IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 1 Level 1 Trigger System for the Belle II Experiment Yoshihito Iwasaki, ByungGu Cheon, Eunil Won, Xin Gao, Luca Macchiarulo, Kurtis Nishimura, and Gary Varner

TABLE I B − − Abstract—The super-KEKB factory currently under con- TOTAL CROSS SECTION AND TRIGGER RATES FOR L = 8 × 1035 cm 2s 1 struction at the KEK High Energy Accelerator Research Orga- FROM VARIOUS PHYSICS PROCESSES AT THE Υ(4S). nization in Japan has a goal of producing 50 ab−1 of integrated luminosity, thus allowing the Belle II experiment to study rare Physics process Cross section (nb) Rate (Hz) decays of B mesons, D mesons, and τ leptons. Such large → ¯ statistics on these decays provide an experimental probe of Υ(4S) BB 1.2 960 + − physics beyond the Standard Model. An online trigger system e e → continuum 2.8 2200 is indispensable to Belle II to reduce the number of beam µ+µ− 0.8 640 background events associated with high and τ +τ − 0.8 640 − ◦ a beam currents, as well as to enhance physics oriented events. Bhabha (θlab ≥ 17 ) 44 350 For this purpose, we have designed the Belle II online trigger ◦ a γγ (θlab ≥ 17 ) 2.4 19 system with two kinds of primary Level 1 trigger components: a b track trigger and an energy trigger. The track trigger is composed 2γ processes ∼ 80 ∼ 15000 of 2−dimensional and 3−dimensional tracking algorithms, and Total ∼ 130 ∼ 20000 the energy trigger implements algorithms based on total energy, a The rate is pre-scaled by a factor of 1/100. b ◦ isolated clusters, and identification of Bhabha events. In addition, θlab ≥ 17 , pt ≥ 0.1GeV/c precise event trigger timing and muon tagging information are provided by the time−of−propagation detector and iron flux return muon detector, respectively. value of 2.11 × 1034 cm−2s−1 of the KEKB , with Index Terms—Belle II experiment, Level I hardware trigger, an eventual goal of integrating 50 ab−1 of luminosity by super−KEKB factory. 2020. Essential features of the Belle II upgrade [7] permit access to several crucial measurements, reviews of which can I.INTRODUCTION be found elsewhere [8], and which are complementary to searches for New Physics at the Large Hadron Collider (LHC) HE B factory experiments, Belle [1] at the KEKB collider experiments [9]. at the High Energy Accelerator Research Organization T The total cross sections and trigger rates of several physical (KEK) in Japan and BaBar [2] at the PEP II collider at processes interest at the target luminosity of × 35 −2 −1 the SLAC National Accelerator Laboratory in the USA, were 8 10 cm s are listed in Table I. Samples of Bhabha and events will performed primarily to measure large mixing-induced charge- γγ be used to measure the luminosity and to calibrate detector parity (CP) violations in B meson decays. Most of the results response. Since at this luminosity the