- Email: [email protected]
The performance of the ATLAS trigger
Nuclear Instruments and Methods in Physics Research A 453 (2000) 405}411
The performance of the ATLAS trigger S. Tapprogge CERN-EP Division, 1211 Geneva 23, Switzerland Accepted 21 June 2000
Abstract This paper describes the expected performance of the ATLAS trigger system, which has to reduce the interaction rate in pp collisions at (s"14 TeV of 10 Hz (at design luminosity) to about 100 Hz going to mass storage. 2000 Elsevier Science B.V. All rights reserved. Keywords: LHC; ATLAS; Trigger; Performance
1. Introduction Proton}proton collisions with (s"14 TeV at the LHC will be used to understand electroweak symmetry breaking (searching for one or more Higgs bosons), to search for new physics beyond the Standard Model (e.g. Supersymmetry) and to perform precision measurements of processes within the Standard Model (e.g. the = boson mass, the top quark mass and the proton structure) and beyond it. At the LHC, a bunch crossing will occur every 25 ns. For the design luminosity of 10 cm\ s\, each bunch crossing contains on average 23 inelastic interactions (`minimum biasa events). The cross-section range for various processes spans more than 10 orders of magnitude. At 10 cm\ s\, the total interaction rate amounts to 1 GHz. The rate for bb production is about
On behalf of the ATLAS T/DAQ community.
7 MHz, the one for = (t quark) production is 2 kHz (80 Hz). For a Standard Model Higgs boson of m "150 GeV the expected rate is about 3 Hz. & No branching ratios for detectable "nal states have been included in the above. In the case of the Higgs boson e.g. the decay HPcc is a rare one with a BR of +10\. More details on the physics at LHC and the performance of ATLAS can be found in Ref. [1]. The ATLAS trigger has thus to be very selective (about 1 : 10) and to provide a high rejection against background events, while keeping excellent e$ciency for signal events. The ATLAS detector is a multi-purpose detector which has been optimised for detection and precise measurement of leptons, photons, jets and missing transverse energy. The interaction region is surrounded by a tracking system (Inner Detector) consisting (moving from inside outwards) of silicon pixel and silicon strip detectors and a transition radiation tracker (providing electron identi"cation). The tracking detectors cover the range "g"(2.5 and are located inside a 2 T solenoid,
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 6 7 3 - 2
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which is surrounded by calorimetry. The region "g"(2.5 is equipped for the electromagnetic section with a "ne grained liquid Argon (LAr) calorimeter. The coverage of the hadronic calorimeter (a combination of scintillator tile and LAr components) extends up to "g""5. Muon detection and measurement is performed with an air-core toroid system, being equipped with muon chambers covering "g"(2.7. In total, the detector consists of more than 10 electronic channels and the average event size is about 1 Mbyte.
2. Overview of the ATLAS trigger system The ATLAS trigger consists of three levels, as shown in Fig. 1. At the "rst level (LVL1), the interaction rate of 1 GHz (at a bunch crossing rate of 40 MHz) has to be reduced to about 75 (upgradable to 100) kHz, with a maximum latency of 2.5 ls. The second level trigger (LVL2) will provide a further reduction by two orders of magnitude to about 1 kHz and the event "lter (EF) as the third stage will reduce the rate to about 100 Hz going to mass storage. LVL2 and the EF make up the higher level trigger (HLT). The "rst-level trigger [2] will be realized in hardware, using FPGA's and ASIC's. It will use only
information from the calorimeters in coarse granularity and from the trigger chambers of the muon system. Its task is to identify high-p objects 2 such as muons, electromagnetic clusters, q's (single hadrons) and jets and to provide information on missing and on total transverse energy. LVL1 operates deadtime free. During the LVL1 latency events are stored in frontend pipeline memories (using analog as well as digital data). After a LVL1 accept, the event fragments are transferred from the frontend pipeline memories to ReadOut Bu!ers (ROB), where they are stored until a LVL2 accept (or reject) is issued. In case of a LVL2 accept, the fragments are assembled (event building) and sent to a processor farm, which constitutes the event "lter. The second-level trigger [3] has to provide a fast rejection. To do so, it makes use of the Region-ofInterest (RoI) concept. In this approach, LVL1 provides guidance to LVL2 about regions in the detector (in the g} plane), where high-p objects 2 have been found. LVL2 then accesses the full granularity information of all relevant sub-detectors in this region and combines the information from di!erent sub-detectors, allowing for a sequential selection. Only a few per cent of the total event data have to be tranferred from the ROBs to the LVL2 processor, thus reducing the necessary bandwidth. In order to keep the average latency for a LVL2 decision to about 10 ms, optimised code for the trigger algorithms (in terms of execution time and amount of data needed) has to be used. The event "lter has access to the full event information and can run o%ine reconstruction code. An average latency of O(1 s) should be achieved. At the EF, the latest and detailed calibration and alignment information will be available.
