- Email: [email protected]
Performance of the all-silicon CMS Tracker
Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
Performance of the all-silicon CMS Tracker Michela Lenzi Universita! e Sezione INFN di Firenze, Largo E. Fermi 2, I-50125 Firenze, Italy On behalf of the CMS Tracker Collaboration
Abstract The Compact Muon Solenoid (CMS) Tracker Collaboration has recently revised the tracking detector layout. While the previous design relied on Micro Strip Gas Chamber and silicon detectors, the new layout implements solid state sensors as the sole technological choice. The new all-silicon layout is presented and the projected performance is discussed in terms of several benchmark topologies. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Track; Silicon detectors; CMS tracker
1. All-silicon tracker layout In December 1999, the CMS Collaboration decided to abandon the previous baseline design [1] in favor of a tracking system entirely based on silicon detectors [2]. A longitudinal view of the allsilicon CMS tracker is shown in Fig. 1. Starting from the inner part and moving outwards, the tracker will be composed of silicon pixel and silicon microstrip devices, distributed in a cylindrical volume of 6 m length and with an outer diameter of 2:4 m: In the central rapidity region, detectors are arranged in a barrel geometry, while in the forward regions, they are arranged as disks, segmented into radial petals. The all-silicon tracker has approximately the same total number of readout channels but less instrumented surface than the previous baseline layout: there are 10 rather than 11 barrel layers and 9 rather than 11 disks in each outer end-cap. Detailed studies have been
E-mail address: [email protected] (M. Lenzi).
carried on to investigate the effect of this reduction in surface area on the tracker performance. In addition, in the original layout a central support tube was located in the middle of the tracker in order to provide well separated thermal volumes for the silicon and Micro Strip Gas Chamber (MSGC) detectors: the former were to operate at 103 C; while the other at room temperature. Since this constraint no longer exists, the support tube has been moved outside the tracker. This allows the use of fully instrumented disks in the forward part in a radial range between 20 and 110 cm avoiding the unfavorable radial gap through which all cables and services for the inner tracker must otherwise be routed. For this reason, accurate studies have started to redefine the arrangement of cables and services so to optimize the material budget. In Fig. 2, the number of layers of silicon microstrip detectors is reported as a function of pseudorapidity. Critical regions in terms of hermeticity are the gap between barrel and end-caps and the high pseudorapidity regions. However, the
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 1 6 - 0
32
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
Fig. 1. Longitudinal section of the all-silicon tracker layout. Thick lines indicate double sided detectors, while thin lines mean single sided ones.
2. Construction considerations
Fig. 2. Average number of detectors intersected by infinite momentum tracks as a function of pseudorapidity in Silicon Strip Detectors.
hit multiplicity for microstrip devices is at least 8 for jZjo2:4 and about 50% of hits are double sided ones. Concerning the pixel detector layout, several optimization studies are currently going on regarding mainly the number, position and dimensions of layers. The tracker performance has been analyzed by using three pixel layers in the barrel and two disks for the forward part. The layers and disks disposition has been optimized to ensure three hits per track on average in pixel detectors for the whole Z coverage.
The possibility of constructing such a large silicon tracker relies crucially on a few key elements. The first one is manufacturing sensors using 600 instead of 400 industrial production lines, having at least the same quality and high volume capacity. This results in significant cost savings for the sensors and also allows the use of large area modules in the outer part of the tracker chosen in order to keep the number of readout channels with respect to the old layout unchanged. Indeed, the detectors in the outer tracker will have a strip length of 19 cm to be compared with the 12 cm in the inner part. The longer strip length will increase the noise by 20%. The effect on the signal-to-noise ratio will be compensated through the use of 500 mm thick sensors, thus increasing the collected charge. In addition, only minor modifications to the MSGC barrel structure are required in order to install the outer silicon detector modules. Therefore, we can exploit the extensive studies performed so far on mechanical, thermal, electronics and other system aspects. Finally, the large progress in the automation of module assembly and use of the new high throughput wire bonding machines allow considerable time reduction during the production phase [3].
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
33
Fig. 3. Contribution of the CMS Tracker subsystems to the radiation and interaction length. The label ‘Outside’ refers to all materials installed beyond the active volume of the Tracker.