Bhabha and cross are in good agreement with the Standard Model (SM) pre- γγ sections – and thus trigger rates – are large, a pre-scale factor dictions of the Cabibbo-Kobayashi-Maskawa (CKM) structure of 100 or more is applied to these triggers. of quark flavor mixing and CP violation [3]. However, the experiments indicated several hints of discrepancies between the SM predictions and the experimental measurements [4], II. BELLE II EXPERIMENT [5]. Accordingly, a much larger data sample is required to Because of expected increases in beam−gas−induced back- investigate further whether these are truly indicative of New grounds, the Belle II detector must tolerate higher occupancy Physics effects. Therefore, an upgrade to the Belle experi- and radiation damage than the original Belle detector. In ment, designated Belle II, at the super-KEKB collider [6] addition, the increased event rate puts a high demand on was approved with an instantaneous luminosity goal of 8 × triggering, data acquisition and computing. To cope with the 1035 cm−2s−1. This rate is 40 times higher than the peak conditions at super−KEKB, most of the components of the Belle detector will be replaced with new ones, as shown in Manuscript received May 26, 2010. This work was supported in part by Fig. 1. the National Research Foundation of Korea under Grant No. 2009-0076449. Y. Iwasaki is with the Institute of Particle and Nuclear Studies, High Energy The innermost part of the tracking system consists of two Accelerator Research Organization (KEK), Tsukuba, 305-0801, Japan (e-mail: layers of silicon pixel sensors (PXD), based on depleted yoshihito.iwasaki@.jp). p−channel field effect transistor (DEPFET) technology, and B.G. Cheon as a corresponding author is with the Department of Physics, − College of Natural Sciences, Hanyang University, Seoul, 133-791, Korea (e- four layers of double sided silicon strip sensors (DSSD) to mail: [email protected]). measure the decay vertex position of B mesons and other E. Won is with the Department of Physics, Korea University, Seoul, 136- particles. Precise determination of trajectories, momenta and 701, Korea (e-mail: [email protected]). X. Gao and L. Macchiarulo are with the Department of Electrical En- dE/dx of charged tracks is provided by a central drift chamber gineering, University of Hawaii, Honolulu, HI 96822, USA (e-mail: xin- (CDC). Improvements in the momentum resolution compared [email protected]; [email protected]). to the Belle CDC are achieved by using a larger outer radius K. Nishimura and G. Varner are with the Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USA (e-mail: kur- and a smaller cell size. For the identification of charged [email protected]; [email protected]). hadrons, a time−of−propagation (TOP) detector is used. The IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 2

