D. Brill

The kaon spectrometer at SIS

A double-focussing QD magnetic spectrometer has been developed and installed at the SIS heavy-ion facility at GSI in Darmstadt. The spectrometers primary purpose is to study meson production in energetic nucleus-nucleus collisions in detail. Its compact design is matched to the requirements of kaon detection with short flight path (5–6.5 m), large solid angle (up to 35 msr), wide momentum acceptance (pmaxpmin ⋍ 2), maximum momentum 1.6 GeV/c (1.9 GeV/c at reduced solid angle) and reasonable momentum resolution (⋍ 1% without and ⋍ 10−3 with ray tracing). A focal-plane length of about 1.5 m allows the efficient use of the detectors necessary for particle identification and ray-tracing, involving wire chambers, time-of-flight scintillators and Cherenkov detectors. Collisions can be characterized by two multiple-module plastic-scintillator hodoscopes detecting reaction fragments in the forward hemisphere. While the primary purpose for the construction of the spectrometer is the measurement of kaons, it can serve as a general purpose magnetic spectrometer. Its large solid angle also allows the study of two-particle correlations.

Enhanced out-of-plane emission of K+ mesons observed in Au + Au collisions at 1-A/GeV

The azimuthal angular distribution of K+ mesons has been measured in Au + Au collisions at 1 AGeV. In peripheral and semi-central collisions, K+ mesons preferentially are emitted perpendicular to the reaction plane. The strength of the azimuthal anisotropy of K+ emission is comparable to the one of pions. No in-plane flow was found for K+ mesons near projectile and target rapidity.

Emission pattern of high-energy pions: A new probe for the early phase of heavy-Ion collisions

The emission pattern of charged pions has been measured in Au+Au collisions at 1 GeV/nucleon incident energy. In peripheral collisions and at target rapidities, high-energy pions are emitted preferentially towards the target spectator matter. In contrast, low-energy pions are emitted predominantly in the opposite direction. The corresponding azimuthal anisotropy is explained by the interaction of pions with projectile and target spectator matter. This interaction with the spectator matter causes an effective shadowing which varies with time during the reaction. Our observations show that high-energy pions stem from the early stage of the collision whereas low-energy pions freeze out later.

Properties of high-energy pions emitted from heavy-ion collisions at 1 GeV/nucleon

Positively charged pions and protons from collisions of Ne+NaF and Au+Au at 1 GeV/nucleon incident energy were measured near midrapidity. The center-of-mass pion spectra deviate from a Maxwell-Boltzmann distribution. The slope of the high-energy part of the pion spectra varies significantly with the system mass and little with the size of the reaction zone. While the total pion yield rises linearly with the number of participant nucleons, the highenergy component increases more than linearly.

Kaon production in heavy ion collisions and the nuclear equation of state

Abstract Double differential cross sections for protons, π + and K + are measured in Au + Au collisions at 1.0 GeV/u and Θ lab =44°±4° with a new magnetic spectrometer at SIS. The pion spectra show an enhancement at low momenta with respect to a thermal distribution and little dependence on the collision centrality. The high-energetic pions measured in semi-central collisions at approximately midrapidity exhibit an anisotropic azimuthal angular distribution, peaking at 90° and 270° with respect to the reaction plane. The same feature is observed for protons. The kaon to proton ratio increases with the number of participating nucleons, i.e. with the centrality of the collision, whereas the pion to proton ratio stays constant. The first data on subthreshold K + production in a heavy mass collision system are discussed with respect to nuclear matter properties by a comparison to microscopic transport calculations.

Evidence for different freeze-out radii of high- and low-energy pions emitted in Au+Au collisions at 1 A.GeV

Abstract Double differential production cross sections of π − and π + mesons and the number of participating protons have been measured in central Au+Au collisions at 1 A·GeV. At low pion energies the π − yield is strongly enhanced over the π + yield. The energy dependence of the π − / π + ratio is assigned to the Coulomb interaction of the charged pions with the protons in the reaction zone. The deduced Coulomb potential increases with increasing pion c.m. energy. This behavior indicates different freeze-out radii for different pion energies in the c.m. frame.

Subthreshold kaon production in Au on Au collisions at 1 GeV/u

Subthreshold kaon production in197Au+197Au collisions at 1.0 GeV/u has been investigated with the Kaon Spectrometer at SIS. At Θlab=44±4∘ we found aK+/p ratio of>3 · 10−4 for the momentum range 650 MeV/c to 1150 MeV/c.

Pion and kaon emission from the fireball formed in Ne + NaF collisions at 1–2 GeV/nucleon☆

Abstract Double differential cross sections for proton, π + and K + meson production have been measured in Ne + NaF collisions at 1 and 2 GeV/nucleon. Parameterizations of the meson spectra with Maxwell-Boltzmann distributions yield common inverse slope parameters for kaons and high-energy pions at each beam energy. No evidence for different freeze out temperatures can be deduced from the spectral slopes of the emitted particles.

Kaon production in hadronic matter

Subthreshold kaon production has been studied in symmetric nucleus-nucleus collisions as a function of the nucleus mass, beam energy and centrality. In Au+Au collsions at 1 AGeV theK + multiplicity increases more than linearly with increasing number of participating nucleons. Transport calculations have to assume a soft equation of state in order to reproduce the data. The in-mediumK − cross section measured in Ni+Ni collisions is enhanced by about a factor of 7 as compared to the free cross section when using theK + cross section at equivalent beam energies as a normalization.

Meson Production in Heavy-Ion Collisions at 1 GEV/Nucleon

Relati vistic heavy-ion collisions provide a unique tool to study the properties of hot and dense nuclear matter. The high stopping power of nuclei in nuclear matter leads to a rapid heating and to a compression of the medium. Two classes of observables are available. One ofthem tests the dynamics of the nuclear matter expressed by pictorial words like flow, squeeze out and side splash, whereas the other class of observables explores the properties of hot and dense nuclear matter via particle production.