The past 10 days have been extremely exciting for every one at CERN or working for the CERN experiments. The accelerator injected first one beam, then two beams in the tunnel; a bunch of proton was spinning in each directions and made to collide in the centres of the different experiments. During these first tests, the energy of the beam in the LHC is the same as at the injection (450 GeV), nevertheless it proved to be a very successful performance of the machine and of the experiments. Because of the low energy, no new particles can be produced, but the data collected are crucial for understanding the detector timing. The electronics sitting in the detector reads the signal produced by the particles flying through. When should the readout start? The accelerator propagates a signal (the “clock”) to indicate when collisions are likely to happen. However, if the detector is not properly timed, the electronics won’t read any signal out of the different sensors. Roughly 10 days ago, so called “splash events” were provided by the accelerator. In this case. The beam collimators at the door of the experiments are closed; one beam hits them producing a splash of particles shining the detector. The understanding of these type of events is challenging, because they are geometrically different from what the experiments is designed to record. The particles fly from the side of the detector, and not radially from the centre. The electronics and the software needed to be adjusted to use these events as “calibration” tools.
The appearance of signal in the detector caused quite some excitement in the ATLAS Control room! Even if it is the second time I see an experiment starting (after my CDF experience), I was extremely happy to feel the same empathy and the same joy. People in operations devoted their life for years (decades?) to see the apparatus working. Being in the same room with experts staring at their system taking data was extraordinary.
The tests and fast processing of the splash events were successful. ATLAS was finally ready for the first LHC Collisions of 2009! LHC and the 4 experiments joined into a celebration on Thursday: during a very crowded seminar held in the CERN auditorium knowledge about the status of the machine and of the experiments, lessons learnt with the first events was shared … and above all the excitement for the new beginning.
