LHC Experiments: refinements for the restart

While the sudden stop of LHC operation may have seemed like a dark cloud over CERN, there was one silver lining: with an extra year for tests, repairs and improvements, ATLAS and CMS will be in better shape than ever when the LHC restarts later this year.

CMS


The CMS pre-shower being installed in the cavern. Although in the world of LHC detectors the pre-shower seems rather modest in dimension, it is still a factor of ten larger than any previous silicon-based high energy physics detector.

CMS

CMS was already well prepared for the first beam last year, but one of the things they hadn’t had a chance to do was run the complete detector for a long period. "We had had several week-long test runs before the first beam, but nothing longer," says Tiziano Camporesi, CMS Commissioning Coordinator. In the weeks following 19 September, CMS performed its most thorough cosmic ray test run to date, running flat out 24/7 as a complete experiment for over a month.

"We were pleasantly surprised to find that we could use the cosmic data to align the detector to a level we assumed we could only achieve from the first collisions," explains Camporesi. "So we are starting another CRAFT (Cosmic Rays At Four Tesla) run on 22 July and we’ll run 24/7 all the way until the first week of September." As well as being crucial for aligning the detectors, these cosmic ray runs are also important for the personnel to gain operational experience.

The shutdown has also given CMS a chance to install the pre-shower, the final sub-detector. The pre-shower, which sits in front of the endcap calorimeters, can pinpoint the position of photons much more accurately than the larger crystal detectors. This will allow the distinction to be made between two low-energy photons and one high-energy photon, a crucial task when trying to spot certain Higgs decay products.

The shutdown period has obviously also been a chance for consolidation work on the detector. One of the most important item of work was the complete refurbishment of the cooling system for the tracker. Numerous other repairs have been carried out, including on some of the muon detector channels, which were removed, repaired and replaced. With these small problems ironed out, and the improved alignment from cosmic rays, the path to physics results once the LHC restarts will be that much easier.

ATLAS


Aligning the beam pipe before final closure of the ATLAS End Cap Toroid Magnet on 26 May 2009.

ATLAS

ATLAS has just finished its second successful cosmic ray run since last September. Like CMS, the experiment started taking cosmic ray data immediately following the LHC’s unexpected stop. "Last year we were ready to run, so even after the breakdown we decided to go straight into a relatively long cosmic run with the entire detector online," explains Marzio Nessi, ATLAS Technical Coordinator. "Then, in November, we opened the detector again," he continues. "This was mainly for repairs and maintenance; there were some single points of failure that we didn’t want to carry with us."

In total more than 250 different consolidation activities have been scheduled in the ATLAS cavern since shutdown. While most of the individual items of work has been relatively minor, together they ensure that the detector is even more prepared for the LHC restart. Amongst the most crucial have been repairs to the inner detector cooling system, which had structural problems with the compressor due to vibrations. There have also been repairs to the front-end electronic cards for the electromagnetic calorimeters.

"Now we know that the detector is even better prepared than last year. Previously we had problems on the level of a percent of the active systems, such as electronic channels that didn’t respond or gas chambers that were leaking too much. So while we had the opportunity we wanted to fix all these things, and ideally get these problems down to just 0.1 %. Now we’ve pretty much achieved that," confirms Nessi.

The extra shutdown period has also allowed some schedules to jump forwards. For instance, the very forward muon chambers (EE chambers) have already been partially installed, even though this was not planned until the 09/10 shutdown. There have also been several upgrades to prepare higher luminosity, such as replacement of optical fibres on the muon systems ready for higher radiation.

The detector was closed again in June ready for the second cosmic ray run. After the final repairs have been done the detector will be completely closed in preparation for the restart . "In the next 4-5 weeks we will be moving the forward shields into place, so after that nothing will be moving again. Then it’s all the final checks before the LHC restart."

