7,691 research outputs found
Single top or bottom production associated with a scalar in \gamma p collision as a probe of topcolor-assisted technicolor
In the framework of the topcolor-assisted technicolor (TC2) models, we study
the productions of a single top or bottom quark associated with a scalar in
\gamma-p collision, which proceed via the subprocesses c\gamma -> t\pi_t^0,
c\gamma -> t h_t^0 and c\gamma -> b\pi^+_t mediated by the anomalous top or
bottom coupling tc\pi_t^0, tch_t^0 and bc\pi_t^+. These productions, while
extremely suppressed in the Standard Model, are found to be significantly
enhanced in the large part of the TC2 parameter space, especially the
production via c\gamma -> b\pi^+ can have a cross section of 100 fb, which may
be accessible and allow for a test of the TC2 models.Comment: 13 pages, 4 figures, comments and references adde
Effects of accidental microconstriction on the quantized conductance in long wires
We have investigated the conductance of long quantum wires formed in
GaAs/AlGaAs heterostructures. Using realistic fluctuation potentials from donor
layers we have simulated numerically the conductance of four different kinds of
wires. While ideal wires show perfect quantization, potential fluctuations from
random donors may give rise to strong conductance oscillations and degradation
of the quantization plateaux. Statistically there is always the possibility of
having large fluctuations in a sample that may effectively act as a
microconstriction. We therefore introduce microconstrictions in the wires by
occasional clustering of donors. These microconstrictions are found to restore
the quantized plateaux. A similar effect is found for accidental lithographic
inaccuracies.Comment: 4 pages, 2 figures, paper for NANO2002 symposium, will appear in SPIE
proceeding
Quantum authentication with key recycling
We show that a family of quantum authentication protocols introduced in
[Barnum et al., FOCS 2002] can be used to construct a secure quantum channel
and additionally recycle all of the secret key if the message is successfully
authenticated, and recycle part of the key if tampering is detected. We give a
full security proof that constructs the secure channel given only insecure
noisy channels and a shared secret key. We also prove that the number of
recycled key bits is optimal for this family of protocols, i.e., there exists
an adversarial strategy to obtain all non-recycled bits. Previous works
recycled less key and only gave partial security proofs, since they did not
consider all possible distinguishers (environments) that may be used to
distinguish the real setting from the ideal secure quantum channel and secret
key resource.Comment: 38+17 pages, 13 figures. v2: constructed ideal secure channel and
secret key resource have been slightly redefined; also added a proof in the
appendix for quantum authentication without key recycling that has better
parameters and only requires weak purity testing code
Kondo Effect in a Quantum Antidot
We report Kondo-like behaviour in a quantum antidot (a submicron depleted
region in a two-dimensional electron gas) in the quantum-Hall regime. When both
spin branches of the lowest Landau level encircle the antidot in a magnetic
field ( T), extra resonances occur between extended edge states via
antidot bound states when tunnelling is Coulomb blockaded. These resonances
appear only in alternating Coulomb-blockaded regions, and become suppressed
when the temperature or source-drain bias is raised. Although the exact
mechanism is unknown, we believe that Kondo-like correlated tunnelling arises
from skyrmion-type edge reconstruction. This observation demonstrates the
generality of the Kondo phenomenon.Comment: 9 pages, 3 figures (Fig.3 in colour), to appear in Phys. Rev. Let
Rotating light, OAM paradox and relativistic complex scalar field
Recent studies show that the angular momentum, both spin and orbital, of
rotating light beams possesses counter-intuitive characteristics. We present a
new approach to the question of orbital angular momentum of light based on the
complex massless scalar field representation of light. The covariant equation
for the scalar field is treated in rotating system using the general
relativistic framework. First we show the equivalence of the U(1) gauge current
for the scalar field with the Poynting vector continuity equation for paraxial
light, and then apply the formalism to the calculation of the orbital angular
momentum of rotating light beams. If the difference between the co-, contra-,
and physical quantities is properly accounted for there does not result any
paradox in the orbital angular momentum of rotating light. An artificial
analogue of the paradoxical situation could be constructed but it is wrong
within the present formalism. It is shown that the orbital angular momentum of
rotating beam comprising of modes with opposite azimuthal indices corresponds
to that of rigid rotation. A short review on the electromagnetism in
noninertial systems is presented to motivate a fully covariant Maxwell field
approach in rotating system to address the rotating light phenomenon.Comment: No figure
Operation of EMEP ‘supersites’ in the United Kingdom. Annual report for 2008.
