Project Description: Acquisition of a Hadron Blind Detector for
Diagnostics of the Quark-Gluon Plasma at RHIC
Research Activities
Introduction
During the first 10 microseconds following the Big Bang, the temperature of the universe
was so high that ordinary hadrons such as protons and neutrons could not form. Instead, the
dominant form of matter was unbound quarks and gluons in a state referred to as quark-gluon
plasma. Such a plasma interacts via the strong interaction, rather than electromagnetic, but is
expected to manifest many of the same features as classical plasmas, such as screening and
collective effects. The extraordinarily high temperature of the epoch just after the Big Bang,
approximately 2 x 1012 K (200 MeV in energy units), is achievable today only via
accelerator-based experiments, which collide heavy nuclei at very high energies. The energy
density of this kind of matter far exceeds that of normal nuclear matter, and indeed of other
plasmas currently accessible. The study of the physics of quark gluon plasma has been
identified as a scientific priority in the National Academy Reports “Connecting Quarks with
the Cosmos: Eleven Science Questions for the New Century” and “Frontiers in High Energy
Density Physics -The X-Games of Contemporary Science”.
Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National
Laboratory probe this form of matter inside the fireball created when two massive nuclei
collide at high energy, by carefully examining a subset of the thousands of particles that
emerge from the collision. Present estimates based on the striking initial results from RHIC
indicate an energy density near 2 x 1030 J/cm3, exceeding that of ordinary nuclear matter by
two orders of magnitude, and more than a factor of 10 above the threshold estimated for
quark-gluon plasma formation. While the lifetime of the fireball is only about 1.5-3 x 10-23
sec, precision probes of the plasma state generated early in the collision by high momentum
transfer scattering of quarks and gluons in the incoming nuclei can be used to map the
evolution of the matter. To find the properties of this new kind of plasma, it is important to
determine its temperature, collision frequency, thermal conductivity, color dielectric
properties, radiation rate, radiative-absorptive coefficients and opacity. First glimpses into
many of these properties are becoming available from the data collected at RHIC so far.
However, there is as yet no experimental information whatsoever about the temperature
reached by the plasma.
Measuring the temperature, one of the most basic plasma properties, requires detecting
thermal radiation consisting of real and virtual photons emitted by the plasma. Unfortunately,
the enormous backgrounds from decays of the thousands of hadrons (particles formed later in
the collision) have prevented reliable measurement of the thermal radiation to date. This
proposal aims to solve this problem by acquisition of a detector using novel technology to
directly measure and reject the otherwise overwhelming background of electrons and
positrons from unstable hadron decays. This will allow detection of electron-positron pairs
from virtual photons; their distribution in energy reflects the temperature of quark gluon
plasma.
Of at least equal importance, rejection of the background will allow a search for evidence of
the restoration of chiral symmetry. As the universe cooled down from the quark gluon
plasma state, the vacuum ground state became filled with a condensate of quarks that
spontaneously broke the chiral symmetry present at high temperature. This step gave rise to
hadron masses which are large compared to the masses of light quarks. The underlying
mechanism which connects symmetry breaking, mass generation and ground state properties
is thought to account for more than 90 % of the visible mass in the Universe. Extensive
theoretical work predicts that chiral symmetry should be restored at the high temperatures
reached at RHIC, and that the properties of particles can be significantly different. Decays of
resonances such as the  and into electron-positron pairs allow study of mediuminduced changes of the resonance masses and widths. Since resonance masses trace the chiral
symmetry order parameter, their measurement addresses directly the origin of chiral
symmetry breaking.
To solve the background problem, we propose to acquire an instrument: a Hadron Blind
Detector (HBD). Our proposal is for support for construction of this novel instrument, by a
consortium of groups, including Stony Brook, the Weizmann Institute, Brookhaven National
Laboratory, Florida Institute of Technology, and the University of Tokyo. All R&D to
develop the HBD concept and design has been completed. Consequently, this proposal is to
acquire the parts for the detector, specialized tooling for evaporation of CsI and assembly at
Stony Brook, monitoring systems for operation of the detector, and support for undergraduate
students to participate, along with physicists and engineers from collaborating institutions, in
the construction and testing. Upon completion, the HBD will be taken to Brookhaven,
installed as a new subsystem of the PHENIX experiment, and used to study heavy ion
collisions at BNL’s unique RHIC facility.
