Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

The diffusivityD _μ of positive muons (μ⁺) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ⁺SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsD _μ through the variation of the magnetic field felt by the μ⁺ during their diffusion through the crystals. For μ⁺ near the centres of the flux lines inNb it givesD _μ(4.6 K)=(8±2)·10⁻¹¹m²S⁻¹. The positive temperature coefficient ofD _μ indicates that at liquid-helium temperatures the diffusivity of μ⁺ inNb is mainly due to phonon-assisted tunnelling processes.     

Chiral SU(4) × SU(4) breaking: masses and decay constants of charmed hadrons

The (4, 4*) ⊕ (4*, 4) model of broken chiral SU (4) × SU (4) symmetry has been used to calculate the third-order coupling constants involving charmed and ordinary pseudoscalar mesons. These coupling constants are exploited to derive some interesting new relations among the masses and decay constants of these charmed particles. Using the known masses and decay constants as inputs, we exploit these relations to predict:F _D = −1·41F _π ,F _F = −1·13F _π ,F _D/F_F = 1·25,m(D _s) = 1·43 GeV,m(F _s) = 1·39 GeV andm(K _s) = 1·02 GeV.     

Fast nonequilibrium induction detectors based on thin superconducting films

A new type of fast detector is proposed. The operation of the detector is based on the change induced in the kinetic inductance of NbN and YBa₂Cu₃O_7−δ superconducting films by nonequilibrium quasiparticles produced by electromagnetic radiation. The speed of a NbN detector is essentially temperature-independent and is less than 1 ps. A model based on the Omen-Scalapino scheme describes well the experimental dependence of the voltage-power sensitivity of a NbN detector on the modulation frequency of the radiation. A low equilibrium quasiparticle density and a high quantum yield give detecting power D*=10¹²W⁻¹ · cm · Hz^1/2 at temperature T=4.2 K and D*=10¹⁶ W⁻¹ · cm · Hz^1/2 at temperature T=1.6 K. The time constant of the low-temperature YBaCuO induction detector is determined only by the electron-phonon interaction time τ _e-ph ^d in the nodal regions. Zh. Tekh. Fiz. 68, 63–69 (October 1998)     

Spin exchange and chemi-ionization during collisions of polarized metastable helium atoms with ground-state cesium atoms

The kinetics of the optical orientation of atoms in a helium-cesium gas-discharge plasma are considered, and kinetic equations describing the optical orientation of atoms in the case of two simultaneously occurring processes, viz., an elastic process (spin exchange) and an inelastic process (chemi-ionization), are derived. The rate constants of these processes are determined experimentally: C _se=(2.8±0.8)×10⁻⁹ cm³s⁻¹, C _ci=(1.0±0.3)×10⁻⁹ cm³s⁻¹. Zh. Tekh. Fiz. 69, 36–40 (September 1999)     

Comments on preliminary data on the reactions e + e −→φ→γ f 0(980)→γπ0π0

Preliminary data on the reactions e ⁺ e ⁻→φ→γ f ₀(980)→γπ⁰π⁰ together with data on ππ scattering and the reactions J/ψ→φπ⁺π⁻ and K ⁻ p→π ⁺π⁻(Λ,Σ) are analyzed. The analysis shows that the mass of the f ₀(980) meson is = 950 MeV, and BR(φ→γf ₀→γπ⁰π⁰)≃ 1× 10⁻⁴, indicating that the f ₀(980) resonance is of a four-quark nature. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 445–449 (10 April 1998)     

Decay rate measurements of upper laser level in He-Cd+ laser

The decay rate of² D _5/2 level of Cd II has been measured by the magnetic-field power-dip method. The decay rate at the zero-pressure limit is found to be 2.4·10⁶s⁻¹. The calculated collision cross section for excited Cd ions with He atoms equals 0.91·10⁻¹⁵ cm². This work was supported by the Institute of Quantum Electronics. WAT Warszawa, within the project 06.2.3.     

Experimental determination of the rate constant for spin exchange in collisions of polarized metastable helium atoms with ground-state cesium atoms

The rate constant for spin exchange in a system consisting of a metastable helium atom and an alkali-metal atom is determined. An experiment on optical orientation of atoms established that the rate constant for spin exchange in a collision of a metastable 2³ S ₁ helium atom with a cesium atom in the 6² S _1/2 ground state equals (2.8±0.8)×10⁻⁹ cm³ s⁻¹. The rate constant for chemoionization of cesium atoms by metastable helium atoms was determined at the same time to be (1.0±0.3)×10⁹ cm³s⁻¹. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 145–148 (10 August 1997)     

