- Email: [email protected]
ATLAS trigger for B-physics
Nuclear Instruments and Methods in Physics Research A 462 (2001) 259–264
ATLAS trigger for B-physics John Baines Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, UK Representing the ATLAS collaboration
Abstract The unprecedented rate of bb% production at the Large Hadron Collider opens up the possibility to study rare decays including those interesting for CP-violation measurements. The trigger must have very good selectivity in order to reduce the interaction rate by about six orders of magnitude for recording. About one collision in every hundred will produce a bb% quark pair. Therefore, in addition to rejecting background from non-bb% events, the trigger must have the ability to identify and select those events containing the B-decay channels of specific interest. The trigger must be flexible to allow for future evolution, both to study different channels and as luminosity increases. The ATLAS trigger consists of three levels. Level-one is implemented in hardware, whilst the higher levels are based on general-purpose processors. In this paper the ATLAS trigger strategy is described and is illustrated using the following three channels: þ B0d ! pþ p and B decays including the subsequent decays D and J=c ! eþ e . # 2001 Elsevier s ! fðK K Þp Science B.V. All rights reserved. PACS: 29.90.+r Keywords: B-physics; Trigger; Real-time; Track Trigger
1. Introduction In this paper, the ATLAS trigger is described with particular reference to the selection of events for B-physics studies. ATLAS is an experiment at the CERN Large Hadron Collider, LHC, which is due to start taking data in 2005. The LHC luminosity will increase with time up to the design luminosity of 1034 cm2 s1 . The focus for the Bphysics programme is a period of running at an initial luminosity of 1033 cm2 s1 which would give an integrated luminosity of 30 fb1 after three years. The LHC has a 40 MHz bunch-crossing frequency. At the initial luminosity there are about E-mail address: [email protected] (J. Baines).
2.3 interactions per bunch crossing giving an interaction rate of about 90 MHz. The ATLAS B-physics programme [1] will continue to develop over the coming years. Important areas include CP-violation studies with the channels B0d ! pþ p and B0d ! J=cK0s (with both J=c ! eþ e and J=c ! mþ m ); measure0 þ ments of Bs oscillations in B0s ! D s p and Bs ! þ þ Ds a1 with Ds ! fðK K Þp ; rare decays of the type Bd;s ! mþ m X; b-production measurements [2] and precision measurements with B hadrons. The components of the ATLAS detector important for the b-trigger are the muon trigger chambers and muon precision chambers, the calorimeter (for electron and muon identification) and the Inner Detector, ID. The ID consists of the
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 1 1 9 - X
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Fig. 2. Schematic showing the three levels of the ATLAS trigger. Fig. 1. An event display for a simulated event in the ATLAS Inner Detector.
pixel detector nearest the beam-pipe; the SemiConductor Tracker (SCT), consisting of silicon micro-strip detectors, at intermediate radii and the Transition Radiation Tracker (TRT), consisting of straw tubes, at the outer radii. An event display showing a simulated B-physics event in the ID is shown in Fig. 1. The TRT has an electronidentification capability based on transition-radiation information. The ATLAS detector [3] and ID [4] are described in more detail elsewhere in these proceedings.
2. Trigger overview The ATLAS trigger [5–7] consists of three levels, shown schematically in Fig. 2. The level-1 trigger utilizes information from the calorimeter and muon trigger chambers. Data are pipe-lined pending the level-1 decision. The pipe-line length limits the decision time to less than 2:5 ms (the target is 2 ms). As a result the level-1 trigger algorithms are implemented in hardware on Application Specific Integrated Circuits (ASICs) or Field Programmable Gate Arrays (FPGAs).
