A limit on the lepton-family number violating process π → μe

A FNAL E799 Collaboration has carried out a search for the lepton-family number violating decay π⁰ → μ^±e using π⁰'s produced from K_L → π⁰π⁰π⁰ decays in flight. No events were observed. Assuming that lepton-family number violation is charge independent, the 90% confidence level upper limit on

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was determined to be 8.6 × 10⁻⁹.     

The first eigenvalue of the Dirac operator on Kähler manifolds

If M^2m is a closed Kähler spin manifold of positive scalar curvature R, then each eigenvalue λ of type r (r {1, …, [(m + 1)/2]}) of the Dirac operator D satisfies the inequality λ² ≥ rR₀/4r − 2, where R₀ is the minimum of R on M^2m. Hence, if the complex dimension m is odd (even) we have the estimation

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for the first eigenvalue of D. In the paper is also considered the limiting case of the given inequalities. In the limiting case with m = 2r − 1 the manifold M^2m must be Einstein. The manifolds S², S² × S², S² × T², P³( ), F( ), P³( ) × T² and F( ³) × T², where F( ³) denotes the flag manifold and T² the 2-dimensional flat torus, are examples for which the first eigenvalue of the Dirac operator realizes the limiting case of the corresponding inequality. In general, if M^2m is an example of odd complex dimension m, then M^2m × T² is an example of even complex dimension m + 1. The limiting case is characterized by the fact that here appear eigenspinors of D² which are Kählerian twistor-spinors.     

Tilting the brane, or some cosmological consequences of the brane Universe

We discuss theories in which the standard-model particles are localized on a brane embedded in space–time with large compact extra dimensions, whereas gravity propagates in the bulk. In addition to the ground state corresponding to a straight infinite brane, such theories admit a (one parameter) family of stable configurations corresponding to branes wrapping with certain periodicity around the extra dimension(s) when one moves along a noncompact coordinate (tilted walls). In the effective four-dimensional field-theory picture, such walls are interpreted as one of the (stable) solutions with the constant gradient energy, discussed earlier [1 and 2]. In the cosmological context their energy “redshifts” by the Hubble expansion and dissipates slower than the one in matter or radiation. The tilted wall eventually starts to dominate the Universe. The upper bound on the energy density coincides with the present critical energy density. Thus, this mechanism can become significant any time in the future. The solutions we discuss are characterized by a tiny spontaneous breaking of both the Lorentz and rotational invariances. Small calculable Lorentz noninvariant terms in the standard model Lagrangian are induced. Thus, the tilted walls provide a framework for the spontaneous breaking of the Lorentz invariance.     

Ferromagnetic resonance in Sr doped rare earth orthocobaltites, Ln1−xSrxCoO3†

FMR measurements have been carried out on several members of the Ln_1−xSrxCoO₃ (Ln = Rare earth) system. The results show that g_eff in these systems is around 1.25 independent of x as well as the rare earth ion. It is suggested that this unusual value of g_eff is due to the localized intermediate-spin Co³⁺ ions (t⁵_2g e¹_g) located at the top of the π^* band.     

Neutrino masses, mixing and hierarchy

We build a model to describe neutrinos based on strict hierarchy, incorporating as much as possible, the latest known data, for Δ_sol and Δ_atm, and for the mixing angles determined from neutrino oscillation experiments, including that from KamLAND. Since the hierarchy assumption is a statement about mass ratios, it lets us obtain all three neutrino masses. We obtain a mass matrix, M_ν and a mixing matrix, U, where both M_ν and U are given in terms of powers of Λ, the analog of the Cabibbo angle λ in the Wolfenstein representation, and two parameters, ρ and κ, each of order one. The expansion parameter, Λ, is defined by

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, and ρ expresses our ignorance of the lightest neutrino mass m₁, (m₁=ρΛ⁴m₃), while κ scales s₁₃ to the experimental upper limit, s₁₃=κΛ²≈0.16κ. These matrices are similar in structure to those for the quark and lepton families, but with Λ about 1.6 times larger than the λ for the quarks and charged leptons. The upper limit for the effective neutrino mass in double β-decay experiments is 4×10⁻³ eV if s₁₃=0 and 6×10⁻³ eV if s₁₃ is maximal. The model, which is fairly unique, given the hierarchy assumption and the data, is compared to supersymmetric extension and texture zero models of mass generation.     

Nuclear effects in the F3 structure function for finite and asymptotic Q

We study nuclear effects in the structure function F₃ which describes the parity violating part of the charged-current neuitrino nucleon deep inelastic scattering. Starting from a covariant approach we derive a factorized expression for the nuclear structure function in terms of the nuclear spectral function and off-shell nucleon structure functions valid for arbitrary momentum transfer Q and in the limit of weak nuclear binding, i.e. when a nucleus can be treated as a non-relativistic system. We develop a systematic expansion of nuclear structure functions in terms of a Q⁻² series caused by nuclear effects (“nuclear twist” series). Basing ourselves on this expansion we calculate nuclear corrections to the Gross-Llewellyn-Smith sum rule as well as to higher moments of F₃. We show that corrections to the GLS sum rule due to nuclear effects cancel out in the Bjorken limit and calculate the corresponding Q⁻² correction. Special attention is paid to the discussion of the off-shell effects in the structure functions. A sizable impact of these effects both on the Q² and x dependence of nuclear structure functions is found.     

