Measurement of at

We present a measurement of the cross section for Z boson production times the branching fraction to tau lepton pairs in collisions at . The measurement is performed in the channel in which one tau lepton decays into a muon and neutrinos, and the other tau lepton decays hadronically or into an electron and neutrinos. The data sample corresponds to an integrated luminosity of 1.0 fb⁻¹ collected with the DØ detector at the Fermilab Tevatron Collider. The sample contains 1511 candidate events with an estimated 20% background from jets or muons misidentified as tau leptons. We obtain , which is consistent with the standard model prediction.     

superconformal Newton–Hooke algebra and many-body mechanics

A representation of the conformal Newton–Hooke algebra on a phase space of n particles in arbitrary dimension which interact with one another via a generic conformal potential and experience a universal cosmological repulsion or attraction is constructed. The minimal superconformal extension of the Newton–Hooke algebra and its dynamical realization in many-body mechanics are studied.     

Tree-level contributions to the rare decays , , and in the Standard Model

The tree-level contributions to the rare decays , , and are analyzed and compared to those occurring in , , and . It is shown that these purely long-distance contributions, arising from the exchange of a charged lepton, can be significant in B⁺ decays for an intermediate τ, potentially blurring the distinction between the modes used to extract B⁺→τ⁺ν_τ and those used to probe the genuine short-distance and FCNC transitions. Numerically, the tree-level contributions are found to account for 98%, 12% and 14% of the total , , and rates, respectively.     

First-principles study of pressure effects on and

We have performed first-principles calculations to investigate the pressure effects on CaFe₂As₂ and BaFe₂As₂. Our calculations show that in CaFe₂As₂, the orthorhombic structure transforms to a collapsed tetragonal structure at 0.4 GPa with a volume collapse of 9.5%, which is in agreement with experiments. Together with the structural phase transition, CaFe₂As₂ undergoes a magnetic transition from the stripe antiferromagnetic ordering to the nonmagnetic state. For BaFe₂As₂, we predict that the orthorhombic structure transforms to the tetragonal structure at 9.4 GPa. Unlike CaFe₂As₂, the magnetic moments of Fe ions in BaFe₂As₂ are not zero and the stripe antiferromagnetic ordering transforms to the checkerboard antiferromagnetic ordering together with the structural phase transition. The stability of the orthorhombic structure up to 9.4 GPa suggests that superconductivity and magnetism coexist in BaFe₂As₂.     

Multiplication for solutions of the equation

Linear first-order systems of partial differential equations (PDEs) of the form f=Mg, where M is a constant matrix, are studied on vector spaces over the fields of real and complex numbers. The Cauchy–Riemann equations belong to this class. We introduce on the solution space a bilinear *-multiplication, playing the role of a nonlinear superposition principle, that allows for algebraic construction of new solutions from known solutions. The gradient equation f=Mg is a simple special case of a large class of systems of PDEs, admitting a *-multiplication of solutions. We prove that any gradient equation has the exceptional property that the general analytic solution can be expressed as *-power series of certain simple solutions.     

Observation of the state of CH2 by optical–optical double resonance

We report the observation of levels in the state of CH₂ via optical–optical double resonance spectroscopy. Direct transitions between the lowest singlet state and the state are allowed by symmetry, but weak because they correspond to a two electron excitation in the single configuration approximation to the electronic wavefunction. The observed transitions involve sequential single photon absorptions at visible and near infrared wavelengths using state intermediate levels. Recent ab initio results (S.N. Yurchenko et al., J. Mol. Spectrosc. 208 (2001), 136) predicted the positions of some of the levels which are confirmed by the present results. The new spectra provide accurate energies for rotational levels in the , l = 0 level of the state.     

Search for the rare decays , , and

Rare decay modes , J/ψ→D⁻π⁺+c.c., and are searched for using events collected with the BESII detector at the BEPC. No signal above background is observed. We present upper limits on the branching fractions of , B(J/ψ→D⁻π⁺)<7.5×10⁻⁵, and at the 90% confidence level.     

Infrared safe observables in super Yang–Mills theory

The infrared structure of MHV gluon amplitudes in planar limit for super Yang–Mills theory is considered in the next-to-leading order of PT. Explicit cancellation of the infrared divergencies in properly defined cross-sections is demonstrated. The remaining finite parts for some inclusive differential cross-sections in planar limit are calculated analytically. In general, contrary to the virtual corrections, they do not reveal any simple structure.     

