####### B Project Description

####### 

####### Gas Cherenkov Detector for High Momentum Charged Particle Identification

####### 

####### University of Illinois at Chicago

Physics Department, 845 West Taylor St., Chicago IL 60304

Edmundo García, Russell Betts, Dave Hofman, Olga Barannikova

##### Yale University

##### Physics Department, P.O. Box 208120 New Haven, CT 06520

Nikolai Smirnov

####### Instituto de Ciencias Nucleares - Universidad Nacional Autonoma de Mexico

Circuito de la Investigación Científica, Ciudad Universitaria

A.P. 70543, CP 04510, Mexico, D.F.

Guy Paic, Pedro Podesta, Oleksiy Sokolov, Eleazar Cuautle Flores

####### Instituto de Fisica - Universidad Nacional Autonoma de Mexico 

Circuito de la Investigación Científica, Ciudad Universitaria

CP 04510, México, D.F.

Jose Ruben Alfaro Molina

####### Instituto Nazionale di Fisica Nucleare

INFN, Sez. Bari,

Via G. Amendola 173, I-70125 Bari, Italy

Eugenio Nappi, Domenico Di Bari, Giacomo Volpe

####### Organisation Européenne pour la Recherche Nucléaire

CERN CH-1211 Genève 23, Switzerland\
Fabio Formenti, Antonio Di Mauro, Paolo Martinengo, Vladimir Peskov,
Diego Perini

**B.1 Introduction**

In the next decade, the experiments at the Large Hadron Collider (LHC)
will be at the frontier of high-energy pp and heavy-ion collision
research \[1\]. LHC data from Pb + Pb collisions at the unprecedented
nucleon-nucleon center of mass energy ($\sqrt{s_{\text{NN}}}$) of 5.5
TeV will complement and expand the Relativistic Heavy Ion Collider
(RHIC) program. As confirmed by RHIC results, one of the most
interesting features of these collisions will be the study of particle
yields in the available transverse momentum (p~T~) range. Today, in
ALICE, the reach of proton identification on a track-by-track basis is
up to 5 GeV/c, and statistical identification is possible up to 30 GeV/c
using the relativistic rise of the Time Projection Chamber (TPC) \[2\].
This proposal, requests support for the construction and test of a
prototype gas Cherenkov detector for track-by-track charged particle
identification in the p~T~ range of 10 -- 30 GeV/c.

Particle production at large p~T~ in pp collisions is understood in
terms of parton fragmentation after hard scattering, so that the proton
to pion ratio can be related to the ratio of the corresponding
fragmentation functions which is of the order of 0.1 \[3\]. Figure 1
shows the p/ ratio as function of p~T~ from RHIC data. In central
heavy-ion collisions the p/ratio is greater than 1 at p~T~ of around 3
GeV/c before diminishing at higher p~T~ \[4,5\]. This seems to indicate
that an alternative mechanism of hadronization is taking place at
intermediate p~T~. This form of hadronization is not fully understood,
and at the moment there are different explanations for it. Among them is
the hydrodynamic approach based on flow \[6\] and the parton
recombination model \[7\]. For the last, the recombination of shower and
thermal partons stands in the range of 2 \< p~T~ \< 5 GeV/c between
hadronization of soft thermal partons at lower p~T~ and fragmentation of
hard partons at higher p~T~ \[8\].

![](media/image1.png){width="3.313888888888889in"
height="2.1152777777777776in"}

![](media/image2.png){width="3.013888888888889in"
height="2.0555555555555554in"}

Figure 1: (Left) p/ratio as a function of p~T~ \[3\]. (Right) particle
to pp yield ratio (nuclear modification factor - RPC) as a function of
p~T~ \[4\].

The determination of shower parton distributions (SPD) has been achieved
by studying the fragmentation functions (FF), which are independent of
the colliding system. Using the SPDs, it is predicted that the p~T~
range for parton recombination at the LHC will be between 10 and 30
GeV/c \[9\]. Alternatively, some experiments at the Intersection Storage
Ring (CERN-ISR) indicate that a substantial part of the proton wave
function may consist of diquarks as beared out by the high fraction of
proton yield in pp collisions \[10\].