The level-1 accept rate is limited to a maximum of 75 kHz (upgradable to 100 kHz). Level-1 provides guidance to level-2 by defining parts of the event, Regions of Interest (RoI), to be investigated at level-2. Events accepted by level-1 are stored in Read Out Buffers (ROBs) pending a level-2 decision. Data from one event are distributed over many ROBs. Following a level-2 accept, the event is assembled in the Event Builder before a further level of selection at the Event Filter (EF). Events passing the EF selection are written to mass storage at a rate of about 100 Hz. The ATLAS High Level Trigger (HLT), comprising level-2 and the EF, is implemented on general-purpose processors, possibly with the addition of FPGA co-processors at level-2 [8]. The two stages differ in that level-2 requests event fragments from the ROBs, as required, whilst the EF has access to the full assembled event. Level-2 must process triggers at the level-1 accept rate and so makes use of fast custom-developed algorithms. The EF input rate is about 1 kHz which allows the use of off-line type algorithms. The ATLAS HLT is still in the design phase. The split of tasks between level-2 and the EF remains to be optimized, especially for the Bphysics selection. In order to optimize the design,
J. Baines / Nuclear Instruments and Methods in Physics Research A 462 (2001) 259–264
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studies of physics performance are being carried out in conjunction with test-bed measurements and full system models which take into account data-flow considerations.
3. B-trigger strategy A schematic diagram of the B-physics selection strategy is shown in Fig. 3. The selection is initiated by a level-1 muon trigger. Since the cross-section for inclusive muon production from pion and kaon decays falls more rapidly with pT than that for prompt muon production from b decays, see Fig. 4, an appropriate choice of pT threshold gives a powerful reduction of the trigger rate due to background processes. A threshold of pT > 6 GeV has been chosen, which gives a level-1 muon trigger rate of 20 kHz at the initial luminosity of 1033 cm2 s1 . The muon trigger system covers the pseudo-rapidity range jZj52:4. The cross-section for B ! mX decays with muon pT > 6 GeV produced within the muon system acceptance is 2:3 mb, giving a rate for this process of 2:3 kHz at the initial luminosity. Most of the
Fig. 3. Schematic illustrating the ATLAS B-physics trigger strategy.
Fig. 4. Differential cross-section ds=dpT for inclusive muon production in ATLAS. The pseudo-rapidity range is jZj52:7.
level-1 muon trigger rate is due to muons with true pT below threshold originating from pion and kaon decay, a large proportion of which can be rejected at level-2 on the basis of more precise track measurements. At level-2 the muon is confirmed or rejected on the basis of data from detectors lying within the RoI defined by level-1. Information from the muon precision chambers is used to improve the pT resolution of the track parameters and so reduce the trigger rate to 9 kHz. By including information from the ID, a further improvement in pT resolution is obtained, particularly near the threshold. Additional background reduction is achieved by detecting a mismatch between the track parameters determined in the muon detectors and those of the parent pion of kaon track measured in the ID. After this selection the trigger rate is about 5 kHz of which about one third is due to b ! m decays. Further rate reduction is achieved by selecting specific channels of interest for physics studies [9]. This selection is kept as inclusive as possible at level-2 with some more exclusive selections at the EF. The rate reduction is achieved by identifying V. DETECTOR/TRIGGER/COMPUTING
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decay products, for example Ds or J=c, characteristic of the channels of interest. In some cases an exclusive reconstruction is necessary, e.g. for B0d ! pþ p .
4. Track reconstruction Since the level-1 trigger muon provides no guidance on the position of the other b-jet in the event, tracks must be reconstructed within the entire volume of the ID in order to identify the final-state decay products. The pT threshold for this search, pT > 0:5 GeV, is determined by the requirement of the J=c ! eþ e trigger for good electron reconstruction efficiency at pT > 1 GeV. ID track reconstruction is initiated by a search of the whole volume of either the pixel detector (pixel-scan) or the TRT (TRT-scan). The seed tracks produced by these scans are extrapolated into the SCT and, in the case of TRT seeds, continued into the pixel detector. The TRT-seeded method benefits from the large ð 36Þ number of TRT measurements along a track and the early identification of electron candidate tracks, which can then be treated differently in the extrapolation. The pixels, on the other hand, provide three highprecision three-dimensional measurements which, being at low radius, are less affected by material interactions and so provide better guidance for extrapolation into the SCT. Distributions of execution time are shown in Fig. 5 for the pixel-scan and TRT-scan as a function of the number of pixel and TRT hits, respectively. The TRT-scan uses a histogramming method which has an execution time that scales linearly with occupancy. The mean execution time, measured on a 450 MHz Pentium-III, is 230 ms. The pixel-scan uses a combinatorial method. As a result the execution time has contributions which scale as the second and third power of occupancy. However, an early rejection of hit-pairs inconsistent with the interaction point (defined by the beam position in the transverse plane and by the extrapolated trigger muon track in the beam direction) limits the number of combinations and so execution time. As a result the mean execution time is only 43 ms.
