- Email: [email protected]
Performance of the High Momentum Particle IDentification detector of ALICE during the LHC run period 2015–2016
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Performance of the High Momentum Particle IDentification detector of ALICE during the LHC run period 2015–2016 G. De Cataldo, on behalf of ALICE collaboration INFN Bari, It, Via Amendola 4, 70126, Bari, Italy
A R T I C L E I N F O
A BS T RAC T
Keywords: RICH CsI photocathodes Particle identification
In the period June 2015–September 2016 the LHC has delivered pp and Pb-Pb collisions respectively at √s=13 TeV and √sNN=5.02 TeV for a total integrated luminosity in ALICE of 14 pb−1. The High Momentum Particle IDentification detector (HMPID) is part of the ALICE experiment. It is based on seven Ring Imaging Cherenkov (RICH) modules, 1.3×1.3 m2 each, with a proximity focusing geometry. The Cherenkov photon detection is achieved by pad segmented photocathodes, coated with 300 nm thick Caesium Iodide layer, installed in multiwire proportional chambers. Liquid C6F14 (perfluorohexane) with n=1.2989 at λ=175 nm is used as Cherenkov radiator. The HMPID identifies with three sigma separation charged π and K in the momentum range 1–3 GeV/c and protons in the range 1.5–5 GeV/c. The scatter plots of the Cherenkov angle for both pp and Pb-Pb events are presented as preliminary Particle Identification (PID) performance. After the good results shown during 2010–2013, in this paper the HMPID performance in the period 2015–2016, with emphasis on the CsI quantum efficiency stability, are presented. Finally the perspectives of the detector operation in the period HL-LHC 2019–2022, are briefly discussed.
1. Introduction ALICE has been designed to characterize the properties of the strongly interacting matter under extremely high energy density and temperature conditions where a state called Quark-Gluon Plasma (QGP) is expected. Such state of the matter prevailed in the early Universe, a few microseconds after its formation. For this task ALICE has been endowed with enhanced particle identification capabilities in a wide momentum range from hundreds MeV/c up to 100 GeV/c combining different detector techniques [1]. The HMPID [2] with its 5% of central barrel acceptance in the pseudo rapidity range |η|=0.5 provides a three sigma separation PID capabilities in the range 1– 5 GeV/c. The detector is based on seven 1.3×1.3 m2 modules (RICH0-6) for a total CsI active area of about 10.3 m2. The Cherenkov photon detection is achieved by pad segmented photocathodes (PC's) coated with 300 nm thick Caesium Iodide layer, installed in multiwire proportional chambers (MWPC) operated with CH4. Liquid C6F14 (perfluorohexane) with n=1.2989 at λ=175 nm is used as Cherenkov radiator. The HMPID identifies with three sigma separation charged π and K in the momentum range 1–3 GeV/c and protons in the range 1.5–5 GeV/c. In this paper the detector performance and the longevity of CsI photocathodes are presented. After 6 years of operation and 10 years
after the PC's production, emphasis is given to the stability of the quantum efficiency (QE) of CsI photocathodes with respect to the specific charge dose from avalanche ion bombardment, polluting gases and possible physicochemical aging effects. 2. The detector performance in the LHC run period 2015– 2016 The HMPID performance during 2010–2013 can be found in [3,4]. During 2015–2016 (LHC Run2), the electronics, the power supply systems and the C6F14 transparency system have operated stably. At the beginning of 2016 the HV sector 1 on RICH1 failed and the detector acceptance decreased from 72% to 70%. The reason could be a loose anode wire, a few hundred volts can be applied before the overcurrent switches off the HV channel. The stability of the CsI QE can be affected by contaminants such as O2 and water or by the specific charge dose from ion bombardment, or also by physicochemical aging effects on the production date of the photocathodes. The O2 and water level in the MWPC gas outlet have been monitored by the Detector Control System (DCS) via a dedicated device. The measured levels at ~10 ppm, were well below the safety threshold of several hundred ppm so no degradation of the CsI QE is expected [5].