ECL KLM CDC TSF Track Track Count 2−dim Track Opening Angle Back−to−Back Timing 3−dim Track Track

Trigger Cell ECL Cluster Count Timing Cluster High Threshold Energy Sum Low Threshold Bhabha

Hit BPID Multiplicity Topology PXD+DSSD Global Decision Logic CDC TOP+ARICH Fine Timing

Fig. 1. Layout of the main constituent parts of the Belle II detector. KLM Muon hit Forward Backward Barrel compact design of this barrel particle identification device 3D Muon Track is made possible by precise spatial and timing detection of Fig. 2. Schematic overview of the Belle II trigger system. Four sub−trigger internally reflected Cherenkov light. The forward region is system outputs are sent to the Global Decision Logic (GDL), where the final instrumented with a new ring imaging Cherenkov detector online trigger decision is performed. (ARICH) that uses aerogel layers with different refractive indices to generate Cherenkov rings with a common radius for each layer. The crystals of the Belle electromagnetic logic is configurable rather than hard−wired. All data flows calorimeter (ECL) are reused for Belle II. For the endcap through high speed serial links, not parallel (ribbon) cables, region, a replacement calorimeter with faster, more responsive, which enables us to funnel a huge amount of information, and higher radiation−tolerant crystals is being considered as a equivalent to O(1000) channels, to one FPGA. future upgrade option. To improve the signal−to−background The schematic overview of the Belle II trigger system is separation under higher super−KEKB background conditions, shown in Fig. 2. The CDC detector trigger provides charged waveform sampling electronics will be used. In the barrel track information such as momentum, position, charge, mul- region, muons and K0 mesons will continue to be identified L tiplicity and so on. The ECL detector trigger provides en- using resistive plate chambers (KLM). To cope with the higher ergy deposit information, energy cluster information, Bhabha background rates due to neutrons, the endcap regions will identification, and cosmic-ray identification. The Barrel PID be upgraded to scintillator strips with embedded wavelength (BPID) detector trigger provides precise timing and hit topol- shifting fibers. To record collision events of interest in all subsystems, a fast and reliable trigger system and a data ogy information. The KLM detector trigger gives muon track information. The global decision logic (GDL) receives all of acquisition system are essential. this sub-trigger information and makes the final decision. A positive decision is sent to a sequence controller (SEQ) as a III. BELLE II ONLINE TRIGGER SYSTEM trigger signal. The total latency in the trigger system is about The requirements for the trigger system are high efficiency 5 µs. To achieve high trigger efficiency for hadronic events, for hadronic events from Υ(4S) → BB¯ and continuum we use two independent triggers, the CDC and the ECL, as production of quark pairs, a maximum average trigger rate the main trigger components. of 30 kHz, a fixed latency of about 5 µs, a timing precision of Background conditions in Belle II are expected to be worse less than 10ns, a minimum two−event separation of 200 ns, than Belle due to higher instantaneous luminosities and smaller and a trigger configuration that is flexible and robust. To meet transverse dimensions of beam profile. In addition, based upon these requirements, we adopt the Belle triggering scheme [10]. experience with Belle, we expect significantly worse back- The Belle triggering scheme consists of four detector trigger ground conditions at the beginning of Super-KEKB accelerator systems that feed into the final-decision logic. Each detec- operation, when vacuum conditions and other accelerator tor trigger summarizes the trigger information from its own parameters are not yet optimized. To satisfy the trigger system system and sends it to the final−decision logic, which then requirements in the presence of such backgrounds, it is impor- combines all the detector triggers and issues a final trigger tant to have several independent and effective trigger strategies when its criteria are satisfied. This was quite successful in to reduce the Level 1 trigger rate. In this way, we can tune the to achieve high efficiency for hadronic the collection of triggers to minimize the dead−time of the events. In Belle II, we use the same concept implemented data acquisition system without sacrificing trigger efficiency in replacement hardware components. Each component has for recording physics events of interest. These strategies are a field−programmable gate array (FPGA) so that the trigger described in the following sections. IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 3

