- Email: [email protected]
The pattern recognition method for the CsI-RICH detector in ALICE
Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304
The pattern recognition method for the CsI-RICH detector in ALICE Domenico Di Bari Dipartimento Interateneo di Fisica dell’Universita" di Bari and INFN, Bari 70126, Italy For the ALICE Collaboration
Abstract The High Momentum Particle Identification Detector in the ALICE experiment is based on a Ring Imaging Cherenkov (RICH) detector with a CsI photocathode. The identification of Cherenkov photons, especially in conditions of high occupancy as will be the case in Pb–Pb collisions at the Large Hadron Collider, requires efficient pattern recognition algorithms. The results of an algorithm based on the Hough transform, that maps the pad coordinate space directly to the Cherenkov angle parameter space, will be shown. The performance of the method on real events collected in the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven (USA), where a prototype of the RICH detector developed in the ALICE framework has been successfully operated, will also be described. r 2003 Elsevier Science B.V. All rights reserved. Keywords: ALICE; LHC; RICH; Pattern recognition
1. Introduction An important part of the experimental program at the future Large Hadron Collider (LHC) at CERN intends to investigate in laboratory the quark-gluon plasma phase (QGP), which is likely produced in central heavy ion collisions at very high energy. The study of such collisions represents a formidable tool to test QCD at its natural scale (LQCD ) and solve the fundamental questions on the color confinement and the breaking of the chiral symmetry, strictly connected to the properties of the QGP.
E-mail address: [email protected] (D. Di Bari).
The ALICE experiment at LHC [1] is devoted to study the strongly interacting matter under extreme conditions of temperature and density in central Pb–Pb collisions at a center-of-mass energy of 2.75 TeV per nucleon with a luminosity of about 1027 cm2 s1. The particle identification will be provided by a large area array of time of flight detectors (TOF) and a single arm Ring Imaging Cherenkov detector (RICH) of a small acceptance (jZjo0:9), DjB36 ) (High Momentum Particle Identification Detector (HMPID)). Both devices will extend the range of identification of lower momentum particles obtained by dE=dx measurements in the Inner Tracking System (ITS), consisting of six layers of very high granularity position-sensitive
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00292-4
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304 160
100
pady
120
100 80
60
40
40
20
20
0 0
50
100
00
150
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padx 160
ϑ = 20°
140
120
120
100
100
80
60
40
40
20
20 0 0
0
50
150
100
100
150
ϑ = 25°
80
60
0
100 padx
pady
pady
2. Description of the CsI-RICH detector
The distributions of the single photon Cherenkov angle Zc and the reconstructed mean
ϑ = 15°
80
60
140
3. ALICE-RICH performance
140
120
160
150
50
padx
padx
Fig. 1. Maps of the pad hits for different Winc : Charged particles (1000) have been generated with b ¼ 1 (photons produced in the fused silica window can be seen near to the Cherenkov pattern). 1000 100 800
SPS beam events events / 1 mrad
80 events / 2 mrad
The liquid radiator of the ALICE-RICH consists of low chromaticity C6F14 with a refractive index n ¼ 1:2948 at l ¼ 180 nm (T ¼ 25 C), corresponding to a momentum threshold pth ¼ 1:22m GeV/c (where m is the particle mass in GeV/c2). The photodetector is a multiwire proportional chamber (MWPC), with anode wires of 20 mm diameter, 4 mm pitch and 2 mm anode–cathode gap. The MWPC is filled with pure methane at ambience temperature and pressure. The conversion of UV photons into electrons takes place in a solid photocathode, consisting of a thin layer of evaporated CsI onto a pad plane segmented in 8 8.4 mm2. More details about detector design and technical aspects can be found in Ref. [2]. Between the radiator and the photodetector, a region called ‘‘proximity gap’’ ensures a reasonable Cherenkov ring size, in relation to the chromaticity and the spatial resolution of the photodetector, without requiring any focalization device. With this geometry, the Cherenkov ring pattern strongly depends on the incident angle Winc of the charged particle with respect to the radiator plane. It is circular for normal incidence (Winc ¼ 0) and becomes an open pattern for Winc > 12 , as shown in Fig. 1, owing to the total internal reflections in the quartz window.