The pixel scan produces an average of 40 seed tracks to be extrapolated into the SCT. This extrapolation is performed using a Kalman Filter algorithm [10] which takes an average of 0:7 ms per seed. The extrapolated tracks are then associated with the track segments found by the TRT-scan. The pixel seeded method has been found to provide good efficiency for hadron reconstruction, whilst the TRT seeded method has advantages for electron reconstruction. Optionally electron candidate tracks, found by the TRT-scan, may be extrapolated inwards. This takes and average of 1:5 ms per seed as the extrapolation includes both the SCT and pixel detectors. In the final stage of reconstruction tracks are extrapolated outwards to the calorimeter and muon detectors. Tracks with pT > 2 GeV, identified as electron candidates by the TRT, are extrapolated to the calorimeter and are confirmed or rejected as electrons on the basis of electromagnetic calorimeter information. In order to identify muons, tracks with pT > 3 GeV are extrapolated to the calorimeter and muon detector. The reconstructed tracks form the basis of the level-2 B-trigger selection.
5. B-trigger selection The ATLAS B-trigger selection will continue to be developed as plans for the physics programme evolve. The following example is given to demonstrate the selectivity and flexibility of the trigger. Trigger rates have been determined from simulated data samples, corresponding to running at a luminosity of 1033 cm2 s1 , generated using a full detector simulation. For channels containing two final-state muons, for example decays including J=c ! mþ m or rare decays of the type Bd;s ! mm(X), the level-2 selection requirement is a second muon with pT > 5 GeV. This contributes 100 Hz to the level-2 trigger rate at the initial luminosity of 1033 cm2 s1 . These channels may also be triggered by an electron with pT > 4 GeV (from the opposite b-jet) in addition to the level-1 muon. This contributes a further 50 Hz to the level-2
J. Baines / Nuclear Instruments and Methods in Physics Research A 462 (2001) 259–264
Fig. 5. Distribution of execution times for the pixel-scan (top) and TRT-scan (bottom) as a function of the number of hits, measured on a 450 MHz Pentium-III. For each histogram bin, the size of the box is proportional to the number of entries.
trigger rate. The muon–electron trigger is particularly useful in CP-violation measurements in the channel B0d ! J=cK0s with J=psi ! mþ m using opposite-side flavour tagging. Other channels are triggered by combining the reconstructed track parameters in order to identify specific products (e.g. J=c or Ds ) of selected Bmeson decays characterised by invariant mass. For þ example the channel B0s ! D with D s p s ! þ fðK K Þp is triggered by selecting events containing a Ds candidate. First opposite charge-sign track pairs with pT > 1:5 GeV are combined in order to select candidates with invariant mass consistent with a fðKKÞ decay. Then a third track with pT > 1:5 GeV is added and an invariant mass cut is applied to select Ds ðfpÞ candidates. This selection gives a level-2 rate of 150 Hz. Similarly, the channel B0d ! J=cK0s with J=c ! eþ e is
263
triggered by selecting events with two oppositesign electron candidates, each with pT > 0:5 GeV, with invariant mass consistent with a J=cðeeÞ decay. This contributes 200 Hz to the level-2 rate. In some cases an exclusive reconstruction of the decay is required, for example to select B0d ! pþ p . Here only a loose mass cut is applied to allow an off-line fit to the different contributions in the mass peak, e.g. B0d ! Kþ p ; Lb ! pK , etc. The combinatorial background is controlled by track cuts, including a cut of pT > 4 GeV for both pion candidates. This trigger contributes 50 Hz to the level-2 accept rate. After the level-2 selection, the rate of B-physics triggers is about 600 Hz. At the EF, track reconstruction is repeated using off-line type algorithms, which give better track-parameter resolution. This allows tighter cuts to be applied than those used at level-2. A further level of selection is performed at this stage which makes use of vertex reconstruction. For the B0d ! pþ p trigger, for example, track pairs passing selection cuts are fitted to a common vertex. In addition to an invariant-mass window, cuts are made on the fit quality and reconstructed transverse decay length, and angular cuts based on the decay topology are applied. Events containing displaced J=c ! eþ e , þ J=c ! mþ m and D s ! fðK K Þp vertexes are selected in a similar way. If events with direct J=c production are required, the transverse decay length cut may be omitted for some predetermined fraction of events. Selections are also made for rare-decay channels of the type Bd;s ! mmðXÞ. Overall the EF selection gives a further factor of about 10 reduction in rate after level-2.
