• Transverse-oblique view comparing the pixel barrel layers in the existing detector (left half) and the upgraded version (right). Image credit: Tai Sakuma

  • The cover of the CMS Pixel Detector Upgrade TDR. Image credit: Sergio Cittolin

  • The top part of the image shows a ceramic holder loaded with 2x2 mm2 SiPMs. The lower part illustrates the proposed depth segmentation, with each colour being a depth read out separately. (The interleaved layers in HB allow optimal use of the SiPM dynamic range.)

  • One half of the HCAL Barrel (foreground) seen here in preparation for mounting inside the CMS magnet in 2006. Photo credit: M. Brice, L. Veillet, D. Lazic

After three years of very successful operation that led to the discovery of a new boson in 2012, the LHC is scheduled for a series of upgrades that will enhance the experimental potential to study the nature of the new particle, and to extend the searches for new physics beyond the Standard Model.

The LHC upgrades will develop in three long shutdowns, designated LS1, LS2, and LS3. In the period 2013–2014 (LS1), the collision centre-of-mass energy will be increased to 14 TeV (or slightly lower). The original performance goal for the LHC to operate at an instantaneous luminosity of 1x1034 Hz/cm2 with 25 ns bunch spacing is likely to be achieved soon after LS1. This is the scenario for which CMS was designed, with an average number of inelastic interactions per crossing (event “pile-up”) of about 25. In the period through LS2 in 2018 the injector chain will be improved to deliver very bright beam bunches into the LHC. It is anticipated that a significant increase in luminosity can be achieved, with a resulting increase in pile-up.

CMS must be prepared to operate, and to extend the full physics programme, with event pile-up of 50 as a baseline, with the possibility that it may be significantly higher at the beginning of LHC fills or if the bunch spacing remains at 50 ns. The goal of the upgrade programme is to provide excellent detector performance under these severe pile-up conditions, which is crucial to support the rich physics programme foreseen. The upgrades prior to LS3 constitute Phase 1. They were outlined in a Technical Proposal published in 2011. There are three major projects planned in Phase 1: a replacement of the pixel tracker, an improvement to the L1-Trigger system and an upgrade of the hadron calorimeter (HCAL). Technical Design Reports have been prepared for the Pixel and HCAL upgrades. A TDR for the Trigger upgrade is currently in preparation.

Pixel Tracker upgrades

The current pixel detector has efficiently recorded data since the LHC first collisions. It provides high track-reconstruction efficiency and precise measurement of the track origin. To preserve the high tracking quality at luminosity and pile-up beyond the original specification, a new detector has been designed with four layers in the barrel and three disks in the endcaps. It will provide an additional fourth space-point measurement over the whole tracking pseudo-rapidity range. The inner layer in the barrel will be closer to the beam axis while the outer layer will be closer to the Tracker Inner Barrel. Along with the use of a new CO2 cooling system and light-weight mechanics, a new cabling scheme for the optical readout and for the powering with DCDC converters will allow reducing the material budget in the tracking acceptance. New readout electronics will allow the pixels to operate at the highest foreseen luminosity without data loss.

With these new features, the pixel detector will provide higher tracking robustness and precision. This has been demonstrated with detailed performance simulations, also showing a significant improvement in b-tagging capability. Up to a pile-up of about 60, the new pixel detector will continue to perform better than the current detector operated at a pile-up of 25. During LS1, a slice of the new detector will be incorporated into the free space available in the current detector, allowing in-situ validation and preparation of the final detector’s commissioning. The full detector is foreseen to be ready for installation in an extended Year-End Technical Stop at the end of 2016.

HCAL upgrades

The hadron calorimeter (HCAL) upgrade is taking advantage of new technologies that have become available since the design of the original detector. It is based on the replacement of the photo-detectors and subsequent readout electronics. The photomultipliers (PMT) of the Hadron Forward calorimeter (HF) will be replaced with multi-anode PMTs and the hybrid photo diodes of the Barrel and Endcap calorimeters (HB/HE) with Silicon Photomultipliers (SiPM). These new devices will minimise the spurious signals generated inside the current photo-detectors and will allow depth segmentation in the HE and HB for improved calibration and better shower-development measurement. In addition, the front-end readout electronics will implement new time digitising capability for further background rejection. New optical links (GBT) and back-end electronics based on the μTCA technology backplanes will allow increasing the data output bandwidth. The installation of the backend electronics is foreseen by the end of 2015 to allow commissioning of the trigger upgrade in parallel to operations. The installation of the front-end will then occur in the following year-end stop for the HF, and during LS2 for the HB and HE.

Testing using simulations of key signals

To demonstrate the strong motivation for the upgrades, the Higgs and SUSY Physics Analyses Groups, and Offline, PPD and Computing Coordination have orchestrated a full campaign of Monte Carlo physics samples production and analyses. Key signals of the Phase 1 physics programme were simulated. The H→ZZ→4l analysis is critical for the measurement of the spin of the new particle. In this channel, it was shown that the HCAL upgrade will significantly improve the lepton isolation, while their higher association efficiency with the new pixel detector will increase the signal rate by 40%. Two fermionic decay modes where also investigated: in the ZH H→bb production, the additional improvement in the b-tagging will allow a signal increase up to 65%; and in the VBF H→ττ case, the increase will reach a factor 2.4, due to the better forward jets and τ measurements. These spectacular improvements were also confirmed in the SUSY domain. For instance, a significant reduction of the background was observed in the “di-photon plus missing ET” final state, due to the lower pixel material budget and the better missing ET measurement with the HCAL upgrade.

The TDRs for Pixel and HCAL were approved by the CMS collaboration and presented to the LHC Committee in September 2012. The LHCC acknowledged the quality of the documents, the strong physics motivations for the upgrades and the level of preparation of the two projects. It also supported the multi-stage approach for the deployment of the upgrades. In the conclusion of its report, the LHCC “endorses the HCAL and Pixel Detector upgrades without reservation”. The two projects are now ready to ramp-up in 2013. The L1-Trigger upgrade project is now preparing a TDR for early 2013; this will complete the description of the technical scope for Phase 1.

— Submitted by Didier Contardo and Jeff Spalding