189 research outputs found
Hard pion bremsstrahlung in the Coulomb region
Hard high-energy pion-nucleus bremsstrahlung, ,
is studied in the Coulomb region, i.e. the small-angle region where the nuclear
scattering is dominated by the Coulomb interaction. Special attention is
focussed on the possibility of measuring the pion polarizability in such
reactions. We study the sensitivity to the structure of the underlying the
pion-Compton amplitude through a model with , , and a_1
exchanges. It is found that the effective energy in the virtual pion-Compton
scattering is often so large that the threshold approximation does not apply.Comment: 18 pages, 5 figure
Production of Polarized Vector Mesons off Nuclei
Using the light-cone QCD dipole formalism we investigate manifestations of
color transparency (CT) and coherence length (CL) effects in electroproduction
of longitudinally (L) and transversally (T) polarized vector mesons. Motivated
by forthcoming data from the HERMES experiment we predict both the A and Q^2
dependence of the L/T- ratios, for rho^0 mesons produced coherently and
incoherently off nuclei. For an incoherent reaction the CT and CL effects add
up and result in a monotonic A dependence of the L/T-ratio at different values
of Q^2. On the contrary, for a coherent process the contraction of the CL with
Q^2 causes an effect opposite to that of CT and we expect quite a nontrivial A
dependence, especially at Q^2 >> m_V^2.Comment: Revtex 24 pages and 14 figure
N=4 central charge superspace at work for supergravity coupled to an arbitrary number of abelian vector multiplets
We present the description in central charge superspace of N=4 supergravity
with antisymmetric tensor coupled to an arbitrary number of abelian vector
multiplets. All the gauge vectors of the coupled system are treated on the same
footing as gauge fields corresponding to translations along additional bosonic
coordinates. It is the geometry of the antisymmetric tensor which singles out
which combinations of these vectors belong to the supergravity multiplet and
which are the additional coupled ones. Moreover, basic properties of
Chapline-Manton coupling mechanism, as well as the SO(6,n)/SO(6)*SO(n) sigma
model of the Yang-Mills scalars are found as arising from superspace geometry.Comment: 18 page
Chiral Anomaly and
Measurement of the process has revealed a possible conflict
with what should be a solid prediction generated by the chiral anomaly. We show
that inclusion of appropirate energy-momentum dependence in the matrix element
reduces the discrepancy.Comment: 8 page standard Latex fil
Magnetooptical Study of Zeeman Effect in Mn modulation-doped InAs/InGaAs/InAlAs Quantum Well Structures
We report on a magneto-photoluminescence (PL) study of Mn modulation-doped
InAs/InGaAs/InAlAs quantum wells. Two PL lines corresponding to the radiative
recombination of photoelectrons with free and bound-on-Mn holes have been
observed. In the presence of a magnetic field applied in the Faraday geometry
both lines split into two circularly polarized components. While temperature
and magnetic field dependences of the splitting are well described by the
Brillouin function, providing an evidence for exchange interaction with spin
polarized manganese ions, the value of the splitting exceeds the expected value
of the giant Zeeman splitting by two orders of magnitude for a given Mn
density. Possible reasons of this striking observation are discussed
Spin-polarized electric currents in diluted magnetic semiconductor heterostructures induced by terahertz and microwave radiation
We report on the study of spin-polarized electric currents in diluted
magnetic semiconductor (DMS) quantum wells subjected to an in-plane external
magnetic field and illuminated by microwave or terahertz radiation. The effect
is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum wells (QWs) and (In,Ga)As/InAlAs:Mn
QWs belonging to the well known II-VI and III-V DMS material systems, as well
as, in heterovalent AlSb/InAs/(Zn,Mn)Te QWs which represent a promising
combination of II-VI and III-V semiconductors. Experimental data and developed
theory demonstrate that the photocurrent originates from a spin-dependent
scattering of free carriers by static defects or phonons in the Drude
absorption of radiation and subsequent relaxation of carriers. We show that in
DMS structures the efficiency of the current generation is drastically enhanced
compared to non-magnetic semiconductors. The enhancement is caused by the
exchange interaction of carrier spins with localized spins of magnetic ions
resulting, on the one hand, in the giant Zeeman spin-splitting, and, on the
other hand, in the spin-dependent carrier scattering by localized Mn2+ ions
polarized by an external magnetic field.Comment: 14 pages, 13 figure
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