CMS Bulletin

HADRON CALORIMETER (HCAL)

All the HCAL calorimeters are ready for data-taking in 2011 and participated fully in the cosmic running and initial beam operations in the last few weeks. Several improvements were made during the winter technical stop, including replacement of the light-guide sleeves in HF, improvements to the low voltage power connections, and separation of HF from HB and HE in the DAQ partitions.

During the 2010 running a form of anomalous noise in the HF was identified as being caused by scintillation when charged particles pass through a portion of the air light-guide sleeve. This portion was constructed from a non-conductive mirror-like material called “HEM”. To suppress these anomalous signals, during the recent winter technical stop all sleeves in the detector were replaced with sleeves made of Tyvek. The detector has been recommissioned with all channels fully operational. Recalibration of the detector will be required due to the differing reflectivity of the new sleeves compared with the HEM sleeves. The necessary techniques for recalibration are in place, using a combination of φ-symmetry and di-electron events from Z boson decay. A timing scan was performed for HF and CASTOR with the first LHC collisions, and only minor adjustments were indicated.

The HO Ring 1 HPDs were operated at 6.5 kV during 2010 since several HPDs suffered discharge problems at nominal voltage. They operated stably throughout the year at this reduced voltage, so it was decided to try operation at 7.5 kV to improve performance for the 2011 physics run, and to carefully monitor the discharge rates. So far, none of the HPDs have started to discharge. If discharge does start, the affected HPD will be reduced to 6.5 kV.

Lumi, the luminosity measurement for CMS, uses data supplied by HF. HF and HB+HE were separated in DAQ partitions to reduce the number of times that electronics problems will require an interruption to the delivery of Lumi data.

A conductive grease was applied to connections at the low voltage power supplies to reduce wear and corrosion. This is a partial mitigation for a long-term problem with increased resistance in connections on the CAEN power supplies used by several CMS detectors.

The cause of the operation problem, which occurred roughly every week or two in 2010, has not been identified, in which synchronisation was lost for an RBX and its data was garbled. This is a particularly difficult problem to troubleshoot, but it is believed that it is most likely due to a susceptibility to clock glitches. Specific monitoring has been added to identify the behavior and improvements to the recovery sequence are being developed.
Preparations continue for the replacement of the HO photodetectors with silicon photomultipliers and for the replacement of the HF PMTs during the next long shutdown. The first batch of SiPMs for HO has been delivered and the full set of electronics boards has been fabricated. The Quality Assurance program has begun, as well as detailed planning for the installation during the shutdown. For the HF PMT replacement, the order for the tubes has been placed and engineering work is underway on the cable plant and readout box changes required to support multianode PMTs. Eight of the new phototubes were installed in the detector in November last year. One of these tubes was configured during the extended technical stop with split-anode readout for evaluation purposes. The results of these in-situ tests will help refine the final design for the multianode modification of HF.

The calibration of HB and HE was improved using φ-symmetry with non-zero suppressed data and η-dependent corrections using isolated track data with the full 2010 data set. More detail is provided in the DPG section of this bulletin. With these corrections applied anomalies were seen in specific locations in HB in splash events taken when the LHC re-established circulating beam. These anomalies appear to be due to misplaced filters on the HB layer-0 fibers for HBminus iphi 6 and 32. The scintillator in layer 0 (L0) is thicker than the other layers, so a filter was installed to reduce the L0 light before combining all layers together in the HPD readout. The post-calibration splash data suggests that the filter is missing in iphi 32, and placed incorrectly in iphi 6, resulting in a non-linear energy response below about 15 GeV in these two sectors. A correction for this problem is now included in the reconstruction software.