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deconfinement is the first and most familiar manifestation of the qcd phase transition . of equal importance
is the chiral transition . the spontaneous breakdown of chiral symmetry responsible for the masses of the hadrons .
the study of low mass vector mesons is an ideal probe of chiral symmetry restoration in heavy ion collisions . as an example
, the lifetime of the @xmath1 is about 50 fm / c in the vacuum and a fraction of @xmath1 s produced in a heavy collision will decay in the fireball .
decays to di - leptons are particularly attractive since the daughter particles are not strongly interacting and can reach the experimental apparatus without rescattering , hence any effects such as mass shifts or broadening of spectral functions due to the onset of chiral symmetry restoration would show themselves in the invariant mass spectra .
the e325 experiment at kek@xcite and ceres at cern @xcite have seen hints of such effects in both pa and aa collisions .    because of the small q value of the kk decay channel , a comparison of the @xmath3 kk/@xmath3 ee rate is a particularly sensitive measure of a mass modification of either the @xmath1 or the kaon  @xcite .
the phenix detector is ideally suited for such measurements in that it has good particle identification for kaons with momentum between 300 mev / c and 2 gev / c and electrons to 5 gev / c , as well as good momentum resolution .
all data presented in this paper are from deuteron - gold collisions at @xmath4=200 gev measured by phenix in 2003 .
about 31 m single - electron triggered events were used for the electron analysis . the trigger required matching hits in the ring imaging cerenkov counter(rich ) and the electromagnetic calorimeter ( emc ) , where the threshold was set to 600 mev . an additional data set with a higher threshold was not used for the present analysis .
electrons were identified by requiring two or more phototubes firing in the rich and an e / p match between the energy measured by the emc and the momentum measured by the tracking system .
specifically we required @xmath5 .
cuts are then made to remove electron candidates coming from conversions .
the invariant mass spectrum of @xmath0 pairs is then formed as shown in figure 1 .    in order to form a background sample ,
opposite sign pairs were taken from different events .
because our sample was triggered , care was taken that the mixed event background had the same characteristics as the events themselves .
in particular minimum bias events were used to form the mixed pair , where one of the electrons was required to pass the trigger requirements .
additionally , event used to form the mixed pair were required to have similar centralities and vertex positions .
normalization of the background distribution was done by matching the data to the background in a sideband region between 850 and 950 mev below the signal and 1100 - 1200 mev above the signal .
other methods of normalization were used to estimate the systematic error from this procedure .
cc     invariant mass spectrum with a fit as described in the text.,width=288 ]     invariant mass spectrum with a fit as described in the text.,width=288 ]    the background subtracted signal was then fit to a relativistic breit - wigner convoluted with a gaussian to account for the experimental resolution ( figure 2 ) .
the width @xmath6 was held fixed at the particle data book value .
values obtained from the fit are shown in table  [ tab : tc ] .
the value of the mass is consistent with the known value of the mass in the vacuum .
in addition the fitted experimental resolution is consistent with simulations of the detector performance .
the signal was then divided into 3 bins of @xmath7 , and corrections were made for acceptance and efficiencies .
a fit was then done to the invariant yield resulting in the yield and inverse slope shown in the table .
major contributors to the systematic error on dn / dy are the normalization of the background and its effect on the inverse slope , and the run - by - run variations from the electron - trigger .
3.4 in     about 62 m minimum bias events were used for the @xmath2 analysis .
kaons were identified in the time - of - flight detector ( tof ) which covers @xmath8 . because of the limited coverage of the tof , the analysis preferentially accepted higher momentum @xmath9 as compared with the electron analysis .
the invariant mass spectrum was generated by combining unlike sign pairs of kaons .
the background shape was formed in a similar manner to the electrons by mixing pairs from events with similar centralities and vertices .
the normalization in this case was determined by adjusting the integral of the background to @xmath10 where @xmath11 and @xmath12 refer to the like sign combinations in each event .
once again the signal was fit to a relativistic breit - wigner convoluted with a gaussian for the experimental resolution . since the resolution of the @xmath1 in the kk decay channel is rather insensitive to momentum effects , we chose to hold the experimental resolution fixed to the value obtained from the simulation - 1.2 mev .
results of the fit are shown in the table and figure 3 .
both the mass and width are consistent with pdg vacuum values . for both decay channels , @xmath6 and @xmath13 are highly correlated , when they are both allowed to vary in the fit .
the data are then divided into bins of @xmath7 , corrections are made for acceptance and efficiencies and a fit is done to obtain the yield . because of the good signal to background in the kk channel , systematic errors are considerably smaller than in the electron channel .
the systematic errors are dominated by the range in @xmath7 over which the fit was done , the run - by - run changes in efficiency , and the corrections due to the fiducial cuts .
figure 4 shows a comparison of the data points from both the electron and kaon analyses .
the fit shown in the figure is to all of the points .
when fits were done to extract yields for comparison , the fits were done separately for the electron and kaon channels .
the two data samples are consistent in both the yield and the inverse slope as can be seen by looking at the comparisons in figures 5 and 6 of the extracted inverse slopes and yields .
within the substantial errors - particularly from the electron analysis - the yields of the @xmath14 and @xmath15 are consistent with one another .
because of the small fraction which decay inside the relevant volume , the effects from chiral symmetry restoration , particularly in dau collisions , are expected to be small .
phenix will make further improvements to this analysis by including the data with a higher trigger threshold and improving systematic errors - particularly from background normalization in the electron channel .
phenix also sees a clear @xmath16 signal , which , with a shorter lifetime than the @xmath1 will increase the number of decays in the fireball .
in addition , effects in au - au collisions could be considerably stronger since energy densities are higher .
further in the future , upgrades to phenix such as the hadron blind detector ( hbd ) will significantly enhance our capabilities to reject conversion and dalitz pairs which are the dominant source of background .
this will open up the possibility of studying the @xmath17 as well as thermal di - electrons .
.[tab : tc]results of fits to the invariant mass spectra and the yields for both the electron and kaon decay channels of the @xmath1 .
statistical errors are listed first , followed by systematic errors . [ cols="<,<,<,<",options="header " , ]
a first measurement of the @xmath0 decay rate of @xmath1 mesons in dau collisions from the phenix detector at rhic and its comparison to the @xmath2 decay channel is described . 
 the comparison of the two decay channels can be sensitive to chiral symmetry restoration .