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Frank Simon | MPI for Physics | Germany

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Matter in the Universe and a Mechanical Masterpiece

The mechanical model of the Belle-II pixel vertex detector, PXD: A bit larger than a soda can. The silicon part (the black modules in the center) would fit in a Red Bull can.

The mechanical model of the Belle-II pixel vertex detector, PXD: A bit larger than a soda can. The silicon part (the black modules in the center) would fit in a Red Bull can.

Another meeting, this time the Belle-II Collaboration meeting at KEK, near Tokyo in Japan. Right now I’m on the way back, just as in my very first post to this blog, on LH 715 bound for Munich from Tokyo’s Narita Airport.

For three days, we were discussion the open issues, and the solutions, for the new detector that is scheduled to start taking data in 2014, with an impressive increase in data rate over the present Belle experiment. Just as the Belle experiment, Belle-II will explore, among other things, a phenomenon called CP violation. At first sight, this seems horrifyingly abstract and complicated (the way the data analysis is performed certainly is), but the subject of study as at the very heart of our existence: CP violation is the mechanism that leads to unequal behavior of matter and antimatter, and is necessary to explain why the universe today is only made out of matter. If matter would not have been preferred over antimatter in the early phases of the universe, it would now be completely empty, just filled with light. So, understanding CP violation is a key goal of particle physics.

Belle-II members crowding around the model of the PXD.

Belle-II members crowding around the model of the PXD.

And here is where Belle-II comes in: The amount of CP-violation that we have observed so far with the existing experiments is by far not enough to explain the amount of matter in the universe today. So we know that there have to be more sources of CP violation than those we have studied so far. These could show up at higher mass scales which we can not study today, but might be able to get at with the LHC. Precision measurements at Belle-II will provide another way. By comparing extremely precise measurements to theoretical expectations, we might be able to detect inconsistencies, which could point to new effects, potentially well beyond what LHC can observe directly.

Precision measurements also need precision instruments. And one such piece is the new pixel vertex detector for Belle-II, which I’ve written about before. The design of this device is constantly progressing, and at this meeting, we had a real highlight: A first mechanical mock-up, full size, with real (however, inactive and unprocessed) silicon modules. And if you look very close, you can see that some of these silicon modules have been thinned down to a thickness of 50 microns, just a bit more than a human hair, with just a small rim of thicker silicon to provide stability. This is done to reduce the interference of our measurement device on the particles we want to study. Part of the mechanical support, which will later provide the precision mounting and the high power cooling, has been fabricated with rapid prototyping, a 3D printing technique. Now still out of plastic, the final product will be made from copper or stainless steel. The nice device certainly drew a lot of attention, and gave everyone a sense of how tine such a masterpiece is: Years of work by many people in a lot of institutes all over Europe, and substantial amounts of funding go into a device that is not much larger than a soda can. But it will be a digital camera, with 8 million pixels, able to take 50 000 pictures a second, providing extremely detailed pictures of the particle interactions going on in Belle-II. And with the first mechanical prototype, a true masterpiece from our engineer, this project starts feeling even more real.

A close look: Mechanical silicon dummies, including a few thinned down to a thickness of 50 microns.

A close look: Mechanical silicon dummies, including a few thinned down to a thickness of 50 microns.

With this last stop, my world tour is coming to an end, for now.

… “In preparation for our landing, please switch off all electronic devices, adjust the backrest of your seat to its full upright position and stow away your tray tables” … Finally!

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