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the photon is a fundamental ingredient of our understanding of elementary particles and their interactions  @xcite .
it can interact either directly ( bare photon ) or in a resolved manner , as a point - like particle ( compton - like scattering ) or via quantum fluctuations to : a lepton pair , a quark anti - quark pair , or a vector boson with the same quantum numbers as the photon ( @xmath5 ) .
photon - induced interactions have been traditionally studied with lepton beams ( e.g. at pep , lep ) and @xmath6 collisions ( at hera & tevatron ) .
ultra - peripheral collisions ( upc ) of hadrons , proton or nuclei collisions where the impact parameter is larger than twice the nuclear radius such as there is no strong interaction , also allow the study of photon - induced reactions .
the main advantatges of hadron vs. lepton colliders are the larger photon luminosity ( the photon spectrum is proportional to @xmath7 , the two - photon luminosity goes as @xmath8 ) and the opportunity to probe strong electromagnetic fields ( coupling @xmath9 instead of @xmath10 ) .
the @xmath2  gev au+au upc at the bnl rhic collider provide means to test photon - nucleon and two - photon interactions at a maximum center of mass energy of @xmath11  gev and @xmath12  gev respectively  @xcite .
this proceeding concentrates on the first measurement ever of @xmath13  and high - mass @xmath1  in upc heavy - ion collisions with the phenix experiment  @xcite .
exclusive @xmath0  photoproduction can proceed via pomeron - exchange ( two - gluon picture ) either through coherent photon - nuclear ( @xmath14 ) or incoherent photon - nucleon ( @xmath15 ) reactions .
the @xmath0  photoproduction cross section is then related to the gluon nuclear distribution functions , @xmath16 .
this allows one to constrain them in the small bjorken-@xmath17 region , @xmath18 which for @xmath19 gives @xmath20 $ ] , a relatively unexplored region ( fig .
[ fig : xvalues_phenix ]  left ) @xcite , and to eventually probe quarkonia propagation in normal nuclear matter ( the so called nuclear absorption ) . on the other hand , exclusive dilepton ( @xmath1 ) pair production can occur through a pure electromagnetic process ( @xmath21 ) and tests qed on a strongly interacting regime .
the results summarized here  @xcite concern data collected in 2004 on au+au collisions at @xmath2  gev with the phenix experiment  @xcite . the apparatus is formed by four spectrometers ( north , south , east , west ) and a few global detectors . the central arms ( east & west spectrometers , fig .
[ fig : xvalues_phenix ]  right ) , are centered at mid - rapidity and measure electrons , photons and hadrons .
the forward arms ( north & south spectrometers ) are for muon detection and identification .
electron tracking and identification is possible thanks to ( from inner to outer ) : the multi - layer drift chambers ( dc ) , the multi - wire proportional chambers ( pc ) , the ring - imaging - cherenkov detectors ( rich ) and the electromagnetic calorimeters ( emcal ) with two different technologies : lead - scintillador sandwich ( pbsc ) , and lead - glass cherenkov ( pbgl ) calorimeters .
left : kinematic @xmath22 versus bjorken-@xmath17 map of the regions explored to probe the nuclear pdfs  @xcite .
right : section of the phenix central arms as they were on year 2004  @xcite . for illustration ,
passage of electrons , positrons , photons and pions through the detectors are depicted .
, title="fig:",scaledwidth=37.0% ]   left : kinematic @xmath22 versus bjorken-@xmath17 map of the regions explored to probe the nuclear pdfs  @xcite .
right : section of the phenix central arms as they were on year 2004  @xcite . for illustration ,
passage of electrons , positrons , photons and pions through the detectors are depicted .
, title="fig:",scaledwidth=51.5% ]    triggering on upc interactions with such a detector , conceived to study the most central heavy - ion collisions , has been a successfully met challenge .
the electromagnetic fields associated with these collisions can excite the nuclei , which will most probably de - excite by emitting neutrons at forward - rapidities .
the probability of @xmath0  coherent ( @xmath23 ) or incoherent ( @xmath24 ) photoproduction in coincidence with au coulomb neutron breakup amounts to @xmath25 or @xmath26 respectively  @xcite .
the trigger strategy was then to :    1 .
detect these neutrons in one or both of the zero degree calorimeters ( zdc ) ; 2 .
reject non - upc ( peripheral or beam - gas ) reactions by imposing a veto on coincident signals in both beam beam counters ( bbc ) , which also selects events with a large rapidity gap ( bbcs coverage is of @xmath27 ) ; and 3 .   using the emcal trigger to select events with one or more electrons of energy @xmath28  gev .
the triggered events ( @xmath29 m events ) were examined after applying data quality criteria ( @xmath30 loss ) .
the subsample of events analyzed are those centered in the detector fiducial area ( @xmath31  cm ) and with only two charged tracks ( electrons ) to optimize the selection of exclusive processes .
electron identification criteria are also applied ( see @xcite for details ) .
a total of @xmath32 @xmath1  pairs and zero like - sign pairs ( @xmath33 ) with @xmath34  gev / c@xmath35 have been measured ( fig .
