HADRON CALORIMETER (HCAL)

Operations and Maintenance

All HCAL sub-detectors participated throughout the recent data taking with 7 TeV collisions. A timing scan of HF was performed to optimize the timing across the detectors and to set the overall time position of the ~10-ns wide signals within the 25-ns integration time slice. This position was chosen to ensure that the trigger primitives in physics events are generated synchronously at the desired bunch crossing, while also providing discrimination between the calorimeter signals and anomalous signals due to interactions within the photomultiplier tubes. This timing discrimination is now used in the standard filter algorithms for anomalous signals.

For HB and HE, once the statistics needed to assess the timing of a sufficient number of channels was accumulated, it was verified that the time settings determined with cosmic, splash events and initial collision data were appropriate for the 7 TeV collision data taking. A further fine-tuning of the HB and HE time settings will be performed once sufficient data at high transverse momentum will be available. This is expected to be after the data set collection for the ICHEP conference. Since the great majority of HB, HE and HF trigger primitives were coming at the correct bunch crossing, the main jet triggers have been enabled at the Level 1 trigger.

While the detector channel status had been stable for many months, two issues have arisen in the last few weeks. The data from one fiber (three channels) in HB is lost; only empty data frames are being sent. This problem is currently under investigation. And one HPD (18 channels) in HB now draws excessive bias current and must be operated at a reduced bias voltage. This problem too is under investigation. It is not known whether the failure is within the HPD itself or in the bias circuit. In either case, repair will not be possible until CMS is opened during a shutdown.

In order to help preserve the lifetime of the HPD the bias voltage was lowered from 80V to 30V. The pulse produced with a lowered bias voltage will tend to be broader. HCAL energy is reconstructed using 4 25ns time samples and we are investigating the impact of reducing the bias voltage on the energy determination. We do observe a broadening of the pulse, however it appears that most of the pulse remains within the 4 time slices. The peak of the pulse occurs at a later time slice and the impact on the trigger efficiency is being evaluated. Additional studies are also needed to correct the standard reconstruction.

HCAL suffered a series of failures in the HV power supply system in April. A review is ongoing, and initial improvements have been made, including additional fusing and improvements to the filtering circuit in the primary power supplies.

The high voltage distribution to 8 channels of HF failed. The board was repaired during a two-day maintenance period.

It has been agreed by CMS that the HPDs in the YB-2 and YB+2 rings of HO will not be operated for the 2010-2011 running period, where very high energy jets will be rare. The HPDs in these rings will be conserved as spares for future operation of HB, HE and HO (inner rings). HO rings 0 and 1 will continue to be operated as normal. The plan is to replace the HPDs in rings 1 and 2 with SiPM-based front-end electronics in 2012.

Several channels in Castor are affected by the magnetic field that is higher than expected in this region. The PMT gains have been equalized for all the non-affected PMTs, and the number of affected channels has been reduced by increasing the HV appropriately, after studying the LED signal and halo muon data with field on and off. An analysis is ongoing to study energy flow at high eta, including Castor and HF.

ZDC is operating routinely. Anomalous PMT hits (similar to those in HF) are seen in 0.1% of collisions but these can be readily identified and filtered. This will not compromise performance in heavy-ion running. The monte-carlo is being improved to better match the energy distribution seen in the data. Studies are underway to develop a ZDC-based trigger for diffractive physics.

Development for Improvements and Upgrades

The HCAL group continues development work on several improvements and upgrades targeted for the shutdowns expected in 2012 and 2015-16. The HPDs in HO rings 1 and 2 will be replaced with SiPM-based front-end electronics in 2012. A plan being prepared for replacement of the HF PMTs in the same shutdown with units having thinner windows, which will reduce the rate of anomalous signals. Design work continues towards replacing by 2016 the HPDs with SiPM-based readout for HB and HE and upgrading the readout to providing depth segmentation and improved signal/noise. There has been a significant focus in the last two months on radiation damage and longevity studies for these new components, both SiPMs and PMTs, including neutron and activation exposures. These studies will conclude in the next two months.

by by J. Spalding and A. Skuja