Ultraperipheral Heavy Ion Collisions Research Articles

Photoproduction in Ultraperipheral Heavy Ion Collisions at RHIC and the LHC

This year HERA has shut down- closing a productive period in the study of photon-hadron interactions. It is interesting to compare electron hadron colliders such as HERA with the LHC -particularly in operation with heavy ions where Z 2 α ∼ 1 and Coulomb interactions are clearly significant. Roughly speaking ion collisions at RHIC cover an equivalent energy range to the fixed target tagged photon experiments at Fermilab in the '80's and those at the LHC will extend the energy range of HERA by a factor of 3–10. It is not suprising then that RHIC results have dealt with vector meson production (ρ and J / ψ ). Similarly we can expect the LHC program with heavy ions to probe the structure of the proton to another order of magnitude in x.

MEASURING PHOTONS AND NEUTRONS AT ZERO DEGREES IN CMS

The CMS Zero Degree Calorimeters, ZDCs, will measure photons and neutrons emitted with |η| ≥ 8.6 from Pb + Pb , p + Pb and p + p collisions at [Formula: see text], 8.8 and 14 TeV respectively. The calorimeter consists of an electromagnetic part segmented in the horizontal direction and an hadronic part segmented into four units in depth. In addition CMS will have access to data from a segmented shower maximum detector being built for luminosity measurements. We will present detailed results from tests beam measurements taken at the CERN SPS. These data will be used to extrapolate the utility of the ZDCs to measure photons, and possibly π0 s in p + p and ultra-peripheral heavy-ion collisions. Data from the hadronic section can be used to estimate the number of participants in heavy ion-collisions. In addition we will discuss plans to use the detector to measure the reaction plane, thereby extending the sensitivity of the central detectors in CMS.

Ultra-peripheral collisions of heavy ions at RHIC and the LHC

This paper deals with so-called Ultra-Peripheral Collisions (UPCs) of heavy ions[C.A. Bertulani, S.R. Klein and J. Nystrand, Ann. Rev. Nucl. Part. Sci. 55 (2005) 271; G. Baur, K. Hencken, D. Trautmann, S. Sadovsky and Y. Kharlov, Phys. Rept. 364 (2002) 359]. These can be defined as collisions in which no hadronic interactions occur because of the large spatial separation between the projectile and target. The interactions are instead mediated by the electromagnetic field. Two types of ultra-peripheral collisions can be distinguished: purely electro-magnetic interactions (two-photon interactions) and photonuclear interactions, in which a photon from the projectile interacts with the hadronic component of the target.

HEAVY QUARKS AND MESON PRODUCTION IN ULTRAPERIPHERAL HEAVY ION COLLISIONS

We report on our recent investigations into photonuclear production of heavy quarks and vector mesons in ultraperipheral heavy ion collisions. In particular, our theoretical predictions are compared with the recent experimental measurements on coherent ρ (STAR) and J/Ψ (PHENIX) photoproduction at RHIC, and estimates for LHC are given.

The QCD pomeron in ultraperipheral heavy ion collisions: V. Double vector meson production in the BFKL approach

In this work the double vector meson production in ultraperipheral heavy ion collisions is investigated within the BFKL approach. The integrated cross sections and event rates for the processes AA --> V1 V2 AA (V_i = rho, omega, phi, J/Psi, Upsilon) are computed and theoretical estimates for scattering on both light and heavy nuclei are given for energies of RHIC and LHC.

Vector meson production in ultraperipheral heavy ion collisions

Ultraperipheral heavy ion collisions (UPCs) are an important alternative for studying QCD dynamics until the next generation of e+e−/ep/eA colliders becomes a reality. Due to the coherent action of all the protons in the nucleus, the electromagnetic field is very strong and the resulting flux of equivalent photons is large, which allows us to study two-photon as well as photonuclear interactions at high energies. In this paper we present a brief review of the vector meson production in UPCs at high energies using the QCD colour dipole approach to describe their photonuclear production and the perturbative QCD Pomeron (BFKL dynamics) to describe the double meson production in photon–photon processes. Predictions for rates and integrated cross sections are presented for energies of RHIC and LHC.

