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The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca$_{3}$Ru$_{2}$O$_{7}$, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a $90^{\circ}$ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca$_{3}$Ru$_{2}$O$_{7}$, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO$_{6}$ octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them., Comment: 12 pages, 6 figures (+ 12 pages, 6 figures of supplemental material)

We analyze the low-energy dynamics of quasi one dimensional, large-$S$ quantum antiferromagnets with easy-axis anisotropy, using a semi-classical non-linear sigma model. The saddle point approximation leads to a sine Gordon equation which supports soliton solutions. These correspond to the movement of spatially extended domain walls. Long-range magnetic order is a consequence of a weak inter-chain coupling. Below the ordering temperature, the coupling to nearby chains leads to an energy cost associated with the separation of two domain walls. From the kink-antikink two-soliton solution, we compute the effective confinement potential. At distances large compared to the size of the solitons the potential is linear, as expected for point-like domain walls. At small distances the gradual annihilation of the solitons weakens the effective attraction and renders the potential quadratic. From numerically solving the effective one dimensional Schr\"oedinger equation with this non-linear confinement potential we compute the soliton bound state spectrum. We apply the theory to CaFe$_{2}$O$_{4}$, an anisotropic $S=5/2$ magnet based upon antiferromagnetic zig-zag chains. Using inelastic neutron scattering, we are able to resolve seven discrete energy levels for spectra recorded slightly below the N\'eel temperature $T_\textrm{N}\approx 200$~K. These modes are well described by our non-linear confinement model in the regime of large spatially extended solitons., Comment: 11 pages, 7 figures

We present a generic mechanism that explains why many Kondo materials show magnetic ordering along directions that are not favoured by the crystal-field anisotropy. Using a renormalization-group (RG) analysis of single impurity Kondo models with single-ion anisotropy, we demonstrate that strong fluctuations above the Kondo temperature drive a moment re-orientation over a wide range of parameters, e.g. for different spin values $S$ and number of Kondo channels $N$. In tetragonal systems this can happen for both easy-plane or easy axis anisotropy. The characteristic crossing of magnetic susceptibilities is not an artefact of the weak-coupling RG treatment but can be reproduced in brute-force perturbation theory. Employing numerical renormalization group (NRG), we show that for an under-screened moment ($S=1$, $N=1$) with easy-plane anisotropy, a crossing of magnetic susceptibilities can also occur in the strong-coupling regime (below the Kondo temperature). This suggests that collective magnetic ordering of such under-screened moments would develop along the magnetic hard axis., Comment: 13 pages, 8 figures

The twin discoveries of the quantum Hall effect, in the 1980's, and of topological band insulators, in the 2000's, were landmarks in physics that enriched our view of the electronic properties of solids. In a nutshell, these discoveries have taught us that quantum mechanical wavefunctions in crystalline solids may carry nontrivial topological invariants which have ramifications for the observable physics. One of the side effects of the recent topological insulator revolution has been that such physics is much more widespread than was appreciated ten years ago. For example, while topological insulators were originally studied in the context of electron wavefunctions, recent work has led to proposals of topological insulators in bosonic systems: in photonic crystals, in the vibrational modes of crystals, and in the excitations of ordered magnets. Here we confirm the recent proposal that, in a weak magnetic field, the dimerized quantum magnet SrCu$_{2}$(BO$_{3}$)$_2$ is a bosonic topological insulator with nonzero Chern number in the triplon bands and topologically protected chiral edge excitations., Comment: 33 pages, 14 figures (preprint format and including supplementary material)

The fermion sign problem is often viewed as a sheer inconvenience that plagues numerical studies of strongly interacting electron systems. Only recently, it has been suggested that fermion signs are fundamental for the universal behavior of critical metallic systems and crucially enhance their degree of quantum entanglement. In this work we explore potential connections between emergent scale invariance of fermion sign structures and scaling properties of bipartite entanglement entropies. Our analysis is based on a wavefunction ansatz that incorporates collective, long-range backflow correlations into fermionic Slater determinants. Such wavefunctions mimic the collapse of a Fermi liquid at a quantum critical point. Their nodal surfaces -- a representation of the fermion sign structure in many-particle configurations space -- show fractal behavior up to a length scale $\xi$ that diverges at a critical backflow strength. We show that the Hausdorff dimension of the fractal nodal surface depends on $\xi$, the number of fermions and the exponent of the backflow. For the same wavefunctions we numerically calculate the second R\'enyi entanglement entropy $S_2$. Our results show a cross-over from volume scaling, $S_2\sim \ell^\theta$ ($\theta=2$ in $d=2$ dimensions), to the characteristic Fermi-liquid behavior $S_2\sim \ell\ln \ell$ on scales larger than $\xi$. We find that volume scaling of the entanglement entropy is a robust feature of critical backflow fermions, independent of the backflow exponent and hence the fractal dimension of the scale invariant sign structure., Comment: 9.5 pages, 10 figures

