1 Gas Cherenkov detector for high momentum charged particle identification in the ALICE experiment at LHC 3rd INT. WORKSHOP ON HIGH-P T PHYSICS AT LHC March, 16-19, 2008 Tokay, Hungary Giacomo Volpe Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy.

2 pioni HMPID RICH, high p T HMPID RICH, high p T ITS Vertexing, low p t tracking and PID with dE/dx ITS Vertexing, low p t tracking and PID with dE/dx TPC Main Tracking, PID with dE/dx TPC Main Tracking, PID with dE/dx TRD Electron ID, Tracking TRD Electron ID, Tracking TOF intermediate p T TOF intermediate p T PHOS ,  0 - ID PHOS ,  0 - ID MUON  -ID MUON  -ID + T0,V0, PMD,FMD and ZDC Forward rapidity region + T0,V0, PMD,FMD and ZDC Forward rapidity region L3 Magnet B= T L3 Magnet B= T ALICE experiment EMCal ALICE is designed to study the physics of strongly interacting matter and the quark- gluon plasma (QGP) in nucleus-nucleus collisions (  s NN = 5.5 TeV) at the LHC. The p-p physics will be study as well as reference data for the nucleus-nucleus analysis. High energy 

3 ALICE experiment ALICE has a unique capability, among the LHC experiments, of charged particle identification, due to the exploiting of different types of detectors:  ITS + TPC : low p T identification (up to p = 600 MeV/c).  TOF : covers intermediate p T region.  TRD : electrons identification.  HMPID : high p T region (1÷5 GeV/c) p (GeV/c) TPC + ITS (dE/dx)  /K /K/K K/ p e /  HMPID (RICH) TOF p (GeV/c) TRD e /  /K/K K/ p High-p T Physics at LHC, 17 March 2008 G. Volpe

4 RICH results: At RHIC has been observed a large enhancement of baryons and antibaryons relative to pions at intermediate p T ≈ GeV/c, while the neutral pions and inclusive charged hadrons are strongly suppressed at those p T. ALICE PID upgrade The key issue is to understand what is the mechanism of the hadronization and the influence of this mechanism on the spectra of baryons and mesons.

5 ALICE PID upgrade The baryon puzzle observed at RICH can be interpreted with the “partons recombination” or “coalescence” mechanism. In the recombination scenario quark-antiquark pair close in the phase space can form a meson at hadronization, while three (anti)quark can form an (anti)baryon. At LHC where the density of jets is very high, a new phenomenon originates where the recombination of shower partons in neighboring jets can make a significant contribution. It is foreseen that the baryon enhancement will be present in a momentum range higher than at RHIC, p T = 10 ÷ 20 GeV/c. (ref. Rudolph C. Hwa, C. B. Yang, arXiv:nucl-th/ v2, 21 Jun 2006)

6 ALICE PID upgrade Other authors using different arguments foresee also change in meson-baryon ratio for p T > 10 GeV/c. Jet quenching can leave signatures not only in the longitudinal and transverse jet energy and multiplicity distributions, but also in the hadrochemical composition of the jet fragments. S. Sapeta and U. A. Wiedemann, arXiv: [hep-ph], July 2007.

7 momentum ? High-p T Physics at LHC, 17 March 2008 G. Volpe The use of the Electromagnetic Calorimeter opens interesting possibility to distinguish quark and gluon jets in gamma - jet events and subsequently the study of the probability of fragmentation in pions, kaons or protons. EMCal  hadrons beam axis TPC ALICE PID upgrade Regardless of the theoretical interpretations it seems important to have the possibility to measure the meson-baryon ratio up to momenta well above the current limits of ALICE for a track-by-track identification.

