- Email: [email protected]
PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2 Giacomo Volpe, On behalf of the ALICE Collaboration Dipartimento Interateneo di Fisica ‘‘M. Merlin’’ and Sezione INFN, Bari, Italy
ARTICLE
INFO
MSC: 00-01 99-00 Keywords: Cherenkov radiation RICH Particle identification
ABSTRACT The ALICE apparatus is devoted collecting proton–proton (pp), proton–nucleus (p–A) and nucleus–nucleus (A– A) collision data provided by the LHC, to study the properties of strongly interacting matter under extremely high temperature and energy density conditions. In ALICE, the momentum of a track is evaluated by exploiting a solenoid magnetic field of 0.5 T. Among the ALICE PID detectors, the HMPID (High Momentum Particle Identification Detector) is devoted to the identification of charged hadrons. It consists of seven identical RICH (Ring Imaging Cherenkov) counters, with liquid C 6 F14 as Cherenkov radiator (n ≈ 1.298 at 𝜆ph = 175 nm). Cherenkov photons and charged particles are detected by a MWPC (Multi Wire Proportional Chamber), equipped with a pad-segmented CsI coated photo-cathode. The HMPID provides 3𝜎 K–𝜋 and p–K separation up to 𝑝T = 3 and 5 GeV/𝑐, respectively. The detector performance depends on the experimental conditions, such as the event multiplicity and the intensity of the solenoid magnetic field. During the LHC Run 2 data-taking period (2015– 2017), the HMPID collected data from different colliding systems with a magnetic field intensity B = 0.5 T and pp data with B = 0.2 T were also recorded. A review of the detector PID performance during the LHC Run 2 data-taking period will be shown. The contribution provided, so far, by the HMPID to the ALICE physics measurements, performed with the LHC Run 2 data, will also be presented.
1. Introduction The ALICE experiment is devoted collecting data from pp, p–A and A–A collisions provided by the LHC, to study the properties of the strongly interacting matter under extremely high energy density and temperature conditions. In central heavy ion collisions at ultra relativistic energies, it is well established that a strongly interacting medium of quarks and gluons is created. The transverse momentum, 𝑝T , distributions of identified hadrons contain valuable information about the collective expansion of the system (𝑝T < 2 GeV/𝑐) and the presence of new hadronization mechanisms like quark recombination (2 < 𝑝T < 8 GeV/𝑐). ALICE has unique particle identification (PID) capabilities among the LHC experiments exploiting different PID techniques, i.e., specific ionization energy loss and time-of-flight measurements, Cherenkov and transition radiation detection, calorimeters and identification by means of topological selections. In particular, for charged hadrons identification, energy loss, time of flight and Cherenkov effect are used. Combining such techniques, ALICE has reported the transverse momentum spectra of charged pions, kaons and (anti-)protons from low (hundreds of MeV/𝑐) to high 𝑝T (20 GeV/𝑐). The ALICE HMPID (High Momentum Particle Identification Detector) [1] is devoted to the identification of charged hadrons, exploiting the Cherenkov effect. It consists of seven identical RICH counters, with liquid C6 F14 as Cherenkov radiator (n ≈ 1.298 at 𝜆ph = 175 nm). Cherenkov photons
and charged particles are detected by a MWPC, coupled with a padsegmented CsI coated photo-cathode. The HMPID detector provides 3𝜎 separation for pions and kaons up to 𝑝T = 3 GeV/𝑐 and for kaons and (anti-)protons up to 𝑝T = 5 GeV/𝑐. 2. The ALICE HMPID pattern recognition performance Particle identification in the HMPID requires the particle’s track to be extrapolated from the central tracking devices of ALICE (ITS, TPC and TRD) and associated with the corresponding cluster of the minimum ionizing particle in the HMPID cathode plane. Starting from the photon cluster coordinates and the extrapolated track parameters, a back-tracing algorithm calculates the corresponding Cherenkov angle. Background discrimination is performed exploiting the Hough Transform Method (HTM) [2]. HTM is an efficient implementation of a generalized template matching strategy for detecting complex patterns in binary images. To each track a Cherenkov angle ⟨𝜃𝐶ℎ ⟩ is associated, obtained as the average of the photon candidate angles in the same ring. The result of this procedure is shown in Fig. 1, where the reconstructed Cherenkov angle is shown as a function of the track momentum in the √ case of pp collisions at 𝑠 = 13 TeV with B = 0.2 T with the continuous which represent the theoretical trend. During the LHC Run 2 datataking period (2015–2017), the HMPID collected data from pp, p–Pb,
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2019.01.030 Received 15 October 2018; Received in revised form 5 January 2019; Accepted 11 January 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.