160
ϑ = 10°
140
pady
silicon detectors, and the large volume Time Projection Chamber (TPC). The HMPID consists of seven modules (B10 m2) arranged in a barrel section located at about 5 m from the beam line. It will provide inclusive measurements of identified pions and kaons in the transverse momentum range 1opT o2:7 GeV/c and protons in 1opT o5 GeV/ c (the upper limits correspond to a 3s separation).
301
600 angle per photon 400
σ = 12.8 mrad
200
(a)
0 0.55
60
angle per track σ = 3.9 mrad
40 20
0.5
0.05 ηs (rad)
0.7
0.75
(b)
0 0.55
0.5
0.05 θs (rad)
0.7
0.75
Fig. 2. Cherenkov angle distributions referred to as (a) photon and (b) track.
Cherenkov angle /Wc S, for particles with normal incidence and b ¼ 1; are shown in Fig. 2a and b, respectively. The resolutions indicated in the panels refer to single particle events collected during a test beam at CERN-SPS. When a high level of background is present and the particles are inclined, the resolution worsens. In Fig. 3, a Monte-Carlo simulation shows the predicted s/Wc S as a function of Winc ; in the case of the highest anticipated charged multiplicity dN=dy=8000 in the central region.
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304
302 10
1
Nent = 13
0.8
7 6 5 4
Cherenkov angle resolution
3
at 50 p.cles/m2
2 1 0 -1
Counts
σs (mrad)
9 8
0
1
2
3
4
5
6
7
0.6
0.4
8
9
0.2
incidence angle (deg.)
Fig. 3. Angular resolution s/Wc S as a function of the particle incidence, at a particle density on the RICH (50 particles/m2) corresponding to dN=dy=8000.
4. Pattern recognition in high multiplicity environments The central Pb–Pb collisions in ALICE are expected to be characterized by high multiplicities. At dN=dy=8000, a RICH pad occupancy of about B13% has been evaluated. High occupancy, presence of detector noise and the non-negligible probability of cluster overlapping make the recognition of Cherenkov patterns and the particle identification by conventional approaches extremely difficult. The pattern recognition strategy discussed in the following is based on a modified Hough Transform method [3]. A Hough counting space could be constructed according to the following transformation: ðx; yÞ-ððxp ; yp ; Wp ; jp Þ; Zc Þ where (x; y) are the coordinates of the reconstructed cluster and (xp ; yp ; Wp ; jp ) are the track parameters extrapolated from the tracking devices up to the radiator entrance. The transformation, therefore, reduces the problem in finding a maximum in a one-dimensional Zc mapping space. As a first step, the detection of the cluster (‘‘mip cluster’’) associated to the primary ionization of the charged track in the gas volume (easily identifiable by the relative greater amount of induced charge than the photon cluster), gives an extremely useful information. In fact the pattern recognition starts, for each charged primary particle, to associate the
0
0
0.1
0.2
0.3 0.4 0.5 Eta Cherenkov (rad)
0.6
0.7
Fig. 4. Distribution of Zc for a single particle. The photon without star labels have been tagged to calculate /Wc S.
impact of the track extrapolated from the internal tracking devices to the radiator entrance, with the nearest mip cluster, if it exists. This ensures that the track has passed through the detector. For each cluster in the event, with a charge compatible with that induced by a photon, the Cherenkov angle Zc is calculated by tracing back to the radiator up to the most probable emission point along the track trajectory. The Zc distribution obtained shows two distinct components: a peak related to the Cherenkov signal and a continuous background. In order to find /Wc S; a ‘‘sliding window’’ of width equal to two times that of the expected s/Wc S (Fig. 3), moves over the full Zc spectrum with a step of 1 mrad. The number of photon clusters is counted at each step in the sliding procedure. The position of the maximum, in the Hough counting space, tags the photons falling in the search band. Finally these photons are weighted and /Wc S is calculated. A complete mathematical description of the method can be found in Ref. [4]. In Fig. 4 an example of the Zc distribution of 13 photons associated to a single particle is shown. The result of the sliding procedure is shown in Fig. 5.
