358 research outputs found
Coherent radiation from neutral molecules moving above a grating
We predict and study the quantum-electrodynamical effect of parametric
self-induced excitation of a molecule moving above the dielectric or conducting
medium with periodic grating. In this case the radiation reaction force
modulates the molecular transition frequency which results in a parametric
instability of dipole oscillations even from the level of quantum or thermal
fluctuations. The present mechanism of instability of electrically neutral
molecules is different from that of the well-known Smith-Purcell and transition
radiation in which a moving charge and its oscillating image create an
oscillating dipole.
We show that parametrically excited molecular bunches can produce an easily
detectable coherent radiation flux of up to a microwatt.Comment: 4 page
Electronic Collective Modes and Superconductivity in Layered Conductors
A distinctive feature of layered conductors is the presence of low-energy
electronic collective modes of the conduction electrons. This affects the
dynamic screening properties of the Coulomb interaction in a layered material.
We study the consequences of the existence of these collective modes for
superconductivity. General equations for the superconducting order parameter
are derived within the strong-coupling phonon-plasmon scheme that account for
the screened Coulomb interaction. Specifically, we calculate the
superconducting critical temperature Tc taking into account the full
temperature, frequency and wave-vector dependence of the dielectric function.
We show that low-energy plasmons may contribute constructively to
superconductivity. Three classes of layered superconductors are discussed
within our model: metal-intercalated halide nitrides, layered organic materials
and high-Tc oxides. In particular, we demonstrate that the plasmon contribution
(electronic mechanism) is dominant in the first class of layered materials. The
theory shows that the description of so-called ``quasi-two-dimensional
superconductors'' cannot be reduced to a purely 2D model, as commonly assumed.
While the transport properties are strongly anisotropic, it remains essential
to take into account the screened interlayer Coulomb interaction to describe
the superconducting state of layered materials.Comment: Final version (minor changes) 14 pages, 6 figure
Spontaneous emission in a planar Fabry-Perot microcavity
Published versio
Measurement of the diffractive structure function in deep inelastic scattering at HERA
This paper presents an analysis of the inclusive properties of diffractive
deep inelastic scattering events produced in interactions at HERA. The
events are characterised by a rapidity gap between the outgoing proton system
and the remaining hadronic system. Inclusive distributions are presented and
compared with Monte Carlo models for diffractive processes. The data are
consistent with models where the pomeron structure function has a hard and a
soft contribution. The diffractive structure function is measured as a function
of \xpom, the momentum fraction lost by the proton, of , the momentum
fraction of the struck quark with respect to \xpom, and of . The \xpom
dependence is consistent with the form \xpoma where
in all bins of and
. In the measured range, the diffractive structure function
approximately scales with at fixed . In an Ingelman-Schlein type
model, where commonly used pomeron flux factor normalisations are assumed, it
is found that the quarks within the pomeron do not saturate the momentum sum
rule.Comment: 36 pages, latex, 11 figures appended as uuencoded fil
Observation of hard scattering in photoproduction events with a large rapidity gap at HERA
Events with a large rapidity gap and total transverse energy greater than 5
GeV have been observed in quasi-real photoproduction at HERA with the ZEUS
detector. The distribution of these events as a function of the
centre of mass energy is consistent with diffractive scattering. For total
transverse energies above 12 GeV, the hadronic final states show predominantly
a two-jet structure with each jet having a transverse energy greater than 4
GeV. For the two-jet events, little energy flow is found outside the jets. This
observation is consistent with the hard scattering of a quasi-real photon with
a colourless object in the proton.Comment: 19 pages, latex, 4 figures appended as uuencoded fil
Search for supersymmetry with a dominant R-parity violating LQDbar couplings in e+e- collisions at centre-of-mass energies of 130GeV to 172 GeV
A search for pair-production of supersymmetric particles under the assumption
that R-parity is violated via a dominant LQDbar coupling has been performed
using the data collected by ALEPH at centre-of-mass energies of 130-172 GeV.
The observed candidate events in the data are in agreement with the Standard
Model expectation. This result is translated into lower limits on the masses of
charginos, neutralinos, sleptons, sneutrinos and squarks. For instance, for
m_0=500 GeV/c^2 and tan(beta)=sqrt(2) charginos with masses smaller than 81
GeV/c^2 and neutralinos with masses smaller than 29 GeV/c^2 are excluded at the
95% confidence level for any generation structure of the LQDbar coupling.Comment: 32 pages, 30 figure
Measurement of the W-pair cross section in collisions at 172 GeV
The e+e- --> W+W- cross section is measured in a data sample collected by ALEPH at a mean centre--of--mass energy of 172.09 GEV, corresponding to an integrated luminosity of 10.65 pb-1. Cross sections are given for the three topologies, fully leptonic, semi-leptonic and hadronic of a W-pair decay. Under the assumption that no other decay modes are present, the W-pair cross section is measured to be 11.7 +- 1.2 (stat.) +- 0.3 (syst.) pb. The existence of the triple gauge boson vertex of the Standard Model is clearly preferred by the data. The decay branching ratio of the W boson into hadrons is measured to be B(W --> hadrons) = 67.7 +- 3.1 (stat.) +- 0.7 (syst.)%, allowing a determination of the CKM matrix element |Vcs|= 0.98 +- 0.14 (stat.) +- 0.03 (syst.)
Measurement of the W mass in collisions at 183 GeV
The mass of the W boson is obtained from reconstructed invariant mass distributions in W-pair events. The sample of W pairs is selected from 57 pb collected with the ALEPH detector in 1997 at a centre-of-mass energy of 183 GeV. The invariant mass distributions of reweighted Monte Carlo events are fitted separately to the experimental distributions in the and all l\nuqqbar channels to give the following W masses: , where the theory error represents the possible effects of final state interactions. The combination of these two measurements, including the LEP energy calibration uncertainty, gives $m_{W} = 80.393 \pm 0.128(stat.)\pm 0.041(syst.) \pm 0.028(theory)\pm 0.021(LEP) GeV/c^2
Search for R-Parity Violating Decays of Supersymmetric Particles in Collisions at Centre-of-Mass Energies near 183 GeV
Searches for pair-production of supersymmetric particles under the assumption that R-parity is violated via a single dominant , or coupling are performed using the data collected by the \ALEPH\ collaboration at centre-of-mass energies of 181--184~\gev. The observed candidate events in the data are in agreement with the Standard Model expectations. Upper limits on the production cross-sections and lower limits on the masses of charginos, sleptons, squarks and sneutrinos are de rived
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