Ultraperipheral Relativistic Heavy Ion Collisions Research Articles

Electromagnetic fields in ultra-peripheral relativistic heavy-ion collisions

Electromagnetic fields in ultra-peripheral relativistic heavy-ion collisions

Magnetic field in relativistic heavy-ion collisions: Testing the classical approximation

It is believed that in noncentral relativistic heavy-ion collisions a very strong magnetic field is formed. There are several studies of the effects of this field, where $\stackrel{P\vec}{B}$ is calculated with the expressions of classical electrodynamics. A quantum field may be approximated by a classical one when the number of field quanta in each field mode is sufficiently high. This may happen if the field sources are intense enough. In heavy-ion physics the validity of the classical treatment was not investigated. In this work we propose a test of the quality of the classical approximation. We calculate an observable quantity using the classical magnetic field and also using photons as input. If the results of both approaches coincide, this will be an indication that the classical approximation is valid. More precisely, we focus on the process in which a nucleon is converted into a delta resonance, which then decays into another nucleon and a pion, i.e., $N\ensuremath{\rightarrow}\mathrm{\ensuremath{\Delta}}\ensuremath{\rightarrow}{N}^{\ensuremath{'}}\ensuremath{\pi}$. In ultraperipheral relativistic heavy-ion collisions this conversion can be induced by the classical magnetic field of one of the ions acting on the other ion. Alternatively, we can replace the classical magnetic field by a flux of equivalent photons, which are absorbed by the target nucleons. We calculate the cross sections in these two independent ways and find that they differ from each other by $\ensuremath{\simeq}10$ % in the considered collision energy range. This suggests that the two formalisms are equivalent and that the classical approximation for the magnetic field is reasonable.

Magnetic excitation in relativistic heavy ion collisions

In this note we study the conversion of nucleons into deltas induced by a strong magnetic field in ultraperipheral relativistic heavy ion collisions. The interaction Hamiltonian couples the magnetic field to the spin operator, which, acting on the spin part of the wave function, converts a spin 1/2 into a spin 3/2 state. We estimate this transition probability and calculate the cross section for delta production. This process can in principle be measured, since the delta moves close to the beam and decays almost exclusively into pions. Forward pions may be detected by forward calorimeters.

Probing two-photon decay widths of mesons at energies available at the CERN Large Hadron Collider (LHC)

Meson production cross sections in ultraperipheral relativistic heavy ion collisions at the CERN Large Hadron Collider are revisited. The relevance of meson models and of exotic QCD states is discussed. This study includes states that have not been considered before in the literature.

Observation of Two-Source Interference in the Photoproduction ReactionAuAu→AuAuρ0

In ultraperipheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a rho{0}. The rho{0} production occurs in two well-separated (median impact parameters of 20 and 40 F for the cases considered here) nuclei, so the system forms a two-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing rho{0} production. Since the rho{0} decays before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled nonlocal wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is 87%+/-5%(stat.)+/-8%(syst.) of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors of 23% at the 90% confidence level.

Coherent photon-photon interactions in very peripheral relativistic heavy ion collisions

Heavy ions at high velocities provide very strong electromagnetic fields for a very short time. The main characteristics of ultraperipheral relativistic heavy ion collisions are reviewed, characteristic parameters are identified. The main interest in ultraperipheral heavy ion collisions at relativistic ion colliders like the LHC is the interactions of very high energy (equivalent) photons with the countermoving (equivalent) photons and hadrons (protons/ions). The physics of these interactions is quite different from and complementary to the physics of the strong fields achieved with current and future lasers.

