Experiment at FAIR_ by pengxiuhui


									               Experiment at FAIR,
  Participation in “Compressed Baryonic Matter
           (CBM)” experiment at FAIR
The exploration of phase diagram of strongly interacting
matter is one of the most challenging fields of modern high
energy physics. Of particular interest is the transition from
hadronic to partonic degrees of freedom which is expected to
occur at high temperatures and/or high baryon densities.
Both phases played an important role in the early universe
and possibly exist in the core of neutron stars.

At ultrarelativistic beam energies provided by RHIC and the
future LHC, partonic matter is expected to be produced at very
high temperatures and at small baryon chemical potentials.
Similar conditions prevailed in the early universe. The
complimentary situation exists in the core of neutron star,
here the baryon density is very high and the temperature very
low. In the laboratory, fireballs at high baryon densities and
moderate temperatures can be created in heavy-ion
collisions at beam energies significantly below top SPS
energies. This region of phase diagram has not been studied in
detail so far.

Recent Lattice QCD calculations predict a smooth
crossover from hadronic to partonic matter at low net
baryon densities, and a first order phase transition with a
critical endpoint at higher net baryon densities. The
discovery of the first order phase transition and/or the
critical endpoint would be a major breakthrough in our
understanding of strongly interacting matter. Another
fundamental issue is the search for signatures of chiral
symmetry restoration. The spontaneously broken chiral
symmetry of QCD is intimately related to the origin of
hadron masses as it corresponds to a phase transition
from almost massless quarks to massive hadrons. A sketch
of the QCD phase diagram is shown in figure 1.

The high-intensity heavy-ion beams of the future FAIR
accelerators, together with the planned Compressed
Baryonic Matter (CBM) experiment, offer excellent
possibilities to produce and to investigate baryonic matter
at highest densities in the laboratory. The research
program comprises the study of the structure and the
equation-of-state of baryonic matter at densities
comparable to the ones in the inner core of neutron stars.
This includes the search for the first order phase
transition from hadronic to partonic matter, the critical
endpoint, and the search for signatures for the onset of
chiral symmetry restoration at high net-baryon densities.

Fig. 1: The phase diagram of strongly interacting matter

The major challenge is to find diagonistic probes which are
connected to chiral symmetry restoration and deconfinement
phase transition. Many signals have been proposed for QGP
and are still under discussion although the hope for finding a
“smoking gun” has not yet become true. The strategy
pursued with the CBM experiment is a systematic and

comprehensive measurement of all relevant observables
with    high     statistics and    high   resolution.   The
measurements include:
   The search for in-medium-modifications of low-mass
     vector mesons (ρ, ω, φ) and of D-mesons (open
     charm). These effects are expected to occur if chiral
     symmetry is restored. The in-medium spectral
     function of short lived vector mesons can be
     measured via their decay into dilepton pairs which
     serve as penetrating probes. A modified in-medium
     mass of D-mesons will influence strongly their
     production cross section, in particular at threshold
     beam energies.
   The search for nonmonotonic behavior of observables
     like the abundance of (multi-) strange particles and
     charmonium (J/ψ, ψ') as function of beam energy
     and/or size of the fireball. Such effects are expected
     when crossing a first order phase transition.
   The search for non-statistical fluctuations of
     observables like particle multiplicities of transverse
     momenta measured event by event. Such fluctuations
     are predicted to occur in the vicinity of the critical
     point, i.e. the signal should show up at a certain
     beam energy.
   The study of collective phenomena like elliptic flow,
     both of bulk particles (K, π, p) and of rare probes (,
     Ξ, Ω, D, J/ψ) as a function of beam energy. Elliptic
     flow is regarded as an observable which is sensitive to
     the very early (possibly partonic) stage of the

CBM experiment and our participation

 CBM experiment is in the design phase and the criteria for
designing the experiment are driven by following observables
(a) open and hidden charm (b) short lived vector mesons (c)
event by event fluctuation (d) strange and multi-strange
particles (e) elliptic flow.