6. Summary and outlook A viable strategy has been demonstrated for the selection of B-physics events in the ATLAS detector. The selection is initiated by a muon trigger at level-1. At level-2 the muon is confirmed followed by a semi-inclusive selection of specific decay channels plus an inclusive muon–electron trigger and a two-muon trigger. At the EF, the rate is further reduced by tighter cuts based on vertex fits plus some additional exclusive and semiV. DETECTOR/TRIGGER/COMPUTING
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inclusive selections to reduce the rate of events passing the level-2 muon–electron and two-muon triggers. It has been shown that this trigger fulfils the requirements of the anticipated physics programme. The HLT selection is based on general-purpose processors which gives the flexibility for the trigger to evolve as the ATLAS physics programme develops. The resources required are about 1000 CPU of 1000 MIPS (i.e. present technology) at level-2 and about the same at the EF [11]. For comparison, the level-2 requirement for non-B-physics triggers at both the initial and design luminosities is about 200 CPU. Therefore B-physics is an important consideration in the design of the ATLAS trigger. An option under consideration is to halve the number of CPU required at level-2 by the use of FPGA co-processors. The division of tasks and resources between level-2 and the EF remains to be optimized. Options for level-2 range from performing only the confirmation of the level-1 muon to a full Btrigger selection as outlined above. The trigger has been documented in a Technical Proposal [7] which has been reviewed and accepted as a proof of principle. Work is continuing on the development and optimization of the trigger system. Two improvements currently under study are a low pT two-muon trigger at level-1 which will increase the acceptance for two-muon channels, and the use of vertex reconstruction or impact parameter cuts at level-2 in order to reduce the input rate to the EF. For the next review stage, the Technical Design Report, some architectural choices will be made based on the results of the
ongoing physics studies, trigger prototyping work and full-system modelling. This will be followed by the start of the final system assembly ready for the first physics in 2005.
References [1] The ATLAS Collaboration, ATLAS Detector and Physics Performance Technical Design Report, CERN-LHCC-9914 and 99-15 (1999). [2] S. Robins, Nucl. Instr. and Meth. A 462 (2001) 184, these proceedings. [3] D. Rousseau, Nucl. Instr. and Meth. A 462 (2001) 189, these proceedings. [4] S. Gadomski, Nucl. Instr. and Meth. A 462 (2001) 285, these proceedings. [5] The ATLAS Collaboration, ATLAS First-level Trigger Technical Design Report, CERN-LHCC-98-14 (1998). [6] The ATLAS Collaboration, ATLAS Trigger Performance Status Report, CERN-LHCC-98-15 (1998). [7] The ATLAS Collaboration, ATLAS High Level Triggers, DAQ and DCS Technical Proposal, CERN-LHCC-200017 (2000). [8] C. Hinkelbein et al., LVL2 Full TRT Scan Feature Extraction Algorithm for B Physics Performed on the Hybrid FPGA=CPU Processor System ATLANTIS: Measurement Results, ATLAS Note ATL-DAQ-2000-012 (2000). [9] J. Baines et al., B-Physics Event Selection for the ATLAS High Level Trigger, ATLAS Note ATL-DAQ-2000-031 (2000). [10] P. Billoir, S. Qian, Simultaneous Pattern Recognition and Track Fitting by the Kalman Filtering Method, Nucl. Instr. and Meth. A 225 (1990) 219. [11] F. Wickens, The ATLAS Level-2 Trigger Pilot Project, in proceedings of the IEEE 2000 Nuclear Science Symposium and Medical Imaging Conference, 15–20 October 2000, Lyon, France, to be published in IEEE Transactions on Nuclear Science.