8 VHMPID  ALICE-HMPID collaboration is studying the possibility to built a new detector to identify charged particles with momentum p > 10 GeV/c  VHMPID (Very High Momentum Particle Identification Detector).  Energy loss or Time of Flight measurements don’t allow to identify track-by- track in such momentum range.  Since the given space in the ALICE detector and the physics requirements it seems inevitable to use gas Cherenkov counters.  To use a gas Cherenkov detector in a magnetic field environment brings about the following key problems: the choice of radiator gas, the photon detection and the detector geometry.  A combination of a gas with low value of refractive index, with the proven concept of large area CsI photocathodes, has been considered.  Depending on the particle momentum values, with VHMPID will be possible to have PID by means pattern recognition method or by threshold counters technique.  Simulation results will be presented. High-p T Physics at LHC, 17 March 2008 G. Volpe

9 Radiator gas CF 4 ( n ≈ ,  th ≈ 31.6) has the drawback to produce scintillation photons (N ph ≈ 1200/MeV), that increase the background. C 4 F 10 (n ≈ ,  th ≈ 18.9) C 5 F 12 (n ≈ 1.002,  th ≈ 15.84) this gas has been used in the DELPHI RICH detector.

10 VHMPID Photon detector Pad-segmented CsI photocathode is combined with a MWPC with the same structure and characteristic of that used in the HMPID detector. The gas used is CH 4, the pads size is 0.8×0.84 cm 2 (wire pitch 4.2 mm), and the average single electron pulse height is of 34 ADC channels (1 ADC = 0.17 fC ≈ 1000 e - ) at 2050 V. The chamber is separated from the radiator by a window (4 mm of thickness). High-p T Physics at LHC, 17 March 2008 G. Volpe

11 Photon detector An other option for the photon detector could be a GEM-like detector combined with a CsI photocathode (higher gain, photons feedback suppression). Principles of operation V. Peskov studies High-p T Physics at LHC, 17 March 2008 G. Volpe

12 VHMPID The simulation has been executed using AliRoot, the official simulation framework of the ALICE experiment; Different geometries has been taken into account; C 5 F 12 as radiator; CaF 2 window. Material photon transmittances and CsI photocathode quantum efficiency High-p T Physics at LHC, 17 March 2008 G. Volpe

13 Proximity-focusing like setup: Charged particles cross the radiator producing Cherenkov photons. On the chamber both charged particle and photon signals are present. Signal topology depends only on the track momentum. Studied setup High-p T Physics at LHC, 17 March 2008 G. Volpe

14 Studied setup Focusing setup: the focusing properties of a spherical mirror of radius R = 240 cm, are exploited. The photons emitted in the radiator are focused in a plane that is located at R/2 from the mirror center, where the photon detector is placed. High-p T Physics at LHC, 17 March 2008 G. Volpe

15 Photon blob topology – proximity focusing setup N ph (  = 1) ≈ (1.4 eV -1 cm -1 )·(3 eV)·(180 cm) ≈ 760, but The number of photons detected is much less because of the absorption in the radiator gas and CsI quantum efficiency. ≈ 43 ≈ 91 3 GeV/c 15 GeV/c High-p T Physics at LHC, 17 March 2008 G. Volpe

16 Photon blob diamater An algorithm to calculate the blob diameter has been implemented. The pad with the largest values of the charge corresponds with the impact particle point. I consider R that contains the 98% of the total charged pads. The values in the figure refers to mean and RMS of a sample of 100 events. High-p T Physics at LHC, 17 March 2008 G. Volpe

17 Photon ring topology: focusing setup N ph (  = 1) ≈ (1.4 eV -1 cm -1 )·(3 eV)·(120 cm) ≈ 500 High-p T Physics at LHC, 17 March 2008 G. Volpe  ≈ 1

18 High-p T Physics at LHC, 17 March 2008 G. Volpe

19 High-p T Physics at LHC, 17 March 2008 G. Volpe

20 Track inclination angle = 10° Orthogonal track displaced 40 cm from the detector center. High-p T Physics at LHC, 17 March 2008 G. Volpe

21 Photodetector Parameters used Blob radius Blob pads number Study of the detector response – proximity focusing like setup Charged particle Cherenkov photons High-p T Physics at LHC, 17 March 2008 G. Volpe

22 Study of the detector response background subtraction algorithm Background produced by Pb-Pb collision event Charged particles It considers pads with charge larger than 200 ADC channels It checks if that pad is a local maximum in pads charge values If the pad considered is a local maximum, it cuts that pad and the adjacent ones (the pads corresponding to the track to identify not are taken into account by this procedure) High-p T Physics at LHC, 17 March 2008 G. Volpe

