A high momentum particle identification detector (HMPID) covering about 50f the ALICE central barrel region has been designed and prototyped.The detector consists of seven RICH modules with a proximity focusing geometry, covering 12m2. The very large density of hits on the detector (80&divide;90 part/m2 in the extreme cases) makes the recognition of the Cherenkov photon patterns a complex and crucial task. A study of the pattern recognition based on the Hough transformation in terms of particle identification efficiency and particle contamination will be presented

Di Bari, D

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1Recognition of Cherenkov ring patterns with the HMPID-RICHdetector in ALICE at LHCThe ALICE-HMPID GroupD. Cozza1, M. Davenport2, G. De Cataldo1, D. Di Bari1, A. Di Mauro2, D. Elia1,A. Franco1, B. Goret2, P. Martinengo2, E. Nappi1;2, G. Paic2;3, F. Piuz2,F. Posa1 and T. D. Williams2.1. Dip. I.A. di Fisica dell’Universita e del Politecnico di Bari and Sez. INFN, Bari, Italy2. CERN, CH-1211 Geneva 23, Switzerland3. Ohio State University, Columbus, Ohio, USAPresented by: Domenico Di Barifor the ALICE CollaborationA high momentum particle identication detector (HMPID) covering about 5% of theALICE central barrel region has been designed and prototyped.The detector consists of seven RICH modules with a proximity focusing geometry, cov-ering 12 m2. The very large density of hits on the detector (8090 part/m2 in the extremecases) makes the recognition of the Cherenkov photon patterns a complex and crucial task.A study of the pattern recognition strategy based on the Hough transformation in termsof particle identication eciency and particle contamination will be presented.1. The CsI-RICH detectorThe technical details of the detector layout have been presented in previous papers[1{3].Here a short description of the basic principle of functioning is reported. When a chargedparticle moves through the RICH (g.1), Cherenkov photons are emitted in the radiatorconsisting of a 10 mm thick layer of C6F14 liquid. The C6F14 refractive index (n=1.29 at170 nm) determines the momentum threshold pth'1.26m GeV/c, where m is the massof the charged particle in GeV/c2. The UV photons are detected by the photodetector,consisting of a conventional MWPC with anode wires of 20 m diameter, 4 mm pitchand 2 mm anode-cathode gap. Photons convert into electrons on a solid photocathodeconsisting of a thin layer of CsI evaporated onto a plane segmented with pads of 88mm2.2                                                                                                                                                        CsI film10 mm5 mm100 mm4 mmfreon radiatorquartz windowcollection wire electrodeproximity gapwire cathodesense wirespad cathode charged particlefrontend electronicswith 500 nmFigure 1. Schematic view of the HMPID CsI-RICH.The space (\proximity gap") between the radiator and the MWPC improves the angu-lar resolution of the measured Cherenkov angle by ensuring the most convenient size ofCherenkov ring images on the photocathode plane with respect to the chromaticity of theradiator and the spatial resolution of the photon detector. The collection wire electrodeplaced in the proximity gap, near the quartz window, prevents the electrons created byionizing particles to enter the MWPC sensitive volume. The analog pad readout allowsan accurate photoelectron localization by means of the reconstruction of cluster centroids.2. Pattern recognition with RICH in ALICEThe ecient recognition of circular (or elliptical) patterns plays a crucial rolein the analysis of high particle density events, anticipated in central Pb-Pb collisions inALICE (dN/dy50008000). The algorithm for the recognition of Cherenkov patternsused in this analysis is based on the Hough Transform Method (HTM). The method, basedon the conformal transformation of the pad coordinate space directly to the Cherenkovangle parameter space, presents the advantage to work regardless of the pattern shapeand/or noise contributions. For more details see ref.[4].The performance of the method has been evaluated on events generated with the de-tector simulation code GALICE[5]. One central Pb-Pb event (13% of pad occupancy)has been fully simulated in the ALICE setup by generating the primary and secondaryparticle flux with the HIJING event generator (g.2). The contribution of neutrons hasbeen simulated with FLUKA[6].In order to study a particular set of ring patterns at xed momentum and incidenceangle in the ALICE multiplicity environment, one ring at the time has been generatedand superimposed to the Pb-Pb event.Pions, kaons and protons of dierent momentum impinging the central HMPID mod-ule[2] at normal incidence have thus been analyzed. Fig.3 shows the distributions of thereconstructed c Cherenkov angle for , K and p at two dierent momenta.The performance of the pattern recognition method has been