CDC Layer 0 TSF Layer 0 2D Track Finder Inner TSF Outer TSF CDC Trigger Output CDC Front-end Multiplexer UT3 TRG UT3 TRG 2D tracks (x3) Board Board

CDC Layer 1 TSF Layer 1

3D Track Finder CDC Front-end Multiplexer UT3 TRG (x3) Board CDC Trigger Output UT3 TRG 3D tracks Board

CDC Front-end

CDC Layer 8 TSF Layer 8 Fig. 4. The geometric shape of a track segment (TS). Each square Multiplexer UT3 TRG corresponds to a wire cell in the CDC (the sense wire being at the center (x6) Board of the square); the IP is downward. Left: a TS for the innermost superlayer. Right: a TS for other superlayers Fig. 3. Hardware configuration of the CDC sub-trigger system. The data flows from left to right. y Y (x, y) Y=AX+B 2 2 2 (x−a) + (y−b) = r IV. CDC TRIGGER

(a, b) B (X, Y) The CDC trigger shown in Fig. 2 finds and characterizes the charged tracks detected by the central drift chamber. α α In the present design, we measure the momentum, position, x X and charge of secondary charged tracks originating near the (b) conformal plane interaction point. Because of the limited solid angle coverage (a) real plane of the chamber, the CDC trigger is not sensitive to charged Fig. 5. Schematic view of the conformal transformation: (a) in CDC tracks in the far−forward and the backward regions. transverse plane, (b) in the conformal plane. At the CDC front−end electronics system, discriminated signals from each of 48 CDC wires are generated in the Y front−end board in each pulse of the 1 GHz clock. These Y=AX+B precise wire−hit signals are down−sampled by a 62.5 MHz clock. A one−bit hit state of the 48 wires on this board is sent to the multiplexer shown in Fig. 3 by high speed serial links, whose data transfer capacity is 12.5 (3.125 × 4) Gbps and (Xi, Yi) actual data rate is 3 Gbps. The multiplexer can receive four Φ sets of such serial links. Each line in Fig. 3 corresponds to one superlayer in the CDC. There are three to six multiplexers per X superlayer, depending on the number of sense wires. After that, the merged wire−hit information by the multi- Fig. 6. The distance R and angle Φ of the point of closest approach to the plexer is sent to the next stage: the track segment finder (TSF). origin of a straight line in the conformal plane. The TSF is realized using one universal trigger board (UT3) per CDC superlayer, with nine such boards in total. in the conformal plane using the following equations. In each TSF, the geometric regions for track segments (TS) are defined as shown in Fig. 4. The number of TS in a 2x 2y X = 2 2 and Y = 2 2 . (1) superlayer depends on the number of wires in a single wire x + y x + y layer. Each TS overlaps the neighboring TS except for the In this transformation, the angle α of point (x, y) on the arc innermost cell of the innermost TS or for the center cell for in Fig. 5 relative to the CDC origin is preserved. An arc of other TS. There are 2336 TS in total. At each tick of the 62.5 a circle passing through the CDC origin is transformed into a MHz clock, the pattern of wire hits in each TS is examined. straight line segment in the conformal plane. This line segment The hit patterns expected for a charged track are predefined is parametrized by the quantities R and Φ in Fig. 6; these are and stored in memory. By a memory look-up method, the related to the arc’s radius and center via presence of a TS hit is determined. The TS hit information, B2 1 1 2336 bits in total, is sent to the 2−dimensional track finding R = = = , (2) rA2 +1 ra2 + b2 r stage at each clock tick. 1 b − tanΦ = − = . (3) In the 2 dimensinal track finding stage, we use a confor- A a mal transformation and Hough transformation [11] to search The polar equation of the straight line though the point (X, Y ) for charged tracks. The schematic image of the conformal in the conformal plane is transformation is shown in Fig. 5. A point (x, y) in the plane transverse to the CDC’s axial wires is transformed into (X, Y ) R = X cosΦ+ Y sinΦ. (4) IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 4

Mean = 0.5044 ± 0.0003 Sigma = 0.01394 ± 0.00019

400

# of Events 200

0 0 0.2 0.4 0.6 0.8 1 pT (GeV)

Fig. 8. Transverse momentum distribution from the 2D fit of tracks in single−muon events. Each muon was generated with pt = 0.5 GeV/c.

