Polarized Cross Sections Research Articles

Measuring effective electroweak couplings in single top quark production at the CERN LHC

We study the mechanism of single top quark production at the CERN LHC in the framework of an effective electroweak Lagrangian, analyzing the sensitivity of different observables to the magnitude of the effective couplings that parametrize new physics beyond the standard model. The observables relevant to the distinction between left and right effective couplings involve in practice the measurement of the spin of the top quark and this can be achieved only indirectly by measuring the angular distribution of its decay products. We show that the presence of effective right-handed couplings implies that the top quark is not in a pure spin state. A unique spin basis is singled out which allows one to connect the top decay products angular distribution with the polarized top differential cross section. We present a complete analytical expression of the differential polarized cross section of the relevant perturbative subprocess including general effective couplings. The mass of the bottom quark, which actually turns out to be more relevant than naively expected, is retained. Finally we analyze different aspects of the total cross section relevant to the measurement of new physics through the effective couplings. The above analysis also applies to top antiquark production in a straightforward way.

Meson photoproduction at GRAAL: preliminary results

A polarised and tagged \(\vec \gamma \) ray beam is produced at GRAAL with energies ranging from 500 MeV up to 1500 MeV. Results of beam polarisation asymmetries and cross sections for the photoproduction of pseudoscalar mesons on the proton are presented. These very precise measurements cover the angular range 30°–150°, providing stringent constraints to theoretical models.

Effects of new neutral currents at linear electron-positron colliders

Effects that are induced by contact four-fermion interactions in the processes e+e− → μ+μ−, \(\bar bb\), and \(\bar cc\) at \(\sqrt s = 0.5\)TeV linear electron-positron colliders are investigated for the case of longitudinally polarized initial beams. This analysis employs new integrated observables constructed from the polarized cross sections for the scattering of final fermions into the forward (σF) and the backward (σB) hemisphere in such a way that they single out the helicity cross sections for the processes in question. This property of the observables makes it possible to perform, in the most general form, a model-independent analysis of contact four-fermion interactions and to set constraints on their parameters. It is also shown that the sensitivity of new polarization observables to contact interactions is noticeably higher than the corresponding sensitivity of canonical observables like σ, AFB, ALR, and ALR,FB.

Polarization effects in inelastic deuteron scattering in the region of baryon-resonance excitation

Within the impulse approximation, polarization effects in inelastic deuteron scattering on nuclei are investigated in the region around the resonance of mass about 2190 MeV/c2. It is shown that the spin-dependent part of the NN → NN*(2190) amplitude plays a significant role at high momentum transfers. Predictions are obtained for some polarization observables and cross sections for various deuteron-spin-flip processes.

Crystal growth and optical assessment of Nd 3+ : GdAl 3(BO 3) 4 crystal

This paper reports the high-temperature top-seeded solution growth (TSSG) of large single crystal of Nd 3+-doped GdAl 3(BO 3) 4 from a K 2Mo 3O 10-LiF flux cooled at a rate of 2 K/day and using a crystal rotation rate of 4.5 rpm. The polarized optical absorption and fluorescence spectra of Nd 3+ : GdAl 3(BO 3) 4 crystal have been investigated. Polarized absorption cross sections are 4.3×10 −20 and 2.6×10 −20 cm 2 with band widths of 8.7 and 6.1 nm for σ- and π-polarization, respectively. The emission cross sections are 6.3×10 −19 and 6.0×10 −19 cm 2 for σ- and π-polarization, respectively, at 1062 nm corresponding to the 4F 3/2→ 4I 11/2 transition. The fluorescence lifetime is 54.3 and 57 μs at 300 and 20 K, respectively.

The Thomas rotation

We review why the Thomas rotation is a crucial facet of special relativity, that is just as fundamental, and just as “unintuitive” and “paradoxical,” as such traditional effects as length contraction, time dilation, and the ambiguity of simultaneity. We show how this phenomenon can be quite naturally introduced and investigated in the context of a typical introductory course on special relativity, in a way that is appropriate for, and completely accessible to, undergraduate students. We also demonstrate, in a more advanced section aimed at the graduate student studying the Dirac equation and relativistic quantum field theory, that careful consideration of the Thomas rotation will become vital as modern experiments in particle physics continue to move from unpolarized to polarized cross sections.

