- Email: [email protected]
Tracking system of the upgraded LHCb
Nuclear Instruments and Methods in Physics Research A 824 (2016) 62–63
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Tracking system of the upgraded LHCb A. Obłąkowska-Mucha, T. Szumlak n AGH – University of Science and Technology, Krakow, Poland
art ic l e i nf o
a b s t r a c t
Available online 19 December 2015
The upgrade of the LHCb experiment will run at an instantaneous luminosity up to 2 1033 cm 2 s 1 with a fully software based trigger, allowing us to read out the detector at a rate of 40 MHz. For this purpose, the full tracking system will be newly developed: the vertex locator (VELO) will be replaced by a pixel-based detector providing an excellent track reconstruction with an efficiency of above 99%. Upstream of the magnet, a silicon micro-strip detector with a high granularity and an improved acceptance, called the Upstream Tracker (UT) will be placed. The tracking system downstream of the magnet will be replaced by the Scintillating Fibre tracker (SciFi), which will consist of 12 layers using 2.5 m long scintillating fibres read out by silicon photo-multipliers. & 2015 Elsevier B.V. All rights reserved.
Keywords: LHCb tracking system LHCb upgrade Upstream Tracker
1. Introduction The presented paper consists of four sections. The first one briefly describes the LHCb (Large Hadron Collider beauty) spectrometer after the upgrade and presents an overview of the physics motivation behind the hardware modernization. The next section gives a more detailed description of one selected tracking sub-systems: The Upstream Tracker (UT). The AGH-Krakow group is involved in design of the readout front-end chip for the latter. The next section discusses the tracking software dedicated to efficient reconstruction of charged particle trajectories at higher upgrade luminosities as well as briefly discussing its expected performance. The paper will be concluded with a short summary section.
2. Upgrade of the LHCb tracking system The process of LHCb spectrometer modernization started effectively in 2011 with the submission of the “Letter of Intent for the LHCb Upgrade” [1]. The LHCb detector will be upgraded along with its data acquisition (DAQ) architecture to cope with the full event 40 MHz readout (at present the limit is 1.1 MHz). In order to attain this ambitious goal the trigger system needs to be rebuilt from scratch. The low level hardware trigger will be completely removed and the task of filtering interesting events, from the perspective of the LHCb physics programme, will be passed on to the new flexible fully software-based trigger [2]. The most striking advantage of such approach is significant increase in data rate (up to 20 kHz of events stored on tape), which n
Corresponding author. E-mail address: [email protected] (A. Obłąkowska-Mucha).
http://dx.doi.org/10.1016/j.nima.2015.12.019 0168-9002/& 2015 Elsevier B.V. All rights reserved.
should subsequently result in a much higher trigger efficiency and in turn improve the annual signal yields with respect to those obtained in Runs I and II. Preliminary analysis performed with dedicated simulated samples showed that these yields would be a factor 10 higher for the leptonic beauty decays and more than 20 for hadronic ones. Apart from changing the triggering system a significant change in the detection setup must be done as well. The entire tracking system will be exchanged, i.e., micro-strip vertex detector will be changed to pixel based one with square 55 μm pixels [3]. For the pixel vertex detector a new VeloPix chip is being designed based on the TimePix3 which works in a trigger-less mode. It will provide initial zero-suppression based on the time-over-threshold measurement. It is expected that the performance of the new VELO will be superior in all aspects (e.g., single hit resolution, primary vertex resolution and impact parameter resolution) comparing to the current one featuring strips. The upstream tracking stations TT (Tracking Turicensis) will be replaced by the brand new Upstream Tracker detector featuring thinner sensors (250 μm versus 500 μm in TT) [4]. Finally the downstream hybrid (silicon micro-strips for the Inner Tracker and straw tubes for the Outer Tracker) tracking stations will be replaced by a novel detector with plastic scintillating fibres [4]. The new T stations will be read out by dedicated silicon photo-multipliers (SiPM) . In the next section a more detailed description of the new UT tracker is given.