3. Material budget
4. Sensor specifications
The amount of material traversed by particles in the tracker has a high impact on the physics performance of the CMS detector and consequently represents a major optimization issue in the definition of the tracker layout, as well as in several technical choices. Indeed, this stringent requirement limits the allowed total number of active layers and determines both the amount and type of material and the cable routing layout. The simulation description of the tracker material is very accurate and yields a careful determination of the tracker material budget in terms of radiation and interaction length. Preliminary evaluation of the tracker materials (material budget) in the new layout are underway. We report in this paper the radiation and interaction length evaluated for the all-silicon layout approved in April 2000 [2]. New elements in terms of service type and layout have been introduced since; consequently deeper investigation is needed to optimize the choice of materials and indeed a large effort is being carried out in this direction. The final evaluation will be released after the optimization phase started during the Engineering Design Review in November 2000: Fractional radiation and interaction length contributed by the tracker materials are shown in Fig. 3 as a function of the pseudorapidity; the information is itemized by sub-system.
A vigorous R&D program has been carried out on silicon detectors in order to ensure full functionality of the tracker for the whole lifetime of the experiment in the hostile LHC environment. One of the main results obtained over the last few years [4] shows that the interstrip capacitance, which is the main detector parameter determining the read-out noise, does not change with irradiation for /1 0 0S crystal lattice orientation. On the contrary, the interstrip capacitance increases considerably for /1 1 1S silicon bulk and only after substantial over depletion of the device, it reaches a value comparable to the one before irradiation. Hence, /1 0 0S silicon will be used to build the devices. In addition, the fluence at which type inversion takes place (and consequently, the operational voltage must change) depends on the bulk resistivity that can be tuned to limit the depletion voltage after irradiation. For this reason, low resistivity wafers have been chosen for the inner tracker where the highest radiation fluences are expected. Finally, the breakdown voltage can be reduced by use of appropriate readout strip and guard ring geometries; in particular, metal overhang and multiguard structure will be adopted to enhance the breakdown performance.
34
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
Fig. 4. Occupancy in the all-silicon tracker as a function of detector radius. Open and closed symbols represent, respectively, the local and global occupancies.
5. Occupancy To solve the pattern recognition problem at the unprecedented high luminosity of LHC, low cell occupancy is required. The detector occupancy has been evaluated by superimposing 24 minimum bias events at each bunch crossing, as expected in the high luminosity phase at LHC. In addition, the simulated pile-up includes 3 early and 2 late full bunch crossings. The occupancy is estimated after digitization and cluster reconstruction and it is defined as Occupancy ¼ Total number of strips in reconstructed clusters : Total number of strips ð1Þ The denominator can be either the total number of channels (global occupancy) or the subset including only the strips of the hit detectors (local occupancy). The first case is related to total data
volume and hence it is relevant for data acquisition, while the local occupancy concerns the pattern recognition as it characterizes local combinatories. In Fig. 4 the occupancy for the microstrip tracker is shown as a function of device radius, both in the barrel and forward part of the detector. A comparison of the occupancy [1] in the outer detectors in the all-silicon tracker with respect to the old layout shows that the local occupancy is reduced by a factor ranging between 2 and 5 in the new design. The global data volume has also improved in the all-silicon layout, since the number of channels is remained unchanged but the global occupancy is lower by at least a factor of 2, mostly thanks to the high speed response and the narrower cluster size of the silicon.
6. Track finding performance The track finding strategy for the CMS Tracker exploits a small number of high precision hits in a
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
35
Fig. 5. Transverse momentum resolution as a function of pseudorapidity for single muons with several pt values for the (a) new and (b) old tracker layout.
very high magnetic field ð4 TÞ: The combinatory problem is solved by using a two-step approach during the pattern recognition phase. The first step of the procedures developed for the CMS Tracker involves the definition of a preselected search road; the hits contained in the road are subsequently analyzed by Kalman filtering. Presently, two backward and one forward propagation packages have been developed. The latter being especially suited to reconstruct short tracks and for fast track finding. A detailed description of the track finding algorithms can be found in Ref. [5]. The efficiency to reconstruct single isolated muon tracks is 100% over most of the pseudorapidity coverage. A dedicated study of the performance at the detector large Z boundaries is in progress. The precision of the track reconstruction performance has been evaluated studying single muon tracks of several pT values. The performance turns out to be comparable with the one expected using the old tracker layout for the parameters that measure the track in the r–f plane. As an example, the precision of the track curvature is shown in Fig. 5a in terms of sðpt Þ=pt as a function of pseudorapidity. This plot has been obtained using the tracker layout version approved in April 2000. For comparison in Fig. 5b, the results obtained with the old tracker version are also reported.
