Long Shutdown 1 (LS1) — the two-year period of maintenance, consolidation and upgrades to prepare the LHC and its particle detectors for collisions at an energy of 13 TeV — recently came to an end as the LHC was successfully restarted earlier this year. Although the term “shutdown” seems to imply a quiet period, this interpretation couldn’t be farther from the truth. Below we present in no particular order some highlights from what was a very productive two years at the CMS experimental site, as we take the final steps towards analysis data at the new energy frontier.
Replacing the LHC beam pipe inside CMS
Particle beams fly around the LHC in clockwise and anti-clockwise directions through narrow beam pipes before colliding within the heart of the CMS detector. The first few layers of detection material in CMS — belonging to the Tracker sub-detector — fit snugly around this beam pipe. The innermost Tracker system is currently made of three concentric cylindrical layers, but will be upgraded in the coming years with the addition of a fourth layer to improve the particle-tracking capabilities of CMS. To incorporate this new layer, the section of the beryllium beam pipe located within CMS needed was replaced with a narrower variant.
- Video: Beam-pipe removal
- Photos: Beam-pipe removal
Cold operation of the Tracker
The aforementioned Tracker is the CMS sub-detector located closest to the particle collisions and faces an onslaught of billions of particles flying through it every second. To mitigate against long-term damage caused by this integrated particle flux, it was decided to operate the Tracker at a much lower temperature than before. New systems were installed at the CMS experimental site to facilitate these operating conditions.
Extraction and re-insertion of the Pixel Tracker
One of two components of the CMS Tracker, called the Pixel Tracker, was removed along with the beam pipe and stored in a separate facility built aboveground. After the new beam pipe was installed, the Pixel Tracker was re-inserted into the CMS detector in a delicate operation that required a few weeks in total.
Repairs to the ECAL Preshower
A small problem detected in one part of the ECAL Preshower system necessitated its removal from the detector for repairs in 2013. Both Preshower discs were successfully repaired and re-installed inside CMS in 2014.
- Photos: Removal of the ECAL Preshower
Improvements to the Muon system
A fourth triggering and measurement station for muons was added in each of the endcaps to help discriminate between interesting low-momentum muons originating from the collision and fake muon signatures caused by backgrounds. Seventy-two new cathode-strip chambers (CSCs) and 144 new resistive-plate chambers (RPCs) were assembled across a three-year period and installed during LS1.
The solenoid awakens after a long slumber
The magnet central to the CMS design (a solenoid, hence the S in CMS) was dormant during LS1 and there was a great deal of activity involved when it was switched back on at the end of 2014.
- Photos: The CMS magnet during LS1
BRIL team installs new detectors
The Beam Radiation Instrumentation and Luminosity (BRIL) project installed several new detectors inside CMS during LS1, including the Pixel Luminosity Telescope, the Beam Condition Monitor and the Beam Halo Monitor, which will help improve measurements of the collision rate within the detector and also the beam-related backgrounds affecting its measurements.
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Outreach activities at the experimental site
Over the course of LS1, nearly 80,000 visitors saw the CMS detector in its underground cavern, either as part of the programme of regular visits or during specially organised events.
CERN Open Days 2013
Around 5,500 visitors saw the CMS detector during the Open Days, making it by far the most-visited underground site at CERN over that particular weekend.
- Photos: CERN Open Days 2013 at CMS
CERN’s 60th anniversary: neighbourhood event
A special invitation was extended to the local community around CERN to participate in the celebrations of CERN’s 60th anniversary. CMS was one of the host sites for the visits which saw 4,500 people ride the underground elevator to see the CMS detector during the weekend.
School Days: special visits for local schools
In May 2014 and February 2015, CMS organised two special sessions of underground visits, providing hundreds of students from schools near CERN with the opportunity of seeing the CMS detector with their classmates.
- Photos: CMS School Day 2015
CMS detector made wheelchair accessible
One of the important milestones at the CMS experimental site was the successful installation of a lift for providing wheelchair access to the detector cavern.
- Photos: The wheelchair lift at CMS
— by Marzena Lapka and Achintya Rao
An article telling the story of LS1 at CMS was published in the CERN Courier in April: http://cerncourier.com/cws/article/cern/60877