21 research outputs found
Consistent Analysis of O(\alpha_s) Corrections to Pion Elastic Form Factor
We examine the role of O(\alpha_s) perturbative corrections to the pion
elastic form factor F_\pi(Q^2). We express the quark current three-point
function in terms of light cone variables and use Borel transformation to
simultaneously model the Feynman mechanism contribution determined by the soft
part of the pion light cone wave function and the hard term involving one gluon
exchange. We find that for Q^2 \sim 5 GeV^2 the total one gluon exchange
contribution may reach 40% of the soft contribution, even though its hard,
factorization scale dependent part remains relatively small.Comment: 15 pages; version to appear in Phys. Rev.
Electron Spin Relaxation in a Semiconductor Quantum Well
A fully microscopic theory of electron spin relaxation by the
D'yakonov-Perel' type spin-orbit coupling is developed for a semiconductor
quantum well with a magnetic field applied in the growth direction of the well.
We derive the Bloch equations for an electron spin in the well and define
microscopic expressions for the spin relaxation times. The dependencies of the
electron spin relaxation rate on the lowest quantum well subband energy,
magnetic field and temperature are analyzed.Comment: Revised version as will appear in Physical Review
Decoherence and Relaxation of a Quantum Bit in the Presence of Rabi Oscillations
Dissipative dynamics of a quantum bit driven by a strong resonant field and
interacting with a heat bath is investigated. We derive generalized Bloch
equations and find modifications of the qubit's damping rates caused by Rabi
oscillations. Nonequilibrium decoherence of a phase qubit inductively coupled
to a LC-circuit is considered as an illustration of the general results. It is
argued that recent experimental results give a clear evidence of effective
suppression of decoherence in a strongly driven flux qubit.Comment: 14 pages; misprints correcte
Theory of weak continuous measurements in a strongly driven quantum bit
Continuous spectroscopic measurements of a strongly driven superconducting
qubit by means of a high-quality tank circuit (a linear detector) are under
study. Output functions of the detector, namely, a spectrum of voltage
fluctuations and an impedance, are expressed in terms of the qubit spectrum and
magnetic susceptibility. The nonequilibrium spectrum of the current
fluctuations in the qubit loop and the linear response function of the driven
qubit coupled to a heat bath are calculated with Bloch-Redfield and rotating
wave approximations. Backaction effects of the qubit on the tank and the tank
on the qubit are analyzed quantitatively. We show that the voltage spectrum of
the tank provides detailed information about a frequency and a decay rate of
Rabi oscillations in the qubit. It is found that both an efficiency of
spectroscopic measurement and measurement-induced decoherence of the qubit
demonstrate a resonant behaviour as the Rabi frequency approaches the resonant
frequency of the tank. We determine conditions when the spectroscopic
observation of the Rabi oscillations in the flux qubit with the tank circuit
can be considered as a weak continuous quantum measurement.Comment: 28 page
Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised protons
Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise. © 2015
A high-power source of ultrawideband pulses of synthesized radiation
A high-power source of ultrawideband radiation based on the synthesis of electromagnetic pulses with different frequency bands has been created. A four-channel former of highly stable bipolar pulses with durations of 0.5 and 1 ns was developed. Each element of a 2 × 2 array of combined antennas was excited from its former channel. Two modes of radiation synthesis were studied: according to the maximum field strength and the maximum spectrum width. For the first mode, radiation pulses with a frequency band of 0.39–2.13 GHz and an effective potential of 600 kV were obtained. Synthesized radiation pulses with an extended frequency band of 0.15–2.7 GHz and an effective potential of 200 kV were obtained in the second mode. The pulse repetition frequency was 100 H
A high-power source of ultrawideband radiation with reflector antenna
Characteristics of a source of linearly-polarize
A high-power source of ultrawideband radiation with reflector antenna
Characteristics of a source of linearly-polarize