As expected, at large Z the resolution worsens as the Tracker lever arm decreases. For low pt tracks, multiple scattering becomes significant and the Z dependence reflects the amount of material traversed by the tracks. On the other hand, the parameters that measure the track in the r–z plane are better estimated in the all-silicon tracker. In Fig. 6a, the resolution for the Z coordinate of the impact point is shown as a function of pseudorapidity for single muons. For comparison in Fig. 6b, the results obtained with the old tracker layout are shown. The Z dependence of the Zimp resolution is due to multiple scattering. An improvement of more than 30% with respect to the old layout is observed for all the samples. This is due to both the additional third pixel layer and the better hit resolution in the z-coordinate measured by the silicon sensors with respect to the gaseous ones.
7. Tracks in jets The jet topology represents a dense hit environment and is a significant benchmark of the tracker performance. To investigate the track finding efficiency and fake rate in such conditions, a sample of ET ¼ 200 GeV di-jet events has been used. In this case, no additional minimum bias pile-up has been taken into account. The track
36
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
Fig. 6. Zimp resolution as a function of pseudorapidity for single muons with different pt values for the (a) new and (b) old tracker layout.
finding efficiency and fake rate are defined as follows: e¼
Number of reconstructed associated tracks Number of generated tracks ð2Þ
stantial reduction of the local occupancy and hence of the out-of-time hits that can easily spoil track reconstruction.
8. Tagging b-jets efake
Number of non associated tracks : ¼ Number of reconstructed tracks
ð3Þ
Tracks have been reconstructed using the CM FKF [5] algorithm. Only tracks satisfying the fiducial requirements pt X0:9; jZjo2:5; vt o3 cm and jvz jo30 cm were taken into account, where vt and vz are, respectively, the transverse and z coordinates of the track origin. Reconstructed tracks are requested to have at least 6 hits and they are associated to the parent helix if they share more than 50% of the hits. The track finding performance in jets is shown in Fig. 7. The track efficiency turns out to be around 90% in the full Z range and the fake rate is lower than 1%. This result indicates that the high granularity of the detector and the algorithms used for track reconstruction are well suited to high density environment, as expected at LHC. The efficiency and fake rate values are not expected to change in presence of pile-up events according to the evaluations obtained with the old tracker layout [1]. This statement is strongly supported by the results reported in Section 5 showing the sub-
In this section, we present some recent results on the CMS tracker capability to distinguish between jets originating from b-quarks (b-jets) and jets from light quarks. Several characteristics of the b-jets event topology can be exploited for this purpose and different approaches have been investigated. All the b-jet identification procedures start by selecting a sample of good quality tracks according to the total number of hits and the number of hits in pixel detectors per reconstructed track, the w2 track parameter and the minimum transverse momentum. Further selection is applied requiring that tracks be withinqaffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cone with Ro0:4 from the jet axis, with R ¼ DZ2 þ Df2 ; where Z is the pseudorapidity and f is the azimuthal angle. The first approach is based on the track Impact Parameter (IP) that is calculated as the minimum distance between the particle trajectory and the beam axis in the transverse plane. To the impact parameter is then attributed the sign of the projection of the impact point on to the jet axis.