PHENIX is a large, multipurpose experiment, built and operated by a 500-person
international consortium. As a PHENIX subsystem, the HBD will be read out by the
PHENIX data acquisition system. Thus the HBD detector construction includes front-end
electronics but not the rest of the readout chain, which will be provided as part of the
PHENIX infrastructure. Integration of the HBD into PHENIX will be done by the institutions
involved in construction, with support from PHENIX collaboration technicians, engineers
and physicists as part of the normal PHENIX operations activities; we do not request funds
for integration in this proposal. The HBD will be operated as an integral part of PHENIX,
with full participation by the PHENIX collaboration in operation, data analysis, and physics
studies. More detail on operational responsibilities and construction plan is given in the
Management Plan below.
The Hadron Blind Detector will be an absolutely unique piece of equipment. Its use as part of
the PHENIX experiment will allow measurements that would otherwise be impossible to
achieve anywhere in the world. The HBD will provide the best opportunity to directly
measure the temperature of the quark gluon plasma and map chiral symmetry restoration at
RHIC. The detector philosophy has been endorsed by the RHIC Detector Advisory
Committee and the PHENIX HBD upgrade is identified as a short-term goal in the NSAC
subcommittee report on Heavy Ion Physics.
Physics opportunities with the HBD
Real and virtual photons, which have no strong interactions with the surrounding quark
gluon plasma or hadronic matter, have long been considered one of the most promising
signals of physics of the early stage of heavy ion collisions; when the temperature and
density are highest, the thermal radiation rate is maximal. Thermal dileptons populate the
continuum region of the e+e- or - invariant mass spectrum, and in the absence of
non-thermal backgrounds, the tail of the transverse momentum spectrum of such pairs
reflects the initial temperature achieved in the collision. Unfortunately, nature is not kind
enough to provide a classic thermal radiation spectrum in particle collisions, as can be
seen in Figure 1, which presents a schematic view of the e+e- mass spectrum measured in
p+p collisions. There are peaks arising from the leptonic decays of hadrons, Drell-Yan
lepton pairs contributing a high mass continuum, and complex hadronic decays that
populate the other continuum regions. The latter are primarily Dalitz decays of 
mesons feeding into the low mass dilepton region, and charmed meson decays
contributing at intermediate masses.
Figure 1. Schematic view of di-electron invariant mass spectrum in p+p collisions.
In heavy ion collisions, the mass spectrum above is expected to be modified. In addition to
the expected presence of thermal radiation, several other modifications have been predicted.
Vector meson masses and widths, especially of the  meson, may be modified due to chiral
symmetry restoration and/or multiple interactions with neighboring particles in a dense
medium. The production of charmed quarks may be enhanced in high temperature matter,
leading to an increased yield of intermediate mass continuum lepton pairs. The yield of J/
may decrease significantly if color screening is present, decreasing the potential between
produced c-cbar pairs.
In the low mass region, the vector meson masses and decay widths should change. Onset of
chiral symmetry restoration may lower the mass of meson inside the dense medium [1]; as
the lifetime of the  is short, many of them decay while the temperature and density of the
medium are large. The dilepton spectrum should then reflect the in-medium mass, as the
decay leptons do not interact with the medium on their way to the detector. It has
alternatively been suggested that collisional broadening in a dense medium may affect the
observed width and mass of vector mesons [2] Such as effect would open additional phase
space below the vacuum lepton mass; both mechanisms lead to additional dileptons around
500 MeV/c2 mass compared to that in p+p collsions. Indeed an excess dilepton yield in this
mass region was observed in heavy ion collisions by the CERES collaboration at the CERN
SPS [3], as illustrated in Figure 2. The dotted lines show the expected sources of di-electrons
from various hadronic decays; their sum (thin solid line) clearly underpredicts the data. The
dashed line shows the expected contribution from unmodified mesons (including feeding of
the channel from  collisions in the hadronic medium), and is incompatible with the
measurement. The heavy solid line and dot-dashed line show expectations from broadened
and lowered-mass  mesons, respectively, and are closer to the data. The CERES
measurement is clear evidence for medium modification of vector mesons, but has
inadequate statistical precision and mass resolution to constrain the extent or type of
modification. The data do show, however, that the mass region between 0.3 and 1 GeV/c2
provides the best sensitivity to medium modification effects.