We analyze a model of itinerant electrons interacting through a quadrupole density-density repulsion in three dimensions. At the mean field level, the interaction drives a continuous Pomeranchuk instability towards $d$-wave, spin-triplet nematic order, which simultaneously breaks the SU(2) spin-rotation and spatial rotational symmetries. This order results in spin antisymmetric, elliptical deformations of the Fermi surfaces of up and down spins. We show that the effects of quantum fluctuations are similar to those in metallic ferromagnets, rendering the nematic transition first-order at low temperatures. Using the fermionic quantum order-by-disorder approach to self-consistently calculate fluctuations around possible modulated states, we show that the first-order transition is pre-empted by the formation of a nematic state that is intertwined with a helical modulation in spin space. Such a state is closely related to $d$-wave bond density wave order in square-lattice systems. Moreover, we show that it may coexist with a modulated, $p$-wave superconducting state., Comment: 15 pages, 9 figures

The newly discovered Ising-type ferromagnet CeRu$_2$Al$_2$B exhibits an additional phase transition at $T_\textrm{N}$ = 14.2 K before entering the ferromagnetic ground state at $T_\textrm{C}$ = 12.8 K. We clarify the nature of this transition through high resolution neutron diffraction measurements. The data reveal the presence of a longitudinal incommensurate spin-density wave (SDW) in the temperature range of $T_\textrm{C}

We examine the effects of disorder on dimerized quantum antiferromagnets in a magnetic field, using the mapping to a lattice gas of hard-core bosons with finite-range interactions. Combining a strong-coupling expansion, the replica method, and a one-loop renormalization group analysis, we investigate the nature of the glass phases formed. We find that away from the tips of the Mott lobes, the transition is from a Mott insulator to a compressible Bose glass, however the compressibility at the tips is strongly suppressed. We identify this finding with the presence of a rare Mott glass phase not previously described by any analytic theory for this model and demonstrate that the inclusion of replica symmetry breaking is vital to correctly describe the glassy phases. This result suggests that the formation of Bose and Mott glass phases is not simply a weak localization phenomenon but is indicative of much richer physics. We discuss our results in the context of both ultracold atomic gases and spin-dimer materials., Comment: 10 pages (including supplementary material), 3 figures

We investigate the interplay of quantum fluctuations and magnetic anisotropies in metallic ferromagnets. Our central result is that fluctuations close to a quantum critical point can drive the moments to point along a magnetic hard axis. As a proof of concept, we show this behavior explicitly for a generic two-band model with local Coulomb and Hund's interactions, and a spin-orbit-induced easy plane anisotropy. The phase diagram is calculated within the fermionic quantum order-by-disorder approach, which is based on a self-consistent free energy expansion around a magnetically ordered state with unspecified orientation. Quantum fluctuations render the transition of the easy-plane ferromagnet first-order below a tricritical point. At even lower temperatures, directionally dependent transverse fluctuations dominate the magnetic anisotropy and the moments flip to lie along the magnetic hard axis. We discuss our findings in the context of recent experiments that show this unusual ordering along the magnetic hard direction., Comment: 5 pages, 3 figures

We investigate the nature of the Bose glass phase of the disordered Bose-Hubbard model in $d>2$ and demonstrate the existence of a glass-like replica symmetry breaking (RSB) order parameter in terms of particle number fluctuations. Starting from a strong-coupling expansion around the atomic limit, we study the instability of the Mott insulator towards the formation of a Bose glass. We add some infinitesimal RSB, following the Parisi hierarchical approach in the most general form, and observe its flow under the momentum-shell renormalization group scheme. We find a new fixed point with one-step RSB, corresponding to the transition between the Mott insulator and a Bose glass phase with hitherto unseen RSB. The susceptibility associated to infinitesimal RSB perturbation in the Mott insulator is found to diverge at the transition with an exponent of $\gamma=1/d$. Our findings are consistent with the expectation of glassy behavior and the established breakdown of self-averaging. We discuss the possibility of measuring the glass-like order parameter in optical lattice experiments as well as in certain spin systems that are in the same universality class as the Bose-Hubbard model., Comment: 6 pages, 3 figures, improved version with additional references

