Nuclear Instruments and Methods in Physics Research A279 (1989) 285-289 North-Holland, Amsterdam 285 Section VII Triggering systems TRIGGER USING TRACK SEARCH AND KINENIATICAL ANALYSIS FOR RARE DECAY CHANNELS AT HIGH RATES The CP-LEAR Collaboration Dominik Andreas TRÖSTER'), Ronny ADLER'), John Richard FRY 8) , Theodoros GERALIS l, Claude JACOBS 3), Emanuel MACHADO 4), Panagiotis PAVLOPOULOS 3), Charles RHEME 6), Daniel SACKER'), Guido TARRACH'), Panagiotis TSILIMIGRAS 3) and Edmond John WATSON 3) 4) sl Delft Basel University, Switzerland, 21 Boston University, USA, 3) CERN, Geneva, Switzerland LIP Coimbra, Portugal, Univer9) 6) 81 sity, The Netherlands, Fribourg University, Switzerland, 7) Ioannina University, Greece, Liverpool University, England, Marseille CPPM, France, 10) Saclay CEN DPhPE, France, 11J Manne Siegbahn Institute, Sweden, 12) PSI, Switzerland 13) Thessaloniki University, Greece, 14) Zürich ETH, Switzerland We report on the definition and construction of the trigger system for the CP violation experiment with tagged K° (PS195) at LEAR, the low energy antiproton machine at CERN The beam is assumed to have a continuous intensity of x 10 antiprotons per second The requirements for fast and efficient rejection are stringent Introduction The detector With improving accelerator technology, both the energy and intensity of available beams increase LEAR, The PS195 detector is mounted in a solenoid of m length and m radius producing a magnetic field of 43 T parallel to the antiproton beam (the z axis) to measure charged particle momenta The detector is similar to a colliding beam experiment Moving radially outwards from the center with a pressurized hydrogen gas target, there are tracking devices (TD) (r = 9-62 cm), particle identification devices (PID) (r = 62-75 cm), and the electromagnetic calorimeter (= 75-100 cm) The whole is surrounded by the aluminium coil A drawing is found in the contribution by Rickenbach [2] The TD consist of two proportional chambers (PC), six drift chambers (DC), and a double layer of streamer tubes for fast information on the z coordinate The chambers are sectorized into 64 parts, each one grouping several wires, whilst the tubes are sectorized into 32 planks The PCs have a pitch of wire/mm The DCs have a single layer of twin wires for on-line left/right ambiguity suppression There is one doublet per cm ; the pitch using the TDCs is mm The PID [2] consist of a Cherenkov detector sandwiched between scintillators They are all read by PM tubes and thus the PIDs are the fastest devices The electromagnetic calorimeter is required for photon detection It is not used for the trigger purposes the low energy antiproton ring at CERN, is optimized to deliver a pure antiproton beam of low energy Each spill lasts about an hour; the time lapse between spills is several minutes This quality of beam enables high statistics experiments, e g to search for small violations of known symmetries This provides an alternative source of knowledge complementing high energy experiments The PS195 experiment, also termed CP-LEAR [1], uses a classical 4m detector like the ones used in colliding beam experiments It should be capable of handling the low energy antiproton beam of LEAR with intensities of up to x 10 s -' The experiment aims to measure CP violating parameters by tagging the strangeness of neutral kaons from the antiproton-proton annihilation pp - (K + m - K ° ) or (K - m + K° ) with a branching ratio of x 10 -3 per antiproton The event size is in the kilobyte range, hence unwanted reactions must be rejected early and efficiently Otherwise, the data rate to be written to mass storage media would exceed present technological capabilities Several triggers for physics, monitoring, and calibration purposes need to be implemented The trigger must be sufficiently modular to offer the flexibility and potential for later refinement and enhancement It is required that the trigger be easily testable at any time between spills in order to reduce the mean time to repair 0168-9002/89/$03 50 C Elsevier Science Publishers B V (North-Holland Physics Publishing Division) The trigger processor Triggering is performed in several levels of complexity, where the earliest decisions are the most primitive VII TRIGGERING SYSTEMS 28 The CP-LEAR Collaboration / Trigger for rare decay channels Log(background/good) -2 -3 ii I , i Log(hme(nsl) i Late Intermediate ~ decisions Early Fig The proportion of background events per good event as a function of time and decision level The dead time due to background events is negligible already after the intermediate decisions This plot is based on simulations using the actual algorithms of the trigger processor ones simply rejecting obviously unusable events Every level is optimized to improve the signal/background ratio while keeping dead time low The decision are taken in three levels : early, intermediate, and late Early decisions operate on detectors