Figure 2. Invariant mass spectrum of electron-positron pairs measured by the
CERES collaboration at the CERN SPS at √sNN = 17 GeV [3].
A number of theoretical studies of conditions in RHIC collisions, and their effect upon
dilepton spectra have been carried out. These calculations use experimental observations to
constrain assumptions about the early stage, including large final baryon and anti-baryon
yields, high initial energy density (and therefore initial temperature, Tinit), large expansion
velocity, and evidence for short lifetime of an equilibrated, cooler hadronic phase. Unlike at
SPS energy, where Tinit is lower and the hadron gas phase rather long-lived, at RHIC the
plasma is predicted to outshine the hadron gas with real photons of energy 1-3 GeV [4]. Rapp
has also calculated the contributions to the dilepton spectra from various sources, including
decay of medium-modified mesons, shown in Figure 3 [5]. Thermal dileptons dominate the
low mass continuum, though the relative contribution from plasma and hot hadron gas phase
depends strongly on the relative lifetimes and Tinit. At intermediate masses, open charm
decays constitute a significant background, but this can be subtracted once the charm
spectrum is measured; the HBD helps reject remaining conversion and decay electronpositron pair backgrounds.
Figure 3. Predicted dilepton mass spectrum at RHIC [5].
It should be noted that the quark gluon plasma at RHIC appears to be strongly coupled [6,7].
This implies a large probability for formation of quasi-bound states at the temperatures
accessible at RHIC of approximately 2 Tc [7], and indeed evidence for such bound states has
been seen in the spectral functions of quarks in high temperature lattice QCD studies for
heavy and strange quarks [8]. Non color-neutral bound states of light quarks may also exist
[7]. The existence of such states in the plasma could have profound effects upon the dilepton
spectrum, adding new peaks at masses determined by the temperature of the plasma and
thermal masses of quarks in the strongly-coupled medium. As the strength of such peaks may
not necessarily be large, dilepton background is crucial to any search for these phenomena.
The apparent short lifetime of the equilibrated hadron gas phase late in the collision suggests
that strongly coupled plasma effects may exceed thermal radiation from the hadronic phase,
however, experimental data is crucial and currently non-existent.
In order to examine the very interesting features of the dilepton mass spectrum, measure the
yield and distribution of thermal dileptons and expected modifications to meson masses from
chiral symmetry restoration, hadronic backgrounds must be suppressed, or measured and
subtracted. The strategy adopted by the PHENIX Collaboration is to use a Hadron Blind
Detector to reconstruct and reject Dalitz decays and conversion pairs from the low-mass
region. The charm decay background in the intermediate mass region will be measured using
a highly segmented silicon microvertex detector to tag vertices displaced from the collision
vertex for electrons from semi-leptonic decays. This measurement will allow subtraction of
the charm decay background from the continuum spectrum. The excellent mass resolution of
PHENIX will allow precise spectroscopic study of the known resonances, even with the
reduced integrated magnetic field due to field-canceling for the HBD.
Hadron Blind Detector to Reject Dalitz Decay and Conversion Background
The Hadron Blind Detector requires large azimuthal coverage to detect both leptons from
hadronic Dalitz decays. The magnetic field in the center of PHENIX is canceled to almost
zero by running the inner coil of the central magnet opposing the larger outer coil, which
allows a very low momentum threshold for electron detection and ensures good acceptance
for the vast majority of Dalitz decays, regardless of asymmetry. This magnetic field
configuration also allows rejection of electrons and positrons from photon conversions, as
they move together and create a double electron signal in the HBD. The HBD will improve
signal to background in both the low and intermediate mass regions, described in detail in the
performance section below.
Experiments utilizing the HBD
The HBD will be installed into PHENIX when its construction and testing are complete,
during one of the planned RHIC shutdown periods. We plan a commissioning and test data
taking period following initial installation, in preparation for physics data production using
the HBD in full energy Au+Au collisions, in p+p collisions to provide a baseline
measurement with no medium modification, and in d+Au collisions to study initial state
effects and the influence of surrounding cold nuclear medium upon the dilepton spectrum.
Though the RHIC running schedule in future years is not firmly known at this time, and
depends upon available funding, schedule guidance is available from the recent NSAC
subcommittee on Heavy Ion Nuclear Physics report. A long run of Au+Au at full RHIC
energy is envisioned for FY2008, optimal commissioning of the HBD would be done using
p+p collisions, which will take place in 2006/2007. Comparison runs with p+p collisions and
d+Au collisions would take place in the following RHIC running periods.
