all the parameters of the model. We combine cosmic microwave background, tomographic weak lensing, redshift distortions and power spectrum probes. We show that next-generation observations can improve the current constraint on the coupling to dark matter by two orders of magnitude; this constraint is complementary to the current Solar System bounds on a coupling to baryons.

The observed acceleration of the present Universe is shown to be well explained by the holographic dark energy characterized by the total comoving horizon of the Universe ($\eta \mathrm{HDE}$). It is of interest to notice that the very large primordial part of the comoving horizon generated by the inflation of the early Universe makes the $\eta \mathrm{HDE}$ behave like a cosmological constant. As a consequence, both the fine-tuning problem and the coincidence problem can reasonably be understood with the inflationary universe and holographical principle. We present a systematic analysis and obtain a consistent cosmological constraint on the $\eta \mathrm{HDE}$ model based on the recent cosmological observations. It is found that the $\eta \mathrm{HDE}$ model gives the best-fit result ${\Omega }_{m0}=0.270$ (${\Omega }_{de0}=0.730$) and the minimal ${\chi }_{\min }^2=542.915$ which is compatible with ${\chi }_{\Lambda \mathrm{CDM}}^2=542.919$ for the $\Lambda \mathrm{CDM}$ model.

We present the results of a weakly modeled burst search for gravitational waves from mergers of nonspinning intermediate mass black holes in the total mass range $100–450{\mathrm{M}}_{\odot }$ and with the component mass ratios between $1:1$ and $4:1$. The search was conducted on data collected by the LIGO and Virgo detectors between November of 2005 and October of 2007. No plausible signals were observed by the search which constrains the astrophysical rates of the intermediate mass black holes mergers as a function of the component masses. In the most efficiently detected bin centered on $88+88{\mathrm{M}}_{\odot }$, for nonspinning sources, the rate density upper limit is 0.13 per ${\mathrm{Mpc}}^3$ per Myr at the 90% confidence level.

Motivated by the recent results from LHC on the di-photon search for a standard model (SM) Higgs-like boson, in this work we discuss the implications of this signal in the framework of the inert Higgs doublet model. Our analysis takes account of the previous limits from Higgs searches at LEP, the Tevatron, and the LHC. We have also considered the bounds coming from perturbativity, unitarity, vacuum stability, and electroweak precision tests. We show that the charged Higgs contributions can interfere constructively or destructively with the W gauge bosons’ loops leading to enhancement or suppression of the di-photon rate with respect to the SM rate. We show also that the invisible decay of the Higgs, if open, could affect the total width of the SM Higgs boson and therefore suppress the di-photon rate.

Global analyses of parton distribution functions (PDFs) have provided incisive constraints on the up and down quark components of the proton, but constraining the other flavor degrees of freedom is more challenging. Higher-order theory predictions and new data sets have contributed to recent improvements. Despite these efforts, the strange quark parton distribution function has a sizable uncertainty, particularly in the small $x$ region. We examine the constraints from experiment and theory, and investigate the impact of this uncertainty on LHC observables. In particular, we study $W/Z$ production to see how the $s$ quark uncertainty propagates to these observables, and examine the extent to which precise measurements at the LHC can provide additional information on the proton flavor structure.

The top quark forward-backward asymmetry measured at the Fermilab Tevatron collider deviates from the standard model prediction. A $W^{\prime }$ boson model is described, where the coupling $W^{\prime }$-$t$-$d$ is fixed by the $t\overline{t}$ forward-backward asymmetry and total cross section at the Tevatron. We show that such a $W^{\prime }$ boson would be produced in association with a top quark at the CERN Large Hadron Collider (LHC), thus inducing additional $t\overline{t}+j$ events. We use measurements of $t\overline{t}+n$-jet production from the LHC to constrain the allowed $W^{\prime }$-$t$-$d$ couplings as a function of $W^{\prime }$ boson mass. We find that this $W^{\prime }$ model is constrained at the $95\%$ confidence level using $0.7\mathrm{f}{\mathrm{b}}^1$ of data from the LHC, and could be fully excluded with $5\mathrm{f}{\mathrm{b}}^1$ of data.

We study how the significance of top quark induced charge asymmetries at the LHC may be enhanced exploiting the $t\overline{t}$ total transverse momentum to enrich the fraction of quark-fusion events in the sample. We combine this variable with previous variables related to the boost and the invariant mass of the $t\overline{t}$ pair to find an optimum cut which maximizes the significance of the asymmetry when systematic and statistic errors are taken into account. We find that including the $t\overline{t}$ transverse momentum in the analysis of the expected 2012 LHC data provides a considerable enhancement in the significance of the asymmetry.

We describe a model-independent search for physics beyond the standard model in lepton final states. We examine 117 final states using $1.1{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions data at $\sqrt{s}=1.96\mathrm{TeV}$ collected with the D0 detector. We conclude that all observed discrepancies between data and model can be attributed to uncertainties in the standard model background modeling, and hence we do not see any evidence for physics beyond the standard model.

Heavy neutrinos at the TeV scale have been searched for at the LHC in the context of left-right models, under the assumption that they couple to the electron, the muon, or both. We show that current searches are also sensitive to heavy neutrinos coupling predominantly to the tau lepton, and present limits can significantly constrain the parameter space of general flavor mixing.

