Weak mesonic currents of second class

A weak current operator of second class which is constructed of mesons only been suggested by Lipkin. If this current is extended to full SU₃ it contributes a second class part to tthe axial vector form factor of ΣΛ decay. This is calculated, using a standard Feynman cut-off to circumvent the logarithmic divergence. Present experiments restrict the coupling of such a current to the leptonic current to about one order of magnitude below the weak four fermi coupling.     

A model for charged second class currents

Choosing a model for the second class axial current composed of an s-wave vector current pseudoscalar meson pair to fix commutation relations, and using vector dominance (B-meson) techniques to calculate, we use recent experimental results to estimate the coupling strength of the second class current. Remarkably, within experimental errors we find that this coupling strength is the same as for the first class current, with the most natural normalization of the second class current. We comment on B-production and Δ production by neutrinos.     

The decay modeτ→πων τ and second class currents

Predictions of a set of equal-time commutators between first and second class currents for the decayτ→πων _τ are discussed. Such experiment should be feasible with the present detectors at SPEAR/PEP and DORIS/PETRA.     

CP violation and the absence of second class currents in charmless B decays

The absence of second class currents together with the assumption of the factorization for non-leptonic B decays provide new constraints on CP observables in the decay . The kinematics of this decay does not allow for interference between the oppositely charged resonances in the Dalitz plot as in . Nonetheless, under the assumption of factorization, the two-body time-dependent isospin analysis leads to a more robust extraction of the angle than in the isospin-pentagon analysis. The absence of second class currents might lead to enhanced direct CP violation and/or allows for a test of some assumptions made in the analysis in other decays like , and . The effects from non-factorizable contributions on the determination of are estimated by means of a numerical study. Received: 3 July 2001 / Revised version: 19 September 2001 Published online: 21 November 2001     

Thresholds for second formant transitions in front vowels

Formant dynamics in vowel nuclei contribute to vowel classification in English. This study examined listeners' ability to discriminate dynamic second formant transitions in synthetic high front vowels. Acoustic measurements were made from the nuclei (steady state and 20% and 80% of vowel duration) for the vowels /i, I, e, epsilon, ae/ spoken by a female in /bVd/ context. Three synthesis parameters were selected to yield twelve discrimination conditions: initial frequency value for F2 (2525, 2272, or 2068 Hz), slope direction (rising or falling), and duration (110 or 165 ms). F1 frequency was roved. In the standard stimuli, F0 and F1-F4 were steady state. In the comparison stimuli only F2 frequency varied linearly to reach a final frequency. Five listeners were tested under adaptive tracking to estimate the threshold for frequency extent, the minimal detectable difference in frequency between the initial and final F2 values, called deltaF extent. Analysis showed that initial F2 frequency and direction of movement for some F2 frequencies contributed to significant differences in deltaF extent. Results suggested that listeners attended to differences in the stimulus property of frequency extent (hertz), not formant slope (hertz/second). Formant extent thresholds were at least four times smaller than extents measured in the natural speech tokens, and 18 times smaller than for the diphthongized vowel /e/.     

First and second order quantum phase transitions in multi-band superconductors

In multi-band and inter-metallic materials superconductivity can be destroyed by applying external pressure in these systems. In many cases the critical temperature is driven continuously to zero, the superconducting to normal transition being associated with a superconducting quantum critical point (SQCP). In this paper we propose a model for this type of SQCP based on the increase of hybridization as pressure is applied in the material. We study a two-band superconductor with hybridization V between these bands. We use a BCS approximation and include both inter- and intra-band attractive interactions. We show that for negligible inter-band interactions, as hybridization increases there is a second order phase transition from a superconductor to a normal state at zero temperature at a critical value of the hybridization V_c. This SQCP can be reached by pressure, since this external parameter controls hybridization in the system. We also find discontinuous transitions at zero temperature and the appearance of a gapless superconducting (GS) phase in a certain range of hybridization in the case of inter-band interactions being dominant.     

A model ferromagnet with first and second order phase transitions

A model ferromagnet with pair and four spin interaction strengths in the ratio x is shown to have classical behaviour for x > 3, and discontinuous magnetization for x ? 3, the point x = 3 corresponding to a tricritical point.     

Chiral two-body currents in nuclei: Gamow-Teller transitions and neutrinoless double-beta decay

We show that chiral effective field theory (EFT) two-body currents provide important contributions to the quenching of low-momentum-transfer Gamow-Teller transitions, and use chiral EFT to predict the momentum-transfer dependence that is probed in neutrinoless double-beta (0νββ) decay. We then calculate for the first time the 0νββ decay operator based on chiral EFT currents and study the nuclear matrix elements at successive orders. The contributions from chiral two-body currents are significant and should be included in all calculations.     