3. Performance of the 5rst-level trigger
Fig. 1. Overview of the ATLAS trigger system.
The input to the LVL1 calorimeter trigger is made up of trigger towers with a size of 0.1;0.1 in *g;* , giving in total 7200 channels. The energy for each trigger tower is provided as an analog sum of the energies of the corresponding cells. For the muon system, information from the fast trigger chambers is used with about 8;10 channels. In
S. Tapprogge / Nuclear Instruments and Methods in Physics Research A 453 (2000) 405}411
the barrel, resistive plate chambers (RPC) are used, for the endcap region thin gap chambers (TGC). 3.1. Selection of electromagnetic clusters The left part of Fig. 2 shows the principle of the algorithm used at LVL1 to select electromagnetic (e.m.) clusters (i.e. electrons and photons). A window of 4;4 trigger towers is moved (in steps of 0.1 in each direction) in the g} plane. The core energy is determined using the central 1;2 adjacent trigger towers (four possible combinations) in the e.m. part. The transverse energy in the ring of 12 trigger towers surrounding the central four electromagnetic towers gives the e.m. isolation, the 16 towers of the hadronic section the hadronic isolation. The e!ect of the two isolation criteria on the inclusive rate at design luminosity is shown in Fig. 2 (right part). The solid curve shows the expected rate as a function of the p threshold on the core trans2 verse energy (1;2 trigger towers). The dotted curve indicates the rate reduction achievable by applying a hadronic isolation, the dashed one is with an electro-magnetic isolation added. By applying these isolation criteria, the inclusive rate can be reduced by about an order of magnitude (suppressing part of the background induced by jets). 3.2. Selection of muons In Fig. 3, the principle of the LVL1 algorithm used to identify muons is shown. The muon trigger
407
system consists of three stations of fast trigger chambers (barrel: RPC's and endcap: TGC's), which are made of two layers each. Depending on the value of the nominal p threshold, roads 2 of di!erent size are used to form coincidences between hits in di!erent layers. In the barrel, the starting point (pivot plane) is the innermost station, for the endcap, the outermost is used. For a low p threshold (nominal value of 2 6 GeV), a coincidence between three (out of four) layers is required, using the two innermost (outermost) stations for the barrel (resp. endcap) system. In the case of high p , the road size is 2 smaller and in addition to the two stations of the low p mode, a coincidence with the third station is 2 required. The dominating source of muons with p '10 GeV is the semi-muonic decay of b quarks, 2 followed by muons from p and K decays. Next, comes the contribution from semi-muonic charm decays. At large transverse momenta ('30 GeV), muons from = decays start to dominate. The contribution from p/K decays dominates the cross-section for p (7 GeV. In Fig. 4, the 2 e$ciency of the LVL1 muon endcap trigger is shown as a function of the muon p (for various 2 nominal thresholds, left: low p mode, right: high 2 p mode). At low luminosity (10 cm\ s\), the 2 expected rate for a threshold of 6 GeV is dominated by muons from p and K decays (16.8 kHz). Decays of b (c) quarks contribute with 4.0 (2.4) kHz.