应用分子子图指数估计与预测烷烃核磁共振碳谱位移和

系统地研究了核磁共振碳谱(¹³C NMR)与化学位移规律及其定量构谱关系(QSSR).本文研究了一组多元素分子子图指数矢量(VMSG),并发现它与烷烃化学位移和(CCSA)有很好线性相关性.采用多元线性回归(MVLR)进行准确估计与预测,结果良好.     

Gamma-ray transitions in Bi following the decay of Po

The decay of 5.7 h ²⁰⁷Po to levels in ²⁰⁷Bi has been studied using a Ge(Li) detector. Combining the relative gamma-ray intensities with previously established conversion electron intensities, K-conversion coefficients are determined. The multipolarities of 29 transitions are deduced from a comparison with theoretical conversion coefficients, thus establishing the following spins and parities for the excited levels:

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. A comparison with a recent shell-model calculation shows good agreement for all negative parity levels.     

On the normalization of Gamow functions

It is proved that the definitions of the norm given by Berggren and Romo are identical. Regularization in ref. ¹) with any convergence factor of the form

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leads to the same result. The existence of the norm is also established for the proton. The norm for neutrons is analytically given.     

Saturation and BFKL dynamics in the HERA data at small-x

We show that the HERA data for the inclusive structure function F₂(x,Q²) for x10⁻² and 0.045Q²45 GeV² can be well described within the color dipole picture, with a simple analytic expression for the dipole–proton scattering amplitude, which is an approximate solution to the non-linear evolution equations in QCD. For dipole sizes less than the inverse saturation momentum 1/Q_s(x), the scattering amplitude is the solution to the BFKL equation in the vicinity of the saturation line. It exhibits geometric scaling and scaling violations by the diffusion term. For dipole sizes larger than 1/Q_s(x), the scattering amplitude saturates to one. The fit involves three parameters: the proton radius R, the value x₀ of x at which the saturation scale Q_s equals 1 GeV, and the logarithmic derivative of the saturation momentum λ. The value of λ extracted from the fit turns out to be consistent with a recent calculation using the next-to-leading order BFKL formalism.     

Mössbauer effect studies of the 74.5 keV level of Dy

Recoilless absorption measurements of the 74.5 keV γ-rays of ¹⁶¹Dy were carried out using

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absorbers. The analysis of the absorption spectrum obtained with the nitrate absorber yielded a value of 1.38 ± 0.02 for the ratio of the g-factor of the 74.5 keV and the ground levels of ¹⁶¹Dy and a value of 0.58 ± 0.04 for the corresponding ratio of the quadrupole moments. Relying on previous measurements of the ground level moments, the values derived for the magnetic and quadrupole moments of the 74.5 level are −0.377 ± 0.012 n.m. and 1.5 ± 0.2 b, respectively. These values are compared with the values predicted by nuclear theory. The isomeric shift between DyNi₂ and absorbers was found to be 0.9 ± 0.2 mm/sec.     

Spectroscopic study of BixEu1−xVO4 and BiyGd1−yVO4 mixed oxides

The mixed oxides Bi_xEu_1−xVO₄ and Bi_yGd_1−yVO₄ crystallize in a zircon-type structure, for 0 <x < 0.6 and 0 < y < 0.64, and in a fergusonite-type structure, for 0.94 < x < 1 and 0.93 < y < 1. A process of competition between the dominant and the constrained effects of the lone-pair 6s² of Bi³⁺ is discussed. The diffuse reflectance spectroscopic studies of these mixed oxides are presented. The observed broad bands are attributed to charge transfer processes and the sharp peaks in the Bi_xEu_1−xVO₄ spectra are ascribed to intra-configurational 4f – 4ftransitions of the Eu³⁺ ion. The broad absorption shift in BiLnVO₄ (Ln : Eu and Gd) compounds to the longer wavelengths range, when Bi is introduced in the LnVO₄ lattice, is ascribed to charge transfer processes in a Bi-VO₄ center and are interpreted assuming a Jahn-Teller effect in the excited state of Bi³⁺. The concept of an internal pressure of Bi³⁺ ions is also used to explain the broad A-band shifts.     