ATLAS trigger for B-physics John Baines Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, UK Representing the ATLAS collaboration
Abstract The unprecedented rate of bb% production at the Large Hadron Collider opens up the possibility to study rare decays including those interesting for CP-violation measurements. The trigger must have very good selectivity in order to reduce the interaction rate by about six orders of magnitude for recording. About one collision in every hundred will produce a bb% quark pair. Therefore, in addition to rejecting background from non-bb% events, the trigger must have the ability to identify and select those events containing the B-decay channels of specific interest. The trigger must be flexible to allow for future evolution, both to study different channels and as luminosity increases. The ATLAS trigger consists of three levels. Level-one is implemented in hardware, whilst the higher levels are based on general-purpose processors. In this paper the ATLAS trigger strategy is described and is illustrated using the following three channels: þ B0d ! pþ p and B decays including the subsequent decays D and J=c ! eþ e . # 2001 Elsevier s ! fðK K Þp Science B.V. All rights reserved. PACS: 29.90.+r Keywords: B-physics; Trigger; Real-time; Track Trigger
1. Introduction In this paper, the ATLAS trigger is described with particular reference to the selection of events for B-physics studies. ATLAS is an experiment at the CERN Large Hadron Collider, LHC, which is due to start taking data in 2005. The LHC luminosity will increase with time up to the design luminosity of 1034 cm2 s1 . The focus for the Bphysics programme is a period of running at an initial luminosity of 1033 cm2 s1 which would give an integrated luminosity of 30 fb1 after three years. The LHC has a 40 MHz bunch-crossing frequency. At the initial luminosity there are about E-mail address: [email protected] (J. Baines).
2.3 interactions per bunch crossing giving an interaction rate of about 90 MHz. The ATLAS B-physics programme [1] will continue to develop over the coming years. Important areas include CP-violation studies with the channels B0d ! pþ p and B0d ! J=cK0s (with both J=c ! eþ e and J=c ! mþ m ); measure0 þ ments of Bs oscillations in B0s ! D s p and Bs ! þ þ Ds a1 with Ds ! fðK K Þp ; rare decays of the type Bd;s ! mþ m X; b-production measurements [2] and precision measurements with B hadrons. The components of the ATLAS detector important for the b-trigger are the muon trigger chambers and muon precision chambers, the calorimeter (for electron and muon identification) and the Inner Detector, ID. The ID consists of the
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 1 1 9 - X
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Fig. 2. Schematic showing the three levels of the ATLAS trigger. Fig. 1. An event display for a simulated event in the ATLAS Inner Detector.
pixel detector nearest the beam-pipe; the SemiConductor Tracker (SCT), consisting of silicon micro-strip detectors, at intermediate radii and the Transition Radiation Tracker (TRT), consisting of straw tubes, at the outer radii. An event display showing a simulated B-physics event in the ID is shown in Fig. 1. The TRT has an electronidentification capability based on transition-radiation information. The ATLAS detector [3] and ID [4] are described in more detail elsewhere in these proceedings.
2. Trigger overview The ATLAS trigger [5–7] consists of three levels, shown schematically in Fig. 2. The level-1 trigger utilizes information from the calorimeter and muon trigger chambers. Data are pipe-lined pending the level-1 decision. The pipe-line length limits the decision time to less than 2:5 ms (the target is 2 ms). As a result the level-1 trigger algorithms are implemented in hardware on Application Specific Integrated Circuits (ASICs) or Field Programmable Gate Arrays (FPGAs).
