Bruinsma, M

Flammer, J

Fleckenstein, H

Gläss, J

Gröpel, A

Hähnel, C

Michetti, A

Männer, R

Nörenberg, M

Pernack, R

Ressing, D

Riu, I

Schwingenheuer, B

Somov, A

Uwer, U

Würtz, A

Bruinsma, M.

Flammer, J.

Fleckenstein, H.

Gläss, J.

Gröpl, A.

Hähnel, C.

Männer, R.

Michetti, A.

Nörenberg, M.

Pernack, R.

Ressing, D.

Riu, I.

Schwingenheuer, B.

Somov, A.

Uwer, U.

Wurz, A.

MPG.PuRe

First Commissioning Results for the Hera-B First Level Trigger

CERN Document Server

First Commissioning Results for the Hera-BFirst Level TriggerM. Bruinsma1, J. Flammer2, H. Fleckenstein2, J. Glass3, A. Gropl3, C. Hahnel3, R. Manner3,A. Michetti2, M. Norenberg2, R. Pernack4, D. Ressing2, I. Riu2, B. Schwingenheuer5, A. Somov2,U. Uwer6, A. Wurz31: NIKHEF, P.O. Box 41882, NL - 1009 DB Amsterdam2: DESY, D-22603 Hamburg, Germany3: Computer Science V, University of Mannheim, D-68131 Mannheim, Germany4: Fachbereich Physik, University of Rostock, D-18051 Rostock5: Max-Planck-Institut fr Kernphysik, D-69117 Heidelberg, Germany6: Physikalisches Institut, University of Heidelberg, D-69120 HeidelbergAbstractIn order to lter rare B decays out of the im-mense data stream produced by the 920GeV pro-ton nucleon interactions at 10MHz in Hera-Bwe developed a very fast track nder. BeingKalman ltering inspired it searches possible par-ticle tracks through up to seven layers of track-ing chambers. The trigger decision is based onthe momenta and masses of track pairs. The im-plementation is a parallel and pipelined systemof about 100 custom made hardware processors.This paper describes its rst commissioning dur-ing the year 2000. As a result of the commission-ing run in 2000 the functionality could be veriedand the performance was estimated.I. RequirementsHera-B has been designed with the main goalto measure CP violation in the B system[1]. TheB mesons are produced from reactions between920GeV protons with a xed target among witha copious background. In order to obtain a suf-cient number of Bs in average 4 interactionstake place at a bunch crossing rate of 10MHz.Since interesting events are rare (<1Hz) a selec-tive trigger system is a key element of Hera-B.The rate of events transmitted from the FontEnd to the readout boards is limited to an av-erage of 50 kHz. Hence the First Level Trig-ger (FLT) has to reduce the rate by three or-ders of magnitude. The length of the Front Endpipelines is 128 slots which allows for a decisiontime of 10s (the rest of the pipeline is neededfor the control systems). In order to limit theamount of detector channels the occupancies areallowed to reach up to 20% at maximal interac-tion rate - which is the major challenge for thetrigger and the other reconstruction systems.The main event selection criteria used in theFLT are: leptons with large transverse momentum(pT) pairs of leptons which form large invariantmassesIn order to minimize the deadtime of the experi-ment the setup of the system cannot exceed a fewminutes. Possible malfunctions must be detectedduring setup and problems must be traceablequickly. An overview about the electronics in-volved in Hera-B is given at this conference[2]. Amore extensive overview of the First Level Trig-ger system can be obtained from[3].II. Trigger StrategyA fast and eÆcient way to implement the pat-tern recognition algorithm specied above is aKalman Filter. It allows iterative processing andhence data required for the processing can be oflocal nature. Our specic implementation makesa few simplications: No drift time but only hit information isused. Multiple scattering estimates for the trackextrapolation are only used in the muon sys-tem. The tracks established during the iterativepattern recognition are not ret.The algorithm as sketched in g. 1 proceeds asfollows:1. An initial search region - a so called Re-gion Of Interest (ROI) is formed by a pro-cessor using particle ID information fromthe muon system[5], the electro-magneticcalorimeter[6] or the high pThadron seekingpad chambers.2. The search region is extrapolated to the nextupstream chamber.3. The ROI is projected simultaneously to all 3stereo views of the chamber and the appro-priate hits are extracted.4. The hit patterns are overlayed and coinci-dences are found.5. The coordinates of the found coincidencestogether with the so far best known pointon the track are used to establish an updatedROI.6. The updated ROI is sent to the next up-stream chamber and the process is iteratedstarting from step 3 - till the last chamber isreached.7. The kinematics of the found tracks is calcu-lated assuming an origin at the main inter-action point and track level cuts are applied.8. All succeeding tracks are stored and all pos-sible track pairs are processed in order tond the number of isolated tracks and theinvariant masses of the pairs.9. In case a suÆcient number of isolated tracksof a certain class or a track pair with properID, charge and mass is found a positive trig-ger information is forwarded to the Hera-BFast Control System - which is responsibleto coordinate the readout of the experiment.Figure 1: Schematic of the track nding algorithm.III. Track FinderimplementationA. HardwareDedicated custom hardware processors based onmassive use of EPLDs, ASICs and Look Up Ta-bles are the core of the system. Approximately100 such processors operate fully pipelined and inparallel. The processors are interconnected with2Gbit=s links[4] in order to