11,102 research outputs found
Implementing Unitarity in Perturbation Theory
Unitarity cannot be perserved order by order in ordinary perturbation theory
because the constraint UU^\dagger=\1 is nonlinear. However, the corresponding
constraint for , being , is linear so it can be
maintained in every order in a perturbative expansion of . The perturbative
expansion of may be considered as a non-abelian generalization of the
linked-cluster expansion in probability theory and in statistical mechanics,
and possesses similar advantages resulting from separating the short-range
correlations from long-range effects. This point is illustrated in two QCD
examples, in which delicate cancellations encountered in summing Feynman
diagrams of are avoided when they are calculated via the perturbative expansion
of . Applications to other problems are briefly discussed.Comment: to appear in Phys. Rev.
Higher-order non-symmetric counterterms in pure Yang-Mills theory
We analyze the restoration of the Slavnov-Taylor (ST) identities for pure
massless Yang-Mills theory in the Landau gauge within the BPHZL renormalization
scheme with IR regulator. We obtain the most general form of the action-like
part of the symmetric regularized action, obeying the relevant ST identities
and all other relevant symmetries of the model, to all orders in the loop
expansion. We also give a cohomological characterization of the fulfillment of
BPHZL IR power-counting criterion, guaranteeing the existence of the limit
where the IR regulator goes to zero. The technique analyzed in this paper is
needed in the study of the restoration of the ST identities for those models,
like the MSSM, where massless particles are present and no invariant
regularization scheme is known to preserve the full set of ST identities of the
theory.Comment: Final version published in the journa
The b Distribution and the Velocity Structure of Absorption Peaks in the Lyman-Alpha Forest
A theory is developed which relates the observed b-parameter of a Lyman-alpha
absorption line to the velocity-curvature of the corresponding peak in the
optical depth fluctuation. Its relation to the traditional interpretation of b
as the thermal broadening width is discussed. It is demonstrated that,
independent of the details of the cosmological model, the differential b
distribution has a high b asymptote of , where , when we make the reasonable assumption that low-curvature fluctuations are
statistically favored over high-curvature ones. There in general always exist
absorption lines much broader than the thermal width. We then develop a linear
perturbative analysis of the optical depth fluctuation, which yields a
single-parameter prediction for the full b distribution: in addition to
exhibiting the general high velocity tail, it explains the observed sharp low b
cut-off. The dependence of the b distribution on cosmological parameters, such
as and the power spectrum, and reionization history as well as
observation/simulation resolution is derived and discussed.Comment: 18 pages, Latex with aaspp4.sty, submitted to Ap
Mutual information--based approach to adaptive homodyne detection of quantum optical states
I propose an approach to adaptive homodyne detection of digitally modulated
quantum optical pulses in which the phase of the local oscillator is chosen to
maximize the average information gain, i.e., the mutual information, at each
step of the measurement. I study the properties of this adaptive detection
scheme by considering the problem of classical information content of ensembles
of coherent states. Using simulations of quantum trajectories and
visualizations of corresponding measurement operators, I show that the proposed
measurement scheme adapts itself to the features of each ensemble. For all
considered ensembles of coherent states, it consistently outperforms heterodyne
detection and Wiseman's adaptive scheme for phase measurements [H.M. Wiseman,
Phys. Rev. Lett. 75, 4587 (1995)].Comment: Submutted to Phys. Rev.
No-Switching Quantum Key Distribution using Broadband Modulated Coherent Light
We realize an end-to-end no-switching quantum key distribution protocol using
continuous-wave coherent light. We encode weak broadband Gaussian modulations
onto the amplitude and phase quadratures of light beams at the Shannon's
information limit. Our no-switching protocol achieves high secret key rate via
a post-selection protocol that utilizes both quadrature information
simultaneously. We establish a secret key rate of 25 Mbits/s for a lossless
channel and 1 kbit/s, per 17 MHz of detected bandwidth, for 90% channel loss.
