Rho0 Photoproduction in Ultra-Peripheral Relativistic Heavy Ion Collisions with STAR

Journal Article · Thu Dec 20 04:00:00 EST 2007 · Physical Review C

OSTI ID:963029

Coll, STAR

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR collaboration presents a measurement of {rho}{sup 0} and direct {pi}{sup +}{pi}{sup -} photoproduction in ultra-peripheral relativistic heavy ion collisions at {radical}s{sub NN} = 200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross-section of {sigma}(AuAu {yields} Au*Au* {rho}{sup 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 {rho}{sup 0} transverse momentum spectrum (p{sub T}{sup 2}) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find {sigma}{sub inc}/{sigma}{sub coh} = 0.29 {+-} 0.03 (stat.) {+-} 0.08 (syst.). The ratio of direct {pi}{sup +}{pi}{sup -} production is comparable to that observed in {gamma}p collisions at HERA, and appears to be independent of photon energy. Finally, the measured {rho}{sup 0} spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

• Research Organization: Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (US)
• Sponsoring Organization: Nuclear Science Division
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 963029
• Report Number(s): LBNL-2101E
• Journal Information: Physical Review C, Journal Name: Physical Review C
• Country of Publication: United States
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
COLOR
COULOMB EXCITATION
DIPOLES
ELECTROMAGNETIC FIELDS
HEAVY IONS
HELICITY
MATRIX ELEMENTS
PHOTONS
PHOTOPRODUCTION
PRODUCTION
S CHANNEL
SPIN
STARS
TARGETS
TRANSVERSE MOMENTUM