N=2 6-dimensional supersymmetric E6 breaking

We study the N=2 supersymmetric E₆ models on the 6-dimensional space–time where the supersymmetry and gauge symmetry can be broken by the discrete symmetry. On the space–time M⁴×S¹/(Z₂×Z₂′)×S¹/(Z₂×Z₂′), for the zero modes, we obtain the 4-dimensional N=1 supersymmetric models with gauge groups SU(3)×SU(2)×SU(2)×U(1)², SU(4)×SU(2)×SU(2)×U(1), and SU(3)×SU(2)×U(1)³ with one extra pair of Higgs doublets from the vector multiplet. In addition, considering that the extra space manifold is the annulus A² and disc D², we list all the constraints on constructing the 4-dimensional N=1 supersymmetric SU(3)×SU(2)×U(1)³ models for the zero modes, and give the simplest model with Z₉ symmetry. We also comment on the extra gauge symmetry breaking and its generalization.     

Effect of Pr and Ca substitution on superconductivity in Y(1−x−yPrxCay)Ba2Cu3O7−δ

Pr substituted at constant Ca concentration for Y in (Y_1−x−yPr_xCa_y)Ba₂Cu₃O_7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of T_c decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of T_c cannot be compensated by appropriate hole doping with Ca.     

The dilambda and the pentaquark in the constituent quark model

We use the non-relativistic constituent quark model and the bag model to study the stability of the dilambda H = (uuddss) and pentaquark states

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. While they are stable in the limit of exact SU(3)_F flavour symmetry between u, d, and s quarks, the H and the P become unbound when a realistic breaking of SU(3)_F is introduced.     

The proton''s gluon distribution

The gluon distribution is dominated by the hard pomeron at small x and all Q², with no soft-pomeron contribution. This describes well not only the DGLAP evolution of the hard-pomeron part of F₂(x,Q²), but also charm photoproduction and electroproduction, and is consistent with what is known about the longitudinal structure function.     

Generalized holonomy of supergravities with 8 real supercharges

We show that the generalized holonomy groups of ungauged supergravity theories with 8 real supercharges must be contained in

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, where is the generalized structure group. Here n=4ν is the number of preserved supersymmetries, so the allowed values are limited to n=0,4,8. In particular, solutions of ungauged supergravities in four, five and six dimensions are examined and found to explicitly follow this pattern. We also argue that the G-structure has to be a subgroup of this generalized holonomy group, which may provide a possible classification for supergravity vacua with respect to the number of supercharges.     

Spectral intensities in the ν1 band of NH3

Intensities have been measured for individual transitions in the Q and R branches of the ν₁ band of NH₃ using a difference-frequency laser spectrometer. The data yield an integrated band strength of S⁰_v=219.36±1.03 cm^-2/MPa at 297 K, corresponding to a transition moment of μ_v = 8.535(20) × 10^-32 C·m, and a Herman-Wallis correction factor, (1 + _jm)², where _j = 0.0209(20). The intensities of a few lines for K 7 were noticeably perturbed by a perpendicular Coriolis interaction with 2ν₄ (E, L = 2), so were excluded from the fit. A small sample of ν₃ band lines occurring in the ν₁ band scans also yields a rough estimate of the ν₃ band intensity with evident irregular perturbations.     

外加电场和Al组分对纤锌矿AlGaN/GaN量子阱中的电子g因子的影响

研究了外加电场和垒层的Al组分对AlGaN/GaN量子阱中的横向和纵向g因子(g⊥和g//)及其各向异性(δg)的影响.纤锌矿体结构的贡献(S_//~(bulk)和g⊥)是构成g⊥=(g//-g_0)=g_//~(bulk)的主要部分,但g_//~(bulk)和g⊥的差值很小且几乎不随外加电场和Al组分改变.当外加电场的方向同极化电场的方向相同(相反)且增加时,g_//~(bulk)和g_⊥~(bulk)的强度同时增加(减小).当外加电场从-1.5×10~8 V·m~(-1)到1.5×10~8 V·m~(-1)变化时,异质结界面对g⊥的贡献(Γ_(Inter))大于0且强度缓慢增加,阱层对g⊥的贡献(Γ_W)小于0且强度也缓慢增加.然而Γ_(Inter)的强度比Γ_w大,且后者的强度随着外加电场的改变增加较快,所以δg0且强度随着外加电场的变化而减小.当垒层的Al组分增加时,如果不考虑应变效应(S_(1,2)=0),g_//~(bulk)和g⊥的强度同时减小,然而考虑应变效应后(S_(1,2)≠0),β_1g⊥和γ1(g_//~(bulk))的强度随着Al组分的增加而增加.随着垒层Al组分的增加,Γ_(Inter)和Γ_w的强度都增加,但Γ_(Inter)的强度较大且增加得较快,所以的的强度缓慢增加.g⊥的强度先随着Al组分的增加而减小,然后又随着Al组分的增加而增加,因为g⊥小于0且强度随着Al组分增加得很快.结果表明,AlGaN/GaN量子阱结构中的电子g因子及其各向异性可以被外加电场、垒层的Al组分、应变效应和量子限制效应共同调制.