Fig. 7. An event display of the 2D track finding stage for a simulated µ−µ− event. Upper left: TS axial hits (dots) in the r-φ plane in the CDC; work of the BaBar experiment [12]. We apply first the the horizontal (vertical) axis is x (y). Upper right: the corresponding TS axial conformal mapping to the track segments originating from hits (dots) in the conformal plane; the horizontal (vertical) axis is X (Y ). Bottom: the corresponding TS axial hits (pixellated curves) in the Hough single track candidates and fit them. Using the geometry of plane; the horizontal (vertical) axis is R (Φ). the CDC, the z0 information is then extracted. Presently the algorithm is being implemented in the C++ language and all the calculational steps are performed with the floating−point The plane with (R, Φ) on orthogonal axes is called the Hough numbers. The minimization process is implemented with the plane. A single point in the conformal plane has an infinite linear regression technique. The present algorithm provides number of straight lines through it and, therefore, an infinite a typical z0 resolution of less than 10 cm, depending on number of curves through it in the Hough plane. However, the particle momentum. However, we expect slightly worse resolution when the algorithm is converted into a hardware a collection of points (Xi, Yi) that lie on a straight line in the conformal plane are mapped by Eq. 4 to curves that pass description language such as VHDL or Verilog. We plan to through a common point (R, Φ) in the Hough plane. We improve the resolution of z0 significantly by utilizing the therefore obtain a solution of the straight line in the conformal timing information of the track segments. plane as the intersection point of the curves in the Hough plane. V. ECL TRIGGER − − An event display of a simulated event with a µ µ The calorimeter is a very important component to generate like−charge pair is shown in Fig. 7. The upper−left figure fast trigger signals that provide a fully efficient trigger for shows the TS hits in the CDC’s transverse plane, with two both neutral and charged particle oriented physics events. curved tracks passing through the interaction point (0, 0). Two primary and complimentary trigger schemes are taken These TS hits are then transformed into two distinct straight into account, namely a total energy trigger and an isolated lines in the conformal plane and, finally, into the family of cluster counting trigger. The former is sensitive to physics curves in the Hough plane (one curve per TS hit). The two events with high electromagnetic energy deposition while the intersections of these Hough−plane curves, at R ≃ 0.015 and latter is sensitive to multi−hadronic physics events that have Φ ≃ 1.9, 5.1, represent the solutions of the reconstructed low−energy clusters and/or minimum ionizing particles. In tracks. The distribution of reconstructed transverse momenta addition, the ECL trigger system could also identify Bhabha for single−muon events generated with pt = 0.5 GeV/c and γγ events at the online level. The online luminosity is shown in Fig. 8. As one can see, there is no apparent measured by Bhabha events is needed to monitor the running bias observed in the fit to this distribution. We also fitted condition of the super−KEKB and will be only available from single−muon samples with other input pt values and found the calorimeter system. no obvious bias in the fits. However, as expected, the fitted pt We had practical experience in implementing and operating resolution rises with pt. the ECL trigger system at the Belle experiment[13]. The In order to suppress the beam-induced background more only distinction between the Belle and Belle II experiments effectively, we also reconstruct 3-dimensional tracks rapidly, is that Belle II must deal with a much higher trigger rate in less than 5 µs, in the online stage using the stereo and axial due to higher luminosity and beam background. To cope information from the CDC. The reconstructed z−coordinate of with this situation and make the ECL trigger performance a given track that is closest to the interaction point (z0) should more flexible at the requisite speed, less than 5 µs, we be consistent with zero, enabling us to veto on beam−induced adopted a new trigger scheme that utilizes a readout electronics background efficiently. This idea originated from the initial architecture with flash analog−to−digital converters (FADC) IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 5 and field−programmable gate array (FPGA) components. The former digitizes a fast analog trigger signal, called a trigger cell (TC), which is a basic element composed of 16 CsI(Tl) DSP FAM TMM crystal counters, while the latter determines the TC peak value x 80 x 9 and the trigger timing of at the peak position of each TC DSP FAM TMM and performs the final trigger decision using programmable x 108 x 9 firmware algorithm logic downloaded into the FPGA. The algorithm logic can be easily updated whenever it needs to DSP FAM TMM be modified, for example due to bad beam conditions at x 108 x 9 super−KEKB. Compared to the Belle trigger implementation, this new scheme uses much more flexible electronic hardware DSP FAM TMM GLobal Decision Logic x 108 x 9 and a much more concise signal cabling structure. Another advantage of this scheme is that we do not need to modify ECL Trigger Mater

