Gas Cherenkov detector for high momentum charged particle identification in the

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3rd INT. WORKSHOP ON HIGH-PT PHYSICS AT
LHCMarch, 16-19, 2008 Tokay, Hungary
Gas Cherenkov detector for high momentum charged
particle identification in the ALICE experiment
at LHC
Giacomo Volpe
Istituto Nazionale di Fisica Nucleare, Sezione di
Bari, Italy.
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ALICE experiment
EMCal
High energy g
pioni
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.
T0,V0, PMD,FMD and ZDC
Forward rapidity region
pioni
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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 pT identification (up to p
  600 MeV/c).
• TOF covers intermediate pT region.
• TRD electrons identification.
• HMPID high pT region (15 GeV/c).

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ALICE PID upgrade
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.

• 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 present in a momentum
range higher than at RHIC, pT 10 20 GeV/c.
(ref. Rudolph C. Hwa, C. B. Yang,
arXivnucl-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 gt 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,
arXiv0707.3494 hep-ph, July 2007.
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ALICE PID upgrade

• 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.

• 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 gt 10 GeV/c ?
  VHMPID (Very High Momentum Particle
  Identification Detector).
• Energy loss or Time of Flight measurements dont
  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.

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• Radiator gas
• CF4 (n 1.0005, gth 31.6) has the drawback
  to produce scintillation photons (Nph
  1200/MeV), that increase the background.
• C4F10 (n 1.0015, gth 18.9)
• C5F12 (n 1.002, gth 15.84) this gas has been
  used in the DELPHI RICH detector.

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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 CH4, the pads size is 0.80.84
  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 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
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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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Studied setup
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.
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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.
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Photon blob topology proximity focusing setup
Nph(b 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.
15 GeV/c
ltNgt 43
ltNgt 91
3 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 setup
Nph(b 1) (1.4 eV-1cm-1)(3 eV)(120 cm) 500
b 1
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Track inclination angle 10
Orthogonal track displaced 40 cm from the
detector center.
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Study of the detector response proximity
focusing like setup
Photodetector
Charged particle
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Study of the detector response background
subtraction algorithm
Charged particles
Background produced by Pb-Pb collision event
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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Study of the detector response focusing setup

• 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.

Photodetector
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Simulation results Cherenkov angle
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Simulation results Cherenkov angle
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Simulation results Pb-Pb background
HIJING generator dNch/dh 4000 at mid rapidity
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Background subtraction algorithm
Hough transform method

• 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 (?p,
  fp), 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, ?p, fp) , hc)
• (xp, yp, ?p, fp) 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 hc bin with a certain width is defined.
• The Cherenkov angle ?c of the particle is
  provided by the average of the hc values that
  fall in the bin with the largest number of
  entries.

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Simulation results Cherenkov angle

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

b 1
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Simulation results Cherenkov angle
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Momentum range for p, K and p identification in
Pb-Pb collisions environment.
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VHMPID in the ALICE apparatus
PHOS
VHMPID modules
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Studied setup

• 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.

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Simulation results Cherenkov angle
b 1
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Simulation results Cherenkov angle
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Momentum range for p, 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