Trigger for the WA92 fixed-target beauty experiment

UCLEAR PHYSIC:

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Nuclear Physics B (Proc. Suppl.) 44 (1995) 435-440

PROCEEDINGS SUPPLEMENTS

Trigger for the WA92 Fixed-Target Beauty Experiment M. Adamovich 6, M. Adinolfi 3, Y. Alexandrov 6, C. Angelini 7, C. Bacci 1°, D. Barberis a, D. Barney 5, J. Batten 5, W. Beusch 2 , C. Bruschini 3, R. Cardarelli 9, A. Cardini 7, V. Casanova 3, F. Ceradini 1°, G. Ciapetti s, M. Dameri 3, G. Darbo a, A. Di Ciaccio 9, A. Duane S, J.P. Dufey 2, Ph. Farthouat 2, V. Flaminio 7, A. Forino 1, B.R. French 2 , A. Frenkel s, C. Gemme 3, R. Gessaroli 1, K. Harrison 3, N. Hummadi 5, R. Hurst 3, A. Kirk 2, F. Lacava s, J.C. Lassalle 2, C. Lazzeroni 7, L. Malferrari 1, S. Maljukov 4, G. Martellotti s, P. Martinengo 2, P. Mazzanti 1 , J.G. McEwen 11 , I. Minashvili 4, P. Musico a, P. Nechaeva 6, P. Novelli a, A. Nisati s, D. Orestano s, B. Osculati 3, M. Passaseo s, G. Penso s, E. Petrolo s, L. Pontecorvo s, A. Quareni 1, P. Ragni s, H. Rotscheidt 2, V. Ryzhov ~, C. Roda 7, L. Rossi 3, N. Russakovich 4, C. Salvo 3, R. Santonico 9, G. Schuler ~, A. Semenov 4, A. Solovjev 4, M. Torelli s, S. Veneziano s, F. Vernocchi 3, M. Verzocchi s, D. Websdale 5, M. Weymann 2, L. Zanello s, M. Zavertyaev 6 Bologna 1, CERN ~, Genova 3, JINR-Dubna 4, London ICSTM 5, Moscow LPI 6, Pisa 7, Roma I s, Roma II 9, Roma III x°, Southampton 11 Presented by Jeremy Batten This report describes the WA92 trigger system which has been designed to collect beauty decays from fixedtarget hadronic interactions. It incorporates muon and high Pt triggers, and a novel pixel processor that searches for secondary vertices downstream of the target.

1. I N T R O D U C T I O N The WAg2 experiment aims at overcoming the disadvantage of low signal-to-noise ratio encountered at fixed-target beauty experiments by employing two strategies: an efficient and selective trigger system, the subject of this report, and the use of a decay detector (DD), described in detail elsewhere [1]. The DD uses silicon planes to form a fast imaging detector. Positioned immediately downstream of the target, it tracks charged particles and produces a high resolution picture of the event topology. In this way reconstruction of beauty decay vertices has been made possible [2]. The experiment uses the CERN Omega Spectrometer, and is supplied with a ~r- beam from the SPS in 2.4 s spills, one every 15 s, at a useful average beam rate of 5 x 106 particles per spill. The highest available energy of 350 GeV is used to maximise the beauty production cross section. When the WAg2 experiment was proposed [3] a maximum data-taking period of six months was foreseen, allowing production of approximately 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSD! 0920-5632(95)00566-8

104 beauty particle pairs with a background of "~ 1010 minimum bias events. To be useful the

trigger needed a high beauty acceptance and high rejection of background coupled with an acceptable dead-time. The principle of the trigger was to use many properties of the beauty decay to give the required acceptance: - - High decay m a s s --4 High Pt products - - H i g h s e m i - l e p t o n i c b r a n c h i n g ratio --+

Secondary muons - - L o n g l i f e t i m e -+ Secondary vertices present This suggested three specific triggers: hightransverse-momentum (PT), muon (MU) and one sensitive to secondary vertices, the beauty contiguity trigger (BCT). These have been implemented in a two-level system.

2. E X P E R I M E N T A L

APPARATUS

A schematic view of the Omega Spectrometer and apparatus specific to WAg2 is shown in figure 1. Following the beam path downstream

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Figure 1. Schematic view of the WA92 apparatus, vertical (X-Z) projection is a silicon beam detector (BMD) consisting ten planes of 20 #m pitch. The target is a 2 mm thick copper disc and has a silicon in-target (IT) detector of 200 /~m pitch attached to its downstream side. This is followed by the DD covering a 3 cm decay region immediately downstream. A silicon vertex detector (VXD) with 12 planes of 25/am pitch and 5 planes of 50/~m pitch covers the region from 5 cm to 60 cm downstream of the target. The standard drift and wire chambers of the Omega Spectrometer (bending power 7 Tin) are used with the addition of three wire chambers of 1 mm pitch for track bridging to the VXD. Two

planes of scintillating hodoscopes for the high Pt trigger are placed at the downstream end of the Omega magnet. A lead glass calorimeter is placed upstream of iron walls for the muon filter. Muon detection is done with six planes, each of 12 m s, of resistive plate chambers (RPC) [4] with a strip pitch of 3 em. 3. F I R S T - L E V E L T R I G G E R The performance of the first-level trigger is defined by the detector strobing requirements and the need for high beauty acceptance and background reduction before the readout and process-

