ZEUS at Hera with open shells to see the calorimeter.
Today I received a present from a colleague who is cleaning out his office. It is actually a mobile (something to hang underneath the sealing moving slowly) with a number of HES sensors from the ZEUS detector at HERA. This reminded me, that we report in this blog a lot on collider experiments just about to be turned on, or experiments to be constructed in the distant future. But ZEUS for example is a beautiful experiment constructed almost 20 years ago, running successfully for 15 years and turned off about 2 years ago. The analysis of the data is still in full swing and we expect many more beautiful papers.
The HERA storage ring was the only one in the world in which two different types of particles were accelerated separately and then brought to collision. In the 6.3-kilometer-long tunnel electrons collided with hydrogen nuclei, i.e. protons from the hadron family, nearly 2000 times heavier. In these electron-proton collisions, the point-like electron acts like a tiny probe that scans the inside of the proton and reveals its inner structure. ZEUS and H1 were the two experiments used to reveal these interactions. A very nice summary of the most important results can be found here.
ZEUS was located in the south hall at the HERA ring and was 10m x 12m x 19m large and had a total weight of 3900 tons. The heart of the ZEUS detector was the uranium scintillator calorimeter (CAL), which measured energies and directions of particles and particle jets with high precision. The CAL was surrounding the tracking detectors consisting of a vertex detector, the central drift chamber, the forward and backward drift chambers, and in the forward direction a transition radiation detector to identify high-energy electrons.
From 2005 to the end of ZEUS data taking in 2007 I was part of the team keeping the detector alive and running, namely the Uranium calorimeter. It was a rather interesting challenge for me, as until then I was working mostly with new detectors in the development phase. But ZEUS was at that time 13 years old, using electronics developed at the end of the 80’s, a complete different time when looking at electronics. No mobile phones, no desktop computers etc. I had to learn how to get the best out of the detector. Very quickly I realized that the calorimeter had a personality. It was like an old car, which stops moving when you shout at it (ok, I admit that my car also has a personality…).
Of course phone calls in the most inappropriate moments, or in middle of the night, were daily business. It happened many times that I had to go to the hall around 3 am and restart the calorimeter as it “tripped” due to a power glitch. A very likely event in the summer time with a lot of thunder storms around. Or some other, for the calorimeter vital system, had a hick-up. But this is in the end a very rewarding job. At least if I got the problem fixed (luckily most of the time the case), I felt very good when going back to bed. And I of course I shared this task with other people and did not have to leave my bed every time there was a problem.
The ZEUS collaboration about 5 minutes after HERA was switched off.
At the end we had to turn ZEUS off, after 15 years of successful operation. This was done with a nice big party. All current and former ZEUS members were invited to do the last shift together. As this many people, about 300, did not fit into the rather small control room, we put up a stage in front of the experimental hall, a beamer, and a screen. All the online plots were displayed on the screen so that we could watch the data taking and the moment of the last beam dump. A tent was organized, and food and drinks were served. We actually had a great time; even so the moment of turning off the detector was a sad moment. You can find a funny summary of this event also at the Symmetry magazine.
By now the south hall is completely empty, but I think I will report on the disassembly of the detector another time.
