Spin Interference Effects Research Articles

NLO corrections to triple vector-boson production in final states with three charged leptons and two jets

Tri-boson production together with vector-boson scattering is a privileged channel to study the electroweak structure of the Standard Model. Upcoming LHC running stages will allow to measure these processes at unprecedented accuracy and for all possible final states, which requires to push theory predictions to still unexplored frontiers. In this work we present the first calculation for the process pp → μ+μ−e+νejj at the LHC in a tri-boson phase space. We evaluate the three LO contributions, namely the O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(α6), which contains the genuine tri-boson signature, along with the O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(αsα5) and O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(αs2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\alpha}_{\ extrm{s}}^2 $$\\end{document}α4), and the two O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(α7) and O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(αsα6) NLO corrections. The calculation is based on full Standard-Model matrix elements, including all resonant and non-resonant terms, complete spin correlations and interference effects. Integrated and differential cross sections are presented for a fiducial region inspired by High Luminosity LHC prospect studies. We find electroweak corrections of −14% for the fiducial cross section, almost twice as large as for other tri-boson processes.

Complete NLO corrections to off-shell tt¯Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{t}\\overline{\ extrm{t}}\ extrm{Z} $$\\end{document} production at the LHC

Measuring precisely top-pair-associated processes at hadron colliders will become possible with the upcoming LHC running stages. The increased data statistics will especially enable differential measurements leading to an improved characterisation of such processes. Aiming at a consistent data-theory comparison, precise Standard-Model predictions are needed, including higher-order corrections and full off-shell effects. In this work we present NLO-accurate predictions for the production and decay of a top-antitop pair in association with a Z boson at the LHC, in the multi-lepton decay channel. The complete set of LO contributions and NLO corrections of EW and QCD origin is included. The calculation is based on full matrix elements, computed with all resonant and non-resonant contributions, complete spin correlations and interference effects. Integrated and differential cross-sections are presented for a realistic fiducial setup.

Spin Interference Effects in a Ring with Rashba Spin-Orbit Interaction Subject to Strong Light–Matter Coupling in Magnetic Field

Electron transport through a one-dimensional quantum ring, subjected to Rashba spin–orbit interaction and connected with two external leads, is studied in the presence of external fields. They include the optical radiation, produced by an off-resonant high-frequency electric field, and a perpendicular magnetic field. By means of the Floquet theory of periodically driven quantum systems the interference effects under these fields are described in detail. It is found analytically the specific conditions to reach the spin-filtering effect, caused by the interplay of the external fields and Rashba spin-orbit interaction.

Tunable strength saddle-point contacts impact on quantum rings transmission

A particular subject of investigation is the role of several sadle-point contact (QPC) parameters on the scattering properties of an Aharonov–Bohm–Aharonov–Casher quantum ring (QR) under Rashba-type spin orbit interaction. We discuss the interplay of the conductance with the confinement strengths and height of the QPC, which yields new and tunable harmonic and non-harmonics patterns, while one manipulates these constriction parameters. This phenomenology may be of utility to implement a novel way to modulate spin interference effects in semiconducting QRs, providing an appealing test-platform for spintronics applications.

Birefringent magnetic field system for experimental checking of neutron electroneutrality by the spin interferometry technique

We describe the structure design and present the results of experimental testing of a birefringent magnetic neutron prisms for spin-echo small-angle neutron scattering (SESANS). The SESANS setup, equipped with perfect single crystals for measuring diffraction-enhanced spin interference effects, must possess unprecedented sensitivity with respect to small external actions on a neutron and is intended for use in experiments for checking the neutron electroneutrality.

High-temperature Aharonov-Bohm-Casher interferometer

We study theoretically the combined effect of the spin-orbit and Zeeman interactions on the tunneling electron transport through a single-channel quantum ring threaded by magnetic flux. We focus on the high-temperature case (temperature is much higher than the level spacing in the ring) and demonstrate that spin-interference effects are not suppressed by thermal averaging. In the absence of the Zeeman coupling, the high-temperature tunneling conductance of the ring exhibits two types of oscillations: Aharonov-Bohm oscillations with magnetic flux and Aharonov-Casher oscillations with the strength of the spin-orbit interaction. For weak tunneling coupling, both oscillations have the form of sharp periodic antiresonances. In the vicinity of the antiresonances, the tunneling electrons acquire spin polarization, so that the ring serves as a spin polarizer. We also demonstrate that the Zeeman coupling leads to appearance of two additional peaks, both in the tunneling conductance and in the spin polarization.

