ATLAS sensitivity to Wtb anomalous couplings in top quark decays

We study the sensitivity of the ATLAS experiment to Wtb anomalous couplings in top pair production with semileptonic decay, pp→tt̄→W⁺bW^-b̄ with one of the W bosons decaying leptonically and the other hadronically. Several observables are examined, including the W helicity fractions and new quantities recently introduced, such as the ratios of helicity fractions and some angular asymmetries defined in the W rest frame. The dependence on anomalous couplings of all these observables has been previously obtained. In this work we show that some of the new observables also have smaller systematic uncertainties than the helicity fractions, with a similar or stronger dependence on anomalous couplings. Consequently, their measurement can significantly improve the limits on anomalous couplings. Moreover, the most sensitive measurements can be combined. In this case, the precision achieved in the determination of Wtb anomalous couplings can be of a few percent in the semileptonic channel alone.     

Spin-2 Fields and Helicity

By considering the irreducible representations of the Lorentz group, an analysis of the different spin-2 waves is presented. In particular, the question of the helicity is discussed. It is concluded that, although from the point of view of representation theory there are no compelling reasons to choose between spin-2 waves with helicity σ=±1 or σ=±2, consistency arguments of the ensuing field theories favor waves with helicity σ=±1.     

s-and t-channel helicity conservation of pion induced nucleon diffraction dissociation at 3.9 GeV/c and the implications of a two-component model

A study of schannel and t-channel helicity conservation has been carried out on a prism plot separated sample of 3.9 GeV/cπ^±p → π (Nπ)_dd diffraction dissociation events. s-channel helicity conservation is found to be strongly violated while t-channel helicity conservation is found to be mildly violated. These results are similar to that found at 11.7 GeV/c. We explain these violations on the basis of a “two-component model” of which the first component is a single exchange amplitude. We postulate that this amplitude is the generalization of the pomeron exchange amplitude which may conserve s-channel helicity. The second component of the model is a double exchange mechanism which strongly violates s-channel helicity. The predictions of this model agree with the experimental data.     

Helicity suppression mechanism of decay amplitudes in relativistic quark model

Structures of the matrix elements of two-and four-quark operators are investigated in connection with the argument of “helicity suppression” in the decay amplitudes. The analysis is made by expanding the quark operators in terms of the constituentquark modes in a relativistic way. It is shown that the naive consideration of helicity suppression can be applied to the case of two-quark operators. The situation is quite different in the case of four-quark operators; sometimes we find maximum value of the decay amplitude, which is suppressed according to the naive helicity suppression. It is made clear which aspects of the constituent-quark picture are essential to cause the difference between our results and those from the free quark picture. Remarks are given on nonleptonic decays ofK-andD-mesons.     

Flow helicity in a simplified statistical model of a fast dynamo

The role of kinetic helicity in small-scale fast dynamo action is investigated by employing a simple statistical model for the underlying flow with statistics that are Gaussian distributed, temporally delta-correlated and spatially homogeneous and isotropic. In order to focus on small-scale dynamo action we restrict our attention to flows possessing no net kinetic helicity. With the help of a diagrammatic technique and a numerical calculation we show that the dynamo growth rate is independent of the kinetic helicity as the magnetic Reynolds number R_m → ∞. It is indicated that the latter enhances the growth of the magnetic energy only for finite R_m.     

Helical turbulent spectra in the presence of energy and helicity fluxes

On the basis of asymptotic approach, the behavior of helicity spectra in turbulent flows has been studied. The obtained expressions relate these spectra to energy spectra on the condition that energy flux ε and helicity flux η are nonzero both in the inertial and dissipative intervals.     

Construction of π± photoproduction amplitudes at high energy from dispersion relations and local duality

We investigate the charged-pion photoproduction amplitudes at all energies. We first show that low-energy s-channel helicity amplitudes exhibit, in most cases, a Bessel-function structure of zeroes. At high energy we therefore parametrize the imaginary parts of helicity amplitudes according to the Harari model, and compute the real parts using fixed-t dispersion relations as proposed recently. Such an evaluation of scattering amplitudes presents some very nice properties. In particular, it allows one to understand from duality arguments how the Born contribution may survive with full strength in the forward direction but not at large t-values. Furthermore FESR are identically satisfied by the model.     

Lorentz-boosted evanescent waves

Polarization, spin, and helicity are important properties of electromagnetic waves. It is commonly believed that helicity is invariant under the Lorentz transformations. This is indeed so for plane waves and their localized superpositions. However, this is not the case for evanescent waves, which are well-defined only in a half-space, and are characterized by complex wave vectors. Here we describe transformations of evanescent electromagnetic waves and their polarization/spin/helicity properties under the Lorentz boosts along the three spatial directions.     

Effect of triple gluon coupling in semi-inclusive polarized scattering processes

We show that the gluon helicity inside a proton and a photon can be deduced from a knowledge of a special combination of cross sections of the semi-inclusive processese+p→e+π+π+... ande ⁺+e ^?→e ⁺+e ^?+π+π+... Such a measurement could thus be used to check the QCD prediction that the gluon helicity increases linearly with lnQ ².     