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
37
Fig. 7. Track finding efficiency and fake rate within jets as a function of jet pseudorapidity. Some increase in fake rate is noticed at large Z:
A jet is tagged if at least Ntracks have a SIP significance (Signed Impact Point divided by its error s) larger than Ns : Typical values for Ntracks and Ns are 2 or 3. Another adopted approach is based on secondary vertex reconstruction. During this stage, the track parameters are successfully improved by constraining them to the estimated vertices position. The conventional SIP algorithm is then applied only to tracks not associated to the primary or two-prong decay vertices. Yet another method uses the parameters of the reconstructed secondary vertices and requires that its radial position be at least three times its error. Further refinements of the b-tagging strategy can be applied, more stringent P for example, imposing P cuts on tracks SIP and=or tracks Pt : The best performance achievable is obtained by optimizing the b-tagging algorithm and the selection cuts according to the energy and the position in pseudorapidity of the jets under study. In Fig. 8, the b-tagging efficiency obtained by applying this optimized procedure is reported for b-jets with transverse energy ranging from 20 to 200 GeV: The mistagging rate, i.e. the probability to tag a light quark jet, is always smaller than 1%. The best results are obtained for 100 GeV energy jets for which the b-tagging efficiency turns
Fig. 8. b-tagging efficiency as a function of jet pseudorapidity. The mistagging rate is smaller than 1%.
out to be 65% in the barrel and drops to 43% in the endcap region. At lower jet transverse energies, the tagging efficiency decreases due to the smaller multiplicity and average pt of reconstructable (i.e. pt > 1 GeV) tracks produced in the decays of b-quarks. The effect of the softer kinematics is also accompanied by a degradation of the impact parameter precision (error is typically 70–100 mm for 1 GeV pt tracks). On the other hand, at high energies the purity deteriorates because of the high track density inside the jets. This affects the hit quality because
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M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
of cluster merging and consequently degrades the pattern recognition performance. In addition, at very high jet transverse energies, b-quarks can be so boosted that the reconstructed track starting points occurs beyond the first pixel layer located at 4 cm from the beam pipe. Accurate studies on 500 GeV energy jets shows that the b-tagging efficiency is 32% in the barrel while keeping the mistagging probability lower than 1%.
9. Summary At the end of 1999, the CMS collaboration opted for a tracking system based entirely on silicon detectors. The number of barrel layers in the silicon strip system has been reduced by one unit and the number of disks has been decreased from 11 to 9 in each outer end-cap, maintaining approximately the same number of read-out channels. On the other hand, one more detector layer has been deployed in the Pixel barrel system. As a consequence, the overall hit multiplicity registered by tracks crossing the detector is comparable to the old layout [1]. The preliminary and still incomplete studies presented in this paper show that the new tracker performance is maintained to the levels obtained in the original layout [1]. Indeed, the addition of a pixel layer and the improved z-resolution of the silicon sensors with respect to MSGCs, result in a substantially improved measurement precision of the track parameter in the r–z plane ðB30%Þ: Furthermore, the faster time response and the narrower charge diffusion of silicon compared to gaseous detectors guarantee smaller occupancy in the detector. We have presented the evaluation of track reconstruc-
tion performance restricted to the core ðR ¼ 0:4Þ of high ET jets, where the hit density is comparable to the one expected at the highest LHC luminosity conditions. The track finding efficiency for 200 GeV jets turns out to be around 90% in the whole Z region. In the central pseudorapidity region, b-tagging efficiency from 47% to 67% are achieved for jet energy ranging from 50 to 200 GeV; with a mistagging probability lower than 1%. Large efforts will be invested in the next months to finalize the evaluation of the overall performance of the CMS tracking detector, keeping into account with special care all the engineering details and changes occurred in the last year. The evaluations reported here indicate that the final CMS Tracker layout is suitable to match the LHC technical and physics challenge. The positive evaluation of the Engineering Design Review (November 2000) has opened the way to the construction phase.
References [1] The Tracker Project, Technical Design Report, CERN=LHCC 98-6 CMS TDR 5, 15 April 1998. [2] CMS Collaboration, Addendum to the CMS Tracker TDR, CERN=LHCC 2000-016. [3] L. Fiore, The role of automation in the construction of CMS silicon strip detector, Nucl. Instr. and Meth. A 473 (2001) 39, these proceedings. [4] R. Dell’Orso, Recent results for the CMS Tracker Silicon Detectors, Proceedings of the 2000 Nuclear Science Symposium and Medical Imaging Conference. [5] A. Caner, et al., Nucl. Instr. and Meth. A 435 (1999) 118.
