The Sector Collector of the CMS DT Trigger system: Installation and Performance

Abstract

Drift Tubes chambers are used for muon detection in the central region of the CMS experiment at LHC. Custom electronics is used for reconstructing muon track segments and for triggering the CMS readout. The trigger Sector Collector modules collect muon segments identified by the on-chamber devices, synchronize the data received from different chambers and convert from LVDS to Optical for transmission to the off-detector electronics. Installation and integration tests were developed for tuning both firmware and hardware of the Sector Collector system: results are reviewed. The system performance during CMS data taking with cosmic rays is discussed. I. DRIFT TUBE TRIGGER SYSTEM A. The Drift Tube detector Drift Tubes (DT) chambers are installed in the central part (“barrel”) of the CMS experiment [1], they are used both for muon track reconstruction and trigger purposes. Chambers are embedded in the return yoke of the magnetic field. The “barrel” iron yoke is segmented in 5 elements (wheels) along the beams direction and in 30° azhimutal sectors in the transverse plane. Four chambers are installed in any sector at increasing distance from the beam pipe; they provide transverse momentum (Pt) measurement using the track bending in the CMS magnetic field. Schematic layout of the DT system for one wheel is shown in Figure 1. Figure 1: Cross section of a CMS wheel. Schematic designs of DT chamber and cell are also shown. Each DT chamber is composed by several layers of drift tubes, performing track segment reconstructions with meantimer technique [2]. 250 DT chambers are installed in the CMS barrel (4 in most of sectors, 5 in the bottom and top ones of each wheel). II. CMS LEVEL-1 TRIGGER SYSTEM The CMS level-1 trigger is designed with custom pipelined electronics, so every collision (BX = 40 MHz bunch crossing frequency) is analyzed with no dead time. Goal of the Level-1 Trigger System is to select interesting events, in order to reduce the total rate of accepted events to less than 100 kHz. If a given event is accepted, a Level-1 Accept signal is issued to the sub-detectors and raw data are transmitted to the following CMS trigger level, called HLT. A. DT Trigger Electronics Overview Goals of the DT-based muon trigger are to perform muon identification, Pt measurement and assignment to the correct BX. The electronics is organized in a logical tree structure. First processing stages are installed on the detector, into aluminium boxes mounted on each DT chamber, called MiniCrates. They have to reconstruct and select two track segments per chamber having higher Pt. Main requirements of the Mini-crate electronics are reliability and radiation tolerance, so that ASICs and pASICs technology have been preferred and redundant designs have been implemented [3]. Following processing stages are performed by track-finder electronics [4]. They are implemented on several custom VME boards which are installed in the underground counting room (UXC), which is separated from the detector hall with a several meters wide concrete wall. Their major tasks are to match segments among DT chambers and select 4 higher Pt muons in the whole barrel. The Sector Collector (SC) system perform link between local trigger (on-detector) and track-finder (in UXC) electronics. III. THE SECTOR COLLECTOR SYSTEM