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 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 production cross section using the pomeron-exchange factorization relations. Received: 20 March 2003 / Published online: 2 June 2003     

Axial vector meson production

Recently available differential cross section and density matrix information on π^?p → B^?p at 4 GeV/c are successfully described in terms of t channel exchanges and s channel absorptive effects. The s channel helicity amplitude which is dominant at small |t| (zero net helicity flip ω exchange) is found to have a zero at ?t = 0.2 GeV². SU(3) and higher symmetries are used to predict cross sections for further axial vector meson production processes, in particular non-diffractive A₁ production. The importance of these processes with regard to Regge phenomenology and meson spectroscopy is emphasized.     

Hydrodynamic description of D meson production in high-energy heavy-ion collisions

The large values and constituent-quark-number scaling of the elliptic flow of low-pT D mesons imply that charm quarks,initially produced through hard processes,...     

The Pomeron-f identity and vector meson production

The Pomeron-f identity scheme proposed by Chew and Rosenzweig is shown to fail badly when compared to vector meson production data. In contrast, a standard exchange degenerate ωf model is satisfactory.     

High energy gamma-ray production in heavy-ion reactions

The doubly-differential cross section for the production of gamma-rays with energies larger than 20 MeV has been measured in the reaction ⁴⁰Ar+¹⁹⁷ Au at 30 MeV/u incident energy. It is found that the spectra are thermal-like, the emitting source having half the beam velocity and an apparent temperature around 7.3 MeV. The non-observation of a collective component is discussed.     

A study of inclusive vector meson production

Data are presented on the reactions π⁺n → ?⁰+X and $\text{K}{}^{\mathrm{?}}\text{p}\text{→}\text{K}{}^1\text{0}\text{(890)+}\text{X}\text{at 6.0}\text{and}\text{7.3}\text{GeV}\text{/c}$, respectively. Comparisons are made with related reactions at higher energies.     

Interference in exclusive vector meson production in heavy-Ion collisions

Vector mesons are produced copiously in peripheral relativistic heavy-ion collisions. Virtual photons from one ion can fluctuate into quark-antiquark pairs and scatter from the second ion, emerging as vector mesons. The emitter and target are indistinguishable, so emission from the two ions will interfere. Vector mesons have negative parity so the interference is destructive, reducing the production of mesons with small transverse momentum. The mesons are short lived, and decay before emission from the two ions can overlap. However, the decay-product wave functions overlap and interfere since they are produced in an entangled state, providing an example of the Einstein-Podolsky-Rosen paradox.     

t-dependences of vector meson diffractive production in ep collisions

We calculate p_T-dependences of diffractive vector meson photo- and electroproduction in electron-proton collisions with and without proton dissociation at the small momenta transfers. The calculated slopes are in good agreement with the data.     

Double vector meson production from the BFKL equation

The double vector meson production in two-photon collisions is addressed assuming that the color singlet t-channel exchange carries large momentum transfer. We consider the non-forward solution of the BFKL equation at high energy and large momentum transfer and estimate the total and differential cross section for the process , where V₁ and V₂ can be any two vector mesons ( ). A comparison between our predictions and previous theoretical results obtained at Born level or assuming the pomeron-exchange factorization relations is presented. Our results demonstrate that the BFKL dynamics implies an enhancement of the cross sections. Predictions for the future linear colliders (TESLA, CLIC and ILC) are given. Received: 17 June 2005, Revised: 1 August 2005, Published online: 6 October 2005 PACS: 12.38.Aw, 13.85.Lg, 13.85.Ni     

Diffractive vector meson production at large momentum transfer

The diffractive process γp→V+X (whereV is a vector meson andX results from the dissociation of the proton) is studied. In particular, we consider the region of large momentum transfer (i.e. |t| ?Λ _QCD ² ) and large centre-of-mass (CM) energy,s. The asymptotic (s→∞,s/|t|?1) behaviour is derived from the BFKL equation and compared to that which is obtained in the Born approximation (two-gluon exchange). We also calculate the corrections to the Born graphs by iterating the BFKL kernel numerically. Improved convergence of the BFKL series is found by summing the logarithms which occur when an exchanged gluon goes nearly on shell. Importantly, we find evidence that the asymptotic solution to the BFKL equation is inappropriate over most of the HERA range and we provide more realistic predictions for the cross section. The predicted cross section is not too small and can be measured at HERA, up to momentum transfers |t|≈10 GeV².     