Utilization for Research, Training and Education
The HBD will produce data usable by all PHENIX collaborators. Regular use of the data for
research and graduate student training will be done under the leadership of the co-PI’s of this
proposal. Personnel who will utilize the HBD heavily for research are given in Table 1. In the
near term, graduate and undergraduate students from Stony Brook, the Weizmann Institute,
and the University of Tokyo will gain hardware training and experience in the construction
and commissioning of the detector. The graduate students, including Sarah Campbell,
Torsten Dahms, Jason Kamin, Michael McCumber and Anne Sickles from Stony Brook, will
take on central roles, and help supervise the work of younger students. Following
commissioning, postdoctoral fellows, and graduate and undergraduate students will utilize
data from the HBD to study thermal photons and chiral symmetry restoration.
Institution
Senior personnel Postdoc. fellows Grad. Students Undergrads
Stony Brook
3
2
5
6
Weizmann
3
1
2
University of Tokyo1
1
2
BNL
2
2
Florida Institute of
2
2
of Technology
Table 1. Researchers utilizing HBD for scientific research in the next three years.
Description of the Research Instrumentation
Detector overview
Figure 4 HBD layout (left); Cherenkov photon detection scheme (right).
The HBD layout is shown in the left panel of fig.4. The two separate volumes are the two
HBD arms around the beam pipe with the entrance mylar window. The RHIC beam pipe is
located at the center, and the PHENIX central arms begin well beyond the outer wall of the
HBD. The volumes are filled with CF4 Cherenkov gas radiator. Inner walls of the final
detector hold 12 photodetection modules in each arm, the arrangement is visible on the left
side of the figure.
Cherenkov photons produced in the radiator by leptons from the beam collisions are
converted into photoelectrons (p.e.) by the CsI photocathode evaporated on the top face of
each photodetector. This is shown in the right panel of fig.4. The CsI quantum efficiency
exceeds 80% at the UV edge of the CF4 transparency region (ħω=11.5 eV). The target
number for a figure of merit N0=800 cm-1 should result in, on average, 36 p.e. per electron
for a radiator length of 50cm; this is aided by a windowless configuration and use of CF4.
The photoelectrons are captured inside the GEM holes by electric field. Each detection
element consists of three layers of Gas Electron Multipliers (GEMs), and provides gas gain
≈104, sufficient to operate the HBD in a safe mode in the environment of heavy ion
collisions. It should be noted that the photon feedback from the last GEM back to the
photocathode on the first GEM is basically impossible due to geometry. The signal is read
out through the pads and further amplified by the electronics placed on the outer surface of
the HBD. The pad size (A=7cm2) is slightly smaller than the size of the spot from Cherenkov
light produced by a single particle, such that one particle normally fires 2-3 pads.
The drift field (Ed) in the gap between the top GEM and the mesh is chosen to repel
ionization electrons due to charged particles in the gap away from the GEMs. At the same
time, the photoelectrons emerging from the surface can be captured into the GEMs with
nearly 100% efficiency, resulting in a high sensitivity to Cherenkov light and “blindness” to
charged particles. An ionizing particle can still produce a signal in the pad above the
threshold (equivalent to approximately 2-3.5 p.e., providing factor of 50 m.i.p rejection at a
threshold of 10 p.e.), however it cannot fire more than a single pad. These values were
confirmed using test beam at KEK into a pre-prototype HBD detection element. Some
background remains from scintillation in CF4, near the edge of the CsI sensitivity. In the
most central heavy ion collisions this introduces a background comparable to the background
from the charged particles. Extensive studies of various HBD elements done at the
Weizmann Institute [9,10] prove the concept to work. The threshold for pions to radiate in
the HBD is approximately p= 4 GeV/c. The most important HBD parameters are:






Response:
Dimensions:
Radiator:
Photocathode:
Detector:
Read-out:
N0~800cm-1; Np.e./lepton ~36; equivalent Np.e./M.I.P. ~ 3
length in Z:0.7m; Ø:1m; split in two; photoactive area 1.5m2
gas: CF4; =28; upper cut-off: 11.5eV; length: ~50cm
material CsI; extraction mode: reflective; lower cut-off 6.0eV
element: triple GEM; working gas: CF4; gain: M~104
Amplitude; 2.4k pads; 1 pad per FEE module
GEM foils
The main working element of the HBD is a Gas Electron Multipliers (GEM). Each GEM is a
Kapton film of ~50um thickness with 5m of Cu on each side. A regular pitch (140um) of
holes with diameter 60-80um is etched through the copper and Kapton. A special coating
(Ni+Au) is be added to the GEMs for CsI evaporation. The GEMs are produced in several
laboratories in the world, however the most reliable and controlled GEM manufacturing is at
CERN. The GEMs for HBD have area of 25x25cm2 and are divided into 25 sectors each.