The level-1 accept rate is limited to a maximum of 75 kHz (upgradable to 100 kHz). Level-1 provides guidance to level-2 by defining parts of the event, Regions of Interest (RoI), to be investigated at level-2. Events accepted by level-1 are stored in Read Out Buffers (ROBs) pending a level-2 decision. Data from one event are distributed over many ROBs. Following a level-2 accept, the event is assembled in the Event Builder before a further level of selection at the Event Filter (EF). Events passing the EF selection are written to mass storage at a rate of about 100 Hz. The ATLAS High Level Trigger (HLT), comprising level-2 and the EF, is implemented on general-purpose processors, possibly with the addition of FPGA co-processors at level-2 [8]. The two stages differ in that level-2 requests event fragments from the ROBs, as required, whilst the EF has access to the full assembled event. Level-2 must process triggers at the level-1 accept rate and so makes use of fast custom-developed algorithms. The EF input rate is about 1 kHz which allows the use of off-line type algorithms. The ATLAS HLT is still in the design phase. The split of tasks between level-2 and the EF remains to be optimized, especially for the Bphysics selection. In order to optimize the design,
J. Baines / Nuclear Instruments and Methods in Physics Research A 462 (2001) 259–264
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studies of physics performance are being carried out in conjunction with test-bed measurements and full system models which take into account data-flow considerations.
3. B-trigger strategy A schematic diagram of the B-physics selection strategy is shown in Fig. 3. The selection is initiated by a level-1 muon trigger. Since the cross-section for inclusive muon production from pion and kaon decays falls more rapidly with pT than that for prompt muon production from b decays, see Fig. 4, an appropriate choice of pT threshold gives a powerful reduction of the trigger rate due to background processes. A threshold of pT > 6 GeV has been chosen, which gives a level-1 muon trigger rate of 20 kHz at the initial luminosity of 1033 cm2 s1 . The muon trigger system covers the pseudo-rapidity range jZj52:4. The cross-section for B ! mX decays with muon pT > 6 GeV produced within the muon system acceptance is 2:3 mb, giving a rate for this process of 2:3 kHz at the initial luminosity. Most of the
Fig. 3. Schematic illustrating the ATLAS B-physics trigger strategy.
Fig. 4. Differential cross-section ds=dpT for inclusive muon production in ATLAS. The pseudo-rapidity range is jZj52:7.
level-1 muon trigger rate is due to muons with true pT below threshold originating from pion and kaon decay, a large proportion of which can be rejected at level-2 on the basis of more precise track measurements. At level-2 the muon is confirmed or rejected on the basis of data from detectors lying within the RoI defined by level-1. Information from the muon precision chambers is used to improve the pT resolution of the track parameters and so reduce the trigger rate to 9 kHz. By including information from the ID, a further improvement in pT resolution is obtained, particularly near the threshold. Additional background reduction is achieved by detecting a mismatch between the track parameters determined in the muon detectors and those of the parent pion of kaon track measured in the ID. After this selection the trigger rate is about 5 kHz of which about one third is due to b ! m decays. Further rate reduction is achieved by selecting specific channels of interest for physics studies [9]. This selection is kept as inclusive as possible at level-2 with some more exclusive selections at the EF. The rate reduction is achieved by identifying V. DETECTOR/TRIGGER/COMPUTING
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decay products, for example Ds or J=c, characteristic of the channels of interest. In some cases an exclusive reconstruction is necessary, e.g. for B0d ! pþ p .
4. Track reconstruction Since the level-1 trigger muon provides no guidance on the position of the other b-jet in the event, tracks must be reconstructed within the entire volume of the ID in order to identify the final-state decay products. The pT threshold for this search, pT > 0:5 GeV, is determined by the requirement of the J=c ! eþ e trigger for good electron reconstruction efficiency at pT > 1 GeV. ID track reconstruction is initiated by a search of the whole volume of either the pixel detector (pixel-scan) or the TRT (TRT-scan). The seed tracks produced by these scans are extrapolated into the SCT and, in the case of TRT seeds, continued into the pixel detector. The TRT-seeded method benefits from the large ð 36Þ number of TRT measurements along a track and the early identification of electron candidate tracks, which can then be treated differently in the extrapolation. The pixels, on the other hand, provide three highprecision three-dimensional measurements which, being at low radius, are less affected by material interactions and so provide better guidance for extrapolation into the SCT. Distributions of execution time are shown in Fig. 5 for the pixel-scan and TRT-scan as a function of the number of pixel and TRT hits, respectively. The TRT-scan uses a histogramming method which has an execution time that scales linearly with occupancy. The mean execution time, measured on a 450 MHz Pentium-III, is 230 ms. The pixel-scan uses a combinatorial method. As a result the execution time has contributions which scale as the second and third power of occupancy. However, an early rejection of hit-pairs inconsistent with the interaction point (defined by the beam position in the transverse plane and by the extrapolated trigger muon track in the beam direction) limits the number of combinations and so execution time. As a result the mean execution time is only 43 ms.