The main tracking device is a set of (7-9) silicon tracking
stations (STS) placed inside a dipole magnet. The tracks
measured in STS are used for particle identification and
momentum determination. The dilepton pairs will be
measured either as electrons or muons. The experimental
difficulty is to identify soft leptons from rare decays in
the environment of heavy-ion collisions with up to 1000
charged particles. The electron measurement suffers from
a large combinatorial background resulting in a moderate
signal-to-background (S/B) ratio for vector mesons. The
muon measurement seems to provide an excellent S/B
ratio for charmonium, but poses the challenge to be
efficient also for soft muons. Figure 2 shows a sketch of
the muon detection system together with the STS inside
the dipole magnet.

FIG.2 : Dipole magnet, Silicon Tracking Stations and Muon
detection system. D-mesons and hyperons can be

identified via their decay topology with the STS only.
Hadron identification (π, K, p, ...) is performed with
tracking and time-of-flight detectors (not shown)
downstream the muon detectors. In this case, the
absorbers of the muon detection systems will be removed.

We propose to simulate, design, fabricate and operate a large
part of muon detection system (fig.2). The muon system is
proposed to be consisted of up to 16 tracking chambers
sandwiched between absorbers of varying thickness and
material. The chamber/absorber system will be placed
downstream the STS. The absorber can be made of iron or
carbon. Depending on the momentum resolution obtained
with the configuration, one option is to go for magnetized iron


For last one year we have been participating in simulation of
CBM experiment aimed at designing muon detectors. This job
has two components, to simulate through full detector
simulation using GEANT and event reconstruction for the
measurement of rare probes via muon decay channels and
optimize the design of muon chambers          for CBM. The
simulation framework includes track finding by Cellular
Automata and track fitting by Kalman Filter approach. We
have installed cbm simulation framework at VECC and have
developed several analyses packages. Below we give our effort
so far at VECC on this simulation.

                  Simulation Framework at CBM

The framework for simulation in CBM (cbmroot) is based on
ROOT, the object-oriented framework developed at CERN to
meet challenges in data analysis for High-Energy Physics
(HEP) Experiments.

Apart from cbmroot, one needs to have Event Generators like
Pythia5, Pythia6, Venus, Pluto etc. for generating events in
CBM environment and transport packages, Geant3 and
Geant4. Also, the VMC - the Virtual Monte Carlo is included in
the framework which allows to run different simulation Monte
Carlo without changing the user code and therefore the input
and output format as well as the geometry and detector
response definition. The schematic design of CbmRoot is as

  Fig. 3 : Framework of CBM simulation

FLUKA is still in trial stage and not used as on date. Fluka
will be used to crosscheck the simulation results with Geant.

                   Simulation Initiatives

In VECC, the framework has been installed and simulation for
proposed Muon Chamber (MuCh) has started in parallel with
simulation initiatives on MuCH at GSI, Germany. For studying
detector responses towards signal (muons) and background,
the event generators PLUTO and UrQMD are used besides

single particle event generator. The base-line collision system
for all simulation for the CBM experiment is Au + Au (fixed
target) at 25 A GeV.

The novel feature of the muon chamber for the CBM
experiment, as compared to other muon detectors in previous
and planned HEP experiments, is that the absorber in MuCh
in CBM is sliced or detectors are placed in between absorbers
to facilitate momentum dependent track detection. This is to
improve upon capturing low momentum muons essentially
from low-mass vector mesons, which would have been
otherwise stopped by thick absorber. Reasonable efficiency for
low momentum muons are very important to reconstruct low
mass vector mesons, needed for studying the in-medium
effect: the broadening or shifting of low-mass vector mesons
expected in compressed baryonic matter. Simulation for
designing muon-chamber for the momentum dependent
tracking is in progress.

                  Some Simulation Results

Initial studies show clear feasibility for having a Muon
Chamber for detecting muons from decay of vector mesons.
Figure 4 shows the reconstructed and background subtracted
invariant mass spectra where J/Psi signal is clearly visible
with signal to background ratio as 100.