3. Upstream Tracker The Upstream Tracker will replace the current upstream tracking device TT. It is supposed to play a critical role in the new High Level Trigger (HLT) system and surpass the performance of the TT detector. It will be composed of four detection planes split
A. Obłąkowska-Mucha, T. Szumlak / Nuclear Instruments and Methods in Physics Research A 824 (2016) 62–63
into two parts called stations. All of them will be placed in a common cold box that will also provide electromagnetic shielding. In order to remove any potential condensation on the cold detection surfaces the box will be flushed with nitrogen or dry air. Each detector plane will consists of single-sided silicon micro-strip sensors with different pitches and strip lengths distributed according to the expected particle flux. Its main role will be to provide a fast estimate of the momentum for charged particles, which should be precise enough to allow efficient rejection of low-momentum tracks. Monte-Carlo studies suggest that rejection capability provided by the UT will speed up the tracking procedure up to three times with respect to the current performance, which taking into account the increase in the instantaneous luminosity is a significant achievement. In order to increase the HLT trigger efficiency, crucial for measuring the rare decays and increase the discovery potential of the LHCb, the UT reach will be extended towards the proton beam. The larger angular acceptance of the UT will be achieved by reducing the distance between the UT sensors and the beam pipe as well as using the innermost sensors with circular shape. The new sensors will have finer granularity and greatly improved radiation hardness compare to their TT predecessors, which is essential since the UT sensors must withstand the integrated luminosity of 50 fb 1. An essential part of the entire detector is a new front-end readout ASIC SALT (Silicon ASIC for LHCb Tracking) [5]. Each chip has 128 fully instrumented readout channels. Each channel will be equipped with a preamplifier and shaper which allows us to achieve peaking time less than 25 ns and a signal reminder smaller than 5% of the maximal signal value 25 ns after the peaking time. This requirement is meant to reduce the spill-over between consecutive events. A SAR ADC block will follow the analogue part of the chip and will be able to operate at 40 MHz with resolution of 6 bits. The digital signal will then be passed to the processing block where a set of specially designed algorithms will perform zero-suppression of the raw signal. Finally, the zerosuppressed data will be compressed and encoded into data banks and transmitted off the chip for further processing in the HLT. The total power consumption of the SALT ASIC must be not greater than 1 W at room temperature.
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for matching hits in the T-stations. After a number of quality criteria (χ2 of a fit, number of hits, outliers rejection) the best candidate is accepted as a long track. In the Upgrade conditions [6] with a ν ¼7.6 visible interaction on average and 3–6 primary vertices, the expected performance was studied with the use of simulated events. The reconstructed efficiency and ghost rate for long tracks is 2–4% lower than for the current detector but it is expected that improvements in algorithms will increase these numbers to already achieved ones. The momentum resolution of the tracking system is about 10–20% better than the current one, mainly due to less material and larger acceptance in the UT. The tracking reconstruction studies showed that in challenging upgrade conditions the LHCb tracking performance will be close to the present one.
5. Conclusion The upgrade of the LHCb detector is vital for maintaining its operation after the Long Shutdown 2 period. Without adding capabilities to read out the full detector at the LHC clock (40 MHz) speed the detector would not be able to operate at higher luminosities for hadronic channels. In order to fulfill this requirement a number of major changes needs to be applied to the hardware structure of the detector. These include the complete replacement of the tracker system and changing the architecture of the event triggering system. The LHCb Collaboration intends to employ a novel approach where the whole trigger will be implemented in high level software capable of performing full event reconstruction in real time. The new tracker will consist of silicon vertex detector equipped with pixel sensors. Also new tracking stations upstream and downstream with respect to the magnet will be installed. The former will be a silicon micro-strip device while the latter will be a novel detector using scintillating fibres read out by silicon photo-multipliers. The software suite that will be used in the new trigger has been tested using dedicated simulated data samples produced for the upgrade conditions. Preliminary studies showed no problems with track reconstruction efficiency or ghost tracks rate.
Acknowledgments 4. Tracking software This research was supported in part by PL-Grid Infrastructure. The tracks in the LHCb spectrometer are of different types according to the type of sub-detector in which they are reconstructed. Long tracks are reconstructed in the VELO and the T-stations, they carry the most valuable information for physics analysis. Since they straddle both sides of the magnet, a combined information from two slopes of the track results in very precise momentum determination. Most of the particles coming out of V0 decays originate beyond the VELO and are reconstructed in the Upstream Tracker and in the T-stations (they are called downstream tracks). On the other hand, a track reconstructed in the VELO and the UT is called upstream, and due to the fringe magnetic field in UT, its momentum can be estimated with a precision better than 25%. The pattern recognition followed by the track fit provides the track parameters by algorithms which take as input various types of tracks. For instance the Forward Tracking algorithm takes the VELO or upstream tracks as an input and searches
References [1] S. Stone, R. Lindner, Letter of intent for the LHCb Upgrade, CERN LHCC-2011001, 2011. [2] P. Campana, et al. LHCb Trigger and Online Upgrade Technical Design Report, CERN-LHCC-2014-016, 2014. [3] P. Collins, et al., LHCb VELO Upgrade Technical Design Report, CERN-LHCC2013-021, 2013. [4] M. Ferro-Luzzi, et al., LHCb Tracker Upgrade Technical Design Report, CERNLHCC-2014-001, 2014. [5] Ch. Parkes, et al., Preliminary Specification of a Silicon Strip Readout Chip for the LHCb Upgrade, CERN-LHCb-PUB-2012-011, 2012. [6] LHCb Collaboration, Framework TDR for the LHCb Upgrade: Technical Design Report, CERN-LHCC-2012–007, 2012.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Tracking system of the upgraded LHCb A. Obłąkowska-Mucha, T. Szumlak n AGH – University of Science and Technology, Krakow, Poland
art ic l e i nf o
a b s t r a c t
Available online 19 December 2015
The upgrade of the LHCb experiment will run at an instantaneous luminosity up to 2 1033 cm 2 s 1 with a fully software based trigger, allowing us to read out the detector at a rate of 40 MHz. For this purpose, the full tracking system will be newly developed: the vertex locator (VELO) will be replaced by a pixel-based detector providing an excellent track reconstruction with an efficiency of above 99%. Upstream of the magnet, a silicon micro-strip detector with a high granularity and an improved acceptance, called the Upstream Tracker (UT) will be placed. The tracking system downstream of the magnet will be replaced by the Scintillating Fibre tracker (SciFi), which will consist of 12 layers using 2.5 m long scintillating fibres read out by silicon photo-multipliers. & 2015 Elsevier B.V. All rights reserved.