We present a detailed analysis of the non-analytic structure of the free energy for the itinerant ferromagnet near the quantum critical point in two and three dimensions. We analyze a model of electrons with an isotropic dispersion interacting through a contact repulsion. A fermionic version of the quantum order-by-disorder mechanism allows us to calculate the free energy as a functional of the dispersion in the presence of homogeneous and spiralling magnetic order. We re-sum the leading divergent contributions, to derive an algebraic expression for the non-analytic contribution to free energy from quantum fluctuations. Using a recursion which relates sub-leading divergences to the leading term, we calculate the full T=0 contribution in $d=3$. We propose an interpolating functional form, which allows us to track phase transition lines at temperatures far below the tricritical point and down to T=0. In $d=2$, quantum fluctuations are stronger and non-analyticities more severe. Using a similar re-summation approach, we find that despite the different non-analytic structures, the phase diagrams in two and three dimensions are remarkably similar, exhibiting an incommensurate spiral phase near to the avoided quantum critical point., Comment: 12 pages, 3 figures, 1 table

We study the effects of quenched charge disorder on the phase reconstruction near itinerant ferromagnetic quantum critical points in three spatial dimensions. Combining a replica disorder average with a fermionic version of the quantum order-by-disorder mechanism, we show that weak disorder destabilizes the ferromagnetic state and enhances the susceptibility towards incommensurate, spiral magnetic ordering. The Goldstone modes of the spiral phase are governed by a 3d-XY model. The induced disorder in the pitch of the spiral generates a random anisotropy for the Goldstone modes, inducing vortex lines in the phase of the helical order and rendering the magnetic correlations short ranged with a strongly anisotropic correlation length., Comment: 7 pages, 4 figures

Inelastic neutron scattering experiments have been performed on single crystals of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x$\approx$0.5. Colossal magnetoresistance (CMR) in the manganites arises from the interplay between a ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating state. Therefore, it appears important to compare these phases concerning their underlying magnetic interaction parameters. Our investigations of the spin-wave disperion in the AFM ordered state of Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ exhibits a strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the site-centered model for charge and orbital ordering in half-doped manganites. Within this model, linear spin-wave theory yields a perfect description of the experimental dispersion. Furthermore, magnetic excitations in the ferromagnetic metallic state of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x=0.49 and x=0.50 exhibit nearly the same magnon dispersion which can be described with a Heisenberg model including nearest-neighbor interactions.

We study the S=1 square lattice Heisenberg antiferromagnet with spatially anisotropic nearest neighbor couplings $J_{1x}$, $J_{1y}$ frustrated by a next-nearest neighbor coupling $J_{2}$ numerically using the density-matrix renormalization group (DMRG) method and analytically employing the Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of the anisotropy, within both methods we find quantum fluctuations to stabilize the N\'{e}el ordered state above the classically stable region. Whereas SBMFT suggests a fluctuation-induced first order transition between the N\'{e}el state and a stripe antiferromagnet for $1/3\leq J_{1x}/J_{1y}\leq 1$ and an intermediate paramagnetic region opening only for very strong anisotropy, the DMRG results clearly demonstrate that the two magnetically ordered phases are separated by a quantum disordered region for all values of the anisotropy with the remarkable implication that the quantum paramagnetic phase of the spatially isotropic $J_{1}$-$J_{2}$ model is continuously connected to the limit of decoupled Haldane spin chains. Our findings indicate that for S=1 quantum fluctuations in strongly frustrated antiferromagnets are crucial and not correctly treated on the semiclassical level., Comment: 10 pages, 10 figures