5. Improvement of the algorithm and applications In order to take into account the background, supposed to be uniformly distributed on the
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304 1
7
protons
contamination
efficiency
5 Counts
0.4
0.8
6
0.6 0.4
4
protons kaons
0.2 3
kaons pions
0.3 0.2 0.1
pions
0
1
0
2 4 momentum (GeV/c)
2
2 4 momentum (GeV/c)
Fig. 6. Efficiency (left panel) and contamination (right panel) as a function of the track momentum in ALICE. 0.1
0.2 0.3 0.4 0.5 0.6 Center of the sample band (rad)
0.7
Fig. 5. The Hough Counting Space. The position of the maximum fixes the center of the search band (here the width of the band was 50 mrad).
photocathode, the photon density is calculated by weighting them with 1=A; where A is the area allowed to that photons falling in the given search band. To minimize the propagation errors of the track parameters (xp ; yp ; Wp ; jp ) on Zc ; the coordinates (xp ; yp ) are replaced by (xmip ; ymip ), representing a better estimate of the impact point. In addition, an iterative procedure to improve the knowledge (Wp ; jp ) is applied After having tagged the candidate photonsp Nffiffiffiffiand /Wc S found, ffi a minimization of the r.m.s./ N with (Wp ; jp ) as free parameters is performed. If N changes, the Hough algorithm is applied again with the new (W0p ; j0p ). The iteration stops when no change in the photons occurs. It has been estimated, by simulation, that this improves the reconstructed Cherenkov angle by the enhancing 20% of the signalto-background photon ratio. In Fig. 6 the p; K and p efficiency and contamination in ALICE has been evaluated by Monte-Carlo simulation [4]. The pattern recognition described above has been applied to the analysis of the STAR-RICH data. In 1999, a prototype of the ALICE-RICH, corresponding to 2/3 of the size of one ALICERICH module (with the same pad size as foreseen for the final design), has been successfully integrated in the STAR apparatus [5]. In Fig. 7 the distribution of /Wc S as a function of the momentum of the charged particles detected
π
0.7
Theta Cerenkov (rad)
00
303
0.6
K
0.5
p
0.4 0.3 0.2 0.1
0
1
1.5
2
2.5
3
3.5
4
4.5
5
Momentum (GeV/c) Fig. 7. Reconstructed Cherenkov angle /Wc S as a function of the particle momentum in STAR. Solid lines indicate the predicted curves for p; k and p:
by the STAR-TPCp is ffiffiffiffiffiffiffiffiffi shown. It refers to the central Au–Au events at SNN ¼ 130 GeV; corresponding to a pad occupancy on the RICH in the range 3–5%. The average track angle /Winc S to the RICH plane is about 8 higher to that predicted in ALICE (/Wc SB5 ). Clear accumulations of p; K and p; with low background, around the relative predicted curves can be easily seen. The inclusive yields can be measured by fitting the plot in Fig. 7 in different momentum intervals. An example is shown in Fig. 8, where the distribution is well fitted with three gaussians (the number of entries is the only constraint in the fit). The calculated p=K and K=p separation are shown in Fig. 9. The lower number of photons detected with respect to the CERN test beam results (higher gas pressure and freon temperature in STAR), justify the lower separation
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304
304
The preliminary results on the physical analysis can be found in Ref. [6].
