We study electronic transitions in highly charged Cf ions that are within the frequency range of optical lasers and have very high sensitivity to potential variations in the fine-structure constant, $\alpha$. The transitions are in the optical range despite the large ionization energies because they lie on the level crossing of the $5f$ and $6p$ valence orbitals in the thallium isoelectronic sequence. ${\mathrm{Cf}}^{16+}$ is a particularly rich ion, having several narrow lines with properties that minimize certain systematic effects. ${\mathrm{Cf}}^{16+}$ has very large nuclear charge and large ionization energy, resulting in the largest $\alpha$ sensitivity seen in atomic systems. The lines include positive and negative shifters.

A search is reported for the pair production of a new quark $b^{\prime }$ with at least one $b^{\prime }$ decaying to a $Z$ boson and a bottom quark. The data, corresponding to $2.0{\mathrm{fb}}^{-1}$ of integrated luminosity, were collected from $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector at the CERN Large Hadron Collider. Using events with a $b$-tagged jet and a $Z$ boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum $b^{\prime }$ candidates is tested for an enhancement. No evidence for a $b^{\prime }$ signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of $b^{\prime }$ quarks with masses $m_{b^{\prime }}<400\mathrm{GeV}$ that decay entirely via $b^{\prime }\to Z+b$. In the case of a vectorlike singlet $b^{\prime }$ mixing solely with the third standard model generation, masses $m_{b^{\prime }}<358\mathrm{GeV}$ are excluded.

The semileptonic decay channel $B\to D\tau \nu$ is sensitive to the presence of a scalar current, such as that mediated by a charged-Higgs boson. Recently, the BABAR experiment reported the first observation of the exclusive semileptonic decay $B\to D{\tau }^{-}\overline{\nu }$, finding an approximately $2\sigma$ disagreement with the standard-model prediction for the ratio $R(D)=\mathrm{BR}(B\to D\tau \nu )/\mathrm{BR}(B\to D\ell \nu )$, where $\ell =e,\mu$. We compute this ratio of branching fractions using hadronic form factors computed in unquenched lattice QCD and obtain $R(D)=0.316(12)(7)$, where the errors are statistical and total systematic, respectively. This result is the first standard-model calculation of $R(D)$ from ab initio full QCD. Its error is smaller than that of previous estimates, primarily due to the reduced uncertainty in the scalar form factor $f_0(q^2)$. Our determination of $R(D)$ is approximately $1\sigma$ higher than previous estimates and, thus, reduces the tension with experiment. We also compute $R(D)$ in models with electrically charged scalar exchange, such as the type-II two-Higgs-doublet model. Once again, our result is consistent with, but approximately $1\sigma$ higher than, previous estimates for phenomenologically relevant values of the scalar coupling in the type-II model. As a by-product of our calculation, we also present the standard-model prediction for the longitudinal-polarization ratio $P_L(D)=0.325(4)(3)$.

A search for new physics is performed in events with two same-sign isolated leptons, hadronic jets, and missing transverse energy in the final state. The analysis is based on a data sample corresponding to an integrated luminosity of $4.98{\mathrm{fb}}^{-1}$ produced in $pp$ collisions at a center-of-mass energy of 7 TeV collected by the CMS experiment at the LHC. This constitutes a factor of 140 increase in integrated luminosity over previously published results. The observed yields agree with the standard model predictions and thus no evidence for new physics is found. The observations are used to set upper limits on possible new physics contributions and to constrain supersymmetric models. To facilitate the interpretation of the data in a broader range of new physics scenarios, information on the event selection, detector response, and efficiencies is provided.

We determine the $\theta$ dependence of the deconfinement temperature of $SU(3)$ pure gauge theory, finding that it decreases in the presence of a topological $\theta$ term. We do that by performing lattice simulations at imaginary $\theta$, then exploiting analytic continuation. We also give an estimate of such dependence in the limit of a large number of colors $N$ and compare it with our numerical results.

The effect of sea quark electromagnetic charge on meson masses is investigated, and first results for full $\mathrm{QED}+\mathrm{QCD}$ low-energy constants are presented. The electromagnetic charge for sea quarks is incorporated in quenched $\mathrm{QED}+\mathrm{full}$ QCD lattice simulations by a reweighting method. The reweighting factor, which connects quenched and unquenched QED, is estimated using a stochastic method on $2+1$ flavor dynamical domain-wall quark ensembles.