We develop an understanding of the anomalous metal state of the parent compounds of recently discovered iron based superconductors starting from a strong coupling viewpoint, including orbital degrees of freedom. On the basis of an intermediate-spin (S=1) state for the Fe^{2+} ions, we derive a Kugel-Khomskii spin-orbital Hamiltonian for the active t_{2g} orbitals. It turns out to be a highly complex model with frustrated spin and orbital interactions. We compute its classical phase diagrams and provide an understanding for the stability of the various phases by investigating its spin-only and orbital-only limits. The experimentally observed spin-stripe state is found to be stable over a wide regime of physical parameters and can be accompanied by three different types of orbital orders. Of these the orbital-ferro and orbital-stripe orders are particularly interesting since they break the in-plane lattice symmetry -- a robust feature of the undoped compounds. We compute the magnetic excitation spectra for the effective spin Hamiltonian, observing a strong reduction of the ordered moment, and point out that the proposed orbital ordering pattern can be measured in resonant X-ray diffraction., Comment: 16 pages, 12 figures

The fermion sign problem is studied in the path integral formalism. The standard picture of Fermi liquids is first critically analyzed, pointing out some of its rather peculiar properties. The insightful work of Ceperley in constructing fermionic path integrals in terms of constrained world-lines is then reviewed. In this representation, the minus signs associated with Fermi-Dirac statistics are self consistently translated into a geometrical constraint structure (the {\em nodal hypersurface}) acting on an effective bosonic dynamics. As an illustrative example we use this formalism to study 1+1-dimensional systems, where statistics are irrelevant, and hence the sign problem can be circumvented. In this low-dimensional example, the structure of the nodal constraints leads to a lucid picture of the entropic interaction essential to one-dimensional physics. Working with the path integral in momentum space, we then show that the Fermi gas can be understood by analogy to a Mott insulator in a harmonic trap. Going back to real space, we discuss the topological properties of the nodal cells, and suggest a new holographic conjecture relating Fermi liquids in higher dimensions to soft-core bosons in one dimension. We also discuss some possible connections between mixed Bose/Fermi systems and supersymmetry., Comment: 28 pages, 5 figures

By combining experimental measurements of the quasiparticle and dynamical magnetic properties of optimally electron-doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ with theoretical calculations we demonstrate that the conventional fermiology approach cannot possibly account for the magnetic fluctuations in these materials. In particular, we perform tunneling experiments on the very same sample for which a dynamical magnetic resonance has been reported recently and use photoemission data by others on a similar sample to characterize the fermionic quasiparticle excitations in great detail. We subsequently use this information to calculate the magnetic response within the conventional fermiology framework as applied in a large body of work for the hole-doped superconductors to find a profound disagreement between the theoretical expectations and the measurements: this approach predicts a step-like feature rather than a sharp resonance peak, it underestimates the intensity of the resonance by an order of magnitude, it suggests an unreasonable temperature dependence of the resonance, and most severely, it predicts that most of the spectral weight resides in incommensurate wings which are a key feature of the hole-doped cuprates but have never been observed in the electron-doped counterparts. Our findings strongly suggest that the magnetic fluctuations reflect the quantum-mechanical competition between antiferromagnetic and superconducting orders., Comment: 10 pages, 9 figures, 1 table

The magnon dispersion in the charge, orbital and spin ordered phase in La(0.5)Sr(1.5)MnO(4) has been studied by means of inelastic neutron scattering. We find an excellent agreement with a magnetic interaction model basing on the CE-type superstructure. The magnetic excitations are dominated by ferromagnetic exchange parameters revealing a nearly-one dimensional character at high energies. The nearest neighbor ferromagnetic interaction in La(0.5)Sr(1.5)MnO(4) is significantly larger than the one in the metallic ferromagnetically ordered manganites. The large ferromagnetic interaction in the charge/orbital ordered phase appears to be essential for the capability of manganites to switch between metallic and insulating phases., Comment: 4 pages, 3 figures

The Proceedings of the 2003 SLAC Workshops on flavor physics with a high luminosity asymmetric e+e- collider. The sensitivity of flavor physics to physics beyond the Standard Model is addressed in detail, in the context of the improvement of experimental measurements and theoretical calculations., Comment: 476 pages. Printed copies may be obtained by request to kwebb@slac.stanford.edu . arXiv admin note: v2 appears to be identical to v1