E-mail addresses: [email protected], [email protected]. http://dx.doi.org/10.1016/j.nima.2017.01.037 Received 1 December 2016; Received in revised form 13 January 2017; Accepted 17 January 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: DE CATALDO, G., Nuclear Instruments and Methods in Physics Research A (2017), http://dx.doi.org/10.1016/j.nima.2017.01.037
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
G. De Cataldo
The specific charge dose on the photocathodes is determined by integrating the anode wire currents measured by the nano-ampere meters of the HV board channels. The selected integration periods are those with the HV values set at 2050 V, with gas gain of ~8×104, summing over all the pp, p-Pb and Pb-Pb runs. The specific charge dose on the CsI PCs is determined using 60% of the total anode wire current, this fraction being deduced by a model that combines the contributions of ionizing particles and CsI photoelectrons. The model assumes that one half of the ion avalanches from the ionizing particles migrates towards the CsI photocathode whereas, conservatively the full ion avalanche from CsI photoelectrons migrates towards the photocathode. Summing the charge dose of 2015–2016 and 2010–2013 [3], a total average value of 0.02 mC/cm2 per PC is reached. This is still below the threshold of 0.2 mC/cm2 for possible CsI QE loss [6,7]. The monitoring of the CsI QE stability cannot be done directly but it can be inferred by monitoring the number of reconstructed Cherenkov photons per ring (Nph) vs. time and after the deconvolution of possible effects as from the specific charge dose, the presence of contaminants and possible physicochemical aging process. Nph is measured at the maximum Cherenkov emission angle. In Fig. 1, the Nph trends vs. time for 34 active PCs, measured in pp and p-Pb collisions over 6 years of operation, are shown. On average, 12 Cherenkov photons per ring are reconstructed. In RICH5 one of the two HV sectors overlapping photocathodes 1 and 2, was off. Therefore the number of reconstructed photons is lower because of not all the
Fig. 2. Distribution of the slopes of the straight line fits to the Nph trends shown in Fig. 1. They are distributed around zero as expected for stable photocathodes without aging effects.
rings are fully contained on the active part of the photocathode. Due to empty radiator vessels (leaking vessels) and HV sectors off, 8 PCs cannot be monitored. All the CsI photocathodes were produced between 2004 and 2006 except PC0 (2001), PC4 and PC5 (2005) installed in RICH2. To check
Fig. 1. Average number of reconstructed photons Nph vs. time (year) per track, at the maximum Cherenkov angle, in the seven RICH modules, in pp and p-Pb collisions. A normalization factor to compensate for the gas gain variation, has been applied.
2
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
G. De Cataldo
pp and Pb-Pb collisions. In pp collisions the background contamination is quite low since the detector occupancy is only 0.1%. In Pb-Pb central collisions due to the higher event multiplicity, the detector occupancy is 30 times higher. After fitting procedures, the background (fake Cherenkov patterns detected) can be easily subtracted. The track-by-track identification is currently used in pp (e.g.: two particle correlation study to evaluate proton/pion ratio in the bulk and jets) whereas due to the higher background level, the extension to Pb-Pb collisions requires a more efficient background rejection. Therefore new algorithms for Cherenkov pattern recognition are currently being explored. A more complete description of the HMPID PID performance in pp, p-Pb and Pb-Pb collisions and the contribution to the ALICE physics program, can be found in [8]. 4. Perspective for HMPID data taking during 2020–2023 Fig. 3. HMPID Cherenkov angle vs track momentum for pp at √s=13 TeV data.
At the end of High Luminosity LHC period (2020–2023 HL-LHC, Run3) a specific charge dose of ~0.5 mC/cm2 should be integrated on the HMPID photocathodes. A CsI QE loss of 8% is expected [5]. This loss could be compensated by increasing the MWPC HV value. In any case, even if no increase is made, no degradation of the detector PID performance is expected. Test results with an upgraded read-out firmware have also shown that in pp collisions an event read-out rate of 10 kHz can be achieved, doubling the actual HMPID rate. In addition the integration of the detector in the new trigger, online and offline systems is progressing. It can therefore be concluded that in the Run3 period, the HMPID will be able of participating in the ALICE physics program. A final decision will be taken at the end of 2018 on the base of the general detector status. 5. Summary In the period 2015–2016 the LHC has delivered pp collisions at √s=13 TeV and Pb-Pb collisions at √sNN=5.02 TeV. The HMPID performance during that period has been stable and in particular the CsI QE. This is consistent with few ppm of O2 and water contaminants measured in the MWPC CH4 gas outlet and an average specific charge dose of 0.02 mC/cm2 on the photocathodes, value well below the threshold of 0.2 mC/cm2 where aging effects could be observed. The scatter plots of the measured Cherenkov angle in pp and Pb-Pb collisions are presented. They show preliminary PID performance for the different particle species in the momentum range 1–5 GeV/c. A more complete description of the HMPID PID capabilities and its contribution to the ALICE physics program, can be found in [8]. Based on the expected integrated luminosity in ALICE at the end of the HL-LHC period 2020–2023, considering the DAQ and Trigger system updating under way, the HMPID will be able of participating in the ALICE physics program. A final decision will be taken at the end of 2018 after considering the general status of the detector.