Fig.4 : Reconstruction of J/Psi by invariant mass of muon

While charmonium signals are very easy to be detected, those
for low-mass vector mesons are definitely a challenge.
Several combinations of absorber and chamber layout have
been tried and fig. 5 shows the latest result where invariant
mass peaks of low mass vector mesons are clearly visible. This
has been obtained using Tungsten absorber at the beginning
along with slices of iron and carbon at later stages.

                                 S/B ~ 1 ()
                                 S/B ~ 0.2 ()
                                 S/B ~ 0.01 ()

Fig. 5: Reconstruction of low mass vector meson using muon

Parallel studies continue with different options of absorber-
type and thicknesses to optimize these parameters for
attending maximum reconstruction efficiency for muons from
low mass vector mesons and for improving upon signal to
background ratio.
Fig. 6 shows the efficiency of detection of muon tracks for
different combinations of absorber slicing. This study is made
using carbon absorber only. This study is important as this
will optimize the number of detector stations which will affect
the cost.

Fig. 6: Muon reconstruction efficiency for various absorber

Layout below shows one of the options of layout where green
boxes represent the absorber and grey ones absorber layers.
Our current simulation activities are concentrating on
optimizing this combination.

Fig. 7: The plot showing the efficiency of hadron detection for
different momentum from 0.5 to 1.5 GeV/c. Total thickness of
carbon absorber corresponds to sum of slices of 10 cm each
with three detector layers in between two slices. It is seen that
with absorber thickness hadron survival proability reduces

Muon Chambers:

We are studying the option of detector technology to be used
for the chamber design. The experimental and technical
challenge is to design and to build a large area, high-
position-resolution detector which has to be operated at
very high particle densities of up to 1 hit/cm2 per event
with an event rate of up to 10 MHz.

Fig. 8 shows the hit density (/cm^2-event) on the surface of 16
chambers using two types of absorbers (Fe and Carbon).
Optimization of absorber material and thickness are being
worked out , but this plot can be taken as guidance for initial
design of the detectors. With 10MHz design rate in CBM, we
get the density up to 16 MHz one first few layers on the
detector setup. Hit density reduces as we for stations at
further distances.

Fig.8: Simulated hit density on various muon stations for
central Au+Au collisions at 25 AGeV.

For the simulations shown above, while constructing hits, a
position resolution of 100micron has been assumed and that
makes another constrain on the detector design. From our
experience in working with gas detectors and in view of huge
expected spin-offs from these detectors, we have studied
various technological options available for muon chamber
design. Table below gives comparative study of the gas
detector technologies available today (some of them are in
R&D phase and MWPC are being used heavily). From this
table it appears that GEM or Micromegas would be suitable
option at least for first few stations and then later on we can
go for standard MWPC option.

           MWPC            GEM                Micromegas
Rate       10^4Hz/mm^2 >5x10^5Hz/mm^2 10^6Hz/mm^2
Gain       High 10^6     low 10^3 (single) High > 10^5
                         >    10^5    (multi
Gain       Drops      at Stable         over Stable    over
stability 10^4Hz/mm^2 5*10^5Hz/mm^2 10^6Hz/mm^2
2D         Yes           Yes                 Yes
Readout ?
Position > 200 µm         50 µm              Good < 80 µm
Time       ~ 100 ns      < 100 ns            < 100 ns
Magnetic High            Low                 Low
Cost       Expensive,    Expensive(?),       Cheap, robust
           fragile       robust

GEM (Gas Electron Multiplier) works on the principle of using
very fine holes on polyimide foil as multiplying unit. Thin
metal-coated polyimide foils are chemically etched to form
high density of holes. On application of a voltage gradient,
electrons which were released on the top side drift into the
hole, multiply in avalanche and transfer to the other side.
Proportional gains above 10^3 are obtained in most common
gases. For obtaining higher gains, one can use multistage

In case of MICROMEGAS, thin metal grids (Cu, Al, Ni etc) of
10-50 micron pitch and 3-10 micron thick placed before the
readout electrode is used for amplification.
Detailed studies are available in the literature on gas
composition, gain variation, discharge probability etc for these
type of detectors. Even though they are being used in some of

the existing experiments, but R&D is ongoing in big way at
various labs for their use in RHIC, ILC etc.