Along with the other two large LHC experiments, both ATLAS and CMS have taken part in a full-scale test of the Grid. After months of preparation and two intensive weeks of 24 x 7 operation they both celebrated the achievement of a new set of goals aimed at demonstrating full readiness for the LHC data taking. Whilst there have been several large-scale data-processing tests in recent years, this was the first production demonstration involving all of the key elements from data taking through to analysis. Records of all sorts have been established: data taking throughput, data import and export rates between the various Grid sites, as well as huge numbers of analysis, simulation and reprocessing jobs – ATLAS alone running close to 1M analysis jobs and achieving 6GB/s of "Grid traffic", the equivalent of a DVD worth of data a second, sustained over long periods.

Outreach

It is not just technical work that has been going on over the past 10 months. With the unprecedented media coverage from the first beam last September, the two experiments have found themselves in the limelight.

CMS


One of many groups of school children that have visited CMS over the past 10 months.

One of the biggest things for the CMS outreach team this year has been visits. "We wanted to get as many people as possible down into the cavern before it closes for good, especially people from the local area. This has been a real success; we’ve had a huge amount of visitors– I dread to think quite how many!", says Dave Barney, CMS Outreach Coordinator. "But this also really highlighted that we’re going to need somewhere for visitors in the future."

With this in mind, much of CMS’s outreach activities have focused on developing a new visitor centre to take over as the main place for visitors once the cavern itself has closed. The new CMS Discovery Centre will be built on the Meyrin site next to the CMS analysis centre (the old PS control room). "It’s a good location because it can easily tie in with tours visiting the LINAC just round the corner, so you can point out the bottle of hydrogen where it all starts." Situated next door to the CMS centre, one of the main attractions of the new centre will be several life-like mock ups of the control desks, where visitors (mainly high-school students) can operate the LHC and CMS and then interpret the data. "The aim is to make it as realistic as possible, so they will be using software as similar as possible to the real thing."

"But we also hope to keep people visiting the CMS site itself," adds Barney. "So, in the long term, there are also plans to open another visitor centre over at Point 5, tied in with new labs that will be built over there. And hopefully we can keep tours visiting underground, even when CMS is operational!" CMS actually has two caverns, one for electronics and the other for the detector; between the two is a 7-metre wall of concrete. "Obviously no-one will be able to see the detector itself but it’s perfectly safe to go down to the other cavern. Even if they can’t see the detector, they can still experience the thrill of going 100 m underground."

CMS also has a new photo book, now available in the CERN shop.

ATLAS


ATLAS’s new high-tech visitor centre.

Since the start-up last year the ATLAS outreach team has had a lot more coverage "Around the first beam our website hits just went through the roof. But even after the peak they stayed consistently much higher than before," explains Erik Johansson, one of ATLAS’s two Outreach Coordinators. "And being featured in a major Hollywood film helped a bit too!" adds Michael Barnett, ATLAS’s other Outreach Coordinator. "There’s even going to be a 15-minute extra clip about CERN and ATLAS on the DVD release of Angels & Demons".

As ATLAS prepares for physics, the outreach activities are shifting in the same direction. "One of the biggest things going on at the moment is the Student Event Analysis," explains Barnett. "The idea is for high school students to be able to analyse real data from ATLAS. At the moment simulated data are being used.The students first identify W and Z bosons, separating them from background events. Then they can search for one Higgs event hidden amongst 200 other events." This may sound pretty complex for high school students, but the system is already up and running and has been tested out on many students. In fact, an EC grant to expand the scheme has been awarded.

The second major development is the new ATLAS visitors centre. The ATLAS cavern has been closed to visitors for some time, so the new visitor centre is there to recreate the incredible experience. And it does so with some impressive technology, including an interactive screen and a 3D film of the detector. The centre, situated at Point 1, was inaugurated on 23 February, and it has been a regular stop for visits ever since.

The outreach team also has a huge number of projects planned for the future. These are too numerous to mention them all, but here’s a taster: ‘Adopt an event’ is a proposal for members of the public to adopt a particular physics event. They will then be given an image of the event and can go on to see if it is included in any papers published later. The winner of an ATLAS multimedia contest will come to CERN as an intern to make films about ATLAS during the restart of the LHC. And the new ATLAS pop-up book will be out in time for a perfect Christmas gift.