Archive for February 8, 2012
Neutrinos coming from the sun offer a window into the inner workings of our star. Scientists have detected the neutrino signature of several different solar nuclear reactions, but other steps in the fusion process have remained elusive. Now, the Borexino Collaboration, which runs a neutrino detector that lies a kilometer below the Gran Sasso mountain in Italy, reports in Physical Review Letters that they have obtained the first evidence of a relatively rare fusion reaction in the sun, while also placing strong limits on another.
Deep in their cores, most stars get their energy by fusing hydrogen into helium. This has been verified by Earth-bound measurements of solar neutrinos, which are one of the by-products of nuclear fusion. For our sun, the dominant pathway is the proton-proton, or pp, reaction chain.
Solar models predict that other reaction pathways occur in the sun. The proton-electron-proton, or pep, reaction produces deuterium that can feed into the pp chain, but only 1 out of 400 deuterium atoms are made through pep. The signature for the pep reaction is a neutrino with a distinct energy of 1.44 mega-electron-volts, and the Borexino experiment was designed to detect neutrinos in this energy range. By carefully removing background signals from cosmic rays and other sources, such as gamma rays from the rocks surrounding the detector and from detector materials, the Borexino Collaboration (Bellini et al.) claims to have seen 3.1 pep neutrinos per day per 100 tons of detector. The team also looked for neutrinos from a separate reaction network, the carbon-nitrogen-oxygen, or CNO, cycle, but was only able to set a stringent upper limit on the flux of these neutrinos. As more data are collected, the researchers may be able to discriminate between competing models of the sun as well as disentangle the different ways neutrino flavors can mix. – Michael Schirber
Mise en place du Masque auto-sauveteur Ce qu'il faut retenir Produced by: CERN Video Productions Director: CERN Video Productions 2:40 min. / 08 February 2012 / © 2012 CERN Language: French
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Deploying the self-rescue mask (SRM) To keep in mind Produced by: CERN Video Productions Director: CERN Video Productions 2:40 min. / 08 February 2012 / © 2012 CERN Language: English www.cern.ch
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Tuesday's sessions were spent looking ahead to the possibilities for 2012. The morning started with the experiments' desiderata for the year with the key requirement being either discovery of Higgs or exclusion at the 95% confidence level down to 115 GeV. To achieve this Atlas and CMS will need an integrated luminosity in the order of 15 fb-1.
Potential improvements to performance and machine availability were then discussed with presentations on maximising the time the LHC is delivering collisions to the experiments, and the potential of the injectors to provide bunches with higher intensities and the smallest possible beam size (these fold directly into higher collision rates). Machine performance will also be improved in 2012 thanks to a number of mitigation measures taken during the Christmas stop aimed at reducing the effects of radiation on the electronics situated in the LHC tunnel.
The possibility for running at a beam energy of 4 TeV was put on the table. Discussions will continue and a final decision will be a clear outcome of the workshop. One of the big successes of 2011 was the squeeze - the reduction of the beam size at the interaction point - which was pushed in the latter part of the year. Squeezing further in 2012 might be possible in combination with the use of tighter collimator settings. This could give a peak luminosity of around 6x1033 cm-2s-1 to be compared with a maximum of 3.6x1033 cm-2s-1 in 2011.
With a bunch spacing of 50 ns, a total of 1380 bunches (as in 2011), 15 fb-1 seems to be in reach if the tighter collimator settings prove to be operationally robust and the impressive performance of the LHC's many hardware systems continues.