Electron-positron pairs as a probe for hadrons in dense matter

In this paper the latest results from the analysis of e ⁺ e ^--pairs emitted in Pb + Au collisions at 40 AGeV/c and a combined analysis of all data available at 158 AGeV/c are presented. The enhancement of low-mass e ⁺ e ^--pairs (m _ee > 0.2 GeV/c ²) with respect to the expected yield from hadron decays first reported at 158 AGeV/c is also found at 40 AGeV/c and is even larger there. Comparing to various theoretical models based on annihilation, the data can only be reproduced, if the properties of the intermediate in the hot and dense medium are modified. Hadron data taken at the same beam energies sheds light on the dynamical evolution of the fireball and addresses the question whether the medium modification is linked to baryon density rather than temperature. In the future e ⁺ e ^--pairs emitted in heavy-ion collisions will be measured at the LHC with the ALICE Transition Radiation Detector (TRD). Here, special emphasis will be put on the production of heavy vector mesons ( , ).Received: 30 September 2002, Published online: 22 October 2003PACS: 25.75.-q Relativistic heavy-ion collisions - 12.38.Mh Quark-gluon plasma - 13.85.Qk Inclusive production with identified leptons, photons, or other nonhadronic particles     

Reactor production of anhydrous18F ion

The direct production of anhydrous¹³F ion has been achieved through the incorporation of a new approach to hot atom reactions for reactor generated radiopharmaceuticals. Capitalizing on the traditional Li₂CO₃ method for hydrated¹⁸F production, a unique target design and geometry utilizing neutron generated energetic tritions emerging from the surface of a thin LiF embedded stainless steel matrix react with gaseous O₂ to produce¹⁸F as a free recoiling anhydrous species. Approximately 10–15 MBq of reactive anhydrous¹⁸F ion absorbed onto a metallic foil has been produced by this method. Verification of the production of¹⁸F was substantiated through decay measurements using Ge(Li) spectroscopic analysis, and by the synthesis of¹⁸F-fluoromethane gas. Cyclotron gas target chemistry which has been instrumental in the preparation of¹⁸F-2FDG and other metabolic tracers may be reproduced using this technique by the addition of 1% F₂ scavenger gas to a recirculating reactor gas handling system.     

Ultrafast photofragmentation dynamics of molecular iodine driven with timed XUV and near-infrared light pulses

Photofragmentation dynamics of molecular iodine was studied as a response to the joint illumination with femtosecond 800 nm near-infrared and 13 nm extreme ultraviolet (XUV) pulses delivered by the free-electron laser facility FLASH. The interaction of the molecular target with two light pulses of different wavelengths but comparable pulse energy elucidates a complex intertwined electronic and nuclear dynamics. To follow distinct pathways out of a multitude of reaction channels, the recoil of created ionic fragments is analyzed. The delayed XUV pulse provides a way of following molecular photodissociation of I(2) with a characteristic time-constant of (55 ± 10) fs after the laser-induced formation of antibonding states. A preceding XUV pulse, on the other hand, preferably creates a 4d(-1) inner-shell vacancy followed by the fast Auger cascade with a revealed characteristic time constant τ(A2)=(23±11) fs for the second Auger decay transition. Some fraction of molecular cationic states undergoes subsequent Coulomb explosion, and the evolution of the launched molecular wave packet on the repulsive Coulomb potential was accessed by the laser-induced postionization. A further unexpected photofragmentation channel, which relies on the collective action of XUV and laser fields, is attributed to a laser-promoted charge transfer transition in the exploding molecule.     

Markov programming by successive approximations with respect to weighted supremum norms

Markovian decision processes are considered in the situation of discrete time, countable state space, and general decision space. By introducing a Banach space with a weighted supremum norm, conditions are derived, which guarantee convergence of successive approximations to the value function. These conditions are weaker then those required by the usual supnorm approach. Several properties of the successive approximations are derived.     

Shortest expected delay routing for Erlang servers

The queueing problem with Poisson arrivals and two identical parallel Erlang servers is analyzed for the case of shortest expected delay routing. This problem may be represented as a random walk on the integer grid in the first quadrant of the plane. An important aspect of the random walk is that it is possible to make large jumps in the direction of the boundaries. This feature gives rise to complicated boundary behavior. Generating function approaches to analyze this type of random walk seem to be extremely complicated and have not been successful yet. The approach presented in this paper directly solves the equilibrium equations. It is shown that the equilibrium distribution of the random walk can be written as an infinite linear combination of products. This linear combination is constructed in a compensation procedure. The starting solutions for this procedure are found by solving the shortest expected delay problem with instantaneous jockeying. The results can be used for an efficient computation of performance criteria, such as the waiting time distribution and the moments of the waiting time and the queue lengths.     

A study of charge distributions in inelastic proton-proton collisions

Charge distributions for the secondaries from inelastic proton-proton collisions are studied by means of new data from an experiment at 12 and 24 GeV/c (2 m HBC at CERN). By comparison with π^±p data at 16 GeV/c, the inclusive charge distributions for the fragments of an incident proton and pion, respectively, are analysed. In addition, the cross sections for charge exchange between cms hemispheres are presented for the various charged-particle multiplicities. For events with no more than one neutral particle, charge and baryon exchange between cms hemispheres are compared to those between clusters. For the inelastic two-prong reactions, the two-dimensional distributions in longitudinal phase space are presented.     

Analysis of the reaction pp → p(ππ)0 p,and the question of double pomeron exchange in high-energy production processes

A low-mass S-wave (ππ)⁰ enhancement is observed in the reaction pp → p (ππ)⁰ p at 12 and 24 GeV/c incident momentum. The mechanism for producing these low-mass pion pairs is investigated, and it is shown that double pomeron exchange can contribute at most 5% to their production cross section at 24 GeV/c.     

Photoproduction of neutral pions on hydrogen at photon energies between 200 and 440 MeV

At the Bonn 500 MeV electron synchrotron the differential cross section of the reaction γ+p → π⁰+p has been measured detecting the recoil proton by a magnetic spectrometer. Angular distributions between pion c.m. angles of 50 ° and 160 ° were taken at photon energies between 220 and 420 MeV.