This granularity in the high voltage has been demonstrated to prevent GEMs from permanent
damage in case of a discharge. For the HBD, 12 detection elements per arm are needed
(2x12x3), plus 30% spare GEMs. Thus the total number of required foils is 96.
Front End Electronics
Signals amplified in the GEMs are collected on the pads and then transferred to the back side
of the detector to hybrid charge sensitive preamplifiers and shapers located on the outer
surface of the HBD. These drive the signals to Front End Electronics (FEE) modules
consisting of shaping amplifiers and digitizing electronics located up to 5m from the detector.
The low noise preamplifiers (equivalent noise less than 0.3p.e.) are designed by the
Instrumentation Division of Brookhaven National Laboratory. The FEE modules, which are
part of the PHENIX infrastructure and not part of this proposal, are being designed and built
by Nevis Lab at Columbia University. The HBD has 2304 readout channels.
Detector performance monitoring systems
The HBD concept uses the same CF4 gas as Cherenkov radiator and working gas for detector
elements. Although the unique properties of CF4 make it a very good choice for both, its
optical properties can be significantly diminished by p.p.m.-level of contamination such as
H20 and 02. Furthermore, some contaminations in the presence of ionizing radiation can react
with the CF4 and form chemically active substances dangerous to the detector. Therefore,
control over the gas quality is absolutely crucial. Other parameters such as gas gain in the
GEMs and CsI quantum efficiency should also be monitored. The gas and gain monitoring
systems are part of this request.
The CF4 optical transparency will be measured at the entrance of the detector and at the exits
of each arm. The system should be located as close as possible to the detector to ensure that
the measurement relates to the detector gas volume itself. Light from an Hg lamp will be
passed through a monochromator selecting a particular wavelength, and then selectively
through one of three volumes with a sample of gas. The light attenuation will be measured by
differential signals from two calibrated PMTs. Sweeping the wavelength, one can measure
gas optical transparency and detect water and oxygen contaminations as well as other
contamination from leaks, outgasing or ionization. This monitoring should be continuous
once the HBD is installed, regardless of the operation status of HBD.
As water and oxygen are the most harmful contaminants to the gas optical properties, and
furthermore, water can permanently damage the hygroscopic CsI photocathodes, additional
devices for measuring oxygen and water content will be installed in the PHENIX Gas Mixing
House, about 70m away from the detector location. Together with the monitoring system
they will ensure that the gas supply lines deliver high purity gas to the detector. The CsI
quantum efficiency will be measured using an existing system at BNL. In-situ monitoring is
still under discussion, but this can be accomplished, for example, by illuminating the detector
with a UV lamp and switching off the GEM field to collect photoelectrons on the mesh.
The HBD gas gain monitoring will utilize a standalone pilot chamber (Canary Chamber)
sharing the same gas supply lines as the HBD. An HBD-type detection element consisting of
three GEMs will be installed to control pressure and temperature variations which affect the
gas amplification. Once the HBD is operational in PHENIX, the gas amplification can also
be measured minimum ionizing particles with the detector operated in the “reverse bias”
configuration (i.e. with the drift field reversed so as to collect the charge produced by
ionizing particles in the drift gap), and with electrons in the data sample. This will provide
an ultimate answer, but routine monitoring during data-taking will rely on the Canary
Chamber.
Detector assembly area
The HBD assembly will be done at Stony Brook, including evaporation of CsI on the GEM
surface, GEM installation and all cabling. Because the CsI is hygroscopic, GEMs with CsI
photocathode cannot be exposed to the atmosphere for extended periods of time (minutes).
