1,464 research outputs found
A next-to-leading order study of photon-pion and pion pair hadro-production in the light of the Higgs boson search at the LHC
We discuss the production of photon-pion and pion pairs with a large
invariant mass at collider energies. We present a study based on a perturbative
QCD calculation at full next-to-leading order accuracy, implemented in the
computer programme DIPHOX. We give estimations for various observables, which
concern the reducible background to the Higgs boson search in the channel H -->
gamma gamma, in the mass range 80-140 GeV at the LHC. We critically discuss the
reliability of these estimates due to our imperfect knowledge of fragmentation
functions at high z and a subtle interplay between higher order corrections and
realistic experimental cuts. We conclude that, whereas the invariant mass
spectrum of photon-pion pairs is theoretically better under control, in the
dipion case large uncertainties remain.Comment: 26 pages Latex, 14 eps figures, replaced by published versio
Large-p_T Inclusive pi^0 Cross Sections and Next-to-Leading-Order QCD Predictions
We review the phenomenology of pi^0 production at large transverse momentum
in proton-induced collisions. Uncertainties in the next-to-leading-order
predictions of Quantum Chromodynamics are discussed. The comparison with data
reveals that the disagreement between theory and experiment lies essentially in
an overall normalization factor. The situation for pi^0 production is
contrasted with that of prompt-photon production in hadronic collisions.Comment: 21 pages (Latex), 13 figures (Postscript
A critical phenomenological study of inclusive photon production in hadronic collisions
We discuss fixed target and ISR inclusive photon production and attempt a
comparison between theory and experiments. The dependence of the theoretical
predictions on the structure functions, and on the renormalization and
factorization scales is investigated. The main result of this study is that the
data cannot be simultaneously fitted with a single set of scales and structure
functions. On the other hand, there is no need for an additional intrinsic
to force the agreement between QCD predictions and experiments, with
the possible exception of one data set. Since the data cover almost overlapping
kinematical ranges this raises the question of consistency among data sets. A
comparative discussion of some possible sources of experimental uncertainties
is sketched.Comment: 22 pages, 3 tables, 10 figures, Late
Spin-Exchange Interaction in ZnO-based Quantum Wells
Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit
unprecedented tunability of the short-range spin exchange interaction. In the
context of large exciton binding energies and electron-hole exchange
interaction in ZnO, this tunability results from the competition between
quantum confinement and giant quantum confined Stark effect. By using
time-resolved photoluminescence we identify, for well widths under 3 nm, the
redistribution of oscillator strengths between the A and B excitonic
transitions, due to the enhancement of the exchange interaction. Conversely,
for wider wells, the redistribution is cancelled by the dominant effect of
internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure
λ/2 fringe-spacing interferometer
International audienceThe precision of interferometry is directly linked to the fringe spacing of the recorded interferogram. Whereas all interferometric devices show a fringe spacing equal to a wavelength of the laser light we present a novel scheme of a two-beam interferometer exhibiting a fringe spacing reduced by a factor of 2; the direct detection of the beat signal is replaced with the monitoring of the fluorescence of a twofold degenerate atomic system resonant with the laser. The λ/2 fringe spacing in the fluorescence signal is demonstrated with a hot sodium vapor excited by a broadband laser tuned to the D1 line. In the saturation regime, the dark fringes are expected to be extremely narrow, leading to the possibility of nanoscale displacement measurements or atom localization
Statistical properties of frequency shifted feedback lasers
International audienceWe evidence experimentally the statistical properties of frequency shifted feedback (FSF) lasers through measurements of the homodyne beat signal and interferometric autocorrelation of a dye FSF laser at the output of a Michelson interferometer. The FSF laser is found to show thermal fluctuations and photon bunching. Moreover whereas the degree of first-order coherence vanishes beyond the coherence length of the FSF source, the degree of second-order coherence exhibits periodic revivals far beyond the coherence length, with a period equal to the cavity roundtrip time. Our observations are in good agreement with the theoretical treatment of Yatsenko et al. [L.P. Yatsenko, B.W. Shore, K. Bergmann, Opt. Comm. 236 (2004) 183] and validate the description of the output field of a FSF laser by a broadband cyclostationary thermal field
Coherence properties of modeless lasers
International audienceMost of classical light sources show a close similarity between their first and second order correlation functions (resp. g(1) and g(2)) functions. We present here the original coherence properties of a peculiar type of laser named modeless laser or Frequency Shifted Feedback (FSF) laser where the g(1) and g(2) functions show a different behaviour. We calculate and evidence experimentally the first and second order correlation functions of modeless lasers, through measurements of the homodyne beat signal and interferometric autocorrelation of a dye FSF laser at the output of a Michelson interferometer. Whereas the degree of first-order coherence vanishes beyond the coherence length of the FSF source, the degree of second-order coherence exhibits periodic revivals far beyond the coherence length, with a period equal to the cavity roundtrip time. Our observations are in good agreement with the theoretical treatment of Yatsenko et al. (Opt. Comm. 282 (2009) 300) [1]
Polarization measurements analysis II. Best estimators of polarization fraction and angle
With the forthcoming release of high precision polarization measurements,
such as from the Planck satellite, it becomes critical to evaluate the
performance of estimators for the polarization fraction and angle. These two
physical quantities suffer from a well-known bias in the presence of
measurement noise, as has been described in part I of this series. In this
paper, part II of the series, we explore the extent to which various estimators
may correct the bias. Traditional frequentist estimators of the polarization
fraction are compared with two recent estimators: one inspired by a Bayesian
analysis and a second following an asymptotic method. We investigate the
sensitivity of these estimators to the asymmetry of the covariance matrix which
may vary over large datasets. We present for the first time a comparison among
polarization angle estimators, and evaluate the statistical bias on the angle
that appears when the covariance matrix exhibits effective ellipticity. We also
address the question of the accuracy of the polarization fraction and angle
uncertainty estimators. The methods linked to the credible intervals and to the
variance estimates are tested against the robust confidence interval method.
From this pool of estimators, we build recipes adapted to different use-cases:
build a mask, compute large maps, and deal with low S/N data. More generally,
we show that the traditional estimators suffer from discontinuous distributions
at low S/N, while the asymptotic and Bayesian methods do not. Attention is
given to the shape of the output distribution of the estimators, and is
compared with a Gaussian. In this regard, the new asymptotic method presents
the best performance, while the Bayesian output distribution is shown to be
strongly asymmetric with a sharp cut at low S/N.Finally, we present an
optimization of the estimator derived from the Bayesian analysis using adapted
priors
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