Total rates of nuclear capture of negative muons in the isotopes 132Xe and 40Ar

The lifetimes of a negative muon in the isotopes ¹³²Xe and ⁴⁰Ar in the solid phase are measured. The lifetime of μ ⁻ in the 1s state of the isotope ¹³²Xe is τ(¹³²Xe)=101.7±1.7 ns, which corresponds to a total nuclear capture rate Λ_c(¹³²Xe)=9.4±0.2 μs⁻¹. The lifetime of μ ⁻ in the isotope ⁴⁰Ar, viz., τ (⁴⁰Ar)=568±6 ns, corresponding to a capture rate Λ_c(⁴⁰Ar)=1.31±0.01 μs⁻¹, is obtained to several times better accuracy as compared to previously published results. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 181–183 (10 February 1999)     

Measurement of atomic parameters for the3 D 1 state of I–II

The decay rates of the³ D ₁ level of I–II and the 3s ₂ level of neon have been measured by the magnetic-field power dip method. The decay rate at the zero-pressure limit is found to be 7.1 MHz/2π for the³ D ₁ level. The collision cross-section for excited I ions with helium atoms equals 0.19×10⁻¹⁵ cm². This work was supported by project MR-1.5-1.05.     

The search for and study of the strange multibaryon states in systems with Λ-hyperon and Ks0 -meson in pA collision at momentum of 10 GeV/c

Exotic strange multibaryon states have been observed in the effective mass spectra of: Λπ^±, Λπ⁺π⁻, Λp, Λpp, Λπp, ΛΛ and ΛK _s ⁰, K _s ⁰ p, K _s ⁰π^± subsystems. The measured Λ/π⁺ ratio for average multiplicities from the pC reaction is equal to (5.3 ± 0.8) × 10⁻², and it is approximately two times larger than the Λ/π⁺ ratio simulated by the FRITIOF model and than that of experimental pp reactions at the same energy. The observed wellknown resonances Σ*⁺ (1385) and K*^± (892) from PDG are good tests of this method. The mean value of the mass for the Σ*⁻(1385) resonance is shifted to 1370 MeV/c ² and the width is two times larger than the same value from PDG. Such kind of behavior for the width and invariant mass of Σ*⁻ (1385) resonance is interpreted as an extensive contribution from stopped Ξ⁻ → Λπ⁻ and medium effect. There is enhancement of the production contribution for all observed hyperons. A few events were registered by hypothesis of light H ⁰ and heavy H ^{0, +} dihyperons. Dihyperons were observed by weak decay channels: (Σ⁻ p), (Λpπ⁻), (Λpπ⁰), and (K ⁻ pp). The text was submitted by the author in English.     

Mixing of excitonic states containing light and heavy holes in an isolated GaAs/AlGaAs quantum well in a magnetic field

The Zeeman splitting of the ground states 1s(hh) and 1s(lh) of excitons with heavy and light holes, respectively, in a 15-nm isolated Al_0.3Ga_0.7As/GaAs quantum well in magnetic fields of up to 20 T is investigated according to the photoluminescence excitation spectra in the Faraday geometry (σ⁺− σ⁻ components). The observed anomalous pattern of nonlinear Zeeman splitting and the nonmonotonic behavior of the effective hole g factor are interpreted in terms of the strong mixing of the magnetoexcitonic states containing light and heavy holes. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 52–56 (10 July 1996)     

Pseudovector heavy quarkonia in e + e −2 collisions

It is shown that BR(χ _b1(1 P)→e ⁺ e ⁻)≃3.3· 10⁻⁷ and BR(χ _c1(1 P)→e ⁺ e ⁻)≃10⁻⁸. This gives realistic possibilities for searching for the production of χ _b1(1 P) and ξ _c1(1 P) states in e ⁺ e ⁻ collisions, even on the present-day colliders, to say nothing of b and c-τ factories. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 569–574 (25 April 1996)     

Enhanced direct <Emphasis Type="Italic">CP</Emphasis> violation and branching ratios in bottom hadron decays

In the framework of factorization we study direct CP violation in the decays of bottom hadrons containing a strange quark or a charm quark, H _b →f ρ ⁰(ω)→f π ⁺ π ⁻ (H _b is the bottom hadron and f is the product hadron) including the effect of ρ–ω mixing. We find that the CP violating asymmetry can be enhanced greatly via the ρ–ω mixing mechanism when the invariant mass of the π ⁺ π ⁻ pair is in the vicinity of the ω resonance. For the processes associated with b→d transitions, e.g. , , B _c ⁻→D ⁻ π ⁺ π ⁻, B _c ⁻→D ^*− π ⁺ π ⁻, Ξ _b ⁰→Λ π ⁺ π ⁻, and Ω _b ⁻→Ξ ⁻ π ⁺ π ⁻, the maximum CP violating asymmetries can reach about −84%, while for the processes associated with b→s transitions, e.g. , , B _c ⁻→D _s ⁻ π ⁺ π ⁻, B _c ⁻→D _s ^*− π ⁺ π ⁻, Ξ _b ⁻→Ξ ⁻ π ⁺ π ⁻, and Ω _b ⁻→Ω ⁻ π ⁺ π ⁻, the CP violating asymmetries can be enhanced to about 95%. Furthermore, taking ρ–ω mixing into account we calculate the b-hadron decay branching ratios. We also discuss the possibility to observe the predicted CP violating asymmetries at the LHC.     