Discriminating top-antitop resonances using azimuthal decay correlations

Top-antitop pairs produced in the decay of a new heavy resonance will exhibit spin correlations that contain valuable coupling information. When the tops decay, these correlations imprint themselves on the angular patterns of the final quarks and leptons. While many approaches to the measurement of top spin correlations are known, the most common ones require detailed kinematic reconstructions and are insensitive to some important spin interference effects. In particular, spin-1 resonances with mostly-vector or mostly-axial couplings to top cannot be easily discriminated from one another without appealing to mass-suppressed effects or to more model-dependent interference with continuum Standard Model production. Here, we propose to probe the structure of a resonance's couplings to tops by measuring the azimuthal angles of the tops' decay products about the production axis. These angles exhibit modulations which are typically O(0.1-1), and which by themselves allow for discrimination of spin-0 from higher spins, measurement of the CP-phase for spin-0, and measurement of the vector/axial composition for spins 1 and 2. For relativistic tops, the azimuthal decay angles can be well-approximated without detailed knowledge of the tops' velocities, and appear to be robust against imperfect energy measurements and neutrino reconstructions. We illustrate this point in the highly challenging dileptonic decay mode, which also exhibits the largest modulations. We comment on the relevance of these observables for testing axigluon-like models that explain the top quark A_FB anomaly at the Tevatron, through direct production at the LHC.

Spin interference and the Fano effect in electron transport through a mesoscopic ringside-coupled with a quantum dot

We investigate the electron transport through a mesoscopic ring side-coupled with aquantum dot (QD) in the presence of Rashba spin–orbit (SO) interaction. It is shown thatboth the Fano resonance and the spin interference effects play important roles in theelectron transport properties. As the QD level is around the Fermi energy, the totalconductance shows a typical Fano resonance line shape. By applying an electrical gatevoltage to the QD, the total transmission through the system can be strongly modulated.By threading the mesoscopic ring with a magnetic flux, the time-reversal symmetry of thesystem is broken, and a spin polarized current can be obtained even though the incidentcurrent is unpolarized.

Electrical manipulation of spins in the Rashba two dimensional electron gas systems

We present our theoretical and experimental studies on manipulation of electron spins based on the Rashba spin-orbit interaction (SOI) in semiconductor heterostructures. Quantum well (QW) thickness dependence of the Rashba SOI strength α is investigated in InP/InGaAs/InAlAs asymmetric QWs by analyzing weak antilocalization. Two different QW thicknesses show inverse Ns dependence of |α| in the same heterostructures. This inverse Ns dependence of |α| is explained by the k⋅p perturbation theory. We confirm that narrow wires are effective to suppress the spin relaxation. Spin interference effects due to spin precession are experimentally studied in small array of mesoscopic InGaAs rings. This is an experimental demonstration of a time reversal Aharonov–Casher effect, which shows that the spin precession angle in an InGaAs channel can be controlled by an electrostatic gate.

MeasuringZ′couplings at the CERN LHC

We study the properties of potential new ${Z}^{\ensuremath{'}}$ gauge bosons produced through the Drell-Yan mechanism at the Large Hadron Collider (LHC). Our analysis is performed using a fully differential next-to-leading-order QCD calculation with spin correlations, interference effects, and experimental acceptances included. We examine the distinguishability of different models and the feasibility of extracting general coupling information with statistical, residual scale, and current parton distribution function error estimates included. We extend a previous parametrization of ${Z}^{\ensuremath{'}}$ couplings to include parity-violating coupling combinations and introduce a convenient technique for simulating new gauge bosons on-peak using the concept of basis models. We illustrate our procedure using several example ${Z}^{\ensuremath{'}}$ models. We find that one can extract reliably four combinations of generation-independent quark and lepton couplings in our analysis. For a ${Z}^{\ensuremath{'}}$ mass of 1.5 TeV, one can determine coupling information very well assuming $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of integrated luminosity, and a precise measurement becomes possible with $1{\mathrm{ab}}^{\ensuremath{-}1}$ at the super-LHC (SLHC). For a 3 TeV mass, a reasonable determination requires the SLHC.

SPIN INTERFERENCE IN RASHBA 2DEG SYSTEMS

The gate controllable SOI provides useful information about spin interference.1 Spin interference effects are studied in two different interference loop structures. It is known that sample specific conductance fluctuations affect the conductance in the interference loop. By using array of many interference loops, we carefully pick up TRS Altshuler-Aronov-Spivak (AAS)-type oscillation which is not sample specific and depends on the spin phase. The experimentally obtained gate voltage dependence of AAS oscillations indicates that the spin precession angle can be controlled by the gate voltage.2 We demonstrate the time reversal Aharonov-Casher (AC) effect in small arrays of mesoscopic rings.3 By using an electrostatic gate we can control the spin precession angle rate and follow the AC phase over several interference periods. We also see the second harmonic of the AC interference, oscillating with half the period. The spin interference is still visible after more than 20π precession angle. We have proposed a Stern-Gerlach type spin filter based on the Rashba SOI.4 A spatial gradient of effective magnetic field due to the nonuniform SOI separates spin up and down electrons. This spin filter works even without any external magnetic fields and ferromagnetic contacts. We show the semiconductor/ferromagnet hybrid structure is an effective way to detect magnetization process of submicron magnets. The problem of the spin injection from ferromagnetic contact into 2DEG is also disicussed. Note from Publisher: This article contains the abstract only.

