151 research outputs found
Moments of semileptonic B decay distributions in the 1/m_b expansion
We report the OPE-based predictions for a number of lepton energy and
hadronic mass moments in the inclusive semileptonic B -> X_c \ell\nu decays
with a lower cut on the charged lepton energy. We rely on the direct OPE
approach where no expansion in the charm mass is employed and the theoretical
input is a limited set of underlying OPE parameters including m_b and m_c. A
Wilsonian treatment with a `hard' cutoff is applied using running low-scale
masses m_Q(\mu) and kinetic expectation value \mu_\pi^2(\mu). This leaves for
perturbative corrections only genuinely short-distance effects and makes them
numerically small. Predictions are also given for the modified hadronic moments
of the kinematic variable \cal N_X^2 which is a combination of M_X^2 and E_X.
Measurement of such moments would allow a more reliable extraction of
higher-order nonperturbative heavy quark parameters from experiment.Comment: 16 pages, LaTeX, three figure
Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2
We present the results of measurements of the thermal conductivity of
Cu2Te2O5Br2, a compound where tetrahedra of Cu^{2+} ions carrying S=1/2 spins
form chains along the c-axis of the tetragonal crystal structure. The thermal
conductivity kappa was measured along both the c- and the a-direction as a
function of temperature between 3 and 300 K and in external magnetic fields H
up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct
features of kappa(T) were observed in the vicinity of T_N=11.4 K in zero
magnetic field. These features are unaltered in external fields which are
parallel to the c-axis, but are more pronounced when a field is applied
perpendicularly to the c-axis. The transition temperature increases upon
enhancing the external field, but only if the field is oriented along the
a-axis.Comment: 5 pages, 3 figure
Coherence length of an elongated condensate: a study by matter-wave interferometry
We measure the spatial correlation function of Bose-Einstein condensates in
the cross-over region between phase-coherent and strongly phase-fluctuating
condensates. We observe the continuous path from a gaussian-like shape to an
exponential-like shape characteristic of one-dimensional phase-fluctuations.
The width of the spatial correlation function as a function of the temperature
shows that the condensate coherence length undergoes no sharp transition
between these two regimes.Comment: 8 pages, 6 figure, submitted to EPJ
K Kbar photoproduction and S-P wave interference
Results of a new analysis of the photoproduction at two photon
energies and with a particular emphasis on the
-wave production are presented. We show that the proper treatment of all the
helicity components of the - and -waves enables one to eliminate the
reported discrepancies in extraction of the -wave photoproduction cross
section from experimental data.Comment: 25 pages, 5 figures, new predictions at 8 GeV added in Fig.
A new analysis of scattering from Roy and Steiner type equations
With the aim of generating new constraints on the OZI suppressed couplings of
chiral perturbation theory a set of six equations of the Roy and Steiner type
for the - and -waves of the scattering amplitudes is derived. The
range of validity and the multiplicity of the solutions are discussed. Precise
numerical solutions are obtained in the range E\lapprox 1 GeV which make use
as input, for the first time, of the most accurate experimental data available
at GeV for both and amplitudes.
Our main result is the determination of a narrow allowed region for the two
S-wave scattering lengths. Present experimental data below 1 GeV are found to
be in generally poor agreement with our results. A set of threshold expansion
parameters, as well as sub-threshold parameters are computed. For the latter,
matching with the SU(3) chiral expansion at NLO is performed.Comment: 45 pages, 17 figures. v2: New title, minor correction
Evidence for an Excess of Soft Photons in Hadronic Decays of Z^0
Soft photons inside hadronic jets converted in front of the DELPHI main
tracker (TPC) in events of qqbar disintegrations of the Z^0 were studied in the
kinematic range 0.2 < E_gamma < 1 GeV and transverse momentum with respect to
the closest jet direction p_T < 80 MeV/c. A clear excess of photons in the
experimental data as compared to the Monte Carlo predictions is observed. This
excess (uncorrected for the photon detection efficiency) is (1.17 +/- 0.06 +/-
0.27) x 10^{-3} gamma/jet in the specified kinematic region, while the expected
level of the inner hadronic bremsstrahlung (which is not included in the Monte
Carlo) is (0.340 +/- 0.001 +/- 0.038) x 10^{-3} gamma/jet. The ratio of the
excess to the predicted bremsstrahlung rate is then (3.4 +/- 0.2 +/- 0.8),
which is similar in strength to the anomalous soft photon signal observed in
fixed target experiments with hadronic beams.Comment: 37 pages, 9 figures, Accepted by Eur. Phys. J.