Reaction e + e ? → π + π ? π + π ? at energies of √ s ≤ 1 GeV

The e ⁺ e ⁻ → π ⁺ π ⁻ π ⁺ π ⁻ cross section is calculated for energies of 0.65 ≤ √s ≤ 1 GeV in the framework of the generalized hidden local symmetry model. The calculations are compared with the data of CMD-2 and BaBaR. It is shown that the inclusion of heavy isovector resonances ρ(1450) and ρ(1700) is necessary for reconciling calculations with the data. It is found that, at √s ≈ 1 GeV, the contributions of the above resonances are much larger, by a factor of 30, than the ρ(770) one, and amount to a considerable fraction ∼0.3–0.6 of the latter at √s ∼ m _ρ. The text was submitted by the authors in English.     

Thermal expansion of thin C60 films

The thermal expansion coefficient a and structure of C₆₀ films with thickness t∼3–10 nm were investigated in the temperature interval from room to liquid-nitrogen temperature by electron-optical methods. The thermal expansion coefficient was determined from the temperature shift of the diffraction maxima in the electron diffraction patterns. The objects of investigation were epitaxial C₆₀ films condensed in vacuum on a (100) NaCl cleavage surface and oriented in the (111) plane. A surface-induced size effect in the thermal expansion coefficient was observed. It was established that as t decreases α _f increases and is described well by the relation α _f=17·10⁻⁶ K⁻¹+8.3·10⁻⁵ nm K⁻¹ t ⁻¹. This relation was used to estimate the linear expansion coefficient α _s of the C₆₀ surface in the (111) plane as α _s=60·10⁻⁶K⁻¹, which is several times larger than the bulk value. The experimental results agree satisfactorily with the theoretical calculations of the mean-square displacements of molecules located in a region near the surface. Zh. éksp. Teor. Fiz. 114, 1868–1875 (November 1998)     

Search for D+→φl+ν and measurement of the branching fraction for D+→φπ+

Using a data sample of integrated luminosity of about 33 pb^-1 collected around 3.773 GeV with the BESII detector at the BEPC collider, the semileptonic decays D⁺→φe⁺ν_e, D⁺→φμ⁺ν_μ and the hadronic decay D⁺→φπ⁺ are studied. The upper limits of the branching fractions are set to be BF(D⁺→φe⁺ν_e)<2.01% and BF(D⁺→φμ⁺ν_μ)<2.04% at the 90% confidence level. The ratio of the branching fractions for D⁺→φπ⁺ relative to D⁺→K^-π⁺π⁺ is measured to be 0.057±0.011±0.003. In addition, the branching fraction for D⁺→φπ⁺ is obtained to be (5.2±1.0±0.4)×10^-3.     

The role of combined electron-deuteron screening ind-d fusion in metals

We propose that thed-d fusion rate in palladium can be enhanced by the combined screening of the electrostatic interactions by the itinerant deuterons and the conduction electrons. The model assumes that, under certain conditions, deuterium exists as a D⁺ ion in palladium. The combined screening by electrons and the D⁺ ions (deuterons) is found to be more effective than that due to electrons alone. The calculated values of thed-d fusion rates, considering screening, for composition PdD at 300 K are 10⁻¹⁶ s⁻¹ and 10⁻¹⁴ s⁻¹ for D ₂ ⁺ ion and D₂ molecule respectively. These values lie in the range suggested by the recent electrochemical experiments.     

Measurement of the rate of nuclear capture of negative muons in the isotopes 84Kr and 136Xe

The lifetime of a negative muon in the 1S state in the isotopes ⁸⁴Kr and ¹³⁶Xe was measured. The values obtained, τ(⁸⁴Kr)=139.2±2.9 ns and τ(¹³⁶Xe)=111.0±4.6 ns, correspond to total nuclear capture rates Λ_c(⁸⁴Kr)=6.75±0.15 μs⁻¹ and Λ _c(¹³⁶Xe)=8.6±0.4 μs⁻¹. Theoretical calculations of the rate of nuclear capture of a negative muon are performed for the Kr isotopes. The experimental results are compared with the theoretical calculations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 302–307 (10 March 1998)     

Effect of unitarization on the amplitudes for the decays K10 → π+π? and K+ → π+π+π?

The unitarization of the amplitude for the decay process K ₁⁰ → π⁺π⁻ and allowance for the rescattering of final-state pions in the decay process K ⁺ → π⁺π⁺π⁻ make it possible to evaluate, by using the parameters extracted from data on K → 2π decays, the K ⁺ → π⁺π⁺π⁻ decay width. The result agrees with the experimental width value at a level of a few percent. Allowance for corrections for higher order terms of the momentum expansion of the amplitude for the decay process K ⁺ → π⁺π⁺π⁻ leads to the slope-parameter value of g ₊₊₋^th = 0.2182, which agrees with its experimental counterpart, g ₊₊₋^exp = 0.2154 ± 0.0035.