Equal Couplings Research Articles

Limits on anomalous trilinear gauge boson couplings from WW, WZ and Wγ production in [formula omitted] collisions at [formula omitted

We present searches for the anomalous γWW and ZWW trilinear gauge boson couplings from WW and WZ production using lepton plus dijet final states and a combination with results from Wγ, WW, and WZ production with leptonic final states. The analyzed data correspond to up to 8.6 fb−1 of integrated luminosity collected by the D0 detector in pp¯ collisions at s=1.96 TeV. We set the most stringent limits at a hadron collider to date assuming two different relations between the anomalous coupling parameters Δκγ, λ, and Δg1Z for a cutoff energy scale Λ=2 TeV. The combined 68% C.L. limits are −0.057<Δκγ<0.154, −0.015<λ<0.028, and −0.008<Δg1Z<0.054 for the LEP parameterization, and −0.007<Δκ<0.081 and −0.017<λ<0.028 for the equal couplings parameterization. We also present the most stringent limits of the W boson magnetic dipole and electric quadrupole moments.

CoGeNT interpretations

Recently, the CoGeNT experiment has reported events in excess of expected background. We analyze dark matter scenarios which can potentially explain this signal. Under the standard case of spin independent scattering with equal couplings to protons and neutrons, we find significant tensions with existing constraints. Consistency with these limits is possible if a large fraction of the putative signal events is coming from an additional source of experimental background. In this case, dark matter recoils cannot be said to explain the excess, but are consistent with it. We also investigate modifications to dark matter scattering that can evade the null experiments. In particular, we explore generalized spin independent couplings to protons and neutrons, spin dependent couplings, momentum dependent scattering, and inelastic interactions. We find that some of these generalizations can explain most of the CoGeNT events without violation of other constraints. Generalized couplings with some momentum dependence allow further consistency with the DAMA modulation signal, realizing a scenario where both CoGeNT and DAMA signals are coming from dark matter. A model with dark matter interacting and annihilating into a new light boson can realize most of the scenarios considered.

Channel capacities of an exactly solvable spin-star system

We calculate the entanglement-assisted and unassisted channel capacities of an exactly solvable spin star system, which models the quantum dephasing channel. The capacities for this non-Markovian model exhibit a strong dependence on the coupling strengths of the bath spins with the system, the bath temperature, and the number of bath spins. For equal couplings and bath frequencies, the channel becomes periodically noiseless.

Stimulated Raman adiabatic passage with unequal couplings: Beyond two-photon resonance

The technique of stimulated Raman adiabatic passage (STIRAP) is a highly efficient tool for coherent population transfer in a chainwise-connected three-state quantum system. It is widely accepted that the two-photon resonance between the two end states of the chain is a crucial condition for STIRAP. This assumption is indeed correct when the two driving fields, pump and Stokes, produce nearly equal couplings for their respective transitions. We show analytically, supported by numeric examples, that when the pump and Stokes couplings differ significantly, the two-photon population transfer profile is distorted and its center is shifted away from the two-photon resonance; hence, the optimal operation of STIRAP demands a two-photon detuning. We derive simple analytic estimates for the center and the width of the two-photon profile. The results are of potential importance in situations when the pump and Stokes fields are of different physical natures (e.g., laser and cavity fields) or when two-photon resonance is unwanted (e.g., in the presence of residual light).

Measurement of trilinear gauge boson couplings fromWW+WZ→lνjjevents inpp¯collisions ats=1.96 TeV

We present a direct measurement of trilinear gauge boson couplings at gammaWW and ZWW vertices in WW and WZ events produced in pp-bar collisions at sqrt{s}=1.96 TeV. We consider events with one electron or muon, missing transverse energy, and at least two jets. The data were collected using the D0 detector and correspond to 1.1/fb of integrated luminosity. Considering two different relations between the couplings at the gammaWW and ZWW vertices, we measure these couplings at 68% C.L. to be kappa_{gamma}=1.07^{+0.26}_{-0.29}, lambda =0.00^{+0.06}_{-0.06} and g_{1}^{Z}=1.04^{+0.09}_{-0.09} in a scenario respecting SU(2)_L x U(1)_Y gauge symmetry and kappa =1.04^{+0.11}_{-0.11} and lambda=0.00^{+0.06}_{-0.06} in an "equal couplings" scenario.

Preheating afterN-flation

We study preheating in $\mathcal{N}$-flation, assuming the Mar\ifmmode \check{c}\else \v{c}\fi{}enko-Pastur mass distribution, equal-energy initial conditions at the beginning of inflation and equal axion-matter couplings, where matter is taken to be a single, massless bosonic field. By numerical analysis we find that preheating via parametric resonance is suppressed, indicating that the old theory of perturbative preheating is applicable. While the tensor-to-scalar ratio, the non-Gaussianity parameters and the scalar spectral index computed for $\mathcal{N}$-flation are similar to those in single-field inflation (at least within an observationally viable parameter region), our results suggest that the physics of preheating can differ significantly from the single-field case.