160
π+
positives
h1 fit
140
Nent = 1147
events 10.005 (rad)
120 100
6. Conclusion
2.1 < pt < 2.5 GeV/c
80
+
K
60
p
40 20 0 0.3
0.35
0.4
0.45 0.5
0.55
0.6
0.65
0.7
0.75
Theta Cherenkov (rad)
Fig. 8. Reconstructed Cherenkov angle /Wc S in a range of restricted pT measured by STAR-TPC. A fit with three gaussians (solid line) has been performed (w2 /NDF=1.2).
10 Preliminary
9
Sep.Sigma (rad)
8 7
K/p
4
Acknowledgements I thank Guy Paic for the useful discussions on this subject. I am grateful to the STAR-RICH Collaboration for testing on data the pattern recognition algorithm described in these proceedings.
Positives Negatives
6 5
A pattern recognition method, based on a modification of the Hough Transform, seems to give promising results in the recognition of Cherenkov circular patterns in a high multiplicity environment. The method, adopted by the ALICE-HMPID group, has been tested on Au– Au events collected by the STAR experiment, where a prototype of the ALICE–RICH has been installed.
π/K
3 2
References
1 0 1.2
1.4
1.6 1.8 2 2.2 Transverse Momentum (GeV/c)
2.4
2.6
Fig. 9. p=K and K=p separation, in units of s; as evaluated in STAR.
(2:1s) in the interval 2:1opTo2:5 GeV/c with respect to ALICE.
[1] ALICE Collaboration, Technical Proposal, CERN/LHCC 95/71. [2] ALICE Collaboration, TDR 1—Detector for High Momentum PID, CERN/LHCC 98/19. [3] D. Cozza et al., Internal Note/RIC ALICE/98-39. [4] D. Cozza, et al., Nucl. Instr. Meth. A 482 (2002) 226. [5] M. Calderon, de la Barca et al., YRHI-98-22. [6] The STAR and STAR-RICH Collaboration, D. Cozza, et al., AIP 602 (2001) 319.
The pattern recognition method for the CsI-RICH detector in ALICE Domenico Di Bari Dipartimento Interateneo di Fisica dell’Universita" di Bari and INFN, Bari 70126, Italy For the ALICE Collaboration
Abstract The High Momentum Particle Identification Detector in the ALICE experiment is based on a Ring Imaging Cherenkov (RICH) detector with a CsI photocathode. The identification of Cherenkov photons, especially in conditions of high occupancy as will be the case in Pb–Pb collisions at the Large Hadron Collider, requires efficient pattern recognition algorithms. The results of an algorithm based on the Hough transform, that maps the pad coordinate space directly to the Cherenkov angle parameter space, will be shown. The performance of the method on real events collected in the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven (USA), where a prototype of the RICH detector developed in the ALICE framework has been successfully operated, will also be described. r 2003 Elsevier Science B.V. All rights reserved. Keywords: ALICE; LHC; RICH; Pattern recognition
1. Introduction An important part of the experimental program at the future Large Hadron Collider (LHC) at CERN intends to investigate in laboratory the quark-gluon plasma phase (QGP), which is likely produced in central heavy ion collisions at very high energy. The study of such collisions represents a formidable tool to test QCD at its natural scale (LQCD ) and solve the fundamental questions on the color confinement and the breaking of the chiral symmetry, strictly connected to the properties of the QGP.
E-mail address: [email protected] (D. Di Bari).
