CMS Bulletin

UPGRADES

There is very good progress in the execution of the LS1 projects and in launching construction of the Phase 1 upgrades. We focus here on two main achievements since the last CMS Week.

The approval of the third Phase 1 TDR

The preparation of the L1 Trigger Upgrade Technical Design Report has been a major effort of the collaboration at the beginning of this year, especially to develop supporting Trigger menu and physics performance studies. These studies have demonstrated the efficiency of the upgraded system to ensure low lepton and jet trigger thresholds, leading to a significant increase of the acceptance for the Higgs measurements, in the associated production mode and in the ττ decays, as well as for the stop searches involving multiple jets in the final state. The TDR was submitted to the LHCC in May and approved at the June committee meeting. It is now a public document, completing the series of the three TDRs describing the Phase 1 upgrades, with the new Pixel system and the HCAL read-out replacement.

The Upgrade Week in DESY and the Phase 2 upgrades

The work on developing the conceptual design for the CMS Phase 2 detector was the main focus of the June upgrade week held at DESY. The meeting was an agreeable and successful week of intense work – many thanks to our DESY and Hamburg University hosts.

The discussions converged on a primary scenario with options for the upgrade, and on the paths towards the shorter- and longer-term deadlines of the project.

The studies of radiation damage and simulation of performance degradation were presented, and clearly demonstrate that new Tracker, and the ECAL and HCAL endcaps (EE and HE respectively) will be needed for the HL-LHC programme. Most likely, all these systems will need replacement after a recorded luminosity of 300 to 500 fb^–1. The ongoing studies will allow further mapping of the operational and performance margins.

The performance of two Tracker configurations, Long Barrel and Barrel Endcap, were compared. A baseline configuration with six outer barrel layers and five forward disks was adopted. A major focus of the project will now be to demonstrate the feasibility of hardware track reconstruction using Associative Memories, and to assess the latency required for implementation in the L1 Trigger.

Two main approaches emerged for the upgrade of the forward calorimetry: replacement of the EE and refurbishment of the HE sensitive material without changing the absorber; or full replacement of both EE and HE, presenting an opportunity for integrated forward calorimetry. An EE shashlik calorimeter and new scintillating or quartz tiles for the HE are considered in the former case, while a dual read-out or imaging calorimetry are investigated in the latter one. Although EB and HE will survive Phase 2 radiation doses, it is proposed to operate the EB at 8–10 ºC to mitigate the increase of noise in the APDs.

The muon system upgrade will include installation of new chambers in the forward region (1.5 < η < 2.4) to improve the L1 Trigger capability. The GEM1/1 project to equip the first station during LS2 is a first step of the project. For farther stations, both GEM and GRPC technologies will be investigated. Due to radiation-induced failures, it is also proposed to replace the DT front-end electronics (minicrates).

To maintain and possibly enhance the L1 Trigger acceptance, it was shown that using Tracker-tracks at this stage will provide significant rate reduction factors. However, to maintain low thresholds for all major triggers, an increase of the overall L1 Trigger rate will be required to allow proper allocation of bandwidth in the Trigger menu. The replacement of the EB front-end electronics (FE), together with the other upgrades foreseen, will allow reaching a 1 MHz rate with an increased latency of 10 µs (increasing the latency beyond this value would require replacement of the CSC FE).

In addition to the main upgrades considered above, pile-up mitigation in the forward region may require specific attention. A preliminary study indicates that an extension of the Pixel system up to η = 4 would almost fully eliminate the large rate of jet-tagging-faking VBF processes due to the high pile-up. R&D on precise timing detectors and their ability to allow discriminating vertex origins are also investigated. Replacing the HE absorber could also allow extended coverage of calorimetry and muon systems up to η = 4 to tag forward tracks associated with a muon. A more complete muon measurement implementing forward toroids in place of the HF could be envisaged if strongly motivated.

The emergence of the upgrade scenario and options will now allow Technical Coordination to establish the subsequent technical constraints at Point 5, simulate the background and radiation generated in the HL-LHC beam conditions and evaluate the scope of work and need for the LS duration. The Tracker and EB FE replacement will be concurrent and will require a shutdown of 26 months. There is also good indication that HE and HF could be removed from the UXC, allowing replacement of the HE absorber. The sequence of work to handle the replacement of Tracker/EB FE, the Forward Calorimeter, the DT minicrate and the YB0 services is being thoroughly investigated. The simulation of the background and radiation conditions will be essential to decide on the extension of the subsystems’ η coverage.

Detector and Physics performance studies are underway to identify main requirements to achieve the HL-LHC physics programme. Benchmark studies for the Snowmass white paper and the ECFA workshop in October are identified. They will use various simulation tools, available or developing. DELPHES will be used for simple implementation of new detector geometries and parameterisation of the measurement performance. This will allow evaluating the benefit of various upgrade options on the timescale of the ECFA workshop. The DELPHES studies will be complemented with fast and full simulations with realistic but less complete detector upgrade descriptions and for a limited set of signals. This will serve as validation of the simplified studies and also to provide performance reach for key physics signals representative of the detector requirements, namely HH→bbγγ and VBF H→ττ. Many new results are expected for discussion during the CMS Week at Taipei.

In parallel, upgrade projects, working group and cross-coordination areas will proceed with cost estimates in preparation of the October 2103 RRB session.  The longer-term goal is to prepare a Technical Proposal for submission to the LHCC in September 2014.