GAUGE-INVARIANT REFORMULATION OF THE VECTOR SCHWINGER MODEL WITH A PHOTON MASS TERM AND ITS HAMILTONIAN, PATH INTEGRAL AND BRST FORMULATIONS

Using the Stueckelberg formalism, we construct a gauge-invariant version of the vector Schwinger model (VSM) with a photon mass term studied by one of us recently. This model describes two-dimensional massive electrodynamics with massless fermions, where the left-handed and right-handed fermions are coupled to the electromagnetic field with equal couplings. This model describing the 2D massive electrodynamics becomes gauge-noninvariant (GNI). This is in contrast to the case of the massless VSM which is a gauge-invariant (GI) theory (as a consequence of demanding the regularization for the theory to be GI). In this work we first construct a GI theory corresponding to this model describing the 2D massive electrodynamics, using the Stueckelberg formalism and then we recover the physical contents of the original GNI theory studied earlier, under some special gauge choice. We then study the Hamiltonian, path integral and BRST formulations of this GI theory under appropriate gauge-fixing. The theory presents a new class of models in the 2D quantum electrodynamics with massless fermions but with a photon mass term.

VECTOR SCHWINGER MODEL WITH A PHOTON MASS TERM: GAUGE-INVARIANT REFORMULATION, OPERATOR SOLUTIONS AND HAMILTONIAN AND PATH INTEGRAL FORMULATIONS

We consider the vector Schwinger model (VSM) describing two-dimensional electrodynamics with massless fermions, where the left-handed and right-handed fermions are coupled to the electromagnetic field with equal couplings, with a mass term for the U(1) gauge field and then study its operator solutions and the Hamiltonian and path integral formulations. We emphasize here that although the VSM has been studied in the literature rather widely but only without a photon mass term (which was a consequence of demanding the regularization for the VSM to be gauge-invariant (GI)). The VSM with a photon mass term is seen to be a gauge-noninvariant (GNI) theory. Using the standard Stueckelberg formalism we then construct a GI theory corresponding to the proposed GNI model. From this reformulated GI theory, we further recover the physical contents of the proposed GNI theory under a very special gauge choice. The theory proposed and studied here presents a new class of models in the two-dimensional quantum electrodynamics with massless fermions but with a photon mass term.

Multicritical behavior of the two-field Ginzburg–Landau model coupled to a gauge field

In this paper, we study the multicritical behavior of the Ginzburg–Landau model in a O ( n 1 ) ⊕ O ( n 2 ) -symmetric version containing ( n 1 / 2 + n 2 / 2 ) -complex order parameters coupled to a gauge field. We develop the RG analysis at a one-loop approximation in the context of the ϵ -expansion approach. The beta functions are obtained, and in the case of equal couplings between the two scalar fields and the gauge field and n 1 = n 2 = n / 2 , the infrared stability of the fixed points is discussed. It is found that the charged infrared-stable fixed point exists for n > 393.2 . Calculations of the relevant critical exponents are also carried out.

Efficient atomic quantum memory for photonic qubits in cavity QED

We investigate a scheme of atomic quantum memory to store photonic qubits of polarization in cavity QED. It is observed that the quantum state swapping between a single-photon pulse and a Λ-type atom can be made via scattering in an optical cavity [T. W. Chen, C. K. Law, P. T. Leung, Phys. Rev. A 69 (2004) 063810]. This swapping operates limitedly in the strong coupling regime for Λ-type atoms with equal dipole couplings. We extend this scheme in cavity QED to present a more feasible and efficient method for quantum memory combined with projective measurement. This method works without requiring such a condition on the dipole couplings. The fidelity is significantly higher than that of the swapping, and even in the moderate coupling regime it reaches almost unity by narrowing sufficiently the photon-pulse spectrum. This high performance is rather unaffected by the atomic loss, cavity leakage or detunings, while a trade-off is paid in the success probability for projective measurement.

