ρ0photoproduction in ultraperipheral relativistic heavy ion collisions atsNN=20

Abstract

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ 0 and direct π + π − photoproduction in ultraperipheral relativistic heavy ion collisions at √ s NN = 200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ (AuAu → Au * Au * ρ 0 ) = 530 ± 19(stat.) ± 57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ 0 transverse momentum spectrum (p 2 T ) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ inc /σ coh = 0.29 ± 0.03 (stat.) ± 0.08 (syst.). The ratio of direct π + π − to ρ 0 production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ 0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

Coherent ρ 0 photoproduction in ultra-peripheral Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

“…There may also be interference between the ρ and ρ production amplitudes, and these amplitudes may be affected by the nuclear environment in a different way [14]. A detailed discussion of models for photoproduction of ρ 0 on complex nuclei based on data from fixed target experiments can be found in [13].The cross sections measured by STAR [8][9][10] at RHIC were found to be about a factor two less than that predicted by the calculation of ref. [15], while in agreement with STARLIGHT [16].…”

“…The ρ 0 is reconstructed using the π + π − decay channel in the rapidity range |y| < 0.5. The rapidity interval corresponds to a γ-nucleon center of mass energy in the range 36 ≤ W γN ≤ 59 GeV with W γN = 48 GeV, about a factor of 4 higher than in any previous measurement [10]. The cross section is measured for the cases of no neutron emission and for at least one emitted neutron.…”

“…The excited nucleus typically decays by the emission of neutrons at very forward rapidities. The signature of these processes is thus a ρ 0 vector meson with very low transverse momentum which may be accompanied by a few neutrons at very forward rapidities but no other particles.Photoproduction of ρ 0 vector mesons on nuclear targets has been studied in fixed target experiments with lepton beams [7], and more recently in ultra-peripheral collisions by the STAR Collaboration at RHIC at √ s NN = 62 [8], 130 [9], and 200 GeV [10]. STAR has also observed coherent photoproduction of the ρ 0 (1700) [11].…”

“…The STAR and ALICE results are consistent within errors if one takes into consideration that b is expected to be ≈4-8% larger for a lead nucleus than for a gold nucleus because of the difference in size (one expects b ∝ R 2 ). The fit was performed for |t| > 0.002 GeV 2 /c 2 to avoid interference effects at very low p T [10].The final sample of coherent ρ 0 → π + π − candidates is corrected for acceptance and efficiency in invariant mass bins. The event sample used to determine the corrections has uniform distributions in invariant mass, rapidity, transverse momentum, and azimuthal angle over the ranges 2m π ≤ M ππ ≤ 1.5 GeV/c 2 , |y| ≤ 1.0, p T ≤ 0.15 GeV/c, and 0 ≤ φ ≤ 2π.…”

“…The measured cross section is in agreement with STARLIGHT [16] and the calculation by Gonçalves and Machado (GM) [34], while the GDL (GlauberDonnachie-Landshoff) prediction [15,35] is about a factor of 2 higher than data. The The STAR Collaboration has published the total coherent ρ 0 photoproduction cross section at three different energies [8][9][10]. To be able to compare the current result to those, one has to integrate dσ/dy over the whole phase space, which can only be done using models.…”