Fig. 4. HMPID Cherenkov angle vs track momentum for Pb-Pb collisions at √sNN=5.02 TeV.
possible PC's aging effects, the Nph trends of Fig. 1 are fitted in the period 2011–2016 with linear functions (2010 data are excluded since taken before a major HMPID HV equalization). For clarity, the fitting lines are not shown in the figure. In Fig. 2 the distribution of the slopes of the fitting lines, is shown. It has a symmetric shape and a mean value around zero. This is consistent with stable CsI photocathodes. The slightly negative slope of PC0, PC2 and PC3 of RICH2 (possible QE loss vs. time) are of the same order of the positive slope (possible QE increase vs. time) in the same module and in other modules. Therefore their values can be considered compatible with stable photocathodes response within the precision of this method. It can therefore be inferred that the CsI QE is stable and up to the present day, there is no significant evidence of degradation. It can also be concluded that after 10 years from production of the majority of the photocathodes (some being older than 10 years), there is no evidence of CsI physicochemical aging process.
References
3. PID performances
[1] [2] [3] [4] [5] [6] [7] [8]
Figs. 3 and 4 show respectively the scatter plots of the Cherenkov angle for pp collisions at √s=13 TeV and Pb-Pb collisions at √sNN=5.02 TeV. With Gaussian fits in different momentum slices of the scatter plots (PID statistical unfolding), the requirement for the particle identification at 3σ can be ensured in the momentum range 1–5 GeV/c, for both
3
K. Aamodt, et al., The ALICE experiment at the CERN LHC, JINST 3 (2008) S08002. T. HMPID, CERN/LHCC 98-19 ALICE TDR 1, 14 August, 1998. G. De Cataldo, Nucl. Instrum. Methods A766 (2014) 19–21 (RICH2013 conference). G. Volpe, Nucl. Instrum. Methods A766 (2014) 259–262 (RICH2013 conference). F. Piuz, Nucl. Instrum. Methods A502 (2003) 76–90. F. Piuz, et al., Nucl. Instrum. Methods A553 (2005) 187–195. H. Hoedlmoser, et al., Nucl. Instrum. Methods A574 (2007) 28–38. Contribution in this conference of G. Volpe, ALICE-HMPID contribution to the ALICE physics program.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Performance of the High Momentum Particle IDentification detector of ALICE during the LHC run period 2015–2016 G. De Cataldo, on behalf of ALICE collaboration INFN Bari, It, Via Amendola 4, 70126, Bari, Italy
A R T I C L E I N F O
A BS T RAC T
Keywords: RICH CsI photocathodes Particle identification
In the period June 2015–September 2016 the LHC has delivered pp and Pb-Pb collisions respectively at √s=13 TeV and √sNN=5.02 TeV for a total integrated luminosity in ALICE of 14 pb−1. The High Momentum Particle IDentification detector (HMPID) is part of the ALICE experiment. It is based on seven Ring Imaging Cherenkov (RICH) modules, 1.3×1.3 m2 each, with a proximity focusing geometry. The Cherenkov photon detection is achieved by pad segmented photocathodes, coated with 300 nm thick Caesium Iodide layer, installed in multiwire proportional chambers. Liquid C6F14 (perfluorohexane) with n=1.2989 at λ=175 nm is used as Cherenkov radiator. The HMPID identifies with three sigma separation charged π and K in the momentum range 1–3 GeV/c and protons in the range 1.5–5 GeV/c. The scatter plots of the Cherenkov angle for both pp and Pb-Pb events are presented as preliminary Particle Identification (PID) performance. After the good results shown during 2010–2013, in this paper the HMPID performance in the period 2015–2016, with emphasis on the CsI quantum efficiency stability, are presented. Finally the perspectives of the detector operation in the period HL-LHC 2019–2022, are briefly discussed.
