We present the results of the combination of searches for the standard model Higgs boson produced in association with a $W$ or $Z$ boson and decaying into $b\overline{b}$ using the data sample collected with the D0 detector in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ at the Fermilab Tevatron Collider. We derive 95% C.L. upper limits on the Higgs boson cross section relative to the standard model prediction in the mass range $100\mathrm{GeV}\le M_H\le 150\mathrm{GeV}$, and we exclude Higgs bosons with masses smaller than 102 GeV at the 95% C.L. In the mass range $120\mathrm{GeV}\le M_H\le 145\mathrm{GeV}$, the data exhibit an excess above the background prediction with a global significance of 1.5 standard deviations, consistent with the expectation in the presence of a standard model Higgs boson.

We present a search for the standard model (SM) Higgs boson produced in association with a $Z$ boson in $9.7{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions collected with the D0 detector at the Fermilab Tevatron Collider at $\sqrt{s}=1.96\mathrm{TeV}$. Selected events contain one reconstructed $Z\to e^{+}{e}^{-}$ or $Z\to {\mu }^{+}{\mu }^{-}$ candidate and at least two jets, including at least one jet identified as likely to contain a $b$ quark. To validate the search procedure, we also measure the cross section for $ZZ$ production in the same final state. It is found to be consistent with its SM prediction. We set upper limits on the $ZH$ production cross section times branching ratio for $H\to b\overline{b}$ at the 95% C.L. for Higgs boson masses $90\le M_H\le 150\mathrm{GeV}$. The observed (expected) limit for $M_H=125\mathrm{GeV}$ is 7.1 (5.1) times the SM cross section.

We present a search for the standard model Higgs boson in final states with a charged lepton (electron or muon), missing transverse energy, and two or three jets, at least one of which is identified as a $b$-quark jet. The search is primarily sensitive to $WH\to \ell \nu b\overline{b}$ production and uses data corresponding to $9.7{\mathrm{fb}}^{-1}$ of integrated luminosity collected with the D0 detector at the Fermilab Tevatron $p\overline{p}$ Collider at $\sqrt{s}=1.96\mathrm{TeV}$. We observe agreement between the data and the expected background. For a Higgs boson mass of 125 GeV, we set a 95% C.L. upper limit on the production of a standard model Higgs boson of $5.2\times {\sigma }_{\mathrm{SM}}$, where ${\sigma }_{\mathrm{SM}}$ is the standard model Higgs boson production cross section, while the expected limit is $4.7\times {\sigma }_{\mathrm{SM}}$.

The interaction of carbonyl sulfide dipolar gas molecules with two time-delayed, single-cycle THz pulses is shown both experimentally and theoretically to induce two-quantum rotational coherences that are significantly enhanced with respect to those induced by one THz pulse, depending on the relative delay of the pulses. The underlying phenomenon is quite general in that it can occur even after a single THz pulse if more than one molecular species is present, since the free induction decay emitted by one species (demonstrated here by atmospheric water vapor) can provide the second field interaction for the other.

We report the first three-dimensional (3D) ion particle simulation of mode conversion from a fast mode compressional wave to kinetic Alfvén waves (KAWs) that occurs when a compressional mode propagates across a plasma boundary into a region of increasing Alfvén velocity. The magnetic field is oriented in the $\widehat{z}$ direction perpendicular to the gradients in the background density and magnetic field ($\widehat{x}$ direction). Following a stage dominated by linear physics in which KAWs with large wave numbers $k_x{\rho }_i\sim 1$ (with ${\rho }_i$ being the ion Larmor radius) are generated near the Alfvén resonance surface, the growth of KAW modes with $k_y{\rho }_i\sim 1$ is observed in the nonlinear stage when the amplitude of KAWs generated by linear mode conversion becomes large enough to drive a nonlinear parametric decay process. The simulation provides a comprehensive picture of mode conversion and shows the fundamental importance of the 3D nonlinear physics in transferring energy to large perpendicular $k_y$ modes, which can provide large transport across plasma boundaries in space and laboratory plasmas.

Alloy discovery and development is slowed by trial and error methods used to identify beneficial alloying elements. This fact has led to suggestions that integrating quantum theory and modeling with traditional experimental approaches might accelerate the pace of alloy discovery. We report here on one such effort, using advances in first principles computation along with an evolving theory that allows for the partitioning of charge density into chemically meaningful structures, alloying elements that improve the adhesive properties of interfaces common to high strength steels have been identified.

