I’m back again from Japan… And I did not have the time to write a post during the trip. That is a good sign: The meetings were very productive. First, a three day Belle-II meeting, followed by the Belle collaboration meeting, where I only attended the first day because I had to go back already on Saturday.
Discussions during the coffee break at the Belle-II meeting. As usual, laptops are everywhere... how could we live without them?
The Belle-II project is currently taking on a more and more concrete shape. We now have our first spokesperson, congratulations to Peter Krizan! For my project, the pixel vertex tracker, the mechanical layout of the interaction region, the area where the two beams (electrons and positrons) collide, is of particular importance. This is still in flux, since the parameters of the accelerator are not fixed yet. However, it emerged that likely the so-called nano-beam option will be chosen. This means that the cross section of the colliding beams will be very small. That is good for high luminosity, and requires lower beam currents and thus a lower power bill than the standard option with high currents. Hopefully also the backgrounds for our detector will be lower. Technically, this option is however quite challenging. The most important consequence of this for me is that the radius of the beam pipe gets smaller, and that means that the inner-most layer of the pixel detector can get closer to the interaction. This means better resolution, and thus better physics performance. Also, it means that the pixel detector will have a bit more space, since the radius of the next detector, the silicon strip detector, is not changing. And that is particularly important for the piece of the pixel detector I’m responsible for: The mechanical support. The electronics of the detector create scary amounts of heat, which is no real surprise considering the roughly 10 million electronics channels, and the data rate of something like 30 GB/s. The consequence of this is a heat load of 300 W, out of a detector with the volume of a red bull can. And most of this volume is needed for high precision support, and for the silicon in the active area where no additional material should be. So the space for cooling is very limited. The new beam option allows to move the inner layer further in, creating the space needed to fit a cooling channel in the detector support. I presented some first ideas at the meeting, but a lot of work is clearly still needed.
Another very good outcome was that we now have a plan for the software framework. Before the meeting, there were two competing proposals. One from the software team of Belle, building on the existing framework, and one from our group in Munich. We proposed to use the ILC software framework, which we have already used for simulations of our new detector. Both ideas have pros and cons. Extending the Belle framework has the nice feature that existing software can be also used for Belle-II, while the ILC framework offers very nice handling of simulations and of detector geometry and so on. After hours of discussions, a plan emerged to combine the advantages of both systems, by having a framework backbone that uses Root and is compatible to the existing Belle software, and using the ILC simulation and geometry handling. This also means that all the development that went into the simulation studies of our pixel detector can be used in the new Belle-II software, which is what we wanted.
A view of the Belle detector, opened for maintenance. The big blue piece is part of the magnetic flux return for the detector, the real high tech is deeper in, closer to the beam pipe.
Apart from these important decisions, a lot of other discussions took place. And getting to know all the people in the collaboration is also a very important aspect of such meetings. And then, I also found the time for a brief visit to the Belle detector, to get a first impression of what it looks like. Of course, compared to the STAR experiment that I worked on for many years, and even more so compared to the LHC experiments or to a future ILC detector, Belle is a quite small detector. This is due to the lower energy of the collisions. None the less, it is a very complex technical masterpiece, full of state-of-the-art subsystems.
Then, on my final day at KEK, the Belle general meeting gave me some new ideas about possible physics analyses to do. I hope that I’ll find the time to also dive into some analysis myself, something that I only rarely get to do nowadays… One of the downsides once you move past the post-doc stage… Maybe a topic for some more thoughts in a future post? Now it is time for some physical exercise, to relax from a week of meetings, and many hours in planes.
