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the upcoming large hadron collider ( lhc ) will reach proton - proton energies an order of magnitude higher than any existing accelerator . because relativistic protons and heavier nuclei accompanied by fields of virtual photons , the lhc can be used to study photonuclear and two - photon interactions at energies far beyond those accessible at hera or other accelerators .
photoproduction is of interest in both pp and heavy - ion collisions .
proton - proton collisions produce photons with the highest energies , and , because of the very high @xmath3 luminosities , good rates .
however , for many channels , the signal to noise ratio may be lower than in ion collisions .
heavy ions are accompanied by very high photon fluxes , and , because of the very strong fields , a single ion - ion collision can induce multiple electromagnetic interactions .
the correlations between the multiple photons lead to the ability to `` tune '' the photon beam , by selecting different photon energy spectra and polarizations .
since these reactions take place at large impact parameters , where no hadronic interactions occur , they are often known as `` ultra - peripheral collisions '' ( upcs ) .
upcs can be used to study a variety of topics @xcite@xcite@xcite .
low@xmath4 gluon distributions can be probed via heavy quark ( including quarkonium ) and jet production .
upcs can be used for many other studies of qcd . at the lhc
, photonuclear interactions can be used to search for new physics .
the strong fields allow for many tests of quantum electrodynamics in the very strong field regime , where perturbation theory may be expected to fail .
many of these topics are already being studied at rhic .
for most reactions , the photon flux from protons or nuclei is well described by the weizscker - williams method of virtual photons .
the photon flux per unit area for an energy @xmath5 at a distance @xmath6 from a relativistic nucleus with charge @xmath7 is @xcite @xmath8 where @xmath9 , @xmath10 is the lorentz boost of the nucleus , @xmath11 is the fine structure constant , and @xmath12 is a modified bessel function .
the total photon flux from an ion with radius @xmath13 is @xmath14 the constrant @xmath15 is usually imposed to eliminate the photon flux inside the nucleus ( where eq .
[ eq : fflux ] fails , and , in any case , most of the flux is not usable ) . for photonuclear or two - photon interactions to be visible
, the two nuclei must not interact hadronically , requiring @xmath16 .
this flux is calculated numerically , but can be approximated within about 15% by requiring @xmath16 in eq .
[ eq : ntot ] @xcite .
the cross section for photonuclear interactions can be written @xcite @xmath17 where @xmath18 is the probability for a photonuclear interaction , @xmath19 where @xmath20 is the cross section for the photonuclear interaction in question .
this formulation is easy to generalize to include multiple interactions between a single ion pair : @xmath21 in general , @xmath22 , so the integrand for a @xmath23photon reaction goes as @xmath24 and the more photons involved in a reaction , the smaller the average impact parameters @xcite . for example , with gold at rhic , the median impact parameter drops from 46  fm for unselected @xmath0 production to 18 fm for @xmath0 accompanied by mutual coulomb excitation @xcite .
the smaller impact parameters harden the photon spectrum , from @xmath25 to independent of @xmath5 . for some reactions , @xmath26 ;
in this case @xmath18 is the mean number of reaction at that @xmath6 .
factorization can be used to simplify triggering on upcs .
one reaction can serve as a trigger for another .
star has studied the reactions @xmath27 and @xmath28 , using signals from the neutrons emitted in the @xmath29 decays to trigger the detector , providing @xmath0 and @xmath2 samples without trigger bias .
the photon polarizations are also correlated .
the electric field of the photon - emitting nucleus parallels the impact parameter vector , so photons are linearly polarized along the impact parameter vector . for multiple interactions between a single ion - pair
, the parallel polarizations can lead to observable angular correlations between decay products @xcite .
the star collaboration has produced final results on @xmath0 production @xcite and on two - photon production of @xmath2 pairs @xcite .
events were selected with two types of triggers : minimum bias triggers that select events with mutual coulomb dissociation , taking advantage of factorization , and topological triggers , that select low multiplicity events with appropriate topologies in the central detector @xcite .
the @xmath30 data is well described by the soft pomeron model , and the previously discussed factorization holds . in the soft pomeron model
, the incident photon fluctuates to a quark - antiquark pair , which then elastically scatters ( via pomeron exchange ) from the target nucleus @xcite .
because the scattering is coherent , the momentum transfer is limited to order @xmath31 .
this low @xmath32 is a distinctive experimental signature ; for gold , most of the signal occurs for @xmath33 mev / c .
figure 1 shows the @xmath34 spectrum of @xmath0 with rapidity @xmath35 , selected with stringent cuts to minimize the background @xcite . at moderate and high @xmath36 ,
the spectrum is well fit by an exponential , @xmath37 .
however , for @xmath38gev@xmath39 , the data drops off .
this drop can be explained by interference between two indisinguishable possibilities : nucleus 1 emits a photon which interacts with nucleus 2 , or vice - versa @xcite . in @xmath3 or @xmath40 collisions ,
these two possibilities are related by a parity transformation . since the @xmath0 is negative parity , the interference is destructive . at mid - rapidity ,
@xmath41 \label{eq : interf}\ ] ] of course , @xmath6 is unknown , and the overall interference depends on the integral over all @xmath6 . away from @xmath42 , the interference is reduced because the photon energies , fluxes , amplitudes _ etc .
_ for the two directions are different .
the solid curve in fig .
1 shows a fit to a functional form based on these factors ; for this sample , the interference is @xmath43 of that expected @xcite .
because the two sources are spatially separated , the final state @xmath44 wave function does not factorize into single - particle wave functions , and the system exhibits the einstein - podolsky - rosen paradox @xcite . for @xmath45 collisions ,
the transformation between the two possibilies is a charge - parity transformation ; vector mesons are @xmath46 positive , so the interference in eq .
[ eq : interf ] is positive @xcite ; this may be studied at the fermilab tevatron .
star has also studied @xmath0 production in @xmath47 collisions .
the photon is usually emitted by the gold nucleus , and the deuteron is the target .
both coherent ( deuteron stays intact ) and incohent ( deuteron dissociates ) interactions have been observed .
the @xmath48 spectrum for the incoherent interactions is similar to that observed in @xmath49 collisions at hera @xcite .
the star @xmath2 data is well described by lowest order quantum electrodynamics and factorization @xcite .
the @xmath32 spectrum of the @xmath2 pairs is not well described by the virtual photon paradigm - the photon virtuality is required to fit the data .
star has also studied 4-prong final states , like @xmath1 .
fig . 2 compares the @xmath32 spectrum of 4-prong events with net charge 0 with those of net charge 2 .
this data was taken in 2002 with the minimum - bias trigger .
a neutral excess is present for @xmath50 mev / c , with the mass spectrum of the excess centered around 1.5 gev / c@xmath39 .
photoproduction can be studied at hadron colliders by using the virtual photons associated with the hadron beams . 
 the lhc will reach photonucleon energies 10 times higher than that available elsewhere . 
 these reactions are already being studied at rhic . after introducing photoproduction at hadron colliders 
 , i will discuss recent results from star on @xmath0 , @xmath1 and @xmath2 production .     
 address = lawrence berkeley national laboratory , berkeley , ca , 94720 , usa