Lepton-pair production from deep inelastic scatteringin peripheral relativistic heavy ion collisions

We calculate the inelastic electron- and muon-pair production in peripheral relativistic heavy ion collisions in the region of large Q 2 of one of the photons. This offers a possibility to study the quark distribution functions in ions in "ultraperipheral heavy ion collisions". The calculations are compared with those making use of the equivalent photon and the equivalent lepton approximation. We compare the results for Pb-Pb and Pb-p collisions at RHIC (γ ≈ 100) and LHC (γ ≈ 3000) energies. Furthermore we include nuclear modifications to the parton distribution functions in our calculations to study their effect on the cross sections.

The QCD pomeron in ultraperipheral heavy ion collisions: IV. Photonuclear production of vector mesons

The photonuclear production of vector mesons in ultraperipheral heavy ion collisions is investigated within the QCD color dipole picture, with particular emphasis on the saturation model. The integrated cross section and the rapidity distribution for the A + A --> V + A + A (V = rho, omega, phi, J/Psi) process are computed and theoretical estimates for scattering on both light and heavy nuclei are given for energies of RHIC and LHC. A comparison with the recent STAR data on coherent production of rho mesons is also presented. Furthermore, we calculate the photoproduction of vector mesons in proton-proton collisions at RHIC, Tevatron and LHC energies.

QCD at high parton density

The high parton density regime of Quantum Chromodynamics (QCD) is briefly discussed. Some phenomenological aspects of saturation are described, mainly focusing on possible signatures of the non-linear QCD dynamics in electron-proton/nucleus collisions. Implications of these effects in central and ultraperipheral heavy-ion collisions are also presented.

PARTON SATURATION APPROACH IN HEAVY QUARK PRODUCTION AT HIGH ENERGIES

The high parton density regime of the quantum chromodynamics (QCD), where the physics of parton saturation is expected to be dominant, is briefly discussed. Some phenomenological aspects of saturation are described, mainly focusing on possible signatures of the nonlinear QCD dynamics in the heavy quark production in electron–proton/nucleus collisions. Implications of these effects in the heavy quark production in ultraperipheral heavy-ion collisions are also presented.

Coherent electromagnetic processes in ultra-peripheral heavy-ion collisions

We report measurements for coherent rho^0 production, AuAu->AuAu rho^0, and coherent rho^0 and e^+e^- pair production accompanied by mutual nuclear Coulomb excitation, AuAu->Au*Au* rho^0 and AuAuee, in ultra-peripheral relativistic gold-gold collisions (UPC). We discuss transverse momentum, mass, and rapidity distributions. The two photon-process of e^+e^- pair production is an important probe of strong field QED because of the large coupling Z*alpha=0.6. At sqrt{s}=200GeV, the e^+e^- production cross section agrees with lowest order QED calculations. The cross sections for coherent rho^0 production at sqrt{s}=130 and 200GeV are in agreement with theoretical predictions. The calculations for both, coherent e^+e^- and rho^0 production treat nuclear excitation as independent process.

Ultra peripheral heavy ion collisions and the energy dependence of the nuclear radius

To estimate realistic cross sections in ultra peripheral heavy ion collisions we must remove effects of strong absorption. One method to eliminate these effects make use of a Glauber model calculation, where the nucleon–nucleon energy dependent cross sections at small impact parameter are suppressed. In another method we impose a geometrical cut on the minimal impact parameter of the nuclear collision (bmin>R1+R2, where Ri is the radius of ion ‘i’). In this last case the effect of a possible nuclear radius dependence with the energy has not been considered in detail up to now. Here we introduce this effect showing that for final states with small invariant mass the effect is negligible. However when the final state has a relatively large invariant mass, e.g., an intermediate mass Higgs boson, the cross section can decrease up to 50%.

The QCD pomeron in ultraperipheral heavy ion collisions: III. Photonuclear production of heavy quarks

We calculate the photonuclear production of heavy quarks in ultraperipheral heavy ion collisions. The integrated cross section and the rapidity distribution are computed employing sound high energy QCD formalisms as the collinear and semihard approaches as well as the saturation model. In particular, the color glass condensate (CGC) formalism is also considered using a simple phenomenological parameterization for the color field correlator in the medium, which allow us to obtain more reliable estimates for charm and bottom production at LHC energies.