with very fast response : the beam counter and the PIDs Intermediate decisions operate on the TDs, but just performing coincidences for fast pattern recognition Late decisions use track parameters and kinematical constraints with full resolution There are decisions which are done in parallel for other annihilation channels or calibration purposes and for minimum biased events If any one of these parallel triggers agrees with the event, it is accepted As time goes by, and trigger levels are passed, the signal to background ratio improves from 0.01 to 300 according to Monte Carlo simulations and as shown in fig The overall trigger efficiency for good events is about 0.15, taking account of losses due to dead time, detector inefficiency and solid angle The trigger decision time is greatly reduced by the use of tables for wire number translation, trigonometric functions etc., and it uses constants for comparisons and computation of kinematical quantities These can be programmed remotely, updated, and verified at any time This gives the trigger the required flexibility in order to search for other decay channels 3.1 Trigger concept The trigger processor is a "pipelined dataflow machine" This is a novel type of processor, a block diagram is shown in fig The term "pipelined" refers to the presence of registers between the functional units These registers decouple them, store intermediate re- sults, and allow independent dialogue with the periphery Due to their programmability, the registers are an important tool for test purposes The term "dataflow" indicates that functional units are put at fixed places, but addresses and data (tagged for validity and particle type) seek their way through Several tracks can be followed through the detector at the same time In each decision level, several tests can be performed in parallel and serially The trigger sequencer element (TS) takes account of the outcome of these tests and decides whether the current event must be evaluated any further or whether it may be rejected The TS is an operating system built in hardware It is a sequencer which operates at 100 MHz Early decisions Early decisions are taken within 60 ns since the charged kaons in the relevant momentum range Fig The trigger processor block diagram The top left column represents the front-end electronics containing discriminators, TDCs and ADCs, and with which the next column, consisting of track follower elements, is performing a dialogue Processing goes from top to bottom The next column, consisting of delay elements, collects the data and groups them for each track Its output is routed to several elements (SELECT, TRACK P, CALIBRA, MOM, COMPENS) for parametrization and stored in appropriate registers These data are then used to calculate missing masses (MIM) and time of flight (TOFU) The CP-LEA R Collaboration / Trigger for rare decay channels (300-750 MeV/c) can be identified by using the PIDs In the early level, the existence of a kaon candidate is checked by requesting, for any segment, coincident hits in the scintillators (to make sure that the track of the kaon candidate crosses the Cherenkov detector between them) and absence of a hit in this particular Cherenkov Simultaneously, the multiplicity of inner scintillator hits is evaluated This is followed by the multiplicity of kaon candidates If there are fewer than two hits in the inner scintillators or no kaon candidate, the event is rejected If the beam counter signals another incoming antiproton, the event is rejected, in order to avoid overlapping signals in the TD Otherwise, a "general strobe" is generated At this level, only events with kaons or slow pions (with a momentum of less than 300 MeV/c) can provide a "general strobe" 3.3 Intermediate decisions Intermediate decisions are taken within 250 ns In the intermediate level, there are several activities Early information of the TDs is used to detect the presence of tracks and to count "primary tracks" in general, and "kaon tracks" in particular (Primary tracks come from charged particles emerging of the annihilation, whilst secondary tracks stem from the decay of neutral primary particles Kaon tracks are primary tracks with the signature of a kaon candidate in the PIDs The other tracks are considered to be pions.) Simultaneously, the transverse momentum of the kaon candidate is estimated Primary tracks have information inside a given range around the position of the corresponding PID in at least one of the PCs, one of DC(1) or DC(2), and one of DC(5) or DC(6) If this is not the case, the track could not be followed reliably through the TDs in the later processor stage The logic is done with coincidences on the crude hit map data, with the detector split into 64 segments If there are fewer than two primary tracks or no kaon track, the event is rejected at this stage The transverse momentum of the kaon is crudely estimated by requiring coincidences of sets of wires in DC(1) and