We study model-independently the implications of non-standard scalar and pseudoscalar interactions for the decays b ->s gamma, b -> s g, b -> s l^+l^- (l=e,mu) and B_s -> mu^+ mu^-. We find sizeable renormalization effects from scalar and pseudoscalar four-quark operators in the radiative decays and at O(alpha_s) in hadronic b decays. Constraints on the Wilson coefficients of an extended operator basis are worked out. Further, the ratios R_H = BR(B -> H mu^+ mu^-)/BR(B -> H e^+ e^-), for H=K^(*), X_s, and their correlations with B_s -> mu^+ mu^- decay are investigated. We show that the Standard Model prediction for these ratios defined with the same cut on the dilepton mass for electron and muon modes, R_H= 1 + O(m^2_mu/m^2_b), has a much smaller theoretical uncertainty (<1%) than the one for the individual branching fractions. The present experimental limit R_K < 1.2 puts constraints on scalar and pseudoscalar couplings, which are similar to the ones from current data on BR(B_s -> mu^+ mu^-). We find that new physics corrections to R_{K*} and R_{X_s} can reach 13% and 10%, respectively., Comment: 28 pages, 6 figures; Table 1 updated, two refs added (to appear in PRD)

We extend our previous analysis of the decays B_{d,s} -> mu^+ mu^- in the Minimal Supersymmetric Standard Model (MSSM) to include gluino and neutralino contributions. We provide analytic formulae, valid at large values of tan(beta), for the scalar and pseudoscalar Wilson coefficients arising from neutral Higgs boson exchange diagrams with gluinos and neutralinos. Together with the remaining contributions (W^{+/-}, H^{+/-}, charginos), and assuming the Cabibbo-Kobayashi-Maskawa (CKM) matrix to be the only source of flavour violation, we assess their implications for the branching fractions Br(B_{d,s} -> mu^+ mu^-). Of particular interest is the quantity R = Br(B_d -> mu^+ mu^-)/Br(B_s -> mu^+ mu^-), since (i) the theoretical errors cancel to a large extent, and (ii) it offers a theoretically clean way of extracting the ratio |V_{td}/V_{ts}| in the Standard Model (SM), which predicts R_SM ~ |V_{td}/V_{ts}|^2 ~ O(10^{-2}). Exploring three different scenarios of modified minimal flavour violation, we find that part of the MSSM parameter space can accommodate large Br(B_{d,s} -> mu^+ mu^-) branching fractions, while being consistent with various experimental constraints. More importantly, we show that the ratio R can be as large as O(1), while the individual branching fractions may be amenable to detection by ongoing experiments. We conclude that within the MSSM with large tan(beta) the decay rates of B_{d} -> mu^+ mu^- and B_{s} -> mu^+ mu^- can be of comparable size even in the case where flavour violation is due solely to the CKM matrix., Comment: 20 pages, 5 figures; version to be published in PRD; Title modified, discussion on flavour violation extended, 2 plots and several references added, results and conclusions remain unchanged

We compute for the first time QCD corrections to the rare decays B -> X_{d,s} nu nubar, B_{d,s} -> l^+ l^-, K -> pi nu nubar and K_L -> mu^+ mu^-, where l=e or mu, in the context of a supersymmetric extension of the Standard Model (SM) with minimal flavour violation and new operators, in addition to those present in the SM. Assuming that the gluino is heavy, we consider an effective theory which consists of charged and neutral Higgs particles, charginos and squarks. We evaluate the QCD corrections to box and Z^0-penguin diagrams with top-quark, charged Higgs boson, chargino and squark exchanges, as well as to neutral Higgs boson penguin diagrams. We provide a compendium of analytic formulae for the Wilson coefficients, which are valid for arbitrary values of tan(beta) (the ratio of the vacuum expectation values of the two Higgs fields) except for the case of the neutral Higgs-boson contributions. These contributions have been obtained at large tan(beta), which may compensate for the inevitable suppression by the masses of the light leptons in decays based on the b -> s (d) l^+ l^- transition. We investigate the dependence of the various branching ratios on the renormalization scale mu, which is the main theoretical uncertainty in the short-distance calculation. We find that the mu dependence of the branching ratios is considerably reduced once the QCD corrections are taken into account. The contributions of new operators are found to be dominant at large tan(beta) in B_{d,s} -> mu^+ mu^- while they are subleading in B -> X_{d,s} nu nubar and completely negligible in kaon decays., Comment: 47 pages, 9 figures; version to appear in Nucl. Phys. B