CsI(Tl) + PD PreAmp DSP FAM TMM the trigger electronics, even in case of a future upgrade to the x 108 x 9 crystals of the endcap detector. To realize the ECL trigger scheme, we prepared three DSP FAM TMM components of the trigger electronics hardware, as shown in x 64 x 9 Online LUM Fig. 9: a fast shaper circuit built in the digital signal processing (DSP) shaper module, a FADC analog module (FAM) and a Fig. 9. Block diagram of the ECL trigger readout electronics system. trigger and monitor module (TMM) [14]. The trigger timing TABLE II latency of each step in the ECL trigger process is described in ESTIMATE OF ECL TRIGGER TIMING LATENCY BASED ON EXPECTED Table II. The expected total latency is fast enough to satisfy PERFORMANCE the budgeted latency for the ECL trigger, which is 4 µs. The ECL trigger should also provide precise resolution of trigger Item Latency(ns) timing to allow triggering on physics events where secondary Peaking time of TC 700 particles hit only the endcap detector region. We expect a ADC pipeline @ FAM 100 resolution with a standard deviation of approximately 20 ns or Peak finding process @ FAM 300 - 400 less, which was achieved with the Belle ECL trigger system. Programmable delay @ FAM 300 Gbit transfer( 200 bits) 100 The ECL trigger system is planned to provide 26 final trigger Optical cable length 200 - 300 output bits and TC hit pattern bits to the GDL, as described in Trigger input alignment @ TMM 100 Table III. Other useful trigger bits will be added after a trigger Stage-1 to stage-2 bit transfer @ TMM 700 simulation study. Trigger decision @ TMM 200 - 300 We expect beam backgrounds to be high in the early stage Total latency 2700 - 3000 of the experiment, while other backgrounds will increase as a function of high beam current or luminosity during stable − TABLE III super KEKB operation. To cope with evolving levels and FINAL ECL TRIGGER OUTPUT TO GDL composition of backgrounds and to reduce the fake trigger rate as much as possible, we prepared a set of easily tunable Item Number of bits options: (1) increase of the TC threshold, (2) increase of the Trigger timing (Final, Fwd, Barrel, Bwd) 4 total energy threshold, (3) increase of the number of isolated Total energy (>0.5, 1.0, 3.0 GeV) 3 clusters, (4) exclusion of a part of the physics trigger region Isolated cluster 4 in the endcap detector and (5) enabling of the cosmic veto Bhabha-type 11 trigger mentioned in the next section. In the early stage of OR-ed Bhabha 1 the experiment, we plan to operate the ECL trigger system Barrel Bhabha 1 by disabling the cosmic veto trigger, as this veto might Prescale Bhabha 1 unintentionally bring down the trigger efficiency for several Cosmic veto 1 interesting low−multiplicity processes, such as the initial state TC hit pattern 576 radiation (ISR) process, the two−photon process and tau−pair Total 26+576 production. However, from the Belle experience, we confirmed that this cosmic veto was very effective in suppressing beam background events, so it must be turned on whenever the ECL and an imaging variant [15] of the time−of−propagation trigger output rate is higher than the controllable level needed (TOP) detector [16] has been chosen as the primary particle by the data acquisition system, which has a maximum readout identification device in the barrel region of Belle II. This rate of 30 kHz. iTOP counter, a compact implementation of the detection of internally reflected Cherenkov light (DIRC) technique [17], VI. PID TRIGGER relies heavily on excellent single photon timing resolution At a Super Flavor Factory, charged hadron flavor iden- obtained with micro-channel plate photomultiplier tubes. We tification is a vital element of the experiment’s success, discuss the design and development of a trigger for the TOP IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 6

Detected pixel LUT_t_1_1 ACC LUT_t_1_2 ACC MAX

Queuer/Encoder LUT_s_1 ACC +

LUT_t_2_1 ACC [1:n] Time/Space LUT_t_2_2 ACC MAX ARGMAX

∆t LUT_s_2 ACC + Queue space

Fig. 11. Conceptual block diagram of the TOP trigger.

the PDFs to determine an optimal estimator of the actual time and position of the charged particle on the TOP counter. Fig. 10. Single TOP detector module and three representative probability A number of trigger algorithm comparisons were performed distribution functions, as an illustration of the type of information available that evaluated items such as time quanitization of each hit and for discriminating charged particle impact position. spatial+timing versus timing-only inputs. Sample PDFs were generated from a sample of 5000 events and evaluated with a separate event sample of the same size. detector that gives precise event timing.