EPR and ENDOR Study of Radiation-Induced Radical Formation in Purines: Sodium Inosine Crystals X-irradiated at 10 K

X-irradiated single crystals of sodium inosine (Na(+)*Inosine(-)*2.5H(2)O), in which the hypoxanthine base is present as the N1-deprotonated anion, were investigated using K-band (24 GHz) electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and ENDOR induced EPR (EIE) techniques at 10 K. At least five different radicals were present immediately after irradiation at 10 K. R1, which decayed upon warming the crystals to 50 K, was identified as the electron-loss product of the parent N1-deprotonated hypoxanthine base. Hyperfine couplings to HC8 and HC2 were fully characterized with ENDOR spectroscopy, and the identification was supported by DFT calculations. R2, which also decayed on warming to 50 K, exhibited nearly equal couplings to HC2 and HC8. Taken in combination with an extensive set of DFT calculations, the experimental results indicate that R2 is the (doubly negative) product of electron-gain by the initially anionic N1-deprotonated hypoxanthine parent. R3, which exhibited hyperfine coupling only to HC8 could not be identified. R4, which persisted on annealing to 260 K, exhibited one large alpha-proton hyperfine coupling which was fully characterized by ENDOR. Based on DFT calculations and the experimental data, R4 was identified as the product of net H-abstraction from C5'. The remaining HC5' was the source of the measured alpha-proton coupling. R5, present at low temperature and the only observable radical after warming the crystals to room temperature, was identified as the C8-H addition radical. The alpha-coupling to HC2 and beta-couplings to the pair of C8 methlyene protons were fully characterized by ENDOR.

Oblique corrections from Higgsless models in warped space

We calculate the tree-level oblique corrections to electroweak precision observables generated in higgless models of electroweak symmetry breaking with a 5D SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group on a warped background. In the absence of brane induced kinetic terms (and equal left and right gauge couplings) we find the S parameter to be ~1.15, while T,U~0, as in technicolor theories. Planck brane induced kinetic terms and unequal left-right couplings can lower S, however for sufficiently low values of S tree-level unitarity will be lost. A kinetic term localized on the TeV brane for SU(2)_D will generically increase S, however an induced kinetic term for U(1)_{B-L} on the TeV brane will lower S. With an appropriate choice of the value of this induced kinetic term S~0 can be achieved. In this case the mass of the lowest Z' mode will be lowered to about ~300 GeV.

Compressible metamagnetic Ising model: Mean-field Curie-Weiss approach and Landau expansion

A layered compressible metamagnetic Ising model is studied within the mean-field Curie-Weiss approach and the Landau expansion. The ferromagnetic and antiferromagnetic couplings depend linearly on the volume as in Domb's ferromagnetic model. For equal spin-lattice couplings of ferromagnetic and antiferromagnetic interactions a closed solution is obtained, suitable for arbitrary values of the compressibility. For a large spin-lattice coupling and a high compressibility, two tricritical points appear in the magnetic field-temperature phase diagram, one at low temperatures, and other at high temperatures, for a low hydrostatic pressure. For an arbitrary ratio of the spin-lattice couplings a perturbation expansion of the free energy on the inverse elastic constant is presented. The effect of pressure on the single tricritical point comprises three possibilities, according to the signs of the derivatives of the tricritical temperature and field, related to the magnitude of the ferromagnetic/antiferromagnetoelastic couplings. In the experimental literature there are some compressible metamagnets whose tricritical behaviors as a function of pressure are in agreement with these findings.

Kondo effect in a quantum dot with even number of electrons: zero-bandwidth approach

The zero-bandwidth limit of the Anderson model is applied to calculate the electrical transport properties through a semiconductor quantum dot, focusing on the regime with two localized electrons inside the dot. The model includes two localized orbitals in the dot, with intra-orbital and inter-orbital Coulomb repulsion energies, coupled to two leads with different electrochemical potentials. We take symmetric hybridization strength (equal couplings between dot and left and right leads) and we consider the exchange energy between two localized electrons in the dot. Magnetic field effects are included by assuming that it produces a splitting in the one-particle energy levels. We study manifestations of the integer-spin Kondo effect in equilibrium (linear-response conductance) and non-equilibrium (differential conductance) properties. The results are directly related and in good qualitative agreement with recent experimental data.

Susceptibility and low-temperature thermodynamics of spin-1/2 Heisenberg ladders.

The temperature dependence of the uniform susceptibility and the ground state energy of antiferromagnetic Heisenberg ladders with up to 6 legs has been calculated, using the Monte Carlo loop algorithm. The susceptibilities of even-leg-ladders show spin gaps while these of odd-leg-ladders remain finite in the zero temperature limit. For small ratios of intra- to inter-leg couplings, odd-leg-ladders can be mapped at low temperatures to single chains. For equal couplings, the logarithmic corrections at low temperatures increase markedly with the number of legs.

