Beam is back!

30 November 2009

The first candidate collision with calorimeter energies as the inset.



After over a year of repairs and detailed preparations, the LHC has finally surpassed the level of progress last seen September 2008. Beams are circulating in the LHC, and splash and collision events are being recorded in the ATLAS detector, with the 900 GeV collisions starting in earnest this week.

Whether you are a registered member of ATLAS or an interested outsider, you can view images of these events online. For a taster, a 3D reconstruction of the first 900 GeV candidate collision event is shown above.

On Thursday, November 19th, Run Coordinator Christophe Clement said of the atmosphere: “It’s not so excited as you would think… Maybe it’s the calm before the storm.”

Perhaps he was right, judging by the elated faces in the Control Room just the next day, when the first splash events for 2009 – displayed in this week’s photo gallery – lit up the ATLAS detector. “It was real fun to see the first splash event in the ATLAS Control Room,” said Adam Trzupek. “I am sure that coming days will be equally or even more interesting.”


Sandra Ciocio and Adam Trzupek of the SCT inspect fresh data. Courtesy of Claudia Marcelloni.



As ATLAS settles in to running with beam, Christophe is pleased to report that the new block-scheduled shift structure is working very well. “We’ve got essentially all the shifts booked to the end of the year,” he says.

He notes that many desks have two people behind them, one the official shifter and the other in training, preparing more members to share the burden of next year’s ten months of run time.

Beam conditions teams are taking special care to understand the “halo” of particles traveling around the main proton beam. Because of the risk that the beam poses until it is fully commissioned, Pixels, SCT, and Muon systems are running in safe mode, with a lower High Voltage. ATLAS’s beam conditions monitor also provides important feedback to the LHC machine teams, helping to determine when the beam is stable.

The trigger teams are working hard to make sure that when the beam goes through, ATLAS is recording. “We have to set up the timing for the triggers so that we can have beautiful displays when we trigger,” says Christophe. The beam position and timing detector, BPTX, is located 140 metres away from ATLAS in the tunnel and gives the trigger a heads-up when beam is coming.

On Monday, November 23rd, Mike Lamont, an LHC machine coordinator, said: “We’ve had an excellent startup so far, incredible progress, in fact.” That afternoon, the LHC circulated its first beams in both directions simultaneously, with bunches crossing at what will be the interaction points.

In the morning, about a hundred and fifty collaborators had gradually crammed into the ATLAS Control Room, trying to keep quiet so that shifters could work, but Marzio Nessi cleared them out anyway around 11:00. By 13:00, many had filtered back in and were present for the first collisions shortly after 14:00.

When they finally came, the low beam energies and intensities rendered the events somewhat anticlimactic. Above the crowd in the Control Room, Markus Elsing and the rest of the Event Scanning crew were hard at work, making the data into the dramatic images that were then projected at the front of the room. These images provided clear evidence that collisions had indeed taken place, alongside many background events, and justified the pouring of champagne.

However, last week was all about controlling beam circulation – the events observed by ATLAS are chance encounters between protons in relatively diffuse bunches. Setting the proton bunches on a collision course as they approach the experimental caverns is on the schedule for this week.

As of November 23rd, one issue was the sensitivity of the new Quench Protection System, and the team responsible has been debugging the system to prevent unnecessary trips. Other odd problems, such as a broken vacuum pump in one of injectors, can cost a few hours here and there.

“There’s a huge amount of things that can go wrong,” says Mike Lamont, but luckily – or rather, due to meticulous workmanship – relatively few of them do. For now, at least, problems guiding the beam won’t be catastrophic; at these current intensities, the protons could hit a magnet and cause a quench without inflicting lasting damage.

After last year’s incident, the LHC team is especially careful to “keep our feet on the ground,” as Mike Lamont put it. Speaking to ATLAS and fellow experimental collaborations, he added: “We know how much effort you guys have put in, so we have to make sure we’ve got a very safe machine before we start pulling the stops out.”

The machine team continues to keep everybody updated on the LHC Commissioning page, including a strategy page with a detailed plan that takes us to the winter break and beyond. The news pages should be updated twice daily, according to Mike Lamont.

In ATLAS’s near future, the solenoid should be on by midweek (toroids already running), and the Pixels, SCT, and muon systems should come out of safe mode, increasing the high voltage. The solenoid has been kept off at the LHC’s request to avoid fringe field effects. And one thing that Mike Lamont and Christophe agreed on: from both detector side and machine side, we’re all looking forward to an opportunity for relaxation, “and the odd beer or two” adds Mike, over the winter break.

All these impressive achievements were presented before a packed and enthusiastic auditorium last Thursday with the first and very quickly produced assessment of detector performances and data quality from each of the four LHC experiments.

 

Katie McAlpine

ATLAS e-News