Photoproduction in Ultra-Peripheral Relativistic Heavy Ion Collisions with STAR

We present a summary of recent photoproduction results in ultra peripheral relativistic heavy ions collisions with STAR. These collisions have impact parameters larger then twice the nuclear radius; the nuclei do not physically collide, but interact via long-range electromagnetic fields. We observe exclusive $\rho^0$ production as well as $AuAu\to Au^*Au^* \rho^0$ with accompanying mutual nuclear excitation at $\sqrt{s_{NN}}=200$ GeV. We report the $\rho^0$ production cross section for both coherent and incoherent coupling accompanied by mutual nuclear excitation. We have studied the cross section as a function of $p_T$, $y_{\rho^0}$ and $M_{\pi\pi}$ and compared it to theoretical models. In addition, we measured the $\rho^0$ helicity matrix elements. They are found to be consistent with s-channel helicity conservation. The ratio of coherent $\rho^0$ and direct $\pi^+\pi^-$ pair photoproduction has been measured and found to be consistent with earlier measurements. The 4-pion final state $AuAu \to \pi^+\pi^-\pi^+\pi^-$ state has also been observed.

Exchange of high and low energy photons in ultraperipheral relativistic heavy-ion collisions

Ultraperipheral collisions at collider energies are a useful tool to study photon-hadron (proton/nucleus) and photon-photon interactions in a hitherto unexplored energy regime. Theoretical tools to study these processes are briefly described. Some current results and problems are discussed

Photoproduction in Ultra-Peripheral Heavy Ion Collisions at STAR

We present recent STAR results on photoproduction in ultra-peripheral relativistic heavy-ion collisions. In these collisions the impact parameter of the beam particles is larger than the sum of their nuclear radii, so that they interact via their long-range Coulomb fields. STAR has measured the production of ρ 0 ( 770 ) mesons in exclusive reactions as well as in processes with mutual nuclear excitation of the beam particles. We present results for the ρ 0 production cross section in Au-Au collisions at s N N = 200 GeV for coherent as well as incoherent coupling. The dependence of the cross section on the ρ 0 rapidity is compared to theoretical models. We also studied the ratio of coherent ρ 0 to direct π + π − production as well as the ρ 0 helicity matrix elements and we observe interference effects in the ρ 0 production. In addition STAR has measured the production of ρ 0 mesons in d -Au collisions at s N N = 200 GeV and that of e + e − -pairs in Au-Au at s N N = 200 GeV . We also see an enhancement around 1510 MeV / c 2 in π + π − π + π − final states in Au-Au collisions at s N N = 200 GeV .

ρ0photoproduction in ultraperipheral relativistic heavy ion collisions atsNN=200GeV

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

Transverse Momentum Distribution of Vector Mesons Produced in Ultraperipheral Relativistic Heavy Ion Collisions

We study the transverse momentum distribution of vector mesons produced in ultraperipheral relativistic heavy ion collisions (UPCs). In UPCs there is no strong interaction between the nuclei, and the vector mesons are produced in photon-nucleus collisions where the (quasireal) photon is emitted from the other nucleus. Exchanging the role of both ions leads to interference effects. A detailed study of the transverse momentum distribution, which is determined by the transverse momentum of the emitted photon, the production process on the target, and the interference effect, is done. We study the unrestricted cross section and the one with an additional electromagnetic excitation of one or both ions; in the latter case small impact parameters are emphasized.

Multi-photon exchange processes in ultraperipheral relativistic heavy-ion collisions

The very strong electromagnetic fields present in ultraperipheral relativistic heavy-ion collisions lead to important higher-order effects of the electromagnetic interaction. These multi-photon exchange processes are studied using perturbation theory and the sudden or Glauber approximation. In many important cases, the multi-photon amplitudes factorize into independent single-photon amplitudes. These amplitudes have a common impact parameter vector, which induces correlations between the amplitudes. Impact-parameter dependent equivalent-photon spectra for simultaneous excitation are calculated, as well as, impact-parameter dependent γγ-luminosities. Excitations, like the multi-phonon giant dipole resonances, vector meson production and multiple e + e −-pair production can be treated analytically in a bosonic model, analogous to the emission of soft photons in QED.

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.