G. Volpe
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
√ Fig. 4. Pion Cherenkov angle measured in the HMPID in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.5 T, for tracks in the transverse momentum range 1.5–1.6 GeV/𝑐. The red line represents the Gaussian fit.
Fig. 1. Cherenkov angle measured in the HMPID as a function of track momentum in pp √ collisions at 𝑠 = 13 TeV with magnetic field B = 0.2 T.
√ Fig. 2. Track inclination angle to the HMPID chamber planes in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.5 T, as a function of the track transverse momentum.
√ Fig. 5. Pion Cherenkov angle measured in the HMPID in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.2 T, for tracks in the transverse momentum range 1.5–1.6 GeV/𝑐. The red line represents the Gaussian fit.
reconstructed with higher efficiency with respect to those coming from inclined tracks. In Figs. 2 and √3 the track inclination angle to the HMPID chambers in pp collisions at 𝑠 = 13 TeV, with a magnetic field intensity of B = 0.5 T and B = 0.2 T, respectively, is shown. To quantify the different performance between the two magnetic field configurations, the angular resolution has been evaluated for a sample of tracks in the same transverse momentum range. In Figs. 4 and 5 the Cherenkov angle distribution of pions in the 𝑝T range 1.5–1.6 GeV/𝑐, in pp collisions with B = 0.5 T and B = 0.2 T, respectively, is shown. The resolution values in the two cases are 𝜎0.5 = 9.95 mrad and 𝜎0.2 = 7.10 mrad, showing the better performance in data collected with lower magnetic field. In pp and p–Pb collisions the track multiplicity in the HMPID acceptance corresponds to an average detector occupancy of ≈ 0.1%. The detector response is Gaussian and the particle yields are then extracted from a three-Gaussian fit to the Cherenkov angle distribution in narrow transverse momentum intervals. Fig. 6 shows the primary track multiplicity measured in the HMPID acceptance in Pb–Pb collisions (red) √ at 𝑠NN = 5.02 TeV with B = 0.5 T and Xe–Xe collisions (blue) at √ 𝑠NN = 5.44 TeV with B = 0.2 T as a function of the collision centrality. The multiplicity of ≈ 90 primary tracks plus secondaries produced in the most central (0%–5%) nucleus–nucleus collisions, corresponds to an average detector occupancy of ≈ 3.5%. In these conditions the probability that the HTM algorithm detects fake Cherenkov patterns increases. This is due to mis-identification in the high occupancy events, larger is the angle value larger is the probability to find background clusters (background uniformly distributed
√ Fig. 3. Track inclination angle to the HMPID chamber planes in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.2 T, as a function of the track transverse momentum.
Pb–Pb collisions with a magnetic field intensity of B = 0.5 T, pp and Xe–Xe collisions data with B = 0.2 T were also recorded. The pattern recognition performance depends on the experimental conditions, such as the intensity of the solenoid magnetic field and the charged particle multiplicity in one event. The track inclination to the HMPID chamber planes depends on the magnetic field intensity and represents a relevant parameter for the pattern recognition procedure. Emission angles of the Cherenkov photons from tracks orthogonal to the camber planes are 2
Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.