Fig. 2. Algorithm for selection of e.m. clusters (left) and expected rate for e.m. clusters at LVL1 as a function of the p threshold (right). 2
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4. Performance of the second-level trigger 4.1. Selection of photons
Fig. 3. Principle of the algorithm for selection of muons at LVL1.
For an inclusive photon trigger, the dominating background is due to jet production, faking a photon signature (e.g. due to jets containing a leading p). The left part of Fig. 5 shows the expected rate (in case of low luminosity) for an inclusive single and a di-photon trigger at LVL2 due to jets after applying several selection criteria in addition to the LVL1 requirements. For a di-photon selection (nominal threshold of 20 GeV for each photon), the shape criteria (no hadronic leakage, compact shower (`2nd samplinga) and p rejection (`1st
Fig. 4. E$ciency for the LVL1 muon endcap trigger as a function of the muon p for various low p thresholds (left) and high 2 2 p thresholds (right). 2
Fig. 5. Expected rate for a single and a di-photon trigger at LVL2 for various selection criteria at low luminosity (left) and at design luminosity (right).
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samplinga)) give an additional rejection of more than two orders of magnitude. A rejection compared to LVL1 of about one order of magnitude is obtained for the inclusive photon trigger with a nominal threshold of 40 GeV. In case of design luminosity (right part of Fig. 5), the achievable reduction is smaller due to the contribution of minimum bias events.
luminosity (left part) and for design luminosity (right part). After the LVL2 calorimeter selection, most of the events contain a leading p or a photon conversion. Adding tracking information removes most of these events and gives a rejection of 8.2}6.2 w.r.t. the LVL2 calorimeter selection.
4.2. Selection of electrons
At LVL2, the information of the trigger chambers (RPC and TGC) is used to initiate the pattern recognition in the precision muon chambers (monitored drift tubes } MDT), which are also shown in Fig. 3. After pattern recognition, the curvature of the muon track is determined and quanti"ed, e.g. by calculating the sagitta of the track. The measured sagitta can then be converted (by using lookup tables) to the momentum of the track. The resolution in p is improved w.r.t. LVL1 and 2 gives sharper thresholds, as shown in Fig. 8. The left (right) part shows the fraction of accepted muons in the barrel system as a function of the muon p for the case of a low (high) nominal 2 p threshold of 6 resp. 20 GeV. 2 A further rejection of the inclusive muon rate (esp. for the low p case) can be obtained by match2 ing the track found in the muon system with a track found in the Inner Detector, where the latter gives also a sharper p cut. This allows to suppress 2 muons originating from p and K decays.
Fig. 6 shows the additional rejection of jet-induced background (w.r.t. LVL1) possible at LVL2 by making use of the full granularity information from the "ne-grained LAr calorimeter of ATLAS. Shown is the rate for an inclusive electron selection as a function of E at LVL2, for the LVL1 selection 2 (`LVL1a) and after adding a cut on the core energy fraction (`E /E a). The next cut shown (`stripsa) uses information from the "rst sampling of the calorimeter, which is segmented into strips of a size of 0.003 in *g. The last cut applied (`E& "a) uses 2 the hadronic energy behind the e.m. cluster. An additional rejection of about 7.5}6.3 is obtained w.r.t. the LVL1 selection. Further rejection can be obtained by searching for charged particle tracks in a region (RoI) of 0.2;0.2 around the cluster direction and requiring at least one track above a given p threshold. Fig. 7 2 shows the e!ect of such a selection on the type of the highest p particle found in the RoI: for low 2
Fig. 6. Expected rate for electrons for various LVL2 calorimeter selections as a function of E . 2
4.3. Selection of muons
4.4. Selection of B-physics events In case of B-physics, events are accepted at LVL1 if they contain a low p muon (nominal threshold 2 of 6 GeV). However, LVL1 does not give further guidance to LVL2. This implies that an unguided search for low momentum charged particles has to be performed in the Inner Detector to reconstruct exclusive "nal states from B hadron decays. Fig. 9 shows the expected e$ciency for reconstructing charged pions in the transition radiation tracker (TRT) as a function of p (left) separately for the 2 barrel and the endcap part of the TRT. The right part of the "gure shows the reconstruction e$ciency for electrons from J/( decays as a function of "g". Electrons are searched to p values of as low 2 as 0.5 GeV, requiring in addition a transition
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Fig. 7. Expected particle type with the highest p for LVL2 electron selections for the case of low luminosity (left) and design luminosity 2 (right).