Performance of the all-silicon CMS Tracker Michela Lenzi Universita! e Sezione INFN di Firenze, Largo E. Fermi 2, I-50125 Firenze, Italy On behalf of the CMS Tracker Collaboration
Abstract The Compact Muon Solenoid (CMS) Tracker Collaboration has recently revised the tracking detector layout. While the previous design relied on Micro Strip Gas Chamber and silicon detectors, the new layout implements solid state sensors as the sole technological choice. The new all-silicon layout is presented and the projected performance is discussed in terms of several benchmark topologies. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Track; Silicon detectors; CMS tracker
1. All-silicon tracker layout In December 1999, the CMS Collaboration decided to abandon the previous baseline design [1] in favor of a tracking system entirely based on silicon detectors [2]. A longitudinal view of the allsilicon CMS tracker is shown in Fig. 1. Starting from the inner part and moving outwards, the tracker will be composed of silicon pixel and silicon microstrip devices, distributed in a cylindrical volume of 6 m length and with an outer diameter of 2:4 m: In the central rapidity region, detectors are arranged in a barrel geometry, while in the forward regions, they are arranged as disks, segmented into radial petals. The all-silicon tracker has approximately the same total number of readout channels but less instrumented surface than the previous baseline layout: there are 10 rather than 11 barrel layers and 9 rather than 11 disks in each outer end-cap. Detailed studies have been
E-mail address: [email protected] (M. Lenzi).
carried on to investigate the effect of this reduction in surface area on the tracker performance. In addition, in the original layout a central support tube was located in the middle of the tracker in order to provide well separated thermal volumes for the silicon and Micro Strip Gas Chamber (MSGC) detectors: the former were to operate at 103 C; while the other at room temperature. Since this constraint no longer exists, the support tube has been moved outside the tracker. This allows the use of fully instrumented disks in the forward part in a radial range between 20 and 110 cm avoiding the unfavorable radial gap through which all cables and services for the inner tracker must otherwise be routed. For this reason, accurate studies have started to redefine the arrangement of cables and services so to optimize the material budget. In Fig. 2, the number of layers of silicon microstrip detectors is reported as a function of pseudorapidity. Critical regions in terms of hermeticity are the gap between barrel and end-caps and the high pseudorapidity regions. However, the
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 1 6 - 0
32
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
Fig. 1. Longitudinal section of the all-silicon tracker layout. Thick lines indicate double sided detectors, while thin lines mean single sided ones.
2. Construction considerations
Fig. 2. Average number of detectors intersected by infinite momentum tracks as a function of pseudorapidity in Silicon Strip Detectors.
hit multiplicity for microstrip devices is at least 8 for jZjo2:4 and about 50% of hits are double sided ones. Concerning the pixel detector layout, several optimization studies are currently going on regarding mainly the number, position and dimensions of layers. The tracker performance has been analyzed by using three pixel layers in the barrel and two disks for the forward part. The layers and disks disposition has been optimized to ensure three hits per track on average in pixel detectors for the whole Z coverage.
The possibility of constructing such a large silicon tracker relies crucially on a few key elements. The first one is manufacturing sensors using 600 instead of 400 industrial production lines, having at least the same quality and high volume capacity. This results in significant cost savings for the sensors and also allows the use of large area modules in the outer part of the tracker chosen in order to keep the number of readout channels with respect to the old layout unchanged. Indeed, the detectors in the outer tracker will have a strip length of 19 cm to be compared with the 12 cm in the inner part. The longer strip length will increase the noise by 20%. The effect on the signal-to-noise ratio will be compensated through the use of 500 mm thick sensors, thus increasing the collected charge. In addition, only minor modifications to the MSGC barrel structure are required in order to install the outer silicon detector modules. Therefore, we can exploit the extensive studies performed so far on mechanical, thermal, electronics and other system aspects. Finally, the large progress in the automation of module assembly and use of the new high throughput wire bonding machines allow considerable time reduction during the production phase [3].
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
33
Fig. 3. Contribution of the CMS Tracker subsystems to the radiation and interaction length. The label ‘Outside’ refers to all materials installed beyond the active volume of the Tracker.
