In the early hours of 14 February 2013, the first period of running for the Large Hadron Collider came to an end. After three very fruitful years of collisions at the LHC, the blank screens of LHC Page 1 seem very odd. “End of Run 1. No beam for a while,” proclaims the remarkable understatement on the webpage that displays the current LHC status. “Access required, time estimate: ~2 years”. The two-year break from operations is the LHC’s first long shutdown, or LS1, and aims at consolidating both the accelerator as well as the detectors. At the end of LS1, the LHC is expected to provide collisions at a centre-of-mass energy of 13–14 TeV, significantly higher than the 8 TeV run of 2012. In addition, the expected separation between successive collisions at this energy will be 25 ns instead of the 50 ns typically used so far.

All CMS subdetectors will benefit from this break by performing crucial tasks necessary to operate the detector at the higher energies and collision rates we will see from 2015 onwards.


The CMS Tracker is based on silicon sensors and is the innermost subdetector, closest to the collision. The inner component of the Tracker known as the Pixel detector is due for a major upgrade that will add an additional tracking layer to the three present at the moment. In preparation for this, a new laboratory has been set up at Point 5 for the Pixels. During LS1, the Pixel detector will be extracted and stored in this lab, to carry out repair work on some damaged parts. Two pilot blades with modules from the Phase 1 upgrade — featuring new electronics — will also be installed. The Pixel detector will be better centred with respect to the beam line when it is reinserted into the Tracker. The outer component of the Tracker, known as the Strips, will undergo repair work on some damaged connections in an attempt to recover as many channels as possible.

In order to mitigate against radiation damage, the main task for the CMS for LS1 will be to lower the Tracker’s operation temperatures significantly. Under humid conditions, lower temperature can lead to condensation that can damage the detector and its service infrastructures. A better dry-gas supply and improved humidity barriers will help prevent such condensation. Furthermore, the software for data acquisition and detector safety will be upgraded along with the offline monitoring and analysis tools.


The second layer in the cylindrical onion that is CMS comprises the Electromagnetic Calorimeter or ECAL, which detects electrons and photons and measures their positions and energies. Two of the three ECAL sub-systems — the endcap crystals (EE) and preshower (ES) — will undergo some minor repairs whilst the barrel (EB) will have its low-voltage cables disconnected to allow the installation of the new HCAL Outer Barrel (HO) photo-detectors. These interventions will be followed by a period of careful re-commissioning. The monitoring system for the crystal transparency will be consolidated, with the addition of a second solid-state 447nm laser and the de-commissioning of two lasers that are more than ten years old.

Off-detector, the data acquisition (DAQ) software, which evolved from the commissioning of ECAL in beam tests around a decade ago, will be completely overhauled and streamlined to ensure long-term sustainability and also to be compliant with developments in the central DAQ and Trigger areas. The activities in the ECAL Detector Performance Group (DPG) are increasing, reflecting the need to automate as much as possible in preparation for the next LHC run. Another area that is already experiencing an upsurge in activity is long-term performance and upgrades, with a view to determining, in the coming years, the optimum ECAL configuration (including possible new detector elements) for Phase 2 of the LHC, from about 2022 onwards.


The Hadron Calorimeter or HCAL detects composite particles such as protons and pions produced in LHC collisions. In LS1, several of the photo-detectors, including all of those in the HCAL Outer Barrel (HO), will be replaced by better, completely different versions to improve the signal-to-noise ratio from the detectors. The photo-detector components have been received and tested at CERN, and the results indicate that they are of high quality, with very few components requiring repair work.

In 2011 and 2012, CMS suffered from a small loss of data due to the so-called “single-event upsets”. This data loss can be minimised in HCAL by replacing components in the Clock and Control Modules (CCMs) of the HCAL Barrel (HB), Endcap (HE) and HO. Since these CCMs reside deep within the subdetector, they can only be accessed when CMS is open, which is why LS1 is the first opportunity to undertake these replacement tasks. In addition, many other tasks are being performed as a first step towards the Phase 1 upgrade of HCAL.


The muon system, as the name suggests, detects muons, heavier cousins of the familiar electron. The CMS muon system is subdivided into three major components — the Drift Tubes (DT), the Resistive Plate Chambers (RPCs) and the Cathode Strip Chambers (CSCs) — each of which has many specific tasks to accomplish during LS1.


LS1 will be the first opportunity to access the Drift Tubes since 2009, and quite possibly they will not be accessible for an even longer time after LS1. Therefore, all problematic DT chambers or electronics will be repaired during this period to guarantee an optimal performance in the future. 120 Trigger Boards will be replaced by ones based on a new technology. The technology used for the existing boards is no longer available, preventing an enlargement of the pool of spares.

Some of the DT electronics will be relocated from inside the experimental cavern to the service cavern. In order to allow the signal to be retransmitted even further, custom copper-to-optical converter modules will be installed. This will lay the foundation for the DT upgrade programme, which will allow the readout electronics of the subdetector to withstand the higher LHC luminosities as well as provide the flexibility needed to implement trigger upgrades.


One of the main activities for the Resistive Plate Chambers is the installation of the fourth disk (RE4), which will complete the RPC endcap region as it was designed in the CMS technical design report. 600 bakelite gaps necessary to build 200 RE4 double-gap chambers are under construction in Korea. The chambers are built and tested at CERN and laboratories in Belgium and India. The front-end, read-out and power system electronics are under construction in Pakistan, Italy and Finland, and will be installed and commissioned during LS1.

The present RPC system, consisting of 912 chambers and around 110,000 electronic channels, will undergo special maintenance and repairing. Many technical interventions have been scheduled to recover the 15 disconnected chambers and the 2% of electronic channels that are not working. A gas leak survey, a power system check and a resistivity measurement will be performed in order to do a complete review of the system.


The Cathode Strip Chambers detect and measure the positions and momenta of muons going through the ends of the CMS detector. The CSC system currently has 468 large chambers, each containing harp-like planes of stretched wires (2.5 million wires in all) kept at several thousand volts. To ensure that muons will be better identified in the higher-energy and higher-rate collisions that will follow LS1, an additional set of 72 chambers produced at CERN will be installed. In addition, the innermost set of CSC will be outfitted with dramatically better electronics to handle the extremely high collision rates.

Other consolidation tasks

CMS will also take advantage of the opportunities presented by LS1 to improve the Trigger and data acquisition (DAQ) as well as the overall cooling infrastructure, among others. Another big activity is to prepare CMS for Phase 2 of the LHC, with the preparation of a Technical Design Report for the upgraded detector.

All in all, LS1 will be as busy a period as ever for CMS, and the collaboration is looking forward to the many new challenges that lie ahead.

— Prepared with contributions from Petra Merkel, Dave Barney, Pawel De Barbaro, Cristina Fernández Bedoya, Pierluigi Paolucci and Jay Hauser.