Fig. 8. E$ciency for reconstructing muons in the barrel as a function of the muon p for a nominal threshold of 6 GeV (left) and of 2 20 GeV (right) at LVL2.
Fig. 9. E$ciency at LVL2 for reconstructing pions in the TRT as a function of p for the barrel and the endcap region (left) and for 2 reconstructing electrons from J/( decays as a function of "g" (right). Also shown in the left part is the generated spectrum of pions.
S. Tapprogge / Nuclear Instruments and Methods in Physics Research A 453 (2000) 405}411 Table 1 E$ciency and jet rejection (cumulative values) for an inclusive electron signature (threshold 30 GeV) at design luminosity at various stages of the trigger selection Selection stage
E$ciency (%)
Jet rejection
LVL1 calorimeter LVL2 calorimeter LVL2 tracking `EFa calorimeter `EFa tracking `EFa matching `EFa transition radiation
96.1 92.1 82.5 81.1 77.2 75.3 67.5
0.9;10 4.8;10 3.7;10 8.4;10 2.3;10 3.6;10 '4.5;10
radiation signal. The track candidates found are then re"ned using the information from the silicon strip and pixel detectors. Several "nal states have been investigated in more detail: BPJ/(#X with J/(Pe>e\(l>l\), B Pp>p\ and B PD ( (K>K\))#X. By ap plying loose mass cuts for these exclusive "nal states, the inclusive rate for 6 GeV muons at LVL1 of 23 kHz can be reduced to less than 1 kHz at LVL2. More details on the performance of these and other algorithms can be found in Ref. [4].
5. Performance of the event 5lter After event building the complete event will be passed to one processor of the event "lter farm. This gives the possibility of doing full event reconstruction and allows for monitoring of all trigger levels (as well as of the detector performance). The result of a study for an inclusive electron selection (nominal threshold 30 GeV) at design luminosity is shown in Table 1, where o%ine reconstruction code (`EFa) has been used as prototype for EF code. With increasing complexity of the
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algorithms the rejection against jets increases. The loss of events due to the LVL1 and LVL2 selection (w.r.t. the `EFa selection) has been estimated to be about 2}5%. The boundary between LVL2 and EF is foreseen to be #exible (e.g. allowing to move more complex algorithms at lower levels) in order to optimise the physics coverage of the HLT.
6. Conclusion The ATLAS trigger system is well suited to the demanding task of triggering in the harsh LHC environment, where an e$cient online rejection of background (about 1 : 10) is needed while maintaining an excellent (and unbiased) e$ciency for (rare) signals. The "rst level trigger will be realised in hardware (maximum latency of 2.5 ls) and provides an inclusive trigger on high-p signatures 2 using information from the calorimeter and from the muon trigger chambers. The higher level triggers (LVL2 and EF) have access to full granularity data from all sub-detectors and will be mostly software based, thus providing a large #exibility in algorithm usage. LVL2 has to provide a fast rejection (about a factor of 100) using mostly only part of the event data, whereas the EF will make use of the full event information to provide a reduction by another order of magnitude.
References [1] ATLAS Collaboration, Detector and Physics Performance Technical Design Report, CERN/LHCC 99-14/15, 1999. [2] ATLAS Collaboration, Level-1 Trigger Technical Design Report, CERN/LHCC 98-14, 1998. [3] ATLAS Collaboration, DAQ, EF, LVL2 and DCS Technical Progress Report, CERN/LHCC 98-16, 1998. [4] ATLAS Collaboration, Trigger Performance Status Report, CERN/LHCC 98-15, 1998.