DC(6) with the kaon candidate signature in the PIDs ; in detail : some 15 wires of DC(6) are in front of each PID module, at least one of them must have fired Assuming circular tracks originating from the center of the target, one of three to nine wires in DC(1) depending on the momentum, must have fired in front of this wire in DC(6), too If such a coincidence if found, it is assumed that a charged kaon is present Otherwise, the event is rejected A threshold of 300 MeV/c on the "kaon candidate" transverse momentum will ensure that all the events with slow pions faking kaons are rejected, whereas events with charged kaons (P< 84) will survive This trigger stage is implemented with ASICs (appli- 28 cation specific integrated circuits) One ASIC contains all circuitry for the coincidences related to one PID module, requiring some 650 gate functions It is made in TTL compatible CMOS and returns the result within 30 ns A solution using standard components would not be faster nor cheaper nor safer, but much more space-consuming 3.4 Late decisions Late decisions are taken within Ws In the late level, the tracks are followed in two passes The first, crude pass uses the segmentation in 64 of the TDs in order to associate hits into tracks The fast pattern recognition algorithm is based on the low track multiplicity of the pp annihilation at rest (maximum 6) and has an efficiency of 98% per track The electronics of the TDs performs real-time digitization (within 700 ns) The precise information (1 mm and mm bin equivalent, respectively) is available through a dedicated controller, of which each TD (and PID) has one The raw track information is then used to acquire the precise digitizations using a simple dialogue mechanism - the second pass This dialogue is done simultaneously in successive TDs for all tracks (for example, track in DC(3), and track in DC(4)) Using these data, the primary tracks are parametrized (momentum, charge, starting angle) through different algorithms via LUTs (look-up tables) and linear approximations The precise kaon momentum is checked against a minimum value If its momentum is too low, it is considered to be a fake kaon The event is rejected If not enough tracks can be parametrized, the event is rejected, too If there are two or four tracks, the charges must be balanced There must be at least one track of each charge sign, but not more than two of the same sign The kaon track is then combined with the pion tracks of opposite charge to get the missing mass of the primary tracks If within the resolution it does not match with the neutral kaon mass, the event is rejected The primary tracks are double-checked (in TOFU) for their identification : the amplitudes in the scintillators are used to correct the time of flight From the particle identification and momenta, the expected time of flight difference in the scintillators is evaluated and compared with the measured value If these quantities not match within their resolution, the event is rejected At this level, the event is considered to be a candidate "good event" and is written to tape 3.5 Trigger monitoring Due to the remotely controllable pipeline registers, the trigger processor is easy to test Characteristic data VII TRIGGERING SYSTEMS 28 The CP-LEAR Collaboration / Trigger for rare decay channels can be inserted at any place to debug the equipment between two programmable elements An autonomous readout processor, called spy, is provided on top of the whole trigger processor The spy has up to 64 parallel data input channels Each channel can acquire trigger processor data independently and at full rate, i.e 30 MHz Any cable between two trigger elements can be fed to the spy for reading out the data on this cable This is very useful in the startup phase and it is vital in the production phase, since the existence of intermediate results on tape speed up off-line evaluation considerably The pattern recognition and track finding are time consuming tasks when programmed on a standard computer The existence of a first guess for the tracks will offer more time for fits which are not foreseen on the trigger processor Elements The electronic modules built for the CP-LEAR trigger processor are called elements The most dedicated ones are described below 4.1 Trigger sequencer The trigger sequencer element is the most dedicated one and will be briefly described first It contains the philosophy of the trigger behaviour which can probably be applied to many other experiments It contains an algorithmic state machine which runs at 100 MHz It has inputs for launching the trigger processor (beam, test); for manipulating the fate of the event under evaluation (accept, good forever, cancel keep mode, panic, skip) ; for synchronization (busy, kinematics done, drift time past, follow done, pretrigger done) It controls any activity of the trigger, from the acceptance of an antiproton to the end of the readout, according to the state diagram in