G. Volpe
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 6. Track multiplicity in the HMPID acceptance as a function of transverse momentum √ in central (0%–5%) Pb–Pb collisions (red) at 𝑠NN = 5.02 TeV with B = 0.5 T and in central √ (0%–5%) Xe–Xe collisions (blue) at 𝑠NN = 5.44 TeV with B = 0.2 T . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Pion PID efficiency as a function of transverse momentum in central (0%–5%) √ Pb–Pb collisions (black) at 𝑠NN = 5.02 TeV with B = 0.5 T and in central (0%–5%) √ Xe–Xe collisions (red) at 𝑠NN = 5.44 TeV with B = 0.2 T . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(yellow line). The efficiency of the pattern recognition for a given particle is quantified by means of the PID efficiency, defined as the following: 𝑖 𝜖PID =
𝑁 𝑖 (signal) 𝑁 𝑖 (signal + background)
(1)
where 𝑖 represents the particle species, 𝑁 𝑖 (signal) is the integral of the corresponding Gaussian fit function and 𝑁 𝑖 (signal + background) are the total entries in the histograms. The PID efficiency is evaluated from a pure sample of a given particle species using Monte Carlo simulations, in which the particle species can be a priori determined. In Fig. 9 the √ PID efficiency for pions in Pb–Pb collisions (black) at 𝑠NN = 5.02 TeV √ with B = 0.5 T and Xe–Xe collisions (red) at 𝑠NN = 5.44 TeV with B = 0.2 T as a function of 𝑝T , is shown. Despite the similar charged particle multiplicity in the HMPID acceptance, the PID efficiency is larger in Xe–Xe collisions than in Pb–Pb collisions thanks to the low intensity of the magnetic field. The presence of the background does not avoid the particle yields extraction by means of the statistical unfolding method [2], but it represents a source of systematic uncertainty on the yield evaluation that increases as the PID efficiency decreases.
Fig. 7. Cherenkov angle measured in the HMPID as a function of track momentum in √ central (0−5%) Xe−Xe collisions at 𝑠NN = 5.44 TeV.
3. Physics results Exploiting the statistical unfolding method, analysis of the HMPID data has provided pions and kaons 𝑝T spectra between 1.5 GeV/𝑐 and 4 GeV/𝑐, (anti-)protons 𝑝T spectra in the transverse momentum range 1.5–6 GeV/𝑐 and the deuteron one in the range 3–8 GeV/𝑐 for the most central Pb–Pb collisions. In combination with the other ALICE PID detectors (ITS, TPC and TOF), the HMPID successfully contributes to the study of the inclusive charged hadron production. The HMPID analysis has been performed for pp, p–Pb and Pb–Pb collision data collected during LHC Run 1 data-taking period and published in Refs. [3–7]. The publication of the results obtained from the analysis of the LHC Run 2 data is in progress. As an example, among the different ALICE measurements where the HMPID has contributed in Run 2 data, Fig. 10 shows the transverse momentum spectra of (anti-)protons measured in √ Pb–Pb collisions at 𝑠NN = 5.02 TeV for different centrality classes.
Fig. 8. Distributions of the Cherenkov angle measured in the HMPID for positive tracks having 𝑝T in two ranges, 1.5−1.6 GeV/𝑐 (left) and 2.5−2.6 GeV/𝑐 (right), in central (0%– √ 5%) Xe−Xe collisions at 𝑠NN = 5.44 TeV. The histograms have been scaled to have a similar maximum value.
on the chamber plane). The three Gaussian distributions for pions, kaons and (anti-)protons in a given transverse momentum interval are convoluted with a background distribution that increases with the Cherenkov angle value [2]. Fig. 7 shows the Cherenkov angle measured in the HMPID as a function of the track momentum in the most central √ (0%–5%) Xe–Xe collisions at 𝑠NN = 5.44 TeV with B = 0.2 T. Despite the three bands of pions, kaons and (anti-)protons being clearly visible a non-negligible background contribution is present as shown in Fig. 8
4. Conclusions ALICE successfully collected the pp, p–Pb, Pb–Pb and Xe–Xe collision data provided so far by LHC during the Run 2 data-taking period. The HMPID detector presented optimal PID performance. It has been studied 3
Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.