0.9 0.5 0.8 0.45 A. Barrel PID Trigger 0.7 Mean=-0.2 ns 0.4 Mean=-0.8 ns StdDev=1.9 ns 0.6 StdDev=1.7 ns 0.35 0.3 The TOP detector chosen for barrel particle identification 0.5 0.25 − 0.4 (PID) has intrinsically good (sub nanosecond) time resolu- 0.2 tion. However sub-100 ps timing is usually only obtained in 0.3 0.15 0.2 0.1 offline reconstruction, and is better than required at the trigger 0.1 0.05 0 0 level, where a couple of ns time resolution for the global -4 -2 0 2 4 -4 -2 0 2 4 decision logic will be used to reduce the data volume from Fig. 12. Trigger timing error for 1 ns and 2 ns hit time quantization. out-of-time hits in the silicon vertex detector. An algorithm for determining the event time resolution from tracks that hit the TOP detector is described below. 0.8 0.6 0.7 Mean=-0.25 ns 0.5 Mean=-0.25 ns Single photon arrival times are recorded by exquisite time 0.6 StdDev=2.1 ns StdDev=4.0 ns resolution photo-sensors and their integrated readout and trig- 0.5 0.4 ger electronics. Giga-bit fiber links carry the position and 0.4 0.3 0.3 0.2 nano-second time-stamp information of these individual hit 0.2 0.1 signals from the 16 TOP detector staves to dedicated trigger 0.1 0 0 processing boards. An algorithm based upon photon arrival -4 -2 0 2 4 -4 -2 0 2 4 patterns for the timing determination of each TOP counter Fig. 13. Trigger time error using only timing information (with and without stave has been studied and is reported in detail elsewhere [18]. 4x times expected background). Fig. 12 shows the distribution of time errors when both time B. Experimental Basis for Detection and position information is used in the estimate, assuming an A GEANT−4 based Monte Carlo study [19] generated input time quantization of 1 (left) and 2 ns (right). data that was used to identify the most effective detection The observed distributions are not Gaussian (have long techniques that could be efficiently implemented in real−time tails), and ≈90% of the estimates are within ±1 ns (±2 ns), hardware. This simulation was used to gather sufficient statis- and correspond to a standard deviation of ≈1.7 (≈ 1.9 ns). tics regarding time and spatial patterns of detected Cherenkov Fig. 12 left illustrates the trigger time resolution based photons generated by charged particles traversing the TOP on timing information alone (1 ns input time quantization). quartz radiators. Figure 10 illustrates how the spatial (X−axis) Notably, the result is not much worse than that when com- and temporal (Y−axis) distributions the fill probability distri- bined with position information. The histogram on the right butions (PDFs) depend upon the particle interaction position illustrates the impact on this algorithm when the background on the counter. A simple time separation−based estimation is noise is increased to a rate at least four times higher than not viable, due to various effects that contribute to the spread expected. Even in this case, ≈80% of tracks are identified in arrival times. For this reason, the information contained in with a time error less than 2 ns. such PDFs is used directly in the estimator. Hits detected in These experiments support the conclusion that an accurate pixels are processed in a series of steps as shown in Fig. 11. identification of the interaction time could be derived based Look-Up Tables (LUTs) contain the weighting information on the timing information only, thus reducing the hardware used to test the photon space and time hit patterns against processing requirements. IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 7

Fig. 15. Schematic view of the GDL system.

D. Future Work on the PID detector Future work will optimize the trigger performance under various background noise and experimental conditions, includ- ing overlapping, multiple track hits.