In this Letter, we aim at making nanoparticles embedded in a host semiconductor with a size comparable to electronic wavelengths “invisible” to the electron transport. Inspired by the recent progress made in optics and working within the framework of the expansion of partial waves, we demonstrate that the opposite effects imposed by potential barriers and wells of a core-shell nanoparticle on the phase shifts associated with the scattered electron wave could make the scattering cross section of the first two partial waves vanish simultaneously. We show that this is sufficient to cloak the nanoparticle from being detected by electrons with specific energy in the sense that a total scattering cross section smaller than 0.01% of the physical cross section can be obtained and a 4 orders of magnitude difference in the total scattering cross section can be presented within an energy range of only 40 meV, indicating possible applications of the “electron cloaks” as novel electronic switches and sensors, and in efficient energy harvesting and conversion technologies.

We use computational and experimental methods to study $d^0$ ferromagnetism at a charge-imbalanced interface between two perovskites. In ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$ superlattice calculations, the charge imbalance introduces holes in the ${\mathrm{SrTiO}}_3$ layer, inducing a $d^0$ ferromagnetic half-metallic 2D hole gas at the interface oxygen $2p$ orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type $d^0$ perovskite interfaces. Atomically sharp epitaxial $d^0$ ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$, ${\mathrm{SrTiO}}_3/{\mathrm{KNbO}}_3$, and ${\mathrm{SrTiO}}_3/{\mathrm{NaNbO}}_3$ interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen $t_{1g}$ band in comparison with the more studied $d$ band $t_{2g}$ symmetry electron gas.

We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ to study the ultrafast dynamics of surface and bulk electrons after photoexcitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, the unique cooling of Dirac fermions in TI by acoustic phonons is manifested through a power law dependence of the surface temperature decay rate on carrier density.

We show that a single Bell’s inequality with two dichotomic observables for each observer, which originates from Hardy’s nonlocality proof without inequalities, is violated by all entangled pure states of a given number of particles, each of which may have a different number of energy levels. Thus Gisin’s theorem is proved in its most general form from which it follows that for pure states Bell’s nonlocality and quantum entanglement are equivalent.

Quantum state tomography is the task of inferring the state of a quantum system by appropriate measurements. Since the frequency distributions of the outcomes of any finite number of measurements will generally deviate from their asymptotic limits, the estimates computed by standard methods do not in general coincide with the true state and, therefore, have no operational significance unless their accuracy is defined in terms of error bounds. Here we show that quantum state tomography, together with an appropriate data analysis procedure, yields reliable and tight error bounds, specified in terms of confidence regions—a concept originating from classical statistics. Confidence regions are subsets of the state space in which the true state lies with high probability, independently of any prior assumption on the distribution of the possible states. Our method for computing confidence regions can be applied to arbitrary measurements including fully coherent ones; it is practical and particularly well suited for tomography on systems consisting of a small number of qubits, which are currently in the focus of interest in experimental quantum information science.

A system responding to a stochastic driving signal can be interpreted as computing, by means of its dynamics, an implicit model of the environmental variables. The system’s state retains information about past environmental fluctuations, and a fraction of this information is predictive of future ones. The remaining nonpredictive information reflects model complexity that does not improve predictive power, and thus represents the ineffectiveness of the model. We expose the fundamental equivalence between this model inefficiency and thermodynamic inefficiency, measured by dissipation. Our results hold arbitrarily far from thermodynamic equilibrium and are applicable to a wide range of systems, including biomolecular machines. They highlight a profound connection between the effective use of information and efficient thermodynamic operation: any system constructed to keep memory about its environment and to operate with maximal energetic efficiency has to be predictive.

Collisions of particles in black hole ergospheres may result in an arbitrarily large center-of-mass energy. This led recently to the suggestion [M. Bañados, J. Silk, and S. M. West, Phys. Rev. Lett. 103, 111102 (2009)] that black holes can act as ultimate particle accelerators. If the energy of an outgoing particle is larger than the total energy of the infalling particles, the energy excess must come from the rotational energy of the black hole and hence, a Penrose process is involved. However, while the center-of-mass energy diverges, the position of the collision makes it impossible for energetic particles to escape to infinity. Following an earlier work on collisional Penrose processes [T. Piran and J. Shaham, Phys. Rev. D 16, 1615 (1977)], we show that even under the most favorable idealized conditions the maximal energy of an escaping particle is only a modest factor above the total initial energy of the colliding particles. This implies that one should not expect collisions around a black hole to act as spectacular cosmic accelerators.