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ing intensive B C T is started. The peaking time of the vertex detector and beam detector electronics [5] that supply the B C T is 250 ns, whilst the decay detector electronics and other apparatus require a trigger at 650 ns. Three triggers are available within these periods; interaction, high Pt and muon. The trigger logic can be seen in figure 2. Interaction t r i g g e r - - INT This uses scintillators to detect the beam and an interaction occuring within the first 60 cm downstream of the target. The IT counter, placed immediately downstream of the target, ensures that most triggered events have primary interactions in the target. This improves the performance of the B C T since its algorithm assumes the primary interaction to have occurred in the central vertical plane of the target. High-transverse-momentum t r i g g e r - - P T This trigger selects, in 350 ns, events richer

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than normal in high Pt tracks. The detector [6] consists of two scintillating "butterfly-shaped" hodoscopes. One of the Omega wire chambers is equipped with a fast readout and used in coincidence with the butterfly hodoscopes to provide greater rejection of background. The effect of the magnetic field is to make the coincidence between the hodoscope segments insensitive to charged particles with Pt _< 0.6 G e V / c with respect to the beam. Muon trigger-- MU This trigger takes advantage of the large semileptonic branching ratio of B mesons. Each of the two RPC hodoscopes has two Z-planes and one Yplane, each plane divided into six chambers. Two triggers are implemented using information from the Z-chambers. The first (FAST-MU) treats a group of 16 strips as one channel, forming a 3/4 majority trigger from four corresponding groups in the four Z-planes. This signal, ready in 150 ns, is used in Level-1 and provides the strobe to the second trigger, the muon processor [7]. This is centred around a programmable coincidence matrix which receives hit information from the individual strips of the two RPC hodoscopes. Tracks pointing to the target are found in 90 ns and the result is used in Level-2.

4. S E C O N D - L E V E L

TRIGGER

The principle component of the second-level is the B C T which is used in coincidence with the P T or MU trigger. The muon processor supplies a di-muon signal which provides a direct Level-2 trigger. Secondary-vertex t r i g g e r - - B C T The BCT, described in detail elsewhere [5,8,9], exploits the relatively long B lifetime which leads to a high average impact parameter (~450 #m). The principle is to search for vertices downstream of the interaction by finding impact parameter tracks, working in the X-Z projection. The impact parameter is taken as the vertical (Z) distance between the primary vertex and a track. Following primary vertex definition, hits from the VXD Z-planes are mapped onto a pixel array. A

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M. Adamovich et al. /Nuclear Physics B (Proc. Suppl.) 44 (1995) 435-440

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pattern of links between elements in the array is formed by a so called contiguity mask; a path of contiguous elements formed by the mask implies a found track. Primary and impact parameter tracks can be searched by making transformations which are parametrized by the distance from the primary vertex. Instructions to the processing elements of the array are received in parallel and give the device an inherently high processing speed. BCT algorithm The algorithm uses the hit positions from eight Z-planes: two from the BMD and six from the VXD. There are four stages to the algorithm: 1) The BMD information is used to find the beam track. This is extrapolated to the centre of the target to give the Z-coordinate of the interaction with the X-coordinate taken as the target centre (figure 3a). 2) A transformation of the coordinates of the

Figure 4. step 3 of the BCT algorithm

hits in the six VXD Z-planes is made. The transformation and its effect can be seen in figure 3b; primary tracks are mapped to straight lines and secondary tracks to hyperbolae. 3) The transformed hits are loaded into the pixel array where the contiguity mask is applied (figure 4b). The array is 2048 pixels by G representing the six VXD planes, with the most upstream plane at the bottom. Black pixels are those with a hit and solid lines are the links generated by the mask. With the bottom register set to one a connective path yields a one in the top register and is counted as a track. The hits along the path are removed by back propagation starting from the top register (figure 4c) leaving unconnected hits for the secondary track search.