Optimal polarized observables for model-independent new-physics search at e +e − colliders

For the processes e +e −→ μ + μ −, τ + τ −, b b ̄ and c c ̄ at a future e +e − collider with s =0.5 TeV, we examine the sensitivity of the helicity cross sections to four-fermion contact interactions. If longitudinal polarization of the electron beam were available, two polarized integrated cross sections would offer the opportunity to separate the helicity cross sections and, in this way, to derive model-independent bounds on the relevant parameters. The measurement of these polarized cross sections with optimal kinematical cuts could significantly increase the sensitivity of helicity cross sections to contact interaction parameters and could give crucial information on the chiral structure of such new interactions.

Angular dependences in electroweak semi-inclusive leptoproduction

We present the leading order unpolarized and polarized cross sections in electroweak semi-inclusive deep inelastic leptoproduction. The azimuthal dependences in the cross section differential in the transverse momentum of the vector boson arise due to intrinsic transverse momenta of the quarks. However, the presented asymmetries are not suppressed by inverse powers of the hard scale. We discuss the different opportunities to measure specific asymmetries as offered by neutral compared to charged current processes and point out the optimal kinematical regions. The present and (proposed) future HERA collider experiments would be most suitable for measuring some of the asymmetries discussed here, especially in case of Λ production.

Direct Measurement ofAbandAcat theZ0Pole Using a Lepton Tag

The parity violation parameters Ab and Ac of the Zbb¯ and Zcc¯ couplings have been measured directly, using the polar angle dependence of the Z0-pole polarized cross sections. Bottom and charmed hadrons were tagged via semileptonic decays. Both the muon and electron identification algorithms take advantage of new multivariate techniques, incorporating for the first time information from the SLD Čerenkov Ring Imaging Detector. Based on the 1993–1995 SLD sample of 150 000 Z0 decays produced with highly polarized electron beams, we measure Ab=0.910±0.068(stat)±0.037(syst), Ac=0.642±0.110(stat)±0.063(syst).Received 4 May 1999DOI:https://doi.org/10.1103/PhysRevLett.83.3384©1999 American Physical Society

Electroweak radiative corrections to polarized Møller scattering at high energies

The cross section for \(\rm e^-e^-\rightarrow e^-e^- \) with arbitrary electron polarizations is calculated within the Electroweak Standard Model for energies large compared to the electron mass, including the complete virtual and soft-photonic \(O(\alpha)\) radiative corrections. The relevant analytical results are listed, and a numerical evaluation is presented for the unpolarized and polarized cross sections as well as for polarization asymmetries. The relative weak corrections are typically of the order of 10%. At low energies, the bulk of the corrections is due to the running of the electromagnetic coupling constant. For left-handed electrons, at high energies the vertex and box corrections involving virtual W bosons become very important. The polarization asymmetry is considerably reduced by the weak radiative corrections.

Study of Gulf Stream features with a multifrequency polarimetric SAR from the Space Shuttle

Using simulations of radar cross section (RCS) based on wave-current interaction calculations, the authors investigate the origin of a prominent enhancement in L-band from signals that were transmitted and received, respectively, with horizontal (H) and vertical (V) polarization radar return. This was observed in imagery of the northern boundary of the Gulf Stream (GS) during the first Shuttle Radar Laboratory (SRL-1) mission. The calculations of surface roughness are based on a one-dimensional (1D) surface current model that closely resembles a current shear that was observed in in situ current measurements, taken at both sides of the GS at the time SRL-1 imaged the GS boundary. In agreement with trends observed in the imagery, significant enhancements in L-band HV polarization cross section occur in the neighborhood of the GS thermal boundary, relative to comparable vertical polarization (VV) cross section signatures at X-, C-, and L-band. The authors also find reasonably good agreement between the simulated and observed magnitudes of the GS signatures (based on calculations of wave action) using two different radar imaging models, and they provide an overview of a number of additional submesoscale features associated with the GS that were present in the image of the GS boundary.

