You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
in this paper we present a summary of the r@xmath1d results and expected performance of a hadron blind detector ( hbd ) which has been developed as an upgrade of the phenix detector at the relativistic heavy ion collider ( rhic ) at bnl
. the hbd will allow the measurement of electron - positron pairs from the decay of the light vector mesons ( @xmath2 , @xmath3 and @xmath4 ) and the low - mass ( @xmath5 1 gev / c@xmath6 ) pair continuum in au+au collisions .
dileptons are valuable probes to diagnose the hot and dense matter formed in relativistic heavy - ion collisions .
the physics potential of this probe is fully confirmed by results from lower energy experiments .
the most prominent result is the strong enhancement of low - mass electron pairs observed by the ceres experiment @xcite at the cern sps and recently confirmed by the na60 results @xcite .
this enhancement triggered a wealth of theoretical activity and was quantitatively reproduced only by invoking the thermal radiation from a high density hadron gas ( @xmath7 ) with in - medium modification of the intermediate @xmath2 meson which could be linked to chiral symmetry restoration @xcite .
the measurement of electron pairs is however a very challenging one .
the main difficulty arises from the huge combinatorial background , i.e. uncorrelated pairs formed by tracks from unrecognized @xmath8 conversions and @xmath9 dalitz decays . for example
, the analysis of run-4 data taken with the present phenix configuration @xcite shows that the signal to background ratio , s / b , at the invariant mass of m @xmath10  500  mev / c@xmath6 is approximately 1/500 , making the measurement of the low - mass pair continuum practically impossible .
an upgrade of the phenix detector is therefore necessary for this measurement .
the present paper is focussed on the comprehensive r&d program that we have carried out over the past two years to develop such an upgrade @xcite .
the upgrade consists of two elements : a ) installation ( foreseen in the original design of phenix ) of an inner coil which results in an almost field - free region close to the vertex , extending out to @xmath10 60 cm in the radial direction ; b ) the major and challenging element of the upgrade is a hadron - blind detector ( hbd ) located in this field free region . fig .
[ fig : hbdinphnx ] shows the layout of the inner part of the phenix detector together with the location of the coils and the proposed hbd .
the main task of the hbd is to recognize and reject @xmath8 conversions and @xmath11 dalitz decays .
the strategy is to exploit the fact that the opening angle of electron pairs from these sources is very small compared to the pairs from light vector mesons . in the field - free region , this angle is preserved and by applying an opening angle cut one can reject more than 90% of the conversions and @xmath11 dalitz decays , while preserving most of the signal .
based on monte carlo simulations , the main hbd specifications are : electron identification with a very high efficiency ( @xmath12 90@xmath13 ) , double hit recognition at a comparable level and moderate @xmath14 rejection factor of @xmath10100 .
we analyzed possible realizations of the hbd detector and finally adopted the following scheme : a windowless erenkov detector , operated with pure cf@xmath0 in a proximity focus configuration and directly coupled to a triple gas electron multiplier ( gem ) @xcite detector with a csi photocathode evaporated on the top face of the first gem foil and with pad readout .
the concept described in the previous section exhibits a number of very attractive features : a ) the choice of cf@xmath0 both as radiator and detector gas in a windowless geometry results in a very large bandwidth ( 6 - 11.5 ev ) and consequently in a very large figure of merit n@xmath15 ( @xmath10800  @xmath16 ) . with these unprecedented numbers ,
the number of photoelectrons n@xmath17 is expected to be of the order of 35 for a 50 cm long radiator ; b ) the readout scheme foresees the detection of the erenkov photoelectrons in a pad plane with hexagonal pads of size slightly smaller than the blob size ( @xmath1010 cm@xmath6 ) such that the probability of a single - pad hit by an electron entering the hbd is negligibly small . on the other hand , whenever a hadron gives a signal in the hbd , it will produce a single pad hit with an almost 100% probability .
this will provide a strong handle in the hadron rejection factor of the hbd ; c ) the relatively large pad size results also in a low granularity and therefore a low cost detector .
in addition , since the photoelectrons produced by a single electron will be distributed between at most three pads , one can expect a primary charge of at least 10 electrons / pad , allowing operation of the detector at a relatively moderate gain of a few times 10@xmath19 .
