696 research outputs found
QCD sum rule analysis of the field strength correlator
The gauge invariant two-point correlator for the gluon field strength tensor
is analysed by means of the QCD sum rule method. To this end, we make use of a
relation of this correlator to a two-point function for a quark-gluon hybrid in
the limit of the quark mass going to infinity. From the sum rules a relation
between the gluon correlation length and the gluon condensate is obtained. We
briefly compare our results to recent determinations of the field strength
correlator on the lattice.Comment: 16 pages, 1 figur
Confining Properties of Abelian-Projected Theories and Field Strength Correlators
We review the string representations of Abelian-projected SU(2)- and
SU(3)-gauge theories and their application to the evaluation of bilocal field
strength correlators. The large distance asymptotic behaviours of the latter
ones are shown to be in agreement with the Stochastic Vacuum Model of QCD and
existing lattice data.Comment: Invited talk given at the Euroconference ``QCD 99'', 7-13th July
1999, Montpellier (France), to appear in Nucl. Phys. B (Proc. Suppl.
SrKZnMnAs: a ferromagnetic semiconductor with colossal magnetoresistance
A bulk diluted magnetic semiconductor (Sr,K)(Zn,Mn)As was
synthesized with decoupled charge and spin doping. It has a hexagonal
CaAlSi-type structure with the (Zn,Mn)As layer forming
a honeycomb-like network. Magnetization measurements show that the sample
undergoes a ferromagnetic transition with a Curie temperature of 12 K and
\revision{magnetic moment reaches about 1.5 /Mn under = 5 T
and = 2 K}. Surprisingly, a colossal negative magnetoresistance, defined as
, up to 38\% under a low field of = 0.1
T and to 99.8\% under = 5 T, was observed at = 2 K. The
colossal magnetoresistance can be explained based on the Anderson localization
theory.Comment: Accepted for publication in EP
Human Cytomegalovirus glycoprotein UL16 causes intracellular sequestration of NKG2D ligands, protecting against NK cell cytotoxicity.
The activating receptor, NKG2D, is expressed on a variety of immune effector cells and recognizes divergent families of major histocompatibility complex (MHC) class I-related ligands, including the MIC and ULBP proteins. Infection, stress, or transformation can induce NKG2D ligand expression, resulting in effector cell activation and killing of the ligand-expressing target cell. The human cytomegalovirus (HCMV) membrane glycoprotein, UL16, binds to three of the five known ligands for human NKG2D. UL16 is retained in the endoplasmic reticulum and cis-Golgi apparatus of cells and causes MICB to be similarly retained and stabilized within cells. Coexpression of UL16 markedly reduces cell surface levels of MICB, ULBP1, and ULBP2, and decreases susceptibility to natural killer cell-mediated cytotoxicity. Domain swapping experiments demonstrate that the transmembrane and cytoplasmic domains of UL16 are important for intracellular retention of UL16, whereas the ectodomain of UL16 participates in down-regulation of NKG2D ligands. The intracellular sequestration of NKG2D ligands by UL16 represents a novel HCMV immune evasion mechanism to add to the well-documented viral strategies directed against antigen presentation by classical MHC molecules
Energy Losses (Gains) of Massive Coloured Particles in Stochastic Colour Medium
The propagation of massive coloured particles in stochastic background
chromoelectric field is studied using the semiclassical equations of motion.
Depending on the nature of the stochastic background we obtain the formulae for
the energy losses of heavy coloured projectile in nonperturbative hadronic
medium and for the energy gains in the stochastic field present, e.g., in the
turbulent plasma. The result appears to be significantly dependent on the form
of the correlation function of stochastic external fieldComment: 9 pages, BI-TP 94/15, plain LaTe
The Extended Chiral Quark Model confronts QCD
We discuss the truncation of low energy effective action of QCD below the
chiral symmetry breaking (CSB) scale, including all operators of dimensionality
less or equal to 6 which can be built with quark and chiral fields. We perform
its bosonization in the scalar, pseudoscalar, vector and axial-vector channels
in the large-N_c and leading-log approximation. Constraints on the coefficients
of the effective lagrangian are derived from the requirement of Chiral Symmetry
Restoration (CSR) at energies above the CSB scale in the scalar-pseudoscalar
and vector-axial-vector channels, from matching to QCD at intermediate scales,
and by fitting some hadronic observables. In this truncation two types of
pseudoscalar states (massless pions and massive Pi'-mesons), as well as a
scalar, vector and axial-vector one arise as a consequence of dynamical chiral
symmetry breaking. Their masses and coupling constants as well as a number of
chiral structural constants are derived. A reasonable fit of all parameters
supports a relatively heavy scalar meson (quarkonium) with the mass \sim 1 GeV
and a small value of axial pion-quark coupling constant g_A \simeq 0.55.Comment: Talk at QCD99, Montpellier, July 1999, 7 pages, Late
Klein Tunnelling and the Klein Paradox
The Klein paradox is reassessed by considering the properties of a finite
square well or barrier in the Dirac equation. It is shown that spontaneous
positron emission occurs for a well if the potential is strong enough. The
vacuum charge and lifetime of the well are estimated. If the well is wide
enough, a seemingly constant current is emitted. These phenomena are transient
whereas the tunnelling first calculated by Klein is time-independent. Klein
tunnelling is a property of relativistic wave equations, not necessarily
connected to particle emission. The Coulomb potential is investigated in this
context: it is shown that a heavy nucleus of sufficiently large will bind
positrons. Correspondingly, it is expected that as increases the Coulomb
barrier will become increasingly transparent to positrons. This is an example
of Klein tunnelling.Comment: 17 page
Odd C-P contributions to diffractive processes
We investigate contributions to diffractive scattering, which are odd under
C- and P-parity. Comparison of p- and p-p scattering indicates that
these odderon contributions are very small and we show how a diquark clustering
in the proton can explain this effect. A good probe for the odderon exchange is
the photo- and electroproduction of pseudo-scalar mesons. We concentrate on the
pi^0 and show that the quasi elastic pi^0-production is again strongly
suppressed for a diquark structure of the proton whereas the cross sections for
diffractive proton dissociation are larger by orders of magnitude and rather
independent of the proton structure.Comment: 18 pages, LaTex2e, graphicx package, 14 eps figures include
Confinement in the Abelian-Higgs-type theories: string picture and field correlators
Field correlators and the string representation are used as two complementary
approaches for the description of confinement in the SU(N)-inspired dual
Abelian-Higgs-type model. In the London limit of the simplest, SU(2)-inspired,
model, bilocal electric field-strength correlators have been derived with
accounting for the contributions to these averages produced by closed dual
strings. The Debye screening in the plasma of such strings yields a novel
long-range interaction between points lying on the contour of the Wilson loop.
This interaction generates a Luescher-type term, even when one restrics oneself
to the minimal surface, as it is usually done in the bilocal approximation to
the stochastic vacuum model. Beyond the London limit, it has been shown that a
modified interaction appears, which becomes reduced to the standard Yukawa one
in the London limit. Finally, a string representation of the SU(N)-inspired
model with the theta-term, in the London limit, can be constructed.Comment: 17 pages, no figures, REVTeX 4; Invited contribution to the
collection of articles devoted to the 70th birthday of Yu.A. Simono
- …