Issues related to the use of a Gaussian-like incident field for low-grazing-angle scattering

We study the suitability of a tapered plane-wave incident field, using both the Gaussian and the more advanced Thorsos tapers for low-grazing-angle rough-surface scattering problems as well as the problem of propagation in the presence of a rough surface. For surface scattering problems it is known that as the angle of incidence approaches grazing incidence the tapered beam waist should be made larger; several criteria relating these two parameters have been proposed for both the Gaussian and the Thorsos tapers. Our two-dimensional scattering simulations with the oceanlike Pierson–Moskowitz surfaces show that when the width of the Gaussian or the Thorsos taper is fixed, the backscatter cross section for TE polarization is characterized by a distinctive and consistent anomalous jump as grazing incidence is approached. This observation has led to a refined version of one of the above-mentioned beam waist–angle of incidence criteria and its robustness is demonstrated. The approximate (non-Maxwellian) nature of the Thorsos–Gaussian taper also becomes evident in over-surface-propagation simulations with use of the boundary integral equation method. A certain inconsistency was observed between the surface field that we obtained by first defining the Thorsos–Gaussian-tapered field on a vertical plane and then propagating it to the surface and that obtained by defining the same tapered field directly on the surface. This effect, not previously appreciated, may be of importance when the rough-surface effects are rigorously incorporated into the propagation problem. We conclude with a detailed derivation of the Thorsos taper that points out all the approximations involved in it and the resulting limitations.

Inelastic photon-neutrino interactions using an effective Lagrangian

We justify the feasibility of substituting a photon leg by a neutrino current in the Euler-Heisenberg Lagrangian to obtain an effective Lagrangian for the process $\gamma\nu\to\gamma\gamma \nu$ and its crossed reactions. We establish the link between these processes and the four-photon scattering in both the Standard Model and the effective theory. As an application, we compute the processes $\gamma\nu\to\gamma\gamma\nu$ and $\gamma\gamma\to\gamma{\nu}\bar\nu$, give their polarized cross sections, and show how to use the $\gamma\gamma\to\gamma\gamma$ results as a check. We settle the question about the disagreement between two computations in the literature concerning the reaction $\gamma\gamma\to \gamma \nu\bar\nu$.

Studies of elastic p– p polarization and differential cross section in a simple parton scattering model

We report here the results of an analysis of p– p elastic polarization and differential cross section data in the framework of a constitutent scattering model where the sharp breaks in the fixed-angle cross section and the growth of polarization with increasing four-momentum transfer −t are attributed to a varying number of constituent scatterings in the different ∣ t∣ regions. On the assumption that over the range ∣ t∣≤1.0 (GeV/c) 2 the scattering process is dominated by a single parton in each of the colliding protons scattering only once, then for the next well defined ∣ t∣ region as determined by the cross section shape we find that the data is consistent with two partons on average participating in the collision, or that one parton in one proton double scatters from partons in the other proton. Moreover, for the next distinct region on average three or more parton-parton scatters occur. The overall growth of the polarization with ∣ t∣ follows from the addition of the polarization in the individual parton-parton scatterings. This analysis suggests that these features of p– p elastic scattering may surprisingly be due to standard low ∣ t∣ high energy processes, and that polarization effects may prove to be an enormously useful tool in exploring what actually happens in the p– p scattering process, even at the constituent level.

Drell-Yan non-singlet spin cross sections and spin asymmetry to O( αs2) (II)

We present predictions for the non-singlet Drell-Yan longitudinal spin cross sections and spin asymmetry, A LL , in proton-proton collisions at large p T at the RHIC energy of 200 GeV at next-to-leading order QCD. The higher order corrections to the non-singlet polarized cross section, σ NS LL , are sizeable and similar to those found for the unpolarized cross section. The non-singlet asymmetry parameter, A LL NS , is very stable against higher order corrections and is a direct measurement of the non-singlet (i.e., valence) polarized quark distributions within the proton.