The nonperturbative mechanism of baryon-antibaryon production due to double quark-pair $(q\overline{q})(q\overline{q})$ generation inside a hadron is considered and the amplitude is calculated as a matrix element of the vertex operator between initial and final hadron wave functions. The vertex operator is expressed solely in terms of first principle input: current quark masses, string tension $\sigma$ and ${\alpha }_s$. In contrast to meson-meson production via single pair generation, in the baryon case a new entity appears in the vertex: the vacuum correlation length $\lambda$, which was computed before through string tension $\sigma$. As an application electroproduction of ${\Lambda }_c{\overline{\Lambda }}_c$ was calculated and an enhancement near 4.61 GeV was found in agreement with recent experimental data.

Nonlinear dynamics in cosmological backgrounds has the potential to teach us immensely about our Universe, and also to serve as prototype for nonlinear processes in generic curved spacetimes. Here we report on dynamical evolutions of black holes in asymptotically de Sitter spacetimes. We focus on the head-on collision of equal mass binaries and for the first time compare analytical and perturbative methods with full blown nonlinear simulations. Our results include an accurate determination of the merger/scatter transition (consequence of an expanding background) for small mass binaries and a test of the cosmic censorship conjecture, for large mass binaries. We observe that, even starting from small separations, black holes in large mass binaries eventually lose causal contact, in agreement with the conjecture.

We study the possibility of growth of the electric and magnetic fields in a force-free plasma due strictly to the gravitational curvature of the spacetime domain where those fields lie. To this end, we identify a total energy by analogy with the results of classical magnetohydrodynamics. After obtaining the general evolution equation for the total energy, we apply to it to the fiducial observers in a number of classical metrics: Schwarzschild, Boyer-Lindquist, Kerr-Schild, Robertson-Walker, and post-Newtonian approximation. As a rule the shift velocity plays the role of minus the fluid velocity in Newtonian MHD, but the details are often highly intricate.

We study the kinematics of timelike geodesic congruences, in the spacetime geometry of rotating black holes in three [the Bañados-Teitelboim-Zanelli (BTZ)] and four (the Kerr) dimensions. The evolution (Raychaudhuri) equations for the expansion, shear and rotation along geodesic flows in such spacetimes are obtained. For the Bañados-Teitelboim-Zanelli case, the equations are solved analytically. The effect of the negative cosmological constant on the evolution of the expansion ($\theta$), for congruences with and without an initial rotation (${\omega }_0$), is noted. Subsequently, the evolution equations, in the case of a Kerr black hole in four dimensions, are written and solved numerically for some specific geodesics flows. It turns out that, for the Kerr black hole, there exists a critical value of the initial expansion below (above) which we have focusing (defocusing). We delineate the dependencies of the expansion on the black hole angular momentum parameter, $a$, as well as on ${\omega }_0$. Further, the role of $a$ and ${\omega }_0$ on the time (affine parameter) of approach to singularity (defocusing/focusing) is studied. While the role of ${\omega }_0$ on the time to singularity is as expected, the effect of $a$ leads to an interesting new result.

We study the dynamics of a spherically symmetric false vacuum bubble embedded in a true vacuum region separated by a “thick wall”, which is generated by a scalar field in a quartic potential. We study the “Farhi-Guth-Guven” (FGG) quantum tunneling process by constructing numerical solutions relevant to this process. The Arnowitt-Deser-Misner mass of the spacetime is calculated, and we show that there is a lower bound that is a significant fraction of the scalar field mass. We argue that the zero mass solutions used to by some to argue against the physicality of the FGG process are artifacts of the thin wall approximation used in earlier work. We argue that the zero mass solutions should not be used to question the viability of the FGG process.

We show that when a topological defect with extended world volume annihilates with an antidefect, there arise topological defects with dimensions less than those of the original defects by one. Domain wall annihilations create vortices while monopole-string annihilations result in instantons. We find that twisted domain wall rings are vortices, whereas twisted monopole rings are instantons.

We use a matching procedure of sum rules relating the electromagnetic form factors to generalized parton distributions and anti-de Sitter modes. In this way, in the framework of an AdS/QCD hard-wall model, the helicity-independent generalized parton distributions of quarks for the nucleon in the zero skewness case are calculated.

Walking technicolor predicts a light composite scalar, techni-dilaton, arising as a pseudo-Nambu-Goldstone boson for the approximate scale symmetry spontaneously broken by techni-fermion condensation. We show that a light techni-dilaton with a mass of around 125 GeV can explain presently observed excesses particularly in the di-photon decay channel at the LHC.

It is known that the neutrino $U_{\mathrm{PMNS}}$ matrix contains the diagonalization of the mass matrix of the charged leptons. In this article we study the influence of a nondiagonal mass matrix for charged leptons on the neutrino phenomenology in two specific $R$-parity violating supersymmetric models. Our analytical and numerical results for those models reveal important corrections due to a nontrivial charged lepton sector. Especially sensitive are the solar and reactor mixing angles.

We study the phase structure of SU(3) lattice gauge theory with $N_f=12$ staggered fermions in the fundamental representation, for both zero and finite temperature at strong gauge couplings. For small fermion masses we find two transitions at finite temperature that converge to two well-separated bulk phase transitions. The phase between the two transitions appears to be a novel phase. We identify order parameters showing that the single-site shift symmetry of staggered fermions is spontaneously broken in this phase. We investigate the eigenvalue spectrum of the Dirac operator, the static potential and the meson spectrum, which collectively establish that this novel phase is confining but chirally symmetric. The phase is bordered by first-order phase transitions, and since we find the same phase structure with $N_f=8$ fermions, we argue that the novel phase is most likely a strong-coupling lattice artifact, the existence of which does not imply IR conformality.