The mechanism of hadronization may reflect itself on the characteristics
of the jets formed in hard parton-parton collisions. There are
indications that the fraction of the total jet energy taken by the
leading hadron will be very different in the case of mesons vs. baryons.
This may result in differences between the corresponding jet
multiplicities. Such studies require a track-by-track identification of
the leading particles.

High p~T~ particle correlations have been used to study jet related
processes in heavy-ion collisions like jet quenching \[11\].
Two-particle high p~T~ correlations usually identify the trigger
particle and the associated particle. The trigger particle has a p~T~
above a certain threshold and it is associated with another particle
with p~T~ threshold below the trigger particle. The correlation function
then shows the angular distributions between these two particles. The
STAR collaboration at RHIC measured the fragmentation properties of
back-to-back dijets using high p~T~ correlations in d+Au and Au+Au
collisions at 200 GeV. A suppressed rate of narrow back-to-back di-jet
peak was observed, which enabled a differential measurement of the
partonic energy loss \[12\].

![](media/image3.jpeg){width="4.700694444444444in"
height="2.3631944444444444in"}

Figure 2. Comparison of deuteron+Au and two samples of Au+Au collisions
showing the increased suppression of back-to-back peaks as the system
size is increased (left to right in figure). In both figures, the rows
show different momentum ranges for associated particles \[9\].

In central Pb+Pb collisions at the LHC, jet rates are expected to be
high at energies at which jets will be distinguished from the background
in the event \[13\]. At intermediate p~T~ the direct identification of
jets will be very hard, given the large background from soft particles.
Thus there is the need to use the correlation methods to study jets'
energy loss and production mechanisms at p~T~ between 10 and 30 GeV/c.
Given the underlying background present at these energies, the
correlation studies will require tracking and exact track-by- track
identification capabilities. The detector we propose will provide these
capabilities, filling the existing gap between low and high p~T~ in the
ALICE's acceptance \[2\]. The use of the relativistic rise in the
ALICE's TPC allows for statistical separation of the baryons and mesons.
However, the track-by-track identification will give a better
interpretation in the details of measurements such as: the
baryon-antibaryon content of individual jets, identification of a baryon
in jets as a leading particle, and the details of the near and far side
jet correlations. Another important measurement is the possible
identification of gluon jets in pp production, coupled with the
multiplicity measurement \[14\].

At LHC the challenge is to use a relatively small detector covering
about 5% of the acceptance of ALICE central barrel with a suitable
trigger. Presuming that sufficient statistics could be collected for
jets with energies between 40 and 80 GeV/c, the leading charged mesons
could carry about 25% of the original parton energy while the protons
would carry about 30 -- 40% . Then the leading protons in the proposed
range could have between 12 and 30 GeV, determining thus the required
range of identification for the detector we propose.

**B.2 Detector Characteristics and Simulations.**

The proposed Cherenkov detector for very high momentum charged particle
identification (VHMPID) is based on the combination of a C~5~F~12~ gas
radiator (refraction index n = 1.0015) with a large area CsI
photon-to-electron converter and a charged particle detector in the form
of either a multi-wire chamber (MWPC) with a pad segmented cathode
\[15\] or a gas electron multiplier (GEM) \[16\]. With these elements
the further characteristics of the VHMPID are based on the experimental
constrains: the space available in ALICE, the 0.5 T magnetic field
surrounding it, and the granularity that allows the discrimination of
background in a central heavy-ion collision at the LHC.

The alternative for the use of the MWPC is a novel, very promising
structure for charge amplification: a TGEM (Thick GEM) or RTGEM
(Resistive Electrode GEM) detector \[16,17\]. The gas electron
multiplier is a composite grid consisting of two metal layers separated
by a thin insulator etched with a regular matrix of open channels
(holes). The metal layers are kept at a suitable difference of
potential, allowing the pre-amplification of the charge drift through
the channels. The GEM concept is illustrated in Fig. 5. The photons are
converted in the CsI, then the electron cascade drifts into the holes
that act as multiplication channels, and finally the electrons' signal
is read out in a position-sensitive device. It is possible to achieve
gains sufficiently high for the detection of single photoelectrons using
a GEM type detector. Thus, the use of a GEM would improve the efficiency
of the VHMPID, given the larger multiplication (gain) of the converted
electrons. The sturdiness of the device when compared to a MWPC is also
an advantage.