Vector meson radiation in relativistic heavy-ion collisions

The (σ, ω) model in the mean-field approximation where the meson fields are treated classically, describes much of observed nuclear structure and has been employed to describe the nuclear equation of state up to the quark-gluon phase transition. The acceleration of the meson sources, for example, in relativistic heavy-ion collisions, should result in bremsstrahlung-like radiation of the meson fields. The many mesons emitted serve to justify the use of classical meson fields. The slowing of the nuclei during the collision is modeled here as a smooth transition from initial to final velocity. Under ultra-relativistic conditions, vector radiation dominates. The angular distribution of energy flux shows a characteristic shape. It appears that if the vector meson field couples to the conserved baryon current, independent of the baryonic degrees of freedom, this mechanism will contribute to the radiation seen in relativistic heavy-ion collisions. The possible influence of the quark-gluon plasma is also considered.     

Dilepton production and scaling

The 1968 BNL dimuon data are reanalyzed in the light of new data in the production of resonances, on the Feynman x behavior of dilepton continua and on the A-dependence of dilepton production. The results show a very high degree of consistency with scaling in the BNL energy domain s = 45–60 GeV². Comparison with Fermilab data at s = 750 GeV² confirms the agreement and indicates that m³ dσ/dm = F(m²/s).     

Direct photon production in heavy-ion reactions at SPS and RHIC

A review on experimental results for direct photon production in heavy ion reactions is given. A brief survey of early direct photon limits from SPS experiments is presented. The first measurement of direct photons in heavy ion reactions from the WA98 collaboration is discussed and compared to theoretical calculations. An outlook on the perspective of photon measurements at RHIC is given.     

Cold production of fast nucleons in central heavy-ion reactions

The transfer of fast nucleons between two ions is investigated as a possible source of preequilibrium emissions. The transferred intensities are given by the tunnelling of a semi classical nuclear matter distribution through the potential barrier. The calculations predict proton-to-neutron ratios larger than one (1.35 for⁴⁰Ca+⁴⁰Ca), linear increases of the multiplicities with the incident energy; the results show too that the flow resulting from the overlap of the frozen density tails cannot account for the characteristics of the tunnelling flow.     

The effect of hadronic scatterings on the measurement of vector meson spin alignments in heavy-ion collisions

Spin alignments of vector mesons and hyperons in relativistic heavy-ion collisions have been proposed as signals of global polarization.The STAR experiment first observed the ∧ polarization.Recently,the ALICE collaboration measured the transverse momentum {PT) and the collision centrality dependence of K*,and Φ spin alignments during Pb-Pb collisions at ~(1/2)sNN=2.76 TeV.A large signal is observed in the low pT region of mid-central collisions for K~*,while the signal is much smaller for Φ,and these have not been understood yet.Since vector mesons have different lifetimes and their decay products have different scattering cross sections,they suffer from different hadronic effects.In this paper,we study the effect of hadronic interactions on the spin alignment of K~*,Φ,and p mesons in relativistic heavy-ion collisions with a multi-phase transport model.We find that hadronic scatterings lead to a deviation of the observed spin alignment matrix element p00 away from the true value for p and K*mesons(with a bigger effect on p) while the effect is negligible for the Φ meson.The effect depends on the kinematic acceptance:the observed p00 value is lower than the true value when the pseudorapidity(η) coverage is small,while there is little effect when the η coverage is large.Hence,this study provides valuable information to understand the vector meson spin alignment signals observed during the experiments.