this is a crucial advantage for the stable operation of a uv photon detector .
however , many elements of the proposed hbd were new and had not been tested before in the laboratory .
a number of issues and questions were raised which required dedicated investigation . to address these questions and to demonstrate the concept validity ,
a comprehensive r&d program was needed .
the results of this effort are summarized below @xcite :    * we have shown that a triple gem detector with a reflective csi photocathode operates in a stable mode at gains up to 10@xmath20 in pure cf@xmath0 . *
a charge saturation effect occurring in cf@xmath0 when the total charge in the avalanche reaches 4@xmath2110@xmath22  e makes the hbd relatively robust against discharges .
* we carried out a test of a triple gem detector operated with pure cf@xmath0 at the proposed location inside the phenix central arm spectrometer .
the detector performed smoothly in the presence of au+au collisions exhibiting no discharges or gain instabilities .
* we have studied the basic parameters which determine the hbd performance . in particular , we measured the device response to mip s and to electrons .
large hadron rejection factors , well in excess of 100 , can be achieved while preserving an electron detection efficiency larger than 90% .
* we confirmed measurements of the csi quantum efficiency over the bandwidth 6 - 8.3  ev and extended them up to 10.3  ev .
extrapolation to the expected operational bandwidth of the device ( 6 - 11.5 ev ) gives a figure of merit n@xmath15=822 @xmath16 .
* aging studies of the gem foils as well as the csi photocathode revealed that there is no significant deterioration of the detector for irradiation levels corresponding to @xmath10  10 years of normal phenix operation at rhic .
these results demonstrated the validity of the proposed hbd concept and paved the way to the incorporation of such a device in the phenix experiment .
the mechanical design of the hbd has been developed and construction procedures have been exercised and optimized on a full - scale prototype .
the hbd consists of two identical arms ( see fig .  [
fig : hbd-3d_qm2005 ] ) , each one covering 135@xmath23 in azimuth and @xmath240.45 units of pseudorapidity . in each arm
the detector element is subdivided in 12 detector modules , 6 along the @xmath4 axis @xmath21 2 along the z axis , with a module size of @xmath25 cm@xmath6 .
the signals are collected in the anode plane consisting of 1152 hexagonal pads in each arm .
the detector vessel has a polygonal shape made of panels glued together .
the full - scale prototype design has been integrated into the phenix standard simulation package .
realistic simulations have been performed using hijing central au+au collisions at @xmath26 gev with embedded @xmath4 mesons .
we chose a very narrow centrality window : impact parameter b  @xmath27 2  fm ( top 2@xmath13 ) which corresponds to an average charged particle rapidity density of dn@xmath28/dy  @xmath29  940 .
[ fig : event_display ] shows the reconstructed clusters after clean - up of such a central collision .
the clean - up is optimized for electron identification and hadron rejection .
it requires : a ) a pad amplitude @xmath30  p.e . ;
b ) a cluster amplitude@xmath31  20  p.e .
; c ) a cluster size with n@xmath32  2 . out of the huge number of hadrons ( @xmath10400 ) going through the hbd only two are reconstructed demostrating excellent hadron rejection .
electrons reconstructed in the central arms and matched to a cluster in the hbd are rejected as a ) likely conversions if the cluster amplitude is larger than 60 p.e . ;
b ) likely @xmath11 dalitz decays if there is another electron cluster in the hbd within a distance of 200  mrad . for the mass region around the @xmath4 meson ,
this results in a reduction of the combinatorial background from these two sources by a factor larger than 100 , compared to the present performance without the hbd . at this level of rejection
the quality of the low - mass pair measurement is not anymore limited by this background but rather by the combinatorial background from the semi - leptonic decays of charmed mesons .
a novel hadron blind detector ( hbd ) has been developed for an upgrade of the phenix experiment at rhic . 
 the hbd will allow a precise measurement of electron - positron pairs from the decay of the light vector mesons and the low - mass pair continuum in heavy - ion collisions . 
 the detector consists of a 50 cm long radiator filled with pure cf@xmath0 and directly coupled in a windowless configuration to a triple gas electron multiplier ( gem ) detector with a csi photocathode evaporated on the top face of the first gem foil .