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

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Gas Cherenkov detector for high momentum Gas Cherenkov detector for high momentum charged particle identification in thecharged particle identification in the

ALICE experiment at LHCALICE experiment at LHC

3rd INT. WORKSHOP ON HIGH-PT PHYSICS AT LHCMarch, 16-19, 2008 Tokay, Hungary

Giacomo Volpe

Istituto Nazionale di Fisica Nucleare, Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Sezione di Bari, ItalyItaly..

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pioni

pioni

HMPIDRICH , PID @ high pT

HMPIDRICH , PID @ high pT

ITSVertexing, low pt tracking and PID with dE/dx

ITSVertexing, low pt tracking and PID with dE/dx

TPCMain Tracking, PID with dE/dx

TPCMain Tracking, PID with dE/dx

TRDElectron ID,Tracking

TRDElectron ID,Tracking TOF

PID @ intermediate pT

TOFPID @ intermediate pT

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=0.2-0.5 TL3 Magnet B=0.2-0.5 T

ALICE experimentEMCal

ALICE is designed to study the physics of strongly interacting matter and the quark-gluon plasma (QGP) in nucleus-nucleus collisions (sNN = 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

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ALICE experimentALICE has a unique capability, among the LHC experiments, of charged particle identification, due to the exploiting of different types of detectors:

ITS + TPC : low pT identification (up to p = 600 MeV/c).

TOF : covers intermediate pT region.

TRD : electrons identification.

HMPID : high pT region (1÷5 GeV/c).

0 1 2 3 4 5 p (GeV/c)

TPC + ITS (dE/dx)

/K

/K

K/p

K/p

e /

HMPID (RICH)

TOF

1 10 100 p (GeV/c)

TRD e /

/K

K/p

High-pT Physics at LHC, 17 March 2008 G. Volpe

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RICH results: At RHIC has been observed a large enhancement of baryons and antibaryons relative to pions at intermediate pT ≈ 2 - 5 GeV/c, while the neutral pions and inclusive charged hadrons are strongly suppressed at those pT.

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.

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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 presentin a momentum range higher than at RHIC, pT = 10 ÷ 20 GeV/c. (ref. Rudolph C. Hwa, C. B. Yang, arXiv:nucl-th/0603053 v2, 21 Jun 2006)

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ALICE PID upgrade

Other authors using different arguments foresee also change in meson-baryon ratio for pT > 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 thejet fragments.

S. Sapeta and U. A. Wiedemann, arXiv:0707.3494 [hep-ph], July 2007.

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momentum ?


• 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.

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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-pT Physics at LHC, 17 March 2008 G. Volpe

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Radiator gas

• CF4 (n ≈ 1.0005, th ≈ 31.6) has the drawback to produce scintillation photons (Nph ≈ 1200/MeV), that increase the background.

• C4F10 (n ≈ 1.0015, th ≈ 18.9)

• C5F12 (n ≈ 1.002, th ≈ 15.84) this gas has been used in the DELPHI RICH detector.

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VHMPIDPhoton 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 CH4, the pads size is 0.8×0.84 cm2 (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).


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Photon detectorAn 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


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VHMPID• The simulation has been executed using AliRoot, the official simulation framework of the ALICE experiment;

• Different geometries has been taken into account;

• C5F12 as radiator;

• CaF2 window.

Material photon transmittances and CsI photocathode quantum efficiency


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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


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Studied setupFocusing 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.


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Photon blob topology – proximity focusing setupNph( = 1) ≈ (1.4 eV-1cm-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.

<N> ≈ 43 <N> ≈ 91

3 GeV/c

15 GeV/c


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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.


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Photon ring topology: focusing setupNph( = 1) ≈ (1.4 eV-1cm-1)·(3 eV)·(120 cm) ≈ 500


≈ 1

1818


1919


2020

Track inclination angle = 10°

Orthogonal track displaced 40 cm from the detector center.


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Photodetector

Parameters used

Blob radius

Blob pads number

Study of the detector response – proximity focusing like setup

Charged particle

Cherenkov photons


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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)


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Pb-Pb collision events, considering the presence of MIPs background


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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


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≈ 1 ≈ 1

Simulation results: Cherenkov angle


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HIJING generator dNch/d= 4000 at mid rapidity

Simulation results: Pb-Pb background


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Hough transform methodBackground 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 (xp, yp) of the charged particles, hitting the detector plane with known incidence angles (θp, φp), 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) → ((xp, yp, θp, φp) , c)

• (xp, yp, θp, φp) 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 θc of the particle is provided by the average of the c values that fall in the bin with the largest number of entries.

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• Hough transform is used to discriminate the signal from the background.

≈ 1

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K

p

p115 < p < 26 GeV/c

p0

K1

K, p0

12.5< p < 8 GeV/c

?0< 2.5 GeV/c

Particle Id.C5F12

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.

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PHOS

VHMPID modules


VHMPID in the ALICE apparatus

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• The available space in the ALICE apparatus is not too much. The goal is to decrease much as possible the detector dimension.

• C5F12 has a boiling point Tb = 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, C4F10 as radiator and SiO2 window has been also investigated.


Studied setup

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≈ 1

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K

p

p117 < p < 24 GeV/c

p0

K1

K, p0

13< p < 9 GeV/c

?0< 3 GeV/c

Particle Id.C4F10

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.

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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.


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Backup

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Study of the detector response

)1(1

)(sin1

)(22

22

2

2

12

2

2

1 pn

mp

n

nN

n

nNpN chpads

Documents

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