VII. KLM TRIGGER In the Belle experiment, the KLM trigger was mainly used for the logic to take µ pair events, which are necessary and important for detector calibrations. For example, because the KLM trigger was independent from the CDC trigger, it was very useful to measure the efficiencies of other trigger logic. In this sense, the KLM trigger in the Belle II experiment is highly recommended. The use of FPGAs and high−speed serial links allow the trigger logic boards developed for other trigger Fig. 14. Overall TOP module firmware block diagram. systems to be adopted for the KLM trigger. 3−dimensional muon tracking in the KLM trigger may be possible if enough TABLE IV information is read out. Such 3−dimensional tracking of NETFIRMWARERESOURCEREQUIREMENTS. muons is expected to improve the trigger efficiency of low + − + − Type of Resource Number of Slices (%) multiplicity events, such as µ µ and τ τ , which pass Slices 14255 (76%) through the very forward and backward regions. Feasibility Slice Flip Flops 21112 (56%) of such a KLM trigger is under discussion. 4-input LUTs 16923 (45%) RAM16 110 (76%) VIII. GLOBAL DESIGN LOGIC SYSTEM The global decision logic (GDL) is the final arbiter of the Level 1 trigger decision. It receives trigger information from C. Firmware Implementation each system shown in Fig. 2, makes logical calculations and − The entire firmware has been implemented on a Virtex 4 pre scaling, and then issues the Level 1 trigger with the FPGA from Xilinx (model number XC4VFX40−10FF672I). appropriate timing. The GDL operates with a pipelined system architecture to ensure no dead−time for logical operations. We The overall flow-diagram of the TOP module firmware is shown in Fig. 14. will use a 62.5 MHz or higher clock as the system clock. As shown in Fig. 15, the GDL receives two types of information: Fast serial fiber optics communication is used to guarantee (1) summary information from each trigger (O(100) bits), and the necessary data rate. Timing information of the detected (2) fine information from each trigger (O(4000) bits). The photons is extracted and stored into a FIFO buffer. A pipelined former is used in the main logic that identifies physics events sorter reads out the time patterns from the FIFOs and combines of interest, and for calibration triggers, which are very similar them into a single sorted stream. This sorted stream of to the trigger summary information of the Belle experiment. time patterns is passed to the trigger logic for trigger event The latter consists of, for example, the track segments (TS) determination. If algorithm threshold is surpassed, the time in the CDC trigger and the energy deposits (TC) in the ECL and position information of the event trigger is broadcast. trigger. This information is used for low−level reconstruction To simplify the implementation and the number of patterns within the GDL: the association of a charged track with an stored, the PDFs stored are indexed with the time interval ECL cluster, the identification of a neutral ECL cluster, and between the event and the photodetector hit, so that the output so on. The detail of the low−level reconstruction is now under of the ARGMAX, when subtracted from the first hit’s time, simulation study. represents directly an estimate of the event time. The GDL processes each trigger information in four stages, The resource usage of the entire firmware is summarized in as shown in Fig. 15. First, the GDL applies a timing adjustment Table IV, where the selected programmable device is shown to compensate for the different latencies of each trigger. to be adequate. Second, the GDL makes the trigger decision and identifies IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. ?, NO. ?, JANUARY 2011 8

TABLE V TRIGGER CATEGORIES AND THEIR TRIGGER LOGICS AT LEVEL 1 ONLINE a receipt of the ECL timing signal. If neither timing signal TRIGGER SYSTEM FOR THE BELLE II EXPERIMENT. arrives, the GDL issues the Level 1 trigger after a fixed delay from the time that the trigger was generated. Category Item Logic name Physics Υ(4S) / continuum trk3+ ⊕ Etot1+ ⊕ ICN4+ IX. CONCLUSION τ pair trk2+ − Calibration Bhabha Bbt The super KEKB collider and the Belle II detector will γ-γ ggt probe for New Physics phenomena in rare B meson, D meson, µ pair trk2+ and τ lepton decays. The collider and detector are now under Random trigger rdmt construction, with plans to begin operation in 2014. The online Veto Beam injection inj trigger system is indispensable for reducing high trigger rates Two photon events twph from unavoidable beam backgrounds during data taking, and for measuring the luminosity delivered from the super−KEKB collider to the Belle II detector. For optimal operation of the Belle II experiment, we have designed a Level 1 online trigger each event with the appropriate physics, calibration, and veto system, based upon ten years of practical experience from logic triggers, shown in Table V. The trk3+ (trk2+) logic is operating the Belle experiment at KEKB, the world’s highest satisfied by the presence of more than two tracks (one track) luminosity collider. This trigger system combines CDC, ECL, in the CDC trigger; the Etot1+ logic is satisfied by an energy PID and KLM sub-detector triggers into a final Level 1 trigger deposit of greater than 1 GeV in the ECL trigger; the ICN4+ decision, performed by the Belle II version of the GDL final logic is satisfied by the presence of more than three isolated decision logic. In this paper we have described the status and clusters in the ECL trigger. 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