Although some authors have claimed that the effect is not detectable, we show experimentally for the first time that as the quantum parameter chi grows beyond 1, an increasingly large part of the ...hard radiation emitted arises from the spin of the electron. Results for the energy loss of electrons in the energy range 35-243 GeV incident on a W single crystal are presented. Close to the axial direction the strong electromagnetic fields induce a radiative energy loss which is significantly enhanced compared to incidence on an amorphous target. In such continuously strong fields, the radiation process is highly nonperturbative for ultrarelativistic particles and a full quantum description is needed. The remarkable effect of spin flips and the energy loss is connected to the presence of a field comparable in magnitude to the Schwinger critical field, E0 = m(2)c(3)/ePlanck's over 2pi, in the rest frame of the emitting electron.
Context.This paper addresses a common problem in astronomy and cosmology: to optimally select a subset of targets from a larger catalog. A specific example is the selection of targets from an imaging ...survey for multi-object spectrographic follow-up. Aims.We present a new heuristic optimisation algorithm, HYBRID, for this purpose and undertake detailed studies of its performance. Methods.HYBRID combines elements of the simulated annealing, MCMC and particle-swarm methods and is particularly successful in cases where the survey landscape has multiple curvature or clustering scales. Results.HYBRID consistently outperforms the other methods, especially in high-dimensionality spaces with many extrema. This means many fewer simulations must be run to reach a given performance confidence level and implies very significant advantages in solving complex or computationally expensive optimisation problems. Conclusions.HYBRID outperforms both MCMC and SA in all cases including optimisation of high dimensional continuous surfaces indicating that HYBRID is useful far beyond the specific problem of optimal target selection. Future work will apply HYBRID to target selection for the new 10 m Southern African Large Telescope in South Africa.
The High Level Trigger (HLT) system of the ALICE experiment is an online event filter and trigger system designed for input bandwidths of up to 25 GB/s at event rates of up to 1 kHz. The system is ...designed as a scalable PC cluster, implementing several hundred nodes. The transport of data in the system is handled by an object-oriented data flow framework operating on the basis of the publisher-subscriber principle, being designed fully pipelined with lowest processing overhead and communication latency in the cluster. In this paper, we report the latest measurements where this framework has been operated on five different sites over a global north-south link extending more than 10,000 km, processing a ldquoreal-timerdquo data flow.
Experimental results for the radiative energy loss of 149, 207, and 287 GeV electrons in a thin Ir target are presented. From the data we conclude that at high energies the radiation length increases ...in accordance with the Landau-Pomeranchuk-Migdal (LPM) theory and thus electrons become more penetrating the higher the energy. The increase of the radiation length as a result of the LPM effect has a significant impact on the behavior of high-energy electromagnetic showers.
The dimuon High Level Trigger Vilakazi, Z Z; Collaboration, the Alice
Journal of physics. Conference series,
11/2006, Letnik:
50, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The CERN-ALICE experiment will produce global data sizes in excess of 75 Mbytes/event, which at event rates of 200–1000 Hz will yield global data streams of 15 Gbyte/s. Online processing–in the form ...of pattern recognition–is necessary to filter interesting (sub-) events and/or compress data correctly by modelling techniques. A layer of trigger–selecting topologically distinct physics signals (J/ψ) and γ–is currently being developed and benchmarked under stringent conditions. An emphasis is placed on the implementation of this high level trigger (HLT) for muon reconstruction efficiency studies. Investigations thus have alluded to the HLT being able to reduce background by a factor of 4 for low transverse momentum muons and a factor of 100 for high Pt single muons. Furthermore the invocation of the HLT will help refine the resolution of the Pt cut efficiency.
The ALICE High Level Trigger comprises a large computing cluster, dedicated interfaces and software applications. It allows on-line event reconstruction of the full data stream of the ALICE ...experiment at up to 25 GByte/s. The commissioning campaign has passed an important phase since the startup of the Large Hadron Collider in November 2009. The system has been transferred into continuous operation with focus on the event reconstruction and first simple trigger applications. The paper reports for the first time on the achieved event reconstruction performance in the ALICE central barrel region.
A few years ago the CERN NA-43 collaboration installed an upgraded detector system which allows a detailed analysis of the particle motion before, during and after penetration of a crystal. Also, ...essentially perfect diamond crystals were produced by the collaborators from Schonland Research Centre. These facts have led to new and very detailed investigations of QED-processes in strong crystalline fields. Along axial directions the radiation emission is enhanced by more than two orders of magnitude. For incidence on a 0.7 mm thick diamond crystal of well-aligned 149 GeV electrons, 35% give rise to a high energy photon peak at ≃120 GeV. For 243 GeV electrons and ≃200 GeV photons, this number decreases to 25% – which may be an indication of quantum suppression. Different measurements of the photon multiplicities show that in most cases positrons and electrons emit equal number of photons. The dramatic radiation emission leads to a strong reduction in transverse energy and all electrons incident within the critical angle are captured to high lying channeling states and exit at channeling angles corresponding to their final energy – a completely new result for negatively charged particles. For the first time, we present an analysis where the photon is used as a `messenger' for the transverse energy of the electron during the formation time and we conclude that the more energetic photons are created closer to the string and emitted in the axial direction – in contrast to earlier calculations using the Dirac equation. The strongly enhanced radiation emission leads to angular cooling for electrons but angular heating for positrons and we show that at higher electron energies the cooling becomes stronger as expected from theory. For electrons, the radiative cooling gives rise to a capture of above-barrier particles into the channeled beam. The total radiative energy loss is shown as not to follow the
γ
2-law expected from classical electrodynamics, but turns over towards the
γ
2/3-law as found from a full QED calculation. Detailed comparisons of doughnut formation for
e
+,
e
− just before radiation emission and behind the crystal show large differences in equalization – evidence that the hard photons are emitted in the first part of the crystal.
The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20–170 GeV. The experimental arrangement ...allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active debate and development. With the approach used in this paper, both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high-energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high-energy SOS photons.
The ALICE High Level Trigger combines and processes the full information from all major detectors in a large computer cluster. Data rate reduction is achieved by reducing the event rate by selecting ...interesting events (software trigger) and by reducing the event size by selecting sub-events and by advanced data compression. Reconstruction chains for the barrel detectors and the forward muon spectrometer have been benchmarked. The HLT receives a replica of the raw data via the standard ALICE DDL link into a custom PCI receiver card (HLT-RORC). These boards also provide a FPGA co-processor for data-intensive tasks of pattern recognition. Some of the pattern recognition algorithms (cluster finder, Hough transformation) have been re-designed in VHDL to be executed in the Virtex-4 FPGA on the HLT-RORC. HLT prototypes were operated during the beam tests of the TPC and TRD detectors. The input and output interfaces to DAQ and the data flow inside of HLT were successfully tested. A full-scale prototype of the dimuon-HLT achieved the expected data flow performance. This system was finally embedded in a GRID-like system of several distributed clusters demonstrating the scalability and fault-tolerance of the HLT.
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