We have made detailed study of GEM and Micromegas options
and following points come which one needs to consider before
taking final decision.


        High (100 mm) pitch
        Direct electron signal, no losses
        Efficient ion collection
        Easy to build
        Robust to aging
        Multi-stage structures provide large gains (103-104)
        Low mass construction no wire frames

Now, issues related to GEM are:
      Only a few sources of supply
      Large area GEM foils are difficult to fabricate. Small
        foils leave large dead areas in the tracking plane.
      Single stage GEM gain is low
      Expensive (relative)

      High (50 mm) pitch
      Direct electron signal no losses
      Funnel effect, so very efficient ion collection
      Easy to build dead zones potentially small
      Robust to aging
      Good electro-mechanical stability large gains (10^3
      Low mass construction no wire frames

    Choice of wire mesh : Woven vs. electro-formed vs.
     etched copper clad kapton

   Bulk MICROMEGAS – pillars of photo resist. Also other
    spacers like fish line
   How to optimize ion backflow? Practical limitations of
    mesh aperture(LPI)
   Minimizing discharges due to heavily ionizing particles (
    nuclear recoils)
   Choice of gas mixtures
   Muon tracking at high rates: how to widen pad response
   Need of resistive coating ( graphite?)
   Other readout schemes : Second anode mesh and a
    separate readout pad plane
   Practical construction :
                   How to paste mesh to frame?
                   How to protect mesh edges?
                   How much Minimum frame width?
                   How to tap HV connection?
                   Frame material, rigidity.
                   Mesh sag due to temperature fluctuations.
                   Invar mesh (shadow mask of CRTs)?
                   Effect of pad ridges on field uniformity.

The research and development activities based on above
discussions will concentrate on the design of large area
detectors composed of GEM and Micromegas modules, and
on the optimization of the module granularity in order to
end up with a reasonable number of readout channels.

We have some experience in working with GEM and now we
have started working on building GEM-based detectors from
few GEM foils procured from CERN. Our plan includes
building two chambers, one with single layer GEM and other
with double layer and then we will test them using radioactive
source. We plan to prepare the readout layout such that fast
chips to be used for CBM can be accommodated. Detector
laboratory at GSI has been collaborating with us in this


Working with high rate front end electronics is a challenge and
integration of these electronics without large dead space and
heat dissipation are to be worked out. These electronics need
to be radiation hard, and the FEE has to be highly integrated
for cost reasons. Detailed R&D is required to optimize the
Application Specific Integrated Circuits (ASICs).
The general plan in CBM is to design an ASIC which will be
used in most of the CBM detectors. One basic criterion is the
chip has to be fast. CBM has collaborated with DETNI
collaboration and has already come up with a version of chip
which can readout at 30MHz speed. We, at VECC will be
receiving some of these chips for testing and GSI has given a
proposal for collaboration in that respect so that we
participate in some of the working groups in design of next
generation of this chip. Our experience in development of
MANAS and INDIPLEX chip will help tremendously.

Indian Contribution:

We plan to contribute from India in most of the chambers in
for muon system. This amounts to design and fabrication of
the chambers after detailed R&D. Based on experience of
working in gas detectors at STAR and ALICE, we will have
several well equipped gas detector laboratories in the
collaborating institutes for production.
We also strongly feel that for acquiring latest detector
technology we need to work on front end electronics and data
acquisition system. Even if the cost constraint allows us to
work on few of the stations, but we would like to work on
complete system e.g. detector fabrication, slow control, FEE ,
DAQ and trigger. We will benefit from a close collaboration
with European institutes which are experienced in the
design of electronics and data acquisition systems. This
effort on working on electronics aspect will need to involve
Indian electronic Industry in a big way.

With our experience on development of MANAS, we believe it
will be possible for us to undertake this part of the project in
collaboration with Indian industries.

Participation in large way for muon project will help us to
achieve following goals:

   We will have a detector which is most important in an
    experiment and we will have the opportunity to get
    maximum credit from its discovery potential.

   Being the most important detector in the experiment, we
    will have access to latest detector and electronics
    technology in collaborative institutes.