23 High-p T Physics at LHC, 17 March 2008 G. Volpe

24 Pb-Pb collision events, considering the presence of MIPs background High-p T Physics at LHC, 17 March 2008 G. Volpe

25 Photodetector In the case of focusing setup the determination of Cherenkov emission angle is possible. Pattern recognition algorithm is needed to retrieve the emission angle. A back-tracing algorithm has been implemented to retrieve the Cherenkov emission angle. It calculates the angle starting from the photon hit point coordinates, on the photon detector. Study of the detector response: focusing setup Charged particle Mirror 120 cm Radiator vessel  High-p T Physics at LHC, 17 March 2008 G. Volpe

26  ≈ 1 Simulation results: Cherenkov angle High-p T Physics at LHC, 17 March 2008 G. Volpe

27 Simulation results: Cherenkov angle High-p T Physics at LHC, 17 March 2008 G. Volpe

28 HIJING generator dN ch /d  = 4000 at mid rapidity Simulation results: Pb-Pb background High-p T Physics at LHC, 17 March 2008 G. Volpe

29 Hough transform method Background subtraction algorithm The Hough Transform Method (HTM) is an efficient implementation of a generalized template matching strategy for detecting complex patterns in binary images. In the case of the Cherenkov pattern recognition, the starting point of the analysis is a bidimensional map with the impact point (x p, y p ) of the charged particles, hitting the detector plane with known incidence angles (θ p, φ p ), and the coordinates (x, y) of hits due to both Cherenkov photons and background sources. A “Hough counting space” is constructed for each charged particle, according to the following transform: (x, y) → ((x p, y p, θ p, φ p ),  c) (x p, y p, θ p, φ p ) is provided by the tracking of the charged particle, so the transform will reduce the problem to a solution in a one-dimensional mapping space. A  c bin with a certain width is defined. The Cherenkov angle θ c of the particle is provided by the average of the  c values that fall in the bin with the largest number of entries.

30 High-p T Physics at LHC, 17 March 2008 G. Volpe Simulation results: Cherenkov angle Hough transform is used to discriminate the signal from the background.  ≈ 1

31 High-p T Physics at LHC, 17 March 2008 G. Volpe Simulation results: Cherenkov angle

32  K p p 1 15 < p < 26 GeV/c p 0  K 1 K, p 0  1 2.5< p < 8 GeV/c ? 0 < 2.5 GeV/c Particle Id.C 5 F 12 8 < p < 14 GeV/c 2.5 < p < 8 GeV/c 8 < p < 14 GeV/c Momentum Momentum range for , K and p identification in Pb-Pb collisions environment.

33 PHOS VHMPID modules High-p T Physics at LHC, 17 March 2008 G. Volpe VHMPID in the ALICE apparatus

34 The available space in the ALICE apparatus is not too much. The goal is to decrease much as possible the detector dimension. C 5 F 12 has a boiling point T b = 28° C at 1 atm, implying a difficult use of it in ALICE setup, where the internal temperature could be more or less the same (heating plant is needed). A setup with radiator length of 80 cm, C 4 F 10 as radiator and SiO 2 window has been also investigated. High-p T Physics at LHC, 17 March 2008 G. Volpe Studied setup

35 High-p T Physics at LHC, 17 March 2008 G. Volpe Simulation results: Cherenkov angle  ≈ 1

36 High-p T Physics at LHC, 17 March 2008 G. Volpe Simulation results: Cherenkov angle

37  K p p1 17 < p < 24 GeV/c p 0  K 1 K, p 0  1 3< p < 9 GeV/c ? 0 < 3 GeV/c Particle Id.C 4 F 10 9 < p < 17 GeV/c 3 < p < 9 GeV/c 9 < p < 14 GeV/c Momentum Momentum range for , K and p identification in Pb-Pb collisions environment.

38 Conclusions & Outlook The focusing setup, since its smaller dimension, is the setup that the collaboration will develop. The goal is to have a small detector performing good PID. The 80 cm setup will be better investigated. To enrich the sample with interesting event, triggering option has been also considered, using a dedicated trigger (see L. Boldizsar talk) and/or photons in the EMCal. High-p T Physics at LHC, 17 March 2008 G. Volpe

39 Backup

40 Study of the detector response