373 research outputs found
Parallel Evaluation of Multi-join Queries
A number of execution strategies for parallel evaluation of multi-join queries have been proposed in the literature. In this paper we give a comparative performance evaluation of four execution strategies by implementing all of them on the same parallel database system, PRISMA/DB. Experiments have been done up to 80 processors. These strategies, coming from the literature, are named: Sequential Parallel, Synchronous Execution, Segmented Right-Deep, and Full Parallel. Based on the experiments clear guidelines are given when to use which strategy.
This is an extended abstract; the full paper appeared in Proc. ACM SIGMOD'94, Minneapolis, Minnesota, May 24–27, 199
Measurement of the branching ratio for beta-delayed alpha decay of 16N
While the 12C(a,g)16O reaction plays a central role in nuclear astrophysics,
the cross section at energies relevant to hydrostatic helium burning is too
small to be directly measured in the laboratory. The beta-delayed alpha
spectrum of 16N can be used to constrain the extrapolation of the E1 component
of the S-factor; however, with this approach the resulting S-factor becomes
strongly correlated with the assumed beta-alpha branching ratio. We have
remeasured the beta-alpha branching ratio by implanting 16N ions in a segmented
Si detector and counting the number of beta-alpha decays relative to the number
of implantations. Our result, 1.49(5)e-5, represents a 24% increase compared to
the accepted value and implies an increase of 14% in the extrapolated S-factor
Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems
Inclusive and exclusive hard-photon (E 30 MeV) production in five
different heavy-ion reactions (Ar+Au, Ag, Ni,
C at 60{\it A} MeV and Xe+Sn at 50{\it A} MeV) has been
studied coupling the TAPS photon spectrometer with several charged-particle
multidetectors covering more than 80% of 4. The measured spectra, slope
parameters and source velocities as well as their target-dependence, confirm
the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon
collisions that accounts for roughly 20% of the total hard-photon yield. The
thermal slopes are a direct measure of the temperature of the excited nuclear
systems produced during the reaction.Comment: 4 pages, 3 figures, Proceedings CRIS 2000, 3rd Catania Relativistic
Ion Studies, "Phase Transitions in Strong Interactions: Status and
Perspectives", Acicastello, Italy, May 22-26, 2000 (to be published in Nuc.
Phys. A
Emission patterns of neutral pions in 40 A MeV Ta+Au reactions
Differential cross sections of neutral pions emitted in 181Ta + 197Au
collisions at a beam energy of 39.5A MeV have been measured with the photon
spectrometer TAPS. The kinetic energy and transverse momentum spectra of
neutral pions cannot be properly described in the framework of the thermal
model, nor when the reabsorption of pions is accounted for in a
phenomenological model. However, high energy and high momentum tails of the
pion spectra can be well fitted through thermal distributions with unexpectedly
soft temperature parameters below 10 MeV.Comment: 16 pages (double-spaced), 5 figures; corrections after referee's
comments and suggestion
Hard photon and neutral pion production in cold nuclear matter
The production of hard photons and neutral pions in 190 MeV proton induced
reactions on C, Ca, Ni, and W targets has been for the first time concurrently
studied. Angular distributions and energy spectra up to the kinematical limit
are discussed and the production cross-sections are presented. From the target
mass dependence of the cross-sections the propagation of pions through nuclear
matter is analyzed and the production mechanisms of hard photons and primordial
pions are derived. It is found that the production of subthreshold particles
proceeds mainly through first chance nucleon-nucleon collisions. For the most
energetic particles the mass scaling evidences the effect of multiple
collisions.Comment: submitted to Phys. Lett.
Evidence for Thermal Equilibration in Multifragmentation Reactions probed with Bremsstrahlung Photons
The production of nuclear bremsstrahlung photons (E 30 MeV) has
been studied in inclusive and exclusive measurements in four heavy-ion
reactions at 60{\it A} MeV. The measured photon spectra, angular distributions
and multiplicities indicate that a significant part of the hard-photons are
emitted in secondary nucleon-nucleon collisions from a thermally equilibrated
system. The observation of the thermal component in multi-fragment
Ar+Au reactions suggests that the breakup of the thermalized
source produced in this system occurs on a rather long time-scale.Comment: Revised version, accepted for publication in Physical Review Letters.
4 pages, 4 fig
Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions
Photon energy spectra up to the kinematic limit have been measured in 190 MeV
proton reactions with light and heavy nuclei to investigate the influence of
the multiple-scattering process on the photon production. Relative to the
predictions of models based on a quasi-free production mechanism a strong
suppression of bremsstrahlung is observed in the low-energy region of the
photon spectrum. We attribute this effect to the interference of photon
amplitudes due to multiple scattering of nucleons in the nuclear medium.Comment: 12 pages, 3 figures, submitted to Phys. Rev. Let
Observation of two new baryon resonances
Two structures are observed close to the kinematic threshold in the mass spectrum in a sample of proton-proton collision data, corresponding
to an integrated luminosity of 3.0 fb recorded by the LHCb experiment.
In the quark model, two baryonic resonances with quark content are
expected in this mass region: the spin-parity and
states, denoted and .
Interpreting the structures as these resonances, we measure the mass
differences and the width of the heavier state to be
MeV,
MeV,
MeV, where the first and second
uncertainties are statistical and systematic, respectively. The width of the
lighter state is consistent with zero, and we place an upper limit of
MeV at 95% confidence level. Relative
production rates of these states are also reported.Comment: 17 pages, 2 figure
Precision measurement of violation in decays
The time-dependent asymmetry in decays is
measured using collision data, corresponding to an integrated luminosity
of fb, collected with the LHCb detector at centre-of-mass energies
of and TeV. In a sample of 96 000 decays, the
-violating phase is measured, as well as the decay widths
and of the light and heavy mass eigenstates of the
system. The values obtained are rad, ps, andps, where the first uncertainty is
statistical and the second systematic. These are the most precise single
measurements of those quantities to date. A combined analysis with decays gives rad. All
measurements are in agreement with the Standard Model predictions. For the
first time the phase is measured independently for each polarisation
state of the system and shows no evidence for polarisation
dependence.Comment: 6 figure
First observation and amplitude analysis of the decay
The decay is observed in a data sample
corresponding to of collision data recorded by the LHCb
experiment during 2011 and 2012. Its branching fraction is measured to be
where the uncertainties are statistical, systematic and from
the branching fraction of the normalisation channel , respectively. An amplitude analysis of the resonant
structure of the decay is used to measure the
contributions from quasi-two-body ,
, and
decays, as well as from nonresonant sources. The
resonance is determined to have spin~1.Comment: 39 pages, 10 figures, submitted to Phys. Rev. D. Updated following
erratum 10.1103/PhysRevD.93.11990
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