We re-examine spontaneous $\cp$ violation (SCPV) at the tree level in the context of the next-to-minimal supersymmetric standard model (NMSSM) with two Higgs doublets and a gauge singlet field. We analyse the most general Higgs potential without a discrete $Z_3$ symmetry, and derive an upper bound on the mass of the lightest neutral Higgs boson. We estimate $\epsi_K$ by applying the mass insertion approximation, finding that in order to account for the for the observed $\cp$ violation in the neutral kaon sector a non-trivial flavour structure in the soft-breaking $A$ terms is required and that the upper bound on the lightest Higgs-boson mass becomes stronger. We also discuss the implications of electric dipole moments of the electron and the neutron in SUSY models with SCPV., Comment: 5 pages, 2 figures, uses JHEP LaTeX. Talk presented by A. Teixeira at the International Europhysics Conference on High Energy Physics (HEP2001), Budapest, Hungary, 12-18 July 2001. To appear in the proceedings

We report on a calculation of Higgs-boson contributions to the decays B_s -> l^+l^- and B -> K l^+l^- (l=e, mu) which are governed by the effective Hamiltonian describing b -> s l^+ l^-. Compact formulae for the Wilson coefficients are provided in the context of the type-II two-Higgs-doublet model (2HDM) and supersymmetry (SUSY) with minimal flavour violation, focusing on the case of large tan(beta). We derive, in a model-independent way, constraints on Higgs-boson-mediated interactions, using present experimental results on rare B decays including b -> s gamma, B_s -> mu^+ mu^-, and B -> K^(*) mu^+ mu^-. In particular, we assess the impact of possible scalar and pseudoscalar interactions transcending the standard model (SM) on the branching ratio of B_s -> mu^+ mu^- and the forward-backward (FB) asymmetry of mu^- in B -> K mu^+ mu^- decay. We find that the average FB asymmetry, which is unobservably small within the SM, and therefore a potentially valuable tool to search for new physics, is predicted to be no greater than 4% for a nominal branching ratio of about 6x10^{-7}. Moreover, striking effects on the decay spectrum of B -> K mu^+ mu^- are already ruled out by experimental data on the B_s -> mu^+ mu^- branching fraction. In addition, we study the constraints on the parameter space of the 2HDM and SUSY with minimal flavour violation. While the type-II 2HDM does not give any sizable contributions to the above decay modes, we find that SUSY contributions obeying the constraint on b -> s gamma can affect significantly the branching ratio of B_s -> mu^+ mu^-. We also comment on previous calculations contained in the literature., Comment: 29 pages, REVTeX, 8 figures. Minor corrections in Eqs. (5.4), (5.11) and (6.3) of the published version

We re-examine spontaneous CP violation at the tree level in the context of the next-to-minimal supersymmetric standard model (NMSSM) with two Higgs doublets and a gauge singlet field. We analyse the most general Higgs potential without a discrete Z_3 symmetry, and derive an upper bound on the mass of the lightest neutral Higgs boson consistent with present experimental data. We investigate, in particular, its dependence on the admixture and CP-violating phase of the gauge singlet field, as well as on tan(beta). To assess the viability of the spontaneous CP violation scenario, we estimate epsilon_K by applying the mass insertion approximation. We find that a non-trivial flavour structure in the soft-breaking A terms is required to account for the observed CP violation in the neutral kaon sector. Furthermore, combining the minimisation conditions for spontaneous CP violation with the constraints coming from K0-K0bar mixing, we find that the upper bound on the lightest Higgs-boson mass becomes stronger. We also point out that the electric dipole moments of electron and neutron are a serious challenge for SUSY models with spontaneous CP violation., Comment: 19 pages, LaTeX2e, 5 figures; matches the published version

The CP-violating asymmetries in the exclusive decay B->K^*l^+l^- (l=e,mu,tau) are predicted to be exceedingly small in the standard model (SM), thereby offering an opportunity to assess various new-physics scenarios. We derive quantitative predictions for various integrated observables in B->K^*mu^+mu^- decay in the presence of physics beyond the SM with additional CP phases and an extended operator basis. In particular, a model-independent analysis of CP asymmetries that require the presence of unitarity phases, in addition to CP violation, is performed. We find that in the low dimuon invariant mass region 2m_mus gamma rate and the upper limit on BR(B^0->K^{*0}mu^+mu^-), CP-violating effects of a few per cent are estimated, even in the presence of new physics with CP phases of O(1). By contrast, in the high dimuon invariant mass region M_{psi'}

