Analogue Input Calibration of the ATLAS Level-1 Calorimeter Trigger : TWEPP-09

Abstract

The ATLAS Level-1 Calorimeter Trigger is a hardware-based pipelined system using custom electronics which identifies, within a fixed latency of 2.5 μs, highly energetic objects resulting from LHC collisions. It is composed of three main subsystems. The PreProcessor system first conditions and digitizes approximately 7200 pre-summed analogue calorimeter signals at the bunch-crossing rate of 40 MHz, and identifies the specific bunch-crossing of the interaction using a digital filtering technique. Pedestal subtraction and noise suppression are applied, and final calibrated digitized transverse energies are transmitted in parallel to the two subsequent processor systems. Several channel-dependent parameters require setting in the PreProcessor system to provide these digital signals which are aligned in time and properly calibrated. The different techniques which are used to derive these parameters are described, along with the quality tests of the analogue input signals and the status of the energy calibration. I. THE ATLAS LEVEL-1 CALORIMETER A. The ATLAS Trigger The ATLAS trigger system consists of three separate components. The task of the ATLAS trigger is to reduce the event rate from 40 MHz to 200 Hz. A schematic of the ATLAS trigger can be seen in Figure 1. The Level-1 trigger is composed of trigger from reduced granularity calorimeter and muon information which are entirely hardware based. The Level-1 system has a requirement that the latency be less than 2.5 μs. The output of the Level-1 trigger are programmable trigger items and regions of interest (RoIs), which are small, energetic areas of η and φ. The Level-1 trigger is itself composed of three sub-systems. L1Calo, the focus of these proceedings, searches for electrons, photons, taus, single hadrons, jets, missing ET and total ET and provides multiplicities per ET threshold with isolation criteria. L1Muon searches for muons. The central trigger processor (CTP) receives information from both L1Calo and L1Muon and generates the Level-1 accept decision which is passed on to Level-2. The Level-2 trigger is software based comprised of approximately 500 CPUs taking the Level-1 RoIs as its input. The Level-2 trigger has access to the full granularity of the ATLAS detector and has a requirement that the latency be less than 40 ms. The Event filter, or Level-3, is a software trigger comprised of approximately 1600 CPUs. The event filter has access to the full event information, calibration constants and offline algorithms. Figure 1: The ATLAS trigger system. B. The Level-1 Calorimeter Trigger The ATLAS Level-1 calorimeter trigger (L1Calo) is fully described elsewhere [2]. L1Calo is a 1 μs fixed latency, pipelined, hardware based system which uses custom electronics. L1Calo consists of nearly 300 VME modules of 10 different types housed in 17 crates. L1Calo is located entirely off detector in the service cavern USA15. Around 250,000 calorimeter cells are summed to 7168 L1Calo channels. The granularity of L1Calo is described in Table B.. Position ∆η x ∆φ |η| < 2.5 0.1 x 0.1 2.5 < |η| < 3.1 0.2 x 0.2 3.1 < |η| < 3.2 0.1 x 0.2 3.2 < |η| < 4.9 0.4 x 0.4125 Table 1: Granularity of L1Calo. L1Calo has three processor types. The PreProcessor (PPr) digitizes the analogue calorimeter pulses, performs bunchcrossing identification and converts ADC counts to energy. The Cluster Processor (CP) identifies electrons, photons and single hadrons. The Jet/Energy-sum processor (JEP) does jet finding and energy sums. C. The PreProcessor (PPr) The calorimeter pulses are obtained through the receiver system which provides input signal conditioning via variable gain amplification. This provides the sin (θ) correction for the E → ET conversion. The calorimeter pulse is sampled at 40 MHz by 10-bit flash-ADC PPrs. The calorimeter pulse is sampled over five bunch-crossings with the pedestal set at 32 ADC counts. Bunch-crossing identification is performed using peak finder which can run in either a linear or saturated mode. A finite impulse response (FIR) filter aids the peak finder by sharpening the signal and improving the signal to noise ratio. The ET is calculated using a look up table which removes the pedestal and provides noise suppression.