A joint Fermilab/SLAC publication

How to turn on the Compact Muon Solenoid

A view of the CMS cavern.

A view of the CMS cavern.

When two proton bunches traveling in opposite directions at close to the speed of light first met on Nov. 23 within the Compact Muon Solenoid detector at the Large Hadron Collider at CERN, 100 million detection elements were ready to record.

The trickle of collisions recorded so far will gradually increase to a flood as LHC operators ramp up the collision energy to seven trillion electronvolts.

Scientists expect this to generate a data stream of about 100 megabytes per second.

Before detection equipment could record any data from those first collisions, scientists actually had to turn the detector on. Read on to find out how hundreds of scientists and maintenance personnel power up the CMS detector.

First step: perform maintenance

The size and complexity of modern detectors makes it difficult for scientists to find defective or malfunctioning parts until they have thoroughly tested the whole detector. Because of these challenges, detectors often have a breaking-in period with crews performing the most maintenance after the first run. The nine-day operational period in 2008, although brief, gave CMS scientists an adequate glimpse of the detector, allowing them to perform most of the maintenance associated with a first run.

"During the first operational period, things move in the magnetic field and you really have to understand how they move," said Fermilab's Jeff Spalding, project manager for the hadron calorimeter sub-detector. "When you open the detector you have to inspect and look for any sign of movement."

During the initial testing period, maintenance crews located and repaired minor leaks in the cooling system and adjusted some of the mounting to account for the effects of the 3.8 Tesla magnetic field, Spalding said. Crews turned on and tested individual parts of the detector as they performed maintenance so if there were problems, they could find out before reassembling the detector.

"What you don't want is to complete all your maintenance, close up the detector, and then find out too late that you have a problem," Spalding said. "Once we close the detector again to a beam-ready configuration, it would take weeks to backtrack to fix something."

Put the CMS back together

"CMS is designed in major sub-detector units that fit together almost like Legos," said Fermilab's Slawek Tkaczyk, maintenance and operation manager for the CMS silicon tracker. To prepare the CMS for beam, crews must carefully align each of the 13 basic components of the 13,800-ton detector into place. Depending on its initial setup, taking the CMS detector from an open maintenance configuration to being ready for beam takes between two weeks and two months.

Synchronize and fine tune sub-detectors

Even after years of planning and precise alignment, CMS scientists can't synchronize every element until the detector is in place and powered on. CERN's Tiziano Camporesi, CMS commissioning and run coordinator, said scientists use cosmic rays interacting with the detector to fine tune and synchronize equipment.

During the shutdown, scientists organized global runs, where all sub-systems operate together to collect cosmic-ray data. Each week scientists held 48-hour midweek global runs, with a continuous six-week run in July.

Begin continuous global runs

The CMS detector has the same turn-on and operational sequence for a cosmic run as it does for receiving actual collision data, so beginning a run with beam is not much different from one without. Months prior to receiving beam, crews have already sealed the collision hall, powered and tested each sub-system, and cooled the magnets. By the time the detector receives beam, it has been operational for weeks. Cumulatively over the life of the detector, about 1 billion cosmic rays have already been recorded.

"The data we collected allowed us to exceed my rosiest predictions because it is extremely high quality," Camporesi said. "The detector is extremely good in terms of resolution and performance."

Such fine-tuning creates higher-quality data. Using an automated system that tracks cosmic rays, Tkaczyk said scientists working on the 220 square meters of silicon tracker have been able to improve synchronization from 25 nanoseconds down to a precision of one nanosecond. Tracker crews also bumped up their signal-to-noise ratio from 27-to-1 in August 2008 to 30-to-1 this October.

The final three-week testing period in November used cosmic rays for last-minute adjustments until beam arrived.

Starting a global run can take as few as 20 people, with some working in the central control room and others joining in from remote centers around the world. Beginning to take data only takes about five minutes, but it can take a few hours of tweaks and adjustments before the detector is running well.

"No matter how well you left the detector the last time, people have been doing local things, improvements, repairs, and changes to software," Camporesi said. "When the detector restarts the process can always be a little bit painful for the first few hours."

The LHC began running at the injection energy of 450 billion electronvolts. Operators will also increase the beam intensity and the energy to a maximum 1.2 trillion electronvolts per beam in 2009, with CMS scientists using collision data to optimize their detector.

by Chris Knight

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