The ultra-peripheral collisions (UPC) program in STAR studies electromagnetic interactions between the nuclei, at impact parameters large enough that no hadronic interactions occur. Purely electromagnetic (two-photon) and photonuclear (photon-Pomeron, for example) interactions are posssible; the collisions can usually be treated as the collision of two exchange particles. For a purely electromagnetic process, the exchange particles are photons, producing two-photon collisions. The nuclear (strong) force exchanges are represented by Pomeron exchange, where the Pomeron represents the absorptive part of the nuclear potential, and is often treated as a colorless particle with the quantum numbers of the vacuum. Photon-Pomeron interactions and double-Pomeron interactions are possible.
Photon-Pomeron production of the rho meson is the most prolific UPC observed by STAR, with a cross section of 500 -1000 mb for gold nuclei at STAR. Our first paper discusses our rho results from the year 2000 data; the year 2001 data analysis is now underway.Two-photon interactions have been studied in e+e- and pp-collisions. In STAR, this process is being studied with nuclear systems. The nucleus-nucleus system offers many advantages. The flux of virtual photons from the electromagnetic fields scale as Z2, and the two-photon interaction rate as Z4. This more than compensates for the lower beam luminosity in heavy-ion accelerators as compared to e+e- accelerators.
Many interesting physics topics can be investigated within the UPC program. The cross sections for vector meson production are sensitive to the vector meson-nucleon cross sections, and hence to the nuclear gluon densities (i.e. shadowing). The vector meson transverse momentum spectrum is sensitive to interference between vector meson production at the two nuclei, and may be used to test quantum mechanics. Processes such as electron positron production with electron capture are sensitive to the atomic physics of high-Z nuclei, and are also important for understanding heavy ion accelerators.
More generally, the RHIC gamma-gamma and gamma-nucleus luminosities are competitive with other accelerators in the invariant mass range up to 2 GeV, and both can be used to study meson spectroscopy, with photonuclear production sensitive to JPC = 1-- final states, with two-photon reactions producting scalar (spin 0) and tensor (spin 2) mesons. In the 1-2 GeV mass region, more states have been observed than fit into the conventional SU(3) quark-antiquark meson spectrum; some of these states may be exotica such as glueballs, hybrids (quark-antiquark-gluon) or four quark states. Glueballs should be produced in hadronic collisions, but, because gluons are uncharged, should rarely be produced in two-photon collisions. So, non-detection in two-photon collisions is an important criteria for confirmation of glueball candidates. Gamma- gamma interactions may also be used to study the production of meson and baryon pairs.
UPCs look very different from the hadronic nucleus-nucleus collisions studied elsewhere in STAR. The multiplicity is low, with two or four charged particle final states common, compared to the hundreds or thousands found in central heavy ion collisions. Despite this difference, STAR is well suited to study this physics, because the STAR TPC and FTPCs have excellent phase space coverage.
The major challenge to studying this type of reactions in a nuclear environment is to separate the interesting processes from the large number of background interactions, with a good signal to noise ratio. This is a problem both in the final analysis as well as at the trigger level. Signals have been studied with the STARlight Monte Carlo discussed in STAR Note 243.
The data shows (see our recent presentations) that a cut on multiplicity, total charge (should be 0) and final state transverse momentum are effective at separating signal from the backgrounds. The observed backgrounds appear somewhat higher than our background simulations of peripheral Nuclear Collisions (hadronic interactions), Beam-gas Interactions, photon-Nucleus Interactions and cosmic rays (the latter only important for triggering). We simulated these backgrounds with various Monte Carlo generators: FRITIOF and VENUS for peripheral nuclear collisions and beam gas interactions, HEMICOSM for cosmic ray muons, and DTUNUC for photo-nuclear interactions.
From these simulations, we have developed efficient algorithms for triggering STAR at Levels 0-3 with good efficiencies and acceptable rates. We have also developed more sophisticated algorithms for achieving good signal to noise ratios in offline analysis.
These simulations are discussed in STAR notes 380 (intended for non-collaborators) and 347 (aimed at collaborators).
We are also considering gluon-gluon collisions at large impact parameters, as described in STAR note 296 and a recent paper in Phys. Rev. C. . This may be a way to measure gluon shadowing in nuclei.
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