G. Volpe
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
multiplicity ones and this represents a source of systematic uncertainty on the yield evaluation. In the lower magnetic field (B = 0.2 T) data, the PID performance is better than in the higher magnetic field, both in terms of angular resolution and PID efficiency in the high multiplicity events. By means of statistical unfolding the HMPID provides charged hadron production measurements, successfully participating to the ALICE physics program. Highlights of the results from LHC Run 2 data have been presented and their publication is now in progress. The study of the PID performance in Run 2 data has shown very good detector stability. This means there are good perspectives for the HMPID operation in LHC Run 3, where the HMPID measurements could in particular contribute to the light nuclei identification: deuteron, triton/3 He, 4 He. References [1] S. Beole, et al., ALICE technical design report: Detector for high momentum PID, CERN-LHCC-98-19. [2] G. Volpe, Pattern recognition and PID procedure with the ALICE-HMPID, Nucl. Instrum. Meth. A766 (2014) 259–262, http://dx.doi.org/10.1016/j.nima.2014.05. 031. [3] B.B. Abelev, et al., Production of charged pions, kaons and protons at large transverse √ momenta in pp and PbPb collisions at 𝑠NN =2.76 TeV, Phys. Lett. B736 (2014) 196–207, http://dx.doi.org/10.1016/j.physletb.2014.07.011, arXiv:1401.1250. [4] J. Adam, et al., Measurement of pion, kaon and proton production in protonproton √ collisions at 𝑠 = 7 TeV, Eur. Phys. J. C75 (5) (2015) 226, http://dx.doi.org/10. 1140/epjc/s10052-015-3422-9, arXiv:1504.00024. [5] J. Adam, et al., Centrality dependence of the nuclear modification factor of charged √ pions, kaons, and protons in Pb-Pb collisions at 𝑠NN = 2.76 TeV, Phys. Rev. C93 (3) (2016) 034913, http://dx.doi.org/10.1103/PhysRevC.93.034913, arXiv:1506. 07287. [6] J. Adam, et al., Multiplicity dependence of charged pion, kaon, and (anti)proton √ production at large transverse momentum in p-Pb collisions at 𝑠NN = 5.02 TeV, Phys. Lett. B760 (2016) 720–735, http://dx.doi.org/10.1016/j.physletb.2016.07. 050, arXiv:1601.03658. [7] S. Acharya, et al., Measurement of deuteron spectra and elliptic flow in PbPb √ collisions at 𝑠NN = 2.76 TeV at the LHC, Eur. Phys. J. C77 (10) (2017) 658, http://dx.doi.org/10.1140/epjc/s10052-017-5222-x, arXiv:1707.07304.
Fig. 10. Transverse momentum spectra of (anti-)protons measured in Pb–Pb collisions √ at 𝑠NN = 5.02 TeV for different centrality classes. Scale factors are applied for better visibility. Statistical and systematic uncertainties are plotted as vertical error bars and boxes around the points, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in different environment conditions, i.e. charged track multiplicity and track inclination (solenoid magnetic field intensity). In high multiplicity events (central A–A collisions) the PID efficiency is lower than in the low
4
Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2 Giacomo Volpe, On behalf of the ALICE Collaboration Dipartimento Interateneo di Fisica ‘‘M. Merlin’’ and Sezione INFN, Bari, Italy
ARTICLE
INFO
MSC: 00-01 99-00 Keywords: Cherenkov radiation RICH Particle identification
ABSTRACT The ALICE apparatus is devoted collecting proton–proton (pp), proton–nucleus (p–A) and nucleus–nucleus (A– A) collision data provided by the LHC, to study the properties of strongly interacting matter under extremely high temperature and energy density conditions. In ALICE, the momentum of a track is evaluated by exploiting a solenoid magnetic field of 0.5 T. Among the ALICE PID detectors, the HMPID (High Momentum Particle Identification Detector) is devoted to the identification of charged hadrons. It consists of seven identical RICH (Ring Imaging Cherenkov) counters, with liquid C 6 F14 as Cherenkov radiator (n ≈ 1.298 at 𝜆ph = 175 nm). Cherenkov photons and charged particles are detected by a MWPC (Multi Wire Proportional Chamber), equipped with a pad-segmented CsI coated photo-cathode. The HMPID provides 3𝜎 K–𝜋 and p–K separation up to 𝑝T = 3 and 5 GeV/𝑐, respectively. The detector performance depends on the experimental conditions, such as the event multiplicity and the intensity of the solenoid magnetic field. During the LHC Run 2 data-taking period (2015– 2017), the HMPID collected data from different colliding systems with a magnetic field intensity B = 0.5 T and pp data with B = 0.2 T were also recorded. A review of the detector PID performance during the LHC Run 2 data-taking period will be shown. The contribution provided, so far, by the HMPID to the ALICE physics measurements, performed with the LHC Run 2 data, will also be presented.