Since our scheme is truly broadband, it can potentially deliver orders of
magnitude higher key rates by extending the encoding bandwidth with higher-end
telecommunication technology.Comment: 5 pages, 3 figures, publishe
Self-aligned nanoscale SQUID on a tip
A nanometer-sized superconducting quantum interference device (nanoSQUID) is
fabricated on the apex of a sharp quartz tip and integrated into a scanning
SQUID microscope. A simple self-aligned fabrication method results in
nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An
aluminum nanoSQUID with an effective area of 0.034 m displays flux
sensitivity of 1.8 \mu_B/\mathrm{Hz}^{1/2}$ and high bandwidth, the SQUID on a tip is a highly
promising probe for nanoscale magnetic imaging and spectroscopy.Comment: 14 manuscript pages, 5 figure
Combined use of zoledronic acid and 153Sm-EDTMP in hormone-refractory prostate cancer patients with bone metastases
Purpose: 153Sm-ethylenediaminetetramethylenephosphonic
acid (EDTMP; Quadramet®) is indicated for the treatment of
painful bone metastases, whereas zoledronic acid (Zometa®)
is indicated for the prevention of skeletal complications.
Because of the different therapeutic effects, combining the
treatments may be beneficial. Both, however, accumulate in
areas with increased osteoblastic activity. Possible drug
interactions were investigated.
Methods: Patients with hormone-refractory prostate cancer
were treated with 18.5 MBq/kg 153Sm-EDTMP in weeks 1
and 3 and with 37 MBq/kg in week 15. Treatment with 4 mg
zoledronic acid began in week 3 and continued every
4 weeks through week 23. In weeks 3 and 15, zoledronic
acid was administered 2 days before 153Sm-EDTMP
treatment. Urine was collected 48 h after injection of
153Sm-EDTMP, and whole-body images were obtained 6,
24 and 48 h post-injection. The effect of zoledronic acid on
total bone uptake of 153Sm-EDTMP was measured indirectly
by the cumulative activity excreted in the urine in weeks 1, 3
and 15. Biodistribution, safety, tolerability and effect on
prostate-specific antigen level were also studied.
Results: The urinary excretion in week 3 divided by the
urinary excretion in week 1 (baseline) times 100% was
mean 98.4±11.6% (median 96.2%). From week 1 to 15,
after four zoledronic acid treatments, the mean ratio was
101.9±10.7% (median 101.8%). Bioequivalence could be
concluded by using a two-sample t test for both perprotocol
(n=13) and full-analysis sets (n=18). Toxicity was
comparable to of monotherapy with 153Sm-EDTMP.
Conclusion: Zoledronic acid treatment does not influence
153Sm-EDTMP skeletal uptake. Combined treatment is
feasible and safe
Landau gauge within the Gribov horizon
We consider a model which effectively restricts the functional integral of
Yang--Mills theories to the fundamental modular region. Using algebraic
arguments, we prove that this theory has the same divergences as ordinary Yang
Mills theory in the Landau gauge and that it is unitary. The restriction of the
functional integral is interpreted as a kind of spontaneous breakdown of the
symmetry.Comment: 17 pages, NYU-TH-93/10/0
Airy Distribution Function: From the Area Under a Brownian Excursion to the Maximal Height of Fluctuating Interfaces
The Airy distribution function describes the probability distribution of the
area under a Brownian excursion over a unit interval. Surprisingly, this
function has appeared in a number of seemingly unrelated problems, mostly in
computer science and graph theory. In this paper, we show that this
distribution also appears in a rather well studied physical system, namely the
fluctuating interfaces. We present an exact solution for the distribution
P(h_m,L) of the maximal height h_m (measured with respect to the average
spatial height) in the steady state of a fluctuating interface in a one
dimensional system of size L with both periodic and free boundary conditions.
For the periodic case, we show that P(h_m,L)=L^{-1/2}f(h_m L^{-1/2}) for all L
where the function f(x) is the Airy distribution function. This result is valid
for both the Edwards-Wilkinson and the Kardar-Parisi-Zhang interfaces. For the
free boundary case, the same scaling holds P(h_m,L)=L^{-1/2}F(h_m L^{-1/2}),
but the scaling function F(x) is different from that of the periodic case. We
compute this scaling function explicitly for the Edwards-Wilkinson interface
and call it the F-Airy distribution function. Numerical simulations are in
excellent agreement with our analytical results. Our results provide a rather
rare exactly solvable case for the distribution of extremum of a set of
strongly correlated random variables. Some of these results were announced in a
recent Letter [ S.N. Majumdar and A. Comtet, Phys. Rev. Lett., 92, 225501
(2004)].Comment: 27 pages, 10 .eps figures included. Two figures improved, new
discussion and references adde
- …