Keywords: LHCb tracking system LHCb upgrade Upstream Tracker
1. Introduction The presented paper consists of four sections. The first one briefly describes the LHCb (Large Hadron Collider beauty) spectrometer after the upgrade and presents an overview of the physics motivation behind the hardware modernization. The next section gives a more detailed description of one selected tracking sub-systems: The Upstream Tracker (UT). The AGH-Krakow group is involved in design of the readout front-end chip for the latter. The next section discusses the tracking software dedicated to efficient reconstruction of charged particle trajectories at higher upgrade luminosities as well as briefly discussing its expected performance. The paper will be concluded with a short summary section.
2. Upgrade of the LHCb tracking system The process of LHCb spectrometer modernization started effectively in 2011 with the submission of the “Letter of Intent for the LHCb Upgrade” [1]. The LHCb detector will be upgraded along with its data acquisition (DAQ) architecture to cope with the full event 40 MHz readout (at present the limit is 1.1 MHz). In order to attain this ambitious goal the trigger system needs to be rebuilt from scratch. The low level hardware trigger will be completely removed and the task of filtering interesting events, from the perspective of the LHCb physics programme, will be passed on to the new flexible fully software-based trigger [2]. The most striking advantage of such approach is significant increase in data rate (up to 20 kHz of events stored on tape), which n
Corresponding author. E-mail address: [email protected] (A. Obłąkowska-Mucha).
http://dx.doi.org/10.1016/j.nima.2015.12.019 0168-9002/& 2015 Elsevier B.V. All rights reserved.
should subsequently result in a much higher trigger efficiency and in turn improve the annual signal yields with respect to those obtained in Runs I and II. Preliminary analysis performed with dedicated simulated samples showed that these yields would be a factor 10 higher for the leptonic beauty decays and more than 20 for hadronic ones. Apart from changing the triggering system a significant change in the detection setup must be done as well. The entire tracking system will be exchanged, i.e., micro-strip vertex detector will be changed to pixel based one with square 55 μm pixels [3]. For the pixel vertex detector a new VeloPix chip is being designed based on the TimePix3 which works in a trigger-less mode. It will provide initial zero-suppression based on the time-over-threshold measurement. It is expected that the performance of the new VELO will be superior in all aspects (e.g., single hit resolution, primary vertex resolution and impact parameter resolution) comparing to the current one featuring strips. The upstream tracking stations TT (Tracking Turicensis) will be replaced by the brand new Upstream Tracker detector featuring thinner sensors (250 μm versus 500 μm in TT) [4]. Finally the downstream hybrid (silicon micro-strips for the Inner Tracker and straw tubes for the Outer Tracker) tracking stations will be replaced by a novel detector with plastic scintillating fibres [4]. The new T stations will be read out by dedicated silicon photo-multipliers (SiPM) . In the next section a more detailed description of the new UT tracker is given.
