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Frank Simon | MPI for Physics | Germany

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An Ambush… And some Thoughts about the ILC

Meeting informally with colleagues working on slightly different topics always bears a certain danger: You might actually get drafted to give a talk on one of the things you are working on, often on very short notice. This is what happened to me Wednesday night, at the monthly informal get-together of the members of the Excellence Cluster ‘Universe’, a newly established research center focusing on nuclear, particle, astro-particle and astro physics. More about this, and my role in the Cluster, in a later post.

Oleg Andreev, a theory fellow at the Cluster working on string theory, is organizing a journal club / informal seminar, and he needed a speaker for Friday morning. He asked me to speak about the International Linear Collider ILC, and would not accept my more or less lame attempts at excuses. So, with about 36 hours left and a lot of other critical things still to do, I started to prepare a presentation on the ILC for a diverse audience, mostly to introduce the concept of this proposed machine. But beyond the stress and extra work, such things are always opportunities to contemplate the bigger picture of what I’m working on.

My focus is the development of detectors for this future machine, but first lets start of with a very brief introduction of what this project is about. The ILC is being discussed as the next big project in high energy particle physics, and will complement the Large Hadron Collider LHC at CERN, that will hopefully deliver its first physics results later this year. While both these science projects aim at discoveries beyond the by now well established Standard Model, they follow different strategies. The LHC, with its high energy (14 TeV in the proton-proton center of mass), is a true discovery machine which opens up new horizons for particle physics. Compared to this energy, the ILC seems rather modest, with initially only 500 GeV (0.5 TeV). However, it will collide electrons and positrons, not protons. Since these are elementary and not composite particles, the full energy is available in the particle collision, and not only a small fraction of it as in the case of the LHC. Also, because of this, the events (that is what we call a “picture” of a collision that is recorded by our detectors) at the ILC will be much cleaner, allowing studies with much higher precision than at the LHC. Together, these machines unfold their full potential, combining energy reach and precision. I am convinced that the interplay of these two colliders will be crucial to establish a new map of the landscape of particle physics beyond our current understanding. The precision measurements at the ILC are only possible with new detector technologies, and this is where my work comes in… to be elaborated on later.

The use of electrons at the ILC brings its own set of challenges, however. Maybe the most striking one is that you can not build it as a circular storage ring like the LHC. The reason for this is the energy loss of charged particles when they travel on a curved path, which increases with decreasing mass of the particle, but to the 4th power. So an electron will lose more than 10 trillion times more energy than a proton on the same path at the same energy, since it is about 2000 times lighter. This kills the beautiful and extremely successful storage ring concept for high energy electron colliders, and is reflected in the name “International Linear Collider”. So, the next generation electron-positron collider will be a long, straight line, accelerating electrons and positrons from oposite ends and colliding them in the middle. Here is a sketch of what it might look like:

Schematic layout of the proposed International Linear Collider.

Schematic layout of the proposed International Linear Collider.

One of the key things here is to get the electrons accelerated as quickly as possible, to keep the machine as short (and thus as cheap) as possible. This requires high acceleration gradients, an interesting topic for a future post. With the presently available technology, the ILC is about 30 km long. This is where the “International” in the name comes in: A project of that size can only be realized as a world-wide enterprise. This is a universal feature of projects at the energy frontier of particle physics, and for me also one of the attractions of the field. Working together across continents and across different cultures and time zones is thus part of the everyday live of particle physicists, something I don’t want to miss.

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