Unified derivation of the dipolar field and relaxation terms in the Bloch-Redfield equations of liquid NMR

The standard theory of NMR relaxation in liquids (with molecular motion described as a classical Brownian motion, and including intermolecular spin–spin couplings) is re-examined, taking great care not to drop significant contributions from the dipolar coupling between distant molecules. This results in ‘‘modified Bloch–Redfield equations’’ for the spins in a single molecule, valid at all spin temperatures, which contain both the usual relaxation terms and a coupling of each spin with a classical average dipolar field. Delicate issues raised in this derivation, like the neglect of quantum correlations between spins on different molecules at (repeated) initial times, are discussed with the help of exact calculations (for all spin temperatures) performed on a simplified model which includes equal couplings between all N spins of a system. The same model is used to compare the merits of different forms of ‘‘high temperature’’ approximation. We also propose an iterative scheme for solving the ‘‘modified Bloch–Redfield equations,’’ in which the starting point is the well understood solution of the problem without dipolar field. Finally, a short discussion is given of the relation between ‘‘quantum correlations’’ and ‘‘quantum coherences’’ in the perspective of multiple-pulse and multiple-quantum experiments. These two notions are very simply related in the strict first approximation of weak order, and have often not been clearly distinguished. However, in the second order approximation, which is required whenever dipolar field effects manifest themselves, unrelated spins may display observable coherences although they are not coupled (hence not correlated).

Efficiencies of Double- and Triple-Resonance J Cross Polarization in Multidimensional NMR

It is generally observed that the performance of double-resonance multiple-pulse-based J cross polarization (DCP) is superior to that of pulsed-free-precession (INEPT)-based sequences for net transfer of coherence between scalar coupled spins. Here, effects of relaxation and radiofrequency held inhomogeneity on transfer efficiency are analyzed for both methods. It is found that relaxation differences are relatively small between INEPT and DCP. RF inhomogeneity effects were found to significantly favor DCP over INEPT, contributing to the observed experimental differences in performance between the two methods. The differences suggest that triple-resonance cross polarization (TCP) between three coupled spins should yield better results than analogous INEPT-based net coherence transfers. The possibilities of TCP are theoretically analyzed by deriving the transfer functions for this type of experiment. It is found that the TCP transfer efficiency is low except in the case of equal scalar couplings, To widen the applications of the potentially interesting TCP method, a scheme involving a concatenation of triple- and double-resonance CP is introduced (concatenated CP of CCP). It is theoretically shown that such a sequence can be tuned to achieve complete in-phase transfer for all ratios of scalar couplings. The transfer times in this scheme are shown to be somewhat shorter than those required for optimally concatenated INEPT in-phase transfers. The transfer efficiencies of CCP were verified with a 3D HACA(N)NH experiment, in which the CA-N-(N)H transfer is driven by the CCP scheme. The experiment was carried out with 15N, 13C-labeled T4 lysozyme (19 kDa), The CCP experiment has much higher sensitivity than a version where the CA-N-(N)H transfer is driven by an INEPT scheme.

The Cauchy-singular integral equation appearing in the theory of chiral spontaneous symmetry breaking

It is shown how the combination of associated hypergeometric functions, in the case of equal vector and axial couplings, satisfies an analytic integral equation on the right-hand cut with Cauchy-singular kernel having the same discontinuity as the original integral equation. This is obtained by using an extension of the quadratic transformation of the Gauss function. A projective quasi-solution is found, fixing an accurate value of the ratioGA/GV=1.154, due to radiative corrections, once favored by Schwinger as twice the Euler numerical constant. Also noticed is a quasi-solution withɛ=d−4=15% andGA=GV indicating possible functioning between Minkowski and de Sitter spaced=5. Finally, evaluation of the conjugate function by the initial—discontinuous—integral operator allows one to understand both the massless confluent limit obtained by the author (J.-P. Lebrun,Nuovo Cimento A,103, 1735 (1990);104, 1071 (1991)) and the one-half the expected rate mentioned in solar-neutrino observations.

Infinite multilevel system coupled to a quantum-field mode

We investigate a multilevel system coupled to a quantum-field mode, neglecting the damping. The model is shown to be exactly solvable in the case of equal couplings and detunings. The eigenvalues and eigensolutions are calculated. We compute the time evolution of the level populations. The behavior is compared with that of the corresponding exactly solvable semiclassical model, in which the quantum mode is replaced with an external classical field, and with that of the Maxwell–Bloch model, in which the ladder and field variables are decorrelated in the lowest nontrivial order. The semiclassical limit of the quantum system is also studied.

The phase diagram of the XXZ model for slab and layered geometries

The phase diagram of the spin 1/2 XXZ model is examined for two different geometries, which can be changed from two- to three-dimensional. The first of these consists of an infinite stack of simple quadratic layers with a variable inter-layer coupling, where the ratio of the inter-layer and intra-layer couplings varies between zero and one. Second, the authors consider a slab geometry consisting of n such layers, with equal couplings in all directions. For both geometries they use the cluster variation method for two-spin clusters to construct the phase diagram of the model, and they examine the changes that take place as the system changes from two- to three-dimensional. For the slab geometry they also calculate the order parameter profile near Tc. It turns out that Dirichlet boundary conditions for the related superfluid wavefunction are most realistic.