1. Introduction The ALICE experiment is devoted collecting data from pp, p–A and A–A collisions provided by the LHC, to study the properties of the strongly interacting matter under extremely high energy density and temperature conditions. In central heavy ion collisions at ultra relativistic energies, it is well established that a strongly interacting medium of quarks and gluons is created. The transverse momentum, 𝑝T , distributions of identified hadrons contain valuable information about the collective expansion of the system (𝑝T < 2 GeV/𝑐) and the presence of new hadronization mechanisms like quark recombination (2 < 𝑝T < 8 GeV/𝑐). ALICE has unique particle identification (PID) capabilities among the LHC experiments exploiting different PID techniques, i.e., specific ionization energy loss and time-of-flight measurements, Cherenkov and transition radiation detection, calorimeters and identification by means of topological selections. In particular, for charged hadrons identification, energy loss, time of flight and Cherenkov effect are used. Combining such techniques, ALICE has reported the transverse momentum spectra of charged pions, kaons and (anti-)protons from low (hundreds of MeV/𝑐) to high 𝑝T (20 GeV/𝑐). The ALICE HMPID (High Momentum Particle Identification Detector) [1] is devoted to the identification of charged hadrons, exploiting the Cherenkov effect. It consists of seven identical RICH counters, with liquid C6 F14 as Cherenkov radiator (n ≈ 1.298 at 𝜆ph = 175 nm). Cherenkov photons
and charged particles are detected by a MWPC, coupled with a padsegmented CsI coated photo-cathode. The HMPID detector provides 3𝜎 separation for pions and kaons up to 𝑝T = 3 GeV/𝑐 and for kaons and (anti-)protons up to 𝑝T = 5 GeV/𝑐. 2. The ALICE HMPID pattern recognition performance Particle identification in the HMPID requires the particle’s track to be extrapolated from the central tracking devices of ALICE (ITS, TPC and TRD) and associated with the corresponding cluster of the minimum ionizing particle in the HMPID cathode plane. Starting from the photon cluster coordinates and the extrapolated track parameters, a back-tracing algorithm calculates the corresponding Cherenkov angle. Background discrimination is performed exploiting the Hough Transform Method (HTM) [2]. HTM is an efficient implementation of a generalized template matching strategy for detecting complex patterns in binary images. To each track a Cherenkov angle ⟨𝜃𝐶ℎ ⟩ is associated, obtained as the average of the photon candidate angles in the same ring. The result of this procedure is shown in Fig. 1, where the reconstructed Cherenkov angle is shown as a function of the track momentum in the √ case of pp collisions at 𝑠 = 13 TeV with B = 0.2 T with the continuous which represent the theoretical trend. During the LHC Run 2 datataking period (2015–2017), the HMPID collected data from pp, p–Pb,
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2019.01.030 Received 15 October 2018; Received in revised form 5 January 2019; Accepted 11 January 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.
G. Volpe
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
√ Fig. 4. Pion Cherenkov angle measured in the HMPID in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.5 T, for tracks in the transverse momentum range 1.5–1.6 GeV/𝑐. The red line represents the Gaussian fit.
Fig. 1. Cherenkov angle measured in the HMPID as a function of track momentum in pp √ collisions at 𝑠 = 13 TeV with magnetic field B = 0.2 T.
√ Fig. 2. Track inclination angle to the HMPID chamber planes in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.5 T, as a function of the track transverse momentum.
√ Fig. 5. Pion Cherenkov angle measured in the HMPID in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.2 T, for tracks in the transverse momentum range 1.5–1.6 GeV/𝑐. The red line represents the Gaussian fit.
reconstructed with higher efficiency with respect to those coming from inclined tracks. In Figs. 2 and √3 the track inclination angle to the HMPID chambers in pp collisions at 𝑠 = 13 TeV, with a magnetic field intensity of B = 0.5 T and B = 0.2 T, respectively, is shown. To quantify the different performance between the two magnetic field configurations, the angular resolution has been evaluated for a sample of tracks in the same transverse momentum range. In Figs. 4 and 5 the Cherenkov angle distribution of pions in the 𝑝T range 1.5–1.6 GeV/𝑐, in pp collisions with B = 0.5 T and B = 0.2 T, respectively, is shown. The resolution values in the two cases are 𝜎0.5 = 9.95 mrad and 𝜎0.2 = 7.10 mrad, showing the better performance in data collected with lower magnetic field. In pp and p–Pb collisions the track multiplicity in the HMPID acceptance corresponds to an average detector occupancy of ≈ 0.1%. The detector response is Gaussian and the particle yields are then extracted from a three-Gaussian fit to the Cherenkov angle distribution in narrow transverse momentum intervals. Fig. 6 shows the primary track multiplicity measured in the HMPID acceptance in Pb–Pb collisions (red) √ at 𝑠NN = 5.02 TeV with B = 0.5 T and Xe–Xe collisions (blue) at √ 𝑠NN = 5.44 TeV with B = 0.2 T as a function of the collision centrality. The multiplicity of ≈ 90 primary tracks plus secondaries produced in the most central (0%–5%) nucleus–nucleus collisions, corresponds to an average detector occupancy of ≈ 3.5%. In these conditions the probability that the HTM algorithm detects fake Cherenkov patterns increases. This is due to mis-identification in the high occupancy events, larger is the angle value larger is the probability to find background clusters (background uniformly distributed
√ Fig. 3. Track inclination angle to the HMPID chamber planes in pp collisions at 𝑠 = 13 TeV with magnetic field B = 0.2 T, as a function of the track transverse momentum.