![](media/image4.wmf) ![](media/image5.wmf)

Fig. 3. Gas electron multiplier GEM concept illustration.

A sketch of the geometry for the VHMPID is shown in Fig. 3. The CsI
photocathode measures the position of Cherenkov photons after they have
been generated in the radiator by a charged particle track. In this
design, the focusing properties of the spherical mirror of radius *R*;
successfully used in many ring imaging Cherenkov counters (RICH); have
been exploited. The photons emitted in the radiator are focused in a
plane that is located at *R/2* (120 cm) from the mirror center. The
interception points of the photons with the focal plane form the ring
image shown in the figure. The dimensions of the radiator's tank are 120
x 100 x 100 cm. The photocathode (labeled as detector in the figure) is
at the track's entrance face, while a spherical mirror is at the other
end of the tank.

The emission angle and the number of photons created in the tank by the
incident charged particle is given by the index of refraction of the
C~5~F~12~. By measuring the emission or Cherenkov angle of the photons
it is possible to identify the particles. Alternatively, by measuring
the radius of the ring pattern produced by the hits in the photocathode
the particles are identified, as shown in data from simulations in Fig.
5.

![](media/image6.png){width="2.859722222222222in"
height="2.477777777777778in"}

Figure 4. Geometry sketch of the VHMPID. The incident particle track is
represented by the gray line, and the Cherenkov photons by the green
shaft. The photon hits in the CsI photocathode (detector) are
represented by red dots forming a ring image at the focal distance.

The response of the proposed VHMPID detector has been studied using
simulation techniques \[18\] based on GEANT 4 \[19\]. The simulations
include the CsI quantum efficiency, the gas transmittance and the
optical characteristics of the proposed materials. As first
approximation, the identification curves were generated by measuring the
radius of the ring image left by the photon hits on the photocathode, no
photoelectron conversion or the response of the MWPC were included in
the simulation at this stage. With this approximation, as shown in Fig.
6 and Table 1, the VHMPID allows the identification in a large momentum
range 9 -- 30 GeV/c for protons, 3 -- 15 GeV/c for pions and 5 -- 15
GeV/c for kaons.

These results involve tracks with incident momentum perpendicular to the
detector's window and aimed at the center of the mirror. For extreme
conditions of larger incident angle or displacement of the track away
from the center of the detector, the basic ring pattern is conserved.
This is shown in Fig. 7, where the entrance point of the original track
outside the ring is included. In the identification procedure, the
incidence angle will be known from the tracking so that the Cherenkov
angle will be reconstructed for each photon individually.

![](media/image7.png){width="2.5208333333333335in"
height="2.298611111111111in"}

Fig. 4. Photon hit position on the CsI photocathode. The rings are from
one event of one incident 16 GeV/c pion (black), one kaon (blue) and one
proton (brown). The entrance point of the original track is included in
the plot (red).

![](media/image8.png){width="3.1430555555555557in"
height="2.4555555555555557in"}

Fig. 5. Identification curves as a function of momentum for kaons, pions
and protons. The radius of the photon hits (ring minor axis) is
calculated from 100 events for each particle and incident energy. Note
that from 9 -- 18 GeV/c the protons will be identified by the absence of
signal.

  ---------------- -- -------------- ---------------------------------------------------
  Table 1.                           
  Identification                     
  momentum range                     

                      With signal    Absence of signal

                      3--15 GeV/c    

  k                   9--15 GeV/c    

  p                   18-30 GeV/c    9--18 GeV/c
  ---------------- -- -------------- ---------------------------------------------------

![](media/image9.png){width="5.715277777777778in"
height="2.173611111111111in"}

Fig. 6. Patterns for single event tracks at 14 degrees incidence angle
(left) and for 60 cm displacement from the center of the mirror. The
entrance point of the original track (mip) is included in the plot
(red).

