CMS Bulletin

ECAL

One half of the ECAL barrel (EB–) and both endcaps (EE) participated in the CMS Global Run in November (GRiN). This was used as an opportunity to exercise the ECAL DAQ and trigger systems following software development work during LS1 and verify the operation of ECAL with an increased latency of +12 BX. It was also used to check the status of EB– following the reconnection of low-voltage cables that were disconnected at the start of LS1 to allow the replacement of HCAL photodetectors. The cables in EB+ will be reconnected and re-commissioned in 2014. The GRiN was also used to verify the successful repair in September 2013 of a region of 75 channels in the positive endcap (EE+), which had not been fully operational since the LHC’s startup in late 2009. A campaign to refurbish all EB/EE low-voltage AC/DC convertors was carried out in parallel with these activities.

Monitoring data (laser and LED light) has been taken regularly to measure the recovery of crystal transparency during LS1. The blue solid-state laser (DP2-447), used to derive the ECAL response corrections, has been re-commissioned at Point 5 following planned refurbishment by the manufacturer, and the laser DAQ software is being upgraded. Humidity sensors have been installed in the ECAL laser room to study the effect of humidity variations on laser performance. A technical test of the application of response corrections in the ECAL barrel at Level-1 and HLT was successfully carried out during the GRiN. These corrections were applied weekly in EE during 2012 and will be extended to EB in 2015.

A serious issue with a connector on the exterior of the minus end of the Preshower (ES–) was discovered after the GRiN. The connector had been damaged beyond repair due to excessive heat. The likely reason for the damage has been understood and a Technical Incident Panel helped to define the path forwards, which involves removing both preshower disks and replacing the damaged connector and three others of the same type. The ES– was removed from CMS in early December and the ES+ due to be removed about a week later. Repairs will take place on the surface, either at P5 or on the Meyrin site, before reinstallation in late spring 2014, prior to the installation of the beam pipe. A detailed Twiki has been prepared and is regularly updated with the latest information and background material.

An ECAL DPG workshop, "Towards ultimate energy calibration and resolution with Run1 Data" was held on 21 and 22 November. The workshop demonstrated a significant amount of progress in understanding the ECAL performance, and in refining our calibration techniques.

The workshop highlighted the importance of improved simulation of the detector noise and better modeling of out-of-time pile-up. Retraining the energy corrections using the more realistic Monte Carlo yields a 6% improvement in resolution for the best-photon category in EB, compared to the 22 January re-reconstruction of 2012 data. In addition, the extra smearing (per electron) needed to match the Monte Carlo is substantially reduced when improved MC and calibrations are used, especially in EE (was 3-4%, depending on EE electron category in the Moriond 2013 dataset; now <2%).

A significant improvement in our understanding of the material upstream of ECAL has been obtained by using several data-driven techniques performed jointly by the ECAL DPG and Tracker POG. A new Tracker material scenario, derived from these results, is being prepared and is expected to reduce the residual discrepancy in resolution between data and MC even further.

Regarding Run 1 data, the reference paper for electron/photon energy scale and resolution using 2010/’11 data, was completed and published in the Journal of Instrumentation in September.

Work is continuing on several fronts in the area of ECAL upgrades, in preparation for the Technical Proposal in 2014 as well as for the planned upgrades in LS3. A series of longevity tests of existing detector components are being carried out. These involve accelerated ageing tests of the on-detector front-end electronics of EB and EE, as well as measurements of the performance of irradiated barrel APDs. Studies of the feasibility of optical bleaching and thermal recovery of irradiated EE crystals are ongoing, and are expected to conclude soon.

A significant amount of work is ongoing to tune the ECAL reconstruction algorithms and evaluate the performance of the current detector at high event pile-up and including detector ageing into the simulation. Finally, following a kick-off meeting held during the October CMS Upgrade week, a set of four working groups has been formed to define the scope and required functionality of the ECAL barrel front-end electronics upgrade (needed after LS3 to provide additional Level-1 Trigger bandwidth and latency).