Doping dependence of the vortex glass and sublimation transitions in the high-TcT_{c} superconductor La2−x_{2-x}Srx_{x}CuO4_{4} as determined from macroscopic measurements

Magnetization and ac-susceptibility measurements are used to characterize the mixed phase of the high-temperature cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ over a large range of doping (0.075 $\leq x\leq$ 0.20). The first order vortex lattice phase transition line $H_{FOT}(T)$, the upper critical field $H_{c2}(T)$ and the second peak $H_{sp}(T)$ have been investigated up to high magnetic fields (8 Tesla applied perpendicular to the $CuO_2$ planes). Our results reveal a strong doping dependence of the magnetic phase diagram, which can mainly be explained by the increasing anisotropy with underdoping. Within our interpretation, the first order vortex lattice phase transition is due to the sublimation (rather than melting) of the vortex lattice into a gas of pancake vortices, whereas the second peak is related to the transition to a more disordered vortex glass state.Comment: 7 pages, 5 figure

Nilpotent Symmetries For A Spinning Relativistic Particle In Augmented Superfield Formalism

The local, covariant, continuous, anticommuting and nilpotent Becchi-Rouet-Stora-Tyutin (BRST) and anti-BRST symmetry transformations for all the fields of a (0 + 1)-dimensional spinning relativistic particle are obtained in the framework of augmented superfield approach to BRST formalism. The trajectory of this super-particle is parametrized by a monotonically increasing parameter \tau that is embedded in a D-dimensional flat Minkowski spacetime manifold. This physically useful one-dimensional system is considered on a three (1 + 2)-dimensional supermanifold which is parametrized by an even element \tau and a couple of odd elements \theta and \bar\theta of the Grassmann algebra. Two anticommuting sets of (anti-)BRST symmetry transformations, corresponding to the underlying (super)gauge symmetries for the above system, are derived in the framework of augmented superfield formulation where (i) the horizontality condition, and (ii) the invariance of conserved quantities on the supermanifold, play decisive roles. Geometrical interpretations for the above nilpotent symmetries (and their generators) are provided.Comment: LaTeX file, 21 pages, a notation clarified, a footnote added and related statements corrected in Introduction, version to appear in EPJ

Lepton flavor violating Z-decays in supersymmetric see-saw model

In supersymmetric see-saw model, the large flavor mixings of sleptons induce the lepton flavor violating (LFV) interactions $\ell_I \bar\ell_J V$ ($V=\gamma, Z$), which give rise to various LFV processes. In this work we examine the induced LFV decays $Z\to\ell_I \bar\ell_J$. Subject to the constraints from the existing neutrino oscillation data and the experimental bounds on the decays $\ell_J\to\ell_I\gamma$, these LFV $Z$-decays are found to be sizable, among which the largest-rate channel $Z\to \tau \bar{\mu}$ can ocuur with a branching ratio of $10^{-8}$ and may be accessible at the LHC or GiagZ experiment.Comment: 8 pages,4 ps files, to appear in EPJ

Renormalization-Group Improved Calculation of the B->Xs+gamma Branching Ratio

Using results on soft-collinear factorization for inclusive B-meson decay distributions, a systematic study of the partial $B\to X_s\gamma$ decay rate with a cut $E_\gamma > E_0$ on photon energy is performed. For values of $E_0$ below about 1.9 GeV, the rate can be calculated without reference to shape functions using a multi-scale operator product expansion (MSOPE). The transition from the shape-function region to the MSOPE region is studied analytically. The resulting prediction for the $B\to X_s\gamma$ branching ratio depends on three large scales: $m_b$, $\sqrt{m_b\Delta}$, and $\Delta=m_b-2E_0$. Logarithms associated with these scales are resummed at next-to-next-to-leading logarithmic order. While power corrections in $\Lambda_{QCD}/\Delta$ turn out to be small, the sensitivity to the scale $\Delta\approx 1.1$ GeV (for $E_0\approx 1.8$ GeV) introduces significant perturbative uncertainties, which so far have been ignored. The new theoretical prediction for the $B\to X_s\gamma$ branching ratio with $E_\gamma\ge 1.8$ GeV is $Br(B\to X_s\gamma)=(3.38_{-0.42-0.30}^{+0.31+0.32})\times 10^{-4}$, where the first error is an estimate of perturbative uncertainties and the second one reflects uncertainties in input parameters. With this cut $(89_{-7}^{+6}\pm 1)$ of all events are contained. The implications of larger theory uncertainties for New Physics searches are briefly explored with the example of the type-II two-Higgs-doublet model, for which the lower bound on the charged-Higgs mass is reduced compared with previous estimates to approximately 200 GeV at 95% confidence level.Comment: 39 pages, 3 figures. Results for two-loop anomalous dimensions (Appendix A.1) corrected, correlations between input parameters included, numerical analysis update

Zero temperature dynamics of Ising model on a densely connected small world network