When a photon hits the CsI an electron is emitted and then amplified so
that a cascade of electrons is produced, these electrons are then
detected in a multi-wire proportional chamber or a GEM detector. In
order to study further the performance of the detector, the response and
pixel size of the MWPC's pads (8 x 8 mm^2^) was included in the Monte
Carlo simulation. The simulation was done with chambers of 122 x 120
pads corresponding to a surface of about 1 m^2^. Furher parameters
applied were those that reproduce satisfactorily the ALICE High Momentum
Particle Identification Detector (HMPID) experimental results.

In addition to the detection of the position of the photoelectron hits
in the MWPC the original particle track leaves an intense minimum
ionizing signal (MIP). Figure 7 shows the photoelectron tracks left in
the MWPC from five superimposed protons, kaons and pion tracks with an
incident momentum of 16 GeV/c, together with the MIP signal.

![](media/image10.png){width="1.9597222222222221in" height="1.80625in"}
![](media/image11.png){width="2.1194444444444445in"
height="1.804861111111111in"}
![](media/image12.png){width="1.9194444444444445in"
height="1.8069444444444445in"}

Fig. 7. Ring trace for 5 superimposed 16 GeV/c events. Protons (left),
kaons (center) and pions (left).

Figure 8, shows the photo electron tracks left in the MWPC for one pion,
one kaon and one proton of 26 GeV/c, superimposed. The particle
identification separation at both energies is visible by eye.
Preliminary results using a patter recognition algorithm confirm these
results numerically, as shown in Fig. 9 where the identification curve
for pions, protons and kaons is shown as a function of the momentum. In
this plot the Cherenkov angle is reconstructed for each photon
individually, then the average value is calculated as
$\theta_{\text{Cherenkov}} = \sum_{}^{}{\theta_{i}/N}$, where N is the
number of photons in each ring.

![](media/image13.png){width="3.0069444444444446in" height="2.29375in"}

Fig. 8. Ring trace for 3 superimposed 16 GeV/c events. Protons (red),
kaons (blue) and pions (black).

![](media/image14.png){width="4.939583333333333in"
height="2.8430555555555554in"}

Fig. 9. Identification curves as a function of momentum for kaons, pions
and protons. The blue circles represent the reconstructed Cherenkov
angle for pions, the green circles for kaons and the black circles for
protons. The Cherenkov angle is reconstructed for each photon, assuming
that the track of the incident particle is known.

Another important factor under consideration to evaluate the VHMPID
performance is the effect of background in the particle identification.
Initial results from Monte Carlo simulations are shown in Fig. 9. The
VHMPID detector is simulated at its proposed location with the ALICE
detector. Then a pion track is embedded in a $\sqrt{s_{\text{NN}}}$= 5.5
TeV Pb+Pb HIJING \[20\] event with a pessimistic charged particle
multiplicity dN~ch~/dy \~ 4000 at mid rapidity. When the full simulation
and reconstruction is carried out, the trace from the pion is clearly
identifiable in the VHMPID above the background, as observed in the
right panel of Fig 10.

![](media/image15.png){width="3.402083333333333in"
height="2.423611111111111in"}
![](media/image16.png){width="2.972916666666667in"
height="2.498611111111111in"}

Fig. 10. On the left is the proposed location within the ALICE detector
for the VHMPID volumes. On the right is the trace left by a simulated 16
GeV/c pion embedded in a HIJIG Pb-Pb.

To finalize this section, it is important to notice that the proposed
location of the VHMPID modules within ALICE geometry is opposite to the
electromagnetic calorimeter (EMCAL), a detector funded by US agencies,
which we expect to use as a trigger for the study of gamma-jets events.

**B.2 Project Organization**

The organizational model for this project in research and education will
be based on the working model of international collaborations. The
common goals and project strategies will be decided by the consensus of
the senior members of the collaboration, and the particular tasks will
be performed according to the capabilities and experience of the partner
institutions. On the other hand, the small size of this project will
allow us to implement a different approach for assigning tasks: when
possible, in order to give early-career researchers and students broader
experience, they will form a single work force to address sub-tasks at
different institutions. This will help them to consolidate relationships
and give them valuable experience to pursue successful careers in the
future.