Onset of perturbative color opacity at smallxand Υ coherent photoproduction off heavy nuclei at LHC

We study photon-induced coherent production of Upsilon in ultraperipheral heavy ion collisions at LHC and demonstrate that the counting rates will be sufficient to measure nuclear shadowing of generalized gluon distributions. This will establish the transition from the regime of color transparency to the regime of perturbative color opacity in an unambiguous way. We argue that such measurements will provide the possibility to investigate the interaction of ultra-small color dipoles with nuclei in QCD at large energies, which are beyond the reach of the electron-nucleon (nucleus) colliders, and will unambiguously discriminate between the leading twist and higher twist scenarios of gluon nuclear shadowing.

Double meson production in ultraperipheral heavy-ion collisions

The double meson production in ultraperipheral heavy ions collisions is addressed, focusing on the particular case of rho-J/Psi from two-photon reactions. The cross section at photon level is obtained using distinct parameterizations for the gluon distribution on the light meson. The resulting estimates for the nuclear case are presented and discussed. As a by product, we estimate the double rho production cross section using the Pomeron-exchange factorization relations.

The QCD pomeron in ultraperipheral heavy ion collisions: II. Heavy quark production

The heavy quark production in ultraperipheral heavy ion collisions is investigated, in particular we focus on the results from the coherent interactions given by the two-photon process. One addresses the heavy quark total cross sections at photon level considering the saturation model and the BFKL dynamics in the color dipole picture. The corresponding cross sections at nuclear level are presented. It is verified that the QCD dynamics implies an enhancement of the cross section in comparison with previous calculations.

Coherent vector meson production in ultra-peripheral heavy-ion collisions at STAR

We report the first observation of coherent rho0 production (AuAu->AuAu rho0) and rho0 production accompanied by mutual nuclear Coulomb excitation (AuAu->Au*Au*rho0),and the observation of coherent e+e- pair production (AuAu->Au*Au*e+e-) in ultra-peripheral relativistic heavy-ion collisions (UPCs). We give transverse momentum, mass, and rapidity distributions. The cross sections for coherent rho0 production at sqrt{s_NN}=130 and 200 GeV are in agreement with theoretical predictions, which treat rho^0 production and nuclear excitation as independent processes.

Coherent rho(0) production in ultraperipheral heavy-ion collisions.

The STAR Collaboration reports the first observation of exclusive rho(0) photoproduction, AuAu-->AuAurho(0), and rho(0) production accompanied by mutual nuclear Coulomb excitation, AuAu-->Au*Au*rho(0), in ultraperipheral heavy-ion collisions. The rho(0) have low transverse momenta, consistent with coherent coupling to both nuclei. The cross sections at sqrt[s(NN)]=130 GeV agree with theoretical predictions treating rho(0) production and Coulomb excitation as independent processes.

Heavy quark photoproduction in ultraperipheral heavy ion collisions

Heavy quarks are copiously produced in ultra-peripheral heavy ion collisions. In the strong electromagnetic fields, c c-bar and b b-bar are produced by photonuclear and two-photon interactions; hadroproduction can occur in grazing interactions. We present the total cross sections, quark transverse momentum and rapidity distributions, as well as the Q Q-bar invariant mass spectra from the three production channels. We consider AA and pA collisions at the Relativistic Heavy Ion Collider and Large Hadron Collider. We discuss techniques for separating the three processes and describe how the AA to pA production ratios might be measured accurately enough to study nuclear shadowing.

Signals for black body limit in coherent ultraperipheral heavy ion collisions

We argue that study of total cross section of photoabsorption and coherent photoproduction of ρ, ρ′-mesons in ultraperipheral heavy ion collisions (UPC) is effective method to probe onset of black body limit (BBL) in the soft and hard QCD interactions. We illustrate the expected features of the onset of BBL using generalized vector dominance model. We show that this model describes very well ρ-meson coherent photoproduction at 6⩽ E γ ⩽10 GeV. In the case of ρ-meson production we find a UPC cross section which is a factor ∼1.5 larger than the one found by Klein and Nystrand. The advantages of the process of coherent dijet production to probe onset of BBL in hard scattering regime where decomposition over the twists becomes inapplicable are explained and relative importance of the γ+pomeron and γ+ γ mechanisms is estimated.