The ALICE experiment at LHC [1] is devoted to study the strongly interacting matter under extreme conditions of temperature and density in central Pb–Pb collisions at a center-of-mass energy of 2.75 TeV per nucleon with a luminosity of about 1027 cm2 s1. The particle identification will be provided by a large area array of time of flight detectors (TOF) and a single arm Ring Imaging Cherenkov detector (RICH) of a small acceptance (jZjo0:9), DjB36 ) (High Momentum Particle Identification Detector (HMPID)). Both devices will extend the range of identification of lower momentum particles obtained by dE=dx measurements in the Inner Tracking System (ITS), consisting of six layers of very high granularity position-sensitive
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00292-4
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304 160
100
pady
120
100 80
60
40
40
20
20
0 0
50
100
00
150
50
padx 160
ϑ = 20°
140
120
120
100
100
80
60
40
40
20
20 0 0
0
50
150
100
100
150
ϑ = 25°
80
60
0
100 padx
pady
pady
2. Description of the CsI-RICH detector
The distributions of the single photon Cherenkov angle Zc and the reconstructed mean
ϑ = 15°
80
60
140
3. ALICE-RICH performance
140
120
160
150
50
padx
padx
Fig. 1. Maps of the pad hits for different Winc : Charged particles (1000) have been generated with b ¼ 1 (photons produced in the fused silica window can be seen near to the Cherenkov pattern). 1000 100 800
SPS beam events events / 1 mrad
80 events / 2 mrad
The liquid radiator of the ALICE-RICH consists of low chromaticity C6F14 with a refractive index n ¼ 1:2948 at l ¼ 180 nm (T ¼ 25 C), corresponding to a momentum threshold pth ¼ 1:22m GeV/c (where m is the particle mass in GeV/c2). The photodetector is a multiwire proportional chamber (MWPC), with anode wires of 20 mm diameter, 4 mm pitch and 2 mm anode–cathode gap. The MWPC is filled with pure methane at ambience temperature and pressure. The conversion of UV photons into electrons takes place in a solid photocathode, consisting of a thin layer of evaporated CsI onto a pad plane segmented in 8 8.4 mm2. More details about detector design and technical aspects can be found in Ref. [2]. Between the radiator and the photodetector, a region called ‘‘proximity gap’’ ensures a reasonable Cherenkov ring size, in relation to the chromaticity and the spatial resolution of the photodetector, without requiring any focalization device. With this geometry, the Cherenkov ring pattern strongly depends on the incident angle Winc of the charged particle with respect to the radiator plane. It is circular for normal incidence (Winc ¼ 0) and becomes an open pattern for Winc > 12 , as shown in Fig. 1, owing to the total internal reflections in the quartz window.
160
ϑ = 10°
140
pady
silicon detectors, and the large volume Time Projection Chamber (TPC). The HMPID consists of seven modules (B10 m2) arranged in a barrel section located at about 5 m from the beam line. It will provide inclusive measurements of identified pions and kaons in the transverse momentum range 1opT o2:7 GeV/c and protons in 1opT o5 GeV/ c (the upper limits correspond to a 3s separation).
301
600 angle per photon 400
σ = 12.8 mrad
200
(a)
0 0.55
60
angle per track σ = 3.9 mrad
40 20
0.5
0.05 ηs (rad)
0.7
0.75
(b)
0 0.55
0.5
0.05 θs (rad)
0.7
0.75
Fig. 2. Cherenkov angle distributions referred to as (a) photon and (b) track.
Cherenkov angle /Wc S, for particles with normal incidence and b ¼ 1; are shown in Fig. 2a and b, respectively. The resolutions indicated in the panels refer to single particle events collected during a test beam at CERN-SPS. When a high level of background is present and the particles are inclined, the resolution worsens. In Fig. 3, a Monte-Carlo simulation shows the predicted s/Wc S as a function of Winc ; in the case of the highest anticipated charged multiplicity dN=dy=8000 in the central region.
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304
302 10
1
Nent = 13
0.8
7 6 5 4
Cherenkov angle resolution
3
at 50 p.cles/m2
2 1 0 -1
Counts
σs (mrad)
9 8
0
1
2
3
4
5
6
7
0.6
0.4
8
9
0.2
incidence angle (deg.)
Fig. 3. Angular resolution s/Wc S as a function of the particle incidence, at a particle density on the RICH (50 particles/m2) corresponding to dN=dy=8000.