The pixel scan produces an average of 40 seed tracks to be extrapolated into the SCT. This extrapolation is performed using a Kalman Filter algorithm [10] which takes an average of 0:7 ms per seed. The extrapolated tracks are then associated with the track segments found by the TRT-scan. The pixel seeded method has been found to provide good efficiency for hadron reconstruction, whilst the TRT seeded method has advantages for electron reconstruction. Optionally electron candidate tracks, found by the TRT-scan, may be extrapolated inwards. This takes and average of 1:5 ms per seed as the extrapolation includes both the SCT and pixel detectors. In the final stage of reconstruction tracks are extrapolated outwards to the calorimeter and muon detectors. Tracks with pT > 2 GeV, identified as electron candidates by the TRT, are extrapolated to the calorimeter and are confirmed or rejected as electrons on the basis of electromagnetic calorimeter information. In order to identify muons, tracks with pT > 3 GeV are extrapolated to the calorimeter and muon detector. The reconstructed tracks form the basis of the level-2 B-trigger selection.
5. B-trigger selection The ATLAS B-trigger selection will continue to be developed as plans for the physics programme evolve. The following example is given to demonstrate the selectivity and flexibility of the trigger. Trigger rates have been determined from simulated data samples, corresponding to running at a luminosity of 1033 cm2 s1 , generated using a full detector simulation. For channels containing two final-state muons, for example decays including J=c ! mþ m or rare decays of the type Bd;s ! mm(X), the level-2 selection requirement is a second muon with pT > 5 GeV. This contributes 100 Hz to the level-2 trigger rate at the initial luminosity of 1033 cm2 s1 . These channels may also be triggered by an electron with pT > 4 GeV (from the opposite b-jet) in addition to the level-1 muon. This contributes a further 50 Hz to the level-2
J. Baines / Nuclear Instruments and Methods in Physics Research A 462 (2001) 259–264
Fig. 5. Distribution of execution times for the pixel-scan (top) and TRT-scan (bottom) as a function of the number of hits, measured on a 450 MHz Pentium-III. For each histogram bin, the size of the box is proportional to the number of entries.
trigger rate. The muon–electron trigger is particularly useful in CP-violation measurements in the channel B0d ! J=cK0s with J=psi ! mþ m using opposite-side flavour tagging. Other channels are triggered by combining the reconstructed track parameters in order to identify specific products (e.g. J=c or Ds ) of selected Bmeson decays characterised by invariant mass. For þ example the channel B0s ! D with D s p s ! þ fðK K Þp is triggered by selecting events containing a Ds candidate. First opposite charge-sign track pairs with pT > 1:5 GeV are combined in order to select candidates with invariant mass consistent with a fðKKÞ decay. Then a third track with pT > 1:5 GeV is added and an invariant mass cut is applied to select Ds ðfpÞ candidates. This selection gives a level-2 rate of 150 Hz. Similarly, the channel B0d ! J=cK0s with J=c ! eþ e is
263
triggered by selecting events with two oppositesign electron candidates, each with pT > 0:5 GeV, with invariant mass consistent with a J=cðeeÞ decay. This contributes 200 Hz to the level-2 rate. In some cases an exclusive reconstruction of the decay is required, for example to select B0d ! pþ p . Here only a loose mass cut is applied to allow an off-line fit to the different contributions in the mass peak, e.g. B0d ! Kþ p ; Lb ! pK , etc. The combinatorial background is controlled by track cuts, including a cut of pT > 4 GeV for both pion candidates. This trigger contributes 50 Hz to the level-2 accept rate. After the level-2 selection, the rate of B-physics triggers is about 600 Hz. At the EF, track reconstruction is repeated using off-line type algorithms, which give better track-parameter resolution. This allows tighter cuts to be applied than those used at level-2. A further level of selection is performed at this stage which makes use of vertex reconstruction. For the B0d ! pþ p trigger, for example, track pairs passing selection cuts are fitted to a common vertex. In addition to an invariant-mass window, cuts are made on the fit quality and reconstructed transverse decay length, and angular cuts based on the decay topology are applied. Events containing displaced J=c ! eþ e , þ J=c ! mþ m and D s ! fðK K Þp vertexes are selected in a similar way. If events with direct J=c production are required, the transverse decay length cut may be omitted for some predetermined fraction of events. Selections are also made for rare-decay channels of the type Bd;s ! mmðXÞ. Overall the EF selection gives a further factor of about 10 reduction in rate after level-2.