Second-class currents in 0−→0+ and unique nuclear β transitions

The induced pseudotensor contribution to 0^? → 0⁺ and uniqueβ transitions has been analyzed starting from two possible forms of the nonrelativistic Hamiltonian. The role of mesonic corrections and mesonic second-class currents has been discussed     

Second class currents and broken SU(3)-symmetry

In the SU(3) limit, a second class, T-normal axial vector current, which transforms like an octet, can give rise to pseudotensor terms in the matrix elements for leptonic decays of baryons. With first order symmetry breaking, this second class current may generate pseudovector and pseudoscalar form factor contributions. It is an anti-hermitian current, but, even with SU(3) breaking, it does not need to disturb the chiral SU(2) × SU(2) algebra of charges.     

Temperature dependence of EXAFS intensities near second order phase transitions

Extended x-ray absorption fine structure (EXAFS) measures near neighbor bond lengths. Thermal damping of the oscillatory absorption modulation is caused by bond-length fluctuations and is predicted to show a weak change in temperature derivative at the temperature T_c of a second order structural phase transition. The size of the slope change is a measure of the sensitivity of T_c to changes in the bond-stretching force constant.     

First and second order phase transitions in gauge theories at finite temperature

We consider in general the nature of the phase transition which occurs in 4D gauge theories coupled to scalar and spinor fields at finite temperature. It is shown that the critical behavior can be isolated in an effective 3D theory of the zero frequency mode whose lagrangian may be calculated explicitly in weak coupling perturbation theory. This lagrangian, in turn, may be investigated by means of standard ?-expansion techniques. Theories with an asymptotically free gauge coupling constant possess no stable fixed point in the ?-expansion and are inferred to have weakly first-order phase transitions; theories not satisfying this condition may have second-order transitions.     

High-energy behavior and second class constraints in quantum gravity

It is proposed that sensible high-energy behavior in a quantum theory of gravity may be achieved in a class of theories in which the connection and metric are independent and unconstrained and where the action is chosen so that no derivatives of the metric appear. This is because in these theories all ten of the metric field equations are realized as second class constraints. These can in principle be solved, expressing the operators g_μν as functions of the operators for the components of the connection and their canonical momenta. Thus, the metric has no independent quantum fluctuations, and the instabilities resulting from the usual curvature squared terms are eliminated. Furthermore, there is no need to assume metric compatibility, as it is automatically restored in the low-energy limit by the dominance of dimension-two terms.In order to explore these ideas a toy model with two degrees of freedom, corresponding to a metric and a connection variable, is quantized and shown to have a sensible high energy limit, while a related model, in which a constraint analogous to metric compatibility is imposed, is found to be unstable at high energies.     

Application of the generalized Landau ansatz to second order phase transitions

The ansatz ofAmit et al. with expansion coefficients which can be nonanalytic functions of either the temperature or the order parameter is used to predict the generalized susceptibility (compressibility) and the surface tension belowT _c and the critical isotherm atT _c from the specific heat, the order parameter at the coexistence curve, and the coherence length. The results agree within 50% with experiments on different phase transitions in ferromagnets, classical gases, quantum gases, binary mixtures, and superfluid helium. The ansatz can be written in two different versions; their results differ only by logarithmic factors.     

From second to first order transitions in a disordered quantum magnet

The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N(c) limit and found to be comparable to that of octet baryons. Large N(c) selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.     

On BRST quantization of second class constraint algebras

A BRST quantization of second-class constraint algebras that avoids Dirac brackets is constructed, and the BRST operator is shown to be related to the BRST operator of first class algebra by a nonunitary canonical transformation. The transformation converts the second class algebra into an effective first class algebra with the help of an auxiliary second class algebra constructed from the dynamical Lagrange multipliers of the Dirac approach. The BRST invariant path integral for second class algebras is related to the path integral of the pertinent Dirac brackets, using the Parisi-Sourlas mechaism. As an application the possibility of string theories in subcritical dimensions is considered.     

Log Mirror Symmetry and Local Mirror Symmetry

We study Mirror Symmetry of log Calabi–Yau surfaces. On one hand, we consider the number of “affine lines” of each degree in ℙ²\B, where B is a smooth cubic. On the other hand, we consider coefficients of a certain expansion of a function obtained from the integrals of dx/x∧dy/y over 2-chains whose boundaries lie on B _φ, where {B _φ} is a family of smooth cubics. Then, for small degrees, they coincide. We discuss the relation between this phenomenon and local mirror symmetry for ℙ² in a Calabi–Yau 3-fold ([CKYZ]). Received: 1 October 1999 / Accepted: 22 November 2000