CMS Bulletin

TRIGGER

Level-1 Trigger Hardware and Software

After the winter shutdown minor hardware problems in several subsystems appeared and were corrected. A reassessment of the overall latency has been made. In the TTC system shorter cables between TTCci and TTCex have been installed, which saved one bunch crossing, but which may have required an adjustment of the RPC timing. In order to tackle Pixel out-of-syncs without influencing other subsystems, a special hardware/firmware re-sync protocol has been introduced in the Global Trigger. The link between the Global Calorimeter Trigger and the Global Trigger with the new optical Global Trigger Interface and optical receiver daughterboards has been successfully tested in the Electronics Integration Centre in building 904. New firmware in the GCT now allows a setting to remove the HF towers from energy sums. The HF sleeves have been replaced, which should lead to reduced rates of anomalous signals, which may allow their inclusion after this is validated. For ECAL, improvements in tower masking are underway. Using 2010 data, a new configuration of the e/γ trigger was studied which rejects anomalous energy deposits in the ECAL APDs. Fine-grain, H/E and isolation criteria were also introduced in the e/γ trigger to reduce the trigger rate, and η-dependent corrections were introduced in the L1 jet trigger. Several improvements were introduced in the muon triggers to improve the efficiency and to reject fake muon tracks. For the RPC, 3/6 patterns are being validated. For the DT and CSC, Track Finder changes in the ghost cancellation schemes have been introduced which significantly reduce the fake di-muon rates.

In the L1 trigger online software, a new central monitoring and alarm infrastructure common to all trigger sub-systems is now in place. New L1 trigger menus for luminosities up to 1 x 10³³ cm^-2s^-1 were prepared and validated. These include cross-triggers, which become important as the basic single object triggers are pre-scaled for the lowest thresholds. In the L1 trigger offline software, the redesign of the L1 trigger emulator DQM based on quality tests was concluded. An effort is ongoing to implement automatic DQM quality tests on trigger rates, synchronisation and η-φ occupancies.

Level-1 Trigger Commissioning and Operations

After the successful increase in LHC luminosity last year, culminating with 348 colliding bunches at the end of the 2010 proton run, the accelerator is starting the 2011 run with a few colliding bunches and at low luminosities but plans to rapidly increase these numbers and surpass last year's values within a matter of weeks. So it is vital to ensure that also the CMS Level-1 Trigger can keep pace with this development.

To ensure smooth operation at high luminosities, almost all Level-1 trigger systems have used the winter shutdown for upgrades of their hardware, firmware and software. The focus is now to validate with the very first data that all these changes really show the expected results and do not introduce any unwanted side effects. Due to the large number of changes it is not always straightforward to disentangle the reasons for observed changes in the behavior of the detector and the electronics. However, most shutdown improvements have been validated by now and there remain only a few open questions of secondary importance.

To check the correct timing of all components we have again used minimum-bias triggers supplied by the beam scintillator telescope (BSC) and zero-bias triggers from the beam pickup system (BPTX). With increasing luminosity these triggers are now turned off or heavily pre-scaled and replaced by the “physics” triggers from the muon systems and the calorimeters. However, very soon also many of these triggers will have to be pre-scaled or replaced by triggers at higher thresholds. In order to prepare for these changes in advance the planned modifications are discussed between the Level-1 Trigger and the High Level Trigger communities on a regular basis so that the required menu changes and applied pre-scale factors on both sides can be matched.

In 2011 the LHC plans to arrive soon at a stable mode of operation, unlike 2010, when the number of bunches and the luminosity were increased every few days or weeks. So the trigger systems will not have to switch menus as frequently as last year. To get the most out of the available luminosity, pre-scale factors will be adjusted during runs so as to compensate for the decrease in luminosity over an LHC fill. This change will be made manually like in 2010 at first but is planned to switch to automatic operation controlled using the online luminosity measurements. So, each trigger configuration table will contain multiple "pre-scale set columns" both for L1 and HLT.

The achieved increase in the level of automation will allow the shifter to concentrate on monitoring the correct functioning of all systems and on checking the quality of the recorded data. Offline shifters will complement this work and allow the Trigger Field Managers to validate the data taken during their turn.

Trigger Studies Group

Since the end of the 2010 data-taking, the Trigger Studies group (TSG) has been preparing for a prolonged 2011-’12 run with luminosities exceeding 5E33 and covering large pile-up exceeding 10 interactions per crossing. To that end, a new series of “Trigger Reviews” took place at the end of November and the beginning of February. MC samples were provided to the physics groups in order to study the impact of high pile-up on trigger rates and signal efficiencies. The emerging conclusion is that the multiple-interaction environment in the 2011 LHC running comes with significant challenges that need to be addressed right away: higher rates (especially for triggers invoking multiple-jets, missing energy and energy sums), reduced rejection power for isolated leptons and significantly increased CPU-performance for HLT algorithms. An initial 5E32 menu with the corresponding datasets has been prepared and validated for the early LHC running, in close collaboration with the physics and detector groups. A follow-up Trigger Review has been scheduled for April for further refinement of the trigger strategy, using the data from the first 2011 collisions and with an eye towards 1E33 and beyond.

With the proliferation of multiple-object triggers, the task of Trigger Performance has become more challenging. Steps toward providing a higher level of automation of the validation and monitoring code have been taken. This aims at easing the burden on both shifters and validation experts, with further improvements expected over the next few months. Development on the monitoring of trigger and primary dataset rates continued with a focus on dynamic pre-scaling, and overall system reliability and redundancy. Validation and support of the HLT and trigger menus in several CMSSW releases continues to occur on a weekly basis.

Operationally, we saw the successful restart of data-taking in February. A dedicated trigger menu was deployed for dealing with off-beam operations, cosmic runs and circulating or ramping beams. Finally, the CMS trigger recorded its first 2011 collisions on Sunday, 13^th March.

It should be noted that a very small team of people carries out the TSG work. In particular, the team of on-call experts dealing with the integration of trigger menus needs to be strengthened. We would like to take this opportunity to thank the retiring on-call experts Dr. Edgar Carrera and Dr. Maurizio Pierini for all their hard work, and welcome three new members in the on-call team: Dr. Stephanie Beauceron, Alex Mott, and Cory Fantasia. The TSG welcomes new members and is searching for new participants in Trigger Performance and Trigger Menu Integration. This is an exciting and challenging job, which offers opportunities to have a big impact in the collaboration!