The members of the collaboration have the relevant background to
undertake this project: Dr. Smirnov was part of the team that recently
installed and commissioned a ring imaging Cherenkov detector in STAR at
RHIC. Prof. Nappi and Paic, have a long tradition in the research on
Cherenkov-light imaging applications and related technological issues.
They are part of the team in charge of the RICH detector under
construction for ALICE. Dr. Alfaro is part of the Mexican team building
the detector V0 for ALICE. He is supervising the manufacturing of this
detector in a state-of-the-art machine shop at the IFUNAM. Dr.
Martinengo a senior scientist at CERN is the current project leader for
the ALICE HMPID, he is also working on the construction of a prototype
GEM detector that will be tested this fall at CERN. Dr. Garcia was the
project manager for the design, construction and commissioning of the
trigger detectors for the PHOBOS experiment at RHIC. Recently he has
worked on the simulations and test beam for the CMS zero degree
calorimeter. Dr. Garcia proposed the focusing design for the VHMPID
detector and generated the simulations shown earlier.

Table 2. is the project timeline. The detector simulation work will be
finalized by the end of the year. At that time we will have defined the
design parameters. It will be also important for the final design to
have the results from the test beam of the GEM prototype, taking place
this Fall CERN. By Spring 2008 a letter of intent (LOI) to build the
VHMPID will be formally presented to the ALICE collaboration. Internal
discussions indicate that the project has the support of the
collaboration. The construction of the VHMPID prototype will start in
the second part of next year; the support from this grant will be a
fundamental to this part and the rest of the project. The culmination of
this work will be the test of the prototype in ALICE followed by the
data analysis and the writing of the proposal to build the full
detector. It is worth mentioning that the full detector is modular, and
that the prototype we are building will be one of the six modules of the
full detector. It is also worth to mention that the installation and
tests of the detector are tributary of the LHC agenda.

Table 3. is the organizational chart for the project. Dr. Paic will be
the coordinator. He will also be the liason to the ALICE collaboration.
Dr. Garcia will be the project manager. The individual institutions,
their senior representatives and the different tasks for the
construction of the VHMPID are also represented in this table. An
advisory council formed by the coordinator, project manager and one
senior member form each institution will serve as steering committee for
the project.

![](media/image17.wmf)

Table 2. Project time line for the construction of the VHMPID prototype.

![](media/image18.png){width="6.495138888888889in"
height="5.424305555555556in"}

Table 3. Organizational chart for the project.

**B.2 Project Outreach**

The project presented here represents the continuation of a proto
collaboration which started in 2004 (RICH conference, Playa del Carmen)
and is now counting with about 20 Ph. D. physicist from INFN Bari, KMKI
Budapest, ICN UNAM, IF UNAM, CERN, UIC and YALE, coordinated by Dr.
Paic. The present proposal aims at reinforcing the US presence in this,
in our view, interesting project, so much for the physics objective as
for its technological complexity and novelty.

The support for this proposal will help to put the U.S. institutions in
a leadership position not only in this project, but also in the context
of the research that will be performed at the LHC in the next few years.
It will contribute to the education of students, exposing them to
diverse research environments, and facilitating their interaction with
world-class physicists. Finally the support for this project will
facilitate the transfer of ideas and technology for the common
advancement of knowledge.

UIC, immersed in the Chicago area, is a center not only of research, but
also of quality teaching and service. UIC's student body reflects the
ethnic and racial diversity of the area, with a high percentage of
African-American and Hispanic students. The high-energy nuclear physics
group has a history in education and research training of students and
postdoctoral associates. The support for this project will strength the
position of Dr. Garcia at UIC, and will allow him to continue his
capacity as role model in the classroom and as a researcher.

Another attribute of this proposal is the complementarity of the VHMPID
in the US lead EMCAL effort. The successful completion of the VHMPID
would further strengthen the presence of the US scientist in the ALICE
experiment. As mentioned, the proposed location for the VHMPID is
opposite to the EMCAL. This opens the opportunity to measure interesting
physics channels using both detectors. For example, in gamma-jet events,
one could determinate the jet composition in the VHMPID while triggering
with the EMCAL.