1. Introduction ALICE has been designed to characterize the properties of the strongly interacting matter under extremely high energy density and temperature conditions where a state called Quark-Gluon Plasma (QGP) is expected. Such state of the matter prevailed in the early Universe, a few microseconds after its formation. For this task ALICE has been endowed with enhanced particle identification capabilities in a wide momentum range from hundreds MeV/c up to 100 GeV/c combining different detector techniques [1]. The HMPID [2] with its 5% of central barrel acceptance in the pseudo rapidity range |η|=0.5 provides a three sigma separation PID capabilities in the range 1– 5 GeV/c. The detector is based on seven 1.3×1.3 m2 modules (RICH0-6) for a total CsI active area of about 10.3 m2. The Cherenkov photon detection is achieved by pad segmented photocathodes (PC's) coated with 300 nm thick Caesium Iodide layer, installed in multiwire proportional chambers (MWPC) operated with CH4. Liquid C6F14 (perfluorohexane) with n=1.2989 at λ=175 nm is used as Cherenkov radiator. The HMPID identifies with three sigma separation charged π and K in the momentum range 1–3 GeV/c and protons in the range 1.5–5 GeV/c. In this paper the detector performance and the longevity of CsI photocathodes are presented. After 6 years of operation and 10 years
after the PC's production, emphasis is given to the stability of the quantum efficiency (QE) of CsI photocathodes with respect to the specific charge dose from avalanche ion bombardment, polluting gases and possible physicochemical aging effects. 2. The detector performance in the LHC run period 2015– 2016 The HMPID performance during 2010–2013 can be found in [3,4]. During 2015–2016 (LHC Run2), the electronics, the power supply systems and the C6F14 transparency system have operated stably. At the beginning of 2016 the HV sector 1 on RICH1 failed and the detector acceptance decreased from 72% to 70%. The reason could be a loose anode wire, a few hundred volts can be applied before the overcurrent switches off the HV channel. The stability of the CsI QE can be affected by contaminants such as O2 and water or by the specific charge dose from ion bombardment, or also by physicochemical aging effects on the production date of the photocathodes. The O2 and water level in the MWPC gas outlet have been monitored by the Detector Control System (DCS) via a dedicated device. The measured levels at ~10 ppm, were well below the safety threshold of several hundred ppm so no degradation of the CsI QE is expected [5].
E-mail addresses: [email protected], [email protected]. http://dx.doi.org/10.1016/j.nima.2017.01.037 Received 1 December 2016; Received in revised form 13 January 2017; Accepted 17 January 2017 0168-9002/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: DE CATALDO, G., Nuclear Instruments and Methods in Physics Research A (2017), http://dx.doi.org/10.1016/j.nima.2017.01.037
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
G. De Cataldo
The specific charge dose on the photocathodes is determined by integrating the anode wire currents measured by the nano-ampere meters of the HV board channels. The selected integration periods are those with the HV values set at 2050 V, with gas gain of ~8×104, summing over all the pp, p-Pb and Pb-Pb runs. The specific charge dose on the CsI PCs is determined using 60% of the total anode wire current, this fraction being deduced by a model that combines the contributions of ionizing particles and CsI photoelectrons. The model assumes that one half of the ion avalanches from the ionizing particles migrates towards the CsI photocathode whereas, conservatively the full ion avalanche from CsI photoelectrons migrates towards the photocathode. Summing the charge dose of 2015–2016 and 2010–2013 [3], a total average value of 0.02 mC/cm2 per PC is reached. This is still below the threshold of 0.2 mC/cm2 for possible CsI QE loss [6,7]. The monitoring of the CsI QE stability cannot be done directly but it can be inferred by monitoring the number of reconstructed Cherenkov photons per ring (Nph) vs. time and after the deconvolution of possible effects as from the specific charge dose, the presence of contaminants and possible physicochemical aging process. Nph is measured at the maximum Cherenkov emission angle. In Fig. 1, the Nph trends vs. time for 34 active PCs, measured in pp and p-Pb collisions over 6 years of operation, are shown. On average, 12 Cherenkov photons per ring are reconstructed. In RICH5 one of the two HV sectors overlapping photocathodes 1 and 2, was off. Therefore the number of reconstructed photons is lower because of not all the
Fig. 2. Distribution of the slopes of the straight line fits to the Nph trends shown in Fig. 1. They are distributed around zero as expected for stable photocathodes without aging effects.
rings are fully contained on the active part of the photocathode. Due to empty radiator vessels (leaking vessels) and HV sectors off, 8 PCs cannot be monitored. All the CsI photocathodes were produced between 2004 and 2006 except PC0 (2001), PC4 and PC5 (2005) installed in RICH2. To check
Fig. 1. Average number of reconstructed photons Nph vs. time (year) per track, at the maximum Cherenkov angle, in the seven RICH modules, in pp and p-Pb collisions. A normalization factor to compensate for the gas gain variation, has been applied.