6. Summary and outlook A viable strategy has been demonstrated for the selection of B-physics events in the ATLAS detector. The selection is initiated by a muon trigger at level-1. At level-2 the muon is confirmed followed by a semi-inclusive selection of specific decay channels plus an inclusive muon–electron trigger and a two-muon trigger. At the EF, the rate is further reduced by tighter cuts based on vertex fits plus some additional exclusive and semiV. DETECTOR/TRIGGER/COMPUTING
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inclusive selections to reduce the rate of events passing the level-2 muon–electron and two-muon triggers. It has been shown that this trigger fulfils the requirements of the anticipated physics programme. The HLT selection is based on general-purpose processors which gives the flexibility for the trigger to evolve as the ATLAS physics programme develops. The resources required are about 1000 CPU of 1000 MIPS (i.e. present technology) at level-2 and about the same at the EF [11]. For comparison, the level-2 requirement for non-B-physics triggers at both the initial and design luminosities is about 200 CPU. Therefore B-physics is an important consideration in the design of the ATLAS trigger. An option under consideration is to halve the number of CPU required at level-2 by the use of FPGA co-processors. The division of tasks and resources between level-2 and the EF remains to be optimized. Options for level-2 range from performing only the confirmation of the level-1 muon to a full Btrigger selection as outlined above. The trigger has been documented in a Technical Proposal [7] which has been reviewed and accepted as a proof of principle. Work is continuing on the development and optimization of the trigger system. Two improvements currently under study are a low pT two-muon trigger at level-1 which will increase the acceptance for two-muon channels, and the use of vertex reconstruction or impact parameter cuts at level-2 in order to reduce the input rate to the EF. For the next review stage, the Technical Design Report, some architectural choices will be made based on the results of the
ongoing physics studies, trigger prototyping work and full-system modelling. This will be followed by the start of the final system assembly ready for the first physics in 2005.
References [1] The ATLAS Collaboration, ATLAS Detector and Physics Performance Technical Design Report, CERN-LHCC-9914 and 99-15 (1999). [2] S. Robins, Nucl. Instr. and Meth. A 462 (2001) 184, these proceedings. [3] D. Rousseau, Nucl. Instr. and Meth. A 462 (2001) 189, these proceedings. [4] S. Gadomski, Nucl. Instr. and Meth. A 462 (2001) 285, these proceedings. [5] The ATLAS Collaboration, ATLAS First-level Trigger Technical Design Report, CERN-LHCC-98-14 (1998). [6] The ATLAS Collaboration, ATLAS Trigger Performance Status Report, CERN-LHCC-98-15 (1998). [7] The ATLAS Collaboration, ATLAS High Level Triggers, DAQ and DCS Technical Proposal, CERN-LHCC-200017 (2000). [8] C. Hinkelbein et al., LVL2 Full TRT Scan Feature Extraction Algorithm for B Physics Performed on the Hybrid FPGA=CPU Processor System ATLANTIS: Measurement Results, ATLAS Note ATL-DAQ-2000-012 (2000). [9] J. Baines et al., B-Physics Event Selection for the ATLAS High Level Trigger, ATLAS Note ATL-DAQ-2000-031 (2000). [10] P. Billoir, S. Qian, Simultaneous Pattern Recognition and Track Fitting by the Kalman Filtering Method, Nucl. Instr. and Meth. A 225 (1990) 219. [11] F. Wickens, The ATLAS Level-2 Trigger Pilot Project, in proceedings of the IEEE 2000 Nuclear Science Symposium and Medical Imaging Conference, 15–20 October 2000, Lyon, France, to be published in IEEE Transactions on Nuclear Science.