Pb–Pb collisions with a magnetic field intensity of B = 0.5 T, pp and Xe–Xe collisions data with B = 0.2 T were also recorded. The pattern recognition performance depends on the experimental conditions, such as the intensity of the solenoid magnetic field and the charged particle multiplicity in one event. The track inclination to the HMPID chamber planes depends on the magnetic field intensity and represents a relevant parameter for the pattern recognition procedure. Emission angles of the Cherenkov photons from tracks orthogonal to the camber planes are 2
Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.
G. Volpe
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 6. Track multiplicity in the HMPID acceptance as a function of transverse momentum √ in central (0%–5%) Pb–Pb collisions (red) at 𝑠NN = 5.02 TeV with B = 0.5 T and in central √ (0%–5%) Xe–Xe collisions (blue) at 𝑠NN = 5.44 TeV with B = 0.2 T . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Pion PID efficiency as a function of transverse momentum in central (0%–5%) √ Pb–Pb collisions (black) at 𝑠NN = 5.02 TeV with B = 0.5 T and in central (0%–5%) √ Xe–Xe collisions (red) at 𝑠NN = 5.44 TeV with B = 0.2 T . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(yellow line). The efficiency of the pattern recognition for a given particle is quantified by means of the PID efficiency, defined as the following: 𝑖 𝜖PID =
𝑁 𝑖 (signal) 𝑁 𝑖 (signal + background)
(1)
where 𝑖 represents the particle species, 𝑁 𝑖 (signal) is the integral of the corresponding Gaussian fit function and 𝑁 𝑖 (signal + background) are the total entries in the histograms. The PID efficiency is evaluated from a pure sample of a given particle species using Monte Carlo simulations, in which the particle species can be a priori determined. In Fig. 9 the √ PID efficiency for pions in Pb–Pb collisions (black) at 𝑠NN = 5.02 TeV √ with B = 0.5 T and Xe–Xe collisions (red) at 𝑠NN = 5.44 TeV with B = 0.2 T as a function of 𝑝T , is shown. Despite the similar charged particle multiplicity in the HMPID acceptance, the PID efficiency is larger in Xe–Xe collisions than in Pb–Pb collisions thanks to the low intensity of the magnetic field. The presence of the background does not avoid the particle yields extraction by means of the statistical unfolding method [2], but it represents a source of systematic uncertainty on the yield evaluation that increases as the PID efficiency decreases.
Fig. 7. Cherenkov angle measured in the HMPID as a function of track momentum in √ central (0−5%) Xe−Xe collisions at 𝑠NN = 5.44 TeV.
3. Physics results Exploiting the statistical unfolding method, analysis of the HMPID data has provided pions and kaons 𝑝T spectra between 1.5 GeV/𝑐 and 4 GeV/𝑐, (anti-)protons 𝑝T spectra in the transverse momentum range 1.5–6 GeV/𝑐 and the deuteron one in the range 3–8 GeV/𝑐 for the most central Pb–Pb collisions. In combination with the other ALICE PID detectors (ITS, TPC and TOF), the HMPID successfully contributes to the study of the inclusive charged hadron production. The HMPID analysis has been performed for pp, p–Pb and Pb–Pb collision data collected during LHC Run 1 data-taking period and published in Refs. [3–7]. The publication of the results obtained from the analysis of the LHC Run 2 data is in progress. As an example, among the different ALICE measurements where the HMPID has contributed in Run 2 data, Fig. 10 shows the transverse momentum spectra of (anti-)protons measured in √ Pb–Pb collisions at 𝑠NN = 5.02 TeV for different centrality classes.