8. ELECTRONICS, TRIGGER2
The performance of the ATLAS trigger S. Tapprogge CERN-EP Division, 1211 Geneva 23, Switzerland Accepted 21 June 2000
Abstract This paper describes the expected performance of the ATLAS trigger system, which has to reduce the interaction rate in pp collisions at (s"14 TeV of 10 Hz (at design luminosity) to about 100 Hz going to mass storage. 2000 Elsevier Science B.V. All rights reserved. Keywords: LHC; ATLAS; Trigger; Performance
1. Introduction Proton}proton collisions with (s"14 TeV at the LHC will be used to understand electroweak symmetry breaking (searching for one or more Higgs bosons), to search for new physics beyond the Standard Model (e.g. Supersymmetry) and to perform precision measurements of processes within the Standard Model (e.g. the = boson mass, the top quark mass and the proton structure) and beyond it. At the LHC, a bunch crossing will occur every 25 ns. For the design luminosity of 10 cm\ s\, each bunch crossing contains on average 23 inelastic interactions (`minimum biasa events). The cross-section range for various processes spans more than 10 orders of magnitude. At 10 cm\ s\, the total interaction rate amounts to 1 GHz. The rate for bb production is about
On behalf of the ATLAS T/DAQ community.
7 MHz, the one for = (t quark) production is 2 kHz (80 Hz). For a Standard Model Higgs boson of m "150 GeV the expected rate is about 3 Hz. & No branching ratios for detectable "nal states have been included in the above. In the case of the Higgs boson e.g. the decay HPcc is a rare one with a BR of +10\. More details on the physics at LHC and the performance of ATLAS can be found in Ref. [1]. The ATLAS trigger has thus to be very selective (about 1 : 10) and to provide a high rejection against background events, while keeping excellent e$ciency for signal events. The ATLAS detector is a multi-purpose detector which has been optimised for detection and precise measurement of leptons, photons, jets and missing transverse energy. The interaction region is surrounded by a tracking system (Inner Detector) consisting (moving from inside outwards) of silicon pixel and silicon strip detectors and a transition radiation tracker (providing electron identi"cation). The tracking detectors cover the range "g"(2.5 and are located inside a 2 T solenoid,
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 6 7 3 - 2
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which is surrounded by calorimetry. The region "g"(2.5 is equipped for the electromagnetic section with a "ne grained liquid Argon (LAr) calorimeter. The coverage of the hadronic calorimeter (a combination of scintillator tile and LAr components) extends up to "g""5. Muon detection and measurement is performed with an air-core toroid system, being equipped with muon chambers covering "g"(2.7. In total, the detector consists of more than 10 electronic channels and the average event size is about 1 Mbyte.
2. Overview of the ATLAS trigger system The ATLAS trigger consists of three levels, as shown in Fig. 1. At the "rst level (LVL1), the interaction rate of 1 GHz (at a bunch crossing rate of 40 MHz) has to be reduced to about 75 (upgradable to 100) kHz, with a maximum latency of 2.5 ls. The second level trigger (LVL2) will provide a further reduction by two orders of magnitude to about 1 kHz and the event "lter (EF) as the third stage will reduce the rate to about 100 Hz going to mass storage. LVL2 and the EF make up the higher level trigger (HLT). The "rst-level trigger [2] will be realized in hardware, using FPGA's and ASIC's. It will use only
information from the calorimeters in coarse granularity and from the trigger chambers of the muon system. Its task is to identify high-p objects 2 such as muons, electromagnetic clusters, q's (single hadrons) and jets and to provide information on missing and on total transverse energy. LVL1 operates deadtime free. During the LVL1 latency events are stored in frontend pipeline memories (using analog as well as digital data). After a LVL1 accept, the event fragments are transferred from the frontend pipeline memories to ReadOut Bu!ers (ROB), where they are stored until a LVL2 accept (or reject) is issued. In case of a LVL2 accept, the fragments are assembled (event building) and sent to a processor farm, which constitutes the event "lter. The second-level trigger [3] has to provide a fast rejection. To do so, it makes use of the Region-ofInterest (RoI) concept. In this approach, LVL1 provides guidance to LVL2 about regions in the detector (in the g} plane), where high-p objects 2 have been found. LVL2 then accesses the full granularity information of all relevant sub-detectors in this region and combines the information from di!erent sub-detectors, allowing for a sequential selection. Only a few per cent of the total event data have to be tranferred from the ROBs to the LVL2 processor, thus reducing the necessary bandwidth. In order to keep the average latency for a LVL2 decision to about 10 ms, optimised code for the trigger algorithms (in terms of execution time and amount of data needed) has to be used. The event "lter has access to the full event information and can run o%ine reconstruction code. An average latency of O(1 s) should be achieved. At the EF, the latest and detailed calibration and alignment information will be available.