fig To avoid confusion, if more than one event happens before the end of the early state, the sequencer resets via the multi(hit) state 4.2 Simple logic The terminator/converter and the differentiator/deglitcher take the role of "glue" to other logic (NIM, TTL) and standardize signal shapes There are several types of coincidences (32 X inputs, 16 X inputs, 16 X (2 inputs, veto) and similar OR gates There is a maskable input AND/OR/ NAND/NOR with logic fan All these elements have about ns delay The multiplicity unit for adding 36 1-bit inputs gives the precise count and signals even/ odd, n=2, n=4, and n = or n = within 20 ns Fig The trigger sequencer state diagram It shows an infinite loop which is characteristic of any operating system The bubbles show states in which the experiment can be while running, the arrows show possible transitions When switched on, the system starts in panic state in order to set all equipment into a known - idle - state While idle, any kind of event is accepted, but not more than one at a time If the event survives the different trigger levels, it is finally read out There is an adder with four inputs and an arithmetic element with a 16-bit ALU (arithmetic logic unit) (15 ns) and an independent 16 X 16-bit multiplier (30 ns) The adder operates on 12-bit numbers and has outputs for (a+ b), (c+d), and (a+b+c+d) 4.3 Programmable logic Some elements are memory or register programmable, others are programmable via their inputs The first kind includes pipeline registers, timer/scalers, approximately equal windows (an element with the dual functionality of approximately equal comparator and window comparator), look-up tables, and flexible memories They are controlled via LORCOM, the system test network which is based in CAMAC The other kind is represented by the data selector, the delay, the register control, the timing generator, the track follower, and the trigger sequencer The spy is somewhere in between The data selector is an extendable multiplexer which selects data according to their validity The delay element (see fig 2) is used for synchronization of data acquired at different times The register control puts track parameters into the right slots of the missing The CP-LEAR Collaboration / Trigger for rare decay channels fig The crate offers the two low buses as standard VME buses The upper bus is for power distribution 43 cm (power 289 onlv) Summary (o IEI tionall CPVME i 16 cm Fig The CP-VME crate dimensions The crate is 9U high and is shown with the optional 6U high VME partition Each station may dissipate A at -5 V, A at -2 V, A at +5 V on average Of the three buses, the topmost is dedicated to power distribution mass/invariant mass calculator The timing generator provides the system clock, 10-30 MHz, by respecting the speed of the slowest active element 4 The CP-VME form factor In order to match our specific requirements, the elements used in the trigger have a particular shape They are fastbus high, but only VME deep Appropriately, this format is termed CP-VME Most logic requires between 64 and 110 contacts on the front panel On the other hand, by making use of the high circuit density offered by 100K ECL logic, VME depth is good for most, if not all, applications Following the VME standard allows us to mix VME cards and CP-VME cards in one crate, as shown in The design of a trigger processor for real-time physics evaluation has been sketched The task is feasible, but involves the design of lots of new hardware (and software) According to Monte Carlo studies, the efficiency and timing requirements are met It has been shown that a new size of printed circuit boards is advantageous Acknowledgements We would like to thank the electronics workshops of Basel, the ELD groups of CERN, Coimbra, and Fribourg for their highly valuable suggestions, and their efficiency We are much in debt to the whole CP-LEAR group for the time spent in fruitful discussions The summer students S Andouche, B Edholm, P Lahary, K Peters, S Vlachos each did a good job References [11 L Adiels et al , Proposal for the experiment PS195, CERN/PSCC/85-6/P82, PSCC/85-30/P82/Add 1, PSCC/85-43/P82/Add 2, PSCC/86-34/M263, PSCC/87-14/M272 [2] R Rickenbach et al., these Proceedings (Int Conf on Advanced Technology and Particle Physics, Como, Italy, 1988) Nucl Instr and Meth A279 (1989) 305 VII TRIGGERING SYSTEMS ... kinematical quantities These can be programmed remotely, updated, and verified at any time This gives the trigger the required flexibility in order to search for other decay channels 3.1 Trigger. .. R Collaboration / Trigger for rare decay channels (300-750 MeV/c) can be identified by using the PIDs In the early level, the existence of a kaon candidate is checked by requesting, for any segment,... i 16 cm Fig The CP-VME crate dimensions The crate is 9U high and is shown with the optional 6U high VME partition Each station may dissipate A at -5 V, A at -2 V, A at +5 V on average Of the