Search for pair production of first-generation leptoquarks in collisions at

A search for pair production of first-generation leptoquarks (LQ) is performed with data collected by the DØ experiment in collisions at at the Fermilab Tevatron Collider. In a sample of data corresponding to 1 fb⁻¹ the search has been performed on the final states with two electrons and two jets or one electron, two jets and missing transverse energy. We find our data consistent with standard model expectations. The results are combined with those found in a previous analysis of events with two jets and missing transverse energy to obtain scalar LQ mass limits. We set 95% C.L. lower limits on a scalar LQ mass of 299 GeV, 284 GeV and 216 GeV for β=1, β=0.5 and β=0.02 respectively, where β is the LQ branching ratio in the eq channel. This improves the results obtained with a lower luminosity sample from Run II of the Tevatron. Lower limits on vector LQ masses with different couplings from 357 GeV to 464 GeV for β=0.5 are also set using this analysis.     

Magnetic transitions and magnetoresistance of -type () compounds

Magnetic transitions and magnetotransport properties of polycrystalline Tb_1−xGd_xMn₆Ge₆ (x=0.2–1.0) compounds have been investigated by magnetic property and resistivity measurements in an applied magnetic field up to 50 kOe. The cell parameter a,c and cell volume V of compounds (x=0.2–1.0) increase with an increasing Gd content. The compounds (x=0.2–1.0) show a rich variety of magnetic behavior, such as antiferromagnetic, ferrimagnetic and paramagnetic state with increasing temperature. Their Curie temperatures increase almost linearly with an increasing Gd content from 460 K for x=0.2 to 484 K for x=1.0. The compounds (x=0.2–1.0) display the field induced metamagnetic transitions, and the threshold fields first increase and then decrease with an increasing Gd content. The magnetoresistance curves of the Tb_0.4Gd_0.6Mn₆Ge₆ compounds in an applied magnetic field up to 50 kOe are presented and the magnetoresistance effects are related to the metamagnetic transitions.     

Search for scalar leptoquarks in the acoplanar jet topology in collisions at

A search for leptoquarks has been performed in 310 pb⁻¹ of data from collisions at a center-of-mass energy of 1.96 TeV, collected by the D0 detector at the Fermilab Tevatron Collider. The topology analyzed consists of acoplanar jets with missing transverse energy. The data show good agreement with standard model expectations, and a lower mass limit of 136 GeV has been set at the 95% C.L. for a scalar leptoquark decaying exclusively into a quark and a neutrino.     

Anomalous dimension with wrapping at four loops in SYM

In this paper we give all the details of the calculation that we presented in our previous paper [F. Fiamberti, A. Santambrogio, C. Sieg, D. Zanon, Wrapping at four loops in SYM, arXiv: 0712.3522], concerning the four-loop anomalous dimension of the Konishi descendant tr(ZZ−ZZ) in the SU(2) sector of the planar SYM theory. We explicitly consider all the wrapping diagrams that we compute using an superspace approach and Gegenbauer polynomial x-space techniques.     

On the low-temperature properties of

Magnetic, electrical transport and heat capacity measurements have been performed on polycrystalline sample of the compound TmRu₂Si₂, which crystallizes with the tetragonal ThCr₂Si₂-type crystal structure. The results show paramagnetic behaviour down to 0.3 K, at variance with the literature data. On the basis of the collected data, possible origins of non-magnetic behaviour are discussed. Mixed valence of Tm ions or hybridisation between Tm 4f and Ru 4d states are the most probable reasons for paramagnetic behaviour of the TmRu₂Si₂.     

Islanding mechanism and band edges in InAs/GaAs nanostructures

Within the continuum elasticity theory, we investigate the substrate orientation effects on the Stranski–Krastanov (SK) growth mode and the band edges in InAs/GaAs nanostructures. Attending to the first part of the study, we estimate the transition thickness and the accumulated stress for substrate orientations (1 1 3) and (1 1 5) and we compare with the values obtained previously in InAs/GaAs (0 0 1) nanostructures. Assuming a coherent behavior at the substrate/film interface in the investigated substrate orientations, we consider a sigmoidal-type function for the dependence of the lattice parameter on the height. To evaluate the transition thickness, a minimization of the total free energy density with the slope is made. Similar to the orientation (0 0 1), in (1 1 3) and (1 1 5)-oriented substrates, two different stability regimes for the total free-energy density as well as for the accumulated stress are obtained. These regimes are directly related to the two stages of the SK growth mode. Although the relief strain mechanisms seem similar in all orientations, a delay in the 2D–3D phase transition is induced for high Miller indexes of the substrate. The non-rigid substrate approximation applied in InAs/GaAs nanostructures yields successful results for the transition thicknesses; the fraction of strained substrate being greater in InAs/GaAs (1 1 3). On the other hand, within our model, quantum dots formation is predicted for InAs/GaAs (1 1 1) heterostructures. Attending to the second, we find for all substrate orientations investigated, the strain-induced shifts lower the energies of the conduction band edges, while they raise the energies of the heavy- and light-holes band edges. In addition, the energy shifts are less significant for orientation (1 1 3), where the compressive stresses are smaller.