4. Pattern recognition in high multiplicity environments The central Pb–Pb collisions in ALICE are expected to be characterized by high multiplicities. At dN=dy=8000, a RICH pad occupancy of about B13% has been evaluated. High occupancy, presence of detector noise and the non-negligible probability of cluster overlapping make the recognition of Cherenkov patterns and the particle identification by conventional approaches extremely difficult. The pattern recognition strategy discussed in the following is based on a modified Hough Transform method [3]. A Hough counting space could be constructed according to the following transformation: ðx; yÞ-ððxp ; yp ; Wp ; jp Þ; Zc Þ where (x; y) are the coordinates of the reconstructed cluster and (xp ; yp ; Wp ; jp ) are the track parameters extrapolated from the tracking devices up to the radiator entrance. The transformation, therefore, reduces the problem in finding a maximum in a one-dimensional Zc mapping space. As a first step, the detection of the cluster (‘‘mip cluster’’) associated to the primary ionization of the charged track in the gas volume (easily identifiable by the relative greater amount of induced charge than the photon cluster), gives an extremely useful information. In fact the pattern recognition starts, for each charged primary particle, to associate the
0
0
0.1
0.2
0.3 0.4 0.5 Eta Cherenkov (rad)
0.6
0.7
Fig. 4. Distribution of Zc for a single particle. The photon without star labels have been tagged to calculate /Wc S.
impact of the track extrapolated from the internal tracking devices to the radiator entrance, with the nearest mip cluster, if it exists. This ensures that the track has passed through the detector. For each cluster in the event, with a charge compatible with that induced by a photon, the Cherenkov angle Zc is calculated by tracing back to the radiator up to the most probable emission point along the track trajectory. The Zc distribution obtained shows two distinct components: a peak related to the Cherenkov signal and a continuous background. In order to find /Wc S; a ‘‘sliding window’’ of width equal to two times that of the expected s/Wc S (Fig. 3), moves over the full Zc spectrum with a step of 1 mrad. The number of photon clusters is counted at each step in the sliding procedure. The position of the maximum, in the Hough counting space, tags the photons falling in the search band. Finally these photons are weighted and /Wc S is calculated. A complete mathematical description of the method can be found in Ref. [4]. In Fig. 4 an example of the Zc distribution of 13 photons associated to a single particle is shown. The result of the sliding procedure is shown in Fig. 5.
5. Improvement of the algorithm and applications In order to take into account the background, supposed to be uniformly distributed on the
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304 1
7
protons
contamination
efficiency
5 Counts
0.4
0.8
6
0.6 0.4
4
protons kaons
0.2 3
kaons pions
0.3 0.2 0.1
pions
0
1
0
2 4 momentum (GeV/c)
2
2 4 momentum (GeV/c)
Fig. 6. Efficiency (left panel) and contamination (right panel) as a function of the track momentum in ALICE. 0.1
0.2 0.3 0.4 0.5 0.6 Center of the sample band (rad)
0.7
Fig. 5. The Hough Counting Space. The position of the maximum fixes the center of the search band (here the width of the band was 50 mrad).