The ALICE experiment has measured the inclusive $J/\psi$ production in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$ down to zero transverse momentum in the rapidity range $2.5<y<4$. A suppression of the inclusive $J/\psi$ yield in Pb-Pb is observed with respect to the one measured in $pp$ collisions scaled by the number of binary nucleon-nucleon collisions. The nuclear modification factor, integrated over the 0%–80% most central collisions, is $0.545\pm 0.032(\mathrm{stat})\pm 0.083(\mathrm{syst})$ and does not exhibit a significant dependence on the collision centrality. These features appear significantly different from measurements at lower collision energies. Models including $J/\psi$ production from charm quarks in a deconfined partonic phase can describe our data.

The pseudospin symmetry (PSS) is a relativistic dynamical symmetry connected with the small component of the Dirac spinor. The origin of PSS in single particle bound states in atomic nuclei has been revealed and studied extensively. By examining the zeros of Jost functions corresponding to the small components of Dirac wave functions and phase shifts of continuum states, we show that the PSS in single particle resonant states in nuclei is conserved when the attractive scalar and repulsive vector potentials have the same magnitude but opposite sign. The exact conservation and the breaking of the PSS are illustrated for single particle resonances in spherical square-well and Woods-Saxon potentials.

We present a theory of nuclear magnetic resonance shielding tensors for electronic states with arbitrary degeneracy. The shieldings are here expressed in terms of generalized Zeeman ($g^{(k)}$) and hyperfine ($A^{(k)}$) tensors, of all ranks $k$ allowed by the size of degeneracy. Contrary to recent proposals [T. O. Pennanen and J. Vaara, Phys. Rev. Lett. 100, 133002 (2008)], our theory is valid in the strong spin-orbit coupling limit. Ab initio calculations for the fourfold degenerate ${\Gamma }_8$ ground state of lanthanide-doped fluorite crystals ${\mathrm{CaF}}_2\mathrm{\text{:}}\mathrm{Ln}$ ($\mathrm{Ln}={\mathrm{Pr}}^{2+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Sm}}^{3+}$, and ${\mathrm{Dy}}^{3+}$) show that previously neglected contributions can account for more than 50% of the paramagnetic shift.

We demonstrate strong coupling between molecular excited states and surface plasmon modes of a slit array in a thin metal film. The coupling manifests itself as an anticrossing behavior of the two newly formed polaritons. As the coupling strength grows, a new mode emerges, which is attributed to long-range molecular interactions mediated by the plasmonic field. The new, molecular-like mode repels the polariton states, and leads to an opening of energy gaps both below and above the asymptotic free molecule energy.

The ground state energies of universal $N$-body clusters tied to Efimov trimers, for $N$ even, are shown to be encapsulated in the statistical distribution of two particles interacting with a background auxiliary field at large Euclidean time when the interaction is tuned to the unitary point. Numerical evidence that this distribution is log normal is presented, allowing one to predict the ground state energies of the $N$-body system.

Using a simple model of strong-field ionization of atoms that generalizes the well-known 3-step model from 1D to 3D, we show that the experimental photoelectron angular distributions resulting from laser ionization of xenon and argon display prominent structures that correspond to electrons that pass by their parent ion more than once before strongly scattering. The shape of these structures can be associated with the specific number of times the electron is driven past its parent ion in the laser field before scattering. Furthermore, a careful analysis of the cutoff energy of the structures allows us to experimentally measure the distance between the electron and ion at the moment of tunnel ionization. This work provides new physical insight into how atoms ionize in strong laser fields and has implications for further efforts to extract atomic and molecular dynamics from strong-field physics.

A theoretical model is presented describing the confinement-induced resonances observed in the recent loss experiment of Haller et al. [Phys. Rev. Lett. 104, 153203 (2010)]. These resonances originate from possible molecule formation due to the coupling of center-of-mass and relative motion. A corresponding model is verified by ab initio calculations and predicts the resonance positions in 1D as well as in 2D confinement in agreement with the experiment. This resolves the contradiction of the experimental observations to previous theoretical predictions.