A unified treatment of electromagnetic decays,electroproduction, and pair production in e+e− annihilation for hadrons of arbitrary spin

We gather together in a unified notation formulas for electromagnetic decay rates of resonances and cross sections for electroproduction and e⁺e^? annihilation for all those processes which can be expressed in terms of matrix elements of the electromagnetic current between single particle (or resonant) helicity states. We show in complete generality how to decompose such helicity matrix elements into form factors which are free of all kinematic singularities and constraints simultaneously at both physical thresholds, and relate them to the familiar multipole moments and to some others which have been used in data analysis. Tables are given of the form factor decomposition of the helicity matrix elements for many cases of actual and potential practical interest.     

Hole core in superconductors and the origin of the Spin Meissner effect

It is proposed that superconductors possess a hidden ‘hole core’ buried deep in the Fermi sea. The proposed hole core is a small region of the Brillouin zone (usually at the center of the zone), where the lowest energy states in the normal state reside. We propose that in the superconducting state these energy states become singly occupied with electrons of a definite spin helicity. In other words, that holes of a definite spin helicity condense from the top to the bottom of the band in the transition to superconductivity, and electrons of that spin helicity ‘float’ on top of the hole core, thus becoming highly mobile. The hole core has radius q₀ = 1/2λ_L, with λ_L the London penetration depth, and the electrons expelled from the hole core give an excess negative charge density within a London penetration depth of the real space surface of the superconductor. The hole core explains the development of a spin current in the transition to superconductivity (Spin Meissner effect) and the associated negative charge expulsion from the interior of metals in the transition to superconductivity, effects we have proposed in earlier work to exist in all superconductors and to be at the root of the Meissner effect.     

Does ψ photoproduction conserve helicity?

We suggest the experimental determination of the spin dependence of ψ photoproduction by measuring the decay angular distribution for ψ → l⁺l^?. Theoretical expectations for the density matrix are explored in the context of several models for ψ photoproduction. In the threshold region, the phenomenological models indicate a substantial breaking of helicity conservation whereas vector-gluon exchange models conserve helicity. Spin measurements can thus provide a test for the gluon exchange approach.     

Covariant trace formalism for heavy mesons-wave top-wave transitions

Heavy meson,s- top-wave, weakb→c transitions are studied in the context of the heavy quark effective theory using covariant meson wave functions. We use the trace formalism to evaluate the weak transitions. As expected from heavy quark symmetry, the eight transitions betweens- andp-wave states are described in terms of only two universal form factors which are given in terms of explicit wave function overlap integrals. We present our results in terms of both invariant and helicity amplitudes. Using our helicity amplitude expressions we discuss rate formulae, helicity structure functions and joint angular decay distributions in the decays \(\bar B \to D^{**} ( \to (D,D^* ) + \pi ) + W^ - ( \to l^ - \bar v_l )\) . The heavy quark symmetry predictions for the one pion transitionsD ^**→(D,D ^*)+π are similarly worked out by using trace techniques.     

Measurement of A LL and cross section of high p T charged π meson production for constraining ΔG at PHENIX

PHENIX at RHIC measured invariant differential cross section and double helicity asymmetry A _LL of high p _T charged pions production in polarized p + p collision at √s = 200 GeV. Interpretations of preliminary results regarding constraining gluon helicity distribution are discussed.     

Polarized electron-proton scattering at highQ 2

We show that polarized electron-proton scattering is an excellent tool to measure helicity changing amplitudes. The asymmetry of polarized protons measured with longitudinally polarized electrons determines the ratio of Pauli (F ²) to Dirac (F ¹) form factors. For the leading Fock state the Pauli form factor originates only from helicity changing quarkgluon interactions which are zero for zero quark masses. Therefore at high momentum transfer the ratioF ²/F ¹ depends essentially on the up and down quark masses.     

Pion-nucleon backward scattering: The location of the amplitude zeros

The complex zero trajectories for the πN helicity amplitudes with definite u-channel isospin are determined from phase shifts. Some of these trajectories display systematic behaviour, having fixed values of (u?u₀), where u₀ corresponds to the backward direction, and small values of Im u. On the assumption that this systematic behaviour will persist to higher energies one can make predictions about the locations of zeros in these helicity amplitudes at high energies. These zero locations are very relevant to an understanding of the mechanisms involved in πN backward scattering. The results, which show quite a striking pattern for the zero positions in the various amplitudes, are discussed from the point of view (a) of the experimental data for πN backward scattering and (b) of a theoretical interpretation of the interaction mechanism.     

Spin structure of the pion in a light-cone representation

The spin structure of the pion is discussed by transforming the wave function for the pion in the naive quark model into a light-cone representation. It is shown that there are higher helicity (λ ₁+λ ₂=±1) states in the full light-cone wave function for the pion besides the ordinary helicity (λ ₁+λ ₂=0) component wave functions as a consequence from the Melosh rotation relating spin states in light-front dynamics and those in instantform dynamics. Some low energy properties of the pion, such as the electromagnetic form factor, the charged mean square radius, and the weak decay constant, could be interrelated in this representation with reasonable parameters.     

Relativistic harmonic oscillator model and electroproduction of low-lying resonances

By using the four-dimensional relativistic harmonic oscillator model, the Q²-evolution of the helicity amplitudes of the low-lying resonances P₃₃(1232), S₁₁(1535), and D₁₃(1520) are estimated. The results show that the Lorentz contraction causes remarkable effects on those helicity amplitudes.     

Lifting of the four-fold em degeneracy and PT asymmetry

Solutions to Maxwell's equations which lift the Lorentz degeneracies of propagation direction (T asymmetry) and helicity (P asymmetry) have been determined from the analysis of a valence four constitutive tensor density, χ^λνσ?.