Measurement of the Strong Coupling alpha s from Four-Jet Observables in e+e- Annihilation
Data from e+e- annihilation into hadrons at centre-of-mass energies between
91 GeV and 209 GeV collected with the OPAL detector at LEP, are used to study
the four-jet rate as a function of the Durham algorithm resolution parameter
ycut. The four-jet rate is compared to next-to-leading order calculations that
include the resummation of large logarithms. The strong coupling measured from
the four-jet rate is alphas(Mz0)=
0.1182+-0.0003(stat.)+-0.0015(exp.)+-0.0011(had.)+-0.0012(scale)+-0.0013(mass)
in agreement with the world average. Next-to-leading order fits to the
D-parameter and thrust minor event-shape observables are also performed for the
first time. We find consistent results, but with significantly larger
theoretical uncertainties.Comment: 25 pages, 15 figures, Submitted to Euro. Phys. J.
ATLAS sensitivity to top quark and W boson polarization in events
Stringent tests on top quark production and decay mechanisms are provided by
the measurement of the top quark and W boson polarization. This paper presents
a detailed study of these two measurements with the ATLAS detector, in the
semileptonic (ttbar -> W W b bbar -> l nu j1 j2 b bbar) and dileptonic (ttbar
-> W W b bbar -> l nu l nu b bbar) ttbar channels. It is based on leading-order
Monte Carlo generators and on a fast simulation of the detector. A particular
attention is paid to the systematic uncertainties, which dominate the
statistical errors after one LHC year at low luminosity (10 fb^{-1}), and to
the background estimate. Combining results from both channel studies, the
longitudinal component of the W polarization (F\_0) can be measured with a 2%
accuracy and the right-handed component (F\_R) with a 1% precision with 10
fb^{-1}. Even though the top quarks in ttbar pairs are not polarized, a large
asymmetry is expected within the Standard Model in the like-spin versus
unlike-spin pair production. A 4% precision on this asymmetry measurement is
possible with 10 fb^{-1}, after combining results from both channel studies.
These promising results are converted in a sensitivity to new physics, such as
tWb anomalous couplings, top decay to charged Higgs boson, or new s-channels
(heavy resonance, gravitons) in ttbar production.Comment: 41+2 pages, 20 figures, ATLAS scientific note SN-ATLAS-2005-05
Planck Intermediate Results II: Comparison of SunyaevâZeldovich measurements from Planck and from the Arcminute Microkelvin Imager for 11 galaxy clusters
A comparison is presented of SunyaevâZeldovich measurements for 11 galaxy clusters as obtained by Planck and by the ground-based interferom- eter, the Arcminute Microkelvin Imager. Assuming a universal spherically-symmetric Generalised Navarro, Frenk & White (GNFW) model for the cluster gas pressure profile, we jointly constrain the integrated Compton-Y parameter (Y500) and the scale radius (Ξ500) of each cluster. Our resulting constraints in the Y500 â Ξ500 2D parameter space derived from the two instruments overlap significantly for eight of the clusters, although, overall, there is a tendency for AMI to find the SunyaevâZeldovich signal to be smaller in angular size and fainter than Planck. Significant discrepancies exist for the three remaining clusters in the sample, namely A1413, A1914, and the newly-discovered Planck cluster PLCKESZ G139.59+24.18. The robustness of the analysis of both the Planck and AMI data is demonstrated through the use of detailed simulations, which also discount confusion from residual point (radio) sources and from diffuse astrophysical foregrounds as possible explanations for the discrepancies found. For a subset of our cluster sample, we have investigated the dependence of our results on the assumed pressure profile by repeating the analysis adopting the best-fitting GNFW profile shape which best matches X-ray observations. Adopting the best-fitting profile shape from the X-ray data does not, in general, resolve the discrepancies found in this subset of five clusters. Though based on a small sample, our results suggest that the adopted GNFW model may not be sufficiently flexible to describe clusters universally
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