We study rare exclusive B decays based on the quark-level transition b->s(d)l^+l^-, where l=e or mu, in the context of supersymmetric theories with minimal flavour violation. We present analytic expressions for various mixing matrices in the presence of new CP-violating phases, and examine their impact on observables involving B and \bar{B} decays. An estimate is obtained for CP-violating asymmetries in B->K^(*)l^+l^- and B->rho(pi)l^+l^- decays for the dilepton invariant mass region 1.2 GeV < M_{l^+l^-}< M_{J/psi}. As a typical result, we find a CP-violating partial width asymmetry of about -6% (-5%) in the case of B->pi (B->rho) in effective supersymmetry with phases of O(1), taking into account the measurement of the inclusive b->s gamma branching fraction. On the other hand, CP asymmetries of less than 1% are predicted in the case of B->K^(*). We argue that it is not sufficient to have additional CP phases of O(1) to observe large CP-violating effects in exclusive b->s(d)l^+l^- decays., Comment: 34 pages, REVTeX, 6 figures, final version to appear in Phys. Rev. D, with some minor additions

The short-distance Hamiltonian describing b->s(d)e^-e^+ in the standard model is used to obtain the decay spectrum of \bar{B}->K^-pi^+e^-e^+ and \bar{B}->pi^-pi^+e^-e^+, assuming the Kpi and pipi systems to be the decay products of K^* and rho respectively. Specific features calculated are (i) angular distribution of K^- (or pi^-) in the K^-pi^+ (or pi^-pi^+) centre-of-mass (c.m.) frame; (ii) angular distribution of e^- in the e^-e^+ c.m. frame; and (iii) the correlation between the meson and lepton planes. We also derive CP-violating observables obtained by combining the above decays with the conjugate processes B->K^+pi^-e^-e^+ and B->pi^-pi^+e^-e^+., Comment: 19 pages, REVTeX, no figures. Equations (2.19a), (2.19b), (5.5)-(5.7) have been corrected; all results remain unchanged. These changes will appear in an Erratum submitted to Phys. Rev. D

We study the possible existence of a real Cabibbo-Kobayashi-Maskawa (CKM) matrix, with CP violation originating from physics beyond the standard model (SM). We show that present experimental data allow for a real CKM matrix provided that new physics also contributes to Delta m_{B_d} by at least 20% of the SM contribution (for rho > 0), besides generating CP violation in the kaon sector. The naturalness of a real CKM matrix is studied within the framework of general multi-Higgs-doublet models with spontaneous CP violation. As an example, we discuss a specific two-Higgs-doublet model and its implications for CP asymmetries in non-leptonic neutral B-meson decays., Comment: 19 pages, LaTeX2e, 1 figure, uses amssymb, elsart, psfig, and subeqn style files

We summarize the results of two papers in which we have studied CP violation in inclusive and exclusive decays b -> d e^+e^-. Two CP-violating effects are calculated: the partial rate asymmetry between b and \bar{b} decay, and the asymmetry between e^- and e^+ spectra for an untagged B/\bar{B} mixture. These asymmetries, combined with the branching ratio, can potentially determine the parameters (\rho, \eta) of the unitarity triangle. We also summarize a paper by Browder et al. on a possible CP-violating asymmetry in the inclusive reaction B -> K^-(K^{*-})X., Comment: 4 pages, LaTeX, 1 figure, uses prhep97.sty (included) and psfig.sty. Talk presented by L. M. Sehgal at the International Europhysics Conference on High Energy Physics, 19-26 August 1997, Jerusalem, Israel. To appear in the Proceedings