Perturbation series in quantum field theory (QFT) are generally divergent asymptotic series which are also typically not Borel resummable in the sense that the resummed series is ambiguous. The ambiguity is associated with singularities in the Borel plane on the positive real axis. In quantum mechanics there are cases in which the ambiguity that arises in perturbation theory cancels against a similarly ambiguous contribution from instanton—anti-instanton events. In asymptotically free gauge theories, this mechanism does not suffice because perturbation theory develops ambiguities associated with singularities in the Borel plane which are closer to the origin by a factor of about $N$ (the rank of the gauge group) compared to the singularities realized by instanton events. These are called IR renormalon poles, and on ${\mathbb{R}}^4$ they do not possess any known semiclassical realization. By using continuity on ${\mathbb{R}}^3\times S^1$, and by generalizing the works of Bogomolny and Zinn-Justin to QFT, we identify saddle point field configurations, e.g., bion-antibion events, corresponding to singularities in the Borel plane which are of order $N$ times closer to the origin than the four-dimensional instanton–anti-instanton singularities in the Borel plane. We conjecture that these are the leading singularities in the Borel plane and that they are the incarnation of the elusive renormalons in the weak coupling regime.

The second Fourier component $v_2$ of the azimuthal anisotropy with respect to the reaction plane is measured for direct photons at midrapidity and transverse momentum ($p_T$) of $1–12\mathrm{GeV}/c$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Previous measurements of this quantity for hadrons with $p_T<6\mathrm{GeV}/c$ indicate that the medium behaves like a nearly perfect fluid, while for $p_T>6\mathrm{GeV}/c$ a reduced anisotropy is interpreted in terms of a path-length dependence for parton energy loss. In this measurement with the PHENIX detector at the Relativistic Heavy Ion Collider we find that for $p_T>4\mathrm{GeV}/c$ the anisotropy for direct photons is consistent with zero, which is as expected if the dominant source of direct photons is initial hard scattering. However, in the $p_T<4\mathrm{GeV}/c$ region dominated by thermal photons, we find a substantial direct-photon $v_2$ comparable to that of hadrons, whereas model calculations for thermal photons in this kinematic region underpredict the observed $v_2$.

Low-energy ($E_0=54\mathrm{eV}$) electron impact single ionization of molecular hydrogen (${\mathrm{H}}_2$) has been investigated as a function of molecular alignment in order to benchmark recent theoretical predictions [Colgan et al., Phys. Rev. Lett. 101, 233201 (2008) and Al-Hagan et al., Nature Phys. 5, 59 (2009)]. In contrast to any previous work, we observe distinct alignment dependence of the $(e,2e)$ cross sections in the perpendicular plane in good overall agreement with results from time-dependent close-coupling calculations. The cross section behavior can be consistently explained by a rescattering of the ejected electron in the molecular potential resulting in an effective focusing along the molecular axis.

We analyze the use of a driven nonlinear cavity to make a weak continuous measurement of a dispersively coupled qubit. We calculate the backaction dephasing rate and measurement rate beyond leading-order perturbation theory using a phase-space approach which accounts for cavity noise squeezing. Surprisingly, we find that increasing the coupling strength beyond the regime describable by leading-order perturbation theory (i.e., linear response) allows one to come significantly closer to the quantum limit on the measurement efficiency. We interpret this behavior in terms of the non-Gaussian photon number fluctuations of the nonlinear cavity. Our results are relevant to recent experiments using superconducting microwave circuits to study quantum measurement.

Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths ($L_p/\lambda <1$), the absolute efficiency of the harmonics declines for the steepest plasma density scale length $L_p\to 0$, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of ${10}^{-4}–{10}^{-6}$ of the laser pulse energy for photon energies ranging from 20–40 eV, with the best results being obtained for an intermediate density scale length.

We measure the power spectrum of the density distribution of a freely expanding two-dimensional (2D) degenerate Bose gas, where irregular density modulations gradually develop due to initial phase fluctuations in the sample. The spectrum has an oscillatory shape, where the peak positions are found to be independent of temperature and show scaling behavior in the course of expansion. The relative intensity of phase fluctuations is estimated from the normalized spectral peak strength and observed to decrease at lower temperatures, confirming the thermal nature of the phase fluctuations. We investigate the relaxation dynamics of nonequilibrium states using the power spectrum. Free vortices are observed with ring-shaped density ripples in a perturbed sample after a long relaxation time.

Recently, there has been much interest in simulating quantum field theory effects of matter and gauge fields. In a recent work, a method for simulating compact quantum electrodynamics (CQED) using Bose-Einstein condensates has been suggested. We suggest an alternative approach, which relies on single atoms in an optical lattice, carrying $2l+1$ internal levels, which converges rapidly to CQED as $l$ increases. That enables the simulation of CQED in $2+1$ dimensions in both the weak and the strong coupling regimes, hence, allowing us to probe confinement as well as other nonperturbative effects of the theory. We provide an explicit construction for the case $l=1$ which is sufficient for simulating the effect of confinement between two external static charges.