The HBD assembly must be done in a dry gas atmosphere. The best solution is to keep the
detector elements in a Glove Box (GB) under continuous flow of a dry inert gas. Since the
HBD assembly is to be done inside the GB, we require sufficient space for all relevant clean
tools inside the GB. The approximate dimensions of the glove box are 2.5m x. 1.3m x 1.5m
Access on both sides, a door to pass the detector through, and an antechamber which can be
flushed with gas are required. The GB and its gas supply will be installed in the existing
clean space at Stony Brook, which was used 4 years ago for drift chamber construction.
A large volume evaporator (0.15 cubic meters) which can be used to deposit CsI on the GEM
foils exists at Stony Brook. It utilizes, however, an old and unreliable diffusion pump. We
would prefer to upgrade the evaporator with a turbopump for this project.
HBD Performance Studies: Proof of Principle
Tests were performed with a triple-GEM detector element in CF4, demonstrating that the
detector can operate in a mode which is blind to hadrons. The results are shown in fig.5.
Figure 5. Demonstration of hadron blindness. Blue and red points are normalized
photoelectron efficiencies at drift field Ed=0 for two GEM voltage settings. The green
curves show the ionization charge collection efficiency in the gap above the first GEM
as the function of Ed.
The measurements shown in red and blue are normalized signals from the detector element
when the CsI photocathode is illuminated with UV light, taken at two different GEM voltage
settings. The transfer field between the GEMs was varied accordingly to ensure that the
number of electrons transferred from one GEM layer to another is the same. Both curves
show the same behavior, which basically does not depend on the Ed around zero. The picture
drastically changes for charge produced by ionizing particles, shown in green. Even a rather
weak negative field applied between the mesh and the first GEM causes reduction of the
ionization signal by a factor about 10, making the detector blind to ionizing particles while
leaving its sensitivity to the UV at more than 95%.
The electron avalanche development in pure CF4 was also studied in great detail. The HBD is
designed to work in the environment of heavy ion collisions, which produce many strongly
ionizing particles with the potential to cause a discharge and damage the detector. Detector
aging in CF4 was thoroughly studied, and neither the photocathode efficiency nor the GEM
gain exhibited any aging effects [10]. It was also demonstrated that CF4 gas possesses an
avalanche quenching property as shown in fig.6.
Figure 6 Avalanche quenching in CF4
The measurements were initially done with a calibrated preamplifier (circles), such that the
total avalanche charge induced by strongly ionizing α-particles is known. The preamplifier
saturates when total charge in the avalanche is above 107. Next, the preamplifier was
removed and the signal measured directly from the pad (open and closed boxes), so that the
avalanche charge is known at a given GEM voltage. One can see that at any voltage the
avalanche never grows above 108 electrons, i.e. GEMs in CF4 have a natural limit on the
charge in the avalanche. This greatly reduces the probability of detector discharge and
damage. This result has been recently published [10].
Background rejection.
Tests done with cosmic ray muons at the Weizmann Institute show that the number of
photoelectrons for Cherenkov-emitting particles is consistent with the expected number of
36. This result and the measured response to the ionizing particle measurement were used in
the full detector Monte-Carlo simulation integrated into the PHENIX standard simulation. A
single central HIJING event, displayed using the HBD event monitoring software, is shown
in fig.7. HIJING calculates only the background for dilepton measurements. Half of the
electrons in the figure are from conversions or Dalitz decays, and are not separated from their
partner due to the field cancellation in the HBD. The rest are not from the vertex. The left
panel shows pad clusters (groups of adjacent pads) produced by the Cherenkov photons;
color coding of the charge in each pad is shown on the right. Single red dots are the positions
of all charged particles on the detector plane. The right panel shows the result of the cluster
reconstruction algorithm. Clusters containing a red dot arise from electrons detected in the
existing PHENIX setup. Other clusters are produced by electrons which cannot be detected in
the current PHENIX. Reconstruction of two close or merged clusters indicate that the
electron measured in the Central arm has a partner and is likely produced by π0-Dalitz decay
or -conversion, and should be rejected. Merged clusters, which correspond to more than one
electron, are identified by the sum of the pad amplitudes in the cluster.
Figure 7. Central HIJING event visualized in HBD event display. Left panel shows
clusters from the Cherenkov light and small red dots represent the positions of the
charged particles on the detector plane. The same event with only reconstructed
clusters is shown on the right. The red dot indicates the electron detected in the
PHENIX Central arm.