As part of its commitment to the UN-ECE Convention on Long-range Transboundary Air Pollution the United Kingdom operates two ‘supersites’ reporting data to the Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe (EMEP).
This report provides the annual summary for 2008, the second full calendar year of operation of the first EMEP ‘supersite’ to be established in the United Kingdom. Detailed operational reports have been submitted to Defra every 3 months, with unratified data. This annual report contains a summary of the ratified data for 2008.
The EMEP ‘supersite’ is located in central southern Scotland at Auchencorth (3.2oW, 55.8oN), a remote rural moorland site ~20 km south-west of Edinburgh. Monitoring operations started formally on 1 June 2006.
In addition to measurements made specifically under this contract, the Centre for Ecology & Hydrology also acts as local site operator for measurements made under other UK monitoring networks: the Automated Urban and Rural Network (AURN), the UK Eutrophication and Acidification Network (UKEAP), the UK Hydrocarbons Network, and the UK Heavy Metals Rural Network. Some measurements were also made under the auspices of the ‘Air Pollution Deposition Processes’ contract. All these associated networks are funded by Defra.
This report summarises the measurements made between January and December 2008, and presents summary statistics on average concentrations.
The site is dominated by winds from the south-west, but wind direction data highlight potential sources of airborne pollutants (power stations, conurbations).
The average diurnal patterns of gases and particles are consistent with those expected for a remote rural site.
The frequency distributions are presented for data where there was good data capture throughout the whole period. Some components (e.g. black carbon) show log-normal frequency distributions, while other components (e.g. ozone) have more nearly normal frequency distributions.
A case study is presented for a period in June 2008, showing the influence of regional air pollutants at this remote rural site.
All the data reported under the contract are shown graphically in the Appendix
Polynomial sequences for bond percolation critical thresholds
In this paper, I compute the inhomogeneous (multi-probability) bond critical
surfaces for the (4,6,12) and (3^4,6) lattices using the linearity
approximation described in (Scullard and Ziff, J. Stat. Mech. P03021),
implemented as a branching process of lattices. I find the estimates for the
bond percolation thresholds, p_c(4,6,12)=0.69377849... and
p_c(3^4,6)=0.43437077..., compared with Parviainen's numerical results of p_c
\approx 0.69373383 and p_c \approx 0.43430621 . These deviations are of the
order 10^{-5}, as is standard for this method, although they are outside
Parviainen's typical standard error of 10^{-7}. Deriving thresholds in this way
for a given lattice leads to a polynomial with integer coefficients, the root
in [0,1] of which gives the estimate for the bond threshold. I show how the
method can be refined, leading to a sequence of higher order polynomials making
predictions that likely converge to the exact answer. Finally, I discuss how
this fact hints that for certain graphs, such as the kagome lattice, the exact
bond threshold may not be the root of any polynomial with integer coefficients.Comment: submitted to Journal of Statistical Mechanic
Photon wave mechanics and position eigenvectors
One and two photon wave functions are derived by projecting the quantum state
vector onto simultaneous eigenvectors of the number operator and a recently
constructed photon position operator [Phys. Rev A 59, 954 (1999)] that couples
spin and orbital angular momentum. While only the Landau-Peierls wave function
defines a positive definite photon density, a similarity transformation to a
biorthogonal field-potential pair of positive frequency solutions of Maxwell's
equations preserves eigenvalues and expectation values. We show that this real
space description of photons is compatible with all of the usual rules of
quantum mechanics and provides a framework for understanding the relationships
amongst different forms of the photon wave function in the literature. It also
gives a quantum picture of the optical angular momentum of beams that applies
to both one photon and coherent states. According to the rules of qunatum
mechanics, this wave function gives the probability to count a photon at any
position in space.Comment: 14 pages, to be published in Phys. Rev.
Coulomb Charging Effects in an Open Quantum Dot
Low-temperature transport properties of a lateral quantum dot formed by
overlaying finger gates in a clean one-dimensional channel are investigated.
Continuous and periodic oscillations superimposed upon ballistic conductance
steps are observed, when the conductance G of the dot changes within a wide
range 0<G<6e^2/h. Calculations of the electrostatics confirm that the measured
periodic conductance oscillations correspond to successive change of the total
charge of the dot by . By modelling the transport it is shown that the
progression of the Coulomb oscillations into the region G>2e^2/h may be due to
suppression of inter-1D-subband scattering. Fully transmitted subbands
contribute to coherent background of conductance, while sequential tunneling
via weakly transmitted subbands leads to Coulomb charging of the dot.Comment: 12 pages, RevTeX, 15 eps figures included, submitted to Phys. Rev.
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