M. Adamovich et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 435-440

The mask widens for upstream planes (the lower rows) to find the paths of tracks with impact parameter within 4- 100 pro. These are defined as primary. 4) The secondary track search is carried out for impact parameter windows of 4-50 # m by repeating steps 2 and 3 using varying values for primary vertex Z-position in the transformation. Tracks are found with a modified contiguity mask corresponding to the smaller impact parameter window. Nine windows are searched covering the range 100-1000 # m with positive and negative values searched at the same time. All the elements execute the same instruction stream making the processor a single-instruction multiple-data (SIMD) machine. It is important to note that the processing time of the trigger is not dependent on the number of hits and tracks, but only on the number of impact parameter windows that are used. Implementation The microstrip detectors used are two beam planes with 512 strips of 20 # m pitch and six vertex detector planes with 2048 strips of 25 # m pitch. These are equipped with fast amplification, encoding and readout stages [5] which use two specially designed Application Specific Integrated Circuits (ASIC). The whole readout process takes an average of 10 #s. The algorithm is implemented by three types of fastbus modules: - - detector-processor interface: F B C T Two modules perform stage 1, the beam and primary vertex finding, with 300 ns average processing time which is data dependent. - - the B C T processing slice: BCPS Eight modules, each with 24 ASICs, perform stages 2, 3 and 4. An overall processing time of 32 ps is achieved with a 20 MHz clock: 6 #s to find, count and erase primary tracks and 2.9 #s for each impact parameter window. - - control and trigger output: BCPC This single module generates the control signals for the whole B C T and counts the primary and secondary tracks, making the trigger decision. A trigger is issued if at least three primary tracks and two secondary tracks with impact parameter

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Table 1 Level-2 acceptances (% of INT) minimum bias c~ bb Trigger data - simulationBCT 5.4% 5.4% 13.5% 49.3% IPT 41.0% 41.0% 45.0% 62.7% 1MU 2.7% 2.5% 8.0% 18.3% 2MU 0.08% 0.06% 0.1% 1.6% Table 2 Beauty and minimum bias acceptance minimum bias Trigger data simulation INT 60% 62% Level-2 2.3% 1.9% beauty INT 71% Level-2 30%

greater than 100 # m and less than 1000 # m are found. 5. T R I G G E R

PERFORMANCE

Event generation and tracking A full simulation of the experiment has been done. Minimum bias events are generated with Fluka as interfaced to G E A N T 3.21 [10]. Events that involve the creation of a charm or beauty quark-antiquark pair are generated in two stages. Pythia 5.4 Ill] and Jetset 7.3 create hard process products, accounting for up to 98.5% of the beam momentum, and then the fraction remaining is passed to Fluka to produce further products from soft processes. The tracking of particles through the experimental apparatus is done with Geant 3.21. Trigger performance The acceptances of the Level-2 triggers to INT triggered events are shown in table 1. Simulation has been done for minimum bias, c6 and bb events, and the agreement between minimum bias data and simulation is good. It can be seen that the BCT has a beauty acceptance close to 50%. INT and overall Level-2 rates are shown in table 2. The Level-2 acceptance to beauty is 30% giving

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M. Adamovich et aL/Nuclear Physics B (Proc. Suppl.) 44 (1995) 435-440

7. C O N C L U S I O N The WA92 experiment has triggered on and isolated the first beauty particles whose flight path and decay are observed in an electronic detector. The trigger system has been designed for a high beauty acceptance and achieves 30% with an enrichment of 13. It has high-transverse-momentum and muon triggers and a secondary-vertex trigger which has a beauty acceptance of 50% and an average processing time of 32/ts. REFERENCES

Figure 5. Level-2 beauty acceptances (% of INT)

an overall enrichment from the trigger of 13. Figure 2 shows how the Level-2 trigger is formed, and this is reflected in the figure 5, which shows the acceptances for beauty as a percentage of the INT trigger. INT has a beauty acceptance of 71%. The trigger is represented by the shaded regions. The total beauty acceptance is 41.8% for Level-2 and 30% for the overall trigger. 6. T H E B E A U T Y

SIGNAL

The experiment has collected a total of 150 naillion events. A subset of these, 40 million events collected with the trigger described in this paper, have been analysed following full track reconstruction. The data have been classified into two streams; those with multi-vertex topologies, and those with a muon originating from a secondary vertex. Following graphical scanning of events we have made a preliminary measurement [12] of the beauty production cross section of 6.6 ± 1.8(stat) 5= 2.0(syst) nb/N, assuming a linear A-dependence.

1. M. Adinolfi et al., Nucl. Instr. and Meth. A329 (1993) 117-124. 2. M. Verzocchi et al., Nucl. Instr. and Meth. A351 (1994) 222-224. 3. M. Adamovich et al., CERN/SPSC 90-10 4. C. Bacci et al., Nucl. Instr. and Meth. A324 (1993)83-92. 5. A. Beer et al., Nucl. Instr. and Meth. A337 (1994) 280-294. 6. W. Beusch et al., Nucl. Instr. and Meth. A249 (1986) 391-398. 7. E. Petrolo and S. Vene~iano, Nucl. Instr. and Meth. A315 (1992) 95-101. 8. G. Darbo and L. Rossi, Nucl. Instr. and Meth. A289 (1990) 584-591. 9. G. Darbo et al., Nucl. Instr. and Meth. A351 (1994) 225-227. 10. GEANT Detector Description and Simulation Tool, CERN Program Library Long Writeup W5013 (1994). 11. T. Sj6strand, Comput. Phys. Commun. 82 (1994) 74. 12. L. Rossi et al. (Beatrice Collaboration), A measurement of the beauty production cross section, to appear 'in Proc. of the 27th Int. Conf. on High Energy Physics, July 1994, Glasgow, UK.