3. Material budget
4. Sensor specifications
The amount of material traversed by particles in the tracker has a high impact on the physics performance of the CMS detector and consequently represents a major optimization issue in the definition of the tracker layout, as well as in several technical choices. Indeed, this stringent requirement limits the allowed total number of active layers and determines both the amount and type of material and the cable routing layout. The simulation description of the tracker material is very accurate and yields a careful determination of the tracker material budget in terms of radiation and interaction length. Preliminary evaluation of the tracker materials (material budget) in the new layout are underway. We report in this paper the radiation and interaction length evaluated for the all-silicon layout approved in April 2000 [2]. New elements in terms of service type and layout have been introduced since; consequently deeper investigation is needed to optimize the choice of materials and indeed a large effort is being carried out in this direction. The final evaluation will be released after the optimization phase started during the Engineering Design Review in November 2000: Fractional radiation and interaction length contributed by the tracker materials are shown in Fig. 3 as a function of the pseudorapidity; the information is itemized by sub-system.
A vigorous R&D program has been carried out on silicon detectors in order to ensure full functionality of the tracker for the whole lifetime of the experiment in the hostile LHC environment. One of the main results obtained over the last few years [4] shows that the interstrip capacitance, which is the main detector parameter determining the read-out noise, does not change with irradiation for /1 0 0S crystal lattice orientation. On the contrary, the interstrip capacitance increases considerably for /1 1 1S silicon bulk and only after substantial over depletion of the device, it reaches a value comparable to the one before irradiation. Hence, /1 0 0S silicon will be used to build the devices. In addition, the fluence at which type inversion takes place (and consequently, the operational voltage must change) depends on the bulk resistivity that can be tuned to limit the depletion voltage after irradiation. For this reason, low resistivity wafers have been chosen for the inner tracker where the highest radiation fluences are expected. Finally, the breakdown voltage can be reduced by use of appropriate readout strip and guard ring geometries; in particular, metal overhang and multiguard structure will be adopted to enhance the breakdown performance.
34
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
Fig. 4. Occupancy in the all-silicon tracker as a function of detector radius. Open and closed symbols represent, respectively, the local and global occupancies.
5. Occupancy To solve the pattern recognition problem at the unprecedented high luminosity of LHC, low cell occupancy is required. The detector occupancy has been evaluated by superimposing 24 minimum bias events at each bunch crossing, as expected in the high luminosity phase at LHC. In addition, the simulated pile-up includes 3 early and 2 late full bunch crossings. The occupancy is estimated after digitization and cluster reconstruction and it is defined as Occupancy ¼ Total number of strips in reconstructed clusters : Total number of strips ð1Þ The denominator can be either the total number of channels (global occupancy) or the subset including only the strips of the hit detectors (local occupancy). The first case is related to total data
volume and hence it is relevant for data acquisition, while the local occupancy concerns the pattern recognition as it characterizes local combinatories. In Fig. 4 the occupancy for the microstrip tracker is shown as a function of device radius, both in the barrel and forward part of the detector. A comparison of the occupancy [1] in the outer detectors in the all-silicon tracker with respect to the old layout shows that the local occupancy is reduced by a factor ranging between 2 and 5 in the new design. The global data volume has also improved in the all-silicon layout, since the number of channels is remained unchanged but the global occupancy is lower by at least a factor of 2, mostly thanks to the high speed response and the narrower cluster size of the silicon.
6. Track finding performance The track finding strategy for the CMS Tracker exploits a small number of high precision hits in a
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
35
Fig. 5. Transverse momentum resolution as a function of pseudorapidity for single muons with several pt values for the (a) new and (b) old tracker layout.
very high magnetic field ð4 TÞ: The combinatory problem is solved by using a two-step approach during the pattern recognition phase. The first step of the procedures developed for the CMS Tracker involves the definition of a preselected search road; the hits contained in the road are subsequently analyzed by Kalman filtering. Presently, two backward and one forward propagation packages have been developed. The latter being especially suited to reconstruct short tracks and for fast track finding. A detailed description of the track finding algorithms can be found in Ref. [5]. The efficiency to reconstruct single isolated muon tracks is 100% over most of the pseudorapidity coverage. A dedicated study of the performance at the detector large Z boundaries is in progress. The precision of the track reconstruction performance has been evaluated studying single muon tracks of several pT values. The performance turns out to be comparable with the one expected using the old tracker layout for the parameters that measure the track in the r–f plane. As an example, the precision of the track curvature is shown in Fig. 5a in terms of sðpt Þ=pt as a function of pseudorapidity. This plot has been obtained using the tracker layout version approved in April 2000. For comparison in Fig. 5b, the results obtained with the old tracker version are also reported.
