We identify a class of chiral models where the one-loop effective potential for Higgs scalar fields is finite without any requirement of supersymmetry. It corresponds to the case where the Higgs fields are identified with the components of a gauge field along compactified extra dimensions. We present a six-dimensional model with gauge group U (3) x U ( 3) and quarks and leptons accommodated in fundamental and bi-fundamental representations. The model can be embedded in a D-brane configuration of type I string theory and, upon compactification on a T-2/Z(2) orbifold, it gives rise to the standard model with two Higgs doublets.
We identify a class of chiral models where the one-loop effective potential for Higgs scalar fields is finite without any requirement of supersymmetry. It corresponds to the case where the Higgs fields are identified with the components of a gauge field along compactified extra dimensions. We present a six dimensional model with gauge group U(3)xU(3) and quarks and leptons accomodated in fundamental and bi-fundamental representations. The model can be embedded in a D-brane configuration of type I string theory and, upon compactification on a T^2/Z_2 orbifold, it gives rise to the standard model with two Higgs doublets.
Quantum key distribution can be performed with practical signal sources such as weak coherent pulses. One example of such a scheme is the Bennett-Brassard protocol that can be implemented via polarization of the signals, or equivalent signals. It turns out that the most powerful tool at the disposition of an eavesdropper is the photon-number splitting attack. We show that this attack can be extended in the relevant parameter regime so as to preserve the Poissonian photon number distribution of the combination of the signal source and the lossy channel.
We demonstrate storage and manipulation of one qubit encoded into a decoherence-free subspace (DFS) of two nuclear spins using liquid state nuclear magnetic resonance techniques. The DFS is spanned by states that are unaffected by arbitrary collective phase noise. Encoding and decoding procedures reversibly map an arbitrary qubit state from a single data spin to the DFS and back. The implementation demonstrates the robustness of the DFS memory against engineered dephasing with arbitrary strength as well as a substantial increase in the amount of quantum information retained, relative to an un-encoded qubit, under both engineered and natural noise processes. In addition, a universal set of logical manipulations over the encoded qubit is also realized. Although intrinsic limitations prevent maintenance of full noise tolerance during quantum gates, we show how the use of dynamical control methods at the encoded level can ensure that computation is protected with finite distance. We demonstrate noise-tolerant control over a DFS qubit in the presence of engineered phase noise significantly stronger than observed from natural noise sources.
The electronic structure of artificial Mn atom arrays on Ag( 111) is characterized in detail with scanning tunnelling spectroscopy and spectroscopic imaging at low temperature. We demonstrate the degree to which variations in geometry may be used to control spatial and spectral distributions of surface state electrons confined within the arrays, how these are influenced by atoms placed within the structure and how the ability to induce spectral features at specific energies may be exploited through lineshape analyses to deduce quasiparticle lifetimes near the Fermi level. Through extensive comparison of dI/dV maps and spectra we demonstrate the utility of a model based upon two-dimensional s-wave scatterers for describing and predicting the characteristics of specific resonators.
We have tuned the whispering gallery modes of a fused silica microresonator over nearly 1 nm at 800 nm, i.e. over half of a free spectral range or the equivalent of 10(6) linewidths of the resonator. This has been achieved by a new method based on the stretching of a two-stem microsphere. The devices described below will permit new cavity-QED experiments with this ultra high finesse optical resonator when it is desirable to optimize its coupling to emitters with given transition frequencies. The tuning capability demonstrated is compatible with both UHV and low temperature operation, which should be useful for future experiments with laser cooled atoms or single quantum dots. A general overview of the current state of the art in microspheres is given as well as a more general introduction.
We have studied the electronic structure of the semimetal surface Bi( 110) by high-resolution angle-resolved photoemission using synchrotron radiation. Several surface states are found very close to the Fermi level giving rise to a complex Fermi surface. As a consequence, the surface is a much better metal than the bulk, a fact which could help to explain the observation of superconductivity in granular systems built from small Bi clusters.