3. Performance of the 5rst-level trigger
Fig. 1. Overview of the ATLAS trigger system.
The input to the LVL1 calorimeter trigger is made up of trigger towers with a size of 0.1;0.1 in *g;* , giving in total 7200 channels. The energy for each trigger tower is provided as an analog sum of the energies of the corresponding cells. For the muon system, information from the fast trigger chambers is used with about 8;10 channels. In
S. Tapprogge / Nuclear Instruments and Methods in Physics Research A 453 (2000) 405}411
the barrel, resistive plate chambers (RPC) are used, for the endcap region thin gap chambers (TGC). 3.1. Selection of electromagnetic clusters The left part of Fig. 2 shows the principle of the algorithm used at LVL1 to select electromagnetic (e.m.) clusters (i.e. electrons and photons). A window of 4;4 trigger towers is moved (in steps of 0.1 in each direction) in the g} plane. The core energy is determined using the central 1;2 adjacent trigger towers (four possible combinations) in the e.m. part. The transverse energy in the ring of 12 trigger towers surrounding the central four electromagnetic towers gives the e.m. isolation, the 16 towers of the hadronic section the hadronic isolation. The e!ect of the two isolation criteria on the inclusive rate at design luminosity is shown in Fig. 2 (right part). The solid curve shows the expected rate as a function of the p threshold on the core trans2 verse energy (1;2 trigger towers). The dotted curve indicates the rate reduction achievable by applying a hadronic isolation, the dashed one is with an electro-magnetic isolation added. By applying these isolation criteria, the inclusive rate can be reduced by about an order of magnitude (suppressing part of the background induced by jets). 3.2. Selection of muons In Fig. 3, the principle of the LVL1 algorithm used to identify muons is shown. The muon trigger
407
system consists of three stations of fast trigger chambers (barrel: RPC's and endcap: TGC's), which are made of two layers each. Depending on the value of the nominal p threshold, roads 2 of di!erent size are used to form coincidences between hits in di!erent layers. In the barrel, the starting point (pivot plane) is the innermost station, for the endcap, the outermost is used. For a low p threshold (nominal value of 2 6 GeV), a coincidence between three (out of four) layers is required, using the two innermost (outermost) stations for the barrel (resp. endcap) system. In the case of high p , the road size is 2 smaller and in addition to the two stations of the low p mode, a coincidence with the third station is 2 required. The dominating source of muons with p '10 GeV is the semi-muonic decay of b quarks, 2 followed by muons from p and K decays. Next, comes the contribution from semi-muonic charm decays. At large transverse momenta ('30 GeV), muons from = decays start to dominate. The contribution from p/K decays dominates the cross-section for p (7 GeV. In Fig. 4, the 2 e$ciency of the LVL1 muon endcap trigger is shown as a function of the muon p (for various 2 nominal thresholds, left: low p mode, right: high 2 p mode). At low luminosity (10 cm\ s\), the 2 expected rate for a threshold of 6 GeV is dominated by muons from p and K decays (16.8 kHz). Decays of b (c) quarks contribute with 4.0 (2.4) kHz.