2
Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx
G. De Cataldo
pp and Pb-Pb collisions. In pp collisions the background contamination is quite low since the detector occupancy is only 0.1%. In Pb-Pb central collisions due to the higher event multiplicity, the detector occupancy is 30 times higher. After fitting procedures, the background (fake Cherenkov patterns detected) can be easily subtracted. The track-by-track identification is currently used in pp (e.g.: two particle correlation study to evaluate proton/pion ratio in the bulk and jets) whereas due to the higher background level, the extension to Pb-Pb collisions requires a more efficient background rejection. Therefore new algorithms for Cherenkov pattern recognition are currently being explored. A more complete description of the HMPID PID performance in pp, p-Pb and Pb-Pb collisions and the contribution to the ALICE physics program, can be found in [8]. 4. Perspective for HMPID data taking during 2020–2023 Fig. 3. HMPID Cherenkov angle vs track momentum for pp at √s=13 TeV data.
At the end of High Luminosity LHC period (2020–2023 HL-LHC, Run3) a specific charge dose of ~0.5 mC/cm2 should be integrated on the HMPID photocathodes. A CsI QE loss of 8% is expected [5]. This loss could be compensated by increasing the MWPC HV value. In any case, even if no increase is made, no degradation of the detector PID performance is expected. Test results with an upgraded read-out firmware have also shown that in pp collisions an event read-out rate of 10 kHz can be achieved, doubling the actual HMPID rate. In addition the integration of the detector in the new trigger, online and offline systems is progressing. It can therefore be concluded that in the Run3 period, the HMPID will be able of participating in the ALICE physics program. A final decision will be taken at the end of 2018 on the base of the general detector status. 5. Summary In the period 2015–2016 the LHC has delivered pp collisions at √s=13 TeV and Pb-Pb collisions at √sNN=5.02 TeV. The HMPID performance during that period has been stable and in particular the CsI QE. This is consistent with few ppm of O2 and water contaminants measured in the MWPC CH4 gas outlet and an average specific charge dose of 0.02 mC/cm2 on the photocathodes, value well below the threshold of 0.2 mC/cm2 where aging effects could be observed. The scatter plots of the measured Cherenkov angle in pp and Pb-Pb collisions are presented. They show preliminary PID performance for the different particle species in the momentum range 1–5 GeV/c. A more complete description of the HMPID PID capabilities and its contribution to the ALICE physics program, can be found in [8]. Based on the expected integrated luminosity in ALICE at the end of the HL-LHC period 2020–2023, considering the DAQ and Trigger system updating under way, the HMPID will be able of participating in the ALICE physics program. A final decision will be taken at the end of 2018 after considering the general status of the detector.
Fig. 4. HMPID Cherenkov angle vs track momentum for Pb-Pb collisions at √sNN=5.02 TeV.
possible PC's aging effects, the Nph trends of Fig. 1 are fitted in the period 2011–2016 with linear functions (2010 data are excluded since taken before a major HMPID HV equalization). For clarity, the fitting lines are not shown in the figure. In Fig. 2 the distribution of the slopes of the fitting lines, is shown. It has a symmetric shape and a mean value around zero. This is consistent with stable CsI photocathodes. The slightly negative slope of PC0, PC2 and PC3 of RICH2 (possible QE loss vs. time) are of the same order of the positive slope (possible QE increase vs. time) in the same module and in other modules. Therefore their values can be considered compatible with stable photocathodes response within the precision of this method. It can therefore be inferred that the CsI QE is stable and up to the present day, there is no significant evidence of degradation. It can also be concluded that after 10 years from production of the majority of the photocathodes (some being older than 10 years), there is no evidence of CsI physicochemical aging process.
References
3. PID performances
[1] [2] [3] [4] [5] [6] [7] [8]
Figs. 3 and 4 show respectively the scatter plots of the Cherenkov angle for pp collisions at √s=13 TeV and Pb-Pb collisions at √sNN=5.02 TeV. With Gaussian fits in different momentum slices of the scatter plots (PID statistical unfolding), the requirement for the particle identification at 3σ can be ensured in the momentum range 1–5 GeV/c, for both
3
K. Aamodt, et al., The ALICE experiment at the CERN LHC, JINST 3 (2008) S08002. T. HMPID, CERN/LHCC 98-19 ALICE TDR 1, 14 August, 1998. G. De Cataldo, Nucl. Instrum. Methods A766 (2014) 19–21 (RICH2013 conference). G. Volpe, Nucl. Instrum. Methods A766 (2014) 259–262 (RICH2013 conference). F. Piuz, Nucl. Instrum. Methods A502 (2003) 76–90. F. Piuz, et al., Nucl. Instrum. Methods A553 (2005) 187–195. H. Hoedlmoser, et al., Nucl. Instrum. Methods A574 (2007) 28–38. Contribution in this conference of G. Volpe, ALICE-HMPID contribution to the ALICE physics program.