The scattering and depolarization of electromagnetic waves by random rough surfaces with different scales of roughness: New full wave solutions

A new unified full-wave approach is used to determine the like and cross polarized cross sections of composite random rough surfaces with significantly different scales of roughness. Traditionally, the solutions for such composite rough surfaces are expressed as sums of solutions based on the perturbation and the physical optics approaches. The full-wave solutions account for both specular point scattering and Bragg scattering in a unified, self-consistent manner. With the new unified full-wave approach, the incoherent radar cross sections are obtained by regarding the composite (multiple scale) rough surface as an ensemble of individual large scale scattering elements (patches) with arbitrary orientations. Thus the scatter cross sections, obtained from the original full-wave solution in a tilted co-ordinate system, are averaged over the large scale slopes (in and perpendicular to the plane of incidence) to obtain the cross sections of the composite surfaces. The rough surface height joint probability density function is assumed to be Gaussian.

Effects of non-standard tri-linear couplings in photon-photon collisions (I) γγ → W+Wt-

The effect of anomalous couplings in γγ → W + W t- is studied for different energies of the γγ mode of the next linear collider. The analysis based on the maximum likelihood method exploits the variables in the four-fermion semi-leptonic final state. Polarized differential cross sections based on the complete set of diagrams for these channels with the inclusion of anomalous couplings are used and compared to an approximation based on γγ → W + W t- with full spin correlations. To critically compare these results with those obtained in e + e t- we perform an analysis based on the complete calculation of the four-fermion semi-leptonic final state. The anomalous couplings that we consider are derived from the next-to-leading order operators in the non-linear realization of symmetry breaking.

POLDIS A Monte Carlo for POLarized (semi-inclusive) Deep Inelastic Scattering

POLDIS is a Monte Carlo program for polarized (semi-inclusive) deep inelastic scattering (DIS). Unpolarized DIS events are generated with the existing lepto-production event generators LEPTO for DIS and AROMA for Heavy Flavor production. The relevant spin asymmetries are computed at partonic level to first order in α s for each generated event, and are then convoluted with the corresponding parton polarization. This procedure provides a polarization weight for each event. The average of these polarization weights gives the polarized cross section spin-asymmetry for the generated sample. The code consists of a set of subroutines to be linked with LEPTO and/or AROMA. No modification to these programs is required. Some existing polarized parton distribution functions are also included.

Absolute cross sections with polarization effects inNe*(2p53p)+Hecollisions: A detailed comparison between theory and experiment

Quantal calculations are presented for intramultiplet mixing collisions of ${\mathrm{Ne}}^{*}{(2p}^{5}3p)$ with $\mathrm{He}{(}^{1}{S}_{0})$ in the 60--1250 meV collision energy range. The coupled equations are solved in a diabatic representation where the coupling terms are obtained using the model potential calculations of Hennecart and Masnou-Seeuws [J. Phys. B 18, 657 (1985)] for the interaction of the ${\mathrm{Ne}}^{*}$ outer electron with the two cores and values fitted on the spectroscopic data of Dabrowski and Herzberg [Mol. Spectrosc. 73, 183 (1978)] for the core-core interaction. An extensive comparison with the experimental data of the Eindhoven group is presented for absolute polarized cross sections and anisotropy parameters. Very good agreement is obtained with thermal energy data of Manders et al. [Phys. Rev. A 39, 4467 (1989)], while the systematic 30--40 % discrepancy with superthermal data of Boom et al. [Phys. Rev. A 49, 4660 (1994)] might be attributed to a calibration problem in the experiment rather than to an inaccuracy in the long-range potentials.

Measurements of A b, R b and the B±,0 lifetimes at the Z0 resonance by SLD

R b , A b and the lifetimes of the B + ( B u ) and B 0 ( B d ) mesons have been measured using a sample of 150,000 hadronic Z 0 decays collected by the SLD experiment at the SLC between 1993 and 1995. The analyses make use of a novel topological vertexing technique to obtain charge, decay length and vertex mass. The asymmetry A b is measured with a self-calibrating technique employing momentum-weighted track charge from both hemispheres in the tagged events as well as by using the polar angle dependency of the Z 0 — pole polarized cross section and tagging the bottom and charmed hadrons in the semileptonic decay channel.