We report Doppler-free measurements of the positronium (Ps) Lyman-$\alpha$ transition using saturated absorption spectroscopy. In addition to a Lamb dip at wavelength ${\lambda }_L=243.0218\pm 0.0005\mathrm{nm}$, we also observed a crossover resonance at ${\lambda }_C=243.0035\pm 0.0005\mathrm{nm}$, arising from the excitation of $1{}^{3}S_1$ atoms to Zeeman mixed $2P$ states, followed by stimulated emission to the $1{}^{1}S_0$ ground state. Since $({\lambda }_L\mathrm{\text{−}}{\lambda }_C)$ is related to the Ps hyperfine interval $E_{\mathrm{hfs}}$, this observation constitutes the first optical measurement of this quantity and yields $E_{\mathrm{hfs}}=198.4\pm 4.2\mathrm{GHz}$. We describe improvements to the methodology that could lead to the $\sim \mathrm{ppm}$ level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states.

Recent advances in attosecond science have relied upon the nearly instantaneous response of free electrons to an external field. However, it is still not clear whether bound electrons are able to rearrange on sublaser cycle time scales. Here, we probe the optical Stark shifts induced by a few-cycle near infrared laser field in helium bound states using isolated attosecond pulses in a transient absorption scheme and uncover a subcycle laser-induced energy level shift of the laser-dressed $1s3p$ state.

Here we investigate theoretically and numerically the coupling between surface plasmon polaritons (SPPs) in monolayer graphene sheet arrays that have a period much smaller than the wavelength. We show that when the collective SPP is excited with an out-of-phase illumination, the beam tends to propagate toward the opposite direction of the Bloch momentum, reflecting a negative coupling between the constituent SPPs. In contrast, for in-phase illumination, the incident beam is split into two collective SPPs that are highly collimated and display low propagation loss. Moreover, the coupling between the individual SPPs results in a reduction of the modal wavelength of the SPP in comparison with that of a single graphene sheet.

In the Leidenfrost effect, liquid drops deposited on a hot surface levitate on a thin vapor cushion fed by evaporation of the liquid. This vapor layer forms a concave depression in the drop interface. Using laser-light interference coupled to high-speed imaging, we measured the radius, curvature, and height of the vapor pocket, as well as nonaxisymmetric fluctuations of the interface for water drops at different temperatures. The geometry of the vapor pocket depends primarily on the drop size and not on the substrate temperature.

Classical vortex generators, known for their efficiency in delaying or even inhibiting boundary layer separation, are here shown to be coveted devices for transition to turbulence delay. The present devices are miniature with respect to classical vortex generators but are tremendously powerful in modulating the laminar boundary layer in the direction orthogonal to the base flow and parallel to the surface. The modulation generates an additional term in the perturbation energy equation, which counteracts the wall-normal production term and, hence, stabilizes the flow. Our experimental results show that these devices are really effective in delaying transition, but we also reveal their Achilles’ heel.

We investigate the effect of weak rotation on the large-scale circulation (LSC) of turbulent Rayleigh-Bénard convection, using the theory for cessations in a low-dimensional stochastic model of the flow previously studied. We determine the cessation frequency of the LSC as a function of rotation, and calculate the statistics of the amplitude and azimuthal velocity fluctuations of the LSC as a function of the rotation rate for different Rayleigh numbers. Furthermore, we show that the tails of the reorientation PDF remain unchanged for rotating systems, while the distribution of the LSC amplitude and correspondingly the cessation frequency are strongly affected by rotation. Our results are in close agreement with experimental observations.

We report generation of density modulation at terahertz (THz) frequencies in a relativistic electron beam through laser modulation of the beam longitudinal phase space. We show that by modulating the energy distribution of the beam with two lasers, density modulation at the difference frequency of the two lasers can be generated after the beam passes through a chicane. In this experiment, density modulation around 10 THz was generated by down-converting the frequencies of an 800 nm laser and a 1550 nm laser. The central frequency of the density modulation can be tuned by varying the laser wavelengths, beam energy chirp, or momentum compaction of the chicane. This technique can be applied to accelerator-based light sources for generation of coherent THz radiation and marks a significant advance toward tunable narrow band THz sources.