As expected, at large Z the resolution worsens as the Tracker lever arm decreases. For low pt tracks, multiple scattering becomes significant and the Z dependence reflects the amount of material traversed by the tracks. On the other hand, the parameters that measure the track in the r–z plane are better estimated in the all-silicon tracker. In Fig. 6a, the resolution for the Z coordinate of the impact point is shown as a function of pseudorapidity for single muons. For comparison in Fig. 6b, the results obtained with the old tracker layout are shown. The Z dependence of the Zimp resolution is due to multiple scattering. An improvement of more than 30% with respect to the old layout is observed for all the samples. This is due to both the additional third pixel layer and the better hit resolution in the z-coordinate measured by the silicon sensors with respect to the gaseous ones.
7. Tracks in jets The jet topology represents a dense hit environment and is a significant benchmark of the tracker performance. To investigate the track finding efficiency and fake rate in such conditions, a sample of ET ¼ 200 GeV di-jet events has been used. In this case, no additional minimum bias pile-up has been taken into account. The track
36
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
Fig. 6. Zimp resolution as a function of pseudorapidity for single muons with different pt values for the (a) new and (b) old tracker layout.
finding efficiency and fake rate are defined as follows: e¼
Number of reconstructed associated tracks Number of generated tracks ð2Þ
stantial reduction of the local occupancy and hence of the out-of-time hits that can easily spoil track reconstruction.
8. Tagging b-jets efake
Number of non associated tracks : ¼ Number of reconstructed tracks
ð3Þ
Tracks have been reconstructed using the CM FKF [5] algorithm. Only tracks satisfying the fiducial requirements pt X0:9; jZjo2:5; vt o3 cm and jvz jo30 cm were taken into account, where vt and vz are, respectively, the transverse and z coordinates of the track origin. Reconstructed tracks are requested to have at least 6 hits and they are associated to the parent helix if they share more than 50% of the hits. The track finding performance in jets is shown in Fig. 7. The track efficiency turns out to be around 90% in the full Z range and the fake rate is lower than 1%. This result indicates that the high granularity of the detector and the algorithms used for track reconstruction are well suited to high density environment, as expected at LHC. The efficiency and fake rate values are not expected to change in presence of pile-up events according to the evaluations obtained with the old tracker layout [1]. This statement is strongly supported by the results reported in Section 5 showing the sub-
In this section, we present some recent results on the CMS tracker capability to distinguish between jets originating from b-quarks (b-jets) and jets from light quarks. Several characteristics of the b-jets event topology can be exploited for this purpose and different approaches have been investigated. All the b-jet identification procedures start by selecting a sample of good quality tracks according to the total number of hits and the number of hits in pixel detectors per reconstructed track, the w2 track parameter and the minimum transverse momentum. Further selection is applied requiring that tracks be withinqaffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cone with Ro0:4 from the jet axis, with R ¼ DZ2 þ Df2 ; where Z is the pseudorapidity and f is the azimuthal angle. The first approach is based on the track Impact Parameter (IP) that is calculated as the minimum distance between the particle trajectory and the beam axis in the transverse plane. To the impact parameter is then attributed the sign of the projection of the impact point on to the jet axis.
