Trigger on track!

22 September 2008

Some of the central trigger team at work in USA15 on

One hundred metres below the Control Room, the four-strong Central Trigger team in USA15 must have felt like they were half a world away from the media scrum taking place above their heads on September 10th. Safely waiting 30 metres away from the beam, hidden behind a thick concrete wall and isolated from all the hustle and bustle above, all their attentions were focussed on catching the first signals of beam coming through on a scope.

“Our aim was to produce a useful beam-related trigger signal to read out the detector exactly when particle bunches entered ATLAS,” explains Thilo Pauly. “When the first bunches came, we saw a pulse coming from the beam pick-up detectors, as well as signals from the simple scintillator detectors that are very sensitive to any particles close to the interaction region,” he continues, adding, “We saw clear signals that came from particles arriving along with the beam.”

“It was really them that saw the first beam as it came to ATLAS,” says Shift Leader Thorsten Wengler. According to Thilo, the team in the underground counting room felt a heady mix of excitement at seeing the first beam and nerves about unforeseen problems. Thankfully, the worst they had to deal with was a little initial trouble with the intensity of the beam being too low, which meant they had to adjust their threshold settings. “After a few injections, it worked fine,” says Thilo, who is impressed with the rapid progress that the LHC made during its first two days in action.

Now the Central Trigger team is busy making sure that all the different trigger inputs align with respect to time, and is working on providing a stable collision trigger to the sub- detectors. “We also have to understand the rates, efficiencies and purities of the triggers, as well as the luminosity of the machine,” says Thilo of the task ahead.

An oscilloscope picture showing a few runs of beam through the LHC. The first channel shows the beam pick-ups, which see well-defined bunches. The remaining signals are from the MBTS (minimum bias trigger scintillators), which show signals when the beam degrades after a few runs.

Ceri Perkins

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