studied in term of particleidentication eciency and particle contamination. If Nfoundi is the number of the recon-3Figure 2. Left: hit map of central HMPID module in ALICE generated with the GALICEpackage for one Pb-Pb central event (dN/dy=8000). Right: a detail of the previous hitmap showing the region where the generated ring has been merged to the event (the ringpads are in black). The reconstructed Cherenkov ring is also shown.Figure 3. Distributions of reconstructed Cherenkov angle c for 1000 of , K and p attwo dierent momenta.structed rings with c in a selected range for a given particle, while Ntoti is the number ofthe total simulated rings in the same c range, then the eciency Ei and contaminationConti are dened as follows:Ei =NfoundiNtoti; Conti =Nfoundj +NfoundkNfoundi +Nfoundj +Nfoundki6=j 6=k (i = ; K; p)where the expected particle production ratios, estimated with HIJING, have been appliedto calculate Conti (see g.4).The choice of the c interval in which Ei and Conti have been evaluated is a resultof a compromise between a good eciency and a low contamination, depending on thephysics signal under study.Table 1 shows the eciencies and contaminations evaluated for , K and p in themomentum range 23 GeV/c. The calculations have been performed implanting full rings400.10.20.32 3 4 5 6Momentum (GeV/c) RatioFigure 4. Expected yields (not including jet quenching) in central Pb-Pb events in ALICE.in a very high density particle event (dN/dy=8000), and lower density (dN/dy=5400),with two dierent proximity gap sizes, corresponding to R=122 mm and R=155 mm ringradii at normal incidence. We clearly observe a strong decrease of the contamination withthe density, and also an evident preference for the smaller ring radii. The eciency forprotons of 50% at low momenta (near to the momentum threshold) is due to the lownumber of emitted photons, making the pattern recognition dicult.The choice of the c interval to compute the eciency and the contamination is, how-ever, related to the physics signal under study with the HMPID. As an example, thecontamination of 20% for charged kaons, with an eciency of 50%, is still acceptable forstudying the KK pair correlation with a low background[7].Table 1. Eciency and contamination for , K and p in the momentum range 23 GeV/c.Events with Cherenkov patterns fully contained in the pad plane have been considered.p = 2 GeV/c  K pe. (%) 80 60 52cont. high dens. R=155mm(%) 5.1 13.0 6.6cont. low dens. R=155mm(%) 2.0 4.7 2.4cont. high dens. R=122mm(%) 2.3 7.2 1.5p = 2.5 GeV/c  K pe. (%) 80 40 67cont. high dens. R=155mm(%) 6.5 17.7 4.6cont. low dens. R=155mm(%) 2.6 9.0 0.9cont. high dens. R=122mm(%) 3.2 12.8 2.5p = 3 GeV/c  K pe. (%) 80 37 74cont. high dens. R=155mm(%) 8.2 25.0 6.1cont. low dens. R=155mm(%) 3.3 14.0 1.6cont. high dens. R=122mm(%) 5.0 23.0 1.8The eciency and contamination when circular patterns are partially outside the RICHplane have also been computed. The results are shown in g.5. The eciency for , Kand p decreases of about 1520% and the contamination increase of about a factor 1.52.500.20.40.60.811.5 2 2.5 3 3.5momentum(GeV/c)Ring part. outsideRing inside00.050.10.151.5 2 2.5 3 3.5momentum(GeV/c)contamination00.20.40.60.811.5 2 2.5 3 3.5momentum(GeV/c)00.10.20.30.40.51.5 2 2.5 3 3.5momentum(GeV/c)contamination00.20.40.60.811.5 2 2.5 3 3.5momentum(GeV/c)00.050.10.151.5 2 2.5 3 3.5momentum(GeV/c)contaminationFigure 5. Eciency and contamination for rings fully contained (squares) and partiallycontained (triangles) in the HMPID photocathode plane.3. ConclusionThe performance of the recognition strategy of circular Cherenkov patterns based on theHough transformation, in a fully simulated central Pb-Pb ALICE event (dN/dy=8000),has been studied in terms of particle identication eciency and particle contamination.An eciency of 80% and a contamination of about 3.5% has been achieved for pionsin the interval 2p3 GeV/c, at normal track incidence to the detector plane and withthe ring fully accepted by the detector, while an eciency from 60% to 40% and a con-tamination from 7% to 23% has been evaluated for kaons. The eciency for protonsincreases from 52% to 74% and remains nearly 80% above 3 GeV/c, with a contamina-tion of about 2%. In addition, when the ring is partially accepted by the detector, theeciency decreases of 1520% and the contamination increase of a factor 1.52 for ,K and p.REFERENCES1. F. Piuz, Nucl. Instr. and Meth. A371(1996) 96.2. ALICE Coll., Technical Design and Report, Detector for High Momentum PID,CERN/LHCC 98-19.3. E. Nappi et al., Internal Note ALICE 98-20.4. D. Cozza et al., Internal Note ALICE 98-39.5. http://www.cern.ch/ALICE/Projects/oine/Simulation/galice.6. http://www.cern.ch/ALICE/Projects/oine/Simulation/fluka.7. D. Ferenc, private communication.

Recognition of Cherenkov Ring Patterns with the HMPID-RICH Detector in ALICE at LHC

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