communicate the pro-cess data and the detector data is received overmany 500Mbit=s optical bers. All processorscontain a MC86020 CPU as controller and are ac-cessible by way of VME. All hardware is housedin 9u VME crates. The VME crates are con-trolled by Power PC computers running Lynx-OS. The setup and control processes are executedon PCs operated by Linux.B. Control software architectureThe Unix based processes are communicating byway of a message system based upon UDP-IP.The same message system is used to reach thecontrollers on the VME processors now based ona mail box protocol over VME. On top of thecommunication layer a standard set of controland monitor methods is implemented allowinghigh level access to the processors. These meth-ods are used by the run setup and control pro-cesses as well as by dedicated test procedures.C. System VericationInitially every system component (typicallyboards) are completely veried in a stand alonemode prior to installation. For example each oftrack nding processors undergo a 1 week testwhich veries every single o-chip signal in everyprocess cycle in a dynamic fashion. This fea-ture clearly had to be carefully designed into theboards.The most important goal of the system setupis to verify the entire system functionality. Thisis achieved by comparing every intermediate pro-cessing result at each pipeline step of the hard-ware processors with the prediction of a simula-tion. The most important part is the operation ofall processors in a synchronous single step mode{ at a suÆcient rate to map out the required pa-rameter space of all tracks.For speed reasons a simple serial operating ofall processors is excluded. We implemented asystem where the local controllers operate theirprocessor in parallel to the others. Inter proces-sor synchronization is ensured by dedicated mes-sages clocking the system. In this way we areable to process events with 500Hz allowing theemulation of a real time second in about one day.Detector data are inserted on the y. The datacan origin from previously recorded detector dataor Monte Carlo generators.In this way we were able to verify the function-ality of the system except for the data input linkssince we are not able to write event data back tothe front end - which we are clearly missing. Thedata inputs can only be checked by independentmore static test procedures.IV. ResultsAs a result of the commissioning run in theyear 2000 we could verify the functionality of theFLT as track nder.Figure 2: Hit track residualsall x-res. sl PC2 0 [cm]020004000600080001000012000140001600018000-10 -8 -6 -4 -2 0 2 4 6 8 10Figure 2 shows the distribution of the residu-als between tracks found by the FLT and hits ina superlayer of the tracker which is not used inthe FLT. A clear peak with the expected widthis observed. The same plot can be produced us-ing the simulation of the trigger algorithm on therecorded data. The non-at background close tothe peak can be explained by correlated hits inthe tracking device. The at underlying back-ground is estimated using tracks from one eventand his from an other event. The backgroundlevel is consistent with a ghost rate of 25%.Figure 3: Hit track residualsTDU   TPU  PC 1  PC 4   TC 1  TC 2           MU 1  MU 3  MU 4        00.10.20.30.40.50.60.70.80.91: realistic MC: DataTrack Efficiency per Superlayer      If one follows the tracking in the simulationstep by step one can estimate the eÆciency of thealgorithm for each update. As a reference sampletracks which are decay products from identiedJ= s are used. Figure 3 represents this eÆciencyboth for taken data and for a simulation on arealistic detector Monte Carlo. The two plotsclearly agree. A decomposition of the eÆcienciesattributes about half of the ineÆciency to detec-tor cell ineÆciencies and the other half to short-comings in the algorithm of the FLT; these can(and are currently) clearly be improved by morecareful adaption of the Look Up Table coding tothe existing detector geometry.V. ConclusionWe have designed, constructed and commis-sioned a deadtime-less level one track nderwhich concludes in about 10s. A key featureof all the hardware is built in self testing capa-bilities which allow for in situ system verica-tion. An intense commissioning run of almostone year was still required to setup all the controland monitoring structures such that the systemfunctionality could be veried - clearly to a largeextend complicated by the ongoing commission-ing of the tracking devices themselves. This isthe rst time a trigger system of a complexitycomparable to the systems designed for the LHCexperiments has been operated.References[1] E.Hartouni et al., HERA B Design Report,DESY-PRC 95/01, (1995).[2] B. Schwingenheuer, these proceedings.[3] T.Fuljahn et al., Concept of the FirstLevel Trigger for HERA B, IEEETrans. Nucl. Sci. NS-45 (1998) 1782-1786.[4] J.Gla et al., Terabit per Second DataTransfer for the HERA BFirst Level Trig-ger, Proceedings of IEEEE Conference onRealtime Systems, pp 38/42, Valencia,Spain, June 2001.[5] M.Bocker et al., The Muon PretriggerSystem of the HERA BExperiment, IEEETrans. Nucl. Sci. NS-48 (2001) TNS-00118-2000.[6] C.Baldanza et al., The HERA BElectronPre-Trigger System, Nucl. Instr. Meth. A409(1998) 643.

First Commissioning results for the HERA-B First Level Trigger

First Commissioning results for the HERA-B First Level Trigger

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