The coordinated and efficient distribution of limited resources by individual decisions is a fundamental, unsolved problem. When individuals compete for road capacities, time, space, money, goods, etc, they normally make decisions based on aggregate rather than complete information, such as TV news or stock market indices. In related experiments, we have observed a volatile decision dynamics and far-from-optimal payoff distributions. We have also identified methods of information presentation that can considerably improve the overall performance of the system. In order to determine optimal strategies of decision guidance by means of user-specific recommendations, a stochastic behavioural description is developed. These strategies manage to increase the adaptibility to changing conditions and to reduce the deviation from the time-dependent user equilibrium, thereby enhancing the average and individual payoffs. Hence, our guidance strategies can increase the performance of all users by reducing overreaction and stabilizing the decision dynamics. These results are highly significant for predicting decision behaviour, for reaching optimal behavioural distributions by decision support systems and for information service providers. One of the promising fields of application is traffic optimization.
Effective field theories with (large) extra dimensions are studied within a physical regularization scheme provided by string theory. Explicit string calculations then allow us to consistently analyse the ultraviolet sensitivity of Kaluza-Klein theories in the presence or absence of low-energy supersymmetry.
It is a common macroscopic observation that knotted ropes or fishing lines under tension easily break at the knot. However, a more precise localization of the breakage point in knotted macroscopic strings is a difficult task. In the present work, the tightening of knots was numerically simulated, a comparison of strength of different knots was experimentally performed and a high velocity camera was used to precisely localize the site where knotted macroscopic strings break. In the case of knotted spaghetti, the breakage occurs at the position with high curvature at the entry to the knot. This localization results from joint contributions of loading, bending and friction forces into the complex process of knot breakage. The present simulations and experiments are in agreement with recent molecular dynamics simulations of a knotted polymer chain and with experiments performed on actin and DNA filaments. The strength of the knotted string is greatly reduced (down to 50%) by the presence of a knot, therefore reducing the resistance to tension of all materials containing chains of any sort. The present work with macroscopic strings revels some important aspects, which are not accessible by experiments with microscopic chains.
After reviewing the observables of QGP we perform an analysis of m(perpendicular to) spectra of strange hadrons measured as function of centrality in 156 A GeV Pb-Pb interactions. We show that there is a good agreement between the chemical and thermal freeze-out conditions, providing additional evidence for the formation and sudden disintegration of a supercooled QGP fireball.
The quark molecular dynamics model is used to study microscopically the dynamics of the coloured quark phase and the subsequent hadron formation in relativistic S+Au collisions at the CERN-SPS. Particle spectra and hadron ratios are compared to both data and the results of hadronic transport calculations. The non-equilibrium dynamics of hadronization and the loss of correlation among quarks are studied.
We analyse multiparticle production in a thermal framework for seven central nucleus + nucleus collisions, e(+) + e(-) annihilation into hadrons on the Z resonance and four hadronic reactions (p + p and p + (p) over bar with partial centrality selection), with centre of mass energies ranging from roots = 2. 6 GeV (per nucleon pair) to 1.8 TeV. Thermodynamic parameters at chemical freeze-out (temperature and baryon and strangeness fugacities) are obtained from appropriate fits, generally improving in quality for reactions subjected to centrality cuts. All systems with non-vanishing fugacities are extrapolated along trajectories of equal energy density, density and entropy density to zero fugacities. The so-obtained temperatures extrapolated to zero fugacities as a function of initial energy density in universally show a strong rise followed by a saturating limit of T-lim = 155 +/- 6 +/- 20 MeV. We interpret this behaviour as mapping out the boundary between quark gluon plasma and hadronic phases. The ratio of strange antiquarks to light ones as a function of the initial energy density in shows the same behaviour as the temperature, saturating at a value of 0.365 +/- 0.033 +/- 0.07. No distinctive feature of 'strangeness enhancement' is seen for heavy ion collisions relative to hadronic and leptonic reactions, when compared at the same initial energy density.