3. Upstream Tracker The Upstream Tracker will replace the current upstream tracking device TT. It is supposed to play a critical role in the new High Level Trigger (HLT) system and surpass the performance of the TT detector. It will be composed of four detection planes split
A. Obłąkowska-Mucha, T. Szumlak / Nuclear Instruments and Methods in Physics Research A 824 (2016) 62–63
into two parts called stations. All of them will be placed in a common cold box that will also provide electromagnetic shielding. In order to remove any potential condensation on the cold detection surfaces the box will be flushed with nitrogen or dry air. Each detector plane will consists of single-sided silicon micro-strip sensors with different pitches and strip lengths distributed according to the expected particle flux. Its main role will be to provide a fast estimate of the momentum for charged particles, which should be precise enough to allow efficient rejection of low-momentum tracks. Monte-Carlo studies suggest that rejection capability provided by the UT will speed up the tracking procedure up to three times with respect to the current performance, which taking into account the increase in the instantaneous luminosity is a significant achievement. In order to increase the HLT trigger efficiency, crucial for measuring the rare decays and increase the discovery potential of the LHCb, the UT reach will be extended towards the proton beam. The larger angular acceptance of the UT will be achieved by reducing the distance between the UT sensors and the beam pipe as well as using the innermost sensors with circular shape. The new sensors will have finer granularity and greatly improved radiation hardness compare to their TT predecessors, which is essential since the UT sensors must withstand the integrated luminosity of 50 fb 1. An essential part of the entire detector is a new front-end readout ASIC SALT (Silicon ASIC for LHCb Tracking) [5]. Each chip has 128 fully instrumented readout channels. Each channel will be equipped with a preamplifier and shaper which allows us to achieve peaking time less than 25 ns and a signal reminder smaller than 5% of the maximal signal value 25 ns after the peaking time. This requirement is meant to reduce the spill-over between consecutive events. A SAR ADC block will follow the analogue part of the chip and will be able to operate at 40 MHz with resolution of 6 bits. The digital signal will then be passed to the processing block where a set of specially designed algorithms will perform zero-suppression of the raw signal. Finally, the zerosuppressed data will be compressed and encoded into data banks and transmitted off the chip for further processing in the HLT. The total power consumption of the SALT ASIC must be not greater than 1 W at room temperature.
63
for matching hits in the T-stations. After a number of quality criteria (χ2 of a fit, number of hits, outliers rejection) the best candidate is accepted as a long track. In the Upgrade conditions [6] with a ν ¼7.6 visible interaction on average and 3–6 primary vertices, the expected performance was studied with the use of simulated events. The reconstructed efficiency and ghost rate for long tracks is 2–4% lower than for the current detector but it is expected that improvements in algorithms will increase these numbers to already achieved ones. The momentum resolution of the tracking system is about 10–20% better than the current one, mainly due to less material and larger acceptance in the UT. The tracking reconstruction studies showed that in challenging upgrade conditions the LHCb tracking performance will be close to the present one.
5. Conclusion The upgrade of the LHCb detector is vital for maintaining its operation after the Long Shutdown 2 period. Without adding capabilities to read out the full detector at the LHC clock (40 MHz) speed the detector would not be able to operate at higher luminosities for hadronic channels. In order to fulfill this requirement a number of major changes needs to be applied to the hardware structure of the detector. These include the complete replacement of the tracker system and changing the architecture of the event triggering system. The LHCb Collaboration intends to employ a novel approach where the whole trigger will be implemented in high level software capable of performing full event reconstruction in real time. The new tracker will consist of silicon vertex detector equipped with pixel sensors. Also new tracking stations upstream and downstream with respect to the magnet will be installed. The former will be a silicon micro-strip device while the latter will be a novel detector using scintillating fibres read out by silicon photo-multipliers. The software suite that will be used in the new trigger has been tested using dedicated simulated data samples produced for the upgrade conditions. Preliminary studies showed no problems with track reconstruction efficiency or ghost tracks rate.
Acknowledgments 4. Tracking software This research was supported in part by PL-Grid Infrastructure. The tracks in the LHCb spectrometer are of different types according to the type of sub-detector in which they are reconstructed. Long tracks are reconstructed in the VELO and the T-stations, they carry the most valuable information for physics analysis. Since they straddle both sides of the magnet, a combined information from two slopes of the track results in very precise momentum determination. Most of the particles coming out of V0 decays originate beyond the VELO and are reconstructed in the Upstream Tracker and in the T-stations (they are called downstream tracks). On the other hand, a track reconstructed in the VELO and the UT is called upstream, and due to the fringe magnetic field in UT, its momentum can be estimated with a precision better than 25%. The pattern recognition followed by the track fit provides the track parameters by algorithms which take as input various types of tracks. For instance the Forward Tracking algorithm takes the VELO or upstream tracks as an input and searches
References [1] S. Stone, R. Lindner, Letter of intent for the LHCb Upgrade, CERN LHCC-2011001, 2011. [2] P. Campana, et al. LHCb Trigger and Online Upgrade Technical Design Report, CERN-LHCC-2014-016, 2014. [3] P. Collins, et al., LHCb VELO Upgrade Technical Design Report, CERN-LHCC2013-021, 2013. [4] M. Ferro-Luzzi, et al., LHCb Tracker Upgrade Technical Design Report, CERNLHCC-2014-001, 2014. [5] Ch. Parkes, et al., Preliminary Specification of a Silicon Strip Readout Chip for the LHCb Upgrade, CERN-LHCb-PUB-2012-011, 2012. [6] LHCb Collaboration, Framework TDR for the LHCb Upgrade: Technical Design Report, CERN-LHCC-2012–007, 2012.