The zero temperature quenching dynamics of the ferromagnetic Ising model on a densely connected small world network is studied where long range bonds are added randomly with a finite probability $p$. We find that in contrast to the sparsely connected networks and random graph, there is no freezing and an initial random configuration of the spins reaches the equilibrium configuration within a very few Monte Carlo time steps in the thermodynamic limit for any $p \ne 0$. The residual energy and the number of spins flipped at any time shows an exponential relaxation to equilibrium. The persistence probability is also studied and it shows a saturation within a few time steps, the saturation value being 0.5 in the thermodynamic limit. These results are explained in the light of the topological properties of the network which is highly clustered and has a novel small world behaviour.Comment: Replaced by accepted version in Eur. Phys. J

Brownian motion near a liquid-like membrane

The dynamics of a tracer molecule near a fluid membrane is investigated, with particular emphasis given to the interplay between the instantaneous position of the particle and membrane fluctuations. It is found that hydrodynamic interactions creates memory effects in the diffusion process. The random motion of the particle is then shown to cross over from a ``bulk'' to a ``surface'' diffusive mode, in a way that crucially depends on the elastic properties of the interface.Comment: 7 pages, 1 figur

Color suppressed contributions to the decay modes B_{d,s} -> D_{s,d} D_{s,d}, B_{d,s} -> D_{s,d} D^*_{s,d}, and B_{d,s} -> D^*_{s,d} D^*_{s,d}

The amplitudes for decays of the type $B_{d,s} \to D_{s,d} D_{s,d}$, have no factorizable contributions, while $B_{d,s} \to D_{s,d} D^*_{s,d}$, and $B_{d,s} \to D^*_{s,d} D^*_{s,d}$ have relatively small factorizable contributions through the annihilation mechanism. The dominant contributions to the decay amplitudes arise from chiral loop contributions and tree level amplitudes which can be obtained in terms of soft gluon emissions forming a gluon condensate. We predict that the branching ratios for the processes $\bar B^0_d \to D_s^+ D_s^-$, $\bar B^0_d \to D_s^{+*} D_s^-$ and $\bar B^0_d \to D_s^+ D_s^{-*}$ are all of order $(2- 3) \times 10^{-4}$, while $\bar B^0_s \to D_d^+ D_d^-$, $\bar B^0_s \to D_d^{+*} D_d^-$ and $\bar B^0_s \to D_d^+ D_d^{-*}$ are of order $(4- 7) \times 10^{-3}$. We obtain branching ratios for two $D^*$'s in the final state of order two times bigger.Comment: 15 pages, 4 figure

Radiative Breaking Scenario for the GUT gauge symmetry

The origin of the GUT scale from the top down perspective is explored. The GUT gauge symmetry is broken by the renormalization group effects, which is an extension of the radiative electroweak symmetry breaking scenario to the GUT models. That is, in the same way as the origin of the electroweak scale, the GUT scale is generated from the Planck scale through the radiative corrections to the soft SUSY breaking mass parameters. This mechanism is applied to a perturbative SO(10) GUT model, recently proposed by us. In the SO(10) model, the relation between the GUT scale and the Planck scale can naturally be realized by using order one coupling constants.Comment: 5 pages, no figure, final version to be published in Eur. Phys. J.

Hard scattering and jets--from p-p collisions in the 1970's to Au+Au collisions at RHIC

Hard scattering in p-p collisions, discovered at the CERN ISR in 1972 by the method of leading particles, proved that the partons of Deeply Inelastic Scattering strongly interacted with each other. Further ISR measurements utilizing inclusive single or pairs of hadrons established that high pT particles are produced from states with two roughly back-to-back jets which are the result of scattering of constituents of the nucleons as described by Quantum Chromodynamics (QCD), which was developed during the course of these measurements. These techniques, which are the only practical method to study hard-scattering and jet phenomena in Au+Au central collisions, are reviewed, with application to measurements at RHIC.Comment: 4 pages, 5 figures, Proceedings of Hard Probes 2004, International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions, Nov 4-10, 2004, to appear in EPJ

Shiva diagrams for composite-boson many-body effects : How they work

The purpose of this paper is to show how the diagrammatic expansion in fermion exchanges of scalar products of $N$-composite-boson (``coboson'') states can be obtained in a practical way. The hard algebra on which this expansion is based, will be given in an independent publication. Due to the composite nature of the particles, the scalar products of $N$-coboson states do not reduce to a set of Kronecker symbols, as for elementary bosons, but contain subtle exchange terms between two or more cobosons. These terms originate from Pauli exclusion between the fermionic components of the particles. While our many-body theory for composite bosons leads to write these scalar products as complicated sums of products of ``Pauli scatterings'' between \emph{two} cobosons, they in fact correspond to fermion exchanges between any number P of quantum particles, with $2 \leq P\leq N$. These $P$-body exchanges are nicely represented by the so-called ``Shiva diagrams'', which are topologically different from Feynman diagrams, due to the intrinsic many-body nature of Pauli exclusion from which they originate. These Shiva diagrams in fact constitute the novel part of our composite-exciton many-body theory which was up to now missing to get its full diagrammatic representation. Using them, we can now ``see'' through diagrams the physics of any quantity in which enters $N$ interacting excitons -- or more generally $N$ composite bosons --, with fermion exchanges included in an \emph{exact} -- and transparent -- way.Comment: To be published in Eur. Phys. J.