Fig. 8. Distributions of the Cherenkov angle measured in the HMPID for positive tracks having 𝑝T in two ranges, 1.5−1.6 GeV/𝑐 (left) and 2.5−2.6 GeV/𝑐 (right), in central (0%– √ 5%) Xe−Xe collisions at 𝑠NN = 5.44 TeV. The histograms have been scaled to have a similar maximum value.
on the chamber plane). The three Gaussian distributions for pions, kaons and (anti-)protons in a given transverse momentum interval are convoluted with a background distribution that increases with the Cherenkov angle value [2]. Fig. 7 shows the Cherenkov angle measured in the HMPID as a function of the track momentum in the most central √ (0%–5%) Xe–Xe collisions at 𝑠NN = 5.44 TeV with B = 0.2 T. Despite the three bands of pions, kaons and (anti-)protons being clearly visible a non-negligible background contribution is present as shown in Fig. 8
4. Conclusions ALICE successfully collected the pp, p–Pb, Pb–Pb and Xe–Xe collision data provided so far by LHC during the Run 2 data-taking period. The HMPID detector presented optimal PID performance. It has been studied 3
Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.
G. Volpe
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multiplicity ones and this represents a source of systematic uncertainty on the yield evaluation. In the lower magnetic field (B = 0.2 T) data, the PID performance is better than in the higher magnetic field, both in terms of angular resolution and PID efficiency in the high multiplicity events. By means of statistical unfolding the HMPID provides charged hadron production measurements, successfully participating to the ALICE physics program. Highlights of the results from LHC Run 2 data have been presented and their publication is now in progress. The study of the PID performance in Run 2 data has shown very good detector stability. This means there are good perspectives for the HMPID operation in LHC Run 3, where the HMPID measurements could in particular contribute to the light nuclei identification: deuteron, triton/3 He, 4 He. References [1] S. Beole, et al., ALICE technical design report: Detector for high momentum PID, CERN-LHCC-98-19. [2] G. Volpe, Pattern recognition and PID procedure with the ALICE-HMPID, Nucl. Instrum. Meth. A766 (2014) 259–262, http://dx.doi.org/10.1016/j.nima.2014.05. 031. [3] B.B. Abelev, et al., Production of charged pions, kaons and protons at large transverse √ momenta in pp and PbPb collisions at 𝑠NN =2.76 TeV, Phys. Lett. B736 (2014) 196–207, http://dx.doi.org/10.1016/j.physletb.2014.07.011, arXiv:1401.1250. [4] J. Adam, et al., Measurement of pion, kaon and proton production in protonproton √ collisions at 𝑠 = 7 TeV, Eur. Phys. J. C75 (5) (2015) 226, http://dx.doi.org/10. 1140/epjc/s10052-015-3422-9, arXiv:1504.00024. [5] J. Adam, et al., Centrality dependence of the nuclear modification factor of charged √ pions, kaons, and protons in Pb-Pb collisions at 𝑠NN = 2.76 TeV, Phys. Rev. C93 (3) (2016) 034913, http://dx.doi.org/10.1103/PhysRevC.93.034913, arXiv:1506. 07287. [6] J. Adam, et al., Multiplicity dependence of charged pion, kaon, and (anti)proton √ production at large transverse momentum in p-Pb collisions at 𝑠NN = 5.02 TeV, Phys. Lett. B760 (2016) 720–735, http://dx.doi.org/10.1016/j.physletb.2016.07. 050, arXiv:1601.03658. [7] S. Acharya, et al., Measurement of deuteron spectra and elliptic flow in PbPb √ collisions at 𝑠NN = 2.76 TeV at the LHC, Eur. Phys. J. C77 (10) (2017) 658, http://dx.doi.org/10.1140/epjc/s10052-017-5222-x, arXiv:1707.07304.
Fig. 10. Transverse momentum spectra of (anti-)protons measured in Pb–Pb collisions √ at 𝑠NN = 5.02 TeV for different centrality classes. Scale factors are applied for better visibility. Statistical and systematic uncertainties are plotted as vertical error bars and boxes around the points, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in different environment conditions, i.e. charged track multiplicity and track inclination (solenoid magnetic field intensity). In high multiplicity events (central A–A collisions) the PID efficiency is lower than in the low
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Please cite this article as: G. Volpe, PID performance of the High Momentum Particle IDentification (HMPID) detector during LHC-Run 2, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.030.