photocathode, the photon density is calculated by weighting them with 1=A; where A is the area allowed to that photons falling in the given search band. To minimize the propagation errors of the track parameters (xp ; yp ; Wp ; jp ) on Zc ; the coordinates (xp ; yp ) are replaced by (xmip ; ymip ), representing a better estimate of the impact point. In addition, an iterative procedure to improve the knowledge (Wp ; jp ) is applied After having tagged the candidate photonsp Nffiffiffiffiand /Wc S found, ffi a minimization of the r.m.s./ N with (Wp ; jp ) as free parameters is performed. If N changes, the Hough algorithm is applied again with the new (W0p ; j0p ). The iteration stops when no change in the photons occurs. It has been estimated, by simulation, that this improves the reconstructed Cherenkov angle by the enhancing 20% of the signalto-background photon ratio. In Fig. 6 the p; K and p efficiency and contamination in ALICE has been evaluated by Monte-Carlo simulation [4]. The pattern recognition described above has been applied to the analysis of the STAR-RICH data. In 1999, a prototype of the ALICE-RICH, corresponding to 2/3 of the size of one ALICERICH module (with the same pad size as foreseen for the final design), has been successfully integrated in the STAR apparatus [5]. In Fig. 7 the distribution of /Wc S as a function of the momentum of the charged particles detected
π
0.7
Theta Cerenkov (rad)
00
303
0.6
K
0.5
p
0.4 0.3 0.2 0.1
0
1
1.5
2
2.5
3
3.5
4
4.5
5
Momentum (GeV/c) Fig. 7. Reconstructed Cherenkov angle /Wc S as a function of the particle momentum in STAR. Solid lines indicate the predicted curves for p; k and p:
by the STAR-TPCp is ffiffiffiffiffiffiffiffiffi shown. It refers to the central Au–Au events at SNN ¼ 130 GeV; corresponding to a pad occupancy on the RICH in the range 3–5%. The average track angle /Winc S to the RICH plane is about 8 higher to that predicted in ALICE (/Wc SB5 ). Clear accumulations of p; K and p; with low background, around the relative predicted curves can be easily seen. The inclusive yields can be measured by fitting the plot in Fig. 7 in different momentum intervals. An example is shown in Fig. 8, where the distribution is well fitted with three gaussians (the number of entries is the only constraint in the fit). The calculated p=K and K=p separation are shown in Fig. 9. The lower number of photons detected with respect to the CERN test beam results (higher gas pressure and freon temperature in STAR), justify the lower separation
D. Di Bari / Nuclear Instruments and Methods in Physics Research A 502 (2003) 300–304
304
The preliminary results on the physical analysis can be found in Ref. [6].
160
π+
positives
h1 fit
140
Nent = 1147
events 10.005 (rad)
120 100
6. Conclusion
2.1 < pt < 2.5 GeV/c
80
+
K
60
p
40 20 0 0.3
0.35
0.4
0.45 0.5
0.55
0.6
0.65
0.7
0.75
Theta Cherenkov (rad)
Fig. 8. Reconstructed Cherenkov angle /Wc S in a range of restricted pT measured by STAR-TPC. A fit with three gaussians (solid line) has been performed (w2 /NDF=1.2).
10 Preliminary
9
Sep.Sigma (rad)
8 7
K/p
4
Acknowledgements I thank Guy Paic for the useful discussions on this subject. I am grateful to the STAR-RICH Collaboration for testing on data the pattern recognition algorithm described in these proceedings.
Positives Negatives
6 5
A pattern recognition method, based on a modification of the Hough Transform, seems to give promising results in the recognition of Cherenkov circular patterns in a high multiplicity environment. The method, adopted by the ALICE-HMPID group, has been tested on Au– Au events collected by the STAR experiment, where a prototype of the ALICE–RICH has been installed.
π/K
3 2
References
1 0 1.2
1.4
1.6 1.8 2 2.2 Transverse Momentum (GeV/c)
2.4
2.6
Fig. 9. p=K and K=p separation, in units of s; as evaluated in STAR.
(2:1s) in the interval 2:1opTo2:5 GeV/c with respect to ALICE.
[1] ALICE Collaboration, Technical Proposal, CERN/LHCC 95/71. [2] ALICE Collaboration, TDR 1—Detector for High Momentum PID, CERN/LHCC 98/19. [3] D. Cozza et al., Internal Note/RIC ALICE/98-39. [4] D. Cozza, et al., Nucl. Instr. Meth. A 482 (2002) 226. [5] M. Calderon, de la Barca et al., YRHI-98-22. [6] The STAR and STAR-RICH Collaboration, D. Cozza, et al., AIP 602 (2001) 319.