M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
37
Fig. 7. Track finding efficiency and fake rate within jets as a function of jet pseudorapidity. Some increase in fake rate is noticed at large Z:
A jet is tagged if at least Ntracks have a SIP significance (Signed Impact Point divided by its error s) larger than Ns : Typical values for Ntracks and Ns are 2 or 3. Another adopted approach is based on secondary vertex reconstruction. During this stage, the track parameters are successfully improved by constraining them to the estimated vertices position. The conventional SIP algorithm is then applied only to tracks not associated to the primary or two-prong decay vertices. Yet another method uses the parameters of the reconstructed secondary vertices and requires that its radial position be at least three times its error. Further refinements of the b-tagging strategy can be applied, more stringent P for example, imposing P cuts on tracks SIP and=or tracks Pt : The best performance achievable is obtained by optimizing the b-tagging algorithm and the selection cuts according to the energy and the position in pseudorapidity of the jets under study. In Fig. 8, the b-tagging efficiency obtained by applying this optimized procedure is reported for b-jets with transverse energy ranging from 20 to 200 GeV: The mistagging rate, i.e. the probability to tag a light quark jet, is always smaller than 1%. The best results are obtained for 100 GeV energy jets for which the b-tagging efficiency turns
Fig. 8. b-tagging efficiency as a function of jet pseudorapidity. The mistagging rate is smaller than 1%.
out to be 65% in the barrel and drops to 43% in the endcap region. At lower jet transverse energies, the tagging efficiency decreases due to the smaller multiplicity and average pt of reconstructable (i.e. pt > 1 GeV) tracks produced in the decays of b-quarks. The effect of the softer kinematics is also accompanied by a degradation of the impact parameter precision (error is typically 70–100 mm for 1 GeV pt tracks). On the other hand, at high energies the purity deteriorates because of the high track density inside the jets. This affects the hit quality because
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M. Lenzi / Nuclear Instruments and Methods in Physics Research A 473 (2001) 31–38
of cluster merging and consequently degrades the pattern recognition performance. In addition, at very high jet transverse energies, b-quarks can be so boosted that the reconstructed track starting points occurs beyond the first pixel layer located at 4 cm from the beam pipe. Accurate studies on 500 GeV energy jets shows that the b-tagging efficiency is 32% in the barrel while keeping the mistagging probability lower than 1%.
9. Summary At the end of 1999, the CMS collaboration opted for a tracking system based entirely on silicon detectors. The number of barrel layers in the silicon strip system has been reduced by one unit and the number of disks has been decreased from 11 to 9 in each outer end-cap, maintaining approximately the same number of read-out channels. On the other hand, one more detector layer has been deployed in the Pixel barrel system. As a consequence, the overall hit multiplicity registered by tracks crossing the detector is comparable to the old layout [1]. The preliminary and still incomplete studies presented in this paper show that the new tracker performance is maintained to the levels obtained in the original layout [1]. Indeed, the addition of a pixel layer and the improved z-resolution of the silicon sensors with respect to MSGCs, result in a substantially improved measurement precision of the track parameter in the r–z plane ðB30%Þ: Furthermore, the faster time response and the narrower charge diffusion of silicon compared to gaseous detectors guarantee smaller occupancy in the detector. We have presented the evaluation of track reconstruc-
tion performance restricted to the core ðR ¼ 0:4Þ of high ET jets, where the hit density is comparable to the one expected at the highest LHC luminosity conditions. The track finding efficiency for 200 GeV jets turns out to be around 90% in the whole Z region. In the central pseudorapidity region, b-tagging efficiency from 47% to 67% are achieved for jet energy ranging from 50 to 200 GeV; with a mistagging probability lower than 1%. Large efforts will be invested in the next months to finalize the evaluation of the overall performance of the CMS tracking detector, keeping into account with special care all the engineering details and changes occurred in the last year. The evaluations reported here indicate that the final CMS Tracker layout is suitable to match the LHC technical and physics challenge. The positive evaluation of the Engineering Design Review (November 2000) has opened the way to the construction phase.
References [1] The Tracker Project, Technical Design Report, CERN=LHCC 98-6 CMS TDR 5, 15 April 1998. [2] CMS Collaboration, Addendum to the CMS Tracker TDR, CERN=LHCC 2000-016. [3] L. Fiore, The role of automation in the construction of CMS silicon strip detector, Nucl. Instr. and Meth. A 473 (2001) 39, these proceedings. [4] R. Dell’Orso, Recent results for the CMS Tracker Silicon Detectors, Proceedings of the 2000 Nuclear Science Symposium and Medical Imaging Conference. [5] A. Caner, et al., Nucl. Instr. and Meth. A 435 (1999) 118.