[ fig : invmass_pt ] left ) . despite the poor statistics , the trend is compatible with starlight  @xcite montecarlo simulations and a full reconstruction with our experimental setup .
the invariant mass distribution is fitted with a continuum ( exponential ) plus a @xmath0  ( gaussian ) shape which allows one to deduce the number of @xmath0 , @xmath36=9.9 \pm 4.1 { \rm ( stat)}\pm 1.0 { \rm ( syst)}$ ] , and of continuum @xmath1  counts , @xmath37=13.7 \pm 3.7 { \rm ( stat)}\pm 1.0 { \rm ( syst)}$ ] for @xmath38 $ ]  gev / c@xmath35 . the statistical and systematical @xmath1  continuum uncertainties are computed by varying its subtraction method , changing its shape to a power law form and studying the dependence on the slope parameters .
those uncertainties are represented by dashed - lines in fig .
[ fig : invmass_pt ] left .
@xmath1  invariant mass distribution as measured in au+au upc at @xmath2  gev with phenix .
the left plot depicts its decomposition into continuum and @xmath0 . the right figure presents the invariant mass versus pair @xmath4 correlation . , title="fig:",scaledwidth=49.5% ]
@xmath1  invariant mass distribution as measured in au+au upc at @xmath2  gev with phenix .
the left plot depicts its decomposition into continuum and @xmath0 .
the right figure presents the invariant mass versus pair @xmath4 correlation .
, title="fig:",scaledwidth=49.5% ]    the dielectron ( @xmath39 ) cross section per invariant mass range is presented in tab .
[ tab : xs_results ] . both , the pairs low transverse momentum , @xmath4 , ( fig .
[ fig : invmass_pt ] right ) and the photoproduction cross sections , are in accordance with the starlight predictions ( tab .  [
tab : xs_results ] ) for the coherent process , @xmath21 .
consequently , @xmath1  photoproduction measurements agree with lo qed theoretical ( starlight ) calculations even in this strongly interacting regime .
the caveats to this statement are the lack of comparisons with other predictions in the kinematical region of interest , and the fact that the nlo corrections influence on the calculations remain uncertain  @xcite .    .
[ tab : xs_results ] measured @xmath1 continuum photoproduction cross sections at mid - rapidity in upc au+au collisions ( accompanied with forward neutron emission ) at @xmath2  gev . for comparison , the starlight predictions are also quoted  @xcite . [ cols="^,^,^ " , ]     the @xmath0  @xmath4 distribution ( fig .
[ fig : invmass_pt ] right ) suggests the presence of both coherent ( @xmath23 ) and incoherent ( @xmath24 ) contributions , consistent with low and intermediate to high @xmath4 respectively , and in agreement with the theoretical expectations  @xcite . at mid - rapidity , the @xmath0  photoproduction
cross section section accompanied with forward neutron emission ( @xmath40 ) is @xmath41 = 76 \pm 31 \ , { \rm(stat ) } \pm 15 \ , { \rm(syst ) } \ , \
this value is shown to be consistent with various theoretical calculations  @xcite in fig .
[ fig : predictions ] .
athough the current precision prevents more quantitative statements , the influence of the coherent and incoherent contributions to the predictions can be observed by comparing the results of calculations where both of them are included , labeled as `` coh.+incoh . '' , and those where only the coherent component is considered , labeled only as `` coh . '' .
in addition , fig .
[ fig : predictions ]  right illustrates the measurement s sensitivity to the nuclear pdfs by examining the filho _
et al _ prediction  @xcite for coherent @xmath0  photoproduction with different shadowing parameterizations .
despite the poor statistics , which does not allow one to rule out any hypothesis , this is the first measurement of @xmath0  photoproduction in heavy - ion collisions and it shows itself to be a promising tool to learn about @xmath0  production processes and to probe the gluon nuclear pdfs at low bjorken-@xmath17 .
comparison of the measured @xmath42  photoproduction cross section with various theoretical calculations .
some of them include both coherent and incoherent components ( labeled as `` coh .
+ incoh . '' ) : strikman _ et al _
@xcite , ivanov _ et al _
the rest only compute the coherent contribution : starlight  @xcite , gonalves _
@xcite , filho _ et al
_  @xcite .
, title="fig:",scaledwidth=42.5% ]   comparison of the measured @xmath42  photoproduction cross section with various theoretical calculations .
some of them include both coherent and incoherent components ( labeled as `` coh .
+ incoh . '' ) : strikman _ et al _
@xcite , ivanov _ et al _  @xcite .
the rest only compute the coherent contribution : starlight  @xcite , gonalves _
@xcite , filho _ et al _  @xcite .
, title="fig:",scaledwidth=42.5% ]
the phenix experiment has proven its versatility by presenting the first measurement ever of high - mass @xmath1  and @xmath0  photoproduction in ultra - peripheral heavy - ion reactions  @xcite in au+au collisions at @xmath2  gev ( accompanied by au coulomb nuclear breakup ) .
dielectron photoproduction cross sections are in agreement with theoretical starlight lo qed calculations for coherent two - photon production , @xmath21 .
@xmath0  photoproduction cross section and its @xmath4 pattern are consistent with the expectations  @xcite and favour the possibility of both coherent ( @xmath14 ) and incoherent ( @xmath15 ) contributions .
even though the poor statistics prevents one from ruling out any hypothesis , it shows itself to be a promising tool to learn about @xmath0  production processes and probe the gluon nuclear pdfs at low-@xmath17 .
future analysis at rhic with higher luminosities and the imminent lhc collisions will provide more discriminating tools .
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@xmath0  and @xmath1  photoproduction measurements in ultra - peripheral au+au collisions at @xmath2  gev with the phenix experiment are presented  @xcite . 
 the triggered events correspond to those accompanied with forward neutron emission from the @xmath3 dissociation . 
 @xmath0  and high - mass @xmath1  cross sections and @xmath4 distributions are consistent with various theoretical calculations .