The HIJING event generator is used to simulate Au+Au events to allow study of high
multiplicity effects on the detector performance. The background rejection with the HBD,
using HIJING central Au+Au collisions with multiplicity of 940 and 650 charged particles
per unit rapidity is illustrated in Figures 8 and 9, respectively. The three lines in Figure 8 are
Signal (blue), Background (red), and their ratio at each rejection step. The signal used for this
figure is φe+e- measured in PHENIX; this mass region has large background levels which
the HBD should improve. The PHENIX detector response was fully simulated, along with
the HBD. The situation without the HBD is shown as step number 1, and gives the
performance level to which all three curves are normalized, including detector effects such as
electron misidentification. In step two, PHENIX-detected electrons are matched to the HBD.
In the third step clusters with amplitudes >90fC (~60 p.e.) are eliminated to remove double
electrons. Finally, in the fourth step, clusters with a partner within 200mrad are rejected.
Approximately 50% of the signal survives all cuts. The S/B ratio increases by a factor of
about 100, showing that the low mass pair region (<1GeV/c2) becomes accessible.
Figure 9 shows the combinatorial background with and without the HBD, compared to the
expected signal level (without thermal radiation or medium modification), shown as the solid
black line. The solid red line compared to the blue points illustrates the impact of the HBD.
Figure 8. Background rejection steps. See text for definition of each rejection step.
Figure 9. Dielectron invariant mass spectrum showing effect upon combinatorial
background of rejection with the HBD.
The HBD reduces combinatorial background by a factor of 100 in the low mass region;
signal to background ratios of approximately 1:2, 1:5 and 1:20 will be achievable at 300, 400
and 500 MeV mass, respectively. At higher masses the signal to background ratio improves
by a factor of 25 and reaches 1:10. Thus the improvement from the HBD will be very
significant. Figure 9 assumes ideal electron identification both with and without the HBD and
the lower (more realistic) charged particle multiplicity, so the improvements appear less
striking than in Figure 8.
Impact of Infrastracture Projects
The Hadron Blind Detector will be an absolutely unique piece of equipment. Its use as part of
the PHENIX experiment will allow measurements that would otherwise be impossible to
achieve anywhere in the world. The HBD will provide the best opportunity to directly
measure the temperature of the quark gluon plasma and map chiral symmetry restoration at
RHIC. The involvement of Stony Brook University in the project will attract graduate and
undergraduate students and give them experience with forefront technology, with potential
applications in other areas of basic and applied research. We note also, that other applications
of GEM detectors are already being investigated, and are of interest for medical imaging. We
plan to continue our collaboration with the group of Dr. Wei Zhao in the Radiology
department at Stony Brook [11]. She is particularly interested in GEMs for breast imaging
readout. Joint R&D in the future would utilize the infrastructure developed for this project.
The construction and testing activities will be carried out at Stony Brook. The project will
provide unique opportunities for students at Stony Brook to work on building cutting edge
equipment. Based on our excellent experience with students, in particular undergraduate
students, working on previous PHENIX detector construction projects carried out by our
group, we expect that 4 to 6 students will become an integral part of the proposed project.
Specifically we anticipate that they will run the CsI evaporator, map CsI photocathode
quality, and perform other Q/A tests on the detector. The group has a strong tradition of
recruitment of a diverse group of students. Currently the group has 5 women: 3 students (1
undergraduate and 2 graduate), a postdoc and a professor out of just over 20 members.
Management Plan
The HBD project is part of the PHENIX project and its construction and operation are
integrated into the PHENIX management structure, as specified in the PHENIX bylaws. The
HBD operation and maintenance will be performed as a standard part of the PHENIX
experimental activities. The HBD will become a subsystem similar to other, already
operational, parts of PHENIX. Management of PHENIX operations is provided by the
PHENIX Detector Council, with a representative for each detector subsystem. The PHENIX
Detector Council (DC) also advises PHENIX management on design, construction and
integration of new subsystems, including the HBD. The DC is co-chaired by the PHENIX
operations manager (E.O'Brien) and upgrades manager (A.Drees). The responsibility for the
HBD subsystem will be shared by the subsystem leader, Prof. Itzhak Tserruya (Weizmann
Institute) and his deputy, Dr. A. Milov (SBU and BNL). The subsystem leader reports to
PHENIX management and represents the HBD consortium in the DC. We note the important
and visible role of Prof. Tserruya and Dr. Milov in the operation as well as construction of
the HBD. They are key co-investigators on this proposal. Their biographical sketches are
attacked as supplemental information.