   Due to large nature of the job involved (design and
    building of 16 chambers itself is big job apart from FEE
    design), all Indian collaborators will have the opportunity
    to get involved in big way in the form of setting up of labs
    in each institute etc.

Participation in analysis

We have already started simulation in big way and with our
existing knowledge on use of grid technology in ALICE we will
be able to take a leading role in data analysis, in particular in
the analysis of the dimuon data which will be a highlight
of the CBM physics program.

 A tier-2 centre has been setup at VECC for working with data
from ALICE and this centre is already in operation. VECC tier-
2 centre is executing jobs submitted from all over the LHC
grid. As CBM will also collect large amount of data and will
likely execute them in grid format, so we can make use of our
grid in extended format for that work.

 We have gathered substantial expertise in Object Oriented
(OO) programming and ROOT which is being used as

framework for CBM simulation and will likely to be used for
CBM data analysis. We therefore strongly feel that we can
make use of our expertise in this area and can take a lead role
in data analysis.


 We have build up an Indian CBM collaboration consisting of
several Indian institutes and Universities.

Partial list is given below, in the bracket approximate number
of participants from each institute is given.

     Variable Energy Cyclotron Centre(14)
     Saha Institute of Nuclear Physics (4)
     Institute of Physics(3)
     Panjab University(3)
     Rajashthan University(2)
     Aligarh Muslim University(5)
     Calcutta University(1)
     Banaras Hindu University(2)
     Indian Institute of Technology-Kharagpur (1)
     Guahati Univ (2)
     Srinagar Univ (2)

Apart from them following institutes have shown interest,
   Bhabha Atomic Research Centre, Mumbai
   Jammu University, Jammu.

In addition to these Indian collaborators, CBM collaboration
has following International participants.

Univ. Heidelberg, Phys. Inst., Univ. HD, Kirchhoff Inst.
Univ. Frankfurt, Univ. Kaiserslautern, Univ. Mannheim
,Univ. Marburg, Univ. Münster, FZ Rossendorf, GSI Darmstadt

Krakow Univ.,Warsaw Univ.,Silesia Univ. Katowice

LIP Coimbra
NIPNE Bucharest

Wuhan Univ.

Nikosia Univ.

Czech Republic:
CAS, Rez,Techn. Univ. Prague

IReS Strasbourg

KFKI Budapest, Eötvös Univ. Budapest

Korea Univ. Seoul, Pusan National Univ.

Univ. Bergen

RBI, Zagreb


And several institutes from Russia and Ukraine.

Collaborators for Muon Projects:

It is expected that with time active participants in muon
projects will be large fraction of CBM collaboration, as muon
detectors will be most important detectors in CBM. Currently
following institutes are actively working on this project.
GSI- Germany, Univ of Heidelberg, several Russian Institutes.


The spin-offs of this participation apart from interesting
physics goals are manifold.

  (1) Use of advanced gas detector technology for medical
      imaging. It has been demonstrated by several authors
      that GEM and MICROMEGAS can be successfully used
      for obtaining high resolution images and as they can be
      operated very fast, so will reduce the dose for this kind
      imaging significantly. They can also be used for
      radiography for different materials.
  (2) Collaboration in ASIC development is another area
      where this participation will help tremendously. As all
      CBM detectors will be very fast, so electronics
      development has to be very fast and from design point
      of view that is a challenge. We will be part of the
      collaboration involving advanced labs in Europe, so this
      will give us the opportunity to gather expertise in this

Communication to CBM Collaboration so far

We are in constant communication with CBM collaboration
and most of the Indian collaborators are voted as official CBM
collaborators. We have attended 2 CBM collaboration meetings
and presented our simulation and detector works.

CBM has chosen Subhasis Chattopadhyay as co-ordinnator
for an EU-sponsored project for “development of muon
detectors in CBM”. EU will provide upto 2MEuro for this R&D
project, it will start in 2007 and will end by 2011 before the
building of detector starts.

CBM has some positive decisions in deciding the muon option
as their dilepton option. Muon results are more promising
compared to electron results and muon detectors are feasible
to be made.


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