Fig. 2. Algorithm for selection of e.m. clusters (left) and expected rate for e.m. clusters at LVL1 as a function of the p threshold (right). 2
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4. Performance of the second-level trigger 4.1. Selection of photons
Fig. 3. Principle of the algorithm for selection of muons at LVL1.
For an inclusive photon trigger, the dominating background is due to jet production, faking a photon signature (e.g. due to jets containing a leading p). The left part of Fig. 5 shows the expected rate (in case of low luminosity) for an inclusive single and a di-photon trigger at LVL2 due to jets after applying several selection criteria in addition to the LVL1 requirements. For a di-photon selection (nominal threshold of 20 GeV for each photon), the shape criteria (no hadronic leakage, compact shower (`2nd samplinga) and p rejection (`1st
Fig. 4. E$ciency for the LVL1 muon endcap trigger as a function of the muon p for various low p thresholds (left) and high 2 2 p thresholds (right). 2
Fig. 5. Expected rate for a single and a di-photon trigger at LVL2 for various selection criteria at low luminosity (left) and at design luminosity (right).
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samplinga)) give an additional rejection of more than two orders of magnitude. A rejection compared to LVL1 of about one order of magnitude is obtained for the inclusive photon trigger with a nominal threshold of 40 GeV. In case of design luminosity (right part of Fig. 5), the achievable reduction is smaller due to the contribution of minimum bias events.
luminosity (left part) and for design luminosity (right part). After the LVL2 calorimeter selection, most of the events contain a leading p or a photon conversion. Adding tracking information removes most of these events and gives a rejection of 8.2}6.2 w.r.t. the LVL2 calorimeter selection.
4.2. Selection of electrons
At LVL2, the information of the trigger chambers (RPC and TGC) is used to initiate the pattern recognition in the precision muon chambers (monitored drift tubes } MDT), which are also shown in Fig. 3. After pattern recognition, the curvature of the muon track is determined and quanti"ed, e.g. by calculating the sagitta of the track. The measured sagitta can then be converted (by using lookup tables) to the momentum of the track. The resolution in p is improved w.r.t. LVL1 and 2 gives sharper thresholds, as shown in Fig. 8. The left (right) part shows the fraction of accepted muons in the barrel system as a function of the muon p for the case of a low (high) nominal 2 p threshold of 6 resp. 20 GeV. 2 A further rejection of the inclusive muon rate (esp. for the low p case) can be obtained by match2 ing the track found in the muon system with a track found in the Inner Detector, where the latter gives also a sharper p cut. This allows to suppress 2 muons originating from p and K decays.
Fig. 6 shows the additional rejection of jet-induced background (w.r.t. LVL1) possible at LVL2 by making use of the full granularity information from the "ne-grained LAr calorimeter of ATLAS. Shown is the rate for an inclusive electron selection as a function of E at LVL2, for the LVL1 selection 2 (`LVL1a) and after adding a cut on the core energy fraction (`E /E a). The next cut shown (`stripsa) uses information from the "rst sampling of the calorimeter, which is segmented into strips of a size of 0.003 in *g. The last cut applied (`E& "a) uses 2 the hadronic energy behind the e.m. cluster. An additional rejection of about 7.5}6.3 is obtained w.r.t. the LVL1 selection. Further rejection can be obtained by searching for charged particle tracks in a region (RoI) of 0.2;0.2 around the cluster direction and requiring at least one track above a given p threshold. Fig. 7 2 shows the e!ect of such a selection on the type of the highest p particle found in the RoI: for low 2
Fig. 6. Expected rate for electrons for various LVL2 calorimeter selections as a function of E . 2
4.3. Selection of muons
4.4. Selection of B-physics events In case of B-physics, events are accepted at LVL1 if they contain a low p muon (nominal threshold 2 of 6 GeV). However, LVL1 does not give further guidance to LVL2. This implies that an unguided search for low momentum charged particles has to be performed in the Inner Detector to reconstruct exclusive "nal states from B hadron decays. Fig. 9 shows the expected e$ciency for reconstructing charged pions in the transition radiation tracker (TRT) as a function of p (left) separately for the 2 barrel and the endcap part of the TRT. The right part of the "gure shows the reconstruction e$ciency for electrons from J/( decays as a function of "g". Electrons are searched to p values of as low 2 as 0.5 GeV, requiring in addition a transition
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Fig. 7. Expected particle type with the highest p for LVL2 electron selections for the case of low luminosity (left) and design luminosity 2 (right).