Routine maintenance and operating costs for PHENIX are supplied as part of the RHIC
operations funds provided by the Department of Energy to BNL. Operation of the HBD is
part of operating the PHENIX experiment, and routine work and performance monitoring
will be performed by the scientists on PHENIX data taking shifts. The consortium building
the HBD will provide the leadership, expertise, and scientific manpower for the more
complex aspects of HBD operation and maintenance. The groups already have in place the
technical expertise in photon detection, detector operation and troubleshooting, gas systems,
and monitoring and analysis software development. The same people who build the detector
will commission it and provide expertise to the PHENIX collaboration to operate the HBD
and provide physics data for the collaboration to use. With current funding levels, we do not
envision a need to request supplemental operations resources.
The construction and testing activities as well as the detector commissioning and operation
will be carried out in a joint effort of a consortium between Stony Brook, The Weizmann
Institute, Center for Nuclear Studies/University of Tokyo, Brookhaven National Laboratory
personnel, and Florida Institute for Technology. Leadership responsibility for construction
tasks is as follows:
o Custom detector components(vessel and GEM detectors): Dr. I. Ravinovich (Weizmann)
o Assembly and testing: Dr. A. Milov and Prof. T. Hemmick or Prof. B. Jacak (SBU)
o Monitoring systems: Dr. C. Woody (BNL)
o Front end electronics: Dr. C. Woody (BNL)
The planned schedule of HBD construction is as follows:
o Production of mechanical parts at Weizmann Institute: begins summer 2005, ready for
shipment in September/October 2005.
o Production of GEM panels at Weizmann: similar schedule to mechanical parts.
o Glove box at Stony Brook: order in September 2005, ready for operations in Nov. 2005.
o CsI evaporation at Stony Brook: December 2005 and January 2006
o Assembly into detector modules at Stony Brook: in parallel with evaporation in assembly
line fashion. All modules complete in mid-February 2006.
o Q/A on parts proceeds in parallel with CsI evaporation and module assembly
o Final detector assembly and testing: February 2006 for completion in early March 2006
o Preamplifier production: begin September 2005, complete in 1-2 months.
o Monitoring systems: 4 months delivery time; complete in January 2006
As all PHENIX subsystems, integration of the HBD into the PHENIX experiment will be
provided by BNL and is outside the scope of this proposal, which is to acquire the detector
itself. Integration includes the readout of the detector HBD information into the PHENIX
DAQ system, the gas supply system (which become part of the PHENIX gas system) and
mechanical integration into the PHENIX experiment. Installation activities will be carried out
by members of the HBD consortium in close collaboration with the PHENIX operations
group at BNL.
References
1.
2.
3.
4.
5.
6.
7.
G.E. Brown and Mannque Rho, Phys. Rept. 269, 333 (1996).
J. Wambach and R. Rapp, Nucl. Phys. A638 ,171 (1998).
G. Agakichiev, et al. (CERES Collaboration), Phys. Lett. B422, 405 (1998).
S. Turbide, R. Rapp and C. Gale, Phys. Rev. C69, 014903 (2004).
R.Rapp, nucl-th/0204003 (2002).
M. Gyulassy and L. McLerran, nucl-th/0405013, accepted in Nucl. Phys. A (2004).
E.V. Shuryak and I. Zahed, Phys. Rev. D70, 054507 (2004) and hep-ph/0406100
(2004).
8. H. Asakawa and T. Hatsuda, Phys.Rev.Lett. 92, 012001 (2004) and Prog. Theor.
Phys. Suppl. 149, 42 (2003).
9. A. Kozlov, I. Ravinovich, L. Shekhtman, Z. Fraenkel, M Inuzuka and I Tserruya,
Nucl. Instr. and Meth. A523, 345 (2004).
10. Z. Fraenkel, et al, “A Hadron Blind Detector for PHENIX at RHIC”, submitted to
NIM, January, 2005.
11. Mary F. Ifferte, Wei Zhao, Barbara V. Jacak, Paul O’Connor, Bo Yu and Pavel
Rehak, Medical Physics 29, 319 (2002) ”The X-ray Sensitivity of Amorphous
Selenium for Mammography”