It has now been more than two years that Antares1, the underwater telescope installed in the depths of the abyssal plains 2500 m under the Mediterranean, is scanning the skies through the Earth in search of neutrinos. Over a thousand of them have already been observed until today, making it possible to establish the first views of the heavens to search for high-energy cosmic neutrinos, particles that may be able to teach us more about the most violent phenomena in the Universe.
Neutrinos are particles that interact very little with matter. Emitted by the most violent cataclysms of the Universe, they could prove that these phenomena are responsible for cosmic rays, mainly protons, which are constantly bombarding the Earth. These protons actually reach us after having been diverted from their paths by intergalactic magnetic fields, which prevents us from identifying their origin.
Their very weak interaction with matter makes neutrinos hard to detect. This problem can only be overcome by using enormous detectors, shielded against the cosmic rays. Antares, installed off the coast of Toulon, is protected against this radiation by a natural shield - 2000 m of water. Work on the detector took two years and its deployment was achieved in May 2008. Today 885 "eyes", together with the electronic equipment designed and built by CEA-IRFU (Institute of Research into the Fundamental Laws of the Universe) to read and process the data, are strung out in groups of threes along 12 flexible lines 450 m high. These lines are higher than the Eiffel Tower and are anchored to the seabed covering an area as large as four football fields.
Since the accident which occurred on the LHC accelerator several days after its commissioning in 2008, the ATLAS collaboration has been impatient to observe "true" events produced at the centre of the detector, and to make the equipment function under real conditions. On 23 November, following several days of tests with a single beam, Atlas recorded its first proton-proton collisions, at the injection energy into the LHC (450 GeV per beam, i.e. 900 GeV in the centre of mass reference frame of the collision). Analysis has then been able to reconstruct known unstable particles by detecting their disintegration products, demonstrating that the detectors and associated software are functioning correctly. The Atlas group, from the Particle Physics Department at IRFU, has also been able to check the behaviour of the muon detector sub-systems and the electromagnetic calorimetry, for which it is responsible. Collisions at 2.38 TeV (1.19 TeV per beam) were recorded before the LHC shutdown on 16 December, establishing a new world-record as of the most powerful particle accelerator in the world. The LHC will start up again in February 2010 after a short technical break aiming to produce collisions with higher energies and at higher intensity. Dark matter may be made of new particles, which could be produced in the high-energy proton-proton collisions at LHC. In this case Atlas would be able to discover them.
The Baryon Oscillation Spectroscopic Survey - known as Boss - delivered its first data during the night of 14-15 September. This experiment, devoted to the search for baryon oscillations, heralds the start of a new era of research into dark energy and the evolution of the Universe. Several teams are involved in BOSS, in particular from IN2P3(1)/CNRS, INSU(2)/CNRS and CEA.
Since the restart of the LHC on 20 November, CMS has taken advantage of the excellent operating performance of the collider to record a large amount of useful data. This is now being used to check its correct operation and calibration. During this period, CMS has demonstrated the stability of the detectors' working conditions as well as the efficiency of the data analysis system, which sends data from the detector to analysis teams around the world, and this in spite of very rapidly changing beam conditions.
Since researchers have been confronting the standard model of particle physics with experimentation, nothing has been able to shake it. Of all particles it describes, only the Higgs Boson has not yet been discovered. But the standard model is probably not the ultimate theory: it does not cover gravitation and numerous experimental observations remain unexplained.
A new invariance, called supersymmetry, was suggested during the 1970s. It associates particles with different spins (integer spin bosons and half-integer spin fermions). It is possible to create supersymmetric extensions of the standard model, elegantly resolving the mathematical problems that emerge during calculation of the Higgs Boson mass.
D01 experiment accumulating data from Fermilab's Tevatron (United States) just published2 results relating to the Higgs Boson research needed for supersymmetric extensions to the standard model. All currently available data has been analysed, representing more than one and a half billion events.
In the Tevatron, a high-energy proton-antiproton collider, large quantities of Higgs Bosons could be produced if they are sufficiently light. A useful channel for detecting them is their production associated with a bottom quark3 (b), H0b. In 90% of cases, supersymmetric light Higgs Bosons are supposed to disintegrate into two bottom quarks. That is why research in this area is based on identifying those events involving at least three jets4 resulting from bottom quarks in the final state.
Until the advent of the LHC, the Tevatron at the Fermi National Accelerator Laboratory, Fermilab (close to Chicago, USA), will remain the world's most powerful collider and the only location where the top1 quark can be produced.
The DØ experiment recently published2 results on the measurement of the rate of production of top-antitop quark pairs. This quantity, which is dependent on the value taken for the mass of the top quark, enables a prediction to be made for that mass using the standard model3. The top quark, which was discovered at Fermilab in 1995, remains the subject of very active research. Methods of analysis and the quantity of data are forever improving, which is resulting in subsequent improvements in the accuracy of the measurement of the top quark mass. The precise measurement of this value, combined with results from other precision measurements, enables the most probable mass of the Higgs Boson to be estimated. Hence improved measurements of the mass of the top quark is tightening the vice in the search for the Higgs boson.
The second phase of the Double Chooz international experiment officially began on Wednesday 20 May. The Declaration of Intent signed by the four partners (CEA, CNRS, EDF, Champagne-Ardenne Region) is the first step in the plan to build a second detector devoted to neutrino research next to the Chooz nuclear power plant. Prior to signing the DOI, the participants visited the site of the first detector, currently under construction. By the end of the year, the detector should pick up the first neutrinos emitted by the plant and attempt to measure the disappearance of primary flux neutrinos. The second detector, which will be operational two years from now, will provide precise measurements of the flux and spectrum of the neutrinos emitted and greatly enhance measurement control and precision. Contact
The Supernova Legacy Survey (SNLS) team at the Canada-France-Hawaii Telescope facility has just obtained the world's best measurement of the explosion rate of massive stars when the Universe was only 10 billion years old. A research team at IRFU's particle physics department at the CEA-Saclay centre worked on the first three years of SNLS data to obtain this result, which makes a crucial contribution to our understanding of the origins and evolution of chemical elements in the interstellar medium. The measurement seems to show that there are two to four times fewer supernovae today than 3.7 billion years ago. Could the Universe be burning out?
Nathalie PALANQUE-DELABROUILLE
Engineers and physicists from IRFU have successfully assembled and commissioned three large chambers designed to reconstruct charged particle tracks. The chambers will characterize the neutrino beam used in the T2K (Tokai to Kamiokande) experiment. They are the first large Time Projection Chambers (TPCs) to be equipped with micromesh gas detectors (Micromegas). The chambers have a very large sensitive area (nearly 9m²) and a correspondingly high number of electronic channels (124,000). IRFU built the entire detection system of the three TPCs, comprising 72 Micromegas detectors and all the front-end electronics. Engineers from SEDI, a department specialised in detector, electronics and information technology, specially designed a new chip called AFTER and two printed circuit boards for sending digitised signals to the acquisition system via an array of 72 gigabit optical links. The three chambers were tested with a particle beam at TRIUMF in Canada and have been installed in JPARC (Tokai, Japan) at the end of 2009.
The first beam and cosmic ray tests demonstrated that tracks could be reconstructed with the required degree of precision. This achievement represents a major step forward in the construction of a vital detector for the T2K experiment.
The TPC are now detecting tracks produced by neutrino interaction in the near detector of T2K and provide superior information to measure the momentum and to identify these particles.
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Dernière mise à jour : Nov 19th, 2024