Magnetotransport through a ring conductor with Rashba spin-orbit interaction

Magnetotransport in a finite-width ring conductor with Rashba effect has been investigated by the scattering matrix method. The spin-resolved conductance and shot-noise properties are determined by the interplay of both spin and charge interference effects. It is found that the period of conductance oscillation due to external magnetic field will not be changed by the existence of an effective magnetic field due to spin-orbit interaction for a ring conductor with small aspect ratio at low magnetic field. The spin-flipped conductance ${G}_{\ensuremath{\sigma},\ensuremath{-}\ensuremath{\sigma}}$ and shot noise ${S}_{\ensuremath{\sigma},\ensuremath{-}\ensuremath{\sigma}}$, where the spin axis is along the perpendicular direction, are found to be fully suppressed to zero in the Aharonov-Bohm (Aharonov-Casher) interference measurement if the Rashba strength (external magnetic flux) is tuned to values corresponding to the zero conductance in the pure Aharonov-Casher (Aharonov-Bohm) measurements.

Multiple-scattering theory for two-dimensional electron gases in the presence of spin-orbit coupling

In order to model the phase-coherent scattering of electrons in two-dimensional electron gases in the presence of Rashba spin-orbit coupling, a general partial-wave expansion is developed for scattering from a cylindrically symmetric potential. The theory is applied to possible electron flow imaging experiments using a moveable scanning probe microscope tip. In such experiments, it is demonstrated theoretically that the Rashba spin-orbit coupling can give rise to spin interference effects, even for unpolarized electrons at nonzero temperature and no magnetic field.

Modulating unpolarized current in quantum spintronics: Visibility of spin-interference effects in multichannel Aharonov-Casher mesoscopic rings

The conventional unpolarized current injected into a {\em quantum-coherent} semiconductor ring attached to two external leads can be modulated from perfect conductor to perfect insulator limit via Rashba spin-orbit (SO) coupling. This requires that ballistic propagation of electrons, whose spin precession is induced by the Aharonov-Casher phase, takes place through a single conducting channel ensuring that electronic quantum state remains a pure separable one in the course of transport. We study the fate of such spin interference effects as more than one orbital conducting channel becomes available for quantum transport. Although the conductance of multichannel rings, in general, does not go all the way to zero at any value of the SO coupling, some degree of current modulation survives. We analyze possible scenarios that can lead to reduced visibility of spin interference effects that are responsible for the zero conductance at particular values of the Rashba interaction: (i) the transmitted spin states remain fully coherent, but conditions for destructive interference are different in different channels; (ii) the transmitted spins end up in partially coherent quantum state arising from entanglement to the environment composed of orbital degrees of freedom of the same particle to which the spin is attached.

Spin interference and nonideal effects on electron-electron collisions in nonideal plasmas

The spin interference and nonideal effects on electron-electron collisions are investigated in nonideal plasmas. An effective pseudopotential model taking into account the screening and nonideal effects is applied to describe the electron-electron interaction potential in nonideal plasmas. The total spin states of the collision system are considered to obtain high-energy electron-electron collision cross sections. The results show that the differential collision cross section is found to decrease with increasing nonideality plasma parameter γ in the region 0<γ<2 and, however, it increases in the region 2<γ<4. It is found that the spin interference and nonideal screening effects are especially important at the scattering angle θL=π/4. It should also be noted that the differential cross sections have minima near γ=2 and the nonideal effect is found to be more significant at the minima.

Spin interference effects in ring conductors subject to Rashba coupling

Quantum interference effects in rings provide suitable means for controlling spin at mesoscopic scales. Here we apply such control mechanisms to coherent spin-dependent transport in one- and two-dimensional rings subject to Rashba spin-orbit coupling. We first study the spin-induced modulation of unpolarized currents as a function of the Rashba coupling strength. The results suggest the possibility of all-electrical spintronic devices. Moreover, we find signatures of Berry phases in the conductance previously unnoticed. Second, we show that the polarization direction of initially polarized, transmitted spins can be tuned via an additional small magnetic control flux. In particular, this enables to precisely reverse the polarization direction at half a flux quantum. We present full numerical calculations for realistic two-dimensional ballistic microstructures and explain our findings in a simple analytical model for one-dimensional rings.

Quantum beating in the conductance of ballistic rings

We propose and demonstrate experimentally a novel configuration for observing spin interference effects in the conductance of quantum rings. Our configuration, in which the conducting ring and the tangential current lead form a single collimating contact, precludes spin rotation in the contact region. We have observed the spin quantum beating in the Aharonov–Bohm (AB) conductance oscillations. This beating is the result of a superposition of two independent interference signals associated with the intrinsic spin–orbit interactions. Our work provides conclusive evidence of the spin Berry phase in the ring conductance.

Spin and quantum interference effects in hopping conductivity

We present results of high field magnetoconductance (MC) measurements on In 2O 3−x films in the hopping regime. For relatively weak disorder, a purely positive MC is observed. With growing disorder, the MC acquires a negative component that tends to saturate at high fields. The data for the latter case are consistent with the co-existence of two mechanisms for MC; an isotropic, spin-alignment mechanism and an anisotropic, quantum-interference one.

Spin interference effects in the earth's field

The spin interference effects in the earth's field is investigated. It is shown that there exist three effects, which are induced by (a) the Schwarzschild field, (b) the earth's spin and (c) the frame rotation respectively, affecting the resulting spin polarization after interference.