3,905 research outputs found
Prompt Jpsi and b->Jpsi X production in pp collisions
International audienceThe LHCb measurement of the J/Ï production cross-section is presented. The double differential cross-section is measured as a function of the J/Ï transverse momentum pT and of the J/Ï rapidity y in the fiducial region pTâ[0;14],GeV/c and yâ[2.0;4.5]. The analysis is based on a sample of 5.2 pbâ1 collected in September 2010 at the pp Large Hadron Collider at CERN, at a centre-of-mass energy of s = 7TeV. The contributions from prompt J/Ï and J/Ï from b are separated using the J/Ï pseudo-proper time
AMBER on the VLTI: data processing and calibration issues
We present here the current performances of the AMBER / VLTI instrument for
standard use and compare these with the offered modes of the instrument. We
show that the instrument is able to reach its specified precision only for
medium and high spectral resolution modes, differential observables and bright
objects. For absolute observables, the current achievable accuracy is strongly
limited by the vibrations of the Unit Telescopes, and also by the observing
procedure which does not take into account the night-long transfer function
monitoring. For low-resolution mode, the current limitation is more in the data
reduction side, since several effects negligible at medium spectral resolution
are not taken into account in the current pipeline. Finally, for faint objects
(SNR around 1 per spectral channel), electromagnetic interferences in the VLTI
interferometric laboratory with the detector electronics prevents currently to
get unbiased measurements. Ideas are under study to correct in the data
processing side this effect, but a hardware fix should be investigated
seriously since it limits seriously the effective limiting magnitude of the
instrument.Comment: 10 page
Simulation of the LHCb electromagnetic calorimeter response with GEANT4
We report about the simulation of the LHCb Electromagnetic Calorimeter (ECAL) using the Geant4 toolkit. The main characteristics of the detector are presented and compared with data obtained with particle beams
Nonleptonic charmless B_c decays and their search at LHCb
International audienceWe discuss the decay of B_c mesons into two light (pseudoscalar and vector) mesons. All these decay channels come from a single type of diagram, namely tree annihilation which allows us to derive simple relations among these processes. The size of annihilation contributions is an important issue in B physics, and we provide two different estimates in the case of non-leptonic charmless B_c decays, either a comparison with annihilation decays of heavy-light mesons or a perturbative model inspired by QCD factorisation. We finally discuss a possible search for these channels at LHCb
Characterizing receptive field selectivity in area V2
The computations performed by neurons in area V1 are reasonably well understood, but computation in subsequent areas such as V2 have been more difficult to characterize. When stimulated with visual stimuli traditionally used to investigate V1, such as sinusoidal gratings, V2 neurons exhibit similar selectivity (but with larger receptive fields, and weaker responses) relative to V1 neurons. However, we find that V2 responses to synthetic stimuli designed to produce naturalistic patterns of joint activity in a model V1 population are more vigorous than responses to control stimuli that lacked this naturalistic structure (Freeman, et. al. 2013). Armed with this signature of V2 computation, we have been investigating how it might arise from canonical computational elements commonly used to explain V1 responses. The invariance of V1 complex cell responses to spatial phase has been previously captured by summing over multiple âsubunitsâ (rectified responses of simple cell-like filters with the same orientation and spatial frequency selectivity, but differing in their receptive field locations). We modeled V2 responses using a similar architecture: V2 subunits were formed from the rectified responses of filters computing the derivatives of the V1 response map over frequencies, orientations, and spatial positions. A V2 complex cellâ sums the output of such subunits across frequency, orientation, and position. This model can qualitatively account for much of the behavior of our sample of recorded V2 neurons, including their V1-like spectral tuning in response to sinusoidal gratings as well as the pattern of increased sensitivity to naturalistic images
Experimental investigation of tsunami waves generated by granular collapse into water
The generation of a tsunami wave by an aerial landslide is investigated
through model laboratory experiments. We examine the collapse of an initially
dry column of grains into a shallow water layer and the subsequent generation
of waves. The experiments show that the collective entry of the granular
material into water governs the wave generation process. We observe that the
amplitude of the wave relative to the water height scales linearly with the
Froude number based on the horizontal velocity of the moving granular front
relative to the wave velocity. For all the different parameters considered
here, the aspect ratio and the volume of the column, the diameter and density
of the grains, and the height of the water, the granular collapse acts like a
moving piston displacing the water. We also highlight that the density of the
falling grains has a negligible influence on the wave amplitude, which suggests
that the volume of grains entering the water is the relevant parameter in the
wave generation.Comment: 23 pages, 16 figure
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