Fig. 8. E$ciency for reconstructing muons in the barrel as a function of the muon p for a nominal threshold of 6 GeV (left) and of 2 20 GeV (right) at LVL2.
Fig. 9. E$ciency at LVL2 for reconstructing pions in the TRT as a function of p for the barrel and the endcap region (left) and for 2 reconstructing electrons from J/( decays as a function of "g" (right). Also shown in the left part is the generated spectrum of pions.
S. Tapprogge / Nuclear Instruments and Methods in Physics Research A 453 (2000) 405}411 Table 1 E$ciency and jet rejection (cumulative values) for an inclusive electron signature (threshold 30 GeV) at design luminosity at various stages of the trigger selection Selection stage
E$ciency (%)
Jet rejection
LVL1 calorimeter LVL2 calorimeter LVL2 tracking `EFa calorimeter `EFa tracking `EFa matching `EFa transition radiation
96.1 92.1 82.5 81.1 77.2 75.3 67.5
0.9;10 4.8;10 3.7;10 8.4;10 2.3;10 3.6;10 '4.5;10
radiation signal. The track candidates found are then re"ned using the information from the silicon strip and pixel detectors. Several "nal states have been investigated in more detail: BPJ/(#X with J/(Pe>e\(l>l\), B Pp>p\ and B PD ( (K>K\))#X. By ap plying loose mass cuts for these exclusive "nal states, the inclusive rate for 6 GeV muons at LVL1 of 23 kHz can be reduced to less than 1 kHz at LVL2. More details on the performance of these and other algorithms can be found in Ref. [4].
5. Performance of the event 5lter After event building the complete event will be passed to one processor of the event "lter farm. This gives the possibility of doing full event reconstruction and allows for monitoring of all trigger levels (as well as of the detector performance). The result of a study for an inclusive electron selection (nominal threshold 30 GeV) at design luminosity is shown in Table 1, where o%ine reconstruction code (`EFa) has been used as prototype for EF code. With increasing complexity of the
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algorithms the rejection against jets increases. The loss of events due to the LVL1 and LVL2 selection (w.r.t. the `EFa selection) has been estimated to be about 2}5%. The boundary between LVL2 and EF is foreseen to be #exible (e.g. allowing to move more complex algorithms at lower levels) in order to optimise the physics coverage of the HLT.
6. Conclusion The ATLAS trigger system is well suited to the demanding task of triggering in the harsh LHC environment, where an e$cient online rejection of background (about 1 : 10) is needed while maintaining an excellent (and unbiased) e$ciency for (rare) signals. The "rst level trigger will be realised in hardware (maximum latency of 2.5 ls) and provides an inclusive trigger on high-p signatures 2 using information from the calorimeter and from the muon trigger chambers. The higher level triggers (LVL2 and EF) have access to full granularity data from all sub-detectors and will be mostly software based, thus providing a large #exibility in algorithm usage. LVL2 has to provide a fast rejection (about a factor of 100) using mostly only part of the event data, whereas the EF will make use of the full event information to provide a reduction by another order of magnitude.
References [1] ATLAS Collaboration, Detector and Physics Performance Technical Design Report, CERN/LHCC 99-14/15, 1999. [2] ATLAS Collaboration, Level-1 Trigger Technical Design Report, CERN/LHCC 98-14, 1998. [3] ATLAS Collaboration, DAQ, EF, LVL2 and DCS Technical Progress Report, CERN/LHCC 98-16, 1998. [4] ATLAS Collaboration, Trigger Performance Status Report, CERN/LHCC 98-15, 1998.
8. ELECTRONICS, TRIGGER2