As a sequel to the calculation of the CP-violating asymmetry in the decay rates of b -> d l^+l^- and \bar{b} -> \bar{d} l^+l^-, we address in this paper the asymmetry in exclusive channels \bar{B} -> pi e^+e^- and \bar{B} -> rho e^+e^-, using form factors from two different models. In the invariant mass region 1GeV < \sqrt{s} < M_{J/psi}, the partial width asymmetry in the channel \bar{B} -> pi is -6% (-2%), and that in the channel \bar{B} -> rho, for one choice of form factors, is -5% (-2%), assuming CKM parameters eta = 0.34, rho = 0.3 (-0.3). We also calculate the forward-backward asymmetry A_{FB} of the e^- in the e^+e^- centre-of-mass system, and find average values (\bar{B} -> pi) \equiv 0, (\bar{B} -> rho)= -17 %, to be compared with the inclusive result (b -> d)= -9%. There is a CP-violating difference between A_{FB} and the corresponding asymmetry in the antiparticle channel \bar{A}_{FB}. Formulae are given that are applicable to any FCNC channel \bar{B} -> P_q (V_q) l^+l^-, q=s, d, with m_l \neq 0, including lepton spin effects. An approximate procedure is used to incorporate the rho, omega, and J/psi resonances., Comment: 34 pages, LaTeX2e, 8 figures, uses cite, psfig, and subeqn style files. Minor typos corrected, version to appear in Phys. Rev. D

The decay $b\to d e^+e^-$ has an amplitude containing comparable contributions proportional to $V_{tb}V_{td}^{*}$, $V_{cb}V_{cd}^{*}$ and $V_{ub}V_{ud}^{*}$. These pieces involve different unitarity phases produced by $c\bar{c}$ and $u\bar{u}$ loops. The simultaneous presence of different CKM phases and different dynamical phases leads to a calculable asymmetry in the partial widths of $b\to d e^+e^-$ and $\bar{b}\to \bar{d} e^+e^-$. Using the effective Hamiltonian of the standard model, we calculate this asymmetry as a function of the $e^+e^-$ invariant mass. The effects of $\rho$, $\omega$ and $J/\psi$ resonances are taken into account in the vacuum polarization of the $u\bar{u}$ and $c\bar{c}$ currents. As a typical result, an asymmetry of $- 5% (- 2%)$ is predicted in the nonresonant domain $1 GeV < m_{e^+e^-} < m_{J/\psi}$, assuming $\eta = 0.34$ and $\rho= 0.3 (-0.3)$. The branching ratio in this domain is $1.2\times 10^{-7} (3.3\times 10^{-7})$. Results are also obtained in the region of the $J/\psi$ resonance, where an asymmetry of $3\times 10^{-3}$ is expected, subject to certain theoretical uncertainties in the $b\to d J/\psi$ amplitude., Comment: 20 pages, LaTeX, 3 eps figures, uses cite.sty, epsf.sty, and amssym.sty. Version to appear in Phys. Rev. D, with some minor additions

The effective Hamiltonian for the decay $b\to s l^+ l^-$ predicts a characteristic polarization for the final state lepton, which can serve as an important test of the underlying theory. The lepton polarization has, in addition to a longitudinal component $P_L$, two orthogonal components $P_T$ and $P_N$, lying in and perpendicular to the decay plane which are proportional to $m_l/m_b$, and therefore significant for the $\tau^+\tau^-$ channel. The normal polarization component $P_N$ is a $T$-odd effect connected with the nonhermiticity of the effective Hamiltonian, arising mainly from $c\bar{c}$ intermediate states. We calculate all three polarization components for the decay $B\to X_s\tau^+\tau^-$ as a function of the lepton pair mass, and find average values $$ = -0.37, $$ = -0.63, $$ = 0.03. By comparison, the $\mu^-$ polarization is $$ = -0.77, $ = \approx 0$., Comment: 17 pages, LaTeX, 5 eps figures, uses epsf.sty and cite.sty

We consider the effects of quark-binding on the angular distribution and polarization characteristics of the inclusive decays $\Lambda_b \to X_s \gamma$ and $\Lambda_b \to X_c l \bar{\n}_l$ $(l=e, \tau)$, using the methods of heavy quark effective theory., Comment: 3 pages, LaTeX, 4 eps figures, uses epsfig.sty, subeqn.sty and wstwocl.sty. Presented by L. M. Sehgal at the EPS-HEP conference, Brussels, 1995. To appear in the proceedings

We study the angular distribution of photons produced in the inclusive decay of a polarized $\Lambda_b$, $\Lambda_b\to X_s\,\gamma$, using the technique of Heavy Quark Effective Theory. Finite non-perturbative corrections are obtained relative to the free quark decay $b\to s\gamma$. These corrections affect significantly the intensity and polarization of photons emitted at small angles relative to the $\Lambda_b